SEGMENTAL SPECIFICITY OF NEURONAL RECOGNITION DURING SYNAPSE FORMATION BETWEEN IDENTIFIED LEECH NEURONS

An early event in the formation of inhibitory synapses between the ser otonergic Retzius (R) neuron and the pressure-sensitive (P) neuron of the leech is the elimination of an extrasynaptic, depolarizing respons e 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 identifie d leech neurons, including other serotonergic neurons that do not form chemical synapses with the P cell. In the reproductive (fifth and six th) segmental ganglia, the R cells (R((5,6))) exhibit functional and m orphological differences with the standard R cells (R((x))) in the oth er 19 segmental ganglia. In the study presented here, we tested the sp ecificity 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,P-6)) in vivo or in vitr o, nor did they form synapses with P cells from standard ganglia (P-(x )) in vitro except on rare occasions, after long periods in culture. I n contrast to the effects of R((5,6)) neurons, R((x)) neurons readily innervated both P-(x) and P-(5,P-6) cells in vitro, suggesting that th e lack of an R((5,6))-P-(5,P-6) synapse reflects differences in the pr esynaptic 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. We conclude that the P cell is ab le to distinguish between the R((x)) and R((5,6)) neurons through cell surface cues, and that the selection of transmitter responses in the postsynaptic neuron is an early event during neuronal recognition and synapse formation.