Differential actions of the blockade of spinal opioid, adrenergic and serotonergic receptors on the tail-flick inhibition induced by morphine microinjected into dorsal raphe and central gray in rats

Microinjection of morphine sulfate into dorsal raphe, ventrolateral central gray and dorsolateral central gray inhibits spinal nociceptive reflexes. The effects of the blockade of spinal opioid, adrenergic, and serotonergic receptors by intrathecal injection of naloxone, yohimbine and methysergide, respectively, on inhibition of the tail-flick response induced by morphine microinjected into dorsal raphe, ventrolateral central gray and dorsolateral central gray were studied. Naloxone (20 μg) given intrathecally effectively antagonized inhibition of the tail-flick response induced by morphine (4 μg) given into dorsal raphe and ventrolateral central gray, but not dorsolateral central gray. On the other hand, intrathecal injection of yohimbine (30 μg) antagonized inhibition of the tail-flick response induced by morphine given into ventrolateral central gray and dorsolateral central gray, but not dorsal raphe. Intrathecal injection of prazosin (30 μg) did not antagonize inhibition of the tail-flick response induced by morphine given into dorsal raphe or lateral central gray. Intrathecal injection of methysergide (30 μg) only partially antagonized inhibition of the tail-flick response induced by morphine given into dorsal raphe, but not ventrolateral central gray and dorsolateral central gray. It is concluded that the analgesia induced by morphine injected into dorsal raphe is mediated by spinal opioid receptors but not by spinalα 2-adrenergic receptors while the analgesia produced by morphine given into dorsolateral central gray is mediated by spinalα 2-adrenergic receptors. The analgesia induced by morphine given into ventrolateral central gray is mediated in part by both spinalα 2-adrenergic and opioid receptors. Spinal α 1-adrenergic receptors are not involved in morphine-induced analgesia given into dorsal raphe, ventrolateral central gray or dorsolateral central gray. Spinal serotonergic receptors only play a minor role in dorsal raphe and lateral central gray morphine-induced analgesia.