Literature Review

The Multifunctional Mesencephalic Locomotor Region

Article· Literature Review (PDF Available)inCurrent pharmaceutical design 19(24) · January 2013with 1,841 Reads
DOI: 10.2174/1381612811319240011 · Source: PubMed
Abstract
In 1966, Shik, Severin and Orlovskii discovered that electrical stimulation of a region at the junction between the midbrain and hindbrain elicited controlled walking and running in the cat. The region was named Mesencephalic Locomotor Region (MLR). Since then, this locomotor center was shown to control locomotion in various vertebrate species, including the lamprey, salamander, stingray, rat, guinea-pig, rabbit or monkey. In human subjects asked to imagine they are walking, there is an increased activity in brainstem nuclei corresponding to the MLR (i.e. pedunculopontine, cuneiform and subcuneiform nuclei). Clinicians are now stimulating (deep brain stimulation) structures considered to be part of the MLR to alleviate locomotor symptoms (i.e. axial symptoms) of patients with Parkinson's disease. However, the anatomical constituents of the MLR still remain a matter of debate, especially relative to the pedunculopontine, cuneiform and subcuneiform nuclei. Furthermore, recent studies in lampreys have revealed that the MLR is more complex than a simple relay in a serial descending pathway activating the spinal locomotor circuits. It has multiple functions. Our goal is to review the current knowledge relative to the anatomical constituents of the MLR, and its physiological role, from lamprey to man. We will discuss these results in the context of the recent clinical studies involving stimulation of the MLR in patients with Parkinson's disease.
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4448 Current Pharmaceutical Design, 2013, 19, 4448-4470
The Multifunctional Mesencephalic Locomotor Region
Dimitri Ryczko1,2 and Réjean Dubuc1,2,*
1Groupe de Recherche sur le Système Nerveux Central, Département de physiologie, Université de Montréal, Montréal, Canada;
2Groupe de Recherche en Activité Physique Adaptée, Département de kinanthropologie,Université du Québec à Montréal, Montréal,
Canada
Abstract: In 1966, Shik, Severin and Orlovskii discovered that electrical stimulation of a region at the junction between the midbrain and
hindbrain elicited controlled walking and running in the cat. The region was named Mesencephalic Locomotor Region (MLR). Since
then, this locomotor center was shown to control locomotion in various vertebrate species, including the lamprey, salamander, stingray,
rat, guinea-pig, rabbit or monkey. In human subjects asked to imagine they are walking, there is an increased activity in brainstem nuclei
corresponding to the MLR (i.e. pedunculopontine, cuneiform and subcuneiform nuclei). Clinicians are now stimulating (deep brain
stimulation) structures considered to be part of the MLR to alleviate locomotor symptoms of patients with Parkinson’s disease. However,
the anatomical constituents of the MLR still remain a matter of debate, especially relative to the pedunculopontine, cuneiform and subcu-
neiform nuclei. Furthermore, recent studies in lampreys have revealed that the MLR is more complex than a simple relay in a serial de-
scending pathway activating the spinal locomotor circuits. It has multiple functions. Our goal is to review the current knowledge relative
to the anatomical constituents of the MLR, and its physiological role, from lamprey to man. We will discuss these results in the context of
the recent clinical studies involving stimulation of the MLR in patients with Parkinson’s disease.
Keywords: Locomotion, mesencephalic locomotor region, pedunculopontine nucleus, cuneiform nucleus, laterodorsal tegmental nucleus,
lamprey, acetylcholine, Parkinson’s disease.
INTRODUCTION
Locomotion is a fundamental motor act required for several
daily activities. In vertebrates, the neural mechanisms underlying
locomotor control involve both spinal and supraspinal structures.
The complex and stereotyped patterns of muscle contractions un-
derlying locomotor movements are generated by “central pattern
generato rs” (CPGs) located in the spin al cord [1]. These specialized
neural networks can intrinsically generate the locomotor pattern
even when isolated from the brain, muscles, and sensory feedback.
That motor output is then referred to as “fictive locomotion” [2].
The presence of spinal cord CPGs for locomotion was demonstrated
in many vertebrate species including the lamprey, salamander,
Xenopus embryo, zebrafish, dogfish, bird, turtle, mouse, rat, cat,
and rabbit (for review see [3;4]). Studies also suggest that a spinal
CPG for locomotion is present in humans. During the First and
Second World Wars, surgeons observed that soldiers with dramatic
injuries to the sp inal cord (e.g. total spinal transection) could gener-
ate locomotor movements [5;6]. Involuntary, rhythmic, alternating
stepping-like movements of the lower limbs have been described in
a subject with an incomplete spinal co rd injury at the cervical level
[7] and in patients with complete paraplegia [8]. This h as been de-
scribed also in kittens and cats with complete spinal transections [9-
12]. In patients with complete spin al transections, electrical stimu-
lation applied on the spinal cord below the lesion elicited the typical
locomotor muscular contractions in the lower limbs [13]. Further-
more, stimulation of the spinal cord with an electrode array elicited
locomotor bouts and weight support in a patient with a spinal lesion
who showed no detectable signs of supraspinal motor control below
the lesion [14] (for review see [15]). Human newborns generate
locomotor-like patterns when feet are placed on a plan e surface an d
yet they lack mature descending pathways [16;17]. The neural cir-
cuitry for stepping is already operational before intentional walking
is even possible, as shown with infants with weight support that are
*Address correspondence to this author at the Université du Québec à Mon-
tréal, Département de Kinanthropologie, C.P. 8888, Succ. Centre-Ville,
Montréal (Québec), Canada H3C 3P8; Tel: (514) 343 5729;
Fax: (514) 343 2111; E-mail: rejean.dubuc@gmail.com
able to step on a treadmill and respond to disturbances (i.e. limb
grasping or limb loading) by maintaining equilibrium and forward
progression in a bilaterally organized fashion [18]. More recently, a
detailed EMG an alysis revealed that some basic limb muscle activ-
ity patterns that are present in various vertebrates are also present in
human newborn babies and retained in adulthood [19]. Spinal lo-
comotor circuits thus seem to be conserved among vertebrate spe-
cies [20].
The organization of supraspinal structures controlling locomo-
tion also appears to be conserved through the vertebrate phylum.
Studies carried out in various vertebrate species have demonstrated
that stimulation of several supraspinal regions can induce locomo-
tion. However, not all these regions are locomotor centers, i.e. neu-
ronal areas devoted to elicit locomotion in a controllable fashion
[21]. One of these locomotor centers located at the border of mid-
brain and hindbrain was originally characterized physiologically in
the cat by a group of neurophysiologists from Moscow in the
1960's. Shik, Severin and Orlovskii demonstrated that unilateral or
bilateral electrical microstimulation of this region elicited con-
trolled walking, trotting, and galloping in pre-collicular/post mamil-
lary decebrated cats placed on a treadmill with weight support.
They named this region “Mesencephalic Locomotor Region”
(MLR) [22]. Interestingly, the locomotor output was reliably con-
trolled by the intensity of the stimulation. Increasing it allowed the
transition from one locomotor mode to another. Since then, the
MLR was identified physiologically in various species including
the lamprey [23], salamander [24], rat [25], stingray [26], rabbit
[27], guinea-pig [28] and monkey [29]. The detailed neural circuits
underlying the physiological effects of MLR stimulation are only
partially understood in mammals. However, the downstream loco-
motor pathways have been characterized in detail at the system and
the cellular levels in lampreys. It now appears that the MLR is more
complex than a simple relay within serially connected supraspinal
structures. It sends multiple parallel projections that ensure inte-
grated, behaviorally coherent physiological functions.
We now review the current knowledge relative to the organiza-
tion and the role of the MLR in several animal models and in hu-
mans. We will first review the possible contribution of the cunei-
1873-4286/13 $58.00+.00 © 2013 Bentham Science Publishers
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4449
form nucleus (CuN), the subcuneiform nucleus (subCuN), the pe-
dunculopontine nucleus (PPN) as putative MLR components mostly
in mammalian models. We will then review recent advances in
lampreys, in relation to the input-output functions of the MLR. The
role of the laterodorsal tegmental nucleus (LDT) will also be ad-
dressed. Finally, we will discuss the implications relativ e to clinical
studies in which the MLR is stimulated to alleviate the symptoms of
gait diso rders e.g. in p atients with P arkinson’s disease.
1. The MLR in Mammals
The confirmed presence of the MLR from lamprey to man indi-
cates that it is highly conserved in the vertebrate phylum (for re-
view see [30]). The MLR is strategically located b etween forebrain
structures and hindbrain regions that project directly to the spinal
locomotor CPG. The pioneering experiments of Shik, Severin and
Orlovskii (1966) [22] in the cat revealed that the MLR is a region of
central importance in the initiation and control of locomotion. The
Horsley-Clarke coordinates (P2, L4, H0) of the stimulation sites in
the cat brain [31;32] indicated that this “definite region of the mid-
brain” corresponded to the CuN [33]. In mammals the activation of
locomotion following MLR stimulation is believed to activate
mono or disynaptically reticulospinal neurons of the pontine and
medullary reticular formation (e.g. in the nucleus gigantocellularis,
nucleus magnocellularis, and raphe nuclei) [34-38]), but further
research is needed to identify the detailed descending projections in
mammals [39].
The MLR receives inputs from several brain regions, which
lead some to propose that it channels locomotion via different sub-
compartments depending on behavioral contexts [40]. It was pro-
posed to be divided into “exploratory”, “defensive”, and to a lesser
extent “appetitive” subsystems. The forebrain inputs to these three
subsystems differ as they originate respectively from the basal gan-
glia, the medial hypothalamus and the lateral hypothalamus (Fig. 1,
for review see [41]). The motor cortex is known to contribute to
visuomotor coordination and precision walking most probably by
direct projections to reticulospinal neurons (for review see [42]).
Recent data show that cortical projections to the PPN (considered to
be part of the MLR) are present in monkeys [43;44] and humans
[45;46] (Fig. 1 and section 1.2.5), but their role is not known in the
context of locomotor control. It was recently shown that the basal
ganglia circuitry is conserved from lamprey to man [47-53] (see
section 2.2). Functionally, this is consistent with the concept that
the basal ganglia are designed for action selection in vertebrates
[54]. According to this theory, before any action is selected, the
corresponding neural structures are tonically inhibited by the dual
output structures of the basal ganglia [i.e. the substantia nigra pars
reticulata (SNr) and the globus pallidus interna (GPi)]. When an
action is selected, the region underlying this specific action is tem-
Fig. (1). The inputs and outputs of the MLR in mammals. Inputs originate from the basal ganglia, the lateral and the medial hypothalamus. For the sake of
simplicity, the basal ganglia connectivity is simplified, and the extensive connections between the PPN (considered to be part of the MLR) and the substruc-
tures of the basal ganglia are not illustrated. The descending outputs from the MLR to reticulospinal neurons that in turn project to the locomotor CPG are
indicated. Neurons colored in blue are glutamatergic, whereas those in yellow are cholinergic. CPG, central pattern generator; D1, type 1 dopaminergic recep-
tors; D2, type 2 dopaminergic receptors; GPe, globus pallidus externa, GPi, globus pallidus interna; HYPOTH, hypothalamus; PAG, periaqueductal gray;
PPN, pedunculopontine nucleus; RS, reticulospinal; SNc substantia nigra pars compacta; SNr, substantia nigra pars reticulata; STN, subthalamic nucleus;
THAL, thalamus (Adapted from [59;41]).
4450 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
porarily disinhibited. A functional balance between the “direct” and
the “indirect” p athways within the basal ganglia controls the net
inhibition that is sent out [55]. The two pathways originate from the
input structure of the basal ganglia (i.e. the striatum). Initiation of a
motor action involves activation of the striatal (striatonigral) neu-
rons of the “direct pathway”, which then send stronger inhibitory
input onto the SNr, one of the two inhibitory output structures of
the basal ganglia. The net result is desinhibition of the target motor
structure, and thus movement is initiated. On the other hand, sup-
pression of a motor action involves the activation of the striatal
(striatopallidal) neurons of the “indirect pathway” which then send
increased inhibition to the globus pallidus externa (GPe). The latter
sends a weaker inhibition to the subthalamic nucleus (STN) which
then sends stronger excitation to the other inhibitory output struc-
ture of the basal g anglia, the GPi. The net result is inhibition of th e
target motor structures and the end of associated movements. Stri-
atal neurons of these two pathways are modulated by dopaminergic
inputs from the substantia nigra pars compacta (SNc) and ventral
tegmental area (VTA). Dopamine tends to increase the initiation of
motor actions by increasing the excitability of striatal neurons of
the “direct pathway” through D1 receptors, and by decreasing the
excitability of striatal neurons of the “indirect pathway” through D2
receptors (for review see [56]). This is consistent with the hypolo-
comotor effects of dopaminergic depletion, and the hyperlocomotor
effects of dopaminergic stimulation from lamprey [57;50] to man
[58].
