Target-specific factors regulate the formation of glutamatergic transmitter release sites in cultured neocortical neurons

Synapse formation in the mammalian CNS is thought to involve specific targe t recognition processes between presynaptic and postsynaptic neurons leadin g to the establishment of defined neuronal circuits. To study the role of t arget neuron-specific factors in synaptogenesis, we used cocultures of pres ynaptic explants and dissociated target neurons from rat neocortex, which e nabled us to selectively vary the postsynaptic target neurons. Cocultures c ontaining target neurons that were obtained early during development [embry onic day 16 (E16)] were compared to cocultures containing target neurons th at were obtained at a later embryonic stage (E19). Postsynaptic currents (PSCs) were evoked in target neurons by maximal extra cellular stimulation in the presynaptic explant. The mean amplitudes of AMP A and NMDA receptor-mediated PSCs were sixfold reduced in E16 target neuron s, whereas the mean amplitudes of GABA(A) receptor-mediated PSCs did not di ffer between E16 and E19 target neurons. This reduction was in part caused by an apparently twofold reduction in mean quantal amplitude, as shown by r ecording AMPA receptor-mediated miniature PSCs. In addition, a reduced numb er of glutamatergic release sites in E16 target neurons was revealed by syn apsin I immunostaining of dendritic presynaptic terminals. No differences i n mean release probability were observed between E16 and E19 target neurons . Thus, the formation of glutamatergic transmitter release sites was strongly influenced by target neuron-specific factors. The formation of functional GABAergic synapses, however, was independent of the type of target neurons, suggesting specific retrograde signaling during the establishment of gluta matergic synapses.