The GABAergic network comprises of diverse interneuron subtypes that have different morphological and physiological characteristics and localize their synapses onto distinct subcellular locations on the postsynaptic targets. Precisely how activity and molecular-driven mechanisms conspire to achieve the remarkable specificity of GABAergic synapse localization and formation is unknown. The functional maturation of GABA-mediated inhibition is a prolonged process that extends well into adolescence, both in rodents and primates [20
], and correlates with the time course of the critical period for OD plasticity [21
]. Moreover, the inhibitory maturation process strongly depends on sensory experience, since sensory deprivation, induced either by dark rearing or by intraocular tetradotoxin (TTX) injection, significantly retards the morphological and functional maturation of GABAergic synapses [21
]. This dependence of GABAergic synapse maturation on sensory experience is not limited to visual cortex, indeed similar results have been found in the somatosensory cortex [24
What are the cellular and molecular mechanisms linking sensory experience to the maturation of GABAergic synapses? Brain-Derived Neurotrophic Factor (BDNF), an activity-dependent molecule shown to be upregulated following light stimulation in the visual cortex [25
], is one of the first molecules implicated in the formation of GABAergic synapses in hippocampal and cortical cultures [27
]. Most importantly, in transgenic mice with precocious BDNF expression, a marked increase in perisomatic inhibitory innervation in the visual cortex is correlated with a premature onset and closure of ocular dominance plasticity, further supporting the link between GABAergic synapse maturation and onset of critical period plasticity [12
]. Since BDNF is produced only by pyramidal cells, it could work as an intercellular signaling factor that translates pyramidal cell activity to GABAergic synapse density.
Another factor that has been shown to positively regulate GABAergic synapse maturation is GABA itself. Early in development, GABA has been shown to be a trophic factor [30
], involved in cell proliferation, neuronal migration, and neurite growth [31
]. Since GAD67 is the main isoform of GABA synthesizing enzyme, its deletion reduces GABA levels by 90% [32
]. Using transgenic mice to knockdown GAD67 in single basket interneurons during the period of their maturation, recent studies show that intact GABA signaling is critical for the maturation of GABAergic synapses [33
] (). Intriguingly, even a partial reduction of GAD67 was sufficient to cause aberrant perisomatic synapse maturation, underlying the importance of maintaining optimal GABA levels for normal synapse development [33
]. Basket cell perisomatic synapses have an exuberant innervation pattern; a single basket interneuron connects to hundreds of pyramidal cells in its vicinity, making numerous synapses onto each individual pyramidal cell soma. It is therefore important to appreciate that reduced GABA levels compromise not only the number of synapses that are made onto each pyramidal soma, but also drastically reduce the number of pyramidal soma it connects to, causing a potential circuit-wide disruption in connectivity [33
]. This study demonstrates that, in addition to mediating inhibitory transmission, GABA signaling also regulates interneuron axon arborization and synapse development in adolescent brain, which, in turn regulates critical period plasticity. Different aspects of this deficit were rescued by treatment with either GABAa or GABAb agonists, suggesting a receptor-specific effect of GABA-mediated signaling during GABAergic synapse maturation [33
]. Since GABAa and GABAb receptors are present on postsynaptic neurons, GABA terminal themselves, and surrounding glial processes, cell-autonomous activation of presynaptic GABAb receptors, which modulate Ca2+
channels and GABA release, could influence growth cone motility and bouton stability, or GABA signaling through postsynaptic or glia receptors could trigger the release of retrograde factors, which promote axon branching and synapse formation.
Figure 1 Sensory activity regulates perisomatic synapse maturation via multiple pathways. (a) Activity modulates GAD67 enzyme levels thereby ensuring normal GABA signaling for the appropriate downstream signaling events required for perisomatic synapse development. (more ...)
Modulation of GABA synthesis by the GAD67 enzyme plays a central role in regulating GABA-mediated signaling [34
]. GAD67 itself is produced at a limiting level in the brain, since deletion of one copy of the Gad1
gene results in a ~
40% reduction of enzyme activity and GABA content in many brain regions [32
]. Furthermore, the transcription of Gad1
, the key step in the physiological control of GAD67 activity, is highly regulated during brain development [35
], by neuronal activity [36
], and experience [37
]. Activity-dependent production of GAD67 thus results in online adjustment of intracellular pool for GABA release. Since alterations in GAD67 and GABA levels profoundly influence interneuron axon growth, synapse formation and network connectivity during the establishment of inhibitory circuits, neuronal activity might regulate the strength and pattern of inhibitory synaptic innervation through GAD67-mediated GABA synthesis and signaling. Such activity-dependent and cell-wide regulation of a “transmitter resource” implies a novel logic for the maturation and plasticity of GABAergic synapses and innervation. Since subtle variations in GABA levels can cause such dramatic effects on inhibitory circuits, and therefore overall network connectivity, it is critical to understand its implications in neuropsychiatric disorders and strive to regulate optimal GABA levels for proper circuit function.
A recent study by Fiorentino et al. [39
] proposes that the interaction between BDNF and GABA signaling influences GABAergic synapse maturation. The authors demonstrate that activation of metabotropic GABAb receptor triggers secretion of BDNF and promotes the development of GABAergic synapses, in particular, the perisomatic GABAergic synapses, onto CA3 pyramidal neurons in the hippocampus of newborn mice [39
]. Whether a similar mechanism is at play in the visual cortex is still unknown; however, the picture so far indicates a positive interplay between sensory experience, BDNF, and GABA signaling, to induce GABAergic synapse maturation and in turn promote the onset of ocular dominance plasticity.
