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

Sex-specific hypnotic effects of the neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile are mediated by peripheral metabolism into an active hypnotic steroid

BACKGROUND

The novel synthetic neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) blocks T-type calcium channels but does not directly modulate neuronal γ-aminobutyric acid type A (GABAA) currents like other anaesthetic neurosteroids. As 3β-OH has sex-specific hypnotic effects in adult rats, we studied the mechanism contributing to sex differences in its effects.

METHODS

We used a combination of behavioural loss of righting reflex, neuroendocrine, pharmacokinetic, in vitro patch-clamp electrophysiology, and in vivo electrophysiological approaches in wild-type mice and in genetic knockouts of the CaV3.1 T-type calcium channel isoform to study the mechanisms by which 3β-OH and its metabolite produces sex-specific hypnotic effects.

RESULTS

Adult male mice were less sensitive to the hypnotic effects of 3β-OH compared with female mice, and these differences appeared during development. Adult males had higher 3β-OH brain concentrations despite being less sensitive to its hypnotic effects. Females metabolised 3β-OH into the active GABAA receptor positive allosteric modulator (3α,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3α-OH) to a greater extent than males. The 3α-OH metabolite has T-channel blocking properties with sex-specific hypnotic and pharmacokinetic effects. Sex-dependent suppression of the cortical electroencephalogram is more pronounced with 3α-OH compared with 3β-OH.

CONCLUSIONS

The sex-specific differences in the hypnotic effect of 3β-OH in mice are attributable to differences in its peripheral metabolism into the more potent hypnotic metabolite 3α-OH.

The role of voltage-gated calcium channels in the mechanisms of anesthesia and perioperative analgesia

PURPOSE OF REVIEW

A family of neuronal voltage-gated calcium channels (VGCCs) have received only recently a significant consideration regarding the mechanisms of anesthesia because VGCC inhibition may be important in anesthetic action by decreasing neuronal excitability and presynaptic excitatory transmission. The T-type VGCCs channels (T-channels), although rarely involved in synaptic neurotransmitter release, play an important role in controlling neuronal excitability and in generating spontaneous oscillatory bursting of groups of neurons in the thalamus thought to be involved in regulating the state of arousal and sleep. Furthermore, these channels are important regulators of neuronal excitability in pain pathway. This review will provide an overview of historic perspective and the recent literature on the role of VGCCs and T-channel inhibition in particular in the mechanisms of action of anesthetics and analgesics.

RECENT FINDINGS

Recent research in the field of novel mechanisms of hypnotic action of anesthetics revealed significant contribution of the Ca V 3.1 isoform of T-channels expressed in the thalamus. Furthermore, perioperative analgesia can be achieved by targeting Ca V 3.2 isoform of these channels that is abundantly expressed in pain pathways.

SUMMARY

The review summarizes current knowledge regarding the contribution of T-channels in hypnosis and analgesia. Further preclinical and clinical studies are needed to validate their potential for developing novel anesthetics and new perioperative pain therapies.

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.

Isoflurane decreases substantia gelatinosa neuron excitability and synaptic transmission from periphery in the rat spinal dorsal horn

Isoflurane is an inhaled anesthetic, though its actions at the cellular level remain controversial. By using acute spinal cord slices from adult rats and the whole-cell recording technique, we found that aqueous isoflurane at the minimum alveolar concentration decreased postsynaptic neural excitability and enhanced membrane conductance, while suppressing glutamate release from presynaptic afferent onto substantia gelatinosa (lamina II) neurons in the dorsal horn. The data demonstrate that isoflurane modulates synaptic transmission from peripheral to the spinal cord via both pre- and postsynaptic effects and these actions may underlie its spinal anesthesia.

