MYDRIATIC AND ANTINOCICEPTIVE EFFECTS OF INTRATHECAL DEXMEDETOMIDINE IN CONSCIOUS RATS

A comparison was made of the dose-effect relationship of intrathecally applied dexmedetomidine as concerns its mydriatic and antinociceptive effects in rats. Pupil diameter was measured with a photographic meth od without anesthesia. Antinociception was measured by means of the ta il-withdrawal test. The intrathecal (i.t.) administration of dexmedeto midine led to dose-dependent antinociception (P < 0.05) and mydriasis (P < 0.001). Both effects reached their maximum 10 min after drug appl ication. The tail-withdrawal latency was prolonged by 9 mu g dexmedeto midine. Mydriasis appeared on administration of a 1 mu g dose and its degree was proportional to the dexmedetomidine dosage up to 12 mu g. A significant correlation was not found between pupil diameter and tail -withdrawal latency in the same rat after any dexmedetomidine dose. Ba sed on the present experiments, we conclude that the mydriatic effect of i.t. dexmedetomidine appears at a lower dose than its antinocicepti ve effect. Our results suggest that the assessment of pupillary respon se is a sensitive method for estimation of the supraspinal effect of i .t. administered alpha(2)-adrenoceptor agonists.