NMDA RECEPTOR ACTIVATION STIMULATES PHOSPHOLIPASE-A(2) AND SOMATOSTATIN RELEASE FROM RAT CORTICAL-NEURONS IN PRIMARY CULTURES

We have recently shown that glutamate exerts a stimulatory action on somatostatin secretion in cortical neurons essentially through NMDA receptor sites. Here, we investigated whether arachidonic acid release could be modified after NMDA receptor activation in cortical neurons in primary culture. We also studied whether pharmacological manipulation of phospholipase A2 could modify somatostatin release. We found that both glutamate and NMDA (N-methyl-D-aspartate) stimulated [H-3]arachidonic acid release. NMDA-evoked arachidonic acid release was inhibited by MK-801 and TCP (two NMDA receptor-type antagonists), or by mepacrine, an inhibitor of phospholipase A2. NMDA-induced somatostatin release was inhibited by MK-801, mepacrine and by another phospholipase A2 inhibitor, p-bromophenacylbromide (pBPB). However, responses to NMDA were unaffected by H7, NDGA (nordihydroguaiaretic acid), indomethacin or by RHC 80267 (inhibitors of protein kinase C, lipooxygenase, cyclooxygenase and diacylglycerol lipase, respectively). Mepacrine (greater-than-or-equal-to 100-mu-M) decreased NMDA-stimulated phosphatidylinositol (PI) hydrolysis and at higher concentrations (250-mu-M) was also able to inhibit basal release whereas pBPB had no effect in the range of concentrations tested. Neomycin (which inhibits phosphatidylinositol metabolism by binding strongly and selectively to inositol phospholipids) reduced by 30% the NMDA-stimulated somatostatin release, although chronic treatment of neurons with the phorbol ester 12-myristate, 13-acetate (PMA) had no effect on this response. Melittin, an activator of phospholipase A2, was able to stimulate both arachidonic acid release and somatostatin secretion. High-performance liquid chromatography (HPLC) analysis of tritiated metabolites released from cortical neurons under basal or NMDA-stimulated conditions revealed that [H-3]arachidonic acid was the only metabolite detectable. Furthermore, external addition of arachidonic acid increased somatostatin secretion. Our results show a correlation between the two parameters studied.