Both protein kinase G dependent and independent mechanisms are involved in the modulation of glutamate release by nitric oxide in rat hippocampal nerve terminals

Abstract

We compared the effects of sodium nitroprusside (SNP), and of 8-bromo guanosine 3',5'-cyclic monophosphate (8-BrcGMP), on the 4-aminopyridine (4-AP)-evoked Ca2+-dependent release of glutamate from hippocampal nerve terminals and further investigated the role of protein kinase G (PKG) in this mechanism. SNP and 8-BrcGMP dose-dependently inhibited glutamate release, however SNP concentrations ([SNP])>500 [mu]M abolished the 4-AP evoked release, whereas 8-BrcGMP maximally inhibited the release by about 30%. The inhibition of glutamate release at low concentrations of SNP (<=5 [mu]M) was of about 20%, and was reversed by Rp-8(4-chlorophenylthio)guanosine-3',5'-cyclic-monophosphorotioate) (RpCPTcGMP, 50 nM), but the inhibition at higher concentrations (5<SNP<=50 [mu]M) was insensitive to the PKG inhibitor, but sensitive to [1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one] (ODQ), which partially prevented the inhibition. [SNP]>50 [mu]M strongly inhibited glutamate release, and this was not reversed by either inhibitor. Furthermore, [SNP]<=50 [mu]M enhanced cGMP formation, and the observed effects were not related to either decreased Ca2+ entry or ATP/ADP levels. Our results indicate that NO/PKG is the signaling pathway underlying the inhibition of glutamate release at low concentrations of NO, and imply that other NO-dependent, but PKG-independent, mechanisms are activated and have complementary roles at higher NO concentrations.