A new model to study compensatory mechanisms in
MPTP-treated monkeys exhibiting recovery
Ste ¤phanie Mounayar,1,2,3Sabrina Boulet,4,5,6DominiqueTande ¤ ,1,2,3Caroline Jan,1,2,3Mathias Pessiglione,1,2,3
Etienne C. Hirsch,1,2,3Jean Fe ¤ger,1,2,3Marc Savasta,4,5,6Chantal Franc?ois1,2,3and Le ¤ onT remblay1,2,3
1Institut National de la Sante ¤ et de la Recherche Me ¤ dicale,Unite ¤ 679, Paris F-75013,2Universite ¤ Pierre et Marie Curie-Paris6,
Institut Fe ¤ de ¤ratif de Recherche de Neurosciences Unite ¤ Mixte de Recherche S679, Paris F-75013,3Assistance Publique -
Ho “ pitaux de Paris,Groupe Pitie ¤ -Salpe “ trie 're, Paris F-75013,4Institut National de la Sante ¤ et de la Recherche Me ¤ dicale,
Unite ¤ 836,Grenoble Institut des Neurosciences, Equipe Dynamique des Re ¤ seaux Neuronaux du Mouvement,Grenoble
F-38043,Cedex 09,5Universite ¤ Joseph Fourier, 38041Grenoble F- 38041,Cedex 09 and6Centre Hospitalier Universitaire
de Grenoble,Grenoble F-38043,Cedex 09, France
Correspondence to: Le ¤ onTremblay, INSERM UMR679, 47 boulevard de l’ho “ pital, 75651Paris Cedex13, France
The cardinal symptoms in Parkinson’s disease (PD), akinesia, rigidity and tremor, are only observed when
the striatal level of dopamine is decreased by 60^80%. During the preclinical phase of PD, compensatory
mechanisms are probably involved in delaying the appearance of motor symptoms. In a MPTP (1-methyl-
4-phenyl-1,2,3,6-tetrahydropyridine) monkey model of PD, a spontaneous recovery has been reported after
initial intoxication suggesting that compensatory mechanisms are activated in this model as well. Assuming
that mechanisms are similarin these phenomena, the studyofrecoveryin monkeys following MPTPintoxication
may enable identification of compensatory mechanisms involved in the preclinical phase of PD. In order to max-
imize the temporal similarity between PD and the MPTP model, we assessed a new progressive monkey model
in which spontaneous recovery is expressed systematically and to characterize it based on (1) its behavioural
features, and (2) the presence of compensatory mechanisms revealed by an immunohistological approach com-
paring dopaminergic andserotoninergic innervation between monkeys either exhibiting behaviouralrecoveryor
stable motor symptoms.This immunohistological study focused on the substantia nigra, striatum and pallidum,
and their anatomical and functional subdivisions: sensorimotor, associative and limbic.The behavioural analysis
revealed that with progressive MPTP intoxication motor symptoms were initially expressed in all monkeys.
Observable recovery from these symptoms occurred in all monkeys (7/7) within 3^5 weeks after the last
MPTP injection, and most exhibited a full recovery. In contrast, acute intoxication induced stable motor symp-
toms. Despite this obvious behavioural difference, immunohistological methods revealed that the loss of dopa-
minergic cell bodies in substantia nigra was substantial and similar in both MPTP-treated groups. However,
quantification of fibres revealed that recovered monkeys displayed more dopaminergic and serotoninergic
fibres than those with stable motor symptoms in sensorimotor and associative territories of striatum and
more dopaminergic fibres in internal pallidum. This study provides a new model of PD where all monkeys
expressed functional recovery from motor symptoms despite a large dopaminergic neuronal loss.The immuno-
histological results suggest that both dopamine and serotonin could be implicated in the compensatory
Keywords: parkinson’s disease; monkey; MPTP; behaviour; immunohistology; compensation
Abbreviations: TH=tyrosine hydroxylase; DAT=dopamine transporter
Received April16, 2007 . Revised August 5, 2007 . Accepted August 8, 2007
Parkinson’s disease (PD) is characterized by a progressive,
irreversible and ultimately disabling motor deficit, with a
triad of major symptoms: akinesia, rigidity and tremor.
This pathology is related to dopamine (DA) depletion in
the basal ganglia, a consequence of degeneration of
dopaminergic neurons localized in the substantia nigra
pars compacta (SNc).
In order to study the pathophysiology of this disease,
many animal models were developed (Jenner et al., 1984;
doi:10.1093/brain/awm208Brain (2007) Page1of17
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Beal, 2001). Among them MPTP (1-methyl 4-phenyl
probably the most relevant model available. This model
can reproduce not only destruction of DA systems but also
the entire triad of symptoms, for example in African Green
Monkeys, where even resting tremor, the most difficult
symptom to produce in an animal model, can be observed
(Redmond et al., 1985; Bergman et al., 1998). These studies
have greatly increased the understanding of the neural basis
of the various Parkinsonian symptoms (DeLong, 1990;
Bergman et al., 1998; Boraud et al., 2002; Pessiglione et al.,
Nevertheless, while this model closely parallels many
aspects of PD, it has several limitations. First, the severity of
symptoms is highly variable between monkeys even when
using the same protocol and similar doses of MPTP. For
example, in a study by Elsworth (Elsworth et al., 2000)
involving 32 monkeys treated with the same protocol,
symptoms ranged from severe (9 monkeys), to moderate
(8), mild (6) or even absent (9). A second limitation of the
MPTP model to date is the difficulty in producing a
gradual destruction ofdopaminergic
corresponding progressive appearance of symptoms as is
observed in PD. In response to this limitation, researchers
developed long-term progressive protocols, with repetitive
small doses injections spread
(Schneider and Kovelowski, 1990; Hantraye et al., 1993).
Although this did result in a progressive appearance of
symptoms, it required extremely long treatment periods
[e.g. 17 to 21 months (Hantraye et al., 1993)]. A final
aspect of the MPTP model that has been considered as a
limitation is that spontaneous recovery
symptoms has often been reported after MPTP intoxication
phenomenon is not observed in humans suffering from
idiopathic PD, and can interfere with both the study of
stable motor symptoms in the model and evaluation of
therapeutic interventions. While some investigations have
attempted to eliminate this problem by establishing a stable
state of symptom expression for studying ongoing dysfunc-
tions (Taylor et al., 1997), the phenomenon of recovery
itself seems to be worthy of further study, providing insight
into the role of behavioural adaptation versus neural
For example, we have previously shown, in MPTP-
treated monkeys, that visual tracking during execution of
movements (Pessiglione et al., 2003) combined with an
increase in motivational processes (Pessiglione et al., 2004b)
could help to overcome executive dysfunction observed in
the early stages of parkinsonism in the monkey model. Such
behavioural adaptations were not sufficient to explain all
the functional recovery, particularly reductions in some of
the motor symptoms, such as tremor or rigidity, indicating
that other compensatory mechanisms must be involved.