The MLR has been primarily defined functionally and, to this
day, its anatomical substrate is not fully characterized and still re-
mains a matter of debate. Anatomically, there are several ways to
define the MLR. For instance, experiments have been designed to
identify the location of the MLR by activating it electrically, by
activating or inactivating it pharmacologically, by making lesions in
the area, by tracing its connections with other brain structures, and
by investigating the neurochemical nature of its neuronal popula-
tions. Integrating these approaches should ultimately lead to a satis-
fying functional and anatomical definition of the MLR, hopefully
for all vertebrate species. Prim arily considered to be part of the
mammalian MLR are the PPN and the CuN. The role of each of the
constituents is not fully understood. The close proximity of the CuN
and PPN somehow prevented a clear understanding of the contribu-
tion of these two nuclei. In some species, th e subCuN was also
proposed as of central importance. However from one species to
another, the existence of an anatomical structure or the definition of
its borders can vary (e.g. the subCuN is not described in all verte-
brates). The mesopontine cholinergic cells have also been identified
as a key component of the MLR, but because this population spans
over more than one nucleus (e.g. PPN, CuN, LDT), there has been
confusion as to what constitu tes the MLR. On the theoretical side, it
has been proposed that subcomponents of the MLR are associated
with different behavioral contexts [40] (for review see [59;41]).
This entails that the subcomponents (i.e. the PPN and the CuN)
would be recruited by substantially different inputs. In this context,
the design of the experiment is of primary importance, as this will
influence the recruitment of the various subsets of neurons within
the MLR [41]. In the next sections we will review the knowledge
that supports a contribution of the CuN, subCuN, and PPN as parts
of the mamm alian MLR. The role of the cholinergic cells of the
MLR will also be discussed, as well as a possible contribution from
the LDT.
1.1. The Cuneiform and Subcuneiform Nuclei
Originally described in cats as the substrate of the MLR for its
ability to finely control locomotion [22;33], the CuN was proposed
by Sinnamon (1993) [40] as the core of the "defensive system" (for
review, see [59;41]). This subcomponent of the MLR controls not
only locomotion, but also cardiovascular and analgesic responses to
painful or threatening stimuli [60;61]. When a rat smells the odor of
a cat, neurons of the CuN are activated together with other defense-
related structures (i.e. medial amygdala, medial hypothalamus and
periaqueductal gray) as revealed by c-fos immunoreactivity [62].
This stimulus is also known to activate neurons in the posterior
accessory olfactory bulb, a region involved in the processing of
predator odor [63]. In hamsters, an increased c-fos expression oc-
curs in the CuN of subordinate males that experienced a stressful
social interaction against dominant males [64]. The data reviewed
here suggests that typical features of this nucleus can be found in
several mammalian species.
1.1.1. Cellular Morphology
Both the CuN and subCuN nuclei are part of the “area cunei-
formis” described both in the macaque and human brains as consti-
tuted of “intercalated cells and fiber fascicles between the inferior
colliculus and the periventricular gray” [65]. Olszewski and Baxter
(1954) subdivided the cuneiform area into a CuN and a subCuN in
the human [66]. This subdivision is also reported in the cat [67;68]
and merely suggested in the gorilla [69], but certainly not system-
atically for all vertebrate species [70].
The CuN was anatomically described in the human on the basis
of the location and morphology of its neurons [66]. It is located
ventral to the inferior and superior colliculi, in the dorsolateral part
of the mesencephalic tegmentum. The nucleus extends from the
caudal border o f the inferior colliculus to the ro stral border of the
superior colliculus. Its size increases from caudal to rostral, bor-
dered rostrally by the pretectal region. Viewed transversally, the
CuN appears triangular or quadrangular in shape. It is bordered
medially by the mesencephalic central gray matter, dorsally by the
colliculi, laterally by the medial lemn iscus and the nucleus
paralemniscalis, and ventrally by the subCuN. The CuN is com-
posed of small to medium sized triangular, fusiform or oval cells
with short processes. Most neurons lie with long, dorsomedially-
directed axes and are associated w ith glial satellites.
The subCuN is located in the lateral part of the m idmesen-
cephalic tegmentum [66]. On cross section, the subCuN lies with its
axis directed dorsomedially. It is bordered dorsally by the CuN, and
ventrally by the PPN, while its medial and lateral neighbor struc-
tures are mostly similar to those o f the CuN. In humans, the subdi-
vision between the CuN and subCuN is based on three cytoarchitec-
tonic differences [66]. First, the subCuN has a lesser density of cells
than the CuN. Second, among the CuN-like cells, there are large
neurons located in the subCuN. Third, the subCuN exhibits a lesser
degree of glial satellitosis. However, no precise anatomical bound-
ary differentiates the CuN and subCuN [70].
In the gorilla, the area cuneiformis was described as the lateral
and dorsolateral mesencephalic tegmentum ventral to the colliculi
[69], as previously observed in the human and macaque [65]. The
area cuneiformis of the gorilla comprises small to medium sized
ovoid, triangular, fusiform and spindle-shaped multipolar cells,
which are scattered throughout the area. Based on the criteria de-
fined in the human by Olszewski and Baxter (1954) [66], there
seems to also be a subdivision between the CuN and the subCuN in
the gorilla [69]. Nevertheless the author proposed to divide the Cun
and subCuN into two subnuclei rather than two nuclei, as the dif-
ferences he observed “may be more of degree than substance” [69].
In the cat [67;68;71], the CuN extends throughout the rostro-
caudal length of the midbrain where it is located directly ventral to
the superior and inferior colliculi. As in humans, it increases in size
from caudal to rostral and merges rostrally with the pretectal region.
The CuN is bordered medially by the periaqueductal gray, dorsally
by the nucleus nervi trigemini mesencephalicus, laterally by the
lateral lemniscus and nucleus paralemniscalis, and ventrally by the
subCuN [67]. Ventrally, the rostral portion of the CuN is separated
from the brachium conjunctivum by the lateral portion of the PPN,
and in its caudal portion by the lateral parabrachial subnuclei [72].
The nucleus contains small-sized oval or spindle-shaped cells, and
medium-sized fusiform or oval cells [67;68;73]. In the cat, the sub-
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4451
CuN extends “from about the rostral level of the decussation of the
brachium conjunctivum to the rostral level of the nucleus oculomo-
torius principalis. It fills the tegmentum ventral to the CuN” [67].
As in the gorilla and human, this nucleus has a lower density of
cells compared to the CuN. The cells mostly resemble those ob-
served in the CuN. Interestingly, as in the gorilla and human, Taber
observed that some cells were similar to those observed in the PPN.
These large neurons may be some of the cholinergic cells of the
PPN pars compacta, known to extend dorsally and overlap with the
subCuN (see e.g. (Fig. 4) in [74]; for review see e.g. [70]).
In the rat, the borders of the CuN were described in studies that
explored the role of the CuN in pain modulation. On the basis of the
cellular morphology described previously in the cat [67], the CuN
of the rat has been described as a large midbrain reticular structure
extending from the rostral pons to the pretectal thalamus [75;76]).
In the rat, neuronal morphology has been used to distinguish the
CuN and subCuN [77] (see also [78;79]).
Though not a mammal, it is worth to note that in the stingray
[26], the MLR is d escribed as the caud al part o f the CuN and is
bordered medially by the periaqueductal gray and medial longitudi-
nal fasciculus as in the cat [67;68], dorsally by the lateral and me-
dial mesencephalic nuclei which would correspond to the inferior
colliculu s in mamm als, laterally by the lateral lemniscus, and ven-
trally by the subCuN.
Taken together these observations indicate that from cat to man,
the CuN and subCuN share many features, including the same
neighboring structures and similar cell morphology. Interestingly,
the ventral part of the CuN (i.e. the subCuN) in the cat, gorilla, and
man contains cells that are morphologically similar to those of the
PPN located further ventrally.
1.1.2. Neurotransmitters
Neurotransmitter expression in the cuneiform area is heteroge-
neous. In mammals, it includes GABAergic, nitrergic, glutamater-
gic, peptidergic and some cholinergic neurons.
GABAergic neurons [i.e. GABA or glutamic acid decarboxy-
lase (GAD) positive neurons] have been observed in the CuN in
cats [80;72] and rats [81], as well as in the subCuN in cats [80],
with no apparent boundaries between the two structures.
Neurons containing nitric oxide synthase were observed in the
rat, but not frequently encountered [82]. Neurons containing both
nitric oxide synthase and NADPH-diaphorase were described in the
cat [72]. NADPH-diaphorase-reactive neurons did not express the
choline acetyltransferase (ChAT) [72], suggesting that these are not
cholinergic, contrasting with the NADPH-diaphorase-reactive neu-
rons in the PPN and LDT of the rat [83] and monkey [84].
Another group of neurons could use glutamate as a neurotrans-
mitter. A large portion of neurons were immunoreactive to gluta-
mate in the cat [82]. This concurs with previous anatomical studies
in the rat showing that efferent projections from the CuN to the
periaqueductal gray [85;86] and to the medullary nucleus raphe
magnus [87] contain an excitatory amino acid that could be gluta-
mate. In contrast, another study revealed no glutamate immunoreac-
tive neurons in the CuN of the cat [72].
A small number of large cholinergic neurons (i.e. ChAT im-
munoreactive) were observed in the CuN in the rat [88]. These were
considered by the authors as satellite cells of the m ain cholinergic
nucleus located in the PPN, somehow reminding the “misplaced
cells” of the PPN described in the CuN of the gorilla [69]. In the
cat, characterization of the central cholinergic system using ChAT
immunohistochemistry revealed virtually no cholinergic cell in the
CuN [89;90;74], compared to the neighboring PPN and LDT, with
the exception of a few neurons at the very ventral portion of the
CuN, which probably corresponds to the subCuN (see (Fig. 4) in
[74]; see also [91]). Cholinergic neurons were also described in the
subCuN and in the PPN in the monkey [92]. However the authors
called the entire area “PPN area”, thus underlining that the center of
mass of this cholinergic population is not the CuN. Similarly in the
human, the CuN, mostly in its subCuN portion, contains some cho-
linergic cells spreading from the Ch5 cholinergic population whom
center of mass is located in the PPN pars compacta (for review see
[93]).
Peptides like neurokinin b in the cat [94], enkephalin in the rat
[95;96]) and cat [97], neurotensin in the rat [98], substance P in the
rat [96] or corticotropin releasing factor in the rat [99] have been
shown to be present in the CuN. In the cat, the CuN does not seem
to contain catecholaminergic, glycinergic or serotoninergic neurons
[72]. The latter result contrasts w ith the observation in the rat that
serotoninergic neurons are present in the CuN [98].
1.1.3. Descending Projections of the Cuneiform and Subcunei-
form Nuclei
The CuN projects to several areas of the CNS. In the cat, two
distinct descending fiber systems were observed. The first descend-
ing system, called “the ventral tegmental bundle”, is large, compact,
crosses the midline in the midbrain, and projects down to the ven-
tromedial tegmentum of the brainstem. In contrast the second de-
scending system projects ipsilaterally and contains as many or more
fibers, but is loosely organized, with divergent routes. The targets
of all of the descending projections include reticulospinal neurons
(reticularis pontis caudalis,reticularis gigantocellularis and raphe
magnus), nuclei which send ascending projections (locus coeruleus,
nuclei linearis intermedius and linearis rostralis,raphe dorsalis,
periaqueductal gray matter and superior colliculus), nuclei which
send projections to the cerebellum (inferior olive, lateral reticular
nucleus, locus coeruleus,nucleus griseum pontis,nucleus reticu-
laris tegmenti pontis), and cranial nerve nuclei (facial and oculomo-
tor nuclei) (reviewed in [73]). Furthermore connections of the CuN
with the magnocellular reticular nucleus in the cat [100] and in the
monkey [101], as well as with the nucleus raphe magnus in the cat
[100], rat [98] and monkey [101] have been observed. The descend-
ing projections of the MLR have been characterized in the cat by
injections of tritiated amino acids in the ventral portion of the CuN
(P2, L4, H-1) [102]. The authors observed bilateral, but mainly
ipsilateral descending projections to the gigantocellular and magno-
cellular reticular nuclei of the pons and medulla, to the nucleus
raphe magnus (all containing reticulospinal neurons), and to the
dorsal tegmental reticular nucleus (which projects to the cerebel-
lum). Projections to the contralateral CuN were also observed.
These results are consistent with a role of the CuN in locomotion
and pain modulation.
As in the cat, efferent projections of CuN in the rat descend
predominantly ipsilaterally in the medulla and pons, reaching the
gigantocellular reticular and caudal pontine nuclei [103], but also
the nucleus raphe magnus,nucleus magnocellularis, and nucleus
reticularis parvocellularis [76]. Contralateral descending fibers
coursed ventrolaterally down to the nucleus magnocellularis. The
subdivision of the CuN into a rostral and a caudal region was pro-
posed on the basis of different afferent and efferent projection pat-
terns [76]. The CuN of the rat would correspond to the caudal por-
tion of the nucleus defined in the cat by Taber (1961) [67] [75;76].
