Morphine and [D-Met2,Pro5]enkephalinamide do not show specific neuroleptic activity.

Several conventionally used in vivo pharmacological assays were applied to examine whether morphine (M) and a potent enkephalin analogue, [D-Met2,Pro5]enkephalinamide (DMPEA) have haloperidol (H)-like neuroleptic activity. The apomorphine (A)-induced stereotypy and the conditioned reflex activity were inhibited by extremely low doses of H, while somewhat higher doses were needed to induce catalepsy or to suppress the A-elicited turning behaviour in rats with unilateral nigral lesion. M produced these effects only in doses higher than needed for analgesia. DMPEA, however, attenuated the A-elicited stereotypy already at a subanalgesic dose level but it was very weak in the other tests. Furthermore, neither M nor DMPEA inhibited the A-elicited stereotypy completely. Consequently, these drugs exhibit strikingly dissimilar relative potencies in the in vivo assays considered specific for neuroleptics. Our findings, in accordance with much of the data available, suggest that neither M nor DMPEA has a specific neuroleptic activity.