The relationship among brain, spinal cord and anesthetic requirements - Antognini JF; Department of Anesthesiology, University of California, Davis 95616, USA.

The spinal cord is a crucial site wherein anesthetics suppress movement in response to noxious stimuli. The balance of excitatory and inhibitory influences on the spinal cord likely determines the extent of motor response, and is thus important to anesthetic requirements. When the volatile anesthetic isoflurane is selectively delivered to the in situ goat brain (with low concentrations in the torso), anesthetic requirements increase dramatically, but when low isoflurane concentrations are delivered to the brain, anesthetic requirements decrease in the torso. When high, supraclinical concentrations of isoflurane (6-10%) are delivered to the brain and not to the torso, spontaneous movement occurs. These results are best explained by a differential effect of anesthetics on spinal cord neurons and cerebral neurons (midbrain reticular formation). Examination of neurons in the dorsal horn and midbrain reticular formation, and the electromyogram, during differential delivery of isoflurane to brain and spinal cord, will test this hypothesis.

Macroscopic sites of anesthetic action: brain versus spinal cord - Antognini JF, Carstens E. Department of Anesthesiology, University of California, Davis 95616, USA.

General anesthesia is achieved by anesthetic action in the central nervous system (CNS).

Whereas amnesia and unconsciousness are due to anesthetic action in the brain, recent evidence suggests that immobility in response to a noxious stimulus is achieved by anesthetic effects in the spinal cord. The putative spinal cord site(s) include dorsal horn cells and motor neurons.

The extent to which anesthetic action in the brain influences the spinal cord probably varies among anesthetics. Furthermore, anesthetics can indirectly influence the brain by their actions within the spinal cord, i.e. by modulating ascending transmission of sensory information.

Toward a unified theory of narcosis: brain imaging evidence for a thalamocortical switch as the neurophysiologic basis of anesthetic-induced unconsciousness - Alkire MT, Haier RJ, Fallon JH; Department of Anesthesiology, University of California at Irvine, Irvine, California 92697, USA.

…………. Functional-brain-imaging data obtained from 11 volunteers during general anesthesia showed specific suppression of regional thalamic and midbrain reticular formation activity across two different commonly used volatile agents.

The differential effects of halothane and isoflurane on electroencephalographic responses to electrical microstimulation of the reticular formation - Orth M  Bravo E, Barter L, Carstens E  Antognini JF; Department of Anesthesiology and Pain Medicine, Section of Neurobiology, Physiology and Behavior, University of California, Davis, Davis, California 95616, USA.

Isoflurane and halothane cause electroencephalographic (EEG) depression and neuronal depression in the reticular formation, a site critical to consciousness.

Propofol, more than halothane, depresses electroencephalographic activation resulting from electrical stimulation in reticular formation - Antognini JF, Bravo E, Atherley R, Carstens E ; Department of Anesthesiology and Pain Medicine, University of California, Davis, CA 95616, USA.

BACKGROUND:
Halothane and propofol depress the central nervous system, and this is partly manifested by a decrease in electroencephalographic (EEG) activity. Little work has been performed to determine the differences between these anesthetics with regard to their effects on evoked EEG activity. We examined the effects of halothane and propofol on EEG responses to electrical stimulation of the reticular formation.

METHODS:
Rats (n= 12) were anesthetized with either halothane or propofol, and EEG responses were recorded before and after electrical stimulation of the reticular formation. Two anesthetic concentrations were used (0.8 and 1.2 times the amount needed to prevent gross, purposeful movement in response to supramaximal noxious stimulation), and both anesthetics were studied in each rat using a cross-over design.

RESULTS:
Electrical stimulation in the reticular formation increased the spectral edge (SEF) and median edge (MEF) frequencies by approximately 1-2 Hz during halothane anesthesia at low and high concentrations. During propofol anesthesia, MEF increased at the low propofol infusion rate, but SEF was unaffected. At the high propofol infusion rate, SEF and MEF decreased following electrical stimulation in the reticular formation.

CONCLUSIONS:
At immobilizing concentrations, propofol produces a larger decrease than halothane in EEG responses to reticular formation stimulation, consistent with propofol having a more profound depressant effect on cortical and subcortical structures.

Gamma-aminobutyric acid-mediated neurotransmission in the pontine reticular formation modulates hypnosis, immobility, and breathing during isoflurane anesthesia - Vanini G, Watson CJ, Lydic R, Baghdoyan HA; Department of Anesthesiology, University of Michigan, Ann Arbor, Michigan 48109, USA.

BACKGROUND:
Many general anesthetics are thought to produce a loss of wakefulness, in part, by enhancing gamma-aminobutyric acid (GABA) neurotransmission. However, GABAergic neurotransmission in the pontine reticular formation promotes wakefulness. This study tested the hypotheses that (1) relative to wakefulness, isoflurane decreases GABA levels in the pontine reticular formation; and (2) pontine reticular formation administration of drugs that increase or decrease GABA levels increases or decreases, respectively, isoflurane induction time.

