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Neural Plasticity
doi:10.1155/2011/286073
Brain Plasticity and Disease: A Matter of Inhibition
Laura Baroncelli,1 Chiara Braschi,2 Maria Spolidoro,1 Tatjana Begenisic,3
Lamberto Maffei,1, 3 and Alessandro Sale1
1 National
Research Council (CNR), Neuroscience Institute, Via Moruzzi 1, I-56124 Pisa, Italy
of Psychology, Florence University, Pizza San Marco 4, I-50121 Florence, Italy
3 Laboratory of Neurobiology, Scuola Normale Superiore, Pizza Cavalieri 7, I-56126 Pisa, Italy
2 Department
Correspondence should be addressed to Alessandro Sale, sale@in.cnr.it
Received 10 January 2011; Accepted 4 May 2011
Academic Editor: Graziella Di Cristo
Copyright 2011 Laura Baroncelli et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
One major goal in Neuroscience is the development of strategies promoting neural plasticity in the adult central nervous
system, when functional recovery from brain disease and injury is limited. New evidence has underscored a pivotal role for
cortical inhibitory circuitries in regulating plasticity both during development and in adulthood. This paper summarizes recent
findings showing that the inhibition-excitation balance controls adult brain plasticity and is at the core of the pathogenesis of
neurodevelopmental disorders like autism, Down syndrome, and Rett syndrome.
The term plasticity refers to the ability of the nervous
system to reorganize its connections functionally and structurally in response to changes in environmental experience,
underlying the adaptive development of neuronal circuitry.
The existence of time windows in early postnatal life
(critical periods) during which neural circuits display a
heightened plasticity in response to external stimuli has
been established for various brain regions subserving major
behavioural functions (for review, see [1, 2]). After the end
of the critical period, neural plasticity dramatically wanes.
Since the pioneering work by Wiesel and Hubel, the visual
system stands as the prime model for studying experiencedependent plasticity. These authors reported that occluding
one eye early in development (a treatment usually referred
to as monocular deprivation) leads to an ocular dominance
shift of cortical neurons, that is, a reduction in the number of
cortical cells responding to that eye and a robust increment
in the number of neurons activated by the open eye [3]. The
imbalance of activity between the two eyes eventually results
in the loss of synaptic inputs from the thalamic regions
representing the closed eye and in the expansion of those
driven by the open eye [47], accompanied by a remodelling
of cortical horizontal connections [8].
In the last 50 years, great eort has been made to
elucidate cellular and molecular mechanisms underlying
the activation and regulation of critical periods in the
brain. Unravelling these processes may potentially enable
researchers to enhance plasticity in the adult brain. Moreover, a detailed knowledge of the events involved in the
maturation and plasticity of neuronal circuitry would be
a determinant in improving our understanding of the
aetiology of developmental brain disorders.
Although a complete picture in the field is still lacking, a
large body of evidence has been accumulated (see, [9, 10]).
In this paper, we will focus our discussion on intracortical
inhibitory circuitry which convincingly emerges as a key factor not only for defining the boundaries of cortical plasticity
but also in developing of pathological states characterized by
severe intellectual disabilities (see also [11, 12]).
2. GABAergic Inhibition and Ocular
Dominance Plasticity in the Adult
Visual Cortex
By sculpting the pattern and timing of neuronal electrical
activity, inhibitory GABAergic circuits are an ideal candidate for regulating the processes of experience-dependent
synaptic modifications. Taking advantage of gene-targeting
technology, this hypothesis has been directly tested by
abolishing the expression of the 65 kD isoform of GABAsynthetic enzyme, hence reducing activity-dependent GABA
synthesis and release at synaptic terminals. Mice that carry
such a disruption of the GAD65 gene do not exhibit ocular
dominance plasticity in response to monocular deprivation;
only an enhancement of inhibition achieved with local delivery of diazepam enables a full rescue of ocular dominance
plasticity in these mice [13].
Converging results obtained with dierent experimental
approaches have subsequently confirmed the key role of
GABAergic inhibition in brain development and plasticity
(e.g., [1417]). It is noteworthy that BDNF-overexpressing
mice show an accelerated maturation of GABAergic cortical
inhibition paralleled by a faster time course of critical period
for ocular dominance plasticity [13], strongly suggesting that
the progressive development of the inhibitory tone not only
enables the onset of the critical period but subsequently
underlies the closure of neural plasticity gates.
