Mechanisms and functional significance of inhibition of neuronal T-type calcium channels by isoflurane

Toward a brighter constellation: multiorgan neuroimaging of neural and vascular dynamics in the spinal cord and brain

Significance

Pain comprises a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

We aimed to develop and validate tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations was targeted to developing novel imaging hardware that addresses the many challenges of multisite imaging. The second key set of innovations was targeted to enabling bioluminescent (BL) imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity, and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for BL imaging and developed a novel modified miniscope optimized for these signals (BLmini).

Results

We describe "universal" implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of BL signals in both foci and a new miniscope, the "BLmini," which has reduced weight, cost, and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D-printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a coalition of methods for understanding spinal cord-brain interactions. Our work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Towards a Brighter Constellation: Multi-Organ Neuroimaging of Neural and Vascular Dynamics in the Spinal Cord and Brain

Significance

Pain is comprised of a complex interaction between motor action and somatosensation that is dependent on dynamic interactions between the brain and spinal cord. This makes understanding pain particularly challenging as it involves rich interactions between many circuits (e.g., neural and vascular) and signaling cascades throughout the body. As such, experimentation on a single region may lead to an incomplete and potentially incorrect understanding of crucial underlying mechanisms.

Aim

Here, we aimed to develop and validate new tools to enable detailed and extended observation of neural and vascular activity in the brain and spinal cord. The first key set of innovations were targeted to developing novel imaging hardware that addresses the many challenges of multi-site imaging. The second key set of innovations were targeted to enabling bioluminescent imaging, as this approach can address limitations of fluorescent microscopy including photobleaching, phototoxicity and decreased resolution due to scattering of excitation signals.

Approach

We designed 3D-printed brain and spinal cord implants to enable effective surgical implantations and optical access with wearable miniscopes or an open window (e.g., for one- or two-photon microscopy or optogenetic stimulation). We also tested the viability for bioluminescent imaging, and developed a novel modified miniscope optimized for these signals (BLmini).

Results

Here, we describe novel 'universal' implants for acute and chronic simultaneous brain-spinal cord imaging and optical stimulation. We further describe successful imaging of bioluminescent signals in both foci, and a new miniscope, the 'BLmini,' which has reduced weight, cost and form-factor relative to standard wearable miniscopes.

Conclusions

The combination of 3D printed implants, advanced imaging tools, and bioluminescence imaging techniques offers a new coalition of methods for understanding spinal cord-brain interactions. This work has the potential for use in future research into neuropathic pain and other sensory disorders and motor behavior.

Evaluation of potential anticonvulsant fluorinated N-benzamide enaminones as T-type Ca2+ channel blockers

Trifluoromethylated N-benzamide enaminones have been identified as potential anticonvulsants for the treatment of drug-resistant epilepsy. T-type Ca²⁺ channels are an important target for anti-seizure medications. Our laboratory has developed several fluorinated N-benzamide enaminone analogs that were evaluated by their ability to target T-type Ca²⁺ channels. Using whole cell voltage-clamp recordings, we identified two meta-trifluoromethyl N-benzamide enaminones with a significant inhibitory effect on T-type Ca²⁺ channels. These compounds had no effect on voltage-activated Na⁺ channels. We also evaluated the effect of the fluorinated N-benzamide enaminone analogs on the T-type Ca²⁺ channel subunits Cav3.2 and Cav3.3. The meta-trifluoromethyl N-benzamide enaminone lead analogs altered the steady-state inactivation of Cav3.2 T-type Ca²⁺ channels, which resulted in a significant increase in the inactivation recovery time of the channels. There was no effect of fluorinated N-benzamide enaminone analogs on the gating mechanism of T-type Ca²⁺ channels, as proven by the lack of effect on the activation and inactivation time constant of Ca²⁺ currents. On the contrary, the meta-trifluoromethyl N-benzamide enaminone lead analogs altered the gating mechanism of Cav3.3 T-type Ca²⁺ channels, as proven by the reduction in the activation and inactivation time constant of the channels. There was no effect on the inactivation kinetics of Cav3.3 T-type Ca²⁺ channels. The present results demonstrate that meta-substituted trifluoromethyl N-benzamide enaminone analogs target T-type Ca²⁺ channels by different mechanisms depending on the channel subunit. Meta-trifluoromethyl N-benzamide enaminone analogs can potentially lead to the design of more specific blockers of T-type Ca²⁺ channels for the treatment of epileptic seizures.

