Absence of antinociceptive effect of alpha-2 agonists microinjected in the periaqueductal gray of the rat.

The antinociceptive efficacy of clonidine, St-91, tolazoline, phenylephrine or isoproterenol microinjected in the periaqueductal gray (PAG) in the rat was compared to that of morphine administered in the same site. Morphine (5 micrograms) was antinociceptive in all 3 analgesiometric tests employed (tail-flick, hot-plate and tail-shock vocalization). Clonidine (5-20 micrograms) did not significantly alter nociceptive thresholds, while St-91 (20 micrograms) produced a brief hyperalgesia in the tail-flick test. The alpha-2 antagonist tolazoline (20 micrograms) produced a brief antinociception in the tail-flick test. Phenylephrine and isoproterenol were without effect on nociceptive thresholds. It is suggested that the ventrolateral PAG, although a site of opiate-produced antinociception, is not involved in alpha-2-mediated antinociception.