A role for synaptic plasticity in the adolescent development of executive function.
ABSTRACT Adolescent brain maturation is characterized by the emergence of executive function mediated by the prefrontal cortex, e.g., goal planning, inhibition of impulsive behavior and set shifting. Synaptic pruning of excitatory contacts is the signature morphologic event of late brain maturation during adolescence. Mounting evidence suggests that glutamate receptor-mediated synaptic plasticity, in particular long term depression (LTD), is important for elimination of synaptic contacts in brain development. This review examines the possibility (1) that LTD mechanisms are enhanced in the prefrontal cortex during adolescence due to ongoing synaptic pruning in this late developing cortex and (2) that enhanced synaptic plasticity in the prefrontal cortex represents a key molecular substrate underlying the critical period for maturation of executive function. Molecular sites of interaction between environmental factors, such as alcohol and stress, and glutamate receptor mediated plasticity are considered. The accentuated negative impact of these factors during adolescence may be due in part to interference with LTD mechanisms that refine prefrontal cortical circuitry and when disrupted derail normal maturation of executive function. Diminished prefrontal cortical control over risk-taking behavior could further exacerbate negative outcomes associated with these behaviors, as for example addiction and depression. Greater insight into the neurobiology of the adolescent brain is needed to fully understand the molecular basis for heightened vulnerability during adolescence to the injurious effects of substance abuse and stress.
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ABSTRACT: Animal studies have shown that decreased orexin expression changes sleep regulation with normal aging. This study examined orexin A and B expression in the tuberal hypothalamus in infants (0-1 year; n = 8), children (4-10 years; n = 7), young adults (22-32 years; n = 4), and older (48-60 years; n = 7) adults. Neuronal expression was defined by the percentage positive orexin immunoreactive (Ox-ir) neurons in the whole tuberal hypothalamus, and in the dorsal medial (DMH), perifornical, and lateral hypothalamus. In addition, the number of Ox-ir neurons/mm(2), regional distribution, and co-localization were examined. Within the whole tuberal hypothalamic section, there was a 23% decrease in the percentage of Ox-ir neurons between infants and older adults (p < 0.001), and a 10% decrease in older compared with younger adults (p = 0.023). These changes were confined to the DMH and/or perifornical hypothalamus. There was a 9%-24% decrease in Ox neurons/mm(2) in adults compared with infants and/or children (p ≤ 0.001). These results demonstrate a decrease in Ox expression with normal human maturation and aging. This may contribute to changes in sleep regulation during development and with aging.Neurobiology of Aging 08/2014; · 4.85 Impact Factor
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ABSTRACT: Recent evidence implicates adult hippocampal neurogenesis in regulating behavioral and physiologic responses to stress. Hippocampal neurogenesis occurs across the lifespan, however the rate of cell birth is up to 300% higher in adolescent mice compared to adults. Adolescence is a sensitive period in development where emotional circuitry and stress reactivity undergo plasticity establishing life-long set points. Therefore neurogenesis occurring during adolescence may be particularly important for emotional behavior. However, little is known about the function of hippocampal neurons born during adolescence. In order to assess the contribution of neurons born in adolescence to the adult stress response and depression-related behavior, we transiently reduced cell proliferation either during adolescence, or during adulthood in GFAP-Tk mice. We found that the intervention in adolescence did not change adult baseline behavioral response in the forced swim test, sucrose preference test or social affiliation test, and did not change adult corticosterone responses to an acute stressor. However following chronic social defeat, adult mice with reduced adolescent neurogenesis showed a resilient phenotype. A similar transient reduction in adult neurogenesis did not affect depression-like behaviors or stress induced corticosterone. Our study demonstrates that hippocampal neurons born during adolescence, but not in adulthood are important to confer susceptibility to chronic social defeat.Frontiers in Behavioral Neuroscience 08/2014; 8:289. · 4.16 Impact Factor
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ABSTRACT: Impulsivity is a heritable, multifaceted construct with clinically relevant links to multiple psychopathologies. We assessed impulsivity in young adult (N~2100) participants in a longitudinal study, using self-report questionnaires and computer-based behavioral tasks. Analysis was restricted to the subset (N=426) who underwent genotyping. Multivariate association between impulsivity measures and single-nucleotide polymorphism data was implemented using parallel independent component analysis (Para-ICA). Pathways associated with multiple genes in components that correlated significantly with impulsivity phenotypes were then identified using a pathway enrichment analysis. Para-ICA revealed two significantly correlated genotype-phenotype component pairs. One impulsivity component included the reward responsiveness subscale and behavioral inhibition scale of the Behavioral-Inhibition System/Behavioral-Activation System scale, and the second impulsivity component included the non-planning subscale of the Barratt Impulsiveness Scale and the Experiential Discounting Task. Pathway analysis identified processes related to neurogenesis, nervous system signal generation/amplification, neurotransmission and immune response. We identified various genes and gene regulatory pathways associated with empirically derived impulsivity components. Our study suggests that gene networks implicated previously in brain development, neurotransmission and immune response are related to impulsive tendencies and behaviors.Translational psychiatry. 09/2014; 4:e451.
