Dual action of morphine and related drugs on compulsive gnawing of rats

Independent effects of morphine and apomorphine on stereotyped gnawing in the hamster

Eight experiments were conducted to investigate the effects of apomorphine, a dopaminergic agonist, and of morphine, an opiate, on stereotyped behavior in the hamster. Animals were observed at two minute intervals for one hour and incidents of stereotyped gnawing, licking and sniffing were recorded using a time-sampling method. Both morphine and apomorphine produced dose-related increases in stereotyped gnawing. A low dose of the opiate antagonist, naloxone (0.4 mg/kg), blocked morphine-induced gnawing but neither that dose nor higher doses of naloxone (1, 4 and 10 mg/kg) blocked apomorphine-induced gnawing. A low dose of the dopaminergic antagonist, haloperidol (0.05 mg/kg), blocked apomorphine-induced gnawing but did not block morphine-induced gnawing. Further experiments indicated that morphine administration did not sensitize, or influence in any way, subsequent apomorphine-induced stereotyped behavior.

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.