Associative Factors in the Effects of Morphine on Self-stimulation

These experiments tested the hypothesis that the suppressing and facilitating effects of morphine on intracranial self-stimulation (ICS) (measured 1 h and 3 h post-injection, respectively) are influenced by associative, non-pharmacological factors. Experiment 1 confirmed previous demonstrations that the facilitation of ICS by morphine (10 mg/kg) develops with repeated drug exposures. Once ICS facilitation had developed, the effect was mimicked by saline injection in most subjects. In a separate group of animals, previous exposure to morphine in the home cage prevented drug-induced facilitation of ICS. Tolerance to ICS suppression developed after repeated pairings of the drug and the ICS chambers, but not when the drug had previously been received in the home cage. Experiment 2 examined the effect of low (0.3, 1, 3 mg/kg) doses of chronic morphine on ICS. Facilitation was observed with 1 and 3 mg/kg, but only after repeated testing. Naloxone (0.1 and 1 mg/kg) failed to reverse facilitation in a number of these subjects. In Experiment 3, animals receiving their daily injections of morphine were allowed to self-stimulate only at 3 h post-injection (when ICS facilitation is usually maximal), rather than at 1 h and 3 h post-injection. ICS facilitation was not observed, even with repeated testing. These data indicate that the facilitation of ICS by morphine is the outcome of a learned association between drug administration and the ICS procedure, rather than the invariable result of opiate receptor activation. Repeated exposure to morphine is required for the initial establishment of ICS enhancement, but the subsequent expression of this behavior is not directly related to opiate receptor activity.