A role for synaptic plasticity in the adolescent
development of executive function
planning, inhibition of impulsive behavior and set shifting. Synaptic pruning of excitatory contacts is the signature morphologic
event of late brain maturation during adolescence. Mounting evidence suggests that glutamate receptor-mediated synaptic
plasticity, in particular long term depression (LTD), is important for elimination of synaptic contacts in brain development. This
review examines the possibility (1) that LTD mechanisms are enhanced in the prefrontal cortex during adolescence due to
ongoing synaptic pruning in this late developing cortex and (2) that enhanced synaptic plasticity in the prefrontal cortex
represents a key molecular substrate underlying the critical period for maturation of executive function. Molecular sites of
interaction between environmental factors, such as alcohol and stress, and glutamate receptor mediated plasticity are
considered. The accentuated negative impact of these factors during adolescence may be due in part to interference with LTD
mechanisms that refine prefrontal cortical circuitry and when disrupted derail normal maturation of executive function.
Diminished prefrontal cortical control over risk-taking behavior could further exacerbate negative outcomes associated with
these behaviors, as for example addiction and depression. Greater insight into the neurobiology of the adolescent brain is
needed to fully understand the molecular basis for heightened vulnerability during adolescence to the injurious effects of
substance abuse and stress.
Translational Psychiatry (2013) 3, e238; doi:10.1038/tp.2013.7; published online 5 March 2013
Adolescent development of executive function
Adolescence is rather inexactly defined as the period
beginning with the onset of puberty and ending with the
shouldering of adult responsibilities.1It is a time of increased
propensity to engage in risky behaviors that include experi-
mentation with alcohol, tobacco, drugs and sexual behavior.
Dahl1has called the adolescent brain a ‘natural tinderbox’
because gonadal hormones are actively stimulating affective
and appetitive behaviors, such as sexual drive, increased
emotional intensity, and risk taking, yet the brain systems that
regulate and moderate these emotional and appetitive urges
are not yet mature.
internally guided behavior, goal planning, and impulse control,
that form the essence of rational thinking and serve to counter
appetitive urges and check risk-taking behavior.2,3The PFC is
the last brain region to mature,4–7and therefore not surprisingly
the frontal lobe capacity for internally guided behavior, working
memory, and organizational skills do not reach full adult
functional capacity until mid to late adolescence.8–12
Crews et al.13have drawn parallels between adolescence
and early sensory critical periods, which are dependent on
plasticity of developing sensory connectivity and allow for
environmental (sensory) modulation of maturing sensory
connections. Specifically, they have suggested that in
adolescence PFC circuitry may be endowed with similar
plasticity and responsiveness to environmental factors, and
as a consequence with heightened vulnerability to the
detrimental effects of substance abuse and stress.13
This review examines the literature on adolescent develop-
ment across species and focuses on the role that glutamate-
receptor mediated plasticity may play in maturation of PFC
circuitry in adolescence. It is postulated that adolescence
represents a phase of increased activity of long term
depression (LTD) mechanisms that predispose to synaptic
elimination and further that termination of this LTD-permissive
phase marks the transition to adulthood. Finally, consideration
is given to the possibility that greater vulnerability to
substances of abuse and stress may represent an interaction
between these environmental factors and the LTD mechan-
isms of plasticity that are accentuated during adolescence.
