Abstract
Rats were fixed with chronically indwelling bipolar electrodes. Subsequently, they pressed for direct electrical stimulation of the lateral hypothalamus. Then thresholds for the positive intracranial reinforcement were measured daily for 20 days while rats were under the influence of morphine injections (10 mg/Kg given 3.5 h prior to testing). Morphine reduced thresholds after the first few days of dosing and the reductions, once seen, did not wane with further dosing. Furthermore, thresholds were reduced by a 10-mg/kg dose of morphine following a regimen of dosing with morphine that produced physical dependence. It was concluded that morphine reduced thresholds for positive intracranial reinforcement and this capability was not changed by assimilation of relatively large quantities of morphine, that is, the effect did not show tolerance.
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