Alterations in Oscillatory Behavior of Central Medial Thalamic Neurons Demonstrate a Key Role of CaV3.1 Isoform of T-Channels During Isoflurane-Induced Anesthesia

Although the central medial nucleus (CeM) of the thalamus is an essential part of the arousal system for sleep and anesthesia initiation, the precise mechanisms that regulate its activity are not well studied. We examined the role of CaV3.1 isoform of T-type calcium channels (T-channels) in the excitability and rhythmic activity of CeM neurons during isoflurane (ISO)-induced anesthesia by using mouse genetics and selective pharmacology. Patch-clamp recordings taken from acute brain slices revealed that CaV3.1 channels in CeM are inhibited by prototypical volatile anesthetic ISO (250 and 500 μM) and selective T-channels blocker 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2). Both TTA-P2 and ISO attenuated tonic and burst firing modes, and hyperpolarized CeM neurons from wild type (WT) mice. These effects were greatly diminished or abolished in CaV3.1 null mice. Our ensuing in vivo local field potential (LFP) recordings from CeM indicated that the ability of TTA-P2 and anesthetic concentrations of ISO to promote δ oscillation was substantially weakened in CaV3.1 null mice. Furthermore, escalating ISO concentrations induced stronger burst-suppression LFP pattern in mutant than in WT mice. Our results demonstrate for the first time the importance of CaV3.1 channels in thalamocortical oscillations from the non-specific thalamic nuclei that underlie clinically important effects of ISO.

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.

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.

Novel neurosteroid hypnotic blocks T-type calcium channel-dependent rebound burst firing and suppresses long-term potentiation in the rat subiculum

BACKGROUND

Hypnotics and general anaesthetics impair memory by altering hippocampal synaptic plasticity. We recently reported on a neurosteroid analogue with potent hypnotic activity [(3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile; 3β-OH], which does not cause developmental neurotoxicity in rat pups. Here, we investigated the effects of 3β-OH on neuronal excitability in the subiculum, the major output structure of the hippocampal formation, and synaptic plasticity at two key hippocampal synapses in juvenile rats.

METHODS

Biophysical properties of isolated T-type calcium currents (T-currents) in the rat subiculum were investigated using acute slice preparations. Subicular T-type calcium channel (T-channel) subtype mRNA expression was compared using qRT-PCR. Using electrophysiological recordings, we examined the effects of 3β-OH and an endogenous neuroactive steroid, allopregnanolone (Allo), on T-currents and burst firing properties of subicular neurones, and on the long-term potentiation (LTP) in CA3-CA1 and CA1-subiculum pathways.

RESULTS

Biophysical and molecular studies confirmed that Ca_V3.1 channels represent the dominant T-channel isoform in the subiculum of juvenile rats. 3β-OH and Allo inhibited rebound burst firing by decreasing the amplitude of T-currents in a voltage-dependent manner with similar potency, with 30-80% inhibition. Both neurosteroids suppressed LTP at the CA1-subiculum, but not at the CA3-CA1 Schaffer collateral synapse.

CONCLUSIONS

Neurosteroid effects on T-channels modulate hippocampal output and provide possible molecular mechanisms for the amnestic action of the novel hypnotic 3β-OH. Effects on T-channels in the subiculum provide a novel target for amnestic effects of hypnotics.

CaV3.1 isoform of T-type calcium channels supports excitability of rat and mouse ventral tegmental area neurons

Recent data have implicated voltage-gated calcium channels in the regulation of the excitability of neurons within the mesolimbic reward system. While the attention of most research has centered on high voltage L-type calcium channel activity, the presence and role of the low voltage-gated T-type calcium channel (T-channels) has not been well explored. Hence, we investigated T-channel properties in the neurons of the ventral tegmental area (VTA) utilizing wild-type (WT) rats and mice, Ca_V3.1 knock-out (KO) mice, and TH-eGFP knock-in (KI) rats in acute horizontal brain slices of adolescent animals. In voltage-clamp experiments, we first assessed T-channel activity in WT rats with characteristic properties of voltage-dependent activation and inactivation, as well as characteristic crisscrossing patterns of macroscopic current kinetics. T-current kinetics were similar in WT mice and WT rats but T-currents were abolished in Ca_V3.1 KO mice. In ensuing current-clamp experiments, we observed the presence of hyperpolarization-induced rebound burst firing in a subset of neurons in WT rats, as well as dopaminergic and non-dopaminergic neurons in TH-eGFP KI rats. Following the application of a pan-selective T-channel blocker TTA-P2, rebound bursting was significantly inhibited in all tested cells. In a behavioral assessment, the acute locomotor increase induced by a MK-801 (Dizocilpine) injection in WT mice was abolished in Ca_V3.1 KO mice, suggesting a tangible role for 3.1 T-type channels in drug response. We conclude that pharmacological targeting of Ca_V3.1 isoform of T-channels may be a novel approach for the treatment of disorders of mesolimbic reward system.

