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.

ABSTRACT

The alpha2-adrenergic receptor antagonists, yohimbine, atipamezole and tolazoline, are used in veterinary medicine as reversal agents for the sedative/hypnotic effects of alpha2-agonists. Ruminants have increased sensitivity to the sedative/hypnotic effects of alpha2-agonists compared to other species. The receptors mediating the sedative effects of alpha2-agonists are located primarily on locus coeruleus neurons in the pons of the lower brainstem. Four pharmacological subtypes of the alpha2-adrenergic receptor (A,B, C and D) have been identified based on differences in ligand affinity. The aim of this study was to 1) determine the pharmacological profile of atipamezole, yohimbine and tolazoline at the four alpha2-adrenergic receptor subtypes and; 2) determine whether these agents differ in their affinities at the alpha2-adrenergic receptor present in the sheep brainstem. In inhibition binding studies against the selective alpha2-adrenergic receptor ligand [3H]-MK-912, tolazoline showed the lowest affinity for all four alpha2-adrenergic receptor subtypes compared to yohimbine and atipamezole. The affinities of yohimbine and atipamezole were similar at the alpha2A-, alpha2B- and alpha2C-adrenergic receptors but differed by approximately 100 fold at the alpha2D-adrenergic receptor. Atipamezole had a 100 fold higher affinity at the alpha2D-adrenergic receptor when compared to yohimbine. To determine the ligand binding characteristics of these agents at the alpha2-adrenergic receptor in sheep brainstem, membranes were labelled with [3H]-MK-912 and inhibition competition curves were performed. Atipamezole showed approximately a 100 fold higher affinity for the sheep brainstem alpha2-adrenergic receptor compared to yohimbine which was similar to what was observed for the alpha2D-adrenergic receptor in PC12 cells transfected with RG-20. The results from these studies suggest that atipamezole has a high affinity for the alpha2D-adrenergic receptor that appears to be the receptor subtype in sheep brainstem.