Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Non-NMDA Mechanisms of Analgesia in Ketamine Analogs

Despite 50 years of clinical use and experimental endeavor the anesthetic, analgesic, and psychomimetic effects of ketamine remain to be fully elucidated. While NMDA receptor antagonism has been long held as ketamine's fundamental molecular action, interrogation of bespoke ketamine analogs with known absent NMDA binding, yet profound anesthetic and analgesia fingerprints, suggests alternative targets are responsible for these effects. Herein we describe experimental findings utilizing such analogs as probes to explore ketamine-based analgesic molecular targets. We have focused on two-pore potassium leak channels, identifying TWIK channels as a rational target to pursue further. While the totality of ketamine's mechanistic action is yet to be fully determined, these investigations raise the intriguing prospect of separating out analgesia and anesthetic effects from ketamine's undesirable psychomimesis—and development of more specific analgesic medications.

Intense Acute Swimming Induces Delayed-Onset Muscle Soreness Dependent on Spinal Cord Neuroinflammation

Unaccustomed exercise involving eccentric contractions, high intensity, or long duration are recognized to induce delayed-onset muscle soreness (DOMS). Myocyte damage and inflammation in affected peripheral tissues contribute to sensitize muscle nociceptors leading to muscle pain. However, despite the essential role of the spinal cord in the regulation of pain, spinal cord neuroinflammatory mechanisms in intense swimming-induced DOMS remain to be investigated. We hypothesized that spinal cord neuroinflammation contributes to DOMS. C57BL/6 mice swam for 2 h to induce DOMS, and nociceptive spinal cord mechanisms were evaluated. DOMS triggered the activation of astrocytes and microglia in the spinal cord 24 h after exercise compared to the sham group. DOMS and DOMS-induced spinal cord nuclear factor κB (NFκB) activation were reduced by intrathecal treatments with glial inhibitors (fluorocitrate, α-aminoadipate, and minocycline) and NFκB inhibitor [pyrrolidine dithiocarbamate (PDTC)]. Moreover, DOMS was also reduced by intrathecal treatments targeting C-X₃-C motif chemokine ligand 1 (CX₃CL1), tumor necrosis factor (TNF)-α, and interleukin (IL)-1β or with recombinant IL-10. In agreement, DOMS induced the mRNA and protein expressions of CX₃CR1, TNF-α, IL-1β, IL-10, c-Fos, and oxidative stress in the spinal cord. All these immune and cellular alterations triggered by DOMS were amenable by intrathecal treatments with glial and NFκB inhibitors. These results support a role for spinal cord glial cells, via NFκB, cytokines/chemokines, and oxidative stress, in DOMS. Thus, unveiling neuroinflammatory mechanisms by which unaccustomed exercise induces central sensitization and consequently DOMS.

Experimental Trypanosoma cruzi Infection Induces Pain in Mice Dependent on Early Spinal Cord Glial Cells and NFκB Activation and Cytokine Production

The neglected tropical infirmity Chagas disease (CD) presents high mortality. Its etiological agent T. cruzi is transmitted by infected hematophagous insects. Symptoms of the acute phase of the infection include fever, fatigue, body aches, and headache, making diagnosis difficult as they are present in other illnesses as well. Thus, in endemic areas, individuals with undetermined pain may be considered for CD. Although pain is a characteristic symptom of CD, its cellular and molecular mechanisms are unknown except for demonstration of a role for peripheral TNF-α in CD pain. In this study, we evaluate the role of spinal cord glial cells in experimental T. cruzi infection in the context of pain using C57BL/6 mice. Pain, parasitemia, survival, and glial and neuronal function as well as NFκB activation and cytokine/chemokine production were assessed. T. cruzi infection induced chronic mechanical and thermal hyperalgesia. Systemic TNF-α and IL-1β peaked 14 days postinfection (p.i.). Infected mice presented increased spinal gliosis and NFκB activation compared to uninfected mice at 7 days p.i. Glial and NFκB inhibitors limited T. cruzi–induced pain. Nuclear phosphorylated NFκB was detected surrounded by glia markers, and glial inhibitors reduced its detection. T. cruzi–induced spinal cord production of cytokines/chemokines was also diminished by glial inhibitors. Dorsal root ganglia (DRG) neurons presented increased activity in infected mice, and the production of inflammatory mediators was counteracted by glial/NFκB inhibitors. The present study unveils the contribution of DRG and spinal cord cellular and molecular events leading to pain in T. cruzi infection, contributing to a better understanding of CD pathology.

