Intrathecal administration of N-methyl-D-aspartate receptor antagonist reduces the minimum alveolar anaesthetic concentration of isoflurane in rats

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Differential actions of isoflurane and ketamine-based anaesthetics on cochlear function in the mouse

Isoflurane is a volatile inhaled anaesthetic widely used in animal research, with particular utility for hearing research. Isoflurane has been shown to blunt hearing sensitivity compared with the awake state, but little is known about how isoflurane compares with other anaesthetics with regard to hair cell transduction and auditory neurotransmission. The current study was undertaken in C57Bl/6J and C129/SvEv strains of mice to determine whether isoflurane anaesthesia affects hearing function relative to ketamine-based anaesthesia. Cochlear function and central auditory transmission were assessed using auditory brainstem response (ABR) and distortion product otoacoustic emission (DPOAE), comparing thresholds and input/output functions over time, for isoflurane vs. ketamine/xylazine/acepromazine anaesthesia. ABR thresholds at the most sensitive region of hearing (16 kHz) were initially higher under isoflurane anaesthesia. This reduced hearing sensitivity worsened over the 1 h study period, and also became evident with broadband click stimulus. Ketamine anaesthesia provided stable ABR thresholds. Although the growth functions were unchanged over time for both anaesthetics, the slopes under isoflurane anaesthesia were significantly less. Cubic (2f(1)-f(2)) DPOAE thresholds and growth functions were initially similar for both anaesthetics. After 60 min, DPOAE thresholds increased for both groups, but this effect was significantly greater with ketamine anaesthesia. The isoflurane-mediated increase in ABR thresholds over time is attributable to action on cochlear nerve activation, evident as a right-shift in the P1-N1 input/output function compared to K/X/A. The ketamine-based anaesthetic produced stable ABR thresholds and gain over time, despite a right-shift in the outer hair cell - mediated DPOAE input/output function.

Inhibition of voltage-gated sodium channels by emulsified isoflurane may contribute to its subarachnoid anesthetic effect in beagle dogs

BACKGROUND

Volatile anesthetics, in addition to their general anesthesia action, have been proven to produce regional anesthetic effect in various animal models. The major aim of this study was to examine whether emulsified isoflurane (EI) could also produce subarachnoid anesthesia and to investigate its possible mechanism.

METHODS

Beagle dogs were randomly assigned into 5 groups (n = 6/group): intrathecally receiving 1% lidocaine 0.1 mL/kg, 30% intralipid 0.1 mL/kg (control), or 8% EI at doses of 0.05, 0.075, or 0.1 mL/kg, respectively. Consciousness state, motor function of limbs, and response to nociceptive stimulus were observed after drug administration. The effect of EI on voltage-gated Na channel was recorded from isolated spinal neurons of rats, using the whole-cell patch-clamp technique. Inhibition of peak sodium currents and effect of EI on Na channel gating were analyzed.

RESULTS

Emulsified isoflurane produced subarachnoid anesthesia in a dose-dependent manner, and at the dose of 0.1 mL/kg, the effect of 8% EI was similar to 1% lidocaine. Sodium channel currents were inhibited by EI at clinically relevant concentrations, with the IC50 (median inhibitory concentration) at 0.69 ± 0.08 mM. Voltage activation of Na channels was positive, shifted by isoflurane at the concentration of 0.77 mM, and V½ of activation (voltage for half-maximal activation) shifted from -12.4 ± 2.7 mV to -7.3 ± 2.3 mV (P < 0.01).

CONCLUSIONS

Emulsified isoflurane produced dose-dependent subarachnoid anesthesia, and this effect might be mediated by inhibition of EI on voltage-gated Na channels in the spinal cord.

GABA(A) positive modulator and NMDA antagonist-like discriminative stimulus effects of isoflurane vapor in mice

RATIONALE

Several neurotransmitter systems have been hypothesized to be involved in the in vivo effects of volatile anesthetics. Drug discrimination may represent a novel procedure to explore the neurochemical systems underlying the sub-anesthetic behavioral effects of these compounds.

OBJECTIVES

The purpose of the present study was to examine the contribution of GABA(A) and NMDA receptors to the discriminative stimulus effects of a behaviorally active sub-anesthetic concentration of isoflurane vapor.

METHODS

Sixteen B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6,000 ppm isoflurane vapor from air. Substitution tests were conducted with volatile anesthetics, abused vapors, GABA(A) positive modulators, NMDA antagonists, and nitrous oxide.

