SELECTIVITY OF ATIPAMEZOLE, YOHIMBINE AND TOLAZOLINE FOR ALPHA-2-ADRENERGIC RECEPTOR SUBTYPES - IMPLICATIONS FOR CLINICAL REVERSAL OF ALPHA-2-ADRENERGIC RECEPTOR-MEDIATED SEDATION IN SHEEP

The alpha(2)-adrenergic receptor antagonists, yohimbine, atipamezole a nd tolazoline, are used in veterinary medicine as reversal agents for the sedative/hypnotic effects of alpha(2)-agonists. Ruminants have inc reased sensitivity to the sedative/ hypnotic effects of alpha(2)-agoni sts compared to other species. The receptors mediating the sedative ef fects of alpha(2)-agonsts are located primarily on locus coeruleus neu rons in the pens of the lower brainstem, Four pharmacological subtypes of the alpha(2)-adrenergic receptor (A, B, C and D) have been identif ied based on differences in ligand affinity. The aim of this study was to: 1) determine the pharmacological profile of atipamezole, yohimbin e and tolazoline at the four alpha(2)-adrenergic receptor subtypes and : 2) determine whether these agents differ in their affinities at the alpha(2)-adrenergic receptor present in the sheep brainstem. In inhibi tion binding studies against the selective alpha(2)-adrenergic recepto r ligand [H-3]-MX-912, tolazoline showed the lowest affinity for all f our alpha(2)-adrenergic receptor subtypes compared to yohimbine and at ipamezole, The affinities of yohimbine and atipamezole were similar at the alpha(2A)-, alpha(2B)- and alpha(2C)-adrenergic receptors but dif fered by approximately 100 fold at the alpha(2D)-adrenergic receptor. Atipamezole had a 100 fold higher affinity at the alpha(2D)-adrenergic receptor when compared to yohimbine, To determine the ligand binding characteristics of these agents at the alpha(2)-adrenergic receptor in sheep brainstem, membranes were labelled with [H-3]-RK-912, and inhib ition competition curves were performed. Atipamezole showed approximat ely a 100 fold higher affinity for the sheep brainstem alpha(2)-adrene rgic receptor compared to yohimbine which was similar to what was obse rved for the alpha(2D)-adrenergic receptor in PC12 cells transfected w ith RG-20, The results from these studies suggest that atipamezole has a high affinity for the alpha(2D)-adrenergic receptor that appears to be the receptor subtype in sheep brainstem.