The hypothesis put forward in this review, while speculative, is
intended to spark further research into possible molecular
mechanisms associated with adolescent development of the
PFC. Certainly synaptic plasticity has been studied much less
extensively in the PFC than in the hippocampus; nonetheless,
mounting evidence suggests that both long term potentiation
(LTP) and LTD play an important role in cognitive functioning
mediated by the PFC and perhaps when disturbed in diseases
related to malfunction of this cortex.14
Department of Neurobiology, Yale University School of Medicine, New Haven, CT, USA
Correspondence: Dr LD Selemon, Department of Neurobiology, PO Box 208001, New Haven, CT 06520-8001, USA. E-mail: firstname.lastname@example.org
Keywords: alcohol; depression; dopamine; long term depression (LTD); prefrontal cortex; substance abuse
Received 4 September 2012; revised 15 Decemeber 2012; accepted 3 January 2013
Citation: Transl Psychiatry (2013) 3, e238; doi:10.1038/tp.2013.7
& 2013 Macmillan Publishers Limited All rights reserved 2158-3188/13
Preadolescent development and sensory critical periods
The specificity and topography of brain wiring are not entirely
genetically preprogrammed but instead established via
dynamic processes occurring in the developing brain.
Adolescence represents the final epoch in a series of
developmental stages that transform the immature brain into
its adult form. In order to fully understand adolescent
development, it is important to appreciate how it differs from
earlier preadolescent maturation.
The developmental mechanisms that account for major
remodeling of connectivity occur before the onset of adoles-
cence, i.e., before postnatal day 28 (PD28) in rodents, 9
months in cats, and 3 years in non-human primates15–17and
include prominent degeneration of neurons and axons.18,19
Indeed, the immature mammalian brain is distinguished from
its adult counterpart by the presence of connections between
by overlap of terminal fields that are segregated in the adult
brain. For example, in the newborn hamsters and rats,
uncrossed retinocollicular projections, i.e. from the retina to
theipsilateral superior colliculus(SC),not onlyoccupya much
expanded territory in the SC relative to that of the adult brain
but also originate from nasal as well as temporal retinal
ganglion cells.20–22Retraction of the terminal projections is
associated with loss of these nasal, ipsilaterally-projecting
ganglion cells.22More generally, in the central nervous
system overproduction of neurons with ensuing neuronal
death is a common mechanism employed by the developing
brain to ensure that the appropriate balance of projection and
receptive neurons is attained.19,23–25
A second, pervasive form of degeneration in the developing
neurons of origin intact. For instance, in the central nervous
system, cortical callosal projections that are widespread in
kittens and young rats are constricted to the adult patterning
by retraction of callosal axons without cell loss.26–28Quanti-
magnitude of this form of degeneration as the number of
axons in the young non-human primate brain ranges from
of neurons or loss of axons, necessarily are associated with
dissolution of established synapses.32However, these early
developmental events are occurring at a time when, overall,
synapses are increasing in density.33–38The classic example
or early connectional remodeling, that of reduction of
polyneuronal input on a single muscle fiber to a single axon,
illustrates how synapticnumber mayincrease asthesurviving
single axon sprouts a much more elaborate terminal
plexus.18,39Likewise, in the central nervous system regres-
sion of inappropriate synapses is more than compensated by
growth and expansion of appropriate terminal fields.40
A wealth of evidence has established that reorganization of
connections throughout the brain is activity dependent and
therefore mediated by a Hebbian mechanism.41–45Although
normal regression of connections in the visual system can
proceed in the absence of visual input41there exists a period
of plasticity during postnatal development that permits
rewiring in response to altered sensory environments.43,46,47
It is noteworthy that critical periods for sensory plasticity occur
in the same preadolescent period in which remodeling of
Adolescence: synaptic elimination and excitatory/
