CHRONIC ADMINISTRATION OF AN ALPHA(2) ADRENERGIC AGONIST DESENSITIZESRATS TO THE ANESTHETIC EFFECTS OF DEXMEDETOMIDINE

alpha(2) adrenergic agonists are being administered perioperatively to facilitate the anesthetic management of the surgical patient. In some clinical settings, use of alpha(2) adrenergic agonists has been exten ded into the postoperative period to prolong the patients' sedative an d stress-free state. We studied whether the administration of alpha(2) adrenergic agonists over an extended period of time would result in ' 'desensitization'' to the central actions of alpha(2) adrenergic agoni sts. Male Sprague-Dawley rats were administered dexmedetomidine, a hig hly selective alpha(2) adrenergic agonist, at rates varying between 1 and 10 mu g.kg(-1).h(-1) via a chronically implanted SC osmotic pump. Spontaneous locomotor activity, tested in an open-field box, was signi ficantly lower in both 3- and 10-mu g.kg(-1).h(-1) treatment groups bu t returned to normal by the second or sixth day, respectively. The hyp notic response to dexmedetomidine IP was decreased in the 10-mu g.kg(- 1).h(-1) dose group from the second day, and by the fourth day in the 3-mu g.kg(-1).h(-1) group. Recovery from the desensitized state was ra pid and occurred on the third day after pump removal in the 3-mu g.kg( -1).h(-1) group and by the fifth day after pump removal in the 10-mu g .kg(-1).h(-1) dose group. By using a higher dose of dexmedetomidine IP (250 mu g.kg in lieu of 100 mu g/kg) at day 7 in ''tolerant'' rats, t he hypnotic response could partially be ''restored'' towards normal. A n attenuated hypnotic response could still be demonstrated even when d exmedetomidine was administered directly into the locus coeruleus (LC) in rats pretreated chronically with dexmedetomidine. In no case could hypnotic responsiveness be restored to normal by pretreating with the alpha(1) adrenergic antagonist prazosin. The minimum anesthetic conce ntration (MAC) for halothane was not altered in the ''tolerant'' rats. These data indicate that hyporesponsiveness develops to the central d epressant effects of an alpha(2) adrenergic receptor agonist, through a pharmacodynamic mechanism.