Dopamine receptor-mediated spinal antinociception in the normal and haloperidol pretreated rat: effects of sulpiride and SCH 23390.

Nociceptive tail flick latencies (TFL) were recorded in response to noxious thermal stimuli applied to lightly anaesthetized rats. The effects of intrathecally administered dopamine receptor agonists alone and combined with dopamine receptor antagonists were examined upon the TFL. Experiments were repeated on animals made supersensitive to dopamine following withdrawal from 28 day administration of haloperidol. In untreated animals the D2-receptor agonist LY 171555 and apomorphine produced an increase in TFL. In contrast, the Di-receptor agonist SKF 38393 had no significant effect on TFL. TFL. Following haloperidol-induced dopamine-supersensitivity, SKF 38393 produced an increase in TFL. In contrast, LY171555 and apomorphine had minimal effects on TFL in this preparation. In animals not treated with haloperidol, the dopamine receptor antagonists SCH 23390 and (+/-)-sulpiride both blocked the increase in TFL produced by the D2-agonists. SCH23390 and (+/-)-sulpiride also blocked the increase in TFL produced by SKF 38393 in haloperidol-supersensitized animals. The antinociceptive action of intrathecally administered dopamine agonists appears to be mediated via D2-receptors. Whether the antinociception produced by SKF 38393 is exclusively contingent upon the activation of D1-receptors in the dopamine-supersensitive animal is as yet unresolved.