The maturational event most consistently linked to the
adolescent stage of development is reduction of synaptic
density or ‘synaptic pruning.’ Quantitative analyses of
increase in synaptic density in multiple cortical areas that
peaks during the postnatal 3rd month, declines slowly (10%)
untilB2 years of age with a steeper decline (40%) occurring
the timing of peak synaptic density is staggered in different
regions, but the basic pattern of peak synaptic density in early
childhood followed by robust synapse elimination throughout
early (auditory cortex) or mid-adolescence (PFC) is in basic
agreement with non-human primate studies.4,50More recent
data have established that synapse elimination in humans
does not end in adolescence but continues at a lower rate into
early adulthood.51In addition, in human cortex the synaptic
related proteins synaptophysin and postsynaptic density
and decline through adolescence,52though it should be noted
that a recent study found increasing concentrations of
epoch.53Nonetheless, most evidence points to synaptic
pruning as the signature late maturational process associated
with adolescence. Other species have been studied less
extensively but exhibit a comparable pattern. Peak synaptic
density wasobservedbythe7thpostnatalweek inthecat.34In
rat, recent data suggest that peak spine density in the PFC is
present at PD31 with spine density decreasing thereafter until
PD 57 or PD60, i.e., early adulthood.33
Synaptic elimination in adolescence is widely thought to
account for the decline in gray matter volume detected via
longitudinal magnetic resonance imaging (MRI) of human
subjects. Although reduction of synaptic connectivity might be
accompanied by retraction of glial and neuronal processes,
elimination of neuronal cell bodies occurs much earlier in
development.54One of the first longitudinal MRI studies of
human subjects detected divergent developmental growth
patterns in gray and white matter volumes: white matter
volume increased linearly until approximately age 22 whereas
cortical gray matter volume in the frontal and parietal lobes
peaked just prior to adolescence (B10–12 years) and then
declined to adult volumes.5
children and adolescents, including one recent large multi-
center study, also show opposing patterns for gray and white
matter.55–57Interestingly, changing cortical volumes over this
age range are most prominent in the frontal and parietal
lobes.8,58,59Indeed, a recent study indicates that there is a
progression in which higher cortical association areas like the
PFC are last to show reduction of gray matter volume.7
The functional significance of synaptic elimination during
adolescence, though still enigmatic, probably involves adjust-
mentofthe excitatory/inhibitorybalance onindividualneurons
Cross-sectional studies of
Synaptic plasticity in adolescent development
and within networks. The main argument in support of this
hypothesis stems from the specificity of the loss: excitatory
synapses are selectively degenerated whereas inhibitory
synapses are spared.35,37Even loss of chandelier axon
boutons in the PFC, a finding thatwas originally interpreted as
loss of inhibitory synapses,60now supports the elimination of
excitatory input in light of new physiologic data.61Further-
more, recent evidence has established that D2 dopamine
receptors on interneurons undergo a profound maturational
change during adolescence.62–64Prior to adolescence, D2
stimulation elicits either no effect or only weak inhibition on
interneurons. However, in adult animals stimulation of D2
receptors is strongly excitatory and therefore results in robust
firing of interneurons and potent inhibition of their pyramidal
cell targets. As a result, inhibition gains a position of
mediated firing of interneurons as well as a relative gain in
inhibitory/excitatory synapse ratio. In the PFC, neurophysio-
logic studies have established a critical role for inhibitory
synapses in mediating information flow through local net-
gamma oscillations which are essential to cortical computation
in many areas of the cortex and to cognitive processing in the
PFC.67,68Thus, the correct balance of inhibition and excitation
seems to be critical for normative executive function, and
conversely, disturbance of this balance is thought to be a
fundamental component of psychiatric illness.69,70
Molecular mechanisms associated with synaptic
