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.