Thiopental produces immobility primarily by supraspinal actions in rats

Identifying c-fos Expression as a Strategy to Investigate the Actions of General Anesthetics on the Central Nervous System

Although general anesthetics have been used in the clinic for more than 170 years, the ways in which they induce amnesia, unconsciousness, analgesia, and immobility remain elusive. Modulations of various neural nuclei and circuits are involved in the actions of general anesthetics. The expression of the immediate-early gene c-fos and its nuclear product, c-fos protein, can be induced by neuronal depolarization; therefore, c-fos staining is commonly used to identify the activated neurons during sleep and/or wakefulness, as well as in various physiological conditions in the central nervous system. Identifying c-fos expression is also a direct and convenient method to explore the effects of general anesthetics on the activity of neural nuclei and circuits. Using c-fos staining, general anesthetics have been found to interact with sleep- and wakefulness-promoting systems throughout the brain, which may explain their ability to induce unconsciousness and emergence from general anesthesia. This review summarizes the actions of general anesthetics on neural nuclei and circuits based on a c-fos expression.

Paradoxical anesthesia: Sleep-like EEG during anesthesia induced by mesopontine microinjection of GABAergic agents

General anesthetic agents are thought to induce loss-of-consciousness (LOC) and enable pain-free surgery by acting on the endogenous brain circuitry responsible for sleep-wake cycling. In clinical use, the entire CNS is exposed to anesthetic molecules with LOC and amnesia usually attributed to synaptic suppression in the cerebral cortex and immobility and analgesia to agent action in the spinal cord and brainstem. This model of patch-wise suppression has been challenged, however, by the observation that all functional components of anesthesia can be induced by focal delivery of minute quantities of GABAergic agonists to the brainstem mesopontine tegmental anesthesia area (MPTA). We compared spectral features of the cortical electroencephalogram (EEG) in rats during systemic anesthesia and anesthesia induced by MPTA microinjection. Systemic administration of (GABAergic) pentobarbital yielded the sustained, δ-band dominant EEG signature familiar in clinical anesthesia. In contrast, anesthesia induced by MPTA microinjection (pentobarbital or muscimol) featured epochs of δ-band EEG alternating with the wake-like EEG, the pattern typical of natural non-rapid-eye-movement (NREM) and REM sleep. The rats were not sleeping, however, as they remained immobile, atonic and unresponsive to noxious pinch. Recalling the paradoxical wake-like quality the EEG during REM sleep, we refer to this state as "paradoxical anesthesia". GABAergic anesthetics appear to co-opt both cortical and spinal components of the sleep network via dedicated axonal pathways driven by MPTA neurons. Direct drug exposure of cortical and spinal neurons is not necessary, and is probably responsible for off-target side-effects of systemic administration including monotonous δ-band EEG, hypothermia and respiratory depression. SIGNIFICANCE STATEMENT: The concept that GABAergic general anesthetic agents induce loss-of-consciousness by substituting for an endogenous neurotransmitter, thereby co-opting neural circuitry responsible for sleep-wake transitions, has gained considerable traction. However, the electroencephalographic (EEG) signatures of sleep and anesthesia differ fundamentally. We show that when the anesthetic state is generated by focal delivery of GABAergics into the mesopontine tegmental anesthesia area (MPTA) the resulting EEG repeatedly transitions between delta-wave-dominant and wake-like patterns much as in REM-NREM sleep. This suggests that systemic (clinical) anesthetic delivery, which indiscriminately floods the entire cerebrum with powerful inhibitory agents, obscures the sleep-like EEG signature associated with the less adulterated form of anesthesia obtained when the drugs are applied selectively to loci where the effective neurotransmitter substitution actually occurs.

Reduced Sensitivity to Anesthetic Agents upon Lesioning the Mesopontine Tegmental Anesthesia Area in Rats Depends on Anesthetic Type

Background

The brainstem mesopontine tegmental anesthesia area is a key node in circuitry responsible for anesthetic induction and maintenance. Microinjecting the γ-aminobutyric acid–mediated (GABAergic) anesthetic pentobarbital in this nucleus rapidly and reversibly induces general anesthesia, whereas lesioning it renders the animal relatively insensitive to pentobarbital administered systemically. This study investigated whether effects of lesioning the mesopontine tegmental anesthesia area generalize to other anesthetic agents.

