Abstract
An early event in the formation of inhibitory synapses between the serotonergic Retzius (R) neuron and the pressure-sensitive (P) neuron of the leech is the elimination of an extrasynaptic, depolarizing response to 5-HT in the P cell from sites of contact. This effect is induced specifically by contact with the R neuron but not with other identified leech neurons, including other serotonergic neurons that do not form chemical synapses with the P cell. In the reproductive (fifth and sixth) segmental ganglia, the R cells (R(5,6)) exhibit functional and morphological differences with the standard R cells (R(x)) in the other 19 segmental ganglia. In the study presented here, we tested the specificity of P cell recognition of R cells by examining whether there were differences between the R(x) and R(5,6) cells with respect to synapse formation with the P cell. R(5,6) neurons did not innervate P cells in the fifth and sixth ganglia (P(5,6)) in vivo or in vitro, nor did they form synapses with P cells from standard ganglia (P(x)) in vitro except on rare occasions, after long periods in culture. In contrast to the effects of R(5,6) neurons, R(x) neurons readily innervated both P(x) and P(5,6) cells in vitro, suggesting that the lack of an R(5,6)-P(5,6) synapse reflects differences in the presynaptic rather than the postsynaptic cell. In contrast to the R(x) cell, contact with the R(5,6) neurons had no significant effect on the responses of the P cell to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)