DIHYDROPYRAZOLE INSECTICIDES - INTERFERENCE WITH DEPOLARIZATION-DEPENDENT PHOSPHORYLATION OF SYNAPSIN-I AND EVOKED RELEASE OF L-GLUTAMATE IN NERVE-TERMINAL PREPARATIONS FROM MAMMALIAN BRAIN

The action of two dihydropyrazoles, RH-3421 and RH-5529, on depolariza tion-dependent phosphorylation of synapsins Ia and Ib and evoked relea se of the excitatory amino acid neurotransmitter L-glutamate from mous e brain synaptosomes has been evaluated. Exposure of synaptosomes to v eratridine, which activates sodium channels, or elevated K+ to activat e calcium channels, resulted in an increase in synapsin phosphorylatio n and L-glutamate release. The sodium channel-selective blocker tetrod otoxin inhibited veratridine-induced, but not K+-induced increases in phosphorylation and transmitter release. In contrast, the dihydropyraz oles proved effective inhibitors of both veratridine- and K+-induced c hanges to synapsin phosphorylation and glutamate release. Moreover, bl ockade of K+-stimulated increases in synapsin phosphorylation and L-gl utamate release by dihydropyrazoles was observed after inclusion of te trodotoxin in the assay at a concentration sufficient to ensure that a ll sodium channel activity was suppressed. IC(50)s established for blo ckade of evoked release of L-glutamate by dihydropyrazoles indicate th at RH-3421 is approximately 30-fold more potent as an inhibitor of ver atridine-stimulated L-glutamate release than of K+-evoked release. Com pared to RH-3421, RH-5529 is 10-fold less effective as an inhibitor of veratridine-induced release and is of similar potency against K+-medi ated efflux. Our data provide further evidence to support the concept that, in addition to their established sodium channel-blocking actions , dihydropyrazoles have the ability to interfere at higher concentrati ons with the operation of presynaptic calcium channels in mammalian ce ntral nerve terminals. (C) 1996 Academic Press, Inc.