Cytosolic ATP Relieves Voltage-Dependent Inactivation of T-Type Calcium Channels and Facilitates Excitability of Neurons in the Rat Central Medial Thalamus

The central medial nucleus (CeM) is a part of the intralaminar thalamus, which is involved in the control of arousal and sensory processing. However, ionic conductances and mechanisms that regulate the activity of the CeM are not well studied. Here, we used in vitro electrophysiology in acute brain slices from adolescent rats to demonstrate that T-type calcium currents (T-currents) are prominent in the majority of the studied CeM neurons and are critical determinants of low-threshold calcium spikes (LTSs), which in turn regulate excitability of these neurons. Using an ATP-free internal solution decreased T-current density and induced a profound hyperpolarizing shift in steady-state inactivation curves while voltage-dependent activation kinetics were spared. Furthermore, selective pharmacological blockade of T-channels or use of an ATP-free solution reduced both tonic action potential (AP) frequency and rebound burst firing in CeM neurons. Our results indicate that T-channels are critical regulators of a thalamocortical circuit output and suggest that cytosolic ATP could be an endogenous regulatory mechanism in which T-channels may functionally gate sensory transmission and arousal in vivo.

A neurosteroid analogue with T-type calcium channel blocking properties is an effective hypnotic, but is not harmful to neonatal rat brain

BACKGROUND

More than 4 million children are exposed annually to sedatives and general anaesthetics (GAs) in the USA alone. Recent data suggest that common GAs can be detrimental to brain development causing neurodegeneration and long-term cognitive impairments. Challenged by a recent US Food and Drug Administration (FDA) warning about potentially neurotoxic effects of GAs in children, there is an urgent need to develop safer GAs.

METHODS

Postnatal Day 7 (P7) rat pups of both sexes were exposed to six (repeated every 2 h) injections of equipotent hypnotic doses of ketamine or the neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) for 12 h. Loss of righting reflex was used to assess hypnotic properties and therapeutic index; quantitative caspase-3 immunohistochemistry was used to assess developmental neuroapoptosis; patch-clamp recordings in acute brain slices were used to assess the effects of 3β-OH on neuronal excitability and synaptic transmission. Cognitive abilities of rats exposed to ketamine, 3β-OH, or vehicle at P7 were assessed in young adulthood using the radial arm maze.

RESULTS

The neuroactive steroid 3β-OH has a therapeutic index similar to ketamine, a commonly used clinical GA. We report that 3β-OH is safe and, unlike ketamine, does not cause neuroapoptosis or impair cognitive development when administered to P7 rat pups. Interestingly, 3β-OH blocks T-type calcium channels and presynaptically dampens synaptic transmission at hypnotically-relevant brain concentrations, but it lacks a direct effect on γ-aminobutyric acid A or glutamate-gated ion channels.

CONCLUSIONS

The neurosteroid 3β-OH is a relatively safe hypnotic that warrants further consideration for paediatric anaesthesia.

Layer- and subregion-specific differences in the neurophysiological properties of rat medial prefrontal cortex pyramidal neurons

Medial prefrontal cortex (mPFC) is critical for the expression of long-term conditioned fear. However, the neural circuits involving fear memory acquisition and retrieval are still unclear. Two subregions within mPFC that have received a lot of attention are the prelimbic (PL) and infralimbic (IL) cortices (e.g., Santini E, Quirk GJ, Porter JT. J Neurosci 28: 4028-4036, 2008; Song C, Ehlers VL, Moyer JR Jr J Neurosci 35: 13511-13524, 2015). Interestingly, PL and IL may play distinct roles during fear memory acquisition and retrieval but the underlying mechanism is poorly understood. One possibility is that the intrinsic membrane properties differ between these subregions. Thus, the current study was carried out to characterize the basic membrane properties of mPFC neurons in different layers and subregions. We found that pyramidal neurons in L2/3 were more hyperpolarized and less excitable than in L5. This was observed in both IL and PL and was associated with an enhanced h-current in L5 neurons. Within L2/3, IL neurons were more excitable than those in PL, which may be due to a lower spike threshold and higher input resistance in IL neurons. Within L5, the intrinsic excitability was comparable between neurons obtained in IL and PL. Thus, the heterogeneity in physiological properties of mPFC neurons may underlie the observed subregion-specific contribution of mPFC in cognitive function and emotional control, such as fear memory expression. NEW & NOTEWORTHY This is the first study to demonstrate that medial prefrontal cortical (mPFC) neurons are heterogeneous in both a layer- and a subregion-specific manner. Specifically, L5 neurons are more depolarized and more excitable than those neurons in L2/3, which is likely due to variations in h-current. Also, infralimbic neurons are more excitable than those of prelimbic neurons in layer 2/3, which may be due to differences in certain intrinsic properties, including input resistance and spike threshold.

