Striatal cholinergic activity during morphine-induced catalepsy in the rat: development of tolerance

Morphine hydrochloride (10 mg/kg,i.p.) induced marked catalepsy in rats, which was associated with significantly enhanced striatal acetylcholine levels and reduced cholinesterase activity. Rapid tolerance to morphine-induced catalepsy was noted after twice daily administrations, the tolerance being discernible after the third injection and was complete after the eleventh injection. With the development of tolerance, morphine-induced effects on striatal acetylcholine and cholinesterase were progressively reversed, confirming striatal cholinergic involvement in morphine-induced catalepsy in the rat.