Distribution of postsynaptic GABA(A) receptor aggregates in the deep cerebellar nuclei of normal and mutant mice

In the central nervous system, the aggregation of receptors is crucial for synapse formation and function. To study the role of presynaptic terminals in the maintenance of postsynaptic specializations, we analyzed the synaptic contacts between Purkinje cells and neurons of the deep cerebellar nuclei in two in vivo models: the Lurcher and Purkinje cell-deficient (PCD) mutant mice. These mutants lose their Purkinje cells at different postnatal stages. By using confocal scanner microscopy and immunohistochemistry, we studied the distribution of the alpha subunit of the gamma-aminobutyric acid (GABA)(A) receptor (GABA(A)Ralpha1) and gephyrin, one of its anchoring proteins, in relation to the distribution of presynaptic markers, glutamic acid decarboxylase (GAD), or synaptophysin. In Lurcher the distribution of GABA(A) receptor aggregates on the membrane of postsynaptic neurons was not affected by the important loss of GAD-positive terminals, whereas in PCD, the number of large GABA(A) receptor aggregates increased. In both mutants the number of aggregates of gephyrin decreased. Most of these remaining aggregates were clustered to form groups, some of which were in front of GAD-positive terminals. This study shows, for the first time, the localization of GABA(A)R alpha 1 in Lurcher and PCD mutant mice. It clearly establishes that GABA(A)R alpha 1 and gephyrin are differentially affected by deafferentation. Because the receptor aggregates are maintained while the gephyrin aggregates are lost, as a result some receptor aggregates are not associated with any gephyrin. These two postsynaptic components appeared to be regulated by different mechanisms