One of the major challenges in neuroscience is the development of strategies aimed at promoting nervous system
plasticity in adulthood, when recovery from injury and
functional rehabilitation are severely hampered. Recently,
new evidence has challenged the classic dogma that ocular
dominance plasticity is a physiological phenomenon exclusively restricted to the early postnatal development and
pointed to a reduction of intracortical inhibition levels as
a crucial step for the restoration of plasticity processes
in the adult brain. The most direct demonstration that
GABAergic inhibition limits plasticity in the adult visual
cortex derives from a recent study reporting that pharmacological reduction of intracortical inhibition obtained
through the infusion of either MPA (an inhibitor of GABA
synthesis) or picrotoxin (a GABAA antagonist) directly into
the visual cortex reactivates ocular dominance plasticity
in response to monocular deprivation in adult rats [18].
Moreover, this treatment leads to a full rescue of long-term
potentiation (LTP) of layer II-III field potentials induced by
theta-burst stimulation from the white matter, an activitydependent form of synaptic plasticity which is normally
occluded in visual cortical slices from adult animals due
to the maturation of inhibitory transmission [18, 19]. The
reduction of intracortical inhibition is accompanied by
processes of structural plasticity. The visual cortex of MPA or
PTX-treated animals, indeed, shows a decrease in the density
of chondroitin sulfate proteoglycans (CSPGs), indicating the
activation of endogenous mechanisms of extracellular matrix
remodelling which are known to be crucially involved in the
expression of neural plasticity [2022]. It is also possible
that other molecular components of the extracellular milieu
regulating synaptic plasticity in the adult brain, such as
myelin proteins [23] and adhesion molecules [24], may
undergo changes in their expression levels in response to a
reduction of intracortical inhibition.
These results show that a brief reduction of GABAergic
inhibition is sucient to reopen a window of plasticity
in the visual cortex well after the normal closure of the
critical period. Similar conclusions have been drawn from
recent evidence showing that the inhibitory tone is a central
hub for the restoration of plasticity in the adult visual
cortex and that a decrease of intracortical inhibition levels is
required for the reinstatement of neural plasticity triggered
by dierent experimental approaches. We demonstrated that
environmental enrichment, a condition of increased sensorymotor and cognitive stimulation, reactivates juvenile-like
ocular dominance plasticity in the visual cortex of adult rats,
with a shift in ocular dominance of cortical neurons following monocular deprivation clearly detectable using both
visual evoked potentials and single-unit recordings [25].
Recovery of plasticity in enriched animals is paralleled by a
marked reduction of the inhibitory tone in the visual cortex.
Importantly, the decrease of inhibitory neurotransmission is
a crucial molecular mechanism underlying the enhancement
of visual cortex plasticity induced by environmental enrichment: preventing the reduction of GABAergic inhibition
during the period of exposure to environmental enrichment
(via micro-osmotic pumps infusing the GABA agonist
diazepam into the visual cortex), indeed, completely blocks
the ocular dominance shift of cortical neurons in response to
monocular deprivation [25]. The enhanced environmental
stimulation provided by environmental enrichment also
leads to a twofold enhancement of serotoninergic transmission and to an increase in the number of BDNF-expressing
neurons in the visual cortex. Interestingly, infusion of a
serotonin synthesis inhibitor not only blocks plasticity in
response to monocular deprivation but also fully counteracts the eects produced by environmental enrichment on
inhibition and BDNF levels. We suggested a model in which
serotonin is the first trigger in the molecular chain set in
motion by environmental enrichment, eliciting the decrease
of GABA-mediated intracortical inhibition and, in parallel or
in series, the enhancement of BDNF levels [25].
It is interesting to point out that while, during development, environmental enrichment increases BDNF and
accelerates the maturation of inhibition in the visual cortex
[15], in adult animals reared in an enriched environment
increased levels of BDNF are associated with reduced
GABAergic inhibition. One possible explanation for these
apparently contrasting results is that the influence exerted
by the environment on these molecular factors may follow
a temporarily distinct sequence in the adult compared to
the developing brain. Specifically, we propose that the very
early (postnatal day 7, see [15]) increase in BDNF detected
in mice reared from birth in an enriched environment may
be the prime factor that directly drives the development of
inhibitory circuitry in the immature brain; on the contrary,
the enhancement of BDNF expression in animals exposed
to environmental enrichment in adulthood may occur
downstream to the decrease of intracortical inhibition, which
could promote the expression of many activity-dependent
genes involved in neural plasticity.
