Antinociceptive effects of the stereoisomers of nicotine given intrathecally in spinal rats.

Spinalized rats received an intrathecal injection of either (-)-nicotine or (+)-nicotine in order to study the stereoselectivity of antinociception. Pain threshold was measured using the tail-flick test. Both stereoisomers had anti-nociceptive effects, but (-)-nicotine was up to 970 times more potent, depending on test conditions. The antinociceptive action of (-)-nicotine was antagonized by mecamylamine and yohimbine but not by naloxone and atropine. The findings show that spinal mechanisms are highly stereoselective toward nicotine, and suggest that primarily nicotinergic and alpha-adrenergic receptors are involved in its central antinociceptive effects.