Role of nitric oxide in penile erection and yawning induced by 5-HT1c receptor agonists in male rats.

The effect of the intracerebroventricular (i.c.v.) administration of NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, two inhibitors of nitric oxide (NO) synthase, on penile erection and yawning induced by 1-(3-chlorophenyl)-piperazine (m-CPP)- and N-(3-trifluoromethylphenyl)-piperazine (TFMPP), two selective 5HT1c receptor agonists, was studied in male rats. Both NO synthase inhibitors (50-500 micrograms i.c.v.) prevented dose-dependently the behavioural responses induced by m-CPP (0.5 mg/kg s.c.) or by TFMPP (1 mg/kg s.c.), but NG-nitro-L-arginine methyl ester was about 4-5 times more potent than NG-monomethyl-L-arginine. The D-isomer of NG-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 responses was prevented by the administration of L-arginine (1 mg i.c.v.). In contrast, NG-nitro-L-arginine methyl ester (20 micrograms) was ineffective when injected in the paraventricular nucleus of the hypothalamus, a brain area that plays a key role in the expression of these behavioural responses. m-CPP- and TFMPP-induced penile erection and yawning was prevented also by the i.c.v. administration of LY 83583 (50-200 micrograms) or methylene blue (50-400 micrograms), two inhibitors of guanylate cyclase but not by reduced hemoglobin (50-400 micrograms), a NO scavenger. The results suggest that central nitric oxide is involved in the expression of penile erection and yawning induced by 5-HT1c receptor agonists.