In addition to factors promoting GABAergic synapse maturation, recent studies have revealed inhibitory mechanisms that set the appropriate time course for establishment of mature GABAergic innervation patterns and the onset of critical period plasticity. In particular, polysialic acid (PSA), linked to the neural cell adhesion molecule (NCAM), acts as a negative signal to suppress the formation of inhibitory synapses and the onset of OD plasticity in the developing visual cortex [40
]. In the mammalian brain, NCAM is a predominant carrier of the unusual long-chain, polyanionic carbohydrate, PSA, although outside the nervous system more carriers of PSA are known, including neuropilin-2 [41
]. PSA is a long linear homopolymer of α
-2,8-linked sialic acid that is synthesized in the Golgi by two polysialyltransferases, PST (also known as ST8SiaIV) and STX (also known as ST8SiaII), either of which is sufficient for the complete synthesis of PSA chain on a standard asparaginyl-linked core carbohydrate attached to NCAM [42
One of the most studied characteristics of PSA is its ability to act as a de-adhesive factor, causing steric hindrance, between cellular membranes. Cell surface expression of PSA constricts intercellular space between apposing cells [44
], which in turn, decreases homophilic binding between NCAM and other cells adhesion molecules including Cadherins, L1 family, and Integrins [45
], therefore acting as a permissive regulating factor rather than a specific instructive cue. PSA affects distinct developmental processes depending on the location and timing of its expression. For example, in the developing nervous system PSA creates conditions permissive for postmitotic migration of precursor cells. In the adult, migrating cells still retain PSA, such as progenitor cells migrating along rostral migratory stream from the subventricular zone to the olfactory bulb [46
] and newborn granule cells in the hippocampus [47
Recent studies show the ability of PSA to regulate ocular dominance plasticity [40
]. Although PSA expression is highest in the embryonic stages, it is expressed in the postnatal brain at different levels depending on brain region and age. In the mouse visual cortex, PSA expression declines to almost undetectable levels shortly after eye opening, and this decline is attenuated by visual deprivation [40
]. Indeed, PSA levels in visual cortex were higher in mice dark reared from birth compared to littermates reared in a normal light-dark cycle. This effect is echoed in the visual cortex contralateral to the eye that received daily intraocular injection of TTX compared to the ipsilateral cortex [40
]. Since the developmental and activity-regulated expression of PSA inversely correlates with the maturation of GABAergic innervation [21
], it is thus possible that PSA decline might be sufficient for GABAergic synapse maturation. Indeed, premature enzymatic removal of PSA in the developing visual cortex results in precocious maturation of perisomatic innervation by basket interneurons and enhanced inhibitory synaptic transmission. Most importantly, the same treatment causes an earlier onset of critical period plasticity in the visual cortex [40
]. Since PSA removal promotes GABAergic synapse formation, and GABA signaling in turn further promotes the maturation of GABAergic innervation [33
], together GABA signaling and PSA removal may constitute a positive feedback mechanism to accelerate GABAergic synapse formation once sensory experience begins, and consequently to induce the onset of critical period plasticity in the visual cortex. PSA also regulates glutamatergic synapse formation [48
] and affects neuron-glia interactions [50
] thus the possibility of additional mechanisms by which PSA influences ocular dominance plasticity cannot be excluded.
What is the precise role of PSA in GABAergic circuit maturation? One possibility is that developmental and activity-dependent removal of PSA might coordinate the timing of axon and synapse morphogenesis during the maturation of GABAergic innervation; indeed precocious perisomatic synapse formation can be triggered by premature removal of PSA. Excessive, premature synapse formation might constrain axon growth. Higher expression of PSA during the early postnatal weeks might attenuate interactions between basket cell axons and pyramidal neurons, thereby holding off synapse formation and promoting the elaboration of axon arbors. Subsequent activity-dependent removal of PSA might unmask mechanisms that are already in place along basket cell axon, allowing fast responses to local synaptogenic cues. A similar example of PSA regulating the timing of a biological process comes from studies of migrating neuronal precursor. When PSA is enzymatically removed from newly generated cells in the SVZ, they form neuronal processes and begin to express neuronal molecular markers. This premature developmental transition is dependent on cell contact and appears to involve signaling through NCAM and p59Fyn kinase [51
Why is such a mechanism in place and what could be its purpose? Interestingly, long polymers of sialic acid are not found in invertebrates [43
], where neural circuits are to a large extent genetically determined. This raises the possibility that PSA might have evolved to regulate vertebrate-specific developmental processes. An example is the role of PSA in cell migration and differentiation. In invertebrates, the differentiation of neuronal precursors occurs close to the region of their birth and involves interactions with its immediate neighbor cells. On the other hand, in vertebrates, newly generated precursors often migrate long distances before acquiring their fate, and thus need to delay their differentiation till they reach their destination. Here, PSA plays a dual role whereby it (a) promotes cell migration by reducing cell-cell adhesion and (b) blocks differentiation by interfering with contact-dependent signaling until the cells arrive at their final location.
Such multifaceted roles for PSA are well suited for the complex experience-dependent neural circuit fine-tuning that occurs in vertebrate CNS. It is interesting to note that vision-dependent critical period plasticity does not start at the onset of eye opening. Instead, it is hypothesized that the critical period cannot start until the input to the circuit has developed reliability and precision [52
]. Thus, cellular mechanisms underlying critical period are not simply an activity-dependent process; instead, it is a sequence of timed events that appear to be important. PSA might then act as “brake” that holds off the onset of critical period plasticity until input information can be reliably relayed to the cortex. The challenge is to understand what happens if and when this timing is altered, whether onset of critical period before the appropriate time might lead to incorrect refinement of neural circuit based on unreliable, or nonoptimal inputs, and whether and how this would in turn affect behavior.