Background Cholinergic drugs are known to modulate general anesthesia, but anesthesia responses in acetylcholine-deficient mice have not been studied. It was hypothesized that mice with genetic deficiency of forebrain acetylcholine show increased anesthetic sensitivity to isoflurane and ketamine and decreased gamma-frequency brain activity. Methods Male adult mice with heterozygous knockdown of vesicular acetylcholine transporter in the brain or homozygous knockout of the transporter in the basal forebrain were compared with wild-type mice. Hippocampal and frontal cortical electrographic activity and righting reflex were studied in response to isoflurane and ketamine doses. Results The loss-of-righting-reflex dose for isoflurane was lower in knockout (mean ± SD, 0.76 ± 0.08%, n = 18, P = 0.005) but not knockdown (0.78 ± 0.07%, n = 24, P = 0.021), as compared to wild-type mice (0.83 ± 0.07%, n = 23), using a significance criterion of P = 0.017 for three planned comparisons. Loss-of-righting-reflex dose for ketamine was lower in knockout (144 ± 39 mg/kg, n = 14, P = 0.006) but not knockdown (162 ± 32 mg/kg, n = 20, P = 0.602) as compared to wild-type mice (168 ± 24 mg/kg, n = 21). Hippocampal high-gamma (63 to 100 Hz) power after isoflurane was significantly lower in knockout and knockdown mice compared to wild-type mice (isoflurane-dose and mouse-group interaction effect, F[8,56] = 2.87, P = 0.010; n = 5 to 6 mice per group). Hippocampal high-gamma power after ketamine was significantly lower in both knockout and knockdown mice when compared to wild-type mice (interaction effect F[2,13] = 6.06, P = 0.014). The change in frontal cortical gamma power with isoflurane or ketamine was not statistically different among knockout, knockdown, and wild-type mice. Conclusions These findings suggest that forebrain cholinergic neurons modulate behavioral sensitivity and hippocampal gamma activity during isoflurane and ketamine anesthesia. Editor’s Perspective What We Already Know about This Topic Acetylcholine plays a major role in arousal, and pharmacologic interventions that raise the concentrations of this neurotransmitter reduce anesthetic potency Forebrain cholinergic neurons are a major source of acetylcholine, but their role in the modulation of anesthetic sensitivity is incompletely understood What This Article Tells Us That Is New In genetically modified mice lacking the vesicular acetylcholine transporter in the forebrain, lower doses of isoflurane and ketamine were necessary to induce the loss of the righting reflex, a surrogate for loss of consciousness, when compared to wild-type counterparts Hippocampal gamma power was lower in genetically modified mice lacking forebrain acetylcholine than in the wild-type mice during both isoflurane and ketamine anesthesia These observations suggest that forebrain cholinergic neurons modulate anesthetic sensitivity during isoflurane and ketamine anesthesia

Background Previous studies showed that synaptic transmission is affected by general anesthetics, but an anesthetic dose response in freely moving animals has not been done. The hippocampus provides a neural network for the evaluation of isoflurane and pentobarbital on multisynaptic transmission that is relevant to memory function. Methods Male Long-Evans rats were implanted with multichannel and single electrodes in the hippocampus. Spontaneous local field potentials and evoked field potentials were recorded in freely behaving rats before (baseline) and after various doses of isoflurane (0.25 to 1.5%) and sodium pentobarbital (10 mg/kg intraperitoneal). Results Monosynaptic population excitatory postsynaptic potentials at the basal and apical dendrites of CA1 were significantly decreased at greater than or equal to 0.25% (n = 4) and greater than or equal to 1.0% (n = 6) isoflurane, respectively. The perforant path evoked multisynaptic response at CA1 was decreased by ~50% at greater than or equal to 0.25% isoflurane (n = 5). A decreased population excitatory postsynaptic potential was accompanied by increased paired-pulse facilitation. Population spike amplitude in relation to apical dendritic population excitatory postsynaptic potential was not significantly altered by isoflurane. Spontaneous hippocampal local field potential at 0.8 to 300 Hz was dose-dependently suppressed by isoflurane (n = 6), with local field potential power in the 50- to 150-Hz band showing the highest decrease with isoflurane dose, commensurate with the decrease in trisynaptic CA1 response. Low-dose pentobarbital (n = 7) administration decreased the perforant path evoked trisynaptic CA1 response and hippocampal local field potentials at 78 to 125 Hz. Conclusions Hippocampal networks are sensitive to low doses of isoflurane and pentobarbital, possibly through both glutamatergic and γ-aminobutyric acid–mediated transmission. Network disruption could help explain the impairment of hippocampal-dependent cognitive functions with low-dose anesthetic.

