Contact-mediated loss of the nonsynaptic response to transmitter during reinnervation of an identified leech neuron in culture.

We have examined the modification of responses to transmitter during the reformation of a specific synapse by identified leech neurons in culture. Single pressure-sensitive mechanosensory (P) neurons in culture and the soma of this cell in vivo have 2 conductances that are activated by application of 5-HT: a Cl conductance and a monovalent cation conductance (gCations); synaptic release of 5-HT by a serotoninergic Retzius cell in vivo and in culture activates only the Cl conductance and not gCations in the P cell. We have characterized the loss of gCations in P cells by manipulating the culture conditions. When 5-HT was applied from a pipette, innervated P cells and P cells paired with Retzius cells that had not formed synapses had a gCations that was markedly reduced compared with gCations in the single P cell. When 5-HT was included in the culture medium in which single P cells were grown, gCations was not reduced. When Retzius-P cell pairs were treated with reserpine, which was shown to deplete the presynaptic neuron of 5-HT, gCations in the P cell was as low as in untreated pairs. Pairing aldehyde-fixed Retzius cells with untreated P cells also resulted in the loss of gCations. The Retzius cell had both types of receptors, but synapse formation did not affect gCations in the presynaptic neuron. The results demonstrate that the loss of the nonsynaptic response of the P cell to transmitter is due to contact between the neurons rather than to the release of transmitter or trophic factors.(ABSTRACT TRUNCATED AT 250 WORDS)