This could involve dopaminergic compensation in the
striatum: e.g. an increase of synthesis and release from
remaining terminals, sprouting of axonal collaterals from
remaining fibres in, as well as denervation supersensitivity
in the dopaminoceptive striatal neurons. Moreover it is
important to note that, according to the distribution of
cortico-striatal inputs, the striatum can be subdivided into
three anatomo-functional territories (Alexander et al.,
1986): sensorimotor territory receiving cortical inputs
primarily from sensory and motor cortex, associative
territory with cortical inputs primarily from prefrontal
cortex and limbic territory with major inputs from
cingulate cortex as well as the amygdala (Kunishio and
Haber, 1994; Parent and Hazrati, 1995). Corresponding
functional attributes for these three territories (sensor-
imotor integration, cognitive processes and reward and
motivational processing) have been identified through
behavioural studies. In both PD (Kish et al., 1988; Brooks
et al., 1990; Frost et al., 1993) and the MPTP model (Jan
et al., 2003) consideration of these territories had revealed
that striatal dopaminergic degeneration is heterogeneous:
sensorimotor and associative territories of striatum are
severely affected, whereas limbic territory is relatively
preserved, providing a relatively large source of potential
collaterals. It is possible that sprouting of axonal collaterals
and sensorimotor territories is involved in a recovery of
parkinsonian symptoms in the MPTP model. Alternatively,
dopaminergic compensation could be related to the DA
innervation present in another structure of basal ganglia
(Bezard et al., 2001), such as the pallidum which is less
affected than the striatum in both patients and MPTP
monkeys (Jan et al., 2000), also consists of anatomo-
functional territories (Haber et al., 1990; Flaherty and
Graybiel, 1994) and also expresses an heterogeneous
denervation. Finally compensation could be mediated by
non-DA systems in the striatum such as GABAergic
(Schroeder and Schneider, 2002), glutamatergic (Bezard
et al., 1998) and/or serotoninergic systems (Gaspar et al.,
1993). Reviews are available summarizing these and other
Bezard and Gross, 1998).
In humans with PD it is commonly assumed that the
cardinal symptoms are observed only once the striatal DA
level is decreased by 60–80% (Bernheimer et al., 1973;
Hornykiewicz and Kish, 1987). Presumably, compensatory
mechanisms during the preclinical phase of PD delay the
appearance of motor symptoms. Because these mechanisms
may well parallel those involved in recovery of function in
MPTP-treated monkeys, a greater understanding of these
mechanisms might lead to the development of new
therapeutic approaches, especially for early stages of PD.
With this in mind, the first aim of this study was to
develop a progressive protocol for MPTP intoxication in
order to consistently induce a full expression of motor
symptoms followed by substantial functional recovery.
A second aim was to use this model to observe the
temporal evolution of symptom appearance and subsequent
Page 2 of17Brain (2007)S. Mounayar et al.
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recovery. A final aim was to investigate potential compen-
satory mechanisms by comparing DA and serotoninergic
(5HT) innervation in different striatal and pallidal terri-
tories (sensorimotor, associative and limbic) in MPTP-
treated monkeys that exhibited behavioural recovery versus
those expressing stable motor symptoms. Portions of the
data in the current article were previously presented in
abstract form (Mounayar et al., 2005).
Material and methods
Animals and MPTP treatment
Fifteen male vervet monkeys (Cercopithecus aethiops) between 4
and 6 years old (young adults) and weighing 4–7kg, provided by
the Barbados Primate Research Centre (Farley Hill, Barbados,
West Indies) were used in this study. Care and treatment of these
monkeys were in strict accordance with NIH guidelines (1996) as
well as with the European Community Council Directive of 1986
(86/609/EEC) and the recommendations of the French National
Five monkeys served as controls and the other 10 were treated
with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, Sigma,
St Louis, MO, USA). All MPTP injections were intramuscular, 0.4
to 0.6mg/kg and performed under light anaesthesia (ketamine
0.5mg/kg, atropine 0.05ml/kg). Two different protocols of
administration were used: progressive intoxication (injections
spaced by 4 to 5 days) or acute intoxication (daily injections).
The protocol for progressive intoxication (Fig. 1, part 1, A) was
used for seven monkeys. Injections were stopped after appearance
of the triad of symptoms (see ‘Behavioural analysis’ section later).
After behavioural recovery,two
also received a short acute protocol (two or three injections at
0.6mg/kg) in order to produce strong and stable symptomatic
state (Fig. 1, part 1, C). Acute intoxication alone was used for
three monkeys and consisted of two series of daily injections
(Fig. 1, part 1, B). For one of these monkeys, supplementary
injections were required to maintain the level of the clinical
signs. Monkeys were sacrificed either after recovery (progressive
intoxication only) or after at least 1 month (Taylor et al., 1997) of
stable Parkinsonian symptoms (progressive+acute or acute only)
with the exception of one monkey (CA 9) who died of an
infectious disease 3 weeks after the end of MPTP injections.
of these seven monkeys
Before MPTP treatment, monkeys were trained to sit in a primate
chair for 1h daily, 5 days a week. This period allowed observation
of spontaneous behaviour (2 periods of 20min), behavioural
responses during joint manipulations, and performance on a
simple reactiontask (repeated
spontaneous behaviour). Following this, monkeys were observed
in their home cages for half an hour. They were also observed
2 times between MPTP injections for progressive protocol and
every 3 or 4 days after treatment in both protocols. These
evaluations allowed rating of their Parkinsonian symptoms in
order to follow the evolution of symptom onset and recovery.
3 times,before andafter
The severity of parkinsonism was evaluated by using the rating
scale proposed by Schneider and Kovelowski (1990). This scale
includes 12 items rated between 0 and 2 or 3, with a total score of
29. It takes in account classical motor symptoms (bradykinesia,
rigidity, tremor, freezing, posture and arm posture) but also
spontaneous activities (arm movements, spontaneous eyes move-
ments and homecage activity) and other activities (vocalization,
triggered eyes movements and feeding).
The presence of many symptoms was checked for both in the
task and during spontaneous behaviour, both in the primate chair
and the home cage (bradykinesia, tremor, freezing, feeding, arm
movements and arm posture). Posture was rated only in the home
cage because the monkeys had their heads fixed in the chairs.
