A mode of action of morphine on a quickly learned conditioned suppression in mice.

Mice exhibited marked suppression of motor activity when placed in the same environment where they had previously received electric shocks. Morphine-HCl (20 mg/kg) markedly attenuated such a conditioned suppression in mice. The morphine-induced reduction of the conditioned suppression was antagonized by pretreatment with reserpine (0.1 and 0.5 mg/kg), haloperiodol, pimozide and alpha-methyl-p-tyrosine, but not atropine-sulfate, p-chlorophenylalanine, phenoxybenzamine-HCl or propranolol. Thus, it is possible that the potentiation of the dopaminergic neurotransmission in the brain is involved in the morphine-induced reduction of the conditioned suppression in mice.