RESULTS

The volatile anesthetics, enflurane and halothane as well as the abused vapors toluene and 1,1,1-trichloroethane fully substituted for isoflurane. The GABA(A) positive modulators, pentobarbital, midazolam, and zaleplon but not the direct GABA(A) agonist, muscimol, produced high levels of partial substitution for isoflurane. The anticonvulsant, valproic acid fully substituted for isoflurane but a second, tiagabine, did not substitute. The competitive NMDA antagonist, CGS-19755, fully and the non-competitive NMDA antagonist, dizocilpine, partially substituted for isoflurane. The glycine-site NMDA antagonist, L-701,324 did not substitute for isoflurane. Gamma-hydroxybutric acid and nitrous oxide gas also failed to substitute for isoflurane.

CONCLUSIONS

The discriminative stimulus effects of sub-anesthetic concentrations of isoflurane vapor are shared by other vapor anesthetics and abused inhalants. The discriminative stimulus effects of isoflurane vapor appear to be mediated by both positive allosteric modulation of GABA(A) receptors as well as antagonism of NMDA receptors.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.

Contrasting Roles of the N-Methyl-d-Aspartate Receptor in the Production of Immobilization by Conventional and Aromatic Anesthetics

We hypothesized that N-methyl-d-aspartate (NMDA) receptors mediate some or all of the capacity of inhaled anesthetics to prevent movement in the face of noxious stimulation, and that this capacity to prevent movement correlates directly with the in vitro capacity of such anesthetics to block the NMDA receptor. To test this hypothesis, we measured the effect of IV infusion of the NMDA blockers dizocilpine (MK-801) and (R)-4-(3-phosphonopropyl) piperazine-2-carboxylic acid (CPP) to decrease the MAC (the minimum alveolar concentration of anesthetic that prevents movement in 50% of subjects given a noxious stimulation) of 8 conventional anesthetics (cyclopropane, desflurane, enflurane, halothane, isoflurane, nitrous oxide, sevoflurane, and xenon) and 8 aromatic compounds (benzene, fluorobenzene, o-difluorobenzene, p-difluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene) and, for comparison, etomidate. We postulated that MK-801 or CPP infusions would decrease MAC in inverse proportion to the in vitro capacity of these anesthetics to block the NMDA receptor. This notion proved correct for the aromatic inhaled anesthetics, but not for the conventional anesthetics. At the greatest infusion of MK-801 (32 microg x kg(-1) x min(-1)) the MACs of conventional anesthetics decreased by 59.4 +/- 3.4% (mean +/- sd) and at 8 microg x kg(-1) x min(-1) by 45.5 +/- 4.2%, a decrease not significantly different from a 51.4 +/- 19.0% decrease produced in the EC50 for etomidate, an anesthetic that acts solely by enhancing gamma-amino butyric acid (GABA) receptors. We conclude that some aromatic anesthetics may produce immobility in the face of noxious stimulation by blocking the action of glutamate on NMDA receptors but that conventional inhaled anesthetics do not.

Molecular and systemic mechanisms of general anaesthesia: the ‘multi-site and multiple mechanisms’ concept

PURPOSE OF REVIEW

Amnesia, hypnosis and immobility are essential components of general anaesthesia. This review highlights recent advances in our understanding of how these components are achieved at a molecular level.

RECENT FINDINGS

Commonly used volatile anaesthetic agents such as isoflurane or sevoflurane cause immobility by modulating multiple molecular targets predominantly in the spinal cord, including gamma-aminobutyric acidA receptors, glycine receptors, glutamate receptors and TREK-1 potassium channels. In contrast, intravenously applied drugs such as propofol or etomidate depress spinal motor reflexes almost exclusively via enhancing gamma-aminobutyric acidA receptor function. Studies on knock-in animals showed that etomidate and propofol act via gamma-aminobutyric acidA receptors containing beta3 subunits, whereas gamma-aminobutyric acidA receptors including alpha2 and gamma subunits mediate the myorelaxant properties of diazepam. These findings suggest that a large fraction of gamma-aminobutyric acidA receptors in the spinal cord assemble from alpha2, beta3 and most probably gamma2 subunits. The hypnotic actions of etomidate are mediated by beta3-containing gamma-aminobutyric acidA receptors expressed in the brain. In contrast, gamma-aminobutyric acidA receptors harbouring beta2 subunits produce sedation, but not hypnosis. Furthermore, there is growing evidence that extrasynaptic gamma-aminobutyric acidA receptors in the hippocampus containing alpha5 subunits contribute to amnesia.