Home cage activity was additionally evaluated using an activity
digitalizing system (Vigie Primates, Viewpoint, Lyon; see for
details Pessiglione et al., 2003). Muscular rigidity and eyes
movements were assessed only in the primate chair. Rigidity was
assessed by joint manipulation and spontaneous eyes movements
were rated by counting saccades during 3min after each period
of task execution. Triggered eyes movements were tested in
chair especially during the task. Finally, vocalization was not
rated because it was rarely observed in normal contexts with our
Fig.1 Methods. Part1Protocols for MPTP intoxication.Three
different protocols used for MPTP administration in monkeys.
(A) A progressive intoxication protocol with injections spaced by
4 or 5 days, (B) an acute intoxication protocol with two series of
daily injections and (C) progressive followed by acute intoxication
protocols.Both behavioural and histological approaches were used
to characterize the different protocols and compare monkeys that
recovered (R) to those with stable motor symptoms monkeys
(SMS). Part 2 Delineation of territory boundaries used for anato-
mical assessments. Sensorimotor (SM), Associative (Ass) and
Limbic (Lim) territories of striatum, including caudate (Cd),
Putamen (Pu) and nucleus accumbens (Acc), and pallidum, including
external (GPe) and internal segments (GPi) are depicted on four
sections at the indicated distances from the anterior commissure
(AC).Optical density and fibres quantification were performed in
areas indicated by dotted lines inside these territories.
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At the end of the experiments, the monkeys received a lethal
overdose of anaesthesia and then were transcardially perfused with
saline followed by fixative solution. Brains were removed from the
skull, cut into 50mm thick transverse sections and stored for
immunohistochemical procedures (see details in Jan et al., 2000).
Dopaminergic and serotoninergic innervations were studied by
immunocytochemical localization of tyrosine hydroxylase (TH),
dopamine transporter (DAT) and serotonin (5HT). The aim was
to compare the denervations between intoxicated monkeys, in
substantia nigra (SN), striatum, external and internal globus
pallidus (GP). Subregions were identified for SN [the peri- and
retrorubral cell group (A8), the Substantia Nigra Pars Compacta
(SNc) cell group (A9) and the ventral tegmental area cell group
(A10)] as well as for striatum and GP (sensorimotor, associative
and limbicterritories). Anatomo-functional
delineated (Fig. 1, part 2) according to the literature on the
topography of cortico-striatal projections for striatal territories
(Parent and Hazrati, 1995) and striato-pallidal projections (Haber
et al., 1990; Flaherty and Graybiel, 1994). TH positive cells were
counted in SN. Dopaminergic and serotoninergic innervations
were evaluated in striatum by optical density and in striatum and
GP by quantification of TH, DAT and 5HT positive fibres.
Immunoreactivity for TH was localized using the protocol in
Francois et al. (1999). Briefly, sections were incubated in a mouse
anti-TH antibody (1/500 dilution: Incstar, Stillwater, MO, USA),
followed by a secondary biotinylated antibody (1/250 dilution:
goat anti-mouse IgG, Vector Laboratories). Visualization was
achieved using avidin–biotin–peroxidase complex (ABC standard
kit; Vector, 1:125 in Phosphate-Buffered Saline) and diaminoben-
zidine (DAB: Sigma).
For DAT, sections were incubated in a rat anti-DAT antibody
(1/4000 dilution: Chemicon) followed by a secondary biotinylated
antibody (1/200 dilution: goat anti-rat, IgG, Vector Laboratories).
Visualization was achieved using avidin–biotin–peroxidase com-
plex (ABC standard kit; Vector, 1:125 in Phosphate-Buffered
Saline) and DAB (Sigma) and Glucose oxydase.
Finally for 5HT, sections were incubated in rabbit anti-5HT
antibody (1/10000 dilution: Diasorin) followed by a secondary
biotinylated antibody (1/200 dilution: goat anti-rabbit, IgG,
Sigma). Visualization was achieved using Streptavidine peroxidase
(1/400) and DAB (Sigma).
In the SNc, TH-positive cells were counted in nine regularly
spaced sections covering the antero-posterior extent of the
structure with an image analysis system (Mercator, ExploraNova,
La Rochelle, France). The sections were matched anatomically in
each of the animals, verifying that the cross-sections of the
midbrain were similar in controls and MPTP-treated monkeys.
The total number was estimated
Abercrombie method as previously described (Herrero et al.,
1993). The percentage of neuronal loss in the SNc was evaluated
by comparison with control values of intact vervet monkeys
(for details see Francois et al., 1999).
In order to quantify TH, DAT and 5HT-labelled fibres,
functional territories of the striatum and pallidum were first
after correctionby the
delineated in one monkey at four levels. Analogous sections were
then selected for analysis in the others. One rostral section level
[anterior commissure (AC)+4.3mm by comparison to a standard
atlas] permitted measurement on limbic and associative territories
of striatum. At a caudal level (AC-3.7), measurements were
performed for sensorimotor territory of striatum, GPe and GPi.
Associative and limbic territory of GPe were assessed at an
intermediate level (AC+0.5), as was the associative territory of
GPi (AC-1.5). In the striatum, due to the high density of TH,
DAT and 5HT-fibres in control monkeys, dopaminergic and
serotoninergic innervations were evaluated by measuring optical
density of immunostaining using an image analysis system
(Mercator). Optical density is a measure based on the difference
of luminosity between a defined area and reference taken in a
band of fibres. In order to complement these optical density
measurements, individual fibres were quantified visually in
sensorimotor and associative territories only for MPTP-treated
monkeys. This was done by counting the number of fibres
crossing the perimeter of 25 circles (diameter=10mm) pseudo
randomly distributed by the computer (Mercator) within the
drawn limits of the sensorimotor striatum as previously described
(Jan et al., 2000). The same procedure was used to quantify fibres
in different territories of pallidal segments (for details of
quantification methods see Jan et al., 2000).
Optical density and quantification of cells or fibres in different
territories were analysed using Mann–Whitney U-test to compare
control, recovered and stable motor symptoms monkeys. We
considered three levels of significance: P50.05, P50.01 and
P50.001. Unless otherwise specified, values are mean and (?)
standard error of mean.
Correlation coefficients (Pearson’s r) were calculated in order to
investigate the relation between behavioural recovery (motor score
total, motor scores for individual symptoms and recovery time)
and immunohistological indices of DA and 5HT innervations
of the different territories of the basal ganglia (cell loss in
dopaminergic areas of mesencephalon, TH, DAT and 5HT
positive fibres in striatum and TH positive fibres in pallidum).
This test indicates a correlation factor r which is close to r=1
when the correlation is significant. Three levels of significance
were considered: P50.05, P50.01 and P50.001.
