RAT CORTICAL SYNAPTOSOMES HAVE MORE THAN ONE MECHANISM FOR CA2- STUDIES USING THE PHONEUTRIA-NIGRIVENTER TOXIN PHTX2 AND POTASSIUM DEPOLARIZATION( ENTRY LINKED TO RAPID GLUTAMATE RELEASE )

PhTX2, one of the components of the venom of the South American spider Phoneutria nigriventer, inhibits the closure of voltage-sensitive Na channels. Incubation of cerebral-cortical synaptosomes with PhTX2 cau ses a rapid increase in the intrasynaptosomal free Ca2+ concentration and a dose-dependent release of glutamate. This release is made up of a slow component, which appears to be due to reversal of Na+-dependent glutamate uptake, and more rapid component that is dependent on the e ntry of extrasynaptosomal Ca2+. It has previously been shown that memb rane depolarization using KCl can cause rapid Ca2+-dependent release o f glutamate from synaptosomes. This requires Ca2+ entry through a spec ific type of Ca2+ channel that is sensitive to Aga-GI, a toxic compone nt of the venom of the spider Agelenopsis aperta. We have compared the effects of PhTX2 and KCl on elevation of intrasynaptosomal free Ca2and glutamate release, and a number of differences have emerged. First ly, PhTX2-mediated Ca2+ influx and glutamate release, but not those ca used by KCl, are inhibited by tetrodotoxin. Secondly, KCl produces a c lear additional increase in Ca2+ and glutamate release following those elicited by PhTX2. Finally, 500 muM MnCl2 abolishes PhTX2-mediated, b ut not KCI-mediated, glutamate release. These findings suggest that mo re than one mechanism of Ca2+ entry may be coupled to glutamate releas e from nerve endings.