Nitric oxide synthase (NADPH-diaphorase) content in brain neurons of neonatal rats after inhibitory learning and intervention into nitric oxide metabolism.

Four-day-old rat pups were taught to avoid an electrified grid under the influence of increased nitric oxide availability in brain (by a nitric oxide substrate L-arginine) that alleviated learning or decreased nitric oxide (due to the action of a blocker of nitric oxide synthase nitro-L-arginine) that impaired learning. Three hours after criteria meeting, the pups were killed for analysis of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase content in brain cells and neuropil. In the cingulate gyrus, NADPH-diaphorase-positive staining was increased after L-arginine, but an opposite picture was observed in hippocampus and basal ganglia, i.e. an increase after the blocker nitro-L-arginine. A noteworthy accumulation of NADPH-diaphorase in hippocampal cells might be tentatively explained by the blocking effect of nitro-L-arginine not allowing NADPH-diaphorase to leave the cells. Application of L-arginine or nitro-L-arginine provoked only minor changes in the studied structures of non-learned pups with the exception of hippocampus where nitro-L-arginine increased the width of neuropil, but to a lesser degree than in learned animals. These results clearly show that both manipulations, i.e. drug application and learning, only have a significant effect on the changes in NADPH-diaphorase positivity in brain neurons.