The stimulus properties of morphine and ethanol.

The present investigation sought (a) to establish the efficacy of morphine and ethanol as discriminative stimuli when each is paired with the administration of saline and (b) to compare, in a qualitative sense, the stimulus properties of the two drugs. Additional experiments examined the effects of treatment with naloxone or l-propranolol upon morphine and ethanol-mediated discriminated responding. Finally, the stereospecificity of the stimuli produced by morphine was determined by a comparison, in morphine-trained rats, of levorphanol and dextrorphan. Discriminated responding developed rapidly in both the morphine and ethanol groups. In tests in which ethanol was administered to morphine-trained animals and vice versa, no similarity to stimulus properties was apparent. Antagonism of discriminated responding induced by morphine and ethanol was attempted using naloxone and l-propranolol. Naloxone blocked the actions of morphine but was without effect upon ethanol. No evidence of antagonism of either drug by propranolol was found. When a range of doses of levorphanol (0.1-3 mg/kg) and dextrorphan (3-100 mg/kg) was tested in morphine trained animals, only levorphanol was able to substitute for morphine. The present results suggest that the stimulus properties of morphine represent typical opiate effects.