Thalamic T-Type Calcium Channels as Targets for Hypnotics and General Anesthetics

General anesthetics mainly act by modulating synaptic inhibition on the one hand (the potentiation of GABA transmission) or synaptic excitation on the other (the inhibition of NMDA receptors), but they can also have effects on numerous other proteins, receptors, and channels. The effects of general anesthetics on ion channels have been the subject of research since the publication of reports of direct actions of these drugs on ion channel proteins. In particular, there is considerable interest in T-type voltage-gated calcium channels that are abundantly expressed in the thalamus, where they control patterns of cellular excitability and thalamocortical oscillations during awake and sleep states. Here, we summarized and discussed our recent studies focused on the Ca_V3.1 isoform of T-channels in the nonspecific thalamus (intralaminar and midline nuclei), which acts as a key hub through which natural sleep and general anesthesia are initiated. We used mouse genetics and in vivo and ex vivo electrophysiology to study the role of thalamic T-channels in hypnosis induced by a standard general anesthetic, isoflurane, as well as novel neuroactive steroids. From the results of this study, we conclude that Ca_V3.1 channels contribute to thalamocortical oscillations during anesthetic-induced hypnosis, particularly the slow-frequency range of δ oscillations (0.5–4 Hz), by generating “window current” that contributes to the resting membrane potential. We posit that the role of the thalamic Ca_V3.1 isoform of T-channels in the effects of various classes of general anesthetics warrants consideration.

General Anesthesia and the Young Brain: The Importance of Novel Strategies with Alternate Mechanisms of Action

Over the past three decades, we have been grappling with rapidly accumulating evidence that general anesthetics (GAs) may not be as innocuous for the young brain as we previously believed. The growing realization comes from hundreds of animal studies in numerous species, from nematodes to higher mammals. These studies argue that early exposure to commonly used GAs causes widespread apoptotic neurodegeneration in brain regions critical to cognition and socio-emotional development, kills a substantial number of neurons in the young brain, and, importantly, results in lasting disturbances in neuronal synaptic communication within the remaining neuronal networks. Notably, these outcomes are often associated with long-term impairments in multiple cognitive-affective domains. Not only do preclinical studies clearly demonstrate GA-induced neurotoxicity when the exposures occur in early life, but there is a growing body of clinical literature reporting similar cognitive-affective abnormalities in young children who require GAs. The need to consider alternative GAs led us to focus on synthetic neuroactive steroid analogues that have emerged as effective hypnotics, and analgesics that are apparently devoid of neurotoxic effects and long-term cognitive impairments. This would suggest that certain steroid analogues with different cellular targets and mechanisms of action may be safe alternatives to currently used GAs. Herein we summarize our current knowledge of neuroactive steroids as promising novel GAs.

Synthetic neuroactive steroids as new sedatives and anaesthetics: Back to the future

Since the 1990s, there has been waning interest in researching general anaesthetics (anaesthetics). Although currently used anaesthetics are mostly safe and effective, they are not without fault. In paediatric populations and neonatal animal models, they are associated with learning impairments and neurotoxicity. In an effort to research safer anaesthetics, we have gone back to re-examine neuroactive steroids as anaesthetics. Neuroactive steroids are steroids that have direct, local effects in the central nervous system. Since the discovery of their anaesthetic effects, neuroactive steroids have been consistently used in human or veterinary clinics as preferred anaesthetic agents. Although briefly abandoned for clinical use due to unwanted vehicle side effects, there has since been renewed interest in their therapeutic value. Neuroactive steroids are safe sedative/hypnotic and anaesthetic agents across various animal species. Importantly, unlike traditional anaesthetics, they do not cause extensive neurotoxicity in the developing rodent brain. Similar to traditional anaesthetics, neuroactive steroids are modulators of synaptic and extrasynaptic γ-aminobutyric acid type A (GABA_A ) receptors and their interactions at the GABA_A receptor are stereo- and enantioselective. Recent work has also shown that these agents act on other ion channels, such as high- and low-voltage-activated calcium channels. Through these mechanisms of action, neuroactive steroids modulate neuronal excitability, which results in characteristic burst suppression of the electroencephalogram, and a surgical plane of anaesthesia. However, in addition to their interactions with voltage and ligand gated ions channels, neuroactive steroids interact with membrane bound metabotropic receptors and xenobiotic receptors to facilitate signaling of prosurvival, antiapoptotic pathways. These pathways play a role in their neuroprotective effects in neuronal injury and may also prevent extensive apoptosis in the developing brain during anaesthesia. The current review explores the history of neuroactive steroids as anaesthetics in humans and animal models, their diverse mechanisms of action, and their neuroprotective properties.

