The basal forebrain is highly interconnected with a number of brain regions involved in sleep-wake regulation, including cholinergic inputs from the pedunculopontine nucleus and laterodorsal nucleus of the pons, histaminergic inputs from tuberomammillary nucleus (TM) of the posterior hypothalamus, noradrenergic inputs from the locus coeruleus and serotonergic inputs from the raphe. The basal forebrain nuclei consist of the nucleus basalis (NB) that projects to the neocortex and the medial septum/ diagonal band area (MS) that project to the hippocampus and entorhinal cortex. We have used reversible inactivation (by local brain application GABAA receptor agonist muscimol) and permanent lesion of the basal forebrain, and selective lesion of histaminergic neurons in the TM, to study whether these structures participate in the emergence and induction of general anesthesia in rats. The main indicator of general anesthesia was a loss of righting reflex (LORR); tail pinch response and frontal cortical and hippocampal EEGs were also recorded.

Muscimol inactivation of the MS and NB prolonged the duration of LORR induced by both injectable (propofol, pentobarbital) and volatile anesthetic (isoflurane, halothane). The dose of anesthetic that induced slow delta EEG waves in the frontal cortex or suppress high gamma activity (70-100 Hz) in the hippocampus was reduced when either the MS or NB was inactivated. However, high-amplitude neocortical delta waves could occur in a standing, non-anesthetized rat after inactivation of NB, and thus neocortical delta EEG by itself does not indicate general anesthesia. Selective lesion of cholinergic neurons in the MS and NB, by local infusion of cholinotoxin 192 IgG-saporin, prolonged the duration of LORR following an anesthetic. Lesion of the histaminergic neurons in the TMN by orexin-saporin also delayed emergence (recovery of LORR) following isoflurane in addition to enhance the sensitivity to isoflurane. NB application of histamine facilitated, while H1 receptor antagonist triprolidine delayed, emergence from isoflurane anesthesia.

Other than the basal forebrain, bilateral inactivation of structures in the limbic system, including the nucleus accumbens, ventral tegmental area, ventral pallidum supramammillary area and amygdala prolonged the duration of LORR following pentobarbital or halothane. The latter inactivation, as compared to saline infusion, also increased delta waves and decreased hippocampal theta and gamma waves following a general anesthetic. By contrast, infusion of muscimol in the median raphe did not significantly alter the behavioral or EEG effects of halothane or pentobarbital. Bilateral inactivation of the entorhinal or piriform cortex prolonged the duration of LORR induced by pentobarbital but not halothane. The effect of inactivation on anesthetic-induced LORR may be explained in part by the connection of the brain area to the basal forebrain. Limbic system inactivation also suppressed the delirium state induced by a dose of anesthestic, including that induced by halothane, isoflurane, pentobarbital and ketamine.

Our research suggests that subcortical histaminergic and cholinergic inputs participate in activating the brain during wake and general anesthesia, such that removal of histaminergic or cholinergic activation of the forebrain increases anesthetic sensitivity and delays emergence. Other work suggests that norepinephrinergic and orexinergic pathways also participating in brain activation and general anesthesia. Our work highlights the limbic system, including the medial septum, hippocampus and nucleus accumbens, as important participants in general anesthesia, and in mediating the delirium state.