The caudal portion of the CuN in the rat projects extensively to the
ventral medulla (i.e. nucleus raphe magnus and nucleus magnocel-
lularis) [75;76]. Neurotensinergic and serotoninergic projections
from the CuN to the nucleus raphe magnus were described in the
rat [98].
Descending projections from the subCuN include ipsilateral
fibers descending ventrally to the ventral nucleus of the lateral lem-
niscus at ro stral pontine lev el with fibers bifurcating to the nucleus
reticular pontis oralis in the rat [76]. Contralateral descending fi-
bers from the subCuN course near the rubrospinal tract, with bifur-
cating fibers innervating the ventrolateral portion of the nucleus
4452 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
reticularis pontis oralis. No descending fibers were observed in the
caudal pons.
Descending projections of the CuN to the spinal cord were ob-
served in the monkey [104], but not in the cat [68]. In the rat, neu-
rons of the CuN projected to the spinal cord with a strong ipsilateral
dominance via the lateral funiculus. Neurons from the subCuN
projected with a strong dominance to the contralateral spinal cord in
the rat [77].
1.1.4. Physiology
Electrical stimulation of the vicinity of the CuN is associated
with aversive or escape response in freely moving cats (i.e. fast
running and jumps, see [105]) and in freely moving rats (i.e. violent
running and explosive jumps, see [106]) (for review see [59]). In
the rat, a microinjection of glutamate into the cuneiform area in-
duces defensive/aversive reactions including freezing at the first
injection, and fast running with subsequent injections into the same
site [107]. Darting behavior consisting of fast progressions alternat-
ing with freezing sequences, are induced by an injection of gluta-
mate in the CuN [108]. This behavior is typically used by animals
moving in a dangerous environment to escape detection from a
predator [109].
Other studies corroborate the observation made by Shik and
colleagues (1966) [22] that the stimulation of the CuN simply elic-
its locomotion. In freely moving awake cats (i.e., no decerebration),
unilateral or bilateral stimulation of this region (0.1 ms, 25-30 A,
300 Hz) increased the velocity of the performance of cats trained to
cross a runway above the water in search for food [110]. The elec-
trodes were implanted in a “specific tegmental region ventrolateral
to the lateral border of the periaqueductal gray substance which
contained the CuN and the nucleus pontine oralis reticularis”. In
pre-collicular decerebrated cats, unilateral stimulation (0.1 ms,
<200 A, 20 Hz) of the MLR, in a region described as the “caudal
part of the CuN” (Horsley-Clarke coordinates P2, L4, H0), elicited
rhythmic discharge of trunk, cervical and hindlimb muscle nerves
[111;112]. In the macaque monkey decerebrated at the mid-
thalamic level and suspended above a treadmill, stimulation (0.2
ms, 25-200 A, 50 Hz) of a site located “in the vicinity of the CuN
and the superior cerebellar peduncle” elicited controlled rhythmic
coordinated movements of the limbs [29]. Increasing the stimula-
tion intensity increased the frequency of locomotor movements and
even elicited gait transition from walking to galloping. In precol-
licular/post-mammillary rabbits, unilateral stimulation (0.2 ms, 300-
600 A, 30 Hz) of an “area below the inferior colliculus […] corre-
sponding to the CuN” (P12.5, L3, H-3) elicited locomotion moni-
tored by kinematic and EMG recordings [27]. In the guinea-pig,
locomotion monitored by EMG recordings could be induced by
unilateral stimulation (0.5 ms, 20-100 A, 50 Hz) of a “ventral area
of the in ferior colliculus preferentially in the dorsal part of the
CuN” [28]. Though not a mammal, it is worth noting that in a sting-
ray decerebrated above the mesencephalon with cerebellum re-
moved, a unilateral stimulation of the MLR (0.1 ms, 50-200 A,
30-100 Hz) could elicit fictive locomotion and control its frequency
[26]. The electrodes were located in an area of 2 mm wide centered
on the midline, extending in the caudal midbrain. The authors de-
scribe the region as the “caudal portion of the interstitial nucleus of
the medial longitudinal fasciculus and the caudal and medial parts
of the CuN and subCuN”. Recent experiments in the cat revealed
that stimulation of the rostromedial vicinity of the CuN (P0-2, L3,
H0) elicits synaptic responses both in the reticulospinal cells and
spinocerebellar neurons [36]. Interestingly, these same spinocere-
bellar neurons also relay inputs from the corticospinal tract [36].
This convergence of descending inputs onto spinocerebellar neu-
rons could ensure a proper execution of complex motor behaviors
involving locomotion and voluntary movements [36].
Interestingly, physiological data provide support for the CuN
taking part in an integrated defensive system. In addition to loco-
motion, the CuN seems to regulate other physiological functions. It
controls nociception and adjusts the metabolism through regulation
of cardiovascular and respiratory activities. Coordinating these
functions when trying to escape from a predator seems physiologi-
cally coherent.
Activation of the CuN by electrical stimulation [60;61;113-115]
or glutamate injection [116] indeed elicits cardiovascular responses
in the rat, i.e. increased arterial pressure and heart rate. These re-
sponses are associated with increased mRNA expression of c-fos in
various regions of the forebrain [113], midbrain and hindbrain [61]
that are believed to be involved in the control of the cardiovascular
system.
In the decerebrate cat, electrical stimulation of the MLR not
only induces locomotion, but also increases respiratory activity, and
this, several seconds before the initiation of locomotion. The stimu-
lated region corresponds to the CuN (P2, L4, H0, (see Fig. 1) and
(Fig. 2) in [117]; P2, L4, H+1, see [118]) or its vicinity (P1.5, L4,
H+0.5, (see Fig. 5) [117]). This multifunctional effect is associated
with a strong coupling between the locomotor and respiratory
rhythm, especially during galloping [117]. Respiratory increases in
the absen ce of movement-related feedback can also be elicited by
stimulation of the subthalamic locomotor region [119]. The neural
pathway underlying the effects of MLR stimulation on respiration
was recently discovered in lampreys [120] (see section 2.1.6). It
was shown that the dorsal part of the MLR sends direct projections
to the respiratory networks, including the region responsible for
generating the respiratory rhythm. In the rat, similar coupling be-
tween locomotion and respiration may also be present, as retrograde
labeling revealed that neurons from the CuN, PPN and LDT project
to the rostral ventral respiratory group [121].
The CuN is believed to also be a component of the descending
antinociceptive system [76;87; 103;122-125]. Electrical stimulation
of the CuN (200 ms, 100-200 A, 60 Hz) elicits analgesic response
in the nociceptiv e tail flick reflex (i.e. a model of acute pain) in rats.
These effects are believed to involve the descending projections to
both the nucleus raphe magnus and the nucleus magnocellularis
[122;76]. Neurons of the CuN show an increased expression of c-
fos during carbachol-induced active sleep in the cat [72]. This in-
creased activity might be considered as consistent with an antinoci-
ceptive role of the CuN, as pain detection threshold increases dur-
ing sleep (e.g. [126]).
Lesion experiments of the CuN have led to puzzling results.
Bilateral ibotenate lesion of the CuN in the rat did not disrupt either
the spontaneous locomotion or locomotor activity induced by injec-
tions of d-amphetamine into the nucleus accumbens [127]. The
authors concluded that the CuN cannot be considered as an ana-
tomical substrate of the MLR. However, the selectivity of th e lesion
towards neurotransmitter phenotype remains to be determined.
Whether all neuronal subsets of the MLR were disrupted is un-
known [41]. This does not exclude that other locomotor regions
such as the diencephalic locomotor region described in rats (“sub-
thalamic locomotor region” see e.g. [128;129]) and in lampreys
[130;131]) or the cerebellar locomotor region [132] could be re-
sponsible for locomotor control after lesion of the MLR.
1.1.5. Inputs to the Cuneiform and Subcuneiform Nuclei
Whereas its outputs are quite focused, the CuN receives inputs
from several forebrain structures that include the amygdala, zona
incerta (which also projects to the subCuN, see [133]), hypothala-
mus and periventricular gray matter; and midbrain structures that
include the periaqueductal gray matter, substantia nigra pars later-
alis, peripeduncular area, and the contralateral CuN in the rat [103].
As indicated above, the CuN is believed to be the core of a “defen-
sive system” that serves to regulate the locomotor, cardiovascular,
and antinociceptive responses to painful or threatening stimuli (e.g.
[60;61]). In this context, the CuN has been shown to constitute an
output station for the defensive responses (i.e. escape behavior,
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4453
associated with an increased respiration, blood pressure, heart rate
and even analgesia) elicited by stimulation of the dorsal periaque-
ductal gray, superior and inferior colliculus or medial hypothalamus
[134] (for review [59;41]). This is consistent with the description
that the defensive portion of the MLR receives excitatory input
from the periaqueductal gray, which itself receives excitatory input
from the medial hypothalamus [41] (see. Fig. 1).
Stimulation of the dorsal periaqueductal gray is known to elicit
defensive escape reactions in vertebrates (explosive flight or freez-
ing, see references in [135] and in [82]). Stimulation of the periaq-
ueductal gray and medial hypothalamus increases c-fos activity in
the neurons of the CuN [136-138]. In the rat, the CuN receives
significant input from the dorsal periaqueductal gray [139]. Injec-
tion of a GABA antagonist (i.e. bicuculline) or a glutamic acid de-
carboxylase inhibitor in the dorsolateral periaqueductal gray of the
rat respectively induced escape and freezing reactions [140]. The
escape reaction was associated with an increased c-fos expression
in the CuN [140].
The inferior colliculus has reciprocal connections with the dor-
sal periaqueductal gray and is involved in aversive-related functions
(for review see [141]). Escape behavior elicited by electrical stimu-
lation of the inferior colliculus, or injection of bicuculline in the
inferior colliculus, is associated with increased c-fos immunoreac-
tivity in th e CuN and in the dorsal periaqueductal gray in the rat
[135;134].
Antinociceptive effects of local injection of morphine into the
CuN on the tail flick reflex have been observed in rats [123;124].
This effect is reduced when an electrolytic lesion of the nucleus
raphe magnus is performed [142]. This is consistent with the previ-
ously proposed antinociceptive role of the CuN [103].
Other inputs to the CuN, with unknown physiological functions,
have been described. The ventrolateral part of the subCuN receives
a dense input from the precommissural nucleus in the rat [78]. The
precise role of the precommissural nucleus is not known. This
mesodiencephalic group of cells consists of an output station of
several medial hypothalamic nuclei. Its connectivity pattern resem-
bles that of the periaqueductal gray and also that of the neighboring
pretectal nuclei, suggesting a role in visuomotor function or spatial
navigation [78]. It has been also proposed to be involved in a vari-
ety of motivated responses [78]. The CuN and subCuN nuclei also
receive inputs from regions involved in the integration of visuomo-
tor information [78]. Autoradiography coupled with tracer injec-
tions have been used to show that the CuN receives input from the
superior colliculus in the rat [143], rabbit [144] cat [145] and mon-
key [146]. The CuN is also known to receive inputs from the ven-
tral part of the lateral geniculate complex in the rat [147], but also
from pretectal nuclei and the nucleus of the optic tract (see refer-
ences in [78]). Furthermore, projections originating from the pre-
limbic area of the medial prefrontal cortex have been observed in
the CuN of the rat [148]. Substance P positive terminals have been
described in the cat [149].
In summary, results from the literature point to the CuN as con-
stituting a significant part of the MLR. It constitutes a "definite
region in the midbrain" th at allows for fine control of the locomotor
output. In addition, the CuN is a good candidate to ensure a physio-
logically coherent defensive response, by decreasing pain percep-
tion, and increasing the metabolism. Whether this nucleus corre-
sponds to the dorsal portion of the MLR recently characterized in
the lamprey [120] needs further investigation. Also, the functional
role of some of its inputs remains to be determined. As indicated
below, other anatomical structures are good candidates to contrib-
ute, at least in part, to the functions of the MLR.
1.2. The Pedunculopontine Nucleus
The PPN was first identified according to the morphological
criteria of Olszewski and Baxter (1954) [66]. Later, when the no-
menclature of cholinergic cells was established, the Ch5 nucleus
was found to overlap mostly with the PPN [150]. As indicated
above, the CuN was originally proposed as a substrate for the MLR
by Shik and colleagues [22;31;32]. Later, the PPN and/or the many
cholinergic cells that lie within its boundaries were proposed to
constitute a major component of the MLR [151] (for review see
[152]). Several studies tried to compare the roles of the CuN and
the PPN. It was concluded that both nuclei were part of the MLR,
but as two different behaviorally-dependent subcompartments [41].
