Potentiation of the aphrodisiac effect of N-n-propyl-norapomorphine by naloxone.

N-n-Propyl-norapomorphine (NPA), a potent dopamine (DA) receptor stimulant, in doses from 0.4 to 80 micrograms/kg i.p. produced a dose-related sexual stimulant effect characterized by recurrent episodes of penile erection (PE). The number of episodes and percentage of responding subjects were proportional to the dose. However, above the maximal effective dose, the effect decreased in a dose-related fashion until beyond 2.5 mg/kg even the natural occurrence of PE was suppressed. Morphine (5 mg/kg), as well as haloperidol (0.3 mg/kg), prevented NPA stimulation. Naloxone, which per se caused a modest increase in PE, markedly potentiated the stimulant effect of low doses of NPA, reversing the inhibitory component of higher doses. We suggest that NPA stimulation of DNA receptors causes release of opiate peptides, dampening the sexual stimulant response. The combination of DA receptor stimulants with naloxone might offer a new possibility for erection defect therapy.