Abstract
Morphine at 5, 10 and 20 mg/kg intraperitoneally caused a dose-dependent, naloxone-reversible, antagonism of d-amphetamine-induced ipsilateral circling in rats lesioned with 6-hydroxydopamine in one substantia nigra. Morphine (10 mg/kg) weakly antagonized apomorphine-induced contralateral circling. Morphine, levorphanol and phenazocine were potent antagonists of d-amphetamine-induced circling in rats with a 6-hydroxydopamine lesion of one neostriatum. Pentazocine, pethidine and ethyl-ketocyclazocine were weak antagonists. It is proposed that a major action of morphine and some closely related opiate analgesics is to reduce the release of dopamine from nigro-striatal nerve terminals in the rat by acting upon specific opiate receptors.
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