SUMMARY

Clinical anaesthesia is based on drug actions at multiple anatomical sites in the brain. The finding that amnesia, hypnosis and immobility involve distinct molecular targets opens new avenues for developing improved therapeutic strategies in anaesthesia.

In vivo evaluation of [11C]N-(2-chloro-5-thiomethylphenyl)-N′- (3-methoxy-phenyl)-N′-methylguanidine ([11C]GMOM) as a potential PET radiotracer for the PCP/NMDA receptor

The development of imaging methods to measure changes in NMDA ion channel activation would provide a powerful means to probe the mechanisms of drugs and device based treatments (e.g., ECT) thought to alter glutamate neurotransmission. To provide a potential NMDA/PCP receptor PET tracer, we synthesized the radioligand [11C]GMOM (ki = 5.2 +/-0.3 nM; log P = 2.34) and evaluated this ligand in vivo in awake male rats and isoflurane anesthetized baboons. In rats, the regional brain uptake of [11C]GMOM ranged from 0.75+/-0.13% ID/g in the medulla and pons to 1.15+/-0.17% ID/g in the occipital cortex. MK801 (1 mg/kg i.v.) significantly reduced (24-28%) [11C]GMOM uptake in all regions. D-serine (10 mg/kg i.v.) increased [11C]GMOM %ID/g values in all regions (10-24%) reaching significance in the frontal cortex and cerebellum only. The NR2B ligand RO 25-6981 (10 mg/kg i.v.) reduced [11C]GMOM uptake significantly (24-38%) in all regions except for the cerebellum and striatum. Blood activity was 0.11+/-0.03 %ID/g in the controls group and did not vary significantly across groups. PET imaging in isoflurane-anesthetized baboons with high specific activity [11C]GMOM provided fairly uniform regional brain distribution volume (VT) values (12.8-17.1 ml g(-1)). MK801 (0.5 mg/kg, i.v., n = 1, and 1.0 mg/kg, i.v., n = 1) did not significantly alter regional VT values, indicating a lack of saturable binding. However, the potential confounding effects associated with ketamine induction of anesthesia along with isoflurane maintenance must be considered because both agents are known to reduce NMDA ion channel activation. Future and carefully designed studies, presumably utilizing an optimized NMDA/PCP site tracer, will be carried out to further explore these hypotheses. We conclude that, even though [11C]GMOM is not an optimized PCP site radiotracer, its binding is altered in vivo in awake rats as expected by modulation of NMDA ion channel activity by MK801, D-serine or RO 25-6981. The development of higher affinity NMDA/PCP site radioligands is in progress.

Questioning the cardiocirculatory excitatory effects of opioids under volatile anaesthesia

BACKGROUND

Opioid-induced hyperalgesia has been demonstrated in awake animals. We observed an increased haemodynamic reactivity in response to noxious stimuli in rats under sevoflurane anaesthesia treated with a very low dose of sufentanil. The aim of this investigation was to determine whether the two phenomena share a common origin: an opioid-induced excitatory reaction. To address this, we administered several drugs with proven efficacy in opioid hyperalgesia to rats presenting with haemodynamic hyper-reactivity.

METHODS

The MACbar of sevoflurane was measured in controls and in animals treated with sufentanil 0.005 micro g kg(-1) min(-1) before and after administration of i.v. (0.25, 0.5 mg kg(-1)) and intrathecal (i.t.) (250 micro g) ketamine, i.v. (0.5, 1 mg kg(-1)) and i.t. (30 micro g) MK-801(NMDA antagonist), i.v. (0.1, 0.5 mg kg(-1)) naloxone, i.v. (10 mg kg(-1)) and i.t. (50, 100 micro g) ketorolac or i.t. (100, 150 micro g) meloxicam (COX-2 inhibitor).

RESULTS

Sufentanil 0.005 micro g kg(-1) min(-1) significantly increased MACbar (3.2 (sd 0.3) versus 1.9 (0.3) vol%). With the exception of naloxone, all drugs displayed a significant MACbar-sparing effect (>50%) in controls. Naloxone completely prevented haemodynamic hyperactivity. Two patterns of reaction were recorded for the other drugs: either hyper-reactivity was suppressed and the MACbar-sparing effect was maintained (i.t. ketamine, i.t. MK-801, i.t. ketorolac [100 micro g], i.t. meloxicam [150 micro g]) or hyper-reactivity was blocked but MACbar-sparing effect was lost (i.v. ketamine [0.5 mg kg(-1)], i.v. MK-801 [0.5, 1 mg kg(-1)], i.v. ketorolac [10 micro g kg(-1)], i.t. ketorolac [50 micro g], i.t. meloxicam [100 micro g]).

