DIFFERENTIAL PRESYNAPTIC AND POSTSYNAPTIC MODULATION OF CHEMICAL TRANSMISSION IN THE SQUID GIANT SYNAPSE BY TYROSINE PHOSPHORYLATION

To assess the role of tyrosine phosphorylation/dephosphorylation balan ce in synaptic transmission, a set of studies was implemented at the s quid giant synapse, Presynaptic induction of tyrosine phosphorylation, following administration of the tyrosine phosphatase inhibitor pervan adate, produced a sizable increase in presynaptic calcium current and a concomitant and paradoxical decrement of the postsynaptic potential amplitude. Presynaptic microinjection of an active protein tyrosine ki nase dramatically increased calcium currents and incremented postsynap tic potential amplitude, By contrast, the same procedure at the postsy naptic terminal reduced the size of the postsynaptic potential. This d ifferential effect may be prodromic to long-term plasticity, as postsy naptic sensitivity is momentarily deemphasized, whereas presynaptic se cond messenger cascades triggered by increased calcium currents are ac centuated.