Different roles of T-type calcium channel isoforms in hypnosis induced by an endogenous neurosteroid epipregnanolone

BACKGROUND

Many neuroactive steroids induce sedation/hypnosis by potentiating γ-aminobutyric acid (GABA_A) currents. However, we previously demonstrated that an endogenous neuroactive steroid epipregnanolone [(3β,5β)-3-hydroxypregnan-20-one] (EpiP) exerts potent peripheral analgesia and blocks T-type calcium currents while sparing GABA_A currents in rat sensory neurons. This study seeks to investigate the behavioral effects elicited by systemic administration of EpiP and to characterize its use as an adjuvant agent to commonly used general anesthetics (GAs).

METHODS

Here, we utilized electroencephalographic (EEG) recordings to characterize thalamocortical oscillations, as well as behavioral assessment and mouse genetics with wild-type (WT) and different knockout (KO) models of T-channel isoforms to investigate potential sedative/hypnotic and immobilizing properties of EpiP.

RESULTS

Consistent with increased oscillations in slower EEG frequencies, EpiP induced an hypnotic state in WT mice when injected alone intra-peritoneally (i.p.) and effectively facilitated anesthetic effects of isoflurane (ISO) and sevoflurane (SEVO). The Ca_V3.1 (Cacna1g) KO mice demonstrated decreased sensitivity to EpiP-induced hypnosis when compared to WT mice, whereas no significant difference was noted between Ca_V3.2 (Cacna1h), Ca_V3.3 (Cacna1i) and WT mice. Finally, when compared to WT mice, onset of EpiP-induced hypnosis was delayed in Ca_V3.2 KO mice but not in Ca_V3.1 and Ca_V3.3 KO mice.

CONCLUSION

We posit that EpiP may have an important role as novel hypnotic and/or adjuvant to volatile anesthetic agents. We speculate that distinct hypnotic effects of EpiP across all three T-channel isoforms is due to their differential expression in thalamocortical circuitry.

Differential cardiotoxic electrocardiographic response to doxorubicin treatment in conscious versus anesthetized mice

INTRODUCTION

Doxorubicin (DOX), an anticancer drug used in chemotherapy, causes significant cardiotoxicity. This study aimed to investigate the effects of DOX on mouse cardiac electrophysiology, in conscious versus anesthetized state.

METHODS

Male and female C57BL/6 mice were injected with saline, 20 or 30 mg/kg DOX. ECGs were recorded 5 days post-injection in conscious and isoflurane anesthetized states. ECGs were analyzed using a custom MATLAB software to determine P, PR, QRS, QTc, and RR intervals as well as heart rate variability (HRV).

RESULTS

ECGs from the same mouse demonstrated P wave and QTc shortening as well as PR and RR interval prolongation in anesthetized versus conscious saline-treated mice. ECG response to DOX was also modulated by anesthesia. DOX treatment induced significant ECG modulation in female mice alone. While DOX20 treatment caused decrease in P and QRS durations, DOX30 treatment-induced QTc and RR interval prolongation in anesthetized but not in conscious female mice. These data suggest significant sex differences and anesthesia-induced differences in ECG response to DOX. HRV measured in time and frequency domains, a metric of arrhythmia susceptibility, was increased in DOX20-treated mice compared to saline.

CONCLUSIONS

This study for the first time identifies that the ECG response to DOX is modulated by anesthesia. Furthermore, this response demonstrated stark sex differences. These findings could have significant implications in clinical diagnosis of DOX cardiotoxicity.

The Biology of General Anesthesia from Paramecium to Primate

General anesthesia serves a critically important function in the clinical care of human patients. However, the anesthetized state has foundational implications for biology because anesthetic drugs are effective in organisms ranging from paramecia, to plants, to primates. Although unconsciousness is typically considered the cardinal feature of general anesthesia, this endpoint is only strictly applicable to a select subset of organisms that are susceptible to being anesthetized. We review the behavioral endpoints of general anesthetics across species and propose the isolation of an organism from its environment - both in terms of the afferent arm of sensation and the efferent arm of action - as a generalizable definition. We also consider the various targets and putative mechanisms of general anesthetics across biology and identify key substrates that are conserved, including cytoskeletal elements, ion channels, mitochondria, and functionally coupled electrical or neural activity. We conclude with a unifying framework related to network function and suggest that general anesthetics - from single cells to complex brains - create inefficiency and enhance modularity, leading to the dissociation of functions both within an organism and between the organism and its surroundings. Collectively, we demonstrate that general anesthesia is not restricted to the domain of modern medicine but has broad biological relevance with wide-ranging implications for a diverse array of species.

