Differential effects of apomorphine on spinal reflex activity following 6-hydroxydopamine or long-term haloperidol pretreatment.

The thermal cutaneous evoked tail flick response was measured both before and after intrathecal injection of R-apomorphine in spinal rats and in rats with an intact neuraxis pretreated with either long-term haloperidol or intrathecal 6-hydroxydopamine. Intrathecal R-apomorphine produced a dose-dependent increase of tail flick latency in spinal rats but not in intact rats. Long-term haloperidol treatment increased the effect of R-apomorphine in spinal rats while 6-OHDA decreased it. The present findings provide further evidence for the modulatory role of spinal DA receptor populations on spinal reflex activity. Pretreatment with long-term haloperidol or 6-OHDA may alter the conformational status of spinal DA receptors in opposite directions and thereby change the responsiveness of receptors to R-apomorphine.