Given the central role of serotonin in promoting
adult visual cortex plasticity, one might expect that the
eects induced by environmental enrichment should be
reproducible through an artificial modulation of cerebral
levels of this neurotransmitter. This possibility has been
addressed in a study by Maya Vetencourt et al. [26], showing
that the administration of fluoxetine, a selective serotonin
reuptake inhibitor (SSRI) widely prescribed in the treatment
of depression for its capability to enhance extracellular
serotonin levels, reinstates plasticity in the visual cortex
of adult animals, with treated rats exhibiting a marked
shift of ocular dominance in favour of the open eye after
one week of monocular deprivation. Also in this case, a
pronounced reduction of intracortical inhibition has been
detected in the visual cortex, and the osmotic infusion
of the GABA agonist diazepam fully prevents the ocular
dominance shift induced by monocular deprivation, thus
impeding plasticity in fluoxetine-treated animals. Further
support to the notion that diuse projecting systems of
the brainstem aect plasticity in adulthood has been very
recently provided by the demonstration that a genetic
enhancement of nicotinic cholinergic transmission restores
ocular dominance plasticity well after the end of the critical
period. This eect is abolished by diazepam treatment,
suggesting that the cholinergic signalling mechanisms may
adjust excitatory-inhibitory balance [27].
Using an approach quite dierent from environmental
enrichment, a study by He and colleagues reported that
exposing adult animals to complete darkness can also
promote plasticity in the visual cortex [28]. These authors
provided indirect evidence that the enhanced cortical plasticity might be related to a shift in the balance between
inhibition and excitation towards levels more similar to those
found in the immature cortex, caused by a reduced expression of GABAA receptors relative to AMPA receptors. This
suggestion has been confirmed in a very recent study [29]
showing that dark exposure decreases inhibitory synaptic
density and paired-pulse depression and reinstates in the
visual cortex the expression of endocannabinoid-dependent
inhibitory long-term depression, a form of synaptic plasticity
normally restricted to the juvenile age [30].
Two dierent hypotheses, not mutually exclusive, could
be formulated for explaining how the reduction of the
inhibitory tone to juvenile-like levels leads to a recovery
of cerebral plasticity in the adult brain. According to
one hypothesis, the maturation of GABAergic intracortical
transmission sets the point after which the editing activity of
visual cortex pyramidal neurons enables ocular dominance
plasticity; as development proceeds further, the inhibitory
tone surpasses a threshold, and this causes the closure of the
critical period. A reduction of inhibition levels may reinstate
in the adult visual cortex the capability of binocular neurons
to detect the imbalance in retinal inputs induced by the
closure of one eye. According to an alternative hypothesis,
the overall increase of cortical activity due to the shift
in excitation-inhibition balance is the key factor favoring
plasticity recovery. Activity-dependent regulation of gene
expression could induce a genetic transcriptional program
critical for promoting plasticity.
In the barn owl, the optic tectum contains a map of
space consisting of bimodal neurons whose auditory and
visual receptive fields are mutually aligned. In juvenile owls,
alternative maps of interaural time dierence can be acquired
as a result of abnormal experience. The group of Knudsen
and colleagues has demonstrated the existence of a sensitive
period for plasticity in the optic tectum by exposing owls
at dierent ages to prismatic spectacles that cause a large
horizontal shift of the visual field [31]. Owls bearing these
spectacles experience a modification of the visual locations
to which the interaural time dierence values correspond,
eliciting the adjustment of auditory receptive fields according
to the optical displacement [31, 32]. Very interestingly,
the environmental rearing conditions can have a dramatic
impact on this form of plasticity. Indeed, the period during
which owls respond adaptively to prismatic displacement of
the visual field ends at about 70 days of age when owls are
housed in individual cages, while it does not end until 200
days of age when owls are housed in groups and in larger
enriched rooms [31]. At the same manner, also the ability
to recover after restoration of normal visual experience is
strongly aected by the environment, because it ends at 200
days of age when prism-reared owls are housed in small
cages but extends throughout life when they are housed in
group flight rooms. Soon after the characterization of the
sensitive period for visual calibration of the auditory space
map, Zheng and Knudsen demonstrated that when a new
learned map is expressed in the external nucleus of the owl
optic tectum, the neural circuitry underlying the old map
is not structurally inactivated but becomes silent due to a
functional suppression operated by inhibitory connections
and involving GABAA receptors [33].
