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

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Genetic variation of rs12918566 affects GRIN2A expression and is associated with spontaneous movement response during sevoflurane anesthesia induction

N-methyl-D-aspartate (NMDA) receptors mediate excitatory neurotransmission in the nervous system and are preferentially inhibited by general anesthetics such as sevoflurane. Spontaneous movement is a common complication during sevoflurane anesthesia induction and seriously affects operations. In this study, we investigated the relationship between NMDA polymorphisms and spontaneous movement during sevoflurane induction. This prospective clinical study enrolled 393 patients undergoing sevoflurane anesthesia as part of their surgical routine. In the GRIN1, GRIN2A, and GRIN2B genes, 13 polymorphisms that form a heteromeric complex as part of the NMDA receptor were selected using Haploview and genotyped using matrix-assisted laser desorption ionization-time of flight mass spectrometry MassARRAY. Both RNAfold and Genotype-Tissue Expression portals were used to identify gene expression profiles. Our data showed that 35.8% of subjects exhibited spontaneous movement. The GRIN2A rs12918566 polymorphism was associated with spontaneous movement during sevoflurane induction. A logistic regression analysis of additive, dominant, and recessive models indicated a significant association (odds ratio [OR] (95% confidence limit [CI]): 0.58 (0.42-0.80), p = .00086; OR (95% CI): 0.51 (0.31-0.84), p = .0075, and OR (95% CI): 0.47 (0.27-0.81), p = .0060, respectively). After false discovery rate (FDR) correction, the additive model was still significant with a P_FDR =0.010. Bioinformatics demonstrated that the rs12918566 genomic variation affected GRIN2A expression in brain tissue. We also revealed that GRIN2A rs12918566 was significantly associated with spontaneous movement during sevoflurane induction. We believe the NMDA receptor plays an important role in regulating the anesthetic effects of sevoflurane.

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.

Comment on " In Vivo [18F]GE-179 Brain Signal Does Not Show NMDA-Specific Modulation with Drug Challenges in Rodents and Nonhuman Primates"

Schoenberger and colleagues ( Schoenberger et al. ( 2018 ) ACS Chem. Neurosci. 9 , 298 - 305 ) recently reported attempts to demonstrate specific binding of the positron emission tomography (PET) radiotracer, [¹⁸F]GE-179, to NMDA receptors in both rats and Rhesus macaques. GE-179 did not work as expected in animal models; however, we disagree with the authors' conclusion that "the [¹⁸F]GE-179 signal seems to be largely nonspecific". It is extremely challenging to demonstrate specific binding for the use-dependent NMDA receptor intrachannel ligands such as [¹⁸F]GE-179 in animals via traditional blocking, due to its low availability of target sites ( B_max^'). Schoenberger and colleagues anesthetized rats and Rhesus monkeys using isoflurane, which has an inhibitory effect on NMDA receptor function and thus would be expected to further reduce the B_max^'. The extent of glutamate release achieved in the provocation experiments is uncertain, as is whether a significant increase in NMDA receptor channel opening can be expected under anesthesia. Prior data suggest that the uptake of disubstituted arylguanidine-based ligands such as GE-179 can be reduced by phencyclidine binding site antagonists, if injection is performed in the absence of ketamine and isoflurane anesthesia, e.g., with GE-179's antecedent, CNS 5161 ( Biegon et al. ( 2007 ) Synapse 61 , 577 - 586 ), and with GMOM ( van der Doef et al. ( 2016 ) J. Cereb. Blood Flow Metab. 36 , 1111 - 1121 ). However, the extent of nonspecific uptake remains uncertain.

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.

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.

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.

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.