Xenon inhalation increases norepinephrine release from the anterior and posterior hypothalamus in rats

Purpose: To investigate the effect of xenon (Xe) and nitrous oxide (N2O) on norepinephrinergic neuronal activity in the rat medial preoptic area (mPOA ) and posterior hypothalamus (PH) using microdialysis. Methods: Sixty male Wistar rats were equally allocated to two groups: mPOA and PH, A microdialysis probe was implanted into the mPOA or the PH. In bot h groups, each animal was exposed to one of the following inhalations: 25% oxygen (control, n = 6 ), 30% Xe (n = 6), 60% Xe (n = 6), 30% N2O (n = 6) o r 60% N2O (n = 6). Norepinephrine concentration in the perfused artificial cerebrospinal fluid was measured by high pressure liquid chromatography at ten-minute intervals. After plotting the time-norepinephrine concentration curve, the area under the curve (AUC) in each group was calculated. Results: In the mPOA, 30 and 60% Xe, but only 60% N2O significantly increas ed norepinephrine release. The AUC in the 30% Xe, 60% Xe or 60% N2O group w as 160 +/- 9 (P < 0.05), 288 +/- 42 (P < 0.01) or 237 +/- 46 pg(.)min/sampl e (P < 0.01), respectively, compared to that in the control group: 77 +/- 1 4 pg(.)min/sample. In the PH, only 60% Xe significantly increased norepinep hrine release compared to control (AUC: 191 +/- 38 vs 71 +/- 1 pg(.)min/sam ple, P < 0.01). Conclusion: The present data suggest that Xe stimulates norepinephrinergic neurons more potently than N2O; 1.2 times more in the mPOA and 2.5 times mo re in the PH. This stimulant effect may contribute to the hypnotic and symp athotonic effects of Xe in rats.