Nitric oxide synthase inhibitors prevent apomorphine- and oxytocin-induced penile erection and yawning in male rats.

The effect of NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, two inhibitors of nitric oxide synthase, on apomorphine- and oxytocin-induced penile erection and yawning, was studied in male rats after intravenous and intracerebroventricular administration. Both compounds prevented dose-dependently apomorphine and oxytocin responses, when given systemically (5-50 mg/kg) or centrally (30-500 micrograms per rat), but NG-nitro-L-arginine methyl ester was about 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 results suggest that central nitric oxide is involved in the expression of penile erection and yawning induced by apomorphine and oxytocin.