Spinal cord alpha-2 noradrenergic receptors mediate conditioned analgesia

The present experiment investigated the effects of direct spinal administration of the monoaminergic receptor blockers yohimbine, phentolamine and methysergide on the expression of conditioned analgesia. Animals in the Paired group received classical conditioning trials in which one context was paired with footshock administration (1 mA shock for 15 s). Animals in the Unpaired control group were administered shock in a second, different, context. On the test day animals within each condition were administered saline (20 microliters), yohimbine (30 micrograms), phentolamine (30 micrograms), or methysergide (30 micrograms) prior to receiving a hot plate test (50 degrees C) in the context previously used to shock the Paired group. These ligands were administered into the spinal fluid through a chronic, indwelling spinal catheter. Animals in the Paired group which received saline displayed longer paw lick latencies than saline-treated animals in the Unpaired group. Yohimbine, but not phentolamine or methysergide, attenuated this conditioned analgesia. These results suggest that spinal cord noradrenergic substrates mediate conditioned analgesia, and that this mediation occurs specifically through the alpha-2 noradrenergic receptor.