MODULATION AND SELECTION OF NEUROTRANSMITTER RESPONSES DURING SYNAPSEFORMATION BETWEEN IDENTIFIED LEECH NEURONS

1. Serotonin (5-HT) modulates two different responses in the pressure sensitive neurons (P) of the leech: an inhibitory, Cl- dependent synap tic response and a depolarizing extrasynaptic response. 2. Serotonergi c Retzius cells (R) in vivo and in culture elicit inhibitory Cl- depen dent responses in P neurons. Moreover, at discrete sites of contact be tween R and P cells, the excitatory response to 5-HT is gradually lost prior to synapse formation, This phenomenon is specifically mediated by R cells. 3. The extrasynaptic response is mediated by cation channe ls sensitive to protein kinase C (PKC), Cation channels are present at the sites of contact but they become insensitive to PKC. Moreover, ca tion channels from single P cells are no longer modulated by PKC if th ey are inserted (by cramming the patch pipette) into the cytoplasm of a P cell in contact with an R cell. 4. Blockers of tyrosine kinases pr event the uncoupling of cation channel modulation and inhibit synapse formation between the R and the P neurons. 5. We suggest that cell con tact induces an intracellular, tyrosine kinase-dependent signal as par t of the mechanism of neuronal recognition leading to synapse formatio n.