Assessing Changes in Volatile General Anesthetic Sensitivity of Mice after Local or Systemic Pharmacological Intervention

One desirable endpoint of general anesthesia is  the state of unconsciousness, also known as hypnosis. Defining the hypnotic state in animals is  less ...

Perelman School of Medicine, University of Pennsylvania

The purpose of these experiments was to determine whether detecting brief decrements in noise level ("gaps") varies with the spectral content and bandwidth of noise in mice as it does in humans. The behavioral effect of gaps was quantified by their inhibiting a subsequent acoustic startle reflex. Gap durations from 1 to 29 ms were presented in five adjacent 1-octave noise bands and one 5-octave band, their range being 2 kHz to 64 kHz. Gaps ended 60 ms before the startle stimulus (experiment 1) or at startle onset (experiment 2). Asymptotic inhibition was greater for higher-frequency 1-octave bands and highest for the 5-octave band in both experiments, but time constants were related to frequency only in experiment 1. For the lowest band (2-4 kHz) neither noise decrements (experiment 1 and 2) nor increments (experiment 3) had any behavioral consequence, but this band was effective when presented as a pulse in quiet (experiment 4). The lowest frequencies in the most effective 1-octave band were one octave above the spectral region where mice have their best absolute thresholds. These effects are similar to those obtained in humans, and reveal a special contribution of wide band, high-frequency stimulation to temporal acuity.

The behavioral response of mice to gaps in noise depends on its spectral components and its bandwidth.

The neural mechanisms through which the state of anesthesia arises and dissipates remain unknown. One common belief is that emergence from anesthesia is the inverse process of induction, brought about by elimination of anesthetic drugs from their CNS site(s) of action. Anesthetic-induced unconsciousness may result from specific interactions of anesthetics with the neural circuits regulating sleep and wakefulness. Orexinergic agonists and antagonists have the potential to alter the stability of the anesthetized state. In this report, we refine the role of the endogenous orexin system in impacting emergence from, but not entry into the anesthetized state, and in doing so, we distinguish mechanisms of induction from those of emergence. We demonstrate that isoflurane and sevoflurane, two commonly used general anesthetics, inhibit c-Fos expression in orexinergic but not adjacent melanin-concentrating hormone (MCH) neurons; suggesting that wake-active orexinergic neurons are inhibited by these anesthetics. Genetic ablation of orexinergic neurons, which causes acquired murine narcolepsy, delays emergence from anesthesia, without changing anesthetic induction. Pharmacologic studies with a selective orexin-1 receptor antagonist confirm a specific orexin effect on anesthetic emergence without an associated change in induction. We conclude that there are important differences in the neural substrates mediating induction and emergence. These findings support the concept that emergence depends, in part, on recruitment and stabilization of wake-active regions of brain.

An essential role for orexins in emergence from general anesthesia.

Opiates, sleep, and pain: the adenosinergic link.

One major unanswered question in neuroscience is how the brain transitions between conscious and unconscious states. General anesthetics offer a controllable means to study these transitions. Induction of anesthesia is commonly attributed to drug-induced global modulation of neuronal function, while emergence from anesthesia has been thought to occur passively, paralleling elimination of the anesthetic from its sites in the central nervous system (CNS). If this were true, then CNS anesthetic concentrations on induction and emergence would be indistinguishable. By generating anesthetic dose-response data in both insects and mammals, we demonstrate that the forward and reverse paths through which anesthetic-induced unconsciousness arises and dissipates are not identical. Instead they exhibit hysteresis that is not fully explained by pharmacokinetics as previously thought. Single gene mutations that affect sleep-wake states are shown to collapse or widen anesthetic hysteresis without obvious confounding effects on volatile anesthetic uptake, distribution, or metabolism. We propose a fundamental and biologically conserved concept of neural inertia, a tendency of the CNS to resist behavioral state transitions between conscious and unconscious states. We demonstrate that such a barrier separates wakeful and anesthetized states for multiple anesthetics in both flies and mice, and argue that it contributes to the hysteresis observed when the brain transitions between conscious and unconscious states.

A conserved behavioral state barrier impedes transitions between anesthetic-induced unconsciousness and wakefulness: evidence for neural inertia.

General anesthesia has been likened to a state in which anesthetized subjects are locked out of access to both rapid eye movement (REM) sleep and wakefulness. Were this true for all anesthetics, a significant REM rebound after anesthetic exposure might be expected. However, for the intravenous anesthetic propofol, studies demonstrate that no sleep debt accrues. Moreover, preexisting sleep debts dissipate during propofol anesthesia. To determine whether these effects are specific to propofol or are typical of volatile anesthetics, the authors tested the hypothesis that REM sleep debt would accrue in rodents anesthetized with volatile anesthetics.

Rapid eye movement sleep debt accrues in mice exposed to volatile anesthetics.

Hydrogen sulfide (H(2)S) depresses mitochondrial function and thereby metabolic rates in mice, purportedly resulting in a state of "suspended animation." Volatile anesthetics also depress mitochondrial function, an effect that may contribute to their anesthetic properties. In this study, we ask whether H(2)S has general anesthetic properties, and by extension, whether mitochondrial effects underlie the state of anesthesia. We compared loss of righting reflex, electroencephalography, and electromyography in mice exposed to metabolically equipotent concentrations of halothane, isoflurane, sevoflurane, and H(2)S. We also studied combinations of H(2)S and anesthetics to assess additivity. Finally, the long-term effects of H(2)S were assessed by using the Morris water maze behavioral testing 2 to 3 weeks after exposures. Exposure to H(2)S decreases O(2) consumption, CO(2) production, and body temperature similarly to that of the general anesthetics, but fails to produce a loss of righting reflex or muscle atonia at metabolically equivalent concentrations. When combined, H(2)S antagonizes the metabolic effects of isoflurane, but potentiates the isoflurane-induced loss of righting reflex. We found no effect of prior H(2)S exposure on memory or learning. H(2)S (250 ppm), not itself lethal, produced delayed lethality when combined with subanesthetic concentrations of isoflurane. H(2)S cannot be considered a general anesthetic, despite similar metabolic suppression. Metabolic suppression, presumably via mitochondrial actions, is not sufficient to account for the hypnotic or immobilizing components of the anesthetic state. Combinations of H(2)S and isoflurane can be lethal, suggesting extreme care in the combination of these gases in clinical situations.

Is hydrogen sulfide-induced suspended animation general anesthesia?

Despite seventeen decades of continuous clinical use, the neuronal mechanisms through which volatile anesthetics act to produce unconsciousness remain obscure. One emerging possibility is that anesthetics exert their hypnotic effects by hijacking endogenous arousal circuits. A key sleep-promoting component of this circuitry is the ventrolateral preoptic nucleus (VLPO), a hypothalamic region containing both state-independent neurons and neurons that preferentially fire during natural sleep.

Jason T. Moore

Department of Anesthesiology and Critical Care,
Department of Neuroscience,
Center for Sleep and Circadian Neurobiology

Perelman School of Medicine, University of Pennsylvania

Assessing Changes in Volatile General Anesthetic Sensitivity of Mice after Local or Systemic Pharmacological Intervention

Jason T. Moore

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Anesthesiology and Critical Care,Department of Neuroscience,Center for Sleep and Circadian Neurobiology

Perelman School of Medicine, University of Pennsylvania

Assessing Changes in Volatile General Anesthetic Sensitivity of Mice after Local or Systemic Pharmacological Intervention

Department of Anesthesiology and Critical Care,
Department of Neuroscience,
Center for Sleep and Circadian Neurobiology