Dihydropyrazole Insecticides: Interference with Depolarization-Dependent Phosphorylation of Synapsin I and Evoked Release ofl-Glutamate in Nerve-Terminal Preparations from Mammalian Brain

Abstract

The action of two dihydropyrazoles, RH-3421 and RH-5529, on depolarization-dependent phosphorylation of synapsins Ia and Ib and evoked release of the excitatory amino acid neurotransmitterl-glutamate from mouse brain synaptosomes has been evaluated. Exposure of synaptosomes to veratridine, which activates sodium channels, or elevated K+to activate calcium channels, resulted in an increase in synapsin phosphorylation andl-glutamate release. The sodium channel-selective blocker tetrodotoxin inhibited veratridine-induced, but not K+-induced increases in phosphorylation and transmitter release. In contrast, the dihydropyrazoles proved effective inhibitors of both veratridine- and K+-induced changes to synapsin phosphorylation and glutamate release. Moreover, blockade of K+-stimulated increases in synapsin phosphorylation andl-glutamate release by dihydropyrazoles was observed after inclusion of tetrodotoxin in the assay at a concentration sufficient to ensure that all sodium channel activity was suppressed. IC50s established for blockade of evoked release ofl-glutamate by dihydropyrazoles indicate that RH-3421 is approximately 20-fold more potent as an inhibitor of veratridine-stimulatedl-glutamate release than of K+-evoked release. Compared to RH-3421, RH-5529 is 10-fold less effective as an inhibitor of veratridine-induced release and is of similar potency against K+-mediated 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 concentrations with the operation of presynaptic calcium channels in mammalian central nerve terminals.