Acetylcholine release from the midbrain interpeduncular nucleus during anesthesia

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.