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

PURPOSE

To investigate the effect of xenon (Xe) and nitrous oxide (N(2)O) 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 both 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% N(2)O (n=6) or 60% N(2)O (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% N(2)O significantly increased norepinephrine release. The AUC in the 30% Xe, 60% Xe or 60% N(2)O group was 160 +/- 9 (P <0.05), 288 +/- 42 (P <0.01) or 237 +/- 46 pg x min/sample (P <0.01), respectively, compared to that in the control group: 77 +/- 14 pg x min/sample. In the PH, only 60% Xe significantly increased norepinephrine release compared to control (AUC: 191 +/- 38 vs. 71 +/- 1 pg x min/sample, P <0.01).

CONCLUSION

The present data suggest that Xe stimulates norepinephrinergic neurons more potently than N(2)O; 1.2 times more in the mPOA and 2.5 times more in the PH. This stimulant effect may contribute to the hypnotic and sympathotonic effects of Xe in rats.