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Serotoninergic Retzius neurons reform an inhibitory synapse onto pressure-sensitive mechanosensory (P) neurons when the cells are removed from the nervous system of the leech and are juxtaposed in tissue culture. The somas of P cells in situ and single (uninnervated) P cells in culture have both a depolarizing and Cl-dependent hyperpolarizing response to application of the transmitter serotonin (5- HT). Synaptic release of 5-HT by a Retzius cell in situ and in culture evokes a Cl-dependent postsynaptic response but does not appear to activate the depolarizing receptors. We have characterized the ionic currents induced by synaptically released and applied 5-HT in voltage- clamped P cells in culture in order to determine the responses to transmitter and their modifications following innervation. When 5-HT was applied to single P cells, 2 types of channels were activated by 5- HT and could be distinguished by changing the ionic composition of the superfusion solution. In an impermeant cation (TrisCl) solution, a 5-HT- dependent Cl current was activated. When single P cells were superfused with a Cl-free solution (Cl replaced by impermeant SO4), 5-HT activated a monovalent cation current. Following innervation of a P cell by a cocultured Retzius cell, the reversal potential of the peak postsynaptic current depended on the Cl gradient and the synaptic response was blocked by the Cl channel blocker 9-anthracenecarboxylic acid. Thus, synaptic release of 5-HT activated solely the Cl channels and not the cationic channels. Pipette application of 5-HT onto innervated P cells activated a Cl conductance comparable in magnitude to the synaptic response. In contrast, the cationic conductance was reduced roughly 5-fold. It is concluded that innervation of a P cell by a Retzius cell resulted in clustering of the synaptic 5-HT receptors, which activate Cl channels and reduction of the nonsynaptic, cationic response. The result is a selection of receptors in the cultured P cell that mimics the pattern observed in vivo.

Selection of postsynaptic serotonin receptors during reinnervation of an identified leech neuron in culture