Apparent pA2 estimates for an antinociceptive receptor.
The involvement of alpha 2-adrenoceptors in antinociception was investigated and apparent pA2 values determined using the tail flick technique in mice. Among various alpha 2-adrenoceptor agonists employed (viz. clonidine, guanfacine and B-HT 920) only clonidine exhibited antinociception in a dose-dependent manner, an effect analogous to morphine. B-HT 920 and guanfacine failed to show increased antinociception, with corresponding increase in concentration. Yohimbine (1.28 X 10(-8) -2.56 X 10(-7) M/kg) and naloxone (2.75 X 10(-8) -2.2 X 10(-7) M/kg) antagonized the antinociceptive effect of clonidine and morphine, respectively. Higher concentration of yohimbine was required to block the action of morphine. However, naloxone (2.75 X 10(-7) M/kg) did not block the antinociceptive effect of clonidine. In vivo equivalents of pA2 values for morphine-naloxone and clonidine-yohimbine pairs were calculated and found to be 7.09 and 7.45, respectively. The probable involvement of a common antinociceptive site for the observed phenomenon is discussed.
Parale
M P
MP
Kulkarni
S K
SK
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D008297:Male; D051379:Mice; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009619:Nociceptors / Q000187:drug effects*; D011930:Reaction Time / Q000187:drug effects; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D009270:Naloxone; D009020:Morphine; D003000:Clonidine