CONCLUSIONS

We have demonstrated that low-dose sufentanil-induced haemodynamic hyper-reactivity is an excitatory micro -opiate-related phenomenon. This effect is reversed by drugs effective in treating opiate-induced hyperalgesia.

Imaging the PCP site of the NMDA ion channel

The N-methyl-D-aspartate (NMDA) ion channel plays a role in neuroprotection, neurodegeneration, long-term potentiation, memory, and cognition. It is implicated in the pathophysiology of several neurological and neuropsychiatric disorders including Parkinson's Disease, Huntington's Chorea, schizophrenia, alcoholism and stroke. The development of effective radiotracers for the study of NMDA receptors is critical for our understanding of their function, and their modulation by endogenous substances or therapeutic drugs. Since the NMDA/PCP receptor lies within the channel, it is a unique target and is theoretically accessible only when the channel is in the active and "open" state, but not when it is in the inactive or "closed" state. The physical location of the NMDA/PCP receptor not only makes it an important imaging target but also complicates the development of suitable PET and SPECT radiotracers for this site. An intimate understanding of the biochemical, pharmacological, physiological and behavioral processes associated with the NMDA ion channel is essential to develop improved imaging agents. This review outlines progress made towards the development of radiolabeled agents for PCP sites of the NMDA ion channel. In addition, the animal and pharmacological models used for in vitro and in vivo assessment of NMDA receptor targeted agents are discussed.

Inhaled anesthetics and immobility: mechanisms, mysteries, and minimum alveolar anesthetic concentration

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific amino acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., gamma-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and alpha(2)-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-D-aspartate, and sodium) remain credible candidates.

Spinal N-methyl-d-aspartate receptors may contribute to the immobilizing action of isoflurane

We examined whether N-methyl-D-aspartate (NMDA) receptors influence the immobilizing effect of isoflurane by a spinal or supraspinal action. We antagonized NMDA receptors by intrathecal (IT), intracerebroventricular (ICV), and IV administration of MK 801 (a noncompetitive NMDA antagonist) and measured the decrease in isoflurane minimum alveolar anesthetic concentration (MAC). We also measured MK 801 tissue concentrations in homogenates of upper and lower spinal cord, a slice of cerebral cortex, and the whole brain. IT infusion of MK 801 decreased isoflurane MAC more potently than ICV or IV infusions. The change in MAC correlated with the MK 801 concentration in the lower part of the spinal cord (P < 0.01) but not with concentrations in supraspinal tissue. The maximal effect of IT MK 801 reached a plateau without achieving anesthesia. IV doses 270-fold larger than the largest IT dose also did not produce anesthesia in the absence of isoflurane. These results suggest that the capacity of MK 801 to decrease the MAC of isoflurane results from an effect on the spinal cord but that spinal NMDA receptors provide only partial mediation of the immobility produced by isoflurane. Because neither IT nor IV MK 801 provide complete anesthesia, these findings also call into question the notion that NMDA blockade alone suffices to produce anesthesia as defined by immobility in the face of noxious stimulation.

IMPLICATIONS

Spinal cord NMDA receptors may mediate a portion of the immobilizing effect of isoflurane. Blockade of NMDA receptors in the cord by MK 801 has a MAC-sparing effect, but MK 801 does not, by itself, produce complete anesthesia.

The inhibition of the N-methyl-D-aspartate receptor channel by local anesthetics in mouse CA1 pyramidal neurons

Although the effects of local anesthetics on sodium channels and various other channels and receptors have been intensively investigated, there is little information available about their effects on N-methyl-D-aspartate (NMDA) receptors. We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Procaine and tetracaine produced a reversible and concentration-dependent inhibition of NMDA-induced currents, but lidocaine showed little inhibition at 1 mM or less. The half-maximal inhibition values (mM; mean +/- SEM) for procaine, tetracaine, bupivacaine, and lidocaine at -60 mV were 0.296 +/- 0.031, 0.637 +/- 0.044, 2.781 +/- 0.940 (extrapolated data), and 7.766 +/- 14.093 (extrapolated data), respectively. Procaine 0.2 mM reduced the maximal NMDA-induced currents without affecting the 50% effective concentration values for NMDA. The inhibition by procaine exhibited voltage dependence and was more effective at negative potentials. These results indicate a noncompetitive antagonism of procaine on NMDA receptors and suggest that the inhibition is the result of a channel-blocking mechanism.