Whereas the CuN was proposed to be part of a defensive system
(see above), the PPN was believed to be part of an exploratory sys-
tem [40]. The PPN sends ascending and descending projections to
several brain structures [153] (for review see [154-156]). The de-
scending projections of the PPN to the reticular system confer to it
a strategic role in regulating the initiation of movements (for review
see [155]). Thus PPN dysfunction has been proposed to be involved
in akinesia [157]. Recently, the PPN has become a primary target
for deep brain stimulation to alleviate the locomotor symptoms in
gait disorders e.g. Parkinson’s disease or progressive supranuclear
palsy (for review see [70]; also see conclusion). Importantly, the
group of Chantal François demonstrated that a selective lesion of
the cholinergic neurons in the monkey PPN induced severe locomo-
tor deficits that were similar to those observ ed in p atients with
Parkinson’s disease and in MPTP monkeys [158].
1.2.1. Cellular Morphology
The PPN is ventral to the CuN in the midbrain. It is also called
Nucleus tegmenti pedunculopontinus” in the gorilla [69] or cat
[67], or “Pedunculopontine tegmental nucleus (PPTg)” in humans
[159], cats [160], or rats [161] (see [70] for terminology).
The anatomy of the human PPN was originally defined by Ol-
szewski and Baxter (1954) [66]. The nucleus is located in the ven-
trolateral part of the caudal mesencephalic tegmentum, at the junc-
tion between the pons and the midbrain. It is bordered rostrally by
the caudal portion of the red nucleus, medially by the superior cere-
bellar peduncle, dorsally by the CuN and subCuN nuclei, and later-
ally and ventrally by fibers of the medial lemniscus. The nucleus is
composed of medium-sized to large oval or elongated cells. On the
basis of the density of cell distribution there are two subdivisions,
the subnucleus compactus (or pars compacta) and the subnucleus
dissipatus (or pars dissipata). The pars compacta is located in the
“dorsolateral portion of the caudal half of the nucleus”. Most of the
cells are densely arranged in this portion of the PPN. The pars dis-
sipata occupies “the remainder of the nucleus” and exhibits a lower
density of cells.
This nucleus has been observed in various vertebrates. For in-
stance in the gorilla [69], the PPN contains medium to large sized
ovoid, angular to fusiform cells with a global low density. A high
density of the medium to large cells is seen in th e dorsolateral por-
tion of the caudal half of the pars compacta as well as in the pars
dissipata. In the rat, neurons of the PPN display morphological
similarities with those observed in humans [162]. A pars compacta
and a pars dissipata were also observed in the PPN using the mor-
phological criteria defined by Olszewski and Baxter (1954) [66].
1.2.2. Neurotransmitters
From the moment that a large population of cholinergic cells
(Ch5) [150] was described in the mesopontine tegmentum at the
level of the PPN, the PPN was regarded mainly as a cholin ergic
nucleus. Some authors even defined the PPN in the rat as a nucleus
comprising exclusively large multipolar cholinergic neurons [162],
and this generated confusion regarding the true identity of the PPN.
In the human, the highest density of cholinergic cells of the Ch5
group is located in the pars compacta of the PPN, where 90 % of
the large cells are cholinergic [163] (for review see [93]). This
compact core is surrounded by a diffuse interstitial component con-
taining neurons as numerous as in the core, but which are smaller
and intermingled with passing fibers of the superior cerebellar pe-
4454 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
duncle and the central tegmental tract. Some isolated Ch5 neurons
are found in other structures such as the CuN, the parabrachial nu-
cleus, the subcoeruleus zone, and the lateral lemniscus. Two popu-
lations of cholinergic neurons corresponding to the pars compacta
and pars dissipata have also been reported in the monkey [84]. In
the pars compacta, clusters of neurons are located at the dorso-
lateral border of the superior cerebellar peduncle, at the level of the
trochlear nucleus. Neurons are intensely labeled by acetylcho-
linesterase in the PPN pars compacta in the monkey [164]. The
neurons of the PPN pars dissipata are scattered along the superior
cerebellar peduncle from midmesencephalic to midpontine levels.
Other neurochemical markers, and calcium binding proteins
later revealed that the PPN is heterogeneous [165-167] (for review
see [156]) and that it cannot be regarded merely as a cholinergic
nucleus. In support of that, the pars compacta of the rat contained
50 % of glutamatergic, 31 % of cholinergic and 19 % of GABAer-
gic neurons [165]. In the pars dissipata, 37% of the neurons were
glutamatergic, 23 % cholinergic and 40 % GABAergic [165]. This
result is in line with the previous observations that the PPN con-
tained cholinergic and non-cholinergic neurons [88; 168-170; 81].
Cholinergic (i.e. ChAT-positive) and catecholaminergic neurons
(i.e. tyrosine hydroxylase-positive) are intermingled in the PPN
[74]. Some cholinergic neurons also express glutamate in the mon-
key [171]. Whereas in the cat 50 % of ChAT immunoreactive neu-
rons of the PPN also contain GABA [172], in the rat 95 % of the
cholinergic cells were considered not likely to co-release GABA
[165].
The different neurochemical phenotypes are also distributed
heterogeneously across the rostrocaudal axis of the PPN [165;166].
The rostral PPN contains mainly GABAergic neurons and interacts
with the basal ganglia, whereas the caudal PPN contains more cho-
linergic and glutamatergic neurons and send projections to the re-
ticular formation (for review see [156]). Interestingly in the lam-
prey, the well-characterized descending pathway from the MLR to
reticulospinal neurons is composed of glutamatergic and choliner-
gic inputs (e.g. [173]; see section 2.1).
Neurons immunoreactive for ChAT were referred to as Ch5
(preferentially located in the PPN) and Ch6 (preferentially located
in the LDT) according to the nomenclature proposed in the rat
[150], in the macaque [174], and human [163] (for review see [93]).
Because the location of cholinergic cells of the PPN coincided more
tightly with the optimal stimulation sites fo r eliciting locomotion in
the rat, Garcia-Rill and colleagues proposed that these PPN cells
played a significant role in the operation of the MLR [175; 151].
Interestingly, the cells of the Ch5 (PPN) and the neighboring Ch6
(LDT) group constitute a cholinergic complex, which consists of a
continuous group of neurons extending from the caudal border of
the substantia nigra to the rostral portion of the locus coeruleus
[150;176;177], with no apparent constraints related to the bounda-
ries of traditionally-defined nuclei in the area. We will further
elaborate in the discussion about the putative role of the neighbor-
ing Ch6 (LDT) in locomotor control.
1.2.3. Descending Projections of the Pedunculopontine Nucleus
The PPN sends descending projections to midbrain, pontine,
and medullary areas as well as to the spinal cord (for review see
[154-156]).
Both in the cat and in the rat, the reticular nuclei containing
reticulospinal neurons are innervated by cholinergic cells of the
PPN. In the cat, the use of anterograde tracers revealed that the pars
compacta of the PPN sends dense ascending projections, but also
descending projections to the raphe nuclei, the nucleus reticularis
tegmenti pontis, the pontine and medullary reticular formation (in-
cluding the nuclei pontis oralis and caudalis) and the nucleus re-
ticularis gigantocellularis [153;160]. ChAT-positive neurons from
the PPN project mainly ipsilaterally to th e pontine gigan tocellular
tegmental field of the reticular formation [178], known to contain
reticulospinal neurons (for review see [38]). This is consistent with
the observation in the rat th at 47 % (8/17) of recorded PPN cells
sending a descending axon to the pontine reticular formation are
cholinergic [169]. Stimulation of NADPH-containing neurons of
the PPN increases acetylcholine release in the pontine reticular
formation (i.e. gigantocellular tegmental field) [179]. Monosynaptic
projections from PPN neurons to reticulospinal neurons of the me-
dulla oblongata have been reported [180]. Furthermore, injection of
a tracer in the nucleus magnocellularis resulted in retrogradely
labeled cells in the PPN [181].
Similarly in the rat, many cholinergic cells of the PPN (Ch5)
send ascending projections to the thalamus, tectum, basal forebrain,
and basal ganglia [150;182;176]. A significant but less important
number of neurons send descending projections to nearly all reticu-
lar nuclei that contain reticulospinal neurons [177]. As is the case in
the cat, retrograde labeling revealed that cholinerg ic cells of the
PPN project mainly to the pontine reticular nuclei oralis and cau-
dalis and in ventromedial portions of the gigantocellular reticular
nucleus. To a lesser extent, descending projections were observed
dorsally in the gigantocellular nucleus, lateral paragigantocellular,
ventral reticular nucleus of the medulla and lateral reticular nucleus
[183;184]. Projections from the PPN to the spinal cord have been
observed in rats [184;185] but not in cats [153].
The descending cholinergic component of the MLR is believed
to play an important role in locomotor control. However, a non-
cholinergic (i.e. glutamatergic, see lamprey sections 2.1.1 to 2.1.3)
descending drive to reticulospinal neurons is also likely to play an
important role in locomotor control in mammals. How these inputs
act synergistically to control locomotion remains to be determined.
1.2.4. Physiology
Garcia-Rill and colleagues (1987) [151] showed in rats that
controlled locomotion could be induced by stimulation (0.5-1 ms,
15-50 A, 20-60 Hz) of the PPN and of the cholinergic cells in its
vicinity, in animals decerebrated at the precollicular level and
placed on a treadmill with weight support. Controlled stepping was
elicited at the lowest current intensities wh en stimulating within the
caudal PPN and sites in and around the middle portion of the PPN.
As previously shown by the Russian group [22] when cats were
stimulated in the CuN, increases in stepping frequency were relia-
bly elicited wh en in creasing current levels in the PPN. The authors
did not test the anterior part of the PPN. A three dimensional recon-
struction was used to merge the histological location of the
stimulation sites with that of cholinergic cells. Importantly, the
location of cholinergic cells in the PPN coincided with effective
sites for inducing locomotion electrically or chemically [151] (for
review see [175]).
In some studies, the respective contribution of the PPN and of
the CuN to the locomotor outputs elicited by MLR stimulation
could not be definitively determin ed. The group of Mori in Hok-
kaido showed that electrical stimulation (0.2 ms, 40 A, 50 Hz) of
the MLR (P2, L4, H0) in freely moving cats sy stematically elicited
forward fast walking and then running [105]. While moving for-
ward, the cat was able to avoid obstacles and tried to jump over a
fence placed in front. Histological work indicated that the elec-
trodes were located “within and around the CuN, possibly including
a part of the PPN” [105]. Later, Mori and co-workers (1999) [132]
obtained in mesencephalic cats precise locomotor control (i.e. fre-
quency control and gait transition) with unilateral or bilateral elec-
trical stimulation (0.2 ms, 5-50 A, 50 Hz) of the MLR (P1.5-2.5,
L3.5-4.5, H-1-+1). In this study, the caudal portion of the CuN was
stimulated as in the previous work of Shik and co-workers [22;32].
However Mori and colleagues do not exclude the contribution of
the PPN [132]. They further propose that their stimulation elicited
locomotion via descending projections from the “MLR/PPN com-
plex” to the reticulospinal neurons of the magnocellular tegmental
field of the medullary reticular formation. This is consistent with
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4455
the observation that long lasting activity can be elicited in the gi-
gantocellularis tegmental field of cats by electrical stimulation (0.5
ms, <120 A, 20-60 Hz) of the PPN (P2.5, L4, H-1) [186] (see also
[187]). In other studies, the stimulated MLR was described to en-
compass both the PPN and the CuN. Unilateral stimulation (0.5-1.0
ms, 16-48 A, 50-60 Hz) of “the lateral part of the CuN as well as
posterior portions of the PPN”, as verified by histology, elicited
controlled locomotion in a pre-collicular/pre-nigral decerebrated rat
preparation [25]. In another study, the optimal MLR site to induce
locomotion corresponded “closely to CuN and the immediately
surrounding pedunculopontine region”, as determined by the his-
tological location of the electrode and the lowest current intensities
needed to elicit locomotion following unilateral stimulation (0.2
ms, 5-50 A, 50 Hz) [188].
In rats, conflicting results arose when th e locomotor role of the
PPN was addressed by lesion experiments (for review, see [189]).
Ibotenate or NMDA lesions of the PPN did not elicit deficit in
spontaneous locomotion or in exploratory locomotion in an open
field [190-193]. Rats with a lesion of the PPN are still able to move
in a maze [194-196]. The effect of lesion of the PPN in locomotion
generated by experimental manipulation of the basal ganglia led to
puzzling results. The locomotor activity induced by dopamine or
amphetam ine injection into the nucleus accumbens was reduced b y
injection in the PPN of an excitotoxin or cobalt lysine [197], pro-
caine or kainic acid [198]. In contrast, lesion of the PPN with ibote-
nate [191] or NMDA [199] did not modify the locomotion respec-
tively induced by injection of amphetamine into the nucleus ac-
cumbens or by subcutaneous amphetamine, but disrupted condi-
tioned reinforcement paradigm, thus suggesting a role for the PPN
in terms of response selection (see also [192]). However histology
revealed that the lesions in that last study destroyed the rostral,
medial and ventral parts of the cholinergic cells, but left the caudal,
lateral and dorsal regions intact [199]. These intact regions are
known to correspond to the optimal stimulation sites to induce lo-
comotion [188], and coincide with the location of cholinergic cells
[151]. The caudal site also contains the most abundant quantity of
glutamatergic neurons, and is the origin of descending projections
to the reticular nuclei [165] (for review see [156]).