Novel neuroactive steroid with hypnotic and T-type calcium channel blocking properties exerts effective analgesia in a rodent model of post-surgical pain

BACKGROUND AND PURPOSE

Neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) is a novel hypnotic and voltage-dependent blocker of T-type calcium channels. Here, we examine its potential analgesic effects and adjuvant anaesthetic properties using a post-surgical pain model in rodents.

EXPERIMENTAL APPROACH

Analgesic properties of 3β-OH were investigated in thermal and mechanical nociceptive tests in sham or surgically incised rats and mice, with drug injected either systemically (intraperitoneal) or locally via intrathecal or intraplantar routes. Hypnotic properties of 3β-OH and its use as an adjuvant anaesthetic in combination with isoflurane were investigated using behavioural experiments and in vivo EEG recordings in adolescent rats.

KEY RESULTS

A combination of 1% isoflurane with 3β-OH (60 mg·kg^-1 , i.p.) induced suppression of cortical EEG and stronger thermal and mechanical anti-hyperalgesia during 3 days post-surgery, when compared to isoflurane alone and isoflurane with morphine. 3β-OH exerted prominent enantioselective thermal and mechanical antinociception in healthy rats and reduced T-channel-dependent excitability of primary sensory neurons. Intrathecal injection of 3β-OH alleviated mechanical hyperalgesia, while repeated intraplantar application alleviated both thermal and mechanical hyperalgesia in the rats after incision. Using mouse genetics, we found that Ca_V 3.2 T-calcium channels are important for anti-hyperalgesic effect of 3β-OH and are contributing to its hypnotic effect.

CONCLUSION AND IMPLICATIONS

Our study identifies 3β-OH as a novel analgesic for surgical procedures. 3β-OH can be used to reduce T-channel-dependent excitability of peripheral sensory neurons as an adjuvant for induction and maintenance of general anaesthesia while improving analgesia and lowering the amount of volatile anaesthetic needed for surgery.

Hemodynamic and neuronal responses to cocaine differ in awake versus anesthetized animals: Optical brain imaging study

Cocaine is a highly addictive drug with complex pharmacological effects. Most preclinical imaging studies investigating the effects of cocaine in the brain have been performed under anesthesia, which confounds findings. To tackle this problem, we used optical imaging to compare the effects of cocaine in the awake versus the anesthetized states. For this purpose, we customized an air floating mobile cage to fit the multi-wavelength spectral and laser speckle optical imaging system and implanted a multi-layer cranial window over the mouse somatosensory cortex. Results showed significant differences in neuronal activity and hemodynamics at baseline and in response to cocaine between the awake and the anesthetized states (isoflurane anesthesia). Specifically, 1) at baseline isoflurane dilated cerebral vessels, increased cerebral blood flow and depressed neuronal Ca²⁺ activity compared to the awake state; 2) acute cocaine (1 mg/kg iv) vasoconstricted blood vessels (arteries and veins) and decreased cerebral blood flow and oxygenated hemoglobin in the anesthetized state but not in the awake condition; 3) cocaine increased the accumulation of mean intracellular Ca²⁺ in neurons in the anesthetized state but not in the awake condition; and 4) in the awake state acute cocaine increased neuronal activities (increased the frequency of Ca²⁺ transients) and increased neuronal synchronization. We also corroborated that in the awake state cocaine also disrupted neurovascular coupling. These findings indicate that both vascular and neuronal responses to cocaine are influenced by isoflurane anesthesia, which highlights the importance of imaging awake animals when studying the effects of cocaine or other drugs in the brain.

Repetitive transcranial magnetic stimulation regulates L-type Ca(2+) channel activity inhibited by early sevoflurane exposure

BACKGROUND

Sevoflurane might be harmful to the developing brain. Therefore, it is essential to reverse sevoflurane-induced brain injury.

OBJECTIVE

This study aimed to determine whether low-frequency repetitive transcranial magnetic stimulation (rTMS) can regulate L-type Ca(2+) channel activity, which is inhibited by early sevoflurane exposure.

