Abstract
Rats were trained to initiate and terminate lateral hypothalamic or dorsal raphe stimulation. The effects of various doses of morphine on self-stimulation (SS) and stimulation escape (SE) were assessed for 7 h/day. The drug produced a strong attenuation of SE at both placements. This effect did not show tolerance with repeated injections of morphine. Similarly, SS was depressed for 1–2 h following drug injection. This effect did tolerate with chronic treatment. A drug-produced facilitation of SS was observed in some subjects, although the magnitude of this effect varied across animals. The data are interpreted to suggest that morphine acts on both the substrate for SS and the substrate for SE, but in opposite directions.
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