METHODS:
To test hypothesis 1, cats (n = 5) received a craniotomy and permanent electrodes for recording the electroencephalogram and electromyogram. Dialysis samples were collected from the pontine reticular formation during isoflurane anesthesia and wakefulness. GABA levels were quantified using high-performance liquid chromatography. For hypothesis 2, rats (n = 10) were implanted with a guide cannula aimed for the pontine reticular formation. Each rat received microinjections of Ringer's (vehicle control), the GABA uptake inhibitor nipecotic acid, and the GABA synthesis inhibitor 3-mercaptopropionic acid. Rats were then anesthetized with isoflurane, and induction time was quantified as loss of righting reflex. Breathing rate was also measured.

RESULTS:
Relative to wakefulness, GABA levels were significantly decreased by isoflurane. Increased power in the electroencephalogram and decreased activity in the electromyogram caused by isoflurane covaried with pontine reticular formation GABA levels. Nipecotic acid and 3-mercaptopropionic acid significantly increased and decreased, respectively, isoflurane induction time. Nipecotic acid also increased breathing rate.

CONCLUSION:
Decreasing pontine reticular formation GABA levels comprises one mechanism by which isoflurane causes loss of consciousness, altered cortical excitability, muscular hypotonia, and decreased respiratory rate.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia - Tassonyi E, Charpantier E, Muller D, Dumont L, Bertrand D.; Division of Anesthesiology, Department of Anesthesiology, Pharmacology and Surgical Intensive Care (APSIC), Geneva University Hospitals, Geneva, Switzerland.

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor.

The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia.

The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

Nicotinic receptors partly mediate brainstem autonomic dysfunction evoked by the inhaled anesthetic isoflurane. - Wang X ; Departments of Pharmacology and Physiology, The George Washington University, Washington, DC 20037, USA.

Isoflurane is one of the most commonly used volatile anesthetics, yet the cardiorespiratory depression that occurs with its use remains poorly understood. In this study, the author examined isoflurane modulation of postsynaptic gamma-aminobutyric acid (GABA) receptors in parasympathetic cardiac vagal neurons (CVNs) and alterations of GABAergic function by targeting nicotinic acetylcholine receptors on GABAergic presynaptic terminals…..

CONCLUSIONS:
These results suggest clinically relevant concentrations of isoflurane inhibit brainstem respiratory rhythmogenesis, prolong inhibitory GABAergic postsynaptic currents and reduce GABA activity in CVNs. The decrease of GABAergic IPSCs frequency is dependent upon inhibition of presynaptic alpha(4)beta(2) nicotinic receptors.

PMID: 19095841

Disruption of muscarinic receptor-G protein coupling is a general property of liquid volatile anesthetics - Anthony BL, Dennison RL, Aronstam RS ; Department of Pharmacology and Toxicology, Medical College of Georgia, Augusta 30912.

The influences of 3 volatile anesthetics, chloroform, enflurane and isoflurane, on muscarinic acetylcholine receptors in rat brainstem were determined.

Each of the volatile anesthetics increased [3H]methylscopolamine [( 3H]MS) binding affinity, but did not affect the number of [3H]MS binding sites. Carbamylcholine affinity for brainstem muscarinic receptors was not altered after equilibration of brainstem membranes with any of these anesthetics. The ability of guanine nucleotides to depress the high affinity binding of two agonists, carbamylcholine and [3H]oxotremorine-M, was decreased or eliminated after equilibration of brainstem membranes with any of the anesthetics. In each of these actions, these anesthetics resemble halothane and diethyl ether.

These results indicate that interference with muscarinic receptor-G protein interactions is a common property of liquid volatile anesthetics and may represent a general mechanism for the disruption of signal transmission between cells during anesthesia

Anesthetic effects on muscarinic signal transduction - Aronstam RS, Dennison RJr.;

Department of Pharmacology and Toxicology, Medical College of Georgia, Augusta 30912.

A wealth of pharmacological and physiological evidence has established that anesthetics disrupt synaptic transmission at muscarinic and other synapses.

The sequence of molecular events precipitated by agonist binding to the receptors is under intense scrutiny. It appears that at the majority of synapses G proteins serve to mediate the transfer of information from receptors to intracellular mechanisms. The major exception to this scheme is the situation in which an ion channel is incorporated directly in the receptor structure. Binding of an agonist to these receptors produces a conformational change in the receptors which opens an intrinsic ion channel. This situation occurs in nicotinic acetylcholine gamma-amino butyric acid type A (GABAA, and 5-hydroxytryptamine type 3 (5-HT3) receptors).

Assays have been developed to evaluate several steps in the cascade of events involved in synaptic signal transduction, and these assays have been employed to determine the step at which anesthetics act to disrupt synaptic transmission. We have demonstrated that several volatile anesthetics alter the interaction of muscarinic receptors with transducer G proteins.

Ligand-binding experiments suggest that receptor-G protein complexes are stabilized, thereby disrupting G protein GTPase activity and muscarinic control of cellular activity. This "stabilization" does not appear to involve an inhibition of guanine nucleotide binding, the proximal event in receptor-G protein dissociation. Two possibilities warrant further consideration: (1) that GDP release from inactive G protein trimers, which is normally catalyzed by the receptor, is inhibited, and (2) that receptor-G protein complexes fail to dissociate even in response to GTP binding. We are currently examining these possibilities using purified G proteins and receptors in reconstituted systems.