stabilization and synaptic pruning
Synapse stabilization and synapse elimination are primary
players in the maturational processes associated with
preadolescent and adolescent development. The transition
step in synapse stabilization. N-methyl-D-aspartate receptors
(NMDAR) are localized very early to the postsynaptic
membrane, but transition to a more mature synapse state is
characterized by recruitment of alpha-amino-3-hydroxy-5-
methyl-4-isoxazole propionic acid receptors (AMPAR) to the
synapse.71–74Expression of AMPARs on the postsynaptic
membrane is induced by NMDAR-mediated long term
potentiation (LTP), the same mechanism originally described
in hippocampus for learning and memory.73–76A second
NMDAR-mediated process, LTD, results when afferent
stimulation fails to activate a target neuron.76In many
respects LTP and LTD are opposite processes although they
engage distinct intracellular signaling mechanisms.77–80
Essentially stimulation of NMDARs can induce activity-
dependent strengthening of synapses via LTP or weakening
via LTD, and AMPAR insertion or removal from the
postsynaptic membrane is the conduit for this change in
synaptic strength.81,82Importantly, LTP and LTD do not just
strengthen or weaken synaptic connections (short term
plasticity) but actually trigger the addition or loss of synapses
(long term plasticity) even in the adult brain.83–92
establishing mature synaptic connections was recognized,
Constantine-Paton et al.44postulated that activity-dependent
remodeling of connectivity in the developing brain might be
mediated by NMDARs because these receptors are perfectly
suited to detection of synchronized pre- and postsynaptic
activation. Growing evidence now supports the idea that LTP
and LTD are required for generation of whisker barrel field
maps in the primary somatosensory cortex and ocular
dominance columns in the primary visual cortex, both
involving reorganization of thalamic inputs to layer 4.49,93–96
In development, as in learning and memory, plasticity is
bidirectional, i.e., synchronized activity of afferent inputs
may trigger LTP and resulting synapse maturation and
stabilization; conversely, asynchronous activity may diminish
synaptic strength via LTD and predispose the synapse to
Recently, changes in the NMDAR have been linked to
critical periods of early developmental plasticity. NMDAR
subunit composition shifts from NR2B predominant to NR2A
prevalent forms in early development in the visual and
somatosensory cortices.98–100Moreover, the shift in NR2B
to NR2A shows a rough correspondence to critical periods for
sensory plasticity: the beginning of the critical period is
critical period is associated with a decrease in NR2B
expression.100,101Importantly, the switch is not locked to a
specific age but in fact can be delayed by sensory deprivation,
suggesting that it is controlled by activity.93,102–105In turn, the
changeover from NR2B to NR2A receptor subtypes controls
the sensitivity of these connections to stimulation by
NMDARs. For example, in the primary visual cortex of the
ferret, NR2B levels are already high at eye opening and
decline inlayer4attheendofthecritical periodforplasticityof
ocular dominance columns but remain high in layer 2/3.106
Correspondingly, physiologic studies in the cat visual cortex
have shown that cortical layer 4 neurons, but not layer 2/3
cells, exhibit reduced sensitivity of visual and spontaneous
activity to an NMDAR antagonist at the end of the critical
period.107Together these findings suggest that the switch
from NR2B to NR2A dominated receptors terminates the
ment of ocular dominance columns in the visual cortex.
NMDAR-mediated LTP and LTD may also constitute the
molecular underpinnings for adolescent synaptic pruning, albeit
with greater emphasis on LTD and synaptic elimination. How
could the same mechanism account for two very different
developmental processes? Perhaps the period of adolescence
corresponds to a widespread shift in the balance of LTP/LTD
mechanisms and a corresponding prevalence of synaptic
elimination over synaptic addition. In rat hippocampal slices,
an increased NR2A/NR2B ratio has been linked to decreased
spine motility and increased synaptic stabilization, suggesting a
role in the NMDAR subunit composition in halting synaptogen-
esis.108Furthermore, the NR2A state is less conducive to LTP.
