THE EFFECT OF IMIDAZOLINE RECEPTORS AND ALPHA(2)-ADRENOCEPTORS ON THEANESTHETIC REQUIREMENT (MAC) FOR HALOTHANE IN RATS

Background Recent evidences have documented that several pharmacologic actions of alpha(2)-adrenoceptor agonists are mediated aia activation of not only alpha(2)-adrenoceptors, but also by imidazoline receptors , which are nonadrenergic receptors in the central nervous system. How ever, the effect of imidazoline receptors on the anesthesia is not wel l known, and it is important to clarify the effects of both receptors on anesthesia. Methods. Seventy-two rats were anesthetized with haloth ane, and the anesthetic requirement for halothane was evaluated as min imum alveolar concentration (MAC). The MAC for halothane was determine d in the presence of dexmedetomidine (0, 10, 20, and 30 mu g/kg, intra peritoneally [IP]), a selective alpha(2)-adrenoceptor agonist with wea k affinity for imidazoline receptors. Then, the authors evaluated the inhibitory effect of rauwolscine (20 mg/kg, IP), an alpha(2)-adrenocep tor antagonist with little affinity for imidazoline receptors, on the MAC-reducing action of dexmedetomidine (50 mu g/kg). Further, the effe ct of rilmenidine (20, 50, 100, 1000 mu g/kg, IP) a selective imidazol ine receptor agonist, on the MAC for halothane was also investigated. Results. Dexmedetomidine decreased the MAC for halothane dose-dependen tly, and this MAC-reducing action of dexmedetomidine was completely bl ocked by rauwolscine. Rilmenidine alone did not change the MAC for hal othane. Conclusions. The present data indicate that the anesthetic spa ring action of dexmedetomidine is most likely mediated through alpha(2 )-adrenoceptors, and the stimulation of imidazoline receptors exerts l ittle effect on the anesthetic requirement for halothane.