The central nucleus of the amygdala contributes to the production of morphine antinociception in the rat tail-flick test

Abstract

Current models of endogenous pain control circuitry emphasize neural substrates within the brainstem and spinal cord. We have recently shown, however, that the central nucleus of the amygdala (Ce) contributes to morphine-induced suppression of formalin-induced nociceptive behaviors. In the four experiments reported here, we investigated the possibility that the Ce also contributes to morphine-induced suppression of simple, spinally mediated nociceptive reflexes. Bilateral N-methyl-D-aspartate (NMDA)-induced lesions of the rat Ce, but not bilateral lesions centered on either the basolateral or medial amygdaloid nucleus, abolished the antinociception produced by 2.5 mg/kg morphine sulfate in the noxious heat-evoked tail-flick test. Bilateral Ce lesions also abolished the antinociception produced by 2 or 4 mg/kg morphine sulfate, but a relatively large dose of morphine sulfate (10 mg/kg, s.c.) resulted in partial reinstatement of antinociception. It is unlikely that these effects were due to secondary, seizure-induced damage following NMDA injection (e.g., to areas outside the amygdala) since bilateral inactivation of the Ce with the local anesthetic lidocaine also reliably attenuated morphine antinociception. It is also unlikely that these effects were artifacts of lesion-induced hyperalgesia, since Ce lesions failed to result in reliable thermal hyperalgesia, even at baseline tail-flick latencies of 10-12 sec. These data are the first to provide direct evidence that systemically administered morphine requires the integrity of a forebrain area in order to suppress spinally mediated nociceptive reflexes. It is argued that the present results, together with recent evidence linking the Ce to the production of several forms of conditioned and unconditioned environmentally induced antinociception, warrant incorporation of the Ce into current models of endogenous pain control circuitry.