IMPLICATIONS

We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Both procaine and tetracaine produced a reversible and concentration-dependent inhibition of the NMDA-induced currents.

Neither GABA(A) nor strychnine-sensitive glycine receptors are the sole mediators of MAC for isoflurane

Inhaled anesthetics produce immobility (a cardinal aspect of general anesthesia) by an action on the spinal cord, possibly by potentiating the responses of gamma-amino-n-butyric acid (GABA(A)) and glycine receptors to GABA and glycine. In this study, we antagonized GABA(A) and glycine responses by intrathecal administration of picrotoxin (a noncompetitive GABA(A) antagonist), strychnine (a competitive glycine antagonist), or combinations of these drugs. We measured the capacity of antagonist infusion to increase isoflurane MAC (the minimum alveolar concentration of anesthetic that prevents movement in response to noxious stimuli in 50% of subjects). We found that these potent GABA(A) and glycine receptor antagonists had a ceiling effect, either alone or in combination increasing the MAC of isoflurane by at most 47%.

IMPLICATIONS

gamma-amino-n-butyric acid and glycine receptors may in part be responsible for the immobilizing action of isoflurane. They are not, however, the only receptors that contribute to isoflurane-induced immobility (i.e., that determine the MAC of isoflurane).

Intrathecal neurokinin-1 receptor antagonist reduces isoflurane MAC in rats

PURPOSE

To study the effects of intrathecal administration of a neurokinin-I(NK-I) receptor antagonist (CP96,345) on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in anaesthetized rats, and on the locomotive activity of conscious rats.

METHODS

Wistar rats (n = 36) were fitted with indwelling intrathecal catheters, and the MAC of isoflurane was determined following the intrathecal administration of saline (control group) or the NK-I receptor antagonist CP96,345 (CP) at 1, 10 and 100 micrograms. Subsequently a reversal dose of intrathecal Substance P (SP) at 1, 10 and 100 micrograms was administered and MAC isoflurane was redetermined. Conscious rats (n = 35) were also examined for the presence of locomotor dysfunction following intrathecal administration of CP and SP. Animals were randomly assigned to each treatment group and the investigators were blinded.

RESULTS

CP at 10 and 100 micrograms reduced MAC isoflurane by 9.9% and 15.3%, respectively (P < 0.05). Intrathecal administration of SP reversed the decreases in MAC by CP; however, locomotive activity was not changed.

CONCLUSION

These results suggest that the NK-1 receptor plays an important role in determining the MAC of isoflurane by inhibition of pain transmission in the spinal cord.

Intrathecally administered NMDA receptor antagonists reduce the MAC of isoflurane in rats

PURPOSE

We studied the effects of intrathecal administration of an N-methyl-D-aspartate (NMDA) receptor antagonist and an antagonist of the glycine site of the NMDA receptor on the minimum alveolar anaesthetic concentration (MAC) of isoflurane in rats, and on locomotor activity in conscious rats.

METHODS

In Wistar rats fitted with indwelling intrathecal catheters, we determined the MAC of isoflurane after the administration of saline (control group); the competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosponic acid(CPP) at 0.01, 0.1, and 1.0 nM; and the selective antagonist of the glycine site on the NMDA receptor complex 7-chlorokynurenic acid (7CKA) at 0.1, 1.0, and 10 nM. After measurement of MAC following administration of the antagonist, the equipotent reversal dose of NMDA or D-serine was administered. The rats were examined for the presence of locomotor dysfunction by intrathecal administration of NMDA receptor antagonists, NMDA and D-serine in conscious rats. All of the experiments were performed using randomization and masking of drugs.

RESULTS

CPP at 0.1 and 1.0 nM decreased the MAC of isoflurane by 9.9-17.6% (P < 0.05). 7CKA at 1.0 and 10 nM reduced MAC from 10.5-15.5% (P < 0.05). Intrathecal administration of NMDA or D-serine reversed the decreases in MAC to control values. Locomotor activity was not changed.

CONCLUSIONS

We believe that NMDA receptor plays an important role in determining the MAC of isoflurane in the spinal cord.