We investigated whether the neuropeptide galanin affects the nitric oxide synthase/cyclic GMP pathway in rat hippocampus by measuring in vivo the extracellular cyclic GMP levels during microdialysis. Galanin (2.5 and 3.5 nmol; i.c.v.) dose-dependently raised the extracellular levels of cyclic GMP in the ventral but not the dorsal hippocampus. The effect of 3.5 nmol galanin was blocked by local application of tetrodotoxin and inhibited by the high-affinity galanin antagonist M40 (galanin-[1-12]-Pro3-[Ala-Leu]2-Ala amide). The non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine maleate (30 microM infused into the ventral hippocampus or 0.2 mg/kg, i.p.) and the competitive one, 3-([R]-carboxypiperazin-4-yl)-propyl-phosphonic acid (50 microM infused), but not local perfusion of the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM) abolished the galanin-evoked cyclic GMP response in the hippocampus. Inhibitors of nitric oxide synthase, L-Arg(NO2)-OMe.HCl and 7-nitroindazole monosodium salt, applied locally, blocked the galanin-induced increase in hippocampal extracellular cyclic GMP. This increase was also prevented by local application of 1H-(1,2,4)oxadiazolo(4,3a) quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The galanin receptors mediating the rise in cyclic GMP reside outside the hippocampus, as galanin (0.35-3 nmol) locally applied had no effect. The results provide in vivo evidence that galanin stimulates the N-methyl-D-aspartate receptor/nitric oxide synthase/cyclic GMP pathway in the ventral hippocampus, which may be of importance in memory processes.