Blockade of tolerance to morphine analgesia by cocaine.

Tolerance to morphine analgesia was induced in male Sprague-Dawley rats by s.c. implantation of a morphine base pellet (75 mg) on the first and second day and determining the magnitude of tolerance 72 h after the first implant by s.

Stereospecific potentiation of opiate analgesia by cocaine: predominant role of noradrenaline.

Cocaine hydrochloride (50 mg) pellets implanted subcutaneously in male Wistar rats potentiated the analgesia of morphine, levorphanol, methadone and buprenorphine as measured by the tail-withdrawal test. Potentiated opiate analgesia was abolished by naloxone and further enhanced by desipramine and phenoxybenzamine. Yohimbine, alpha-methyl p-tyrosine, haloperidol, zimelidine, methysergide, p-chlorophenylalanine produced no significant effect on potentiated opiate analgesia.