In the mammalian auditory system, a well-defined critical period exists for tone-specific enlargement in the primary
auditory cortex (A1) representation resulting from transient
exposure to sound stimuli [34]. Strikingly, the Merzenichs
group has recently demonstrated that while in adult control
rats this exposure produces no measurable alteration of A1
tonotopy, rats transferred to an environment of continuous
moderate-level noise exhibit a re-establishment of a period
of sound exposure-driven plasticity [35]. This eect, which
is reminiscent of the reopening of critical period plasticity
triggered in the visual system by dark exposure, is paralleled
by a decrease in the expression level of GABAA 1 and 2/3
subunits in A1.
Thus, reduction of GABAergic inhibition may emerge as
a common feature of the strategies that successfully reopen
a period of stimulus exposure-based plasticity in the adult
brain [18, 25, 26, 28, 35].
3. Beyond the Visual Cortex
During the critical period, the high susceptibility of neuronal
connections to experience-dependent changes is essential
for a proper maturation of normal sensory functions. This
high potential for plasticity, however, may also favour the
emergence of developmental pathological states when an
The critical role of GABAergic inhibition in regulating
experience-dependent plasticity is not restricted to the visual
cortex.
4. Pathological Inhibition of Cerebral Function:
The Case of Amblyopia
anomalous perturbation of sensory-driven activity takes
place. A paradigmatic case is that of amblyopia, a widely
diused and still untreatable pathology of the visual system aecting 24% of the total world population [36].
Amblyopia derives from conditions of early abnormal visual
experience in which a functional imbalance between the
two eyes is predominant owing to anisometropia (unequal
refractive power in the two eyes), strabismus (abnormal
alignment of one or both eyes), or congenital cataract,
resulting in a dramatic loss of visual acuity and a broad
range of other perceptual abnormalities, including deficits
in stereopsis and contrast sensitivity [37, 38]. It is worth
stressing that in amblyopic patients the visual impairment
is caused by an abnormal processing of visual information
at the central level; thus, the use of corrective lenses is
completely ineective [3941].
It is currently accepted that, due to a lack of sucient
residual plasticity within the brain, the reinstatement of
visual functions in amblyopic subjects is possible only if
corrective treatment is started early in development. The
classic amblyopia therapy consists in patching or penalizing
the preferred eye, thus forcing the brain to use the visual
input carried by the weaker amblyopic eye [42]. However,
an increasing number of clinical and animal studies are now
challenging these traditional beliefs, reporting that repetitive
visual training based on sensory enrichment procedures
may represent a very useful approach for the treatment of
amblyopia (for a comprehensive review, see [38, 43]).
The mechanisms underlying vision improvements in
adult amblyopic patients remain to be elucidated, since
the activation of cortical plasticity may occur at several
dierent levels of the visual system and through a variety of
neural processes. A number of studies, however, suggested
that an impairment of the balance between excitation and
inhibition could aect visual cortex development and that
cortical overinhibition could underlie the degradation of
spatial vision abilities [4448]. Accordingly, recent advances
in our understanding of the cellular and molecular brakes
that limit amblyopia recovery to a critical period underscored
intracortical inhibition as a main obstacle for reinstatement
of normal visual functions after a period of early abnormal
visual experience. In animal models, amblyopia can be
induced by imposing a long-term reduction of inputs
from one eye by lid suture (i.e., with a protocol of longterm monocular deprivation). Similarly to that observed in
humans, animals rendered amblyopic by long-term monocular deprivation display a permanent loss of visual acuity in
the aected eye and a pronounced ocular dominance shift of
visual cortical neurons in favour of the normal eye (e.g., [49
51]).
Early studies in animal models of amblyopia reported
that the administration of anti-inhibitory compounds (e.g.,
bicuculline) leads to a substantial restoration of binocularity
in the visual cortex [52, 53]. Recently, it has been shown that
the same experimental paradigms discussed in Section 2 and
associated with a reduced inhibition-excitation balance in
the adult cerebral cortex are also able to recover sight from
amblyopia (for review [54, 55]). Among these treatments,
environmental enrichment emerges as a totally non-invasive
approach [56]. We reported that a brief exposure (twothree weeks) of adult amblyopic rats to environmental
enrichment promotes a complete recovery of both visual
acuity and ocular dominance, as demonstrated both with
electrophysiological recordings of visual evoked potentials
from the primary visual cortex and with a standard visual
acuity behavioural test (visual water-box task). The environmental enrichment-induced recovery of visual acuity is
long-lasting, persisting for a minimum of two weeks [56].