Monkeys undergoing progressive intoxication received 3 to
7 (mean 4.7) injections of MPTP for a cumulative dose of
1.2 to 2.5mg/kg (mean 1.8mg/kg). The dose was higher for
monkeys with acute intoxication [6 to 12 injections (mean
7.8); total dose of 2.8 to 5.2mg/kg (mean 3.2mg/kg: see
Table 1 and Fig. 1]. All monkeys under progressive protocol
(7/7) developed motor symptoms and then exhibited
recovery after stopping treatment (Fig. 2). Recovery was
complete for most of the monkeys (6/7), while one monkey
(CA37) presented only a partial recovery with mild motor
symptoms, bradykinesia, postural trouble and tremor
persisting until sacrifice. Monkeys treated with the acute
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symptoms but exhibited a weak recovery at best, remaining
severely symptomatic until sacrifice (Table 1, Figs. 1 and 2).
One of these monkeys (CA2) required supplementary
MPTP injections in order to maintain a high motor
score. With a single exception (the monkey with the lowest
overall symptom score, CA21, who did not exhibit resting
tremor), all these animals developed the complete triad of
Evolution oftotal score over time
With the progressive protocol, symptoms appeared abruptly
after the third MPTP injection, with a maximal effect 4 or 5
days after the last injection (see time=0 on Fig. 2). Even
though most of monkeys developed the triad of motor
symptoms, akinesia, rigidity and tremor, the maximal score
obtained for recovered monkeys (16.6?1.7) was inferior to
the maximal score observed in monkeys with stable motor
symptoms (23.2?0.6). Recovery after the maximal effect
and cessation of MPTP injections was gradual over 3 to 5
weeks (mean 28.6?3.9 days to maximal recovery see
Fig. 2). In general, the greater the maximal effect, the longer
the time to maximal recovery. An exception to this
occurred in monkey CA33 which presented a high score
of 19 and had the fastest recovery, returning to a normal
state in 17 days.
Evolution byindividual symptoms
When averaged across monkeys treated with the progressive
protocol, bradykinesia was the first symptom to appear
whereas tremor was the last (Figs 3 and 4). In fact,
Fig. 2 Evolution of motor score during intoxication and recovery. Data represent the evolution of appearance and disappearance of
symptoms during and after the intoxication (left panel) and the motor score at time of sacrifice (right panel). Monkeys with progressive
protocol are represented with continuous lines andmonkeys with acute intoxicationwith dottedlines.Timelines are aligned such thatday 0
corresponds to the day on which the maximal motor score was obtained from an individual animal.
T able1 MPTP injections and Parkinsonian score
MonkeyGroup (Protocol, effect) State before recoveryState before sacrifice Survival time (months)DA cell loss (%)
P=progressive protocol; A=acute protocol; R=monkey that recovered; SMS=monkey with stable motor symptoms.
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bradykinesia was one of the first symptoms to appear in all
monkeys whereas tremor appeared later, not developing at
all in one case (CA21). Other early appearing and long-
lasting symptoms included a decrease in spontaneous
activities such as homecage activity and spontaneous eyes
movements (Figs 3 and 4).
Symptomsin some cases
asymmetrically (Table 2). For example, symptom expression
appeared locally and/or
Fig. 3 Appearance and disappearance of individual symptoms.This graph represents the average day of appearance and disappearance for
each symptom, centred on the day of maximal score. All averages were computed from the seven animals treated with the progressive
protocol (recovered monkeys). Error bars indicate the SD for appearance and disappearance of symptoms.
Fig. 4 Example of the evolution of Parkinsonian symptoms in one monkey treated with the progressive protocol (CA23, maximal
score=19). Each symptom was rated between 0 and 2 or 3 during the intoxication and the recovery.
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was stronger or earlier for the arms than at the legs in five
monkeys out of six. Symptoms appeared first on the left
side for four monkeys, on the right side for one monkey
and symmetrically for one. The laterality of dysfunction did
not appear to be dependent on the left versus right-side
dominance of the monkeys.
An extensive loss of TH positive neurons was observed in
both groups of MPTP-treated monkeys compared to
control monkeys (Fig. 5, P50.01). In contrast this cell
loss was not statistically different between recovered (62%)
and stable motor symptoms monkeys (73%). This pattern
of loss is found in the three areas: A8, A9 and A10.
However somewhat greater variability was noted in the
group of recovered monkeys, in which CA21, the monkey
with the lowest score has the smallest loss of DA cells (32%;
see Table 3).
TH, DATand 5HTinnervation ofthe striatum
In the striatum, TH labelling measured using optical
density was greatly decreased in all three territories for
monkeys expressing a recovery from Parkinsonian symp-
toms (P50.01) and in sensorimotor and limbic territories
for those with stable motor symptoms (P50.01) compared
to controls (Fig. 6A and B). The decrease of 62% in the
(Fig. 6B). The sensorimotor territory is the most depleted
region for both groups of MPTP-treated monkeys with a
loss of about 80% (83% for stable motor symptoms
monkeys and 78% for recovered monkeys), whereas the
denervation is about 65% for associative and 57% for
limbic territory. In contrast, when comparing the TH
not statistically significant
Fig. 5 TH labelling in the mesencephalon. (A) Depiction of areas used to define the A8, A9 and A10 cells groups and examples of the
distribution of TH immunostaining in the mesencephalon from control, stable motor symptoms and recovered monkeys. Scale bar, 2mm.
(B) Percentage of TH-positive neurons remaining in the mesencephalon of stable motor symptoms and recovered monkeys relative to
control.Results from total counts as well as counts in three subareas, A8, A9 and A10, are presented. Mann and Whitney U:?P50.05;
T able 3 Dopaminergic cell loss
Monkey Dopaminergic cell loss (%)Total
A8=peri- and retrorubral cell group; A9=substantia nigra pars
compacta cell group; A10=ventral tegmental area cell group.
T able 2 Asymmetry of symptoms
Earlier appearance or
CA21did not develop enough symptoms for inclusion in this
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groups, and despite the obvious behavioural difference
between these two groups, there was no significant
difference for any of the three territories (Fig. 6B). In
order to assess the DA innervation at a finer level of
resolution, fibre quantifications were performed in the
sensorimotor and associative territories of striatum. In
contrast to the optical density measures, these measures
revealed that recovered
TH-labelled fibres than stable motor symptoms monkeys
(Fig. 6C and D). Thus recovered monkeys had more
TH fibres than stable motor symptoms monkeys above all
in sensorimotor (23.8 times more for recovered monkeys,
P50.01) but also in associative territory (5.8 times more).
