Stimulation of spinal dopaminergic receptors: differential effects on tail reflexes in rats

The tail-flick reflex to heat and tail-withdrawal reflex to touch were measured in spinal and intact rats given either apomorphine (0.02-1.75 mumol.kg-1) or dopamine (0.02-1.76 mumol.kg-1) in the lumbar subarachnoid space. In spinal rats both apomorphine and dopamine suppressed thermal tail-flick reflex and enhanced tactile tail-withdrawal response in a dose-dependent way. The effect of apomorphine in spinal rats was counteracted by dopaminergic receptor antagonists (cis-flupenthixol and (+)-butaclamol), but not by their stereoisomers. Phenoxybenzamine, propranolol, methysergide and naloxone failed to counteract the effects of apomorphine on tail-reflex responses in spinal rats. (+)-Butaclamol also counteracted effects of dopamine in spinal rats. Neither apomorphine nor dopamine influenced tail reflexes in intact rats, which suggests that effects of spinal dopaminergic mechanisms on these reflexes are influenced by descending supraspinal pathways.