A reduced intracortical inhibition is a crucial mechanism
underlying the enhancement of visual cortex plasticity in
environmental enrichment: preventing the reduction of
GABAergic inhibition during the period of environmental
enrichment, indeed, completely blocks the recovery of
binocularity and visual acuity. These findings draw attention
to the environmental enrichment procedure as a prospective,
injury-free, intervention strategy for amblyopia and further
substantiate a major role for GABAergic transmission in the
control of plasticity windows in the sensory cortices.
5. Inhibition and
While the physiological maturation of GABAergic connections is essential for a tight control of developmental cortical
plasticity and for promoting the acquisition of mature
sensory abilities, it is currently accepted that abnormal
levels of inhibition achieved during development can cause
pathological states of severe brain disability [11, 57, 58]. On
this regard, Rett syndrome, Down syndrome, and autism
disorder stand as the most informative cases (the role of
inhibition in schizophrenia is discussed in another review
published in this issue).
5.1. Rett Syndrome. Rett syndrome is a progressive developmental disorder characterised by mental retardation and
severe dysfunction in motor coordination skills [59], predominantly aecting the female population in early childhood. Using a systematic gene screening approach, lossof-function mutations in the X-linked gene encoding the
methyl-CpG binding protein (MeCP2) have been identified
as the cause of Rett syndrome [60]. MeCP2 is involved in the
regulation of expression of a wide range of genes [61] and
in RNA splicing [62]. Transgenic mice carrying conditional
deletion or neuron specific expression of mutated MeCP2
forms exhibit abnormalities in motor coordination, social
interaction, and cognitive abilities, providing a useful model
for analysing the behavioural and molecular phenotype of
the Rett syndrome [6366].
Detailed electrophysiological analysis of these animal
models showed a reduction of neuronal activity in cortical
and hippocampal neurons due to a shift in the balance
between cortical excitation and inhibition in favour of inhibition [67, 68] and an attenuation of LTP expression in the
hippocampus and in the motor and somatosensory cortex
[68, 69]. These results led to the hypothesis that an anomalous increase in the inhibition/excitation ratio could be
responsible for the motor, behavioural, and cognitive defects
associated with Rett syndrome [11]. This interpretation
is supported by autoradiographic labelling studies on human
postmortem brain samples, showing a significant increase
in the density of GABA receptors that may correlates
with cognitive and motor symptoms of Rett syndrome
[70]. A very recent work by Chao and colleagues further
demonstrated that a dysregulation of GABAergic system has
a role in modulating the pathogenesis of Rett syndrome:
mice lacking MeCP2 selectively in GABA-releasing neurons,
indeed, recapitulate most of the behavioural features of Rett
syndrome [71]. Surprisingly, these mice display a reduced
inhibitory tone, while no data were presented concerning
levels of excitation. Therefore, while these results confirm
that a dysfunction of GABAergic neurons can contribute
to the Rett phenotype, they also outline a more complex
framework for the involvement of inhibitory transmission in
Rett syndrome.
Since the gene encoding BDNF is under MeCP2 regulation [72] and the severity of behavioural symptoms in
MeCP2 deficient mice correlate with levels of circulating
BDNF [73], attempts have been made to rescue the Rett
syndrome phenotype by delivering BDNF. It has been shown
that exogenous BDNF in MeCP2 mutant mice is able to
compensate for deficits at the behavioural, anatomical, and
electrophysiological level [73, 74]. Pre-weaning environmental enrichment, which results in augmented cerebral
BDNF levels, ameliorates motor and cognitive impairment
and reverses cortical LTP deficits [75]. Very interestingly,
environmental enrichment increases the number of cortical
excitatory synapses with no changes found in inhibitory
synaptic density, thus resulting in overall reduction of the
cortical inhibitory tone [75].