In contrast with rat studies, unilateral or bilateral electrolytic
lesion of the PPN in primates reliably produces severe akinesia
during several days [157]. The histology revealed that the choliner-
gic cells and the MLR were lesionned in these animals [157].
Furthermore, PPN lesions with kainate, which targets neurons more
than fibers of passage, also resulted in severe akinesia during the
days following the lesion [200]. Variations in location and neuro-
chemical selectivity of lesions may explain the conflicting results
obtained in rats and monkeys (for review see [189]). However,
strong evidence was recently provided that selective lesion of the
cholinergic neurons of the PPN in the monkey elicits severe loco-
motor deficits that are strikingly similar to those observed in pa-
tients with Parkinson’s disease [158].
Different pattern s of activity were recorded in th e PPN. PPN
neurons are spontaneously active in intact anaesthetized rats, and
exhibit a mean firing rate around 10 Hz as measured with single
unit extracellular recordings [201]. 91 % of the recorded cells fire
regularly, 6 % fire irregularly, and 3 % exhibit bursts of discharges
[201]. Cholinergic neurons of the PPN with ascending and descend-
ing axons can be divided according to short or long spike durations
in vitro in the rat [202]. PPN cells with short or long spike duration
were also reported in patients with Parkinson’s disease or progres-
sive nuclear palsy [203]. It was proposed to divide these cells ac-
cording to their slow spiking or fast spiking pattern in anaesthetized
rats [204] as previously reported for cholinergic PPN neurons with
ascending projections in the non-anaesthetized cat [205]. The activ-
ity of slow and fast spiking cholinergic neurons has been associated
with “up” and “down” cortical states during slow oscillations in
anaesthetized rats [204]. In non-cholinergic PPN neurons, different
firing patterns including quiescent, tonic firing and irregular firing
patterns were associated with cortical activity in anaesthetized rats
[206]. Non-cholinergic neurons displaying irregular firing were
shown to have a descending axon whereas the other types had as-
cending axons [206]. The link between neurochemical subtypes,
firing properties, and connectivity of PPN neurons is still not fully
understood. However, the available data suggest a functional het-
erogeneity, probably linked to the different targets reached by PPN
neurons via their ascending and descending projections.
An increased c-fos activity during REM sleep in cells of the
PPN was reported in the cat [207; 208]. The activity of cholinergic
neurons is associated with cortical slow oscillations during sleep
[204]. The ascending projections of the PPN to the thalamus might
be involved in the switch between sleep and wakefulness [204] (for
review see [209;210]).
1.2.5. Inputs to the Pedunculopontine Nucleus
The PPN receives inputs from a variety of brain structures in-
cluding the cortex, basal ganglia, thalamus, hypothalamus, pons,
cerebellum, medulla, and spinal cord (for review see [156]). The
experiments of Shik and colleagues (1966) [22] showed that MLR
can control stereotyped locomotor patterns in animals deprived of
their forebrain. However, without stimulation, these mesencephalic
cats are immobile, unable of goal-directed behaviors and poorly
adapt to the external conditions (i.e. the treadmill speed) (for review
see [59]). We will mainly review here the inputs from the basal
ganglia and cortex.
Inputs from the Basal Ganglia
In mammals, the PPN is so much interconnected with most
components of the basal ganglia that some authors proposed that
the PPN should be considered as a part of the basal ganglia [211].
To make it short, the PPN is reciprocally connected with the SNr,
the GPi and the STN. It sends projections to the SNc, and receives
projections from the striatum (for review see [211;156]). Addition-
ally, it projects to the thalamus and receives inputs from the cortex,
a pattern of connexion typically observed in the basal ganglia.
At rest, the PPN is believed to be under a tonic inhibitory con-
trol from the basal ganglia output stations, which are tonically ac-
tive at rest (e.g. SNr neurons in the mouse [55] and GPi cells in the
monkey [212]). The basal ganglia would allow selecting between
locomotion initiation/suppression on the basis of the contextual
information provided by cortical and thalamic inputs as well as
dopaminergic modulatory inputs of the SNc/VTA [54]. Activation
of the striatal cells of the direct pathway (expressing D1 receptors)
elicits a temporary reduction of the inhibitory output from the basal
ganglia and elicits locomotion, whereas activation of the striatal
cells of the indirect pathway (expressing D2 receptors) increases the
inhibitory output of the basal ganglia and decreases locomotor ini-
tiation in the mouse [55]. This dual control of the locomotor output
by the basal ganglia may directly involve PPN neurons, as we will
review h ere.
A wide range of experiments indicate that the PPN receives
inhibitory inputs from the SNr. Injections of tritiated amino acids in
the rat SNr revealed fibers terminating in the PPN [213]. SNr axon
terminals make symmetric contacts onto PPN neurons retrogradely
labeled from the medial reticular formation [214]. Anterograde
tracing from the SNr revealed term inals in the ipsilateral and con-
tralateral PPN pars compacta and pars reticulata in the rat [215].
Neurons in the SNr were retrogradely labeled by injections of tracer
in the PPN in the cat [216] and in the rat [217]. Inputs from SNr
neurons onto glutamatergic and to a lesser extent onto cholinergic
cells of th e PPN were reported in the rat u sing electronic micros-
copy coupled with anterograde tracer injection in the SNr [218].
This is in accordance with observation that 20 to 24 % of GABAer-
gic terminals formed symmetrical synapses onto 40-50 % of cho-
linergic cells in the cat PPN, whereas the larger remaining propor-
tion of GABAergic terminals innervated non cholinergic PPN neu-
4456 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
rons [172]. Stimulation of the SNr elicited short latency inhibitory
synaptic inputs in PPN cells recorded intracellularly in anesthetized
cats [219] and in anesthetized rats [220], and in cholinergic and
non-cholinergic neurons in rat brain slices [221]. 70 % of the PPN
cells recorded with patch clamp electrodes received inhibitory in-
puts following stimulation of the SNr in the rat. Among these cells
28 % expressed ChAT ARNm [222] (see also [202]). This inhibi-
tory input is consistent with observation that the spiking activity of
extracellularly recorded PPN cells could be temporarily suppressed
in most cases following stimulation of the SNr in the cat [223].
Altogether, this supports the idea that the SNr inhibits cholinergic
and non-cholinergic PPN neurons.
The other inhibitory output station of the basal ganglia, the GPi,
also sends projections to the PPN. Cells in the GPi (called entope-
duncular nucleus in the rat) were retrogradely labeled from the PPN
[217], confirming previous anatomical observations in the monkey
[224]. GPi neurons send both descending projections to the PPN
and ascending projections to the thalamus, as elegantly shown by
anterograde tracing of single axon coupled to tridimensionnal re-
construction in the monkey [225]. Also shown in the monkey, the
projections from the GPi mainly targeted the non-cholinergic neu-
rons of the pars dissipata of the PPN, making symmetric synapses
[226], confirming the previous observation in the rat that GPi neu-
rons retrogradely labeled from the PPN are GABAergic [227].
The STN also projects to the PPN. Some cells in the STN were
retrogradely labeled following the injection of a tracer in th e ipsi-
lateral PPN in the rat [228-231;217]. Injection of an anterograde
tracer in the STN revealed rather sparse projections terminating in
the rat PPN [228;232] and in the ipsilateral pars compacta of the
monkey PPN [233;234]. Short latency, mostly monosynaptic exci-
tatory post synaptic responses were recorded in neurons in the PPN
area (i.e. neurons were located in the CuN and the PPN) following
electrical stimulation of the STN in hemidecorticated rats [235] (see
also [202]). These results are in accordance with the observation
that STN neurons are glutamatergic in the monkey [236] and in the
cat [237]. In some cases, excitatory synaptic responses were fol-
lowed by an inhibitory one [235], an effect that could be related to
the STN projections to the inhibitory output stations of the basal
ganglia, which in turn inhibit the PPN [238]. STN lesion in the rat
decreased the mean firing rate of PPN cells [201]. Altogether, the
excitatory inputs from the STN and inhibitory inputs from the out-
put stations of the basal ganglia could constitute a functional bal-
ance to control the activity of the PPN [201].
This anatomical and electrophysiological data is corroborated
by physiological studies suggesting that the PPN is under tonic
inhibition provided by the output stations of the basal ganglia. Lo-
comotion could be induced by infusion of GABA antagonists
(picrotoxin or bicuculline) in the MLR (P1-2, L2.5-4.5, H-1-+1) of
the cat [239]. Conversely, chemically-induced, electrically-induced
(10-100 A) or spontaneous locomotion could be blocked by infu-
sion of GABA agonists (muscimol and GABA) in the MLR. The
injection sites encomp assed the CuN, the PPN, portions of the bra-
chium conjunctivum and inferior colliculus, and sometimes th e
lateral central gray [239]. Unilateral injection of picrotoxin in the
subpallidal region (i.e. which receives inhibitory input from the
nucleus accumbens) caused an increase of locomotor activity to-
gether with an increased expression of c-fos in a portion of tissue
encompassing the ventrolateral periaqueductal gray, the region of
the dorsal tegmental bundle, the CuN, and the PPN. The number of
stained cells in the PPN showed the highest correlation with loco-
motor activity [240]. Locomotor activity elicited by a unilateral
injection of amphetamine in the nucleus accumbens was reduced by
GABA injection into the ipsilateral PPN [241]. In MPTP monkeys,
an injection of a GABAA antagonist (bicuculline) unilaterally into
the PPN decreased the akinesia, but did not modify the hourly mo-
tor activity in the intact monkey, despite some indications of motor
hyperactivity [242]. Conversely, injection of a GABAA agonist
(muscimol) caused a decreased motor activity in intact, but not in
MPTP monkeys. This suggests that an increased inhibitory drive
onto the PPN is present in the conditions of Parkinson’s disease
[242]. This increased descending inhibitory drive from the output
stations of the basal ganglia to the PPN is believed to be involved in
akinesia [157].
Interestingly, cells in the SNc and VTA were retrogradely la-
beled following injection of tracers in the PPN and LDT in the rat
[217]. This confirms the previous observations that injection of an
anterograde tracer in the SNc revealed labeled projections in th e
PPN [213]. Dopaminergic terminals are present in the PPN in the
monkey [243]. We recently demonstrated that a dopaminergic in-
nervation of the MLR originates from the homolog of the SNc
and/or VTA in lampreys [Ryczko et al. 20111, 20122]. Also, it
could be noted that an anterograde tracing study revealed that the
ventral striatum sends projections onto the PPN in the monkey
[244].
Inputs from the Cortex
A bilateral removal of the cerebral cortex shortly after birth in
cats did not prevent these animals from displaying complex goal-
directed behaviors involving locomotion [245]. However, the motor
cortex is believed to contribute to the precise modifications of the
muscular activity that are needed when the locomotion “surpasses
the mere rhythm of ordinary walking on a smooth floor” [246] (see
also [247]). The group of Drew in Montréal reported that the py-
ramidal tract neurons of the motor cortex are involved in visuomo-
tor coordination during locomotion, notably to adjust the limb mus-
cles involved in negotiating the obstacle [247;248] (for review see
[42]). The reticulospinal sy stem is believ ed to contribute to the
postural adjustments that accompany the gait modification through
integration of inputs from different sources including the motor
cortex via a corticoreticular pathway [249-251] (for review see
[42]). The planification of the adjustments of the locomotor pattern
may involve the posterior parietal cortex (for review see [252;
253]).
However it is not known whether these voluntary adaptations of
the locomotor pattern to the “exigencies” [247] of the environment
involve the MLR. But interestingly, the activity of some neurons of
the PPN is modified in relation to voluntary arm movements in the
monkey [254], as well as during voluntary or passive arm move-
ments in humans suffering from Parkinson’s disease or progressive
supranuclear palsy [203]. In the cat, retrograde tracing revealed that
the motor cortex, and to a lesser extent premotor cortex, project to
the pars compacta of the PPN [153]. Furthermore, anterograde
tracing studies in the monkey showed that the PPN receives inter-
mingled inputs from the forelimb and hindlimb regions in the pri-
mary motor cortex, supplementary and presupplementary motor
areas [255;43]. This confirmed the previous observations in the
monkey that lesion of the precentral gyrus revealed degenerated
fibers in the PPN [256]. Connections between the motor cortex and
the PPN were also suggested by probabilistic diffusion tractography
in the monkey [44]. In the human, the same technique revealed that
the regions of the primary motor cortex which control the trunk, the
upper and lower extremities had the highest density of connection
with the PPN [45;46]. This presumably excitatory glutamatergic
connection from the motor cortex to the PPN could constitute a
component of a functional balance together with the inhibitory in-
puts from the basal ganglia output stations to control the activity of
the PPN [255;43]. Neurons of the PPN activated by the motor cor-
1 Ryczko D, Auclair F, Dubuc R. Dopaminergic innervation of the mesencephalic
locomotor region: Descending inputs from the posterior tuberculum. Society for Neu-
roscience Abstract, Washington, DC, USA, 2011.