Methods

Cell-selective lesions were made using ibotenic acid, and rats were later tested for changes in the dose–response relation to etomidate, propofol, alfaxalone/alfadolone, ketamine, and medetomidine delivered intravenously using a programmable infusion pump. Anesthetic induction for each agent was tracked using five behavioral endpoints: loss of righting reflex, criterion for anesthesia (score of 11 or higher), criterion for surgical anesthesia (score of 14 or higher), antinociception (loss of pinch response), and deep surgical anesthesia (score of 16).

Results

As reported previously for pentobarbital, on-target mesopontine tegmental anesthesia area lesions reduced sensitivity to the GABAergic anesthetics etomidate and propofol. The dose to achieve a score of 16 increased to 147 ± 50% of baseline in control animals ± SD (P = 0.0007; 7 lesioned rats and 18 controls) and 136 ± 58% of baseline (P = 0.010; 6 lesioned rats and 21 controls), respectively. In contrast, responsiveness to the neurosteroids alfaxalone and alfadolone remained unchanged compared with baseline (94 ± 24%; P = 0.519; 6 lesioned rats and 18 controls) and with ketamine increased slightly (90 ± 11%; P = 0.039; 6 lesioned rats and 19 controls). The non-GABAergic anesthetic medetomidine did not induce criterion anesthesia even at the maximal dose tested. The dose to reach the maximal anesthesia score actually obtained was unaffected by the lesion (112 ± 8%; P = 0.063; 5 lesioned rats and 18 controls).

Conclusions

Inability to induce anesthesia in lesioned animals using normally effective doses of etomidate, propofol, and pentobarbital suggests that the mesopontine tegmental anesthesia area is the effective target of these, but not necessarily all, GABAergic anesthetics upon systemic administration. Cortical and spinal functions are likely suppressed by recruitment of dedicated ascending and descending pathways rather than by direct, distributed drug action.

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Minimum alveolar concentration: Key concepts and a review of its pharmacological reduction in dogs. Part 1

OBJECTIVE

To outline the major components of the minimum alveolar concentration (MAC) and review the literature in regard to pharmacological manipulation of the MAC of halothane, isoflurane, sevoflurane, enflurane, and desflurane in dogs. The pharmacologic agents included are alpha-2 agonists, benzodiazepines, propofol, maropitant, opioids, lidocaine, acepromazine, non-steroidal anti-inflammatory agents, and NMDA antagonists. Part 1 will focus on summarizing the relevance, measurement, and mechanisms of MAC and review the effects of alpha-2 agonists, benzodiazepines, and propofol on MAC.

DATABASES USED

PubMed, Google Scholar, CAB Abstracts. Search terms used: minimum alveolar concentration, MAC, dog, canine, inhaled anesthetic potency, isoflurane, sevoflurane, desflurane, enflurane, and halothane.

CONCLUSIONS

Many drugs reduce the MAC of inhaled anesthetics in dogs, and allow for a clinically important decrease in inhalant anesthetic use. A decrease in MAC may decrease the adverse cardiovascular and pulmonary effects associated with the use of high concentrations of inhaled anesthetics.

Pancuronium enhances isoflurane anesthesia in rats via inhibition of cerebral nicotinic acetylcholine receptors

PURPOSE

This study was conducted to elucidate the mechanism of enhancement of volatile anesthetics by neuromuscular blocking agents in rats and to consider the relevance of this enhancement to clinical anesthesia.

METHODS

Male Sprague-Dawley rats were used. After confirming a movement in response to tail clamping under 1.1 % isoflurane anesthesia, response was determined when the tail clamp was applied at several points after microinjection of pancuronium into the lateral ventricle. Arousal responses to microinjection of nicotine into the lateral ventricle were assessed with or without pretreatment with intraventricular pancuronium. The intravenous 50 % effective dose (ED50) and 95 % effective dose (ED95) for neuromuscular blockade with pancuronium administered in a cumulative fashion at 1.1 % isoflurane were calculated.