Heantos-4, a natural plant extract used in the treatment of drug addiction, modulates T-type calcium channels and thalamocortical burst-firing

Heantos-4 is a refined combination of plant extracts currently approved to treat opiate addiction in Vietnam. In addition to its beneficial effects on withdrawal and prevention of relapse, reports of sedation during clinical treatment suggest that arousal networks in the brain may be recruited during Heantos administration. T-type calcium channels are implicated in the generation of sleep rhythms and in this study we examined whether a Heantos-4 extraction modulates T-type calcium channel currents generated by the Cav3.1, Cav3.2 and Ca3.3 subtypes. Utilizing whole-cell voltage clamp on exogenously expressed T-type calcium channels we find that Heantos inhibits Cav3.1 and Cav3.3 currents, while selectively potentiating Cav3.2 currents. We further examined the effects of Heantos-4 extract on low-threshold burst-firing in thalamic neurons which contribute to sleep oscillations. Using whole-cell current clamp in acute thalamic brain slices Heantos-4 suppressed rebound burst-firing in ventrobasal thalamocortical neurons, which express primarily Cav3.1 channels. Conversely, Heantos-4 had no significant effect on the burst-firing properties of thalamic reticular neurons, which express a mixed population of Cav3.2 and Cav3.3 channels. Examining Heantos-4 effects following oral administration in a model of absence epilepsy revealed the potential to exacerbate seizure activity. Together, the findings indicate that Heantos-4 has selective effects both on specific T-type calcium channel isoforms and distinct populations of thalamic neurons providing a putative mechanism underlying its effects on sedation and on the thalamocortical network.

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.

Dynamic Analysis of the Conditional Oscillator Underlying Slow Waves in Thalamocortical Neurons

During non-REM sleep the EEG shows characteristics waves that are generated by the dynamic interactions between cortical and thalamic oscillators. In thalamic neurons, low-threshold T-type Ca(2+) channels play a pivotal role in almost every type of neuronal oscillations, including slow (< 1 Hz) waves, sleep spindles and delta waves. The transient opening of T channels gives rise to the low threshold spikes (LTSs), and associated high frequency bursts of action potentials, that are characteristically present during sleep spindles and delta waves, whereas the persistent opening of a small fraction of T channels, (i.e., ITwindow) is responsible for the membrane potential bistability underlying sleep slow oscillations. Surprisingly thalamocortical (TC) neurons express a very high density of T channels that largely exceed the amount required to generate LTSs and therefore, to support certain, if not all, sleep oscillations. Here, to clarify the relationship between T current density and sleep oscillations, we systematically investigated the impact of the T conductance level on the intrinsic rhythmic activities generated in TC neurons, combining in vitro experiments and TC neuron simulation. Using bifurcation analysis, we provide insights into the dynamical processes taking place at the transition between slow and delta oscillations. Our results show that although stable delta oscillations can be evoked with minimal T conductance, the full range of slow oscillation patterns, including groups of delta oscillations separated by Up states ("grouped-delta slow waves") requires a high density of T channels. Moreover, high levels of T conductance ensure the robustness of different types of slow oscillations.

Impact of Hyperpolarization-activated, Cyclic Nucleotide-gated Cation Channel Type 2 for the Xenon-mediated Anesthetic Effect: Evidence from In Vitro and In Vivo Experiments

BACKGROUND

The thalamus is thought to be crucially involved in the anesthetic state. Here, we investigated the effect of the inhaled anesthetic xenon on stimulus-evoked thalamocortical network activity and on excitability of thalamocortical neurons. Because hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels are key regulators of neuronal excitability in the thalamus, the effect of xenon on HCN channels was examined.

