EP 60761- and EP 50885-induced penile erection: structure–activity studies and comparison with apomorphine, oxytocin and N-methyl-D-aspartic acid

Abstract

The effect of 10 peptides structurally related to the growth hormone (GH) releasing peptide hexarelin, injected into the paraventricular nucleus of the hypothalamus (PVN), on penile erection was studied in male rats. Six out of the 10 peptides tested induced penile erection in a dose-dependent manner. Among them, the most potent were EP 80661, EP 60761 and EP 91072, which were active at doses of 20–200 ng. The potency of these peptides in inducing penile erection is comparable to that of apomorphine, oxytocin and N-methyl-D-aspartic acid similarly injected into the PVN. Other peptides found active were EP 50885, EP 90101 and EP 91071, which induced penile erection at doses of 200–2000 ng. In contrast, EP 51322, EP 70555, EP 51216 and EP 91073 were inactive, as were hexarelin, EP 40904 and EP 40737 in a previous study. The majority of EP peptides found active when injected into the PVN induced penile erection, although to a lesser extent, also when given systemically (endovenously). The proerectile effect of EP peptides was prevented by the oxytocin receptor antagonist [d(CH₂)₅ Tyr(Me)²-Orn⁸]-vasotocin given into the lateral ventricles but not into the PVN, by the nitric oxide (NO) synthase inhibitor N^G-nitro-l-arginine methyl ester given either into the lateral ventricles or into the PVN, by the N-type Ca²⁺ channel blocker ω-conotoxin GVIA and by morphine, but not by the dopamine receptor antagonist cis-flupenthixol or by the N-methyl- D-aspartic acid receptor antagonist dizolcipine, given into the PVN. As the structure–activity relationship of EP peptides for proerectile activity is different from those of other biological actions of these compounds, ie for GH release and eating behaviour, the present results suggest that EP peptides induce penile erection by acting on specific hypothalamic receptor sites that activate paraventricular oxytocinergic neurons projecting to extrahypothalamic brain areas that mediate this sexual function by a mechanism similar to that of dopamine receptor agonists, oxytocin and N-methyl- D-aspartic acid.