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 method without anesthesia. Antinociception was measured by means of the tail-withdrawal test. The intrathecal (i.t.) administration of dexmedetomidine led to dose-dependent antinociception (P< 0.05) and mydriasis (P < 0.001). Both effects reached their maximum 10 min after drug application. The tail-withdrawal latency was prolonged by 9 μg dexmedetomidine. Mydriasis appeared on administration of a 1 μg dose and its degree was proportional to the dexmedetomidine dosage up to 12 μg. A significant correlation was not found between pupil diameter and tail-withdrawal latency in the same rat after any dexmedetomidine dose. Based on the present experiments, we conclude that the mydriatic effect of i.t. dexmedetomidine appears at a lower dose than its antinociceptive effect. Our results suggest that the assessment of pupillary response is a sensitive method for estimation of the supraspinal effect of i.t. administered α2-adrenoceptor agonists.
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