RESULTS

Intraventricular pancuronium dose-dependently reduced the response to tail clamping, and the dose required to show immobilization of 50 % of rats (intraventricular ED50) was 1.62 µg/kg. Pretreatment with pancuronium at 6 µg/kg significantly reduced the effect of awakening by nicotine under isoflurane anesthesia (P = 0.044). The intravenous ED50 and ED95 for neuromuscular blockade were 63 µg/kg (90 % confidence interval [CI] 52-75 µg/kg) and 133 µg/kg (90 % CI 109-158 µg/kg), respectively. The ratio of intraventricular ED50 to intravenous ED50 was 0.026.

CONCLUSION

Pancuronium microinjection into the lateral ventricle dose-dependently enhances the depth of isoflurane anesthesia, which might be caused by inhibition of neuronal nicotinic acetylcholine receptor transmission in the cerebrum. Intravenous injection of pancuronium at high doses might increase the cerebrospinal concentration to a level at which an effect can be observed.

Xenon alters right ventricular function

BACKGROUND

In contrast to other volatile anesthetics, xenon produces less cardiovascular depression with fewer fluctuations of various hemodynamic parameters, but reduces cardiac output (CO) in vivo. Besides an increase in left ventricular afterload and reduction of heart rate, an impairment of the right ventricular function might be an additional pathophysiological mechanism for the reduction of CO. Therefore, we used an animal model to study the effects of xenon as a supplemental anesthetic on right ventricular function, especially right ventricular afterload.

METHODS

Right ventricular function was monitored with a volumetric pulmonary artery catheter in 11 pigs during general anesthesia with thiopental. Six animals received additional 70% (volume) xenon (equivalent to 0.55 MAC minimum alveolar concentration). Parameters for systolic function, afterload, and preload were calculated at baseline and during 50 min of xenon application, and in a corresponding control group. Significant differences were detected by multivariate analyses of variance for repeated measures.

RESULTS

Xenon reduced CO on average by 30% and increased pulmonary arterial elastance by 60%, which led to a reduction of the right ventricular ejection fraction by 25%. Whereas right ventricular preload remained stable, maximal slope of pulmonary artery pressure and the right ventricular elastance increased. No effect on the ratio of stroke work and end-diastolic volume was found.

CONCLUSION

The reduction in CO during xenon anesthesia was partly due to an impairment of the right ventricular function, mainly caused by an increased afterload, without an impairment of systolic ventricular function.

Neurons in the ventral spinal cord are more depressed by isoflurane, halothane, and propofol than are neurons in the dorsal spinal cord

BACKGROUND

Volatile anesthetics act primarily in the spinal cord to produce immobility but their exact site of action is unclear. Between 0.8 and 1.2 minimum alveolar anesthetic concentration (MAC), isoflurane does not depress neurons in the dorsal horn, suggesting that it acts at a more ventral site within the spinal cord such as in premotor interneurons and motoneurons. We hypothesized that isoflurane, halothane, and propofol would exert a greater depressant effect on nociceptive responses of ventral horn neurons when compared with dorsal horn neurons.

METHODS

Rats were anesthetized with isoflurane or halothane and responses of dorsal (<1200 microm deep) and ventral (>1200 microm) lumbar neurons to noxious mechanical stimulation of the hindpaw were determined at 0.8 and 1.2 MAC. In a third group anesthetized with isoflurane at 0.8 MAC, we administered 5 mg/kg propofol while recording responses from dorsal horn or ventral horn neurons.

RESULTS

Dorsal horn neuronal responses were not significantly affected when either isoflurane or halothane was increased from 0.8 to 1.2 MAC; propofol also had no significant effect. On the other hand, with increased isoflurane or halothane concentration, responses of ventral horn neurons were depressed by 60% and 45%, respectively. Propofol profoundly depressed (>90%) ventral horn neurons.

CONCLUSIONS

These data suggest that, in the peri-MAC range, isoflurane, halothane, and propofol have little or no effect on neuronal responses to noxious mechanical stimulation in the spinal dorsal horn but depress such responses in the ventral horn. Immobility produced in the 0.8-1.2 MAC range by these anesthetics appears to result from a depressant action in the ventral horn.