2 Ryczko D, Grätsch S, Dubé C, Auclair F, Dubuc R. Evidence that descending projec-
tions from lamprey dopaminergic neurons homologous to those in the substantia nigra
and/or ventral tegmental area control locomotion. Society for Neuroscience Abstract,
New Orleans, LA, USA, 2012.
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4457
tex could relay precise information down to the reticulospinal sys-
tem and thus influence interlimb coordination during locomotion
[43].
Other cortical areas project to the PPN (for review see [156]).
Anterograde and retrograde tracing experiments revealed that the
PPN receives input from the prefrontal cortex in the rat [217;148;
257]. Retrograde tracing and immunohistochemistry revealed direct
projections from the prefrontal cortex and primary auditory cortex
to the caudal PPN in the guinea pig [258]. Anterograde labeling
from the auditory cortex revealed direct projections to the choliner-
gic cells of the caudal PPN in the same animal [258].
Inputs from other Regions
Tracing experiments revealed that other regions project to the
PPN (for review see [156]). In the rat, retrograde labeling revealed
that the PPN receives major inputs from the central tegmental field,
periaqueductal grey, lateral hypothalamus/zona incerta region, ra-
phe nucleus, superior colliculus, as well as pontine and medullary
reticular nuclei, but also from the amygdala, bed nucleus of the
stria terminalis, the habenula, substantia innominata, nucleus of the
solitary tract, and thalamic parafascicular nucleus [217; 257]. Af-
ferents from the LDT and the contralateral PPN were also observed
in rats [217]. Injection of an anterograde tracer in the deep cerebel-
lar nuclei revealed direct inputs to the PPN in the monkey [259].
These projections formed asymmetrical synapses onto PPN neu-
rons, including cholinergic ones. The PPN could thus provide a
relay through which the cerebellum could influence not only the
reticulospinal neurons, but also the basal ganglia and/or the thala-
mus which in turn feed back onto motor cortical areas [259]. In rats,
the PPN receives a small input from the precommissural nucleus,
which could be involved in visuomotor functions or spatial naviga-
tion on the basis of its connection pattern [78].
Altogether these studies show that the PPN is capable of elicit-
ing locomotion in a controllable fashion. Contrary to the CuN, the
PPN receives extensive inputs from the forebrain (e.g. basal ganglia
and the cortex). These forebrain inputs to the PPN are believed to
be related to the role of this nucleus as an exploratory system [40].
The cholinergic cells of the PPN appear essential for locomotion
[158]. Their localization corresponds to effective stimulation sites
for eliciting locomotion in the rat [151]. Moreover, the PPN is a
new target to alleviate the symptoms of p atients with gait deficits
(see discussion). Knowledge about the detailed circuitry controlling
locomotion in higher vertebrates remains scarce. In the next section,
we will review how a simple, basal vertebrate allowed deciphering
the role of the cholinergic and non-cholinergic (i.e. glutamatergic)
descending commands from the MLR, that may also play a crucial
role in mammals.
2. THE LAMPREY MLR
The lamprey has proved to be a very useful animal model to
investigate the neural mechanisms underlying locomotor control,
from cells to behavior. As in other vertebrates, the MLR of lam-
preys finely controls the power of the locomotor output [23]. It does
so by activating reticulospinal neurons via monosynaptic choliner-
gic and glutamatergic descending projections [173]. Reticulospinal
neurons, in turn, monosynaptically p roject to the spinal locomo tor
networks [260;261] (for review see [262]). Cholinergic cells were
described in the MLR of lampreys [173] (Fig. 2) and their distribu-
tion is strikingly similar to that of mammals. One of the significant
and unique advantages for using the lamprey model is that the
brainstem mechanisms responsible for initiating and controlling
locomotion can be studied using an array of techniques, with the
added benefit of including all relevant structures needed for loco-
motor control, and the ability to monitor ongoing locomotor behav-
ior while intracellularly recording cells in the supraspinal locomotor
circuits. Our results and that of others have shown that the neural
organization controlling locomotion is highly preserved among
vertebrate species, probably due to the very fundamental nature of
this motor behavior.
Based on the distribution of cholinergic cells, the PPN and LDT
have been tentatively identified in the lamprey. The cholinergic
cells form two groups on the basis of their location, cell body size,
and distribution density [173] (Fig. 2). A first group (yellow
spheres, Fig. 2) constitutes a dense population located at the border
between the pons and the mesencephalon, close to the ventricular
surface (Fig. 2). This cluster comprises 2/3 of the total number of
cholinergic cells. A second group comprises smaller cells; it is lo-
cated more rostrally, mostly in the mesencephalon, and expends
laterally and dorsally to the first group (red spheres, Fig. 2). Cells
are less numerous and distributed more sp arsely compared to th e
first group. Comparing these cholinergic populations to those de-
scribed in cats [178], rats [160], and amphibians [263], it was sug-
gested by Le Ray et al. (2003) [173] that the first caudal group is
homologous to the cholinergic cells centered on the LDT of mam-
mals (Ch6, see discussion) and the second more rostrolateral group
homologous to the cholinergic cells centered on the PPN (Ch5, see
also section 1.2. and [30]). Many neurons in the lamprey PPN and
LDT are retrogradely labeled by tracers injected in hindbrain nuclei
containing reticulospinal neurons (e.g. [264], however see [52]). but
their cholinergic identity is not yet defined. PPN electrical stimula-
tion elicits locomotion in lampreys [23], but the LDT area has been
more often targeted for MLR stimulation, most likely because of
the dense population of cholinergic neurons (i.e. close to the caudal
and medial LDT area) [173;264;265;120]. Interestingly in the sala-
mander the cholinergic cells of the LDT were also identified as the
optimal stimulation site in th e MLR to elicit locomotion [24]. We
will first present the outputs from th e MLR and then its inputs in
the lamprey.
2.1. Multifunctional Output from the MLR in Lampreys
2.1.1. Direct Projections to Reticulospinal Cells
The descending circuitry from the MLR to th e spinal cord CPG
for locomotion is not known in detail in mammals. In the lamprey,
the shortest serial output pathway involves two synapses. The MLR
sends descending projections to reticulospinal neurons of the mid-
dle (MRRN) and posterior (PRRN) rhombencephalic reticular nu-
clei [266;264] that in turn, project to interneurons of the spinal CPG
and motoneurons [261].
The homology between the reticular formation of mammals and
that of the lamprey is partially known [267]. On the basis of the
anatomical organization of the reticular nuclei, the MRRN would
be part of the pontine reticular formation, and the PRRN part of the
medullary reticular formation [266]. As indicated above, the caudal
pontine reticular formation of cats and rats is strongly activated by
stimulation of the PPN [186;187]. Pontine reticulospinal neurons,
with fast-conducting axons, rhythmically discharge during locomo-
tion in cats [268;251;269]. The large Müller cells in the MRRN of
lampreys all send projections through the ventromedial spinal cord
[270] and display rhythmic discharges during locomotion. In
mammals, the MLR also densely projects to the medullary reticular
formation, specifically to the nuclei gigantocellularis and magno-
cellularis [181;271;102]. The medial medullary reticular formation
relays MLR commands to the spinal CPG via the ventrolateral
funiculus [272-275;35;28;34]. In lampreys, inactivation of the
MRRN has a drastic effect on the induction of locomotion from the
MLR [266], indicating that the ventromedial axons in the spinal
cord contribute importantly.
In the lamprey, MLR inputs to reticulospinal neurons are mono-
synaptic and rely on glutamate and acetylcholine transmission
[173]. Local injections of nicotine or acetylcholine induce dose-
dependent depolarizations in reticulospinal neurons. Repeated in-
jections elicit long duration locomotor bouts. Stimulation of the
MLR elicits monosynaptic responses in reticulospinal neurons that
are partially reduced by bath application of a nicotinic antagonist
4458 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
(i.e. D-tubocurarine) or potentiated by a cholinesterase inhibitor
(i.e. physostigmine). The remaining part of the synaptic response is
sensitive to glutamatergic antagonists (CNQX and AP5), revealing
the dual nature of the connection between the MLR and the reticu-
lospinal cells. Whether the contribution of glutamate and acetylcho-
line differs from one reticulospinal cell to another, and whether
single neurons in the MLR contain both neurotransmitters or only
one or the other remains to be established in the lamprey . Furth er-
more, in a semi-intact lamprey preparation in which the activity of
reticulospinal cells can be recorded while the tail is swimm ing in
the recording chamber [276;277;23], locomotor bouts are harder to
elicit by MLR stimulation when a nicotinic antagonist is applied on
the brainstem [173]. This indicates that cholinergic connections to
reticulospinal cells play a significant role in eliciting locomotion.
Altogether, there is strong evidence that cholinergic inputs from
the MLR to reticulospinal cells are involved in locomotor control in
lampreys [173]. This is in agreement with the recent findings in the
monkey and humans that damage to the cholinergic cells in the
MLR is associated with locomotor deficits (see above; [158]).
2.1.2. Differential Recruitment of Reticulospinal Neurons During
Locomotion
The recruitment pattern of reticulospinal cells w as examined
with increasing MLR stimulation intensities [266]. At low MLR
stimulation strengths, reticulospinal cells in the MRRN (pontine
group) were first activated. At higher stimulation strengths, the
MRRN cells reached a maximal activation and another population
of reticulospinal cells, the PRRN (medullary group), was recruited.
As cholinergic inputs, glutamatergic inputs from the MLR to the
reticulospinal cells play an important role in locomotor control. A
selective inactivation of the MRRN by local in jection of glutama-
tergic antagonists (CNQX and AP5) reduced the spiking activity of
the cells in this region, and decreased the swimming frequency and
the size of the EMG bursts. When applying the glutamatergic an-
tagonists prior to MLR stimulation, the amount of current needed to
elicit locomotion dram atically increased. Inactiv ation of the PRRN
reduced the swimming frequency. The reticulospinal cells thus
show a specific recruitment pattern during MLR-induced locomo-
tion. MRRN cells (pontine group) are recruited for initiation and
Fig. (2). Schematic three-dimensional representation of the distribution of ChAT-immunoreactive cells in the MLR of lampreys. Each sphere repre-
sents a ChAT-immunoreactive neuron. The diameter of the cells is illustrated two-fold larger than actual size for the sake of visibility. A: Caudal view of the
two populations of cholinergic neurons in the proximity of the large isthmic Müller cell I1 (green sphere). The brain is delineated by the blue volume. Caudo-
medially, the larger cholinergic cells in the laterodorsal tegmental nucleus (yellow spheres) are densely clustered, whereas rostro-laterally, smaller cholinergic
cells in the pedunculopontine nucleus (red spheres) appear more sparsely distributed. Scale bar, 400 m. B, C: The two cholinergic populations are illustrated
at higher magnification in the caudal view (B) and a lateral view (C). Scale bar, 200 m. C: caudal; L: lateral; M: medial; R: rostral; V: ventral. (Adapted from
[173]).
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4459
maintenance of slow swimming, whereas PRRN cells (medullary
group) are additionnaly recruited for faster swimming frequencies.
2.1.3. The MLR Provides a Bilaterally Symmetrical Output
In the original experiment of Shik and colleagues (1966) [22], a
unilateral stimulation of the MLR efficiently elicited a bilaterally
symmetrical locomotor output. It was later shown in several verte-
brate species that the stimulation of the MLR on one side was suffi-
cient to produce symmetrical locomotion, including in the lamprey
[23], salamander [24], stingray [26], rabbit [27], and guinea-pig
[28]. Recent experiments in the lamprey revealed that the MLR
sends a highly symmetrical synaptic input to reticulospinal neurons
on both sides [264]. Stimulation of the MLR on one side elicited
similar synaptic inputs in pairs of homologous large reticulospinal
neurons recorded simultaneously on both sides. In addition, calcium
imaging revealed that the reticulospinal neurons of all sizes, includ-
ing the small ones, were also symmetrically activated on both sides
when stimulating the MLR on one side. Simultaneous analysis of
two reticu lospin al cells and kinem atic analysis of the locomotor
movements revealed that unilateral MLR stimulation elicited sym-
metrical activity of reticulospinal cells and symmetrical locomotor
output.
The bilateral inputs to reticulospinal neurons were likely mono-
synaptic, as they followed high frequency stimulation in a high-
divalent cation solution. Moreover, the inputs disappeared gradually
and symmetrically when gradually replacing the Ring er’s solution
with a Ca2+-free solution. In mammals, bilateral inputs to reticu-
lospinal cells were described as asymmetrical [278]. In that study,
two different electrodes were used to stimulate the MLR on either
side. The asymetrical synaptic inputs may have resulted from a
slight difference in the position or different properties of the elec-
trodes. Bilateral symmetrical activation of the spinal motoneurons
was observed following unilateral MLR stimulation in cats [279].