(CaMKII), which has a well established role in LTP,109,110binds
preferentially to the NR2B subunit.110–112Accordingly, NR2B
expression on the postsynaptic membrane has been shown to
be necessary for LTP induction, while a role for NR2A in LTP is
not well established.113–115Moreover, NR2A expression is
enhanced by ligand binding to NMDARs and therefore is
activity-modulated whereas NR2B expression is not dependent
Synaptic plasticity in adolescent development
be responsible for metaplasticity of synapses, i.e., a change in
the likelihood of subsequent synaptic plasticity.117,118With age
and activity, NR2A subunits become incorporated into the
postsynaptic membrane, replacing NR2B subunits.116The
resulting increased NR2A/NR2B ratio translates into a higher
threshold for induction of LTP and conversely a state that is
more favorable for induction of LTD.118,119
The role of plasticity in the neocortex is not as well
established as in the hippocampus. However, NMDAR-
mediated LTP and LTD have been described in the visual
neocortex120and at multiple synapses in the PFC.121–123
Notably, LTD mediated through metabotropic glutamate
receptors (mGluRs) has emerged as a major alterative to
NMDAR-mediated LTD in widespread areas of the brain124–126
and therefore deserves consideration as a possible molecular
basis for synaptic pruning in the PFC. In this regard, mGluR
plasticity has been described at the thalamocortical synapse in
the somatosensory cortex,127perhaps indicating that this form
of plasticity is also present at the mediodorsal thalamic
mGluR LTD acts presynaptically to decrease transmitter
release and depress synaptic activity.127Such a mechanism
would be unlikely to result in synapse loss and spine involution
and therefore would not be a strong candidate for LTD-
facilitated synaptic pruning during adolescence. Furthermore,
mGluR LTD atpostsynapticsitesinthehippocampushas been
associated with large spines containing an abundance of
AMPAR.128Unlike the hippocampus where large mushroom
spines are in the majority, thin, filopodial spines predominate in
the PFC.129Thus, strong evidence for mGluR in plasticity
related to PFC synaptic pruning is presently lacking; none-
theless, possible involvement of mGLuR-mediated LTD in
prefrontal adolescent maturation cannot be discounted.
Many questions remain to be answered about the role of
metaplasticity in the PFC as well. As the NR2A subtype
promotes a LTD-receptive state in the synapse and LTD is
associated with synaptic elimination, it would be interesting to
know whether and when the NR2B to NR2A switch occurs in
the PFC and how it relates to the synaptic pruning that refines
connectivity associated with cognitive control of behavior. If
the LTD-receptive state is a hallmark of adolescent develop-
ment, a reasonable presumption is that there exists an
additional molecular switch that greatly curtails the LTD-
receptive state of adolescence into the much less receptive
state of adulthood. This switch, although presently unidenti-
fied, would transform the synapse into a state that is less
receptive to alterations in AMPAR expression on the
postsynaptic membrane. Given that synaptic pruning con-
tinues into early adulthood albeit at a lower level than that of
adolescence,33,51it seems likely that the transition phase is
gradual rather than abrupt resulting in a much less plastic
state by the end of the third decade in humans.
Adolescence development of cognitive function and
Executive functions governed by the PFC exhibit a prolonged
period of maturation reaching fulfillment only in late
Volumetric changes occurring during
adolescence have been correlated with improved cognitive
performance, e.g., verbal and spatial memory performance is
positively correlated with gray matter thinning in the frontal
lobes.6General intelligence has also been shown to bear a
relationship to the trajectory of frontal cortical gray matter
thinning, such that subjects with superior intelligence show a
robust early adolescent increase in gray matter volume
followed by equally robust thinning during later adoles-
cence.131However, too much cortical thinning during adoles-
cence has been associated with diseased states such as
Attention Deficit Hyperactivity Disorder (ADHD).132Thus,
there is an optimal level of synaptic pruning that is essential to
normal development of adult cognitive function.
One recent study addressed the role of AMPAR expression
and LTD in the development of PFC function in the mouse.
Vazdarjanova et al.133utilized a transgenic mouse that over-
expresses calcyon, a protein which mediates activity-depen-
dent AMPAR internalization, and found that calcyon over-
expression over the lifetime of the mouse resulted in marked
impairment of contextual fear extinction (CFE) and working
memory capacity, both dependent on normal PFC function.
