ACTIVITY-DEPENDENT SHORT-TERM ENHANCEMENT OF INTERCELLULAR COUPLING

It was reported previously that repeated brief tetanization of the pos terior eighth nerve can produce long-term homosynaptic potentiations o f the electrotonic and chemical components of the mixed EPSP evoked in the Mauthner cell lateral dendrite by a single stimulus to the nerve. We show here that the same stimulus paradigm can lead, alternatively, to short-term enhancements of both excitatory responses. These transi ent modifications last for similar to 3 min, with a time course simila r to post-tetanic potentiation at chemical synapses. However, a differ ent stimulus pattern that transiently increases the presynaptic calciu m concentration, paired-nerve stimuli, does not have any significant e ffect on electrotonic transmission, whereas it facilitates the chemica lly mediated EPSP. On the other hand, induction of the short-lasting p otentiation of coupling, which depended on the discontinuous or burst- like property of the tetanizing paradigm, required NMDA-receptor activ ation and was blocked by postsynaptic intradendritic injections of the calcium chelator bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic aci d. The ineffectiveness of presynaptic calcium in potentiating electrot onic coupling likely reflects the involvement of a calcium-dependent r egulatory protein in the postsynaptic cell and suggests that hemichann els on the two sides of a gap junction plaque can be modified independ ently. NMDA-mediated modulation of gap junctions could be widespread, because both types of channels coexist during development and in sever al mammalian adult central nervous system structures such as hippocamp us.