METHODS

Rats were randomly divided into three groups: control, sevoflurane, and rTMS groups. A Whole-cell patch clamp technique was applied to record L-type Ca(2+) channel currents. The I-V curve, steady-state activation and inactivation curves were studied in rats of each group at different ages (1 week, 2 weeks, 3 weeks, 4 weeks and 5 weeks old).

RESULTS

In the control group, L-type Ca(2+) channel current density significantly increased from week 2 to week 3. Compared with the control group, L-type Ca(2+) channel currents of rats in the sevoflurane group were significantly inhibited from week 1 to week 3. Activation curves of L-type Ca(2+) channel shifted significantly towards depolarization at week 1 and week 2. Moreover, steady-state inactivation curves shifted towards hyperpolarization from week 1 to week 3. Compared with the sevoflurane group, rTMS significantly increased L-type Ca(2+) channel currents at week 2 and week 3. Activation curves of L-type Ca(2+) channel significantly shifted towards hyperpolarization at week 2. Meanwhile, steady-state inactivation curves significantly shifted towards depolarization at week 2.

CONCLUSIONS

The period between week 2 and week 3 is critical for the development of L-type Ca(2+) channels. Early sevoflurane exposure inhibits L-type Ca(2+) channel activity and rTMS can regulate L-type Ca(2+) channel activity inhibited by sevoflurane.

CaV3.2 calcium channels control NMDA receptor-mediated transmission: a new mechanism for absence epilepsy

CaV3.2 T-type calcium channels, encoded by CACNA1H, are expressed throughout the brain, yet their general function remains unclear. We discovered that CaV3.2 channels control NMDA-sensitive glutamatergic receptor (NMDA-R)-mediated transmission and subsequent NMDA-R-dependent plasticity of AMPA-R-mediated transmission at rat central synapses. Interestingly, functional CaV3.2 channels primarily incorporate into synapses, replace existing CaV3.2 channels, and can induce local calcium influx to control NMDA transmission strength in an activity-dependent manner. Moreover, human childhood absence epilepsy (CAE)-linked hCaV3.2(C456S) mutant channels have a higher channel open probability, induce more calcium influx, and enhance glutamatergic transmission. Remarkably, cortical expression of hCaV3.2(C456S) channels in rats induces 2- to 4-Hz spike and wave discharges and absence-like epilepsy characteristic of CAE patients, which can be suppressed by AMPA-R and NMDA-R antagonists but not T-type calcium channel antagonists. These results reveal an unexpected role of CaV3.2 channels in regulating NMDA-R-mediated transmission and a novel epileptogenic mechanism for human CAE.

T-type calcium channel blockers as neuroprotective agents

T-type calcium channels are expressed in many diverse tissues, including neuronal, cardiovascular, and endocrine. T-type calcium channels are known to play roles in the development, maintenance, and repair of these tissues but have also been implicated in disease when not properly regulated. Calcium channel blockers have been developed to treat various diseases and their use clinically is widespread due to both their efficacy as well as their safety. Aside from their established clinical applications, recent studies have suggested neuroprotective effects of T-type calcium channel blockers. Many of the current T-type calcium channel blockers could act on other molecular targets besides T-type calcium channels making it uncertain whether their neuroprotective effects are solely due to blocking of T-type calcium channels. In this review, we discuss these drugs as well as newly developed chemical compounds that are designed to be more selective for T-type calcium channels. We review in vitro and in vivo evidence of neuroprotective effects by these T-type calcium channel blockers. We conclude by discussing possible molecular mechanisms underlying the neuroprotective effects by T-type calcium channel blockers.

Effects of general anesthetics on substance P release and c-Fos expression in the spinal dorsal horn

BACKGROUND

The authors examined in vivo the effects of general anesthetics on evoked substance P release (primary afferent excitability) and c-Fos expression (neuronal activation) in superficial dorsal horn.

METHODS

Rats received saline, propofol (100 mg/kg), pentobarbital (50 mg/kg), isoflurane (2 minimum alveolar concentration), nitrous oxide (66%), or fentanyl (30 μg/kg). During anesthesia, rats received intraplantar 5% formalin (50 μl) to left hind paw. Ten minutes later, rats underwent transcardial perfusion with 4% paraformaldehyde. Substance P release from small primary afferents was assessed by incidence of neurokinin 1 receptor internalization in the superficial dorsal horn. In separate studies, rats were sacrificed after 2 h and c-Fos expression measured.