Limbic territory was relatively preserved and fibre density
prohibited quantification using this method.
Assessment of DA innervation using immunohistological
visualization of the dopamine transporter was also per-
formed initially using measures of optical density. This
revealed a tendency for a decrease in sensorimotor and
associative territories in both groups of MPTP-treated
monkeys, although only the decrease in the sensorimotor
monkeys displayed more
territory in the stable motor symptom monkeys was
significant (decrease of 45%, P50.01; see Fig. 7A and B).
The labelling was relatively unaffected in limbic territory.
Once more no significant difference was observed between
recovered andstable motor
contrast, quantification DAT fibres revealed a significant
difference between stable motor symptoms and recovered
monkeys in both sensorimotor and associative territories
(Fig. 7C and D). Recovered monkeys had more DAT fibres
than stable motor symptoms monkeys (25 times more in
sensorimotor territory, 3.8 times more in associative
Finally, measures of optical density for 5HT-immunor-
eactivity in the striatum showed a strong tendency for an
increase in recovered monkeys in all three territories
(Fig. 8A and B). However this increase was only significant
for the sensorimotor territory (P50.01 control versus
recovered), and for the associative territory (P50.05
recovered versus stable motor symptoms). There was no
significant difference in limbic territory between the three
groups of monkeys.
Fig. 6 TH labelling in striatum. (A) Examples of distribution of TH immunostaining in sensorimotor, associative and limbic territories of
striatum from control, stable motor symptoms and recovered monkeys. Scale bar,1mm. (B) Optical density measurement of TH immu-
nostaining in the three anatomo-functional territories in control (n=4), stable motor symptoms (n=4) and recovered monkeys (n=5).
Mann and Whitney U:??P50.01. (C) Higher power magnification (?40) examples of TH-positive fibres in the different territories of stria-
tum from control, stable motor symptoms and recovered monkeys. Scale bar, 50mm. (D) Quantification of the number of TH-positive
fibres crossing 30 circles in sensorimotor and associative territories of striatum in stable motor symptoms (n=4) and recovered monkeys
(n=5). Mann and Whitney U:??P50.01.
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Quantification of 5HT fibres helped to confirm this
tendency and allow further analysis (Fig. 8C and D). First, a
significant increase of fibre number was evident in
recovered compared to stable motor symptoms monkeys
in all three territories (P50.01). Moreover, a decrease of
the number of 5HT fibres was observed in stable motor
symptoms monkeys in comparison to control monkeys in
associative and limbic territories (P50.01).
A similar pattern of loss in TH positive fibres was found in
the GPe as in the striatum using the fibre quantification
procedure. Thus a large decrease in the quantity of fibres
was observed in sensorimotor and associative territories of
recovered and stable motor symptoms monkeys compared
to controls. However no significant difference was found
between the MPTP-treated groups. No significant reduction
was noted in the limbic territory (Fig. 9). In the GPi, only
the sensorimotor territory showed a decrease in TH positive
fibres with MPTP treatment (P50.01; both groups). In
addition a significant difference was detected in this
territory between the two MPTP groups, with fibre
number greater in the recovered monkeys (P50.05). The
associative territory did not show any modification in
MPTP-treated monkeys compared to control when only
means were considered. However counts were highly
variable in the group of recovered monkeys and for those
that exhibited the greatest recovery (CA23, 33, 34), counts
were superior to those of control and stable motor
symptoms monkeys. Moreover, fibre counts for the two
monkeys which expressed recovery before receiving an
acute intoxication were greater than those for the three
monkeys that received only the acute protocol.
Correlation between behavioural and
The strongest correlation observed was between motor
score and A8 cell death (r=0.99, P50.01; Fig. 10A). Cell
death in no other area of mesencephalon was significantly
correlated with motor scores or recovery time. In contrast,
in the striatum correlations were detected with recovery
time but not with motor score. There was a correlation on
Fig. 7 DAT labelling in striatum. (A) Examples of distribution of DAT immunostaining in sensorimotor, associative and limbic territories
of striatum from control, stable motor symptoms and recovered monkeys. Scale bar,1mm. (B) Optical density measurement of DAT
immunostaining in the three anatomo-functional territories in control (n=4), stable motor symptoms (n=4) and recovered monkeys
(n=5).Mann and Whitney U:??P50.01. (C) Higher power magnification (?40) of DAT-positive fibres in the different territories of striatum
from control, stable motor symptoms and recovered monkeys. Scale bar, 50mm. (D) Quantification of DAT-positive fibres crossing 30
circles in sensorimotor and associative territories of striatum in stable motor symptoms (n=4) and recovered monkeys (n=5). Mann and
New model to study compensatory mechanisms in MPTP-treated monkeys exhibiting recovery Brain (2007) Page 9 of17
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one hand between recovery time and the number of TH
fibres in associative territory (r=0.94; P50.05; Fig. 10B)
and on the other hand between recovery time and the
number of DAT fibres in sensorimotor territory (r=0.94;
P50.05; Fig. 10C). Finally in pallidum there was a
correlation between the number of TH+fibres in limbic
GPe and the recovery time. There were no significant
correlations at all for GPi. Data concerning stable motor
symptoms monkeys are indicated on the graph with their
real motor score but at an arbitrary recovery time after all
recorded recovery times.
A few correlations between individual symptom scores and
immunohistological variables were significant. These include
those between tremor and number of TH fibres in
sensorimotor territoryofGPe (r=0.91;P50.05)andbetween
homecage activity and number of 5HT fibres in sensorimotor
and associative territories of striatum (r=0.91; P50.05).
This report presents evidence that a progressive protocol of
MPTP administration (injections every 4–5 days) allows the
study of the progressive appearance of parkinsonian
symptoms. Moreover,a recovery
symptoms was observed in all monkeys treated with this
progressive protocol. In contrast, monkeys treated with an
acute protocol (two series of repeated daily injections of
MPTP) failed to show such recovery, exhibiting a stable
level of parkinsonian symptoms. Despite this difference,
immunohistochemical analysis revealed that DA cell death
in the substantia nigra is as extensive for monkeys showing
a recovery as for monkeys with a stable expression of
symptoms. In contrast, indices of striatal DA (TH-IR, DAT-
IR) and 5-HT (5-HT-IR) innervation were greater in several
striatal and pallidal territories of recovered monkeys than
those with stable motor symptoms.
