Neurochemical Modulators of Sleep and Anesthetic States

Abstract

The regulation of consciousness is a fundamental question that has a long and storied association with philosophy. Today, consciousness studies command a central position in contemporary neuroscience 1-3. The complexities of consciousness studies and the pressing demands of clinical care have led most anesthesiologists to focus on research problems with pragmatic outcomes. Yet the ability to accurately assess and manipulate states of consciousness with anesthetic drugs is the ultimate concern for every surgical patient and anesthesia provider. This volume recognizes consciousness studies as a legitimate and accessible concern for anesthesiology. This chapter considers the relationship between molecules known to regulate the loss of consciousness during anesthesia and molecules that regulate the loss of consciousness during physiological sleep. Sleep neurobiology has been shown to provide unique insights into the study of consciousness 4-6. The proposal 7,8 that neuronal networks that evolved to generate states of sleep and wakefulness also contribute to the generation of anesthetic states has been supported by many laboratories 9-19. There is evidence that the loss of consciousness during sleep is caused, in part, by the loss of functional connectivity and information processing 20. Functional connectivity is critically dependent on neurochemical transmission. Therefore, this chapter focuses on intravenous and volatile anesthetics that have been shown to alter endogenous neurotransmitters known to regulate states of consciousness. Reviews on sleep from an anesthesiology perspective are available elsewhere 8,21-24. The present overview is derived from a September 2007 PubMed title search of peer-reviewed papers linking 10 commonly used anesthetics with 11 endogenous molecules known to regulate states of consciousness. The list of intravenous anesthetics includes propofol, pentobarbital, ketamine, etomidate, and midazolam. The list of volatile anesthetics includes isoflurane, sevoflurane, nitrous oxide, xenon, and desflurane. The 11 endogenous molecules known to regulate sleep/wake states 25 include acetylcholine (ACh), gamma-aminobutyric acid (GABA), glutamate, adenosine, dopamine, histamine, serotonin, norepinephrine, hypocretin/orexin, glycine, and galanin. This 10 by 11 matrix was searched from 1950 to 2007 and identified 660 references. A search of the last 10 years (1997 to 2007) revealed a total of 192 references (Tables 1 and 2). I. Intravenous Anesthetics Alter Neurotransmitters that Regulate Sleep and Wakefulness The endogenous neurotransmitters and neuromodulators ACh, GABA, glutamate, the monoamines (dopamine, histamine, serotonin, and norepinephrine), and adenosine contribute to generating and maintaining states of sleep and wakefulness. The following section

γ-Aminobutyric Acid–mediated Neurotransmission in the Pontine Reticular Formation Modulates Hypnosis, Immobility, and Breathing during Isoflurane Anesthesia

“…The fact that different brain regions regulate different neurobehavioral functions means that insights into the mechanisms of anesthetic action can be achieved by deconstructing anesthetic states into their component traits 3,41,42. The present study applied this deconstruction approach by relating measures of pontine reticular formation GABA to the traits of cortical electroencephalogram and muscle hypotonia (fig.…”

Contribution of extrasynapticN-methyl-d-aspartate and adenosine A1 receptors in the generation of dendritic glutamate-mediated plateau potentials

“…Synaptic stimulation pipettes (7 MV) were filled with regular ACSF and glutamate stimulation pipettes (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) were filled with 200 mM Na-glutamate ( pH ¼ 9). A programmable stimulator, Master-8, and a stimulus isolation unit, IsoFlex (A.M.P.I., Jerusalem, Israel), were used to generate current pulses for both synaptic stimulation and glutamate iontophoresis.…”

Dissociation of electromyogram and mechanical response in sleep apnoea during propofol anaesthesia

“…Hence, control of motor tone and respiratory activity should not be fundamentally different between light anaesthesia and sleep. Also, the anaesthetic effects of propofol are mediated by its effects on monoaminergic, c-aminobutyric acid-ergic, glutamatergic, cholinergic and adenosinergic neurotransmission in brain regions known to regulate sleep and wakefulness [56] and EEG patterns during anaesthesia resembles those seen in slow-wave sleep [45]. The loss of connectivity between different brain regions is also similar during sleep and anaesthesia [57].…”

“…Based on these findings and the above considerations, we believe that propofol anaesthesia merely unmasked and augmented, by delaying arousal, the magnitude of the electro-mechanical dissociation present also during sleep. Nevertheless, pharmacological anaesthesia and sleep are not identical and differ in terms of both neurophysiology and neurochemistry [56]. Also, in contrast to anaesthesia, pharyngeal patency can sometimes be restored during sleep without EEG or other signs of arousal [15].…”