An exceptional local rise in metabolism during general anesthesia has been noted previously in the ventral midbrain's highly cholinoceptive interpeduncular nucleus (IPN). We report here a functional correlate. Increased interstitial acetylcholine (ACh) was measured in the IPN of rats through chronically implanted microdialysis probes upon anesthesia by inhalation of 3% halothane (mean 1425% of pre-anesthesia baseline at 30min, n=5) and by i.p. injection of 100mg kg-1 ketamine (mean 387%, n=6). With 50mg kg-1 i.p. pentobarbital (n=8), ACh either climbed or fell repeatably in each animal, a positive correlation (p<0.05) emerged between the baseline pre-anesthetized level and the percentage change after 60min. Mapping of the brainstem under ketamine (n=2) or pentobarbital (n=3) anesthesia showed the ACh source to lie in the IPN. We conclude that physiological responses to the chemically and pharmacologically diverse anesthetics halothane and ketamine, and probably also to pento-barbital, converge to enhance the output of the IPN.
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