Special features of this present progressive
The first aim of our study was to create a new model of
MPTP administration in order to follow the evolution of
symptoms, to observe behavioural recovery and to inves-
tigate potential compensatory mechanisms involved in the
phenomenon of recovery. Recovery has been reported in
Fig. 8 5HT labelling in striatum. (A) Examples of distribution of 5HT immunostaining in sensorimotor, associative and limbic territories of
striatum from control, stable motor symptoms and recovered monkeys. Scale bar,1mm. (B) Optical density measurement of 5HT immu-
nostaining in the three anatomo-functional territories in control (n=4), stable motor symptoms (n=4) and recovered monkeys (n=5).
Mann and Whitney U:?P50.05;??P50.01. (C) Higher power magnification (?40) of 5HT-positive fibres in the territories of striatum from
control, stable motor symptoms and recovered monkeys. Scale bar, 50mm. (D) Quantification of 5HT-positive fibres crossing 30 circles in
sensorimotor, associative and limbic territories of striatum in control (n=4), in stable motor symptoms (n=4) and recovered monkeys
(n=5). Mann and Whitney U:??P50.01.
Page10 of17Brain (2007)S. Mounayar et al.
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monkeys after acute MPTP intoxication but with a large
amount of variability in its occurrence (Eidelberg et al.,
1986; Taylor et al., 1997; Elsworth et al., 2000). In standard
acute intoxication procedures, monkeys received four or
five intramuscular injections of MPTP (0.4mg/kg) over
5 days leading to a total dose between 1.6 and 2.5mg/kg
(Elsworth et al., 2000). As in previous studies (Eidelberg
et al., 1986; Elsworth et al., 1989; Taylor et al., 1997), such
an acute protocol produced a large range in symptom
expression, from the stability of the parkinsonian symptoms
to the expression of recovery. Taylor et al. (1997) reported
that the expression of recovery, when it occurred, was
dependent on the initial severity of symptoms. In the
present progressive protocol, the chosen interval of 4 to
5 days between each injections leaves time for elimination
of MPTP (Przedborski et al., 2001), as well as expression
and evaluation of symptoms resulting from the last
injection. The latter was evident in a previous study
(Pessiglione et al., 2003), using the same interval between
MPTP injections showing that the behavioural effects of a
particular injection, measured by the number of errors in a
motor task, were detectable by the fourth day following an
progressive protocol takes into account this time lag
between injection and maximal effect. This allows much
greater control of the maximal symptom severity in an
individual monkey so that MPTP administration can be
stopped once all core symptoms are evident, but while
recovery is still possible. Additional MPTP administration
could surpass a threshold beyond which full recovery is not
possible, potentially close to the maximal score of CA37,
the only monkey treated with the progressive protocol that
did not exhibit complete recovery. The ability to closely
monitor the effects of an individual MPTP injection and
cease administration in an individual-specific fashion thus
appear to be central to the use of this progressive protocol
for consistently producing monkeys exhibiting recovery.
Another progressive protocol (Bezard et al., 1997)
involving daily injections of MPTP (0.2mg/kg, i.v.) has
been used to produce a stable expression of parkinsonian
symptoms without recovery. This yields a larger cumulative
dose of MPTP (3?0.2mg/kg) than the present progressive
protocol(1.2–2.5mg/kg) or standard acute
(1.6–2.5mg/kg) (Eidelberg et al., 1986; Elsworth et al., 1989;
Taylor et al., 1997). It is noteworthy that a similar stable
expression of parkinsonian symptoms was obtained with
our acute daily injection protocol with a similar cumulative
dose of MPTP (3.2mg/kg). This suggests that with daily
injections, 3mg/kg may be required to obtain a stable
expression of parkinsonian symptoms without recovery.
Two additional factors could explain why the progressive
protocol with spaced injections results in expression of
recovery more consistently than daily injections. First,
MPTP and its metabolites are fully excreted within a period
of 3 days after injection. With injection of MPTP spaced by
4 to 5 days, accumulation of the toxic form of MPTP (the
MPP+) would be avoided (Przedborski et al., 2001).With
daily injections, accumulation of MPP+could lead to an
underestimation of the impact of daily dose of MPTP. This
could explain why in a daily progressive protocol (Bezard
et al., 1997) the expression of symptoms continued to
increase to a full and stable state even after the injections of
MPTP had been stopped. A second, and perhaps more
important, reason why the spaced progressive protocol may
promote recovery is the fact that the interval between
injections may make it possible for compensatory mechan-
isms to be initiated gradually, before the threshold level,
around a loss of 60–70% of dopaminergic neurons,
resulting in the expression of the motor symptoms. We
hypothesize that the compensatory mechanisms in the
progressive protocol are initiated earlier, at the beginning of
dopaminergic lesion (with the first injection), and progress
during the interval between each step of intoxication. They
are pushed over their limit if an excess of MPTP was
administered, but permit a progressive recovery from the
symptoms when MPTP intoxication is stopped before such
excesses are achieved.
Thespacing ofinjections inthe current
Fig. 9 Quantification of TH-positive fibres in the two pallidal
segments, in the three anatomo-functional territories in control,
stable motor symptoms and recovered monkeys. Numbers of
TH-positive fibres crossing 25 circles in the sensorimotor, associa-
tive and limbic territories of GPe, and in the sensorimotor and
associative territories of GPi. Mann and Whitney U:?P50.05;
??P50.01.Graph for associative GPi indicates the number of TH
fibres for each monkey (control with squares, stable motor
symptoms with circles and recovered with triangles.
New model to study compensatory mechanisms in MPTP-treated monkeys exhibiting recovery Brain (2007)Page11of17
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A new monkey model to study early
stages of Parkinson’s disease
Another advantage of this protocol lies in the possibility to
observe a progressive appearance of Parkinsonian symp-
toms. Obviously symptoms emerge faster in this model
than in PD in human. The present protocol is a
compromise between acute protocols and other progressive
protocols leading to a slower progression of symptoms
but requiring an intoxication schedule extended over
several months (Hantraye et al., 1993; Schneider and
Pope-Coleman, 1995). Thus Hantraye et al. (1993) treated
five monkeys over a period of 21 months and observed that
bradykinesia and hypokinesia were often the first symptoms
to appear, whereas resting tremor appeared last. Moreover
several studies by Schneider’s team used a chronic
(Schneider and Kovelowski, 1990; Schneider and Pope-
Coleman, 1995) resulted in cognitive deficits appearing
prior to motor deficits. Although the progressive protocol
used in the present study is much shorter, similar
characteristics can be observed. Bradykinesia is the first
motor symptom to appear, whereas tremor is the last one
(Figs. 3 and 4). Using the same progressive protocol in a
previous study (Pessiglione et al., 2004a), we reported that
before emergence of overt motor symptoms, cognitive
difficulties may produce hypokinesia (lack of initiation),
bradykinesia (abnormal slowness) and executive disorders
(hesitations and freezing). Thus the evolution of symptoms
over time appears to be similar, although condensed in
time, in the present protocol, compared to longer, chronic
intoxication protocols. The ability to study the evolution of
protocol that, atleast insome cases
Fig.10 Correlation between immunohistology and behaviour.Correlations were observed between dopaminergic cell loss in A8 and
motor score (A), and also between recovery time and three different histological variables,TH fibres in associative striatum (B), DAT
fibres in sensorimotor striatum (C) and TH fibres in limbic GPe (D).Full shapes correspond to recovered monkeys and empty shapes
correspond to stable motor symptom monkeys.Continuous lines represent regression lines and dotted lines represent the extrapolation
of regression lines.
