Etomidate Impairs Long-Term Potentiation In Vitro by Targeting α5-Subunit Containing GABAA Receptors on Nonpyramidal Cells

α5GABAA receptor: A potential therapeutic target for perioperative neurocognitive disorders, a review of preclinical studies

Perioperative neurocognitive disorders (PND) are a common complication in elderly patients following surgery, which not only prolongs the recovery period but also affects their future quality of life and imposes a significant burden on their family and society. Multiple factors, including aging, vulnerability, anesthetic drugs, cerebral oxygen desaturation, and severe pain, have been associated with PND. Unfortunately, no effective drug is currently available to prevent PND. α5 γ-aminobutyric acid subtype A (α5GABAA) receptors have been implicated in cognitive function modulation. Positive or negative allosteric modulators of α5GABAA receptors have been found to improve cognitive impairment under different conditions. Therefore, targeting α5GABAA receptors may represent a promising treatment strategy for PND. This review focuses on preclinical studies of α5GABAA receptors and the risk factors associated with PND, primarily including aging, anesthetics, and neuroinflammation. Specifically, positive allosteric modulators of α5GABAA receptors have improved cognitive function in aged experimental animals. In contrast, negative allosteric modulators of α5GABAA receptors have been found to facilitate cognitive recovery in aged or adult experimental animals undergoing anesthesia and surgery but not in aged experimental animals under anesthesia alone. The reasons for the discordant findings have yet to be elucidated. In preclinical studies, different strategies of drug administration, as well as various behavioral tests, may influence the stability of the results. These issues need to be carefully considered in future studies.

Increasing NPYergic transmission in the hippocampus rescues aging-related deficits of long-term potentiation in the mouse dentate gyrus

Loss of neuropeptide Y (NPY)-expressing interneurons in the hippocampus and decaying cholinergic neuromodulation are thought to contribute to impaired cognitive function during aging. However, the interaction of these two neuromodulatory systems in maintaining hippocampal synaptic plasticity during healthy aging has not been explored so far. Here we report profound sex differences in the Neuropeptide-Y (NPY) levels in the dorsal dentate gyrus (DG) with higher NPY concentrations in the male mice compared to their female counterparts and a reduction of NPY levels during aging specifically in males. This change in aged males is accompanied by a deficit in theta burst-induced long-term potentiation (LTP) in the medial perforant path