Whether a physiological bilateral symmetrical input from the MLR
to reticulospinal neurons is also present in mammals needs further
investigation.
Retrogradely labeled cells were seen in the MLR on both sides
when a tracer was injected unilaterally in th e pontine reticulospinal
population (i.e. MRRN) [264], as previously observed in cats [280;
102] and rats [281]. Using two different tracers on each side of the
reticular formation, very few MLR cells were double-labelled indi-
cating that only few projected bilaterally [264]. Interestingly, the
density of the anatomical projections from the MLR to reticulospi-
nal cell populations was not symmetrical in lampreys. More cells
were labeled in the MLR on the side of the injection (i.e. MRRN,
[264]), as in the cat [102]. It remains uncertain how this anatomical
asymmetry is compensated physiologically (see [279] in the cat).
Altogether, these results show that in the lamprey, th e MLR pro-
vides a symmetrical and bilateral output to reticulospinal cell popu-
lations, which is very likely to contribute to elicit a basic symm etri-
cal locomotor output [264].
2.1.4. A Parallel Muscarinic Hyperdrive to Boost the Locomotor
Output
As indicated above, the MLR sends monosynaptic glutamater-
gic and cholinergic (i.e. nicotinic) inputs to reticulospinal cells.
However, muscarinic agonists are also known to powerfully acti-
vate reticulospinal neurons via an indirect pathway. In a recent
study, a parallel output pathway of the MLR, which activates re-
ticulospinal neurons via two synapses, has been identified [265].
The authors found a new population of muscarinoceptive neurons in
the caudal brainstem of lampreys that receive inputs from the MLR
and project back to reticulospinal cells to amplify and extend the
duration of locomotor output. These cells respond to muscarine
with long-lasting bouts of activity, receive direct muscarinic excita-
tion from the MLR, and send glutamatergic excitation to reticu-
lospinal neurons. Blocking the muscarine receptors of these neurons
dramatically reduced MLR-induced excitation of reticulospinal
neurons and slowed locomotion. These results further explain the
previous observations that muscarine elicited sustained depolariza-
tion in reticulospinal neurons [282]. The presence of these muscari-
noceptive neurons forced us to rethink the organization of supraspi-
nal locomotor control to include a sustained feedforward loop that
boosts locomotor output elicited from the MLR. In other words, the
cholinergic “hyperdrive” component of the MLR allows an even
stronger activation of the locomotor system [265].
2.1.5. Controlling Sensory Inputs During Locomotion
Sensory inputs influence markedly locomotor activity (for re-
view see [283;284]). In lampreys, cutaneous sensory stimulation
excites reticulospinal neurons. At low stimulation intensities, the
reticulospinal response linearily co-varies with the stimulation
intensity . At higher intensities, the depolarization switches to large
sustained depolarizations accompanied by spiking activity [276]. It
was shown that cholinergic inputs from the MLR not only play a
key role in the initiation of locomotion, but they can decrease th e
sensory transmission to reticulospinal cells during locomotion. First
it was shown that the response of reticulospinal cells to trigeminal
stimulation can be reduced by local application of a muscarinic
agonist (i.e. pilocarpine), and increased with a muscarinic antago-
nist (i.e. atropine) [285]. Stimulation of the MLR was then shown
to modulate sensory transmission [286]. The synaptic responses of
reticulospinal neurons to electrical stimulation of the sensory tri-
geminal nerve were reduced by MLR stimulation. Furthermore, the
amplitude of the decrease of the response increased with the stimu-
lation intensity of the MLR. Muscarinic receptors were shown to be
involved in the depression of sensory transmission, as it was pre-
vented by bath application of a muscarinic antagonist (i.e. atropine).
Cholinergic inputs elicited by MLR stimulation are thus able of
modulating sensory transmission to reticulospinal cells. This could
contribute to ensure a goal-directed locomotor command, w ith de-
creased perturbation from sensory inputs [285].
The main challenge that remains is to determine whether dis-
tinct cholinergic cell populations within the MLR are responsible
for the depression of sensory transmission, the nicotinic activation
of reticulospinal cells and th e parallel activation of mu scarinocep-
tive cells. Whether the cholinergic cells are organized in sep arated
or intermingled subpopulations also remains to be assessed.
2.1.6. Adjusting the Respiratory Network Activity Prior and Dur-
ing Locomotion
It was recently shown that in addition to controlling locomotor
functions, the MLR also controls the respiratory networks via an-
other pathway [120] (Fig. 3). In freely behaving lampreys and
mammals, the respiration frequency increases prior to, and during
locomotion [120] (for review see [287]). The mechanism responsi-
ble for this adjustment o f the metabolism in an ticipation of th e en-
ergy demand was recently addressed [120]. Stimulation of the MLR
increased the respiratory frequency, for stimulation strengths that
were both under and over the threshold for eliciting locomotion.
Interestingly, a subset of MLR cells (located in the dorsal part of
the MLR) was shown to project directly to the respiratory generator
(called the paratrigeminal respiratory group in lampreys, pTRG)
and to the respiratory motoneurons (Fig. 3). This is consistent with
the observation that stimulation of the MLR resets the respiratory
rhythm. Cells in the pTRG were retrogradely labeled from the res-
piratory motoneuron pool, and were shown to receive a monosyn-
aptic excitatory glutamatergic input from the MLR. Single cells in
the MLR were recorded and labeled with an intracellular dye. They
were shown to project bilaterally to pTRG cells and respiratory
motoneurons, but not to reticular nuclei associated with locomotion.
Activation of these MLR cells by stimulation of the posterior
tuberculum was associated with the activation of the reticular nu-
clei, as would be the case during locomotion. Specific silencing of
only the dorsal MLR with injections of GABA prevented the respi-
ratory increases without affecting locomotion. Altogether, the study
4460 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
of Gariépy et al. (2012) [120] provided evidence that a subset of
neurons of the dorsal MLR that are active prior and during locomo-
tion can adjust respiration by direct connection to the respiratory
network.
2.2. Inputs to the MLR in Lampreys
Recent studies have shown that typical features of the basal
ganglia are present in the lamprey. These features show similarities
with those of avian and mammalian species, from circuit organiza-
tion to cellular properties [47-53]. This important discovery sug-
gests that the basic organization of the basal ganglia was already
present some 560 million years ago, when lampreys diverged from
the vertebrate lineage [288].
The output structures of the lamprey basal ganglia tonically
inhibit the MLR. Indeed, a simple injection of a GABA antagonist
(i.e. gabazine) in the MLR initiates locomotion, whereas injectio n
of a GABA agonist (i.e. muscimol) dramatically decreases the dura-
tion of a locomotor bout elicited by injection of D-glutamate in the
MLR [289]. Injection of retrograde tracer in the MLR labeled many
descending GABAergic neurons in the medial pallium and to a
lesser extent from the striatum and lateral pallium [289]. Among
these potential homologs of the pallidum, a region ventral to the
eminentia thalami was recently proposed as a putative pallidal area,
as it appears to be homologous to the avian and mammalian GPe
and GPi [51]. A putative equivalent for the other output station of
the basal ganglia, the SNr, has been recently proposed [52]. Some
neurons in the two structures are GABAergic and tonically active
[51, 52]. Different pallidal neurons send projections to the
diencephalic and mesencephalic locomotor regions and to the
tectum [51]. SNr neurons send distinct projections to the thalamus
and to the tectum but not to the MLR [52].
The input layer of the lamprey basal ganglia, the striatum,
shows many similarities with that of mammals. The striatum con-
tains neurons immunoreactive to enkephalin, GABA, substance P,
and acetylcholinesterase [47]. Some cellular properties of striatal
neurons show similarities with those of mammals, including a Kir
K+ inward rectifying conductance [49;50]. Recently dopaminergic
D2 receptor mRNA was found in a subpopulation of striatal cells,
in a region that contains tyrosine-hydroxylase positive synaptic
terminals [53]. The protein sequence of this D2 receptor shows a
strong similatity to that of other vertebrate D2 receptors, especially
in the portion of the receptor that binds dopamine [53]. Patch-clamp
recordings of striatal neurons revealed that application of a D2 re-
ceptor agonist (TNPA) decreased the excitability of striatal neurons
(i.e. reduced spiking activity and reduced occurrence of post-
inhibitory rebound spikes) [53]. This feature is typical of the “indi-
rect pathway” described in mammals [55] (for review see [56]).
Interestingly, the D2 receptors were not expressed by neurons ret-
rogradely labeled from the lamprey homolog of the SNr [52] i.e. the
neurons of the “direct pathway” described in mammals. This sug-
gests that in lamprey, as in mammals, th e D2 receptors are ex-
pressed by neurons of the “indirect pathway”.
As mentioned w ith the D2 receptors just previously, typical
circuit features of the direct and indirect pathways have been identi-
Fig. (3). The multifunctional outputs of the MLR in lampreys. There are at present four parallel output pathways known to originate from the lamprey
MLR. The MLR: i) sends a bilaterally symmetrical output to the hindbrain reticulospinal cells that in turn activate the spinal locomotor CPG, ii) recruits a
parallel “hyperdrive” pathway that provides a maximal activation of the locomotor system by sending additional excitatory drive to reticulospinal neurons, iii)
adjusts the activity of the respiratory centers prior and during locomotor movements to regulate metabolism, and iv) decreases the strength of the sensory in-
puts that are fed onto reticulospinal neurons during locomotion to ensure finely controlled locomotion. Blue: glutamatergic neurons/projections. Yellow: cho-
linergic neurons/projections. MLR: mesencephalic locomotor region. MRRN: middle rhombencephalic reticular nucleus. PRRN: posterior rhombencephalic
reticular nucleus, pTRG: paratrigeminal respiratory group.
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4461
fied within the lamprey basal ganglia. In favor of the existence of
the direct pathway, striatal GABAergic neurons expressing sub-
stance P project to the mammalian homolog of the SNr [51;52].
These neurons do not express the mRNA of D2 receptors [53], but
whether these express the D1 receptor is still unknown. Striatal
neurons projecting to the SNr, display inward rectification [52]. The
striatal neurons of the indirect pathway express enkephalin and
project indirectly to the GPi via the GPe and the STN [51]. Alto-
gether, th ese recent findings rev ealed th at typical elements of the
basal ganglia are present in lampreys. It remains to be determined
whether the direct and indirect pathways have opposite effects on
locomotor behavior, as demonstrated in mammals [55].
The striatum receives inputs from dopaminergic cells in the
posterior tuberculum, considered to be homologous to the dopa-
minergic neurons of the SNc (A9) and/or the VTA (A10) of mam-
mals [48;57]. Interestingly, forebrain dopamine depletion in lam-
preys treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydro-
pyridine) resulted in severe locomotor deficits, characterized by a
decrease in the initiation and maintenance of locomotor activity
[57]. In the context of the cu rrently accep ted model of the basal
ganglia, loss of dopaminergic inputs to the striatum would lead to a
decreased activity of the “direct pathway” and an increased activity
of the “indirect pathway”. Taken together, these results suggest that
a loss of dopaminergic input in lamprey would result in an in-
creased inhibition of motor areas targeted by the output structures
of the basal ganglia, as seen in mammals (for review see [290]).
Recently, w e have demonstrated that the MLR also receives
direct descending dopaminergic projections from the lamprey ho-
molog of A9 and/or A10 [Ryczko et al. 20113], and that this input
contributes to locomotor control [Ryczko et al. 20124]. Thus this
dopaminergic system can influence locomotion in two ways: first,
via ascending projections to the basal ganglia which in turn inner-
vate the MLR, or second, via direct descending projections to the
MLR. Further studies are required to determine whether this de-
scending dopaminergic pathway was conserved through evolution.
This may well be the case, as dopaminergic terminals have been
observed in the monkey PPN [243].
Altogether, the lamprey as an animal model has generated a
better understanding of the organization of the locomotor system,
from the spinal to supraspinal structures. Recent studies revealed
that the lamprey MLR is multifunctional. It sends parallel output
projections to respiratory, sensory and motor areas that ensure an
integrated physiological response. It receives projections from the
basal ganglia, and was recently shown to receive olfactory inputs
via the posterior tuberculum (ventrocaudal diencephalon), thus
participating in the transformation of those olfactory inputs into a
locomotor command [291] (for review see [292]). Whether differ-
ent cell populations of the MLR would be recruited depending on
the inputs remains to be demonstrated. This would be consistent
with the proposition by Sinnamon (1993) [40] in mammals that the
MLR can be subdivided in compartments that would be activated in
a context-dependent manner (for review see [59;41]).
3. CLINICAL PERSPECTIVES
The MLR appears to be highly conserved as a locomotor con-
trol structure across the vertebrate phylum. Data obtained from both
healthy subjects and patients suggest that the neural circuits ana-
tomically associated with the MLR are involved in locomotor con-
trol in humans as well. Functional magnetic resonance imaging
3 Ryczko D, Auclair F, Dubuc R. Dopaminergic innervation of the mesencephalic
locomotor region: Descending inputs from the posterior tuberculum. Society for Neu-
roscience Abstract, Washington, DC, USA, 2011.