METHODS

The effects of xenon on thalamocortical network activity were investigated in acutely prepared brain slices from adult wild-type and HCN2 knockout mice by means of voltage-sensitive dye imaging. The influence of xenon on single-cell excitability in brain slices was investigated using the whole-cell patch-clamp technique. Effects of xenon on HCN channels were verified in human embryonic kidney cells expressing HCN2 channels.

RESULTS

Xenon concentration-dependently diminished thalamocortical signal propagation. In neurons, xenon reduced HCN channel-mediated Ih current amplitude by 33.4 ± 12.2% (at -133 mV; n = 7; P = 0.041) and caused a left-shift in the voltage of half-maximum activation (V1/2) from -98.8 ± 1.6 to -108.0 ± 4.2 mV (n = 8; P = 0.035). Similar effects were seen in human embryonic kidney cells. The impairment of HCN channel function was negligible when intracellular cyclic adenosine monophosphate level was increased. Using HCN2 mice, we could demonstrate that xenon did neither attenuate in vitro thalamocortical signal propagation nor did it show sedating effects in vivo.

CONCLUSIONS

Here, we clearly showed that xenon impairs HCN2 channel function, and this impairment is dependent on intracellular cyclic adenosine monophosphate levels. We provide evidence that this effect reduces thalamocortical signal propagation and probably contributes to the hypnotic properties of xenon.

Hyperexcitability of rat thalamocortical networks after exposure to general anesthesia during brain development

Prevailing literature supports the idea that common general anesthetics (GAs) cause long-term cognitive changes and neurodegeneration in the developing mammalian brain, especially in the thalamus. However, the possible role of GAs in modifying ion channels that control neuronal excitability has not been taken into consideration. Here we show that rats exposed to GAs at postnatal day 7 display a lasting reduction in inhibitory synaptic transmission, an increase in excitatory synaptic transmission, and concomitant increase in the amplitude of T-type calcium currents (T-currents) in neurons of the nucleus reticularis thalami (nRT). Collectively, this plasticity of ionic currents leads to increased action potential firing in vitro and increased strength of pharmacologically induced spike and wave discharges in vivo. Selective blockade of T-currents reversed neuronal hyperexcitability in vitro and in vivo. We conclude that drugs that regulate thalamic excitability may improve the safety of GAs used during early brain development.

Awake vs. anesthetized: layer-specific sensory processing in visual cortex and functional connectivity between cortical areas

During general anesthesia, global brain activity and behavioral state are profoundly altered. Yet it remains mostly unknown how anesthetics alter sensory processing across cortical layers and modulate functional cortico-cortical connectivity. To address this gap in knowledge of the micro- and mesoscale effects of anesthetics on sensory processing in the cortical microcircuit, we recorded multiunit activity and local field potential in awake and anesthetized ferrets (Mustela putoris furo) during sensory stimulation. To understand how anesthetics alter sensory processing in a primary sensory area and the representation of sensory input in higher-order association areas, we studied the local sensory responses and long-range functional connectivity of primary visual cortex (V1) and prefrontal cortex (PFC). Isoflurane combined with xylazine provided general anesthesia for all anesthetized recordings. We found that anesthetics altered the duration of sensory-evoked responses, disrupted the response dynamics across cortical layers, suppressed both multimodal interactions in V1 and sensory responses in PFC, and reduced functional cortico-cortical connectivity between V1 and PFC. Together, the present findings demonstrate altered sensory responses and impaired functional network connectivity during anesthesia at the level of multiunit activity and local field potential across cortical layers.

