ANTINOCICEPTION PRODUCED BY MICROINJECTION OF MORPHINE IN THE RAT PERIAQUEDUCTAL GRAY IS ENHANCED IN THE FOOT, BUT NOT THE TAIL, BY INTRATHECAL INJECTION OF ALPHA-1-ADRENOCEPTOR ANTAGONISTS

Antinociception produced by microinjection of morphine in the ventrola teral periaqueductal gray is mediated in part by alpha(2)-adrenoceptor s in the spinal cord dorsal horn. However, several recent reports demo nstrate that microinjection of morphine in the ventrolateral periaqued uctal gray inhibits nociceptive responses to noxious heating of the ta il by activating descending neuronal systems that are different from t hose that inhibit the nociceptive responses to noxious heating of the feet. More specifically, alpha(2)-adrenoceptors appear to mediate the antinociception produced by morphine using the tail-flick test, but no t that using the foot-withdrawal or hot-plate tests. The present study extended these findings and determined the role of alpha(1)-adrenocep tors in mediating the antinociceptive effects of morphine microinjecte d into the ventrolateral periaqueductal gray using both the foot-withd rawal and the tail-flick responses to noxious radiant heating in light ly anesthetized rats. Intrathecal injection of selective antagonists w as used to determine whether the antinociceptive effects of morphine w ere modulated by alpha(1)-adrenoceptors. Injection of the selective al pha(1)-adrenoceptor antagonists prazosin or WB4101 potentiated the inc rease in the foot-withdrawal response latency produced by microinjecti on of morphine in the ventrolateral periaqueductal gray. In contrast, either prazosin or WB4101 partially reversed the increase in the tail- flick response latency produced by morphine. These results indicate th at microinjection of morphine in the ventrolateral periaqueductal gray modulates nociceptive responses to noxious heating of the feet by act ivating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the tail. Mor e specifically, alpha(1)-adrenoceptors mediate a pro-nociceptive actio n of morphine using the foot-withdrawal response, but in contrast, alp ha(1)-adrenoceptors appear to mediate part of the antinociceptive effe ct of morphine determined using the tail-flick test. (C) 1998 Elsevier Science B.V.