SPINAL CHOLINERGIC AND MONOAMINE RECEPTORS MEDIATE THE ANTINOCICEPTIVE EFFECT OF MORPHINE MICROINJECTED IN THE PERIAQUEDUCTAL GRAY ON THE RAT TAIL, BUT NOT THE FEET

The antinociceptive effects of morphine (5 mu g) microinjected into th e ventrolateral periaqueductal gray were determined using both the tai l flick and the foot withdrawal responses to noxious radiant heating i n lightly anesthetized rats. Intrathecal injection of appropriate anta gonists was used to determine whether the antinociceptive effects of m orphine were mediated by alpha(2)-noradrenergic, serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdra wal response latency produced by microinjection of morphine in the ven trolateral periaqueductal gray was reversed by intrathecal injection o f the cholinergic muscarinic receptor antagonist atropine, but was not affected by the alpha(2)-adrenoceptor antagonist yohimbine, the serot onergic receptor antagonist methysergide, or the opioid receptor antag onist naloxone. Ln contrast, the increase in the tail flick response l atency produced by morphine was reduced by either yohimbine, methyserg ide or atropine. These results indicate that microinjection df morphin e in the ventrolateral periaqueductal gray inhibits nociceptive respon ses to noxious heating of the tail by activating descending neuronal s ystems that are different from those that inhibit the nociceptive resp onses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic and alpha(2)-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptor s appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.