Page12 of17Brain (2007) S. Mounayar et al.
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symptoms without having to use such extended protocols
should be of great advantage.
This relatively slow emergence of the entire triad of
symptoms may also allow more appropriate comparisons
with the clinical disorder. Tremor is the most difficult
symptom to produce in monkeys (Bergman et al., 1998;
Guehl et al., 2003), so its presence with this new protocol
supports the similarity of the model to PD at the level of
progressive appearance and expression of symptoms could
be revisited in conjunction with results from this model.
One relatively consistent observation is that bradykinesia
and tremor are generally the most prominent symptoms
(Kang et al., 2005; Uitti et al., 2005). This has led to a
classification of two clinical subtypes in Parkinson’s disease:
akinetic-rigid or tremor-dominant. According to post-
mortem studies (Jellinger, 1999), the akinetic-rigid subtype
is attributed to loss of dopaminergic neurons in the
ventrolateral SNc (A9), whereas cell loss in the medial
SNc and retrorubral field (A8) is associated with the
tremor-dominant subtype. Correlational analysis in the
present study did not show such a clear division. Although
a significant correlation was observed between the loss of
dopaminergic neurons in A8 and the overall severity of
motor impairment (r=0.99, P50.01), the correlation
between tremor and the dopaminergic cell loss in A8,
although strong (r=0.87), was not statistically significant.
Nevertheless, one of the few significant correlations detected
between individual symptoms and histological markers was
the correlation between tremor and the number of
dopaminergic fibres in the sensorimotor GPe, one of the
target structures of A8 (Jan et al., 2000). Taken together,
these results from human and monkey provide convergent
evidence for the possible role of this extra-striatal projection
from A8 to the pallidum in the expression of this particular
symptom, as has been previously suggested by some authors
(Bernheimer et al., 1973; Bergman et al., 1994).
According to clinical studies, Parkinsonian symptoms
may appear first in the upper limb (Schelosky and Poewe,
1990), in the lower limb (Vidailhet et al., 1994) or equally
in the upper and lower limbs (Dickson and Grunewald,
2004). In our study symptoms appeared mostly in the
upper limb initially (5/6). However it is not clear whether
this result is dependent on evaluation strategies used. The
tasks used involve particularly arm movements and when
the monkey is in the primate chair, observation of legs is
limited. This could explain the earlier detection of
symptoms in the upper limbs. Comparison of localization
of initial symptoms in the progressive protocol to the
evaluations made in humans could help to understand the
variability observed in these studies. It is also clear that PD
is commonly an asymmetric disease, as evidenced by the
fact that major ratings scales of Parkinsonian symptoms
take this asymmetry into account (see e.g. Hoehn and Yahr,
1967; Martinez-Martin et al., 1994; Goetz et al., 1994). This
phenomenon had not been described previously in animal
described inPD in the
models, but we were able to observe an asymmetry in the
emergence of symptoms, with one side affected first or
more strongly in five monkeys out of six. No systematic
relation between the affected side and a monkey’s dominant
side was apparent.
Compensatory mechanisms behind recovery
In considering the compensatory mechanisms involved in
delaying the onset of symptoms during preclinical stages of
PD and in animal models, the residual dopaminergic
system has received the most attention. One focus has been
the role of increased DA release (Zigmond et al., 1984;
Snyder et al., 1990; Schneider et al., 1994) or turnover
(Agid et al., 1973; Bernheimer et al., 1973; Zigmond et al.,
1984; Altar et al., 1987) from remaining axons. This
biochemical aspect will be addressed in another study,
already presented in abstract form (Boulet et al., 2005), but
one aspect that can be addressed here is whether any
increase in DA release would come exclusively from fibres
compensation could also be due to a sprouting of
dopaminergic fibres, as has been suggested from results
using both a rat model (Finkelstein et al., 2000) and the
MPTP-treated monkey (Song and Haber, 2000).
The present study indicates that despite a similar loss of
dopaminergic cells, and a substantial loss of TH and DAT
labelling in striatum of both monkeys that recovered and
those with stable motor symptoms, differences could be
detected between the two groups. These differences were
not, however apparent using gross measures of optical
density: it was necessary to utilize a finer level of analysis
involving numerical quantification of fibres. These results
based on fibre quantification revealed that recovered
monkeys displayed more TH and DAT-labelled fibres
than stable motor symptoms monkeys. The possibility
that this reflects a greater sparing of fibers in the recovered
group cannot be entirely ruled out. However, the similarity
in nigral cell loss in our two groups of monkeys, as well as
the studies cited earlier (Finkelstein et al., 2000; Song and
Haber, 2000), suggest that collateral sprouting from
remaining DA fibres makes a contribution to the observed
Thus sprouting of dopaminergic fibres could be impli-
cated in the phenomenon of recovery and, furthermore, in
the compensatory mechanisms in early phases of PD.
