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Stimulation-produced analgesia: development of tolerance and cross-tolerance to morphine.

Analgesia resulting from focal electrical stimulation of the brain of the rat shows tolerance with repeated exposures; this tolerance dissipates after a period of nonstimulation. Addiction to morphine reduces greatly the analgesia produced by electrical stimulation of the brain, which demonstrates cross-tolerance between morphine analgesia and stimulation-produced analgesia. Recovery of the stimulation-produced analgesia is seen after discontinuing administration of morphine. These results suggest that morphine and electrical stimulation produce analgesia by common mechanisms. The fact that tolerance occurs to the analgesic effect of electrical stimulation indicates that tolerance may be an alteration of an endogenous neuronal process.