title: Absence of antinociceptive effect of alpha-2 agonists microinjected in the periaqueductal gray of the rat.
abstract: 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.
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000031:analogs & derivatives / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009619:Nociceptors / Q000187:drug effects*; D010487:Periaqueductal Gray / Q000187:drug effects*; D010656:Phenylephrine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D009435:Synaptic Transmission / Q000187:drug effects*; D014043:Tolazoline / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D010656:Phenylephrine; C011266:ST 91; D009020:Morphine; D014043:Tolazoline; D007545:Isoproterenol; D003000:Clonidine