GABAergic terminals are required for postsynaptic clustering of dystrophin but not of GABA(A) receptors and gephyrin

Altered inhibitory input to Purkinje cells of dystrophin-deficient mice

We investigated evoked EPSPs and spontaneous IPSPs in cerebellar slices from dystrophin-deficient mdx mice. In the presence of the GABA(A) antagonist bicuculline the increase in EPSP amplitude was less in mdx Purkinje cells compared to control, and the amplitude of miniature IPSCs in mdx cells was also significantly less than in controls. This reduced inhibitory input is most likely due to the reported reduction in the size of GABA(A) channel clusters.

Gamma-protocadherins are targeted to subsets of synapses and intracellular organelles in neurons

The clustered protocadherins (Pcdhs) comprise >50 putative synaptic recognition molecules that are related to classical cadherins and highly expressed in the nervous system. Pcdhs are organized into three gene clusters (alpha, beta, and gamma). Within the alpha and gamma clusters, three exons encode the cytoplasmic domain for each Pcdh, making these domains identical within a cluster. Using an antibody to the Pcdh-gamma constant cytoplasmic domain, we find that all interneurons in cultured hippocampal neurons express high levels of Pcdh-gamma(s) in a nonsynaptic distribution. In contrast, only 48% of pyramidal-like cells expressed appreciable levels of these molecules. In these cells, Pcdh-gamma(s) were associated with a subset of excitatory synapses in which they may mediate presynaptic to postsynaptic recognition in concert with classical cadherins. Immunogold localization in hippocampal tissue showed Pcdh-gamma(s) at some synapses, in nonsynaptic plasma membranes, and in axonal and dendritic tubulovesicular structures, indicating that they may be exchanged among synapses and intracellular compartments. Our results show that although Pcdh-gamma(s) can be synaptic molecules, synapses form lacking Pcdh-gamma(s). Thus, Pcdh-gamma(s) and their relatives may be late additions to the classical cadherin-based synaptic adhesive scaffold; their presence in intracellular compartments suggests a role in modifying synaptic physiology or stability.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

Association of gephyrin with synaptic and extrasynaptic GABAA receptors varies during development in cultured hippocampal neurons

Several studies have reported extrasynaptic clusters of GABAA receptors in hippocampal neurons. Yet their functional relevance as well as their evolution in relation with gephyrin during synaptogenesis remain unknown. We have analyzed the expression pattern of the main proteins of the GABAergic synapses during synaptogenesis in cultured hippocampal neurons. We found that GABAergic terminals, characterized by VIAAT and GAD-65 expression, differentiated 3 to 7 days after the glutamatergic endings. At the postsynaptic side, the GABAAR- beta3 subunit was first diffuse and then clustered when GABAergic terminals differentiated and gephyrin formed large clusters. Colocalization of these proteins was high and increased with development. At later stages, GABAAR beta3 clusters colocalized with gephyrin at synaptic but also at extrasynaptic sites. GABAAR gamma2 subunits were directly expressed as clusters which were first extrasynaptic and not associated with gephyrin. Subsequently, the GABAAR gamma2 subunits associated with gephyrin at synaptic and/or extrasynaptic sites. Our data indicate that formation of GABAAR gamma2 subunit clusters is gephyrin independent.

The enigma of transmitter-selective receptor accumulation at developing inhibitory synapses

The control of synaptic inhibition is crucial for normal brain function. More than 20 years ago, glycine and gamma-aminobutyric acid (GABA) were shown to be the two major inhibitory neurotransmitters. They can be released independently from different terminals or co-released from the same terminal to activate postsynaptic glycine and GABA(A) receptors. The anchoring protein gephyrin is involved in the postsynaptic accumulation of both glycine and GABA(A) receptors. In lower brain regions, both receptors can be concentrated in synapses, whereas in higher brain regions, glycine receptors are mostly excluded from postsynaptic sites. The activation of glycine and/or GABA(A) receptors determines the strength and precise timing of inhibition. Therefore, tight regulation of postsynaptic glycine versus GABA(A) receptor localization is crucial for optimizing synaptic inhibition in neurons. This review focuses on recent findings and discusses questions concerning the specificity of postsynaptic inhibitory neurotransmitter receptor accumulation during inhibitory synapse formation and development.

GABAergic and glutamatergic terminals differentially influence the organization of GABAergic synapses in rat cerebellar granule cells in vitro

Synapse formation in CNS neurons requires appropriate sorting and clustering of neurotransmitter receptors and associated proteins at postsynaptic sites. In GABAergic synapses, clustering of GABA(A) receptors requires gephyrin, but it is not known whether presynaptic signals are also involved in this process. To investigate this issue, we analyzed the subcellular distribution of GABA(A) receptors and gephyrin in primary cultures of cerebellar granule cells, by comparing cells receiving GABAergic input with cells devoid of such afferents. Using immunofluorescence staining, we show that the GABA(A) receptor alpha1 and gamma2 subunit, but not alpha6 or delta subunit, form clusters co-localized with gephyrin in granule cell neurites, irrespective of the presence of GABAergic axons. GABAergic terminals typically were surrounded by groups of gephyrin clusters, pointing to the presence of multiple synaptic sites. In contrast, in neurites devoid of GABAergic input, gephyrin clusters were distributed at random and apposed to glutamatergic terminals, suggesting the formation of mismatched synapses. Both populations of gephyrin clusters were co-localized with GABA(A) receptor subunits, indicating that these proteins are associated also in non-GABAergic synapses. To determine whether signaling mediated by GABA(A) receptors is required for the formation of appropriately matched gephyrin clusters, cultures were treated chronically with bicuculline, or with either muscimol or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. All these treatments failed to influence the distribution of gephyrin clusters. We conclude that although GABAergic presynaptic terminals have a preponderant influence on the distribution of gephyrin clusters in dendrites of cerebellar granule cells, GABA transmission is dispensable for postsynaptic clustering of gephyrin and GABA(A) receptors and for the formation of appropriately matched GABAergic synapses.