Autoradiographic localisation of alpha 2 adrenoceptor binding sites in the spinal cord of the sheep.

Alpha 2 adrenoceptor agonists, such as xylazine and detomidine, are used as sedatives, analgesics and premedicants. In this study the localisation of binding sites for alpha 2 agonist drugs in the sheep spinal cord was examined using [3H] clonidine as a ligand and autoradiographic techniques related to our initial histological mapping of the various tracts and laminae. The results indicated a high concentration of binding sites for alpha 2 adrenoceptor agonists in the lamina II region of the dorsal horn of the spinal cord (the substantia gelatinosa) which is an area widely implicated in the transmission of painful stimuli. Possibly therefore some of the analgesic effect of this group of drugs in sheep is mediated at the spinal level.