Background: Cholinergic drugs are known to modulate the response of general anesthesia. However, the sensitivity of isoflurane or other volatile anesthetics after selective lesion of septal cholinergic neurons that project to the hippocampus is not known. Methods: Male Long Evans rats had 192 immunoglobulin G-saporin infused into the medial septum (n = 10), in order to selectively lesion cholinergic neurons, whereas control, sham-lesioned rats were infused with saline (n = 12). Two weeks after septal infusion, the hypnotic properties of isoflurane and ketamine were measured using a behavioral endpoint of loss of righting reflex (LORR). Septal lesion was assessed by counting choline acetyltransferase–immunoreactive cells and parvalbumin-immunoreactive cells. Results: Rats with 192 immunoglobulin G-saporin lesion, as compared with control rats with sham lesion, showed a 85% decrease in choline acetyltransferase–immunoreactive, but not parvalbumin–immunoreactive, neurons in the medial septal area. Lesioned as compared with control rats showed increased isoflurane sensitivity, characterized by a leftward shift of the graph plotting cumulative LORR percent with isoflurane dose. However, lesioned and control rats were not different in their LORR sensitivity to ketamine. When administered with 1.375% isoflurane, LORR induction time was shorter, whereas emergence time was longer, in lesioned as compared with control rats. Hippocampal 62–100 Hz gamma power in the electroencephalogram decreased with isoflurane dose, with a decrease that was greater in lesioned (n = 5) than control rats (n = 5). Conclusions: These findings suggest a role of the septal cholinergic neurons in modulating the sensitivity to isoflurane anesthesia, which affects both induction and emergence. The sensitivity of hippocampal gamma power to isoflurane appears to indicate anesthesia (LORR) sensitivity.

Background The brain histaminergic system plays a critical role in maintenance of arousal. Previous studies suggest that histaminergic neurotransmission might be a potential mediator of general anesthetic actions. However, it is not clear whether histaminergic tuberomamillary nucleus (TMN) is necessarily involved in the sedative/hypnotic effects of general anesthetics. Methods Male Long Evans rats underwent either TMN orexin-saporin/sham lesion or implantation of intracerebroventricular cannula 2 weeks before the experiment. The behavioral endpoint of loss of righting reflex was used to assess the hypnotic property of isoflurane, propofol, pentobarbital, and ketamine in animals. Histaminergic cell loss was assessed by adenosine deaminase expression in the TMN using immunohistochemistry. Results Rats with bilateral TMN orexin-saporin lesion induced an average 72% loss of histaminergic cells compared with sham-lesion rats. TMN orexin-saporin lesion or intracerebroventricular administration of triprolidine (an H1 receptor antagonist) decreased the 50% effective concentration for loss of righting reflex value and prolonged emergence time to isoflurane anesthesia. However, TMN orexin-saporin lesion had no significant effect on the anesthetic sensitivity to propofol, pentobarbital, and ketamine. Conclusions These findings suggest a role of the TMN histaminergic neurons in modulating isoflurane anesthesia and that the neural circuits for isoflurane-induced hypnosis may differ from those of γ-aminobutyric acid-mediated anesthetics and ketamine.

Background The tuberomammillary histaminergic neurons are involved in the sedative component of anesthetic action. The nucleus basalis magnocellularis (NBM) in the basal forebrain receives dense excitatory innervation from the tuberomammillary nucleus and is recognized as an important site of sleep-wake regulation. This study investigated whether NBM administration of histaminergic drugs may modulate arousal/emergence from isoflurane anesthesia. Methods Microinjections of histaminergic agonists and antagonists were made into the NBM of rats anesthetized with isoflurane. The changes in electroencephalographic activity, including electroencephalographic burst suppression ratio and power spectra, as well as respiratory rate, were recorded under basal conditions and after NBM injection. Time to resumption of righting reflex was recorded as a measure of emergence from anesthesia. Results The rats displayed a burst suppression electroencephalographic pattern at inhaled isoflurane concentrations of 1.4-2.1%. Application of histamine (1 microg/0.5 microl) to the NBM reversed the electroencephalographic depressant effect of isoflurane; i.e., electroencephalographic activity shifted from the burst suppression pattern toward delta activity at 1.4% isoflurane, and the burst suppression ratio decreased at 2.1% isoflurane. Histamine-evoked activation of electroencephalography was blocked by NBM pretreatment with a H1 receptor antagonist, triprolidine (5 microg/1 microl), but not by a H2 receptor antagonist, cimetidine (25 microg/1 microl). The respiratory rate was significantly increased after histamine injection. NBM application of histamine facilitated, while triprolidine delayed, emergence from isoflurane anesthesia. Conclusions Histamine activation of H1 receptors in the NBM induces electroencephalographic arousal and facilitates emergence from isoflurane anesthesia. The basal forebrain histaminergic pathway appears to play a role in modulating arousal/emergence from anesthesia.

Background The action of propofol has been studied in vitro and in vivo, but the effects of intravenously administered propofol on synaptic transmission in freely behaving rats have not been studied before. Methods Rats were implanted with recording electrodes in the dentate gyrus and with stimulation electrodes in the medial perforant path (MPP). Paired pulses at different interpulse intervals (IPIs) were delivered to the MPP, and average evoked potentials were recorded in the dentate gyrus before and after a bolus of propofol (10 or 20 mg/kg administered intravenously) or control vehicle was injected via femoral vein cannula. Because of the layered structure of the hippocampus, population excitatory postsynaptic potentials and population spikes could be distinguished and analyzed. Results Propofol has no significant effect on the population excitatory postsynaptic potentials or population spike evoked by a single MPP stimulus pulse. However, paired-pulse inhibition of the dentate population spikes was increased at IPI of 20 and 30 ms. Paired-pulse inhibition of the population spike was most prominent when tail pinch response was lost (deep and moderate anesthesia), but it persisted during light anesthesia. At 200 ms IPI, paired-pulse facilitation of population spikes was observed during moderate anesthesia in most rats. Conclusions In freely behaving rats, intravenous propofol enhanced paired-pulse inhibition at < 50 ms IPI, likely by enhancing gamma-aminobutyric acid A receptor-mediated inhibition. Propofol also increased paired-pulse facilitation at 200 ms IPI through an unknown mechanism, which may contribute to the neuroexcitatory effect of propofol.