6-Hydroxydopamine lesions of the ventral tegmentum abolishd-amphetamine and morphine analgesia in the formalin test but not in the tail flick test

The effect of bilateral 6-hydroxdopamine (with desipramine) lesions of the ventral tegmental area-substantia nigra region on analgesia produced by morphine andd-amphetamine was examined with the formalin and tail flick tests in rats. These lesions depleted dopamine, but not noradrenaline, in the ventral and anterior striatum. Morphine anddamphetamine produced analgesia in the formalin test in sham lesioned rats but not in lesioned rats. In contrast, morphine produced the same degree of analgesia in the tail flick test in sham lesioned and lesioned rats whiled-amphetamine did not produce analgesia in this test. These data suggest that morphine andd-amphetamine analgesia in the formalin test involves dopamine, whereas morphine analgesia in the tail flick test does not.