Stereoselectivity of spinal neurotransmission: effects of baclofen enantiomer on tail-flick reflex in rats.

Spinal rats and rats with an intact neuraxis received an intrathecal injection of an enantiomer of baclofen. The R-enantiomer was 100-1000 times more potent than its antipode in inhibiting the tail-flick reflex, both in intact rats and in spinal rats. Spinalization enhanced the inhibitory effects of both enantiomers without altering their dose-response relations. The findings show that baclofen enantiomers have direct actions on stereo-selective spinal mechanisms and that spinalization fails to alter the stereo-selectivity of spinal mechanisms towards the enantiomers.