A Metabolic Mechanism for Anaesthetic Suppression of Cortical Synaptic Function in Mouse Brain Slices-A Pilot Investigation

Regulation of synaptically located ionotropic receptors is thought to be the main mechanism by which anaesthetics cause unconsciousness. An alternative explanation, which has received much less attention, is that of primary anaesthetic disruption of brain metabolism via suppression of mitochondrial proteins. In this pilot study in mouse cortical slices, we investigated the effect of disrupting cellular metabolism on tissue oxygen handling and cortical population seizure-like event (SLE) activity, using the mitochondrial complex I inhibitor rotenone, and compared this to the effects of the general anaesthetics sevoflurane, propofol and ketamine. Rotenone caused an increase in tissue oxygen (98 mmHg to 157 mmHg (p < 0.01)) before any measurable change in SLE activity. Thereafter, tissue oxygen continued to increase and was accompanied by a significant and prolonged reduction in SLE root mean square (RMS) activity (baseline RMS of 1.7 to 0.7 µV, p < 0.001) and SLE frequency (baseline 4.2 to 0.4 events/min, p = 0.001). This temporal sequence of effects was replicated by all three anaesthetic drugs. In conclusion, anaesthetics with differing synaptic receptor mechanisms all effect changes in tissue oxygen handling and cortical network activity, consistent with a common inhibitory effect on mitochondrial function. The temporal sequence suggests that the observed synaptic depression—as seen in anaesthesia—may be secondary to a reduction in cellular metabolic capacity.

The effect of general anaesthetics on brain lactate release

The effects of anaesthetic agents on brain energy metabolism may explain their shared neurophysiological actions but remain poorly understood. The brain lactate shuttle hypothesis proposes that lactate, provided by astrocytes, is an important neuronal energy substrate. Here we tested the hypothesis that anaesthetic agents impair the brain lactate shuttle by interfering with astrocytic glycolysis. Lactate biosensors were used to record changes in lactate release by adult rat brainstem and cortical slices in response to thiopental, propofol and etomidate. Changes in cytosolic nicotinamide adenine dinucleotide reduced (NADH) and oxidized (NAD⁺) ratio as a measure of glycolytic rate were recorded in cultured astrocytes. It was found that in brainstem slices thiopental, propofol and etomidate reduced lactate release by 7.4 ± 3.6% (P < 0.001), 9.7 ± 6.6% (P < 0.001) and 8.0 ± 7.8% (P = 0.04), respectively. In cortical slices, thiopental reduced lactate release by 8.2 ± 5.6% (P = 0.002) and propofol by 6.0 ± 4.5% (P = 0.009). Lactate release in cortical slices measured during the light phase (period of sleep/low activity) was ~25% lower than that measured during the dark phase (period of wakefulness) (326 ± 83 μM vs 430 ± 118 μM, n = 10; P = 0.04). Thiopental and etomidate induced proportionally similar decreases in cytosolic [NADH]:[NAD⁺] ratio in astrocytes, indicative of a reduction in glycolytic rate. These data suggest that anaesthetic agents inhibit astrocytic glycolysis and reduce the level of extracellular lactate in the brain. Similar reductions in brain lactate release occur during natural state of sleep, suggesting that general anaesthesia may recapitulate some of the effects of sleep on brain energy metabolism.

Ketamine Action in the In Vitro Cortical Slice Is Mitigated by Potassium Channel Blockade

BACKGROUND

Ketamine is a general anesthetic thought to act by antagonizing N-methyl-D-aspartate receptors. However, ketamine acts on multiple channels, many of which are potential targets-including hyperpolarization-activated cyclic nucleotide-gated and potassium channels. In this study we tested the hypothesis that potassium leak channels contribute to the anesthetic action of ketamine.

METHODS

Adult mouse cortical slices (400 µm) were exposed to no-magnesium artificial cerebrospinal fluid to generate seizure-like event activity. The reduction in seizure-like event frequency after exposure to ketamine (n = 14) was quantified as a signature of anesthetic effect. Pharmacologic manipulation of hyperpolarization-activated cyclic nucleotide-gated and potassium channels using ZD7288 (n = 11), cesium chloride (n = 10), barium chloride (n = 10), low-potassium (1.5 mM) artificial cerebrospinal fluid (n = 10), and urethane (n = 7) were investigated.

RESULTS

Ketamine reduced the frequency of seizure-like events (mean [SD], -62 [22]%, P < 0.0001). Selective hyperpolarization-activated cyclic nucleotide-gated channel block with ZD7288 did not significantly alter the potency of ketamine to inhibit seizure-like event activity. The inhibition of seizure-like event frequency by ketamine was fully antagonized by the potassium channel blockers cesium chloride and barium chloride (8 [26]% and 39 [58%] increase, respectively, P < 0.0001, compared to ketamine control) and was facilitated by the potassium leak channel opener urethane (-93 [8]%, P = 0.002 compared to ketamine control) and low potassium artificial cerebrospinal fluid (-86 [11]%, P = 0.004 compared to ketamine control).

