Modulation of glutamate release by a nitric oxide/cyclic GMP-dependent pathway

The mechanism by which changes in cyclic GMP (cGMP) regulate glutamate release was investigated in rat cerebrocortical nerve terminals. The elevation of cGMP levels by inhibition of cGMP-phosphodiesterase with 2-o-propoxy-phenyl-8-azapurin-6-one (zaprinast) reduced the Ca(2+)-dependent glutamate release evoked by depolarization with 30 mM KCl or 1 mM 4-aminopyridine. The nitric oxide (NO) donor S-nitroso-N-acetylpenicillamine also enhanced cGMP and reduced glutamate release. In addition, the membrane-permeable analogs 8-bromoguanosine 3':5'-cyclic monophosphate (8-Br-cGMP) and N,2'-o-dibutyrylguanosine (dbcGMP) at 10 microM also mimic glutamate release inhibition. The reduction in glutamate release was observed with no modifications in the ATP/ADP ratio, and was reversed in the presence of the protein kinases inhibitor [N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide, HCl] (H-8). Interestingly, higher concentrations of dbcGMP (1 mM) abolished the inhibition observed with low concentrations although no facilitation was observed. This finding seems to indicate the existence of a dual role for cGMP in the control of glutamate exocytosis.