Loss of channel modulation by transmitter and protein kinase C during innervation of an identified leech neuron.

When serotonergic Retzius (R) neurons of the leech contact pressure-sensitive (P) neurons in culture, P cells selectively lose a protein kinase C-dependent cationic response to serotonin and the R cell reforms the inhibitory, chloride-dependent synapse seen in vivo. In P cells not contacted by R cells, cell-attached patches contained single cation channels sensitive to serotonin and phorbol ester with characteristic properties and high incidence (present in about one-half of the patches). P cells paired with R cells had a cation channel with similar biophysical properties and incidence, but channel activity was not stimulated by serotonin and phorbol ester. These results suggest that the early clearing of the non-synaptic (excitatory) response to serotonin is due to the loss of activation by protein kinase C (and not the number) of cation channels as a prelude to inhibitory synapse formation.