NITRIC-OXIDE SYNTHASE ACTIVITY IN THE DORSAL PERIAQUEDUCTAL GRAY OF RATS EXPRESSING INNATE FEAR RESPONSES

IMMUNOHISTOCHEMICAL studies have shown nitric oxide synthase (NOS)-pos itive neurons in the dorsolateral sector of periaqueductal gray (FAG), a neural site known to be critical for the expression of defensive re sponses. In the present study, we first characterized the dorsal FAG ( dPAG) NOS, and then examined NOS activity and cyclic GMP (cGMP) accumu lation in the dPAG of rats exposed to a predator (cat) for 15 min. NOS activity evaluated by enzymatic conversion of [H-3]arginine to [H-3]c itrulline in dPAG of exposed rats increased 14.6% and the cGMP radioim munoassay showed an increase of 30.6% in relation to non-exposed rats. These results suggest an involvement of the NO/cGMP pathway in the dP AG during defensive responses.