4 Ryczko D, Grätsch S, Dubé C, Auclair F, Dubuc R. Evidence that descending projec-
tions from lamprey dopaminergic neurons homologous to those in the substantia nigra
and/or ventral tegmental area control locomotion. Society for Neuroscience Abstract,
New Orleans, LA, USA, 2012.
revealed that the PPN and the adjacent CuN on the left side of right-
handed healthy subjects are both activated when healthy subjects
imagine they are walking [293;158]. If they imagine walking faster,
there is a concomitant increased activity in the PPN and CuN [158].
Single unit recordings in patients with Parkinson’s disease revealed
an increase in tonic firing of neurons in the subCuN during men-
tally mimicked locomotion [294]. Furthermore, gait initiation and
maintenan ce were severely impaired (i.e. inability to stand an d
walk) in a subject after a hemorrhage in the right side of the brain at
the level of the pontomesencephalic junction, i.e. an area including
the right PPN and adjacent structures [295].
These results are consisten t with the increasing number of stud-
ies that report that bilateral “deep brain stimulation” [296] of the
PPN can alleviate som e of the locomotor d eficits in patients w ith
Parkinson’s disease, i.e. improvement in gait stability, reduced
episodes of freezing of gait and reduced falls (e.g. [297-299], see
also [300]). Improvement of hand dexterity in patients with Parkin-
son’s disease with severe symptoms has also been described [301].
However, in some cases, no clear positive effects of the stimulation
of the PPN were observed [302;303] (for review see [304]); even
negative effects were at times reported. For instance, urinary incon-
tinence could be elicited by PPN stimulation [305], a phenomenon
also observed in the original experiments of Shik and colleagues in
the cat (1966) [22]. Sleep can also be induced by stimulation of the
PPN [306]. Visual impairment without abnormal eye movements
was reported when stimulating at frequencies below 35 Hz [307].
Above 35 Hz (especially 130 Hz), paresthesia, eyelid elevation, and
diplopia were seen. Recent data suggest that targeting the rostral
versus the caudal part of the PPN produces different clinical effects
[308]. Altogether, these studies show that further research is needed
to better understand the operation of the MLR/PPN region, in order
to optimize its stimulation before using this approach as a standard
deep brain stimulation procedure [309;310].
The mechanisms by which deep brain stimulation exerts its
effects are not yet fully understood. It was initially proposed to
mimic the effects of an ablation (for review see [311]). In patients
with Parkinson’s disease, the mechanism of action of deep brain
stimulation has been proposed to involve a functional local inhibi-
tion of the abnormal rhythmic activity in the stimulated structure
(e.g. STN: [312; 313]). However the effect of deep brain stimula-
tion could be more complex and involve multiple short term and
long term effects locally and distally from the stimulation site (for
review see [314;315]). Based on the knowledge from basic re-
search, we can hypothesize that when stimulating the PPN in pa-
tients with Parkinson’s disease, the goal of the stimulation would be
to activate neurons of the MLR. Whether this would happen via a
direct activation of the MLR cells or a disruption of the tonic inhi-
bition exerted by inhibitory inputs from the basal ganglia is not
known. In animal models, the parameters of the MLR stimulation
that elicit locomotion are quite variable from one species to another.
The pulse durations ranged from 0.1 to 2 ms. Intensities ranged
from 0.1 to 600 A and the frequencies from 2 to 300 Hz. In each
species, however, the frequency range was quite narrow, rarely
exceeding a few tens of Hz. For instance in the cat or rat, 40-60 Hz
was the most efficient range to elicit locomotion when stimulating
the MLR. Interestingly in the rat, PPN cells in brainstem slices
preferentially discharge at gamma band frequency (30-60 Hz) when
depolarized [316;210]. Whether an optimal discharge frequency
range is associated with locomotor initiation and maintenance in
physiological and pathological conditions and how the stimulation
parameters can finely control this optimal activity remains to be
determined, especially in humans. For instance high frequency PPN
stimulation might not be needed to obtain positive motor effects, as
low frequency stimulation d ecreases akinesia in park insonian mon-
keys [317]. Furthermore, as cells degenerate in the PPN of patients
with Parkinson’s disease [318;158], it is possible that stimulating
4462 Current Pharmaceutical Design, 2013, Vol. 19, No. 24 Ryczko and Dubuc
the PPN in patients in various stages of the pathology might have
different effects.
Detailed knowledge of the properties of the cell populations of
the MLR, and of their connectivity, is not available and is difficult
to obtain in mammals. This is where the lamp rey model can be very
useful as a complementary approach to gain rapid and detailed
knowledge of the operation of the MLR, which can then be verified
in mammals. More information about the role of the MLR in bipeds
is also needed. The combination of single unit recordings with
kinematic recordings and electromyograms during locomotion on a
treadmill in the primate is a useful approach to better understand the
role of the MLR during locomotion in normal and pathological
conditions [319]. In addition, this approach could be used to im-
prove stimulation efficiency and explore new pharmacological
treatmen ts.
Interestingly, a central role of the cholinergic neurons of the
PPN on locomotor behavior has been confirmed with recent data
obtained in monkeys and humans [158]. Gait impairment (i.e. bal-
ance deficits and falls) in patients with Parkinson’s disease is corre-
lated with a significant loss of neurons positive for acetylcho-
linesterase in the PPN, but the number of Nissl-stained neurons in
the adjacent CuN are not modified compared to non-faller p atients
[158]. Furthermore, in aging monkeys treated with MPTP, major
balance deficits and abnormal posture were associated with a de-
struction of around 30 % of the cholinergic neurons of the PPN
[158]. A bilateral lesion of around 39 % of cholinergic neurons of
the PPN in normal monkeys resulted in gait and posture deficits that
include a decrease in step length and speed, a decreased knee angle,
an increased back curve, a deviation of the hindquarters and of the
head, as well as axial and limb rigidity [158]. Some of these spe-
cific gait and posture symptoms (namely decreased step length and
speed, and increased back curve) were also observed in MPTP
treated monkeys [158]. Consistent with these results, previous stud-
ies have also characterized neuronal loss in the PPN in p atients
suffering from Parkinson’s disease [320;321;318], but also in pa-
tients suffering from progressive supranuclear palsy [322; 320], a
pathology associated with a parkinsonian-like “extrapyramidal”
symptomatology that includes axial rigidity and gait instability.
Basic research is also need ed to answer the question of whether
another nucleus containing most of the Ch6 cholinergic neuron
population [150;163], the LDT, is a component of the MLR. This
nucleus is a close neighbor of the PPN and CuN. In rodents and
primates, the LDT “lies in the ventrolateral portion of the periaque-
ductal gray, behind the caudal pole of the trochlear nucleus, and
dorsolateral to the dorsal nucleus of Gudden” [84]. The LDT con-
tains cholinergic cells in cats [323;89;324], rats [90;150;325;326],
monkey [174;327;328;84] and humans [163]. As said earlier, there
is no clear demarcation between the Ch5 and Ch6 groups: the cho-
linergic cells from the two populations merge and span over many
traditionally-defined nuclei (for review see [93]). In the rat, cho-
linergic, glutamatergic and GABAergic neurons are intermingled
but heterogeneously distributed within the LDT and the PPN [165]
(see also [81;172]). The pattern of descending projections from the
LDT suggests that this nucleus could control reticulospinal neurons
in mammals. In the cat, cholinergic neurons from the LDT inner-
vate the medial pontine reticular formation [329]. Injections of a
tracer in the nucleus magnocellularis retrogradely labeled cells in
the LDT [181]. In the rat, cholinergic cells of the LDT send de-
scending projections throughout the reticular formation, in nuclei
containing reticulospinal neurons [177]. Cholinergic cells (i.e. im-
munoreactive to ChAT or acetylcholinesterase) were retrogradely
labeled in the LDT, although to a lesser extent than in the PPN,
following injections in the medullary and pontine reticular nuclei.
Anterograde labeling experiments in the rat revealed that a portion
of neurons from the LDT project to the pontine reticular formation
[217]. In rats, chemical desinhibition of the periaqueductal gray
increased c-fos immunoreactivity in non-cholinergic neurons of the
LDT, but not in the PPN [330]. The periaqueductal gray is activated
in the context of a defensive system (F ig. 1). In rats exposed to a
cat, increased c-fos labeling has been reported in the LDT together
with the periaqueductal gray [331]. In the rat, 10 % of the choliner-
gic fibers from the LDT innervate the medioventral medulla [271].
The observation that LDT and PPN have a common afferent / effer-
ent pattern suggests that these structures may constitute a single
functional unit [217]. In the lamprey, stimulation of the cholinergic
nucleus corresponding to the LDT (the caudal, compact cholinergic
nucleus on Fig. 2) reliably and more efficiently controls locomo-
tion. It appears that the role of the Ch6 population associated with
the LDT on the effects of the MLR has been overlooked or at least,
underestimated. Whether stimulating this region could have posi-
tive effects on locomotor deficits requires experimentation in a
model of biped locomotion.
The cholinergic component of the PPN could constitute a target
to improve the locomotor deficits in patients with Parkinson’s dis-
ease. Importantly, these locomotor symptoms respond poorly to
dopaminergic drugs. Cholinergic drugs specifically dedicated to
enhance the strength of the descending cholinergic component of
the PPN could constitute a useful target to decrease the gait insta-
bilities of such patients. So far, increasing cholinergic transmissio n
with cholinesterase inhibitors has been used to improve cognitive
function (i.e. memory and language) in patients suffering from Alz-
heimer’s and Parkinson’s disease [332-334]. Locomotor effects
have not been extensively explored to date. It is worth noting that a
transient increase of tremor was observed when a cholinesterase
inhibitor (i.e., rivastigmine) w as adm inistered to patients with Park-
inson’s disease (for review see [333]). This is consistent with the
fact that anticholinergic drugs are clinically used to decrease tremor
in these patients [333]. Some authors also characterized a worsen-
ing of motor function and mood in a patient with Parkinson’s dis-
ease when treated with the cholinesterase inhibitor rivastigmine
[335]. The lack of specificity of cholinergic pharmacological agents
for the PPN system thus constitutes a significant limitation [158].
Cholinergic agonists of the receptors expressed on the targets of the
descending projections of the PPN could constitute a useful tool to
alleviate the locomotor deficits in these patients [158].
CONCLUSION
The basic scheme of the supraspinal control of locomotion in
vertebrates has traditionally placed the MLR as part of serially in-
terconnected neural structures extending from the basal ganglia to
the spinal CPGs for locomotion. We now have good indication that
the MLR sends mu ltiple parallel pathways that ensure integrated
and behaviorally coherent functions. Stimulation of the MLR in
lampreys can activate symmetrically the descending reticulospinal
cell nuclei to control the locomotor frequency [266;264]. In
addition, there is a parallel “hyperdrive pathway” from the MLR to
muscarinoceptive rhombencephalic cells that in turn provide an
additional boost to the excitation of reticulospinal cells to generate
faster locomotion [265]. In addition, the MLR of lampreys sends
parallel projections to neurons of the respiratory centers, which
increase their activity prior and during locomotor movements to
eventually compensate for the increased energy demand during
movements [120]. Interestingly, stimulation the CuN in mammals
can control both locomotion and respiration [118;117]. Another
parallel pathway decreases the strength of the sensory inputs that
are fed onto reticulospinal neurons during locomotion, to ensure
finely controlled locomotion [285;286]. These multiple outputs may
be used in a behaviorally-dependent manner, as suggested by the
different patterns of inputs that the MLR receives from the basal
ganglia and the hypothalamus [41]. Further research is needed to
acquire detailed knowledge of the operation of the MLR in physio-
logical and pathological conditions in mammals, especially in view
of the recent findings obtained in a basal vertebrate model, the lam-
prey. The information will be useful to improve deep brain stimula-
The Multifunctional Mesencephalic Locomotor Region Current Pharmaceutical Design, 2013, Vol. 19, No. 24 4463
tion protocols or pharmacological treatments in individuals with
motor deficits, such as in the case o f Parkinson’s disease.
CONFLICT OF INTEREST
The authors confirm th at this article content has no conflicts of
interest.
ACKNOWLEDGEMENTS
This work was supported by the Parkinson Society Canada; the
Canadian Institutes of Health Research (CIHR, 15129 and 15176 to
R.D), the Natural Sciences and Engineering Research Council of
Canada (NSERC), and the Fonds de la Recherche en Santé du
Québec (Groupe de Recherche sur le Système Nerveux Central,
GRSNC, 5249). D.R. received fellowships from the Fonds de la
Recherche en S anté du Québec and the GRSNC (Jasper fellow -
ship). We are grateful to Danielle Veilleux for h er technical assis-
tance and François Auclair for his useful comments on the manu-
script.
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