Mismatched appositions of presynaptic and postsynaptic components in isolated hippocampal neurons

To determine whether presynaptic input is necessary for postsynaptic differ entiation, we isolated hippocampal neurons in microisland culture and thus deprived pyramidal cells of GABA input and GABAergic neurons of glutamate i nput. We find that glutamate input is necessary for clustering the AMPA-typ e glutamate receptor but not for clustering the NMDA receptor or the associ ated PSD-95 family scaffold in GABAergic cells; GABA input is not necessary for clustering the GABA(A) receptor or gephyrin in pyramidal cells. Isolat ed neurons showed a surprising mismatch of presynaptic and postsynaptic com ponents. For example, in isolated pyramidal neurons, although GABA(A) recep tor clusters covered <4% of the dendritic surface and presynaptic boutons c overed <12%, a full two-thirds of the GABA(A) receptor clusters were locali zed inappropriately opposite the non-GABAergic, presumed glutamatergic, ter minals. Furthermore, inhibitory and excitatory postsynaptic components were segregated into separate clusters in isolated cells and apposed to separat e boutons of a single axon. Thus, GABAA receptors were clustered opposite s ome terminals, whereas NMDA receptors were clustered opposite other termina ls of a single axon. These results suggest the involvement of a synaptogeni c signal common to glutamate and GABA synapses that permits experimentally induced mismatching of presynaptic and postsynaptic components in isolated neurons, as well as a second specificity-conferring signal that mediates ap propriate matching in mixed cultures.