ROLE OF NITRIC-OXIDE IN PENILE ERECTION AND YAWNING INDUCED BY 5-HT1CRECEPTOR AGONISTS IN MALE-RATS

The effect of the intracerebroventricular (i.c.v.) administration of N -G-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, two inh ibitors of nitric oxide (NO) synthase, on penile erection and yawning induced by 1-(3-chlorophenyl)-piperazine (m-CPP)- and N-(3-trifluorome thylphenyl)-piperazine (TFMPP), two selective 5HT(1c) receptor agonist s, was studied in male rats. Both NO synthase inhibitors (50-500 mu g i.c.v.) prevented dose-dependently the behavioural responses induced b y m-CPP (0.5 mg/kg s.c.) or by TFMPP (1 mg/kg s.c.), but NG-nitro-L-ar ginine methyl ester was about 4-5 times more potent than NG-monomethyl -L-arginine. The D-isomer of N-G-monomethyl-L-arginine, which does not inhibit nitric oxide synthase, was ineffective. The inhibitory effect of NG-nitro-L-arginine methyl ester on m-CPP- and TFMPP-induced respo nses was prevented by the administration of L-arginine (1 mg i.c.v.). In contrast, N-G-nitro-L-arginine methyl ester (20 mu g) was ineffecti ve when injected in the paraventricular nucleus of the hypothalamus, a brain area that plays a key role in the expression of these behaviour al responses. m-CPP- and TFMPP-induced penile erection and yawning was prevented also by the i.c.v. administration of LY 83583 (50-200 mu g) or methylene blue (50-400 mu g), two inhibitors of guanylate cyclase but not by reduced hemoglobin (50-400 mu g), a NO scavenger. The resul ts suggest that central nitric oxide is involved in the expression of penile erection and yawning induced by 5-HT1c receptor agonists.