Sitting position does not alter minimum alveolar concentration for desflurane

PURPOSE

Hypotension is a common complication of the sitting position during anesthesia, and is often counteracted by decreasing anesthetic depth, thereby exposing patients to the risk of being inadequately anesthetized. Baroreceptor unloading and the consequent sympathoexcitation, as during head up tilt, decreases pain threshold and arouses the central nervous system (CNS), whereas hypotension exerts a direct CNS depressant effect. We estimated the minimal alveolar concentration (MAC) of desflurane for immobility in patients undergoing surgery in the sitting position, in comparison to MAC desflurane for patients having a similar type of surgery in the supine position.

METHODS

The Dixon up-and-down method was used to evaluate the MAC for desflurane in patients undergoing cervical spine laminoplasty (n = 24) or discectomy (n = 24) in the sitting and supine positions, respectively. Logistic regression with co-variate adjustment was employed to examine if the two positions (sitting and supine) have different or share the same concentration vs response relationship for immobility. Monte Carlo simulation was used to calculate 95% confidence intervals (CI) for the MAC in each position, and to estimate the difference in MAC (delta MAC) between the sitting and supine positions.

RESULTS

Modeling both sitting [6.54% (6.50-6.66, 95% CI)] and supine [6.70 (6.55-6.81)] patients as having different MAC concentrations did not significantly improve our simplified model, which treats the two patient groups as one [6.61 (6.52-6.70), delta -2 log likelihood = 2.735, P = 0.098]. Mean delta MAC (95% CI) was -0.14 (-0.30, 0.03).

CONCLUSION

The sitting position does not change desflurane anesthetic requirements for immobility.

Involvement of NMDA Receptors in Thiopental-Induced Loss of Righting Reflex, Antinociception and Anticonvulsion Effects in Mice

Potentiation of GABA(A) receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 microg i.c.v.) or bicuculline (0.6 microg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA(A) and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA(A) receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.

Neural pathways associated with loss of consciousness caused by intracerebral microinjection of GABAA-active anesthetics

Anesthesia, slow-wave sleep, syncope, concussion and reversible coma are behavioral states characterized by loss of consciousness, slow-wave cortical electroencephalogram, and motor and sensory suppression. We identified a focal area in the rat brainstem, the mesopontine tegmental anesthesia area (MPTA), at which microinjection of pentobarbital and other GABA(A) receptor (GABA(A)-R) agonists reversibly induced an anesthesia-like state. This effect was attenuated by local pre-treatment with the GABA(A)-R antagonist bicuculline. Using neuroanatomical tracing we identified four pathways ascending from the MPTA that are positioned to mediate electroencephalographic synchronization and loss of consciousness: (i) projections to the intralaminar thalamic nuclei that, in turn, project to the cortex; (ii) projections to several pontomesencephalic, diencephalic and basal forebrain nuclei that project cortically and are considered parts of an ascending "arousal system"; (iii) a projection to other parts of the subcortical forebrain, including the septal area, hypothalamus, zona incerta and striato-pallidal system, that may indirectly affect cortical arousal and hippocampal theta rhythm; and (iv) modest projections directly to the frontal cortex. Several of these areas have prominent reciprocal projections back to the MPTA, notably the zona incerta, lateral hypothalamus and frontal cortex. We hypothesize that barbiturate anesthetics and related agents microinjected into the MPTA enhance the inhibitory response of local GABA(A)-R-bearing neurons to endogenous GABA released at baseline during wakefulness. This modulates activity in one or more of the identified ascending neural pathways, ultimately leading to loss of consciousness.

