INVOLVEMENT OF PHOSPHOLIPASE-A(2) AND ARACHIDONIC-ACID IN THE DEPOLARIZATION-EVOKED ACCUMULATION OF CA2-ENDINGS( IN HIPPOCAMPAL MOSSY FIBERNERVE)

Depolarization-evoked increases in intraterminal free Ca2+ are require d for the induction of neurotransmitter release from nerve terminals. Although the mechanisms that regulate the voltage-induced accumulation of presynaptic Ca2+ remain obscure, there is evidence that the phosph olipase-dependent accumulation of arachidonic acid, or its metabolites , may be involved. Therefore, fura-2 loaded hippocampal mossy fiber ne rve endings were used to investigate the relationships between membran e depolarization, lipid metabolism and presynaptic Ca2+ availability. It was observed that depolarization of the nerve terminals with KCl in duced an increase in intraterminal free calcium that was inhibited mor e than 90% by a combination of voltage-sensitive Ca2+ channel blockers . In addition, the K+-dependent effects on Ca2+ concentrations were at tenuated in the presence of phospholipase A2 inhibitors, but were mimi cked by the phospholipase A2 activator melittin and exogenous arachido nic acid. Both the melittin- and arachidonic acid-induced increases in presynaptic Ca2+ were reduced by voltage-sensitive Ca2+ channel block ers. The stimulatory effects of arachidonic acid appeared to be indepe ndent of its further metabolism to prostaglandins. In fact, inhibition of either cyclooxygenase or lipoxygenase pathways resulted in a poten tiation of the depolarization-evoked increase in intraterminal free Ca 2+. From these results, we propose that some portion of the depolariza tion-evoked increase in intraterminal free calcium depends on the acti vation of phospholipase A2 and the subsequent accumulation of unesteri fied arachidonic acid.