α-MELANOCYTE STIMULATING HORMONE AND OXYTOCIN INDUCED PENILE ERECTIONS, AND INTRACAVERNOUS PRESSURE INCREASES IN THE RAT

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Abstract
α-Melanocyte stimulating hormone (α-MSH; Fluka Chemie AG, Geneva, Switzerland) and oxytocin induce erection in rats after intracerebroventricular administration. We studied possible interactions of α-melanocyte stimulating hormone with mechanisms pertaining to oxytocin or nitric oxide. We used 78 anesthetized male Sprague-Dawley rats. Catheters were implanted in the lateral cerebral ventricle or into the subarachnoid space at L6 to S1. Intracavernous pressure was documented and arterial blood pressure was directly measured. Intracerebroventricular α-MSH (3 μg.) produced a mean of 2.6 ± 0.6 erectile responses (p <0.05) with a mean duration of 3.4 ± 1.1 minutes (p <0.05). Mean peak intracavernous pressure was 114 ± 8 cm. water. An intracerebroventricular dose of 100 μg. N-nitro-L-arginine-methyl ester HCl (Sigma Chemical Co., St. Louis, Missouri) given in intracerebroventricular fashion abolished α-MSH induced erectile responses, whereas intracerebroventricular administration of 500 ng. of the oxytocin receptor antagonist l-deamino, 2-D-Tyr(Oet), 4-Thr, 8-Orn-OT (Ferring AB, Malmö, Sweden) had no effect. Intracerebroventricular oxytocin (30 ng.) induced a mean of 3.2 ± 0.9 erectile responses (p <0.05) with a mean peak intracavernous pressure of 81 ± 8 cm. water and a mean duration of 3.3 ± 1.1 minutes. Intrathecal α-MSH (3 μg.) did not produce any erectile responses, whereas a mean of 5.7 ± 0.9 responses (p <0.001) with a mean peak intracavernous pressure of 142 ± 8 cm. water and mean duration of 5.0 ± 1.3 minutes was obtained with 30 ng. oxytocin intrathecally. Responses induced by intrathecal oxytocin were abolished by 100 μg. N-nitro-L-arginine-methyl ester HCl intrathecally. We confirmed by monitoring intracavernous pressure and blood pressure that supraspinal erectile responses induced by α-melanocyte stimulating hormone involve effects mediated by nitric oxide but are independent of oxytocinergic mechanisms. At the spinal level oxytocin produces erectile responses involving nitric oxide. α-Melanocyte stimulating hormone does not seem to have a spinal site of action.

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