RESULTS

Intraplantar formalin-induced robust neurokinin 1 receptor internalization in ipsilateral dorsal horn (ipsilateral: 54 ± 6% [mean ± SEM], contralateral: 12 ± 2%; P < 0.05; n = 4). Fentanyl, but not propofol, pentobarbital, isoflurane, nor nitrous oxide alone inhibited neurokinin 1 receptor internalization. However, 2 minimum alveolar concentration isoflurane + nitrous oxide reduced neurokinin 1 receptor internalization (27 ± 3%; P < 0.05; n = 5). All agents reduced c-Fos expression (control: 34 ± 4, fentanyl: 8 ± 2, isoflurane: 12 ± 3, nitrous oxide: 11 ± 2, isoflurane + nitrous oxide: 12 ± 1, pentobarbital: 11 ± 2, propofol: 13 ± 3; P < 0.05; n = 3).

CONCLUSION

General anesthetics at anesthetic concentrations block spinal neuron activation through a mechanism that is independent of an effect on small primary afferent peptide release. The effect of fentanyl alone and the synergistic effect of isoflurane and nitrous oxide on substance P release suggest a correlative rationale for the therapeutic use of these anesthetic protocols by blocking nociceptive afferent transmitter release and preventing the initiation of cascade, which is immediately postsynaptic to the primary afferent.

Inhibition of T-type calcium current in rat thalamocortical neurons by isoflurane

Thalamocortical (TC) neurons provide the major sensory input to the mammalian somatosensory cortex. Decreased activity of these cells may be pivotal in the ability of general anesthetics to induce loss of consciousness and promote sleep (hypnosis). T-type voltage-gated calcium currents (T-currents) have a key function regulating the cellular excitability of TC neurons and previous studies have indicated that volatile general anesthetics may alter the excitability of these neurons. Using a patch-clamp technique, we investigated the mechanisms whereby isoflurane, a common volatile anesthetic, modulates isolated T-currents and T-current-dependent excitability of native TC neurons in acute brain slices of the rat. In voltage-clamp experiments, we found that isoflurane strongly inhibited peak amplitude of T-current, yielding an IC(50) of 1.1 vol-% at physiological membrane potentials. Ensuing biophysical studies demonstrated that inhibition was more prominent at depolarized membrane potentials as evidenced by hyperpolarizing shifts in channel availability curves. In current-clamp experiments we found that isoflurane decreased the rate of depolarization of low-threshold-calcium spikes (LTCSs) and consequently increased the latency of rebound spike firing at the same concentrations that inhibited isolated T-currents. This effect was mimicked by a novel selective T-channel blocker 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2). In contrast, isoflurane and TTA-P2 had minimal effect on resting membrane potential and cell input resistance. We propose that the clinical properties of isoflurane may at least partly be provided by depression of thalamic T-currents.

GABAergic excitotoxicity injury of the immature hippocampal pyramidal neurons' exposure to isoflurane

BACKGROUND

Certain anesthetics exhibit neurotoxicity in the brains of immature but not mature animals. γ-Aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, is excitatory on immature neurons via its action at the GABAA receptor, depolarizing the membrane potential and inducing a cytosolic Ca2+ increase ([Ca2+]i), because of a reversed transmembrane chloride gradient. Recent experimental data from several rodent studies have demonstrated that exposure to isoflurane during an initial phase causes neuronal excitotoxicity and apoptosis. GABAA receptor-mediated synaptic voltage-dependent calcium channels' (VDCCs) overactivation and Ca2+ influx are involved in these neural changes.

METHODS

We monitored [Ca2+]i using Fluo-4 AM fluorescence imaging. Using whole-cell patch clamp techniques, IVDCC (voltage-dependent calcium channel currents) were recorded from primary cultures of rat hippocampal neurons (5-day culture) exposed to isoflurane. To further investigate the neurotoxicity of high cytosolic-free calcium after isoflurane in a dose- and time-dependent manner, the possibility of increased caspase-3 levels was evaluated by Western blot and quantitative real-time polymerase chain reaction. Statistical significance was assessed using the Student t test or 1-way analysis of variance followed by the Tukey post hoc test.