Indeed correlations were observed between dopaminergic
fibres quantification and recovery time but not motor
Consideration of the different striatal territories indivi-
dually reveals other details regarding potential compensa-
tory mechanisms. Fibre quantifications and correlations
with recovery time in recovered monkeys suggest that
collateral sprouting in both sensorimotor and associative
territories could be involved. As it is generally assumed that
parkinsonian motor symptoms come from dysfunction of
New model to study compensatory mechanisms in MPTP-treated monkeys exhibiting recoveryBrain (2007)Page13 of17
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neuronal activity in the sensorimotor territory of striatum,
it is not surprising that the few remaining fibres in this
territory could participate to the recovery. More surprising
is the correlation with the fibres present inside the
fibres remaining or sprouting inside this non-motor
territory of the striatum could be also involved in
compensatory mechanisms for delaying or reducing symp-
toms. This raises another question: what is the contribution
of any dysfunction inside the associative territory to the
parkinsonian symptoms? Specific investigations of this new
questioncould shed light
Non-striatal DA systems could also be involved in
compensation. For example, in the pallidum a decrease of
TH labelling has been seen in different territories of
external and internal subdivision in MPTP-treated monkeys
(Jan et al., 2000). However this decrease is less in pallidum
than in striatum, suggesting that the pallidum could
contribute to compensation. The DA could act at different
levels in basal ganglia (striatum and other nuclei) to
increase the selectivity of information that passes through
the entirety of basal ganglia (see Tremblay et al., 1989 and
Pessiglione et al., 2005). The relative preservation of the DA
projection to the GP could help to reduce this loss of
selectivity observed in primate model of Parkinson’s
disease. In humans it has been suggested that GPi in
particular could be implicated in compensatory mechan-
isms (Whone et al., 2003). This study indicated that an
increase of 18F-dopa uptake was seen in early phase of
PD in the GPi but not in the GPe. In the current study,
no statistical difference in TH labelling was detected in
the GPe between recovered monkeys and those with
stable motor symptoms, regardless of the territory con-
sidered. There was, nevertheless, a correlation between
recovery time and the number of TH fibres in the
limbic GPe, indicating that GPe could be implicated in
compensatory mechanisms. Additionally an increase of
TH labelling has been noticed in sensorimotor internal
pallidum of recovered monkeys. Moreover, a strong
tendency to an increase of TH labelling has also been
reported in the associative GPi among recovered monkeys,
notably for those with the most complete recovery. Indeed
low number of TH fibres in comparison to the others.
This data suggests that both these GPi territories could be
implicated in compensatory mechanisms, as has been
shown in early phases of PD (Whone et al., 2003). This
further validates the hypothesis that compensatory mecha-
nisms in recovery in monkeys and in early phases of PD
could be the same.
Systems other than the dopaminergic one could be
involved in compensatory mechanisms and in particular the
serotoninergic system. As was the case for dopaminergic
fibres, we focused on immunohistological than biochemi-
cal data (for this aspect see Boulet et al., 2005).
recovery hada very
Indeed serotoninergic sprouting has been reported in
MPTP-treated monkey (Gaspar et al., 1993). This sprouting
could simply be a reflection of a phenomenon of
competition due to the decrease of the number of
dopaminergic fibres. However, it could be also involved
in the functional recovery as those authors suggest. Studies
in adult rats have produced inconsistent results: some reveal
a serotoninergic hyperinnervation in striatum after a
6-hydroxy-dopamine lesion (Zhou et al., 1991), whereas
others report a decreased serotonin innervation (Takeuchi
et al., 1991). These conflicting results make it difficult to
determine the role of serotonin in compensatory mechan-
isms, but using the progressive protocol, in which the
behavioural recovery can be used as evidence that true
compensation has occurred at some level, the results are
consistent with a positive role of serotonin in compensa-
tion. Thus we have observed an increase in 5HT labelling in
the three functional territories of the striatum of recovered
monkeys, contrasted with a decrease in such labelling in
those monkeys with stable motor symptoms in comparison
to controls. This would appear to rule out the serotoni-
nergic hyperinnervation as a simple artifact of decreased
compensation. The increase in serotoninergic innervation
raises the question of whether treatment of parkinsonian
symptoms with serotoninergic agents might be helpful,
potentially reinforcing endogenous compensatory mecha-
nisms and delaying the increased severity of motor
symptoms. Such a treatment with serotoninergic agents
has been suggested previously by Iravani and collaborators
(Iravani et al., 2003) who reported that the indirect 5HT
agonist MDMA could transiently decrease motor symptoms
in MPTP-treated monkeys and decrease dyskinesia when
necessary before testing such a treatment to have a
better understanding of the role of serotonin in this
system. A study in adult mice (Rozas et al., 1998) produced
apparently contradictory results with respect to behavioural
recovery and striatal 5HT innervation. They reported
that the lower the degree of recovery is, the higher
5HT innervation is. It is not clear whether differences
between species or MPTP treatment protocols or other
variables account for this discrepancy. Even in the current
study, no correlation was found between serotoninergic
fibre quantification and recovery time, whereas a correla-
tion between 5HT fibres and homecage activity was
observed. An additional limitation in interpretation of
these results is the fact that few things are known on
the functional role of 5HT in the striatum, making it
difficult even to speculate as to the mechanism by which
5-HT could compensate for the loss of dopamine in the
striatum. Complementary approaches using the progressive
protocol can be used to elucidate the role of 5HT in
compensatory mechanisms, and potentially in the striatum
Page14 of17Brain (2007)S. Mounayar et al.
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Our study describes a relatively short-term, progressive
protocol for MPTP administration and demonstrates its use
as a model for studying compensatory mechanisms in PD.
This protocol allows production of extensive degeneration
of the central dopaminergic systems, resulting in expression
of three of the major motor symptoms in PD, while also
consistently yielding monkeys that show complete, or nearly
complete, behavioural recovery.
Using this protocol confirmed the importance of the
residual dopaminergic system in compensation in the face
of degeneration, not only in the three territories of striatum
but also in extra-striatal structures and in particular in GPi.
This model also provides preliminary support for the role
of serotonin in compensation. Future studies are needed to
determine which of these changes is causative and which is
simply correlated with the behavioural improvements. This
could then inform us on possible therapeutic interventions
to promote the biochemical changes responsible for
compensation and thus potentially increase compensation
in order to delay the emergence and/or worsening of
symptoms in early phases of PD. Indeed, as we have
indicated, compensatory mechanisms in our model could
be similar to those in early stage of PD. It is important to
note that the absence of behavioural recovery in patients
with PD does not exclude the possibility of compensatory
mechanisms: because PD is a progressive condition,
compensation may manifest itself solely as a slowing of
progression of clinical symptoms. However even if these
specific compensation do not occur naturally in PD,
understanding the mechanisms of recovery in MPTP-
treated monkeys could be nevertheless prove useful in
opening the way to new treatments by artificially activating
compensation in the early phases of PD.
This model could provide a useful experimental tool for
studying other aspects of compensation as well: receptor
up-, or down-, regulation, role of other neurotransmitters
and neuromodulators, electrophysiological aspects, etc.
Finally such a model could be used to study the role of
dopamine and basal ganglia in other brain functions, such
as learning (Matsumoto et al., 1999) and reward-guided
behaviour (Schultz et al., 2003; Nakamura and Hikosaka,
Ste ´phanie Mounayar was supported by Association France
Parkinson and the Fondation pour la Recherche Me ´dicale
Parkinson (France). This study was supported by the
National Parkinson Foundation (Miami, Florida, USA) and
France Parkinson. We wish to thank Jeffrey Hollerman for
helpful suggestions on data analysis and for checking
English. We also appreciate the constructive suggestions
from the referees during peer review.
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