Early exposure to sevoflurane inhibits Ca(2+) channels activity in hippocampal CA1 pyramidal neurons of developing rats

Sevoflurane is one of inhalation anesthetics and has been commonly used in obstetric and pediatric anesthesia. The widespread use of sevoflurane in newborns and infants has made its safety a health issue of concern. Voltage-gated Ca(2+) channels (VGCCs) play an important role in neuronal excitability and are essential for normal brain development. However, the role of sevoflurane on regulating Ca(2+) channels during the period of rapid brain development is still not well understood. The aim of this study is to explore the effects of sevoflurane on voltage-gated Ca(2+) channels for hippocampal CA1 pyramidal neurons during the period of rapid brain development. 1-week-old Sprague-Dawley rats were randomly divided into 3 groups: control group, 2.1% sevoflurane group (exposed to 2.1% sevoflurane for 6h) and 3% sevoflurane group (exposed to 3% sevoflurane for 6h). Whole-cell patch clamp technique was used. I-V curve, steady-state activation and inactivation curves of Ca(2+) channels were studied in rats of the both 3 treated groups at 5 different ages (1 week, 2 weeks, 3 weeks, 4 and 5 weeks old). After anesthesia with sevoflurane at 1-week-old rats, Ca(2+) channels current density was significantly decreased at week 1 and week 2 (p<0.01). And 3% sevoflurane exposure resulted in a rightward shift in steady-state activation curve at week 1 and week 2, as well as the inactivation curve from week 1 to week 3. However, the 2.1% sevoflurane-induced rightward shift was only found in steady-state inactivation curve of Ca(2+) channels at week 1 and week 2. Both the slope factor (k) of Ca(2+) channels activation and inactivation curves increased by 3% sevoflurane at week 1 (p<0.05). Therefore, early exposure to sevoflurane persistently inhibits Ca(2+) channels activity in hippocampal CA1 pyramidal neurons of developing rats but the development of Ca(2+) channels recovers to normal level at juvenile age. Moreover, the inhibition of 3% sevoflurane on VGCCs is greater than that of 2.1% sevoflurane.

Essential thalamic contribution to slow waves of natural sleep

Slow waves represent one of the prominent EEG signatures of non-rapid eye movement (non-REM) sleep and are thought to play an important role in the cellular and network plasticity that occurs during this behavioral state. These slow waves of natural sleep are currently considered to be exclusively generated by intrinsic and synaptic mechanisms within neocortical territories, although a role for the thalamus in this key physiological rhythm has been suggested but never demonstrated. Combining neuronal ensemble recordings, microdialysis, and optogenetics, here we show that the block of the thalamic output to the neocortex markedly (up to 50%) decreases the frequency of slow waves recorded during non-REM sleep in freely moving, naturally sleeping-waking rats. A smaller volume of thalamic inactivation than during sleep is required for observing similar effects on EEG slow waves recorded during anesthesia, a condition in which both bursts and single action potentials of thalamocortical neurons are almost exclusively dependent on T-type calcium channels. Thalamic inactivation more strongly reduces spindles than slow waves during both anesthesia and natural sleep. Moreover, selective excitation of thalamocortical neurons strongly entrains EEG slow waves in a narrow frequency band (0.75-1.5 Hz) only when thalamic T-type calcium channels are functionally active. These results demonstrate that the thalamus finely tunes the frequency of slow waves during non-REM sleep and anesthesia, and thus provide the first conclusive evidence that a dynamic interplay of the neocortical and thalamic oscillators of slow waves is required for the full expression of this key physiological EEG rhythm.

Mapacalcine protects mouse neurons against hypoxia by blocking cell calcium overload

Stroke is one of a major cause of death and adult disability. Despite intense researches, treatment for stroke remains reduced to fibrinolysis, a technique useful for less than 10% of patients. Finding molecules able to treat or at least to decrease the deleterious consequences of stroke is an urgent need. Here, we showed that mapacalcine, a homodimeric peptide purified from the marine sponge Cliona vastifica, is able to protect mouse cortical neurons against hypoxia. We have also identified a subtype of L-type calcium channel as a target for mapacalcine and we showed that the channel has to be open for mapacalcine binding. The two main L-type subunits at the brain level are CaV1.3 and CaV1.2 subunits but mapacalcine was unable to block these calcium channels.Mapacalcine did not interfere with N-, P/Q- and R-type calcium channels. The protective effect was studied by measuring internal calcium level variation triggered by Oxygen Glucose Deprivation protocol, which mimics stroke, or glutamate stimulation. We showed that NMDA/AMPA receptors are not involved in the mapacalcine protection. The protective effect was confirmed by measuring the cell survival rate after Oxygen Glucose Deprivation condition. Our data indicate that mapacalcine is a promising molecule for stroke treatment.