TYROSINE PHOSPHORYLATION DURING SYNAPSE FORMATION BETWEEN IDENTIFIED LEECH NEURONS

1. We have examined whether tyrosine phosphorylation is required for s ynapse formation between identified neurons from the central nervous s ystem of the leech in culture. 2. Within a few hours of contact with t he cell body of the serotonergic Retzius neuron (R cell), the soma of the postsynaptic pressure-sensitive neuron (P cell), but not the R cel l, could be labelled intracellularly with an antibody against phosphot yrosine residues. The labelling seemed specific for P cells contacted by R cells, as it was greatly reduced in pairs of either R or P cells and in single cells. Genistein (20 mu M) and lavendustin A (10 mu M), selective inhibitors of tyrosine kinases, blocked the labelling of con tacted P cells, whereas their ineffective analogues (genistin and lave ndustin B) had no effect on labelling. 3. R, cell contact also induced the loss of an extrasynaptic, depolarizing response (due to modulatio n of cation channels) to serotonin (5-HT) in the P cell within a few d ays of juxtaposing cell bodies and within an hour of contact with grow th cones. Treatment of the neurons with the tyrosine kinase inhibitors (but not the ineffective analogues) prevented the loss of the depolar izing response and of single cation channel modulation by 5-HT. 4. R c ells formed inhibitory, Cl--dependent synapses with P cells. Synapse f ormation was prevented by the tyrosine kinase inhibitors but not by th eir ineffective analogues. These compounds had no obvious effect on ne urite outgrowth or cell adhesion. We conclude that tyrosine phosphoryl ation is a signal during the formation of this synapse.