5.2. Down Syndrome. Down syndrome is caused by triplication of chromosome 21 (Chr21) and is the most
common genetic cause of mental retardation [76]. People
with Down syndrome have moderate to severe cognitive
impairment, with various disturbances in learning and
memory abilities [77, 78]. In search of possible molecular
and cellular processes involved in the pathogenesis of the
syndrome, several murine models have been generated,
carrying triplications of dierent segments of Chr16, which
has a large degree of synteny with human Chr21 [79, 80].
Currently, the prime model is the Ts65Dn transgenic mouse
[81, 82], which recapitulates all main hallmarks of the Down
syndrome phenotype, including characteristic craniofacial
abnormalities, impaired spatial and nonspatial learning
abilities, and attention and visual function deficits (e.g., [83
85]). Anatomical studies indicated that Ts65Dn mice have
a reduced number of cerebellar and hippocampal neurons
[8688], impaired neurogenesis in the dentate gyrus of the
hippocampus (see [86, 89]; see also [90] for similar evidence
in human foetuses), and simplified dendritic branching in
several brain regions, associated with alterations in spine
size and shape [9193]. Moreover, dysfunctions in the
mechanisms driving nerve-growth factor (NGF) retrograde
transport from the hippocampus to the basal forebrain
[94, 95] are responsible for a prominent degeneration of
basal forebrain cholinergic neurons in adult Ts65Dn mice
[96], which is also a hallmark of the Alzheimers disease.
Accordingly, nearly one hundred per cent of persons born
with Down syndrome develop Alzheimers disease if they live
into their fourth decade of life [96, 97].
A large number of studies have shown that the cognitive
impairment displayed by Ts65Dn mice is mainly related
to excessive levels of inhibition in temporal lobe circuitry,
causing a failure of long-term synaptic plasticity in the
hippocampus [98100]. The deficit of synaptic plasticity is
linked to marked morphological changes in the structure of
synapses, with a selective enlargement of the active zones
of symmetric synapses and increased immunostaining for
synaptic proteins localized at inhibitory synapses in cortex
and hippocampus [101, 102]. The central role of overinhibition in Down syndrome pathogenesis has been recently
confirmed by the demonstration that administration of noncompetitive antagonists of GABAA receptors reverses spatial
learning disabilities and LTP deficits in Ts65Dn mice [100].
One of the major challenging tasks in the field of
Down syndrome therapy is unravelling dosage-sensitive
genes whose dysfunction, due to the presence of an extra
copy of chromosome 21, might be responsible for the main
functional and morphological defects. A recent study by
Chakrabarti et al. [103] has shown that two genes, Olig1 and
Olig2, are essentially involved in the syndrome. The authors
first reported that, very early in development, Ts65Dn
mice have a marked increase in the number of forebrain
GABAergic neurons generated in the medial ganglionic
eminence (one of two regions in the ventral telencephalon
where most inhibitory neurons proliferate and dierentiate).
More specifically, an overproduction of two specific classes
of inhibitory neurons (i.e., parvalbumin- and somatostatinpositive neurons) has been detected. This anatomical phenotype is directly related to increased levels of inhibitory
transmission in the forebrain of Ts65Dn mice, as assessed
with electrophysiological methods [103]. Remarkably, a
genetic reinstatement of dysomia at the level of Olig1 and
Olig2 genes (obtained by breeding Ts65Dn mice with a line
having only one copy of each of these genes) was sucient
to rescue the Ts65Dn phenotype, correcting the interneuron overproduction and restoring synaptic transmission
to euploid levels [103]. Even if a behavioural assessment
of the cognitive performance in Ts65Dn mice after reestablishment of dysomia was not reported, these results
suggest that a few dosage-sensitive genes might eventually be
responsible for many of the deficits displayed by people with
Down syndrome and further support a causal link between
aberrant inhibition in cortical and hippocampal circuitries
and cognitive impairment due to Down syndrome.
Despite the increasing knowledge concerning the
molecular mechanisms underlying Down syndrome, a
suitable treatment for this disorder is still lacking. Since
environmental enrichment is particularly eective in reducing GABAergic inhibition [104], it may have a great potential
for therapeutic application to Down syndrome. MartnezCue et al. have reported increased exploratory behaviour and
enhanced spatial learning in enriched Ts65Dn mice, albeit
the eect was gender-specific [105]. Despite these results,
a detailed investigation of the environmental enrichment
eects on Down syndrome pathogenesis is still lacking.