RESULTS

Under control conditions, isoflurane enhanced the GABA-induced [Ca2+]i increase in a dose-dependent manner. Dantrolene and nicardipine markedly inhibited this enhancement mediated by isoflurane. Moreover, in Ca2+-free media, pretreatment with isoflurane did not show any influence on the caffeine-induced increase of [Ca2+]i. Similarly, using whole-cell recording, isoflurane increased the peak amplitude of IVDCC in the cultured neurons from rat hippocampus by depolarization pulses. Isoflurane (0.25, 0.5, 0.75, and 1 minimum alveolar concentration [MAC]) potentiated IVDCC peak current amplitude by 109.11%±9.03%, 120.56%±11.46%, 141.33%±13.87%, and 146.78%±15.87%, respectively. To analyze variation in protein levels, the effect of treatments with isoflurane on caspase-3 activity was dose- and time-dependent, reaching a maximal caspase-3 activity after exposure to 1 MAC for 6 hours (P<0.001). However, in the mRNA levels, hippocampal caspase-3 mRNA levels began to be significantly increased in isoflurane-treated developing rat hippocampal neurons after 6 hours of exposure to 0.25 MAC isoflurane (P<0.001).

CONCLUSIONS

Isoflurane-mediated enhancement of GABA-triggered [Ca2+]i release results from membrane depolarization with subsequent activation of VDCCs and further Ca2+-induced Ca2+ release from the ryanodine-sensitizing Ca2+ store. An increase in [Ca2+]i, caused by activation of the GABAA receptor and opening of VDCCs, is necessary for isoflurane-induced calcium overload of immature rat hippocampal neurons, which may be involved in the mechanism of an isoflurane-induced neurotoxic effect in the developing rodent brain.

Effects of isoflurane anesthesia on the cardiovascular function of the C57BL/6 mouse

Isoflurane (ISO) is the most commonly used inhalational anesthetic for experimental interventions in mice and is preferred for imaging technologies that require the mouse to remain anesthetized for relatively long time periods. This study compares the stability of mean arterial pressure (MAP), heart rate (HR), and body temperature under ISO concentrations of 1%, 1.5%, and 2% (volume-to-volume, v/v) for up to 90 minutes postinduction. At all three levels of anesthesia, we examined evoked physiological responses to fractional inspiratory ratio variations of oxygen (FiO2) and nitrous oxide (N2O). In addition, we determined the hemodynamic effects of anesthesia on pH, glucose, insulin, glucocorticoids, and partial pressure of oxygen and of carbon dioxide in the blood (paO2, paCO2). The results indicate that the most appropriate ISO dose level was 1.5% v/v, yielding stable MAP and HR values comparable to those observed in the animal's conscious state, with a minute-to-minute variability in MAP and HR of .11%. Based on such recordings, the optimal FiO2 appeared to be 50%. The additional use of N2O was associated with higher and more stable values of MAP and HR. Arterial pH values were within the physiological range and varied between 7.20 and 7.43. ISO anesthesia at 1.5% v/v was also associated with mild hyperglycemia (+47%), whereas insulin levels and corticosteroids remained unaltered. We conclude that the application of isoflurane as an inhalational anesthetic in the mouse can be optimized to attain stable hemodynamics by administering it at 1.5% v/v and by supplementing it with N2O.

Are neuronal voltage-gated calcium channels valid cellular targets for general anesthetics?

The effects of anesthetics and analgesics on ion channels have been the subject of intense research since recent reports of direct actions of anesthetic molecules on ion channel proteins.  It is now known that ligand-gated channels, particularly γ-amino-butyric acid (GABAA) and N-methyl-D-aspartate (NMDA) receptors, play a key role in mediating anesthetic actions, but these channels are unable to account for all aspects of clinical anesthesia such as loss of consciousness, immobility, analgesia, amnesia, and muscle relaxation.  Furthermore, an assortment of voltage-gated and background channels also display anesthetic sensitivity and a key question arises: What role do these other channels play in clinical anesthesia? These channels have overlapping physiological roles and pharmacological profiles, making it difficult to assign aspects of the anesthetic state to individual channel types.  Here, we will focus on the function of neuronal voltage-gated calcium channels in mediating the effects of general anesthetics.

The transient receptor potential channel antagonist SKF96365 is a potent blocker of low-voltage-activated T-type calcium channels

BACKGROUND AND PURPOSE

SKF96365 (SKF), originally identified as a blocker of receptor-mediated calcium entry, is widely used diagnostically, as a blocker of transient receptor potential canonical type (TRPC) channels. While SKF has been used as a tool to define the functional roles of TRPC channels in various cell and tissue types, there are notable overlapping physiological and pathophysiological associations between TRPC channels and low-voltage-activated (LVA) T-type calcium channels. The activity of SKF against T-type Ca channels has not been previously explored, and here we systematically investigated the effects of SKF on recombinant and native voltage-gated Ca channel-mediated currents.