CONCLUSIONS

The results of this study show that mechanisms additional to hyperpolarization-activated cyclic nucleotide-gated channel block are likely to explain the anesthetic action of ketamine and suggest facilitatory action at two-pore potassium leak channels.

Understanding the Effects of General Anesthetics on Cortical Network Activity Using Ex Vivo Preparations

General anesthetics have been used to ablate consciousness during surgery for more than 150 yr. Despite significant advances in our understanding of their molecular-level pharmacologic effects, comparatively little is known about how anesthetics alter brain dynamics to cause unconsciousness. Consequently, while anesthesia practice is now routine and safe, there are many vagaries that remain unexplained. In this paper, the authors review the evidence that cortical network activity is particularly sensitive to general anesthetics, and suggest that disruption to communication in, and/or among, cortical brain regions is a common mechanism of anesthesia that ultimately produces loss of consciousness. The authors review data from acute brain slices and organotypic cultures showing that anesthetics with differing molecular mechanisms of action share in common the ability to impair neurophysiologic communication. While many questions remain, together, ex vivo and in vivo investigations suggest that a unified understanding of both clinical anesthesia and the neural basis of consciousness is attainable.

Mitochondrial Function in Astrocytes Is Essential for Normal Emergence from Anesthesia in Mice

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics.Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission.

WHAT THIS ARTICLE TELLS US THAT IS NEW

In a tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse, the induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection. However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were half that of the controls.Similarly, the induction EC50s for loss of righting reflex were similar between the control and astrocyte-specific Ndufs4(KO) mice; concentrations for regain of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were much less than the control.Thus, mitochondrial complex I function within astrocytes is essential for normal emergence from anesthesia.

BACKGROUND

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics similar to that seen with global genetic knockout of Ndufs4. Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission. Therefore, the authors examined the role of astrocytes in the anesthetic hypersensitivity of Ndufs4(KO).

METHODS

A tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse was constructed. The specificity of the astrocyte-specific inducible model was confirmed by using the green fluorescent protein reporter line Ai6. Approximately 120 astrocyte-specific knockout and control mice were used for the experiments. Mice were anesthetized with varying concentrations of isoflurane or halothane; loss of righting reflex and response to a tail clamp were determined and quantified as the induction and emergence EC50s. Because norepinephrine has been implicated in emergence from anesthesia and astrocytes respond to norepinephrine to release gliotransmitters, the authors measured norepinephrine levels in the brains of control and knockout Ndufs4 animals.

RESULTS

The induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection (induction concentration, EC50(ind)-isoflurane: control = 1.27 ± 0.12, astrocyte-specific knockout = 1.21 ± 0.18, P = 0.495; halothane: control = 1.28 ± 0.05, astrocyte-specific knockout = 1.20 ± 0.05, P = 0.017). However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the controls for tail clamp; (emergence concentration, EC50(em)-isoflurane: control = 1.18 ± 0.10, astrocyte-specific knockout = 0.67 ± 0.11, P < 0.0001; halothane: control = 1.08 ± 0.09, astrocyte-specific knockout = 0.59 ± 0.12, P < 0.0001). The induction EC50s for loss of righting reflex were also similar between the control and astrocyte-specific Ndufs4(KO) mice (EC50(ind)-isoflurane: control = 1.02 ± 0.10, astrocyte-specific knockout = 0.97 ± 0.06, P = 0.264; halothane: control = 1.03 ± 0.05, astrocyte-specific knockout = 0.99 ± 0.08, P = 0.207). The emergent concentrations for loss of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the control (EC50(em)-isoflurane: control = 1.0 ± 0.07, astrocyte-specific knockout = 0.62 ± 0.12, P < 0.0001; halothane: control = 1.0 ± 0.04, astrocyte-specific KO = 0.64 ± 0.09, P < 0.0001); N ≥ 6 for control and astrocyte-specific Ndufs4(KO) mice. For all tests, similar results were seen at 7 weeks after 4-hydroxy tamoxifen injection. The total norepinephrine content of the brain in global or astrocyte-specific Ndufs4(KO) mice was unchanged compared to control mice.

CONCLUSIONS

The only phenotype of the astrocyte-specific Ndufs4(KO) mouse was a specific impairment in emergence from volatile anesthetic-induced general anesthesia. The authors conclude that normal mitochondrial function within astrocytes is essential for emergence from anesthesia.