Alpha-2-adrenergic receptors in avian spinal cord: increases in apparent density associated with the sympathetic preganglionic cell column.

Vertebrate spinal cord receives a dense and diversified catecholaminergic innervation from brainstem and diencephalon. Within the spinal gray, the densest terminations appear to be within the neuropil surrounding sympathetic preganglionic neurons (SPNs) in thoracic spinal cord. Results of recent iontophoresis investigations showed that several catecholamines and clonidine, an alpha-2 agonist, uniformly inhibited the maintained discharge activity of SPNs [19]. These experiments raised the possibility that the inhibitory effects might be mediated by activation of an alpha-2-adrenergic receptor. The present series of ligand binding studies provide biochemical evidence suggesting the presence of alpha-2-adrenergic receptors in the SPN cell column. Total specific binding (Bmax) of the radiolabeled agonists clonidine (CLO) and para-amino-clonidine (PAC) (at concentrations above and below apparent KDS) was significantly greater in thoracic spinal cord in comparison with cervical spinal cord (P less than 0.001). The elevated levels in thoracic spinal cord were entirely accounted for by increases in apparent receptor density in dorsal horn and the SPN cell column (inclusive of the adjoining intermediate spinal laminae) (P less than 0.005). Adrenergic receptor subtype specificity of [3H]PAC was tested in competitive inhibition experiments. The results confirmed that [3H]PAC is a preferential alpha-2 agonist in thoracic and cervical spinal cord, and indicated the following rank order of potency for its displacement: norepinephrine = yohimbine much greater than prazosin greater than propranolol.