EXPERIMENTAL APPROACH

Effects of SKF on recombinant Ca channels were studied under whole-cell patch clamp conditions after expression in HEK293 cells. The effect of SKF on cerebellar Purkinje cells (PCs) expressing native T-type Ca channels was also assessed.

KEY RESULTS

SKF blocked recombinant Ca channels, representative of each of the three main molecular genetic classes (Ca(V)1, Ca(V)2 and Ca(V)3) at concentrations typically utilized to assay TRPC function (10 microM). Particularly, human Ca(V)3.1 T-type Ca channels were more potently inhibited by SKF (IC(50) approximately 560 nM) in our experiments than previously reported for similarly expressed TRPC channels. SKF also inhibited native Ca(V)3.1 T-type currents in a rat cerebellar PC slice preparation.

CONCLUSIONS AND IMPLICATIONS

SKF was a potent blocker of LVA T-type Ca channels. We suggest caution in the interpretation of results using SKF alone as a diagnostic agent for TRPC activity in native tissues.

Mechanisms of inhibition of CaV3.1 T-type calcium current by aliphatic alcohols

Many aliphatic alcohols modulate activity of various ion channels involved in sensory processing and also exhibit anesthetic capacity in vivo. Although the interaction of one such compound, 1-octanol (octanol) with different T-type calcium channels (T-channels) has been described, the mechanisms of current modulation and its functional significance are not well studied. Using patch-clamp technique, we investigated the mechanisms of inhibition of T-currents by a series of aliphatic alcohols in recombinant human Ca(V)3.1 (alpha1G) T-channel isoform expressed in human embryonic kidney (HEK) 293 cells and thalamocortical (TC) relay neurons in brain slices of young rats. Octanol, 1-heptanol (heptanol) and 1-hexanol (hexanol) inhibited the recombinant Ca(V)3.1 currents in concentration-dependent manner yielding IC(50) values of 362 microM, 1063 microM and 3167 microM, respectively. Octanol similarly inhibited native thalamic Ca(V)3.1 T-currents with an IC(50) of 287 microM and diminished burst firing without significant effect on passive membrane properties of these neurons. Inhibitory effect of octanol on T-currents in both native and recombinant cells was accompanied with accelerated macroscopic inactivation kinetics and hyperpolarizing shift in the steady-state inactivation curve. Additionally, octanol induced a depolarizing shift in steady-state activation curves of T-current in TC neurons. Surprisingly, the recovery from fast inactivation at hyperpolarized membrane potentials was accelerated by octanol up 3-fold in native but not recombinant channels. Given the importance of thalamocortical pathways in providing sleep, arousal, and anesthetic states, modulation of thalamic T-currents may at least contribute to the pharmacological effects of aliphatic alcohols.

Selective T-type calcium channel block in thalamic neurons reveals channel redundancy and physiological impact of I(T)window

Although it is well established that low-voltage-activated T-type Ca(2+) channels play a key role in many neurophysiological functions and pathological states, the lack of selective and potent antagonists has so far hampered a detailed analysis of the full impact these channels might have on single-cell and neuronal network excitability as well as on Ca(2+) homeostasis. Recently, a novel series of piperidine-based molecules has been shown to selectively block recombinant T-type but not high-voltage-activated (HVA) Ca(2+) channels and to affect a number of physiological and pathological T-type channel-dependent behaviors. Here we directly show that one of these compounds, 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2), exerts a specific, potent (IC(50) = 22 nm), and reversible inhibition of T-type Ca(2+) currents of thalamocortical and reticular thalamic neurons, without any action on HVA Ca(2+) currents, Na(+) currents, action potentials, and glutamatergic and GABAergic synaptic currents. Thus, under current-clamp conditions, the low-threshold Ca(2+) potential (LTCP)-dependent high-frequency burst firing of thalamic neurons is abolished by TTA-P2, whereas tonic firing remains unaltered. Using TTA-P2, we provide the first direct demonstration of the presence of a window component of Ca(2+) channels in neurons and its contribution to the resting membrane potential of thalamic neurons and to the Up state of their intrinsically generated slow (<1 Hz) oscillation. Moreover, we demonstrate that activation of only a small fraction of the T-type channel population is required to generate robust LTCPs, suggesting that LTCP-driven bursts of action potentials can be evoked at depolarized potentials where the vast majority of T-type channels are inactivated.