Most relevant to this discussion, adolescence was the critical
period for production of these deficits. When over-expression
was silenced specifically during the adolescent epoch, normal
CFE function was rescued.133One possible explanation for
these findings is that AMPAR internalization and associated
functions like LTD are more sensitive to regulation during
adolescence and this regulation is turned off, or at least
greatly diminished, in the adult brain. Whether overactive LTD
during adolescence translates into altered synaptic number in
the PFC or elsewhere is currently not known. However, it is
interesting that upregulated calcyon expression has been
found in schizophrenia, a neurodevelopmental disease in
which PFC gray matter deficits are prominent.134–136
In the PFC, synaptic plasticity is highly modulated by
dopamine receptor, especially the D1 receptor.14,122,137This
is not surprising since D1 receptor stimulation has been shown
to trigger phosphorylation of AMPAR, which in turn promotes
trafficking of these receptors to the external membrane.138,139
The D1 receptor is therefore strategically positioned to effect
changes in AMPAR synaptic expression and ultimately in
synaptic strength and/or number. In the adult non-human
primate, long term sensitizing regimens of amphetamine
decrease spine density on pyramidal cells in the PFC and
have detrimental effects on working memory performance.140
Moreover, these effects seem to be due to changes at the D1
receptor because both cognitive and morphologic effects on
PFC pyramidal neurons can be reversed by long term
treatment with a D1 antagonist.141If AMPAR-mediated LTD
expression is in a state of greater sensitivity to modulation in
mechanism couldbe magnifiedduringadolescence resulting in
exaggerated consequences at the synapse. Other known
cannabinoid143receptors, might have similarly increased
potency during the adolescent period.
Adolescence vulnerability to environmental factors
Adolescence has been described as a period of accentuated
opportunity and of enhanced vulnerability.1It has long been
recognized that early onset of substance abuse is associated
Synaptic plasticity in adolescent development
to temporally correlate with the time of greatest vulnerability to
learning and memory pathways in a maladaptive fashion,149,150
but the question of why addiction is more devastating in
adolescence than in adulthood remains unanswered. Adoles-
cence is also associated with onset of mental illness, as for
example depression rates rise in adolescence especially for
females,151and the prodromal phase of psychosis, including
early onset schizophrenia, surfaces during the adolescent
window.152Despite the fact that adolescents are bigger and
stronger than younger children, mortality rates increase more
than 200% from childhood mainly due to accidents, suicide,
substance abuse, and eating disorders.1
One of the most studied environmental effects in adoles-
cence is alcohol abuse. In adults, brain toxicity has been
documented as a consequence of chronic alcohol abuse:
cortical gray matter thinning is most prominent in the PFC153
and associated with changes in neuronal and glia density in
both the orbitofrontal154and superior frontal cortices.155
Alarmingly, the detrimental effects of alcohol consumption
seem to be magnified in adolescence. Studies in human
subjects have shown that impairment of memory function is
more pronounced following even acute exposure to alcohol in
younger (ages 21–24) than in older (ages 25–29) subjects.156
In adolescent rats, ethanol administration selectively impairs
doses.157Moreover, ethanol consumption in rats that simu-
lates binge drinking results in more widespread pathology in
adolescent animals than in adults.158
The basis of the enhanced vulnerability to alcohol in
adolescence is undoubtedly complex and involves interaction
with multiple neurotransmitter systems.159With regard to
neuroplasticity, there are well documented effects of alcohol
on the glutamate system. Acutely, ethanol inhibits NMDAR
neurotransmission whereas long term exposure results in
also growing evidence to suggest that ethanol has a greater
effect on glutamate neurotransmission during adolescence
than in later life. Ethanol exposure at low doses in adolescent
rats is associated with inhibition of NMDAR-mediated EPSCs
in the CA1 region of the hippocampus while high doses are
required to inhibit EPSCs in adults.161Ethanol also blocks
LTP in CA1 neurons of the hippocampus in adolescent but not
adult rats.162Thus, even acute alcohol consumption in
adolescence could disrupt mechanisms of Hebbian plasticity,
and more chronic alcohol consumption in adolescence may
induce homeostatic upregulation of glutamate neurotransmis-
sion that could result in long term changes in synapse number
and dendritic spine morphology.160Homeostatic regulation of
synaptic activity, i.e., increases or decreases synaptic scaling
acrossthe whole population of synapses, is also thought to be
mediated by increased or decreased expression of AMPAR
receptors on the post-synaptic membrane.163This suggests a
potential site of interaction between developmental plasticity
and homeostatic plasticity since both involve trafficking of