Melatonin and anesthesia: a clinical perspective

The hypnotic, antinociceptive, and anticonvulsant properties of melatonin endow this neurohormone with the profile of a novel hypnotic-anesthetic agent. Sublingually or orally administered melatonin is an effective premedicant in adults and children. Melatonin premedication like midazolam is associated with sedation and preoperative anxiolysis, however, unlike midazolam these effects are not associated with impaired psychomotor skills or the quality of recovery. Melatonin administration also is associated with a tendency toward faster recovery and a lower incidence of postoperative excitement than midazolam. Oral premedication with 0.2 mg/kg melatonin significantly reduces the propofol and thiopental doses required for loss of responses to verbal commands and eyelash stimulation. In rats, melatonin and the more potent melatonin analogs 2-bromomelatonin and phenylmelatonin have been found to have anesthetic properties similar to those of thiopental and propofol, with the added advantage of providing potent antinociceptive effects. The exact mechanism(s) by which structurally diverse intravenous and volatile anesthetics produce general anesthesia is still largely unknown, but positive modulation of gamma-aminobutyric acid type A (GABAA) receptor function has been recognized as an important and common pathway underlying the depressant effects of many of these agents. Accumulating evidence indicates that there is interplay between the melatonergic and GABAergic systems, and it has been demonstrated that melatonin administration produces significant, dose-dependent increases in GABA concentrations in the central nervous system. Additional in vitro data suggest that melatonin alters GABAergic transmission by modulating GABAA receptor function. Of greater importance, data from in vivo studies suggest that the central anesthetic effects of melatonin are mediated, at least in part, via GABAergic system activation, as they can be blocked or reversed by GABAA receptor antagonists. Further work is needed to better understand the general anesthetic properties of melatonin at the molecular, cellular, and systems levels.

Projections from the mesopontine tegmental anesthesia area to regions involved in pain modulation

Pentobarbital microinjected into a restricted locus in the upper brainstem induces a general anesthesia-like state characterized by atonia, loss of consciousness, and pain suppression as assessed by loss of nocifensive response to noxious stimuli. This locus is the mesopontine tegmental anesthesia area (MPTA). Although anesthetic agents directly influence spinal cord nociceptive processing, antinociception during intracerebral microinjection indicates that they can also act supraspinally. Using neuroanatomical tracing methods we show that the MPTA has multiple descending projections to brainstem and spinal areas associated with pain modulation. Most prominent is a massive projection to the rostromedial medulla, a nodal region for descending pain modulation. Together with the periaqueductal gray (PAG), the MPTA is the major mesopontine input to this region. Less dense projections target the PAG, the locus coeruleus and pericoerulear areas, and dorsal and ventral reticular nuclei of the caudal medulla. The MPTA also has modest direct projections to the trigeminal nuclear complex and to superficial layers of the dorsal horn. Double anterograde and retrograde labeling at the light and electron microscopic levels shows that MPTA neurons with descending projections synapse directly on spinally projecting cells of rostromedial medulla. The prominence of the MPTA's projection to the rostromedial medulla suggests that, like the PAG, it may exert antinociceptive actions via this bulbospinal relay.

General anesthetics inhibit gap junction communication in cultured organotypic hippocampal slices

Gap junctions are protein channels that directly connect the cytosol of neighboring cells, thus forming electrical synapses and promoting synchronous neuronal activities. Such activities lead to the initiation and propagation of electroencephalogram oscillations implicated in cognition and consciousness. In this study, we investigated the effects of propofol, thiopental, and halothane on gap junction communication in cultured organotypic hippocampal slices by recovery of fluorescence after photo bleaching (FRAP) technique and electrophysiological recordings. Propofol 15 microM and thiopental 10 microM attenuated gap junction communication in slice cultures by 46.7% +/- 4.5% and 48.8% +/- 5.5%, respectively, as measured by FRAP. Smaller concentrations of propofol 5 microM and thiopental 2 microM did not change gap junction coupling. Accompanying the decreased gap junction communication, hippocampus slice cultures exposed to propofol 15 microM and thiopental 10 microM were found to have reduced electrophysiologic spontaneous discharges and primary after discharges evoked by a tetanic train of 50 Hz for 2 s. On the other hand, halothane 0.64 mM, a concentration slightly larger than twice its minimum alveolar concentration had no effect on gap junction coupling while halothane 2.8 mM blocked FRAP by 70%. The current study illustrates that anesthetic concentrations of propofol and thiopental, but not halothane, attenuate gap junction communication in cultured hippocampal slices. Suppression of gap junction function could compound the mechanisms of anesthetic actions.