5.3. Autism. Autism is a heterogeneous developmental disorder characterised by significant impairments in the social,
communicative, and cognitive domain and by the presence of
repetitive patterns of stereotyped activities [106, 107], mostly
aecting males in early childhood [108]. The advent of
magnetic resonance imaging enabled the in vivo investigation
of structural brain morphology in people with autism.
Several regions have been reported to be enlarged or reduced
relative to controls, but a large consensus on these results is
currently missing (for a review, [109]).
The aetiological mechanisms of autism are at present
poorly defined. Despite a likely contribution of environmental causes, genes play a crucial role in the onset of
this pathology with concordance between monozygotic twins
reaching 90%, as compared with less than 10% for dizygotic
twins and siblings [110, 111]. Only recently, considerable
eorts have been focused on understanding the genetic
basis of autism and led to the identification of multiple
chromosomal loci and epigenetic factors associated with
autism heritability (for a review, [112]). Given the complex
repertoire of symptoms characterising autistic syndrome,
it has been proposed that defects in the development
and functioning of multiple and relatively independent
neural systems work together to generate the pathological
phenotype. In particular, neural circuits underlying social
and emotional behaviour, language processing, and higherorder cognition are considered natural candidates [113].
Converging results have pointed to an increased excitation/inhibition ratio in sensory, mnemonic, social, and emotional systems as a core mechanism underlying neurological
and behavioural deficits observed in autistic patients [58].
Consistently, clinical studies showed that epilepsy displays
a good percentage of comorbidity with autism [114]. An
imbalance of neural circuits leading to a disproportionate
high level of excitation could be due to increased glutamatergic transmission or suppressed GABAergic inhibition.
The hypothesis that a reduction of inhibitory neurotransmission shared in common between many systems could
be a key factor in the pathogenesis of autism is consistent
with a large body of evidence [115]. Indeed, a significant
reduction in protein levels of both isoforms of glutamic acid
decarboxylase [116, 117] and GABA receptors [118120] has
been reported in autistic cerebral cortex. Linkage genetic
studies uncovered that polymorphism, copy number, and
epigenetic alterations in chromosomal regions containing
GABA receptor subunit genes are associated with autistic
phenotype [121123].
On the cellular level, it has been shown that in a valproic
acid rat model of autism, the amygdala is hyperreactive
to electrical stimulation and displays enhanced synaptic
plasticity as well as defective inhibitory transmission [124].
Moreover, a direct demonstration that inhibitory circuitries
are activated atypically and are less synchronized in the brain
of autistic people has been provided by studies of functional
magnetic resonance imaging [125, 126].
Since autism is a developmental disorder, the imbalance
in the ratio of excitation versus inhibition could result
from abnormal processes during neural circuit maturation.
Indeed, defects in synaptogenesis and synaptic refinement
have been suggested to be a leading cause of autism, and
mutations of genes that normally control the patterning
of synaptic maturation of specific neuronal subpopulations
have been shown to segregate with the pathological phenotype [127, 128]. Among these genes, Dlx1 and Dlx2 encode
transcription factors exerting a crucial role in the generation
of GABAergic cortical interneurons and lie in a chromosomal region associated with autism susceptibility [129]. In
accordance with the excitation/inhibition model, it has been
proposed that pharmacological agents that reduce neural
excitation, such as anticonvulsivants and benzodiazepines,
could represent a suitable therapeutic treatment for autism
[58]. At present, some evidence that anticonvulsivants could
be eective in ameliorating autistic symptoms is available
(e.g., [130132]).
It should be pointed out, however, that the exact role
of excitation/inhibition balance in autism is still debated.
Indeed, an increased inhibitory synaptic transmission and a
decreased glutamatergic excitation have been also reported
in dierent transgenic mouse models of autism [133, 134].
Altogether the results reviewed here show how dramatic
can be the influence exerted by inhibitory transmission
on brain plasticity. Not only are these findings crucial to
our knowledge about the molecular mechanisms underlying
the expression and regulation of plasticity processes, but
they also have strong implications for the treatment of
neurological disorders related to an aberrant development
of GABAergic circuits. The possibility of rescuing a normal
phenotype in animal models of these pathologies by manipulating levels of intracortical inhibition draws attention
on the GABAergic system as an eligible candidate for the
development of new therapeutic strategies.
The research was supported by a grant from Regione Toscana
(Regional Health Research Program 2009) to Alessandro Sale
and a grant from Fondazione Cassa di Risparmio di Pisa to
Lamberto Maei.
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