AMPARs. Furthermore, sites of homeostatic plasticity corre-
late with lamina that exhibit plasticity during critical periods in
the visual and somatosensory cortices, suggesting a possible
mechanism for heightened vulnerability of selected circuitry
during different phases of development.163
plasticity in adolescence is primarily occurring in the neural
circuitry that mediates executive processing, then disruption
of synaptic plasticity at this time might result in enduring
deficits in control of emotion, logical thinking and inhibition of
impulsivity. In turn this lack of executive control could
exacerbate the addictive tendencies and result in more
The adolescent brain is also more responsive to stress than
the adult brain164and as a consequence may be more
vulnerable to depression.151Analogous to the manner in
which alcohol has age-specific effects that may be depend on
which regions of the brain are most plastic, a recent study has
shown that the effects of sexual abuse, presumably the stress
associated with the abuse, produces different brain pathology
at childhood and adolescent ages.165Notably, frontal gray
matter volume deficits were most pronounced in adult
subjects who experienced sexual abuse at ages 14–16.165
The neural pathways that mediate and modulate the
stressful effects on cognitive function in the PFC involve
monoamine signaling.164Given the prominence of dopamine
neurotransmission in mediating stress, the development of
dopamine innervation of the PFC during late maturation might
provide insight into the enhanced sensitivity to stress at this
age. In the non-human primate, dopamine innervation of the
middle PFC layers peaks near the onset of puberty and then
decreases rapidly to adult levels while innervation of other
layers is stable throughout the postnatal period.166D1
receptor levels also peak and decline to adult levels around
the adult D1 receptor patternis reached early donot appear to
support a role for dopamine in adolescent enhancement of
plasticity. However, in rodent prefrontal cortex, cell specificity
has been observed in the distribution of D1 receptors with
pyramidal cell neurons, but not interneurons, expressing
higher levels of D1 receptors in adolescence than in
adulthood.168These rodent data suggest that changes in D1
receptor expression might accentuate dopamine signaling in
adolescence and thereby account for greater plasticity during
is that the LTD-receptive state of adolescence is more
sensitive to modulators like dopamine and that critical
differences might be found in the mechanisms of glutamate
receptor-mediated synaptic plasticity in the adolescent brain
compared its adult counterpart.
Identifying the molecular basis for synaptic pruning in
adolescence could have wide ranging clinical ramifications.
If NMDA-mediated LTD were proved to underlie reduction of
connectivity, then the intracellular pathways associated with
LTD processes, including those that mediate AMPAR inter-
nalization, could be targeted to curtail excessive synaptic
pruning in diseases such as schizophrenia and ADHD.
Because the D1 receptor is a key modulator of synaptic
plasticity in the PFC and can even determine polarity of
plasticity, i.e. high dopamine levels can predispose prefrontal
synapses to LTD over LTP,137treatment with dopaminergic
Synaptic plasticity in adolescent development
antagonists or drugs that target intracellular dopamine
signaling might also be useful in decreasing overactive LTD
mechanisms. Along these same lines, drugs that impact the
D1 receptor or its signaling pathways could ameliorate the
impact of stress on the adolescent brain of individuals at risk
for depression. Likewise, the involvement of glutamate
receptors, including mGluRs,126in drug and alcohol addiction
raises the possibility that pharmacologic targeting of gluta-
mate signaling might have the potential to diminish the long
term consequences of substance abuse in adolescence. In
the same manner that the discovery of aberrant mGluR5
mechanisms in Fragile X syndrome has spawned new
therapeutic approaches for treating this disease,169–171
greater insight into the molecular substrates of adolescent
maturation of the prefrontal cortex might lead to similar novel
drug development for disorders and environmental exposures
linked to abnormal adolescent development.
The adolescent epoch is a time when refinement of
connectivity establishes the proper excitatory/inhibitory bal-
ance in the PFC, and it is a critical period for normal
maturation of executive functioning. Adolescence is postu-
lated to be a time when LTD-driven synaptic pruning is
occurring at a high rate in regions that govern higher cognitive
function like the PFC. Further, the transition to adulthood is
hypothesized to be marked by changes in the synapse that
make the mature neuron less sensitive to AMPAR internaliza-
tion, less likely to undergo LTD and thus less likely to undergo
retraction of synaptic contacts.
Conflict of interest
The author declares no conflict of interest.
Acknowledgements. I thank Dr Keith Young for his pre-submission
reading of this manuscript and helpful comments.
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Synaptic plasticity in adolescent development