Brain research
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Antinociception produced by microinjection of morphine in the ventrolateral periaqueductal gray is mediated in part by alpha2-adrenoceptors in the spinal cord dorsal horn. However, several recent reports demonstrate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, alpha2-adrenoceptors appear to mediate the antinociception produced by morphine using the tail-flick test, but not that using the foot-withdrawal or hot-plate tests. ⋯ In contrast, either prazosin or WB4101 partially reversed the increase in the tail-flick response latency produced by morphine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray modulates nociceptive responses to noxious heating of the feet by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the tail. More specifically, alpha1-adrenoceptors mediate a pro-nociceptive action of morphine using the foot-withdrawal response, but in contrast, alpha1-adrenoceptors appear to mediate part of the antinociceptive effect of morphine determined using the tail-flick test.