We previously reported that 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP) is endogenously produced via nitric oxide/reactive oxygen species signaling pathways and it reacts with protein thiol residues to add cGMP structure to proteins through S-guanylation. S-Guanylation occurs on synaptosomal-associated protein 25 (SNAP-25), which is a part of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex that regulates exocytosis. However, the biological relevance of 8-nitro-cGMP in the nervous system remains unclear. Here, we investigated the effects of intracerebroventricular (icv) infusion of 8-nitro-cGMP on mouse brain functions. The results of an open-field test and fear-conditioning task revealed that icy infusion of 8-nitro-cGMP decreased the vertical activity and context-dependent fear memory of mice, which are both associated with the hippocampus. Immunohistochemical analysis revealed increased c-Fos-positive cells in the dentate gyrus in 8-nitro-cGMP-infused mice. Further, biochemical analyses showed that icy infusion of 8-nitro-cGMP increased S-guanylated proteins including SNAP-25 and SNARE complex formation as well as decreased complexes containing complexin, which regulates exocytosis by binding to the SNARE complex, in the hippocampus. These findings suggest that accumulation of 8-nitro-cGMP in the hippocampus affects its functions, including memory, via S-guanylation of hippocampal proteins such as SNAP-25. (C) 2019 Elsevier Inc. All rights reserved.