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Sumikura, H. MD; Uchida, K. MD; Ono, T. MD; Noda, M. MD; Sugiuchi, N. MD; Aoki, T. MD

doi: 10.1097/00000539-199802001-00049
Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Anesthesia/OR Economics

Dept. of Anesthesiology, St. Marianna Univ. School of Medicine, KAWASAKI, JAPAN.

Abstract S49

Introduction: It has been suggested that circadian variations of physiologic states affect requirements for inhaled anesthetics [1]. Especially endorphins and GABA which show circadian variations have powerful inhibitory effects on the neural activity of CNS [2]. We previously reviewed circadian variations of MAC for sevoflurane at different phases of photoperiod. And then we studied changes of circadian variations of MAC after administration of naloxone.

Methods: Seventy-six male Sprague-Dawley rats at 6-7 weeks of age were maintained in a laboratory with free access to food and water under a 12:12 light/dark regime (lights on at 06:00 a.m.) for ten days. Sixty rats were divided randomly into four groups (n=15) and MAC for sevoflurane was determined at different periods of the photoperiod (early light period, late light period, early dark period, late dark period) by a standard tail-clamp technique. Sixteen rats were divided into two groups (n=8) and MAC was determined after intraperitoneal administration of naloxone of 0.1mg/kg at the late light period and the early dark period.

Results: MAC for sevoflurane in rats revealed characteristic circadian variations (Table 1). The lowest values occurred in the late light period and the highest values occurred in the early dark period. There were significant differences (P<0.05). After administration of naloxone, MAC was increased significantly in these two groups and circadian variations were disguised.

Table 1

Table 1

Discussion: MAC for sevoflurane showed characteristic circadian variations. And naloxone disguised these circadian variations. It has been suggested that naloxone antagonizes both GABA and endorphins [3]. So we think GABA and endorphins might be responsible for circadian variation of MAC. It has been reported that GABA shows the highest value in the light period and endorphins show the highest value in the dark period. So it appears that GABA contributes to the decrease of MAC more than endorphines.

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1. Anesthesiology 1970;32:507
2. Nature 1994;367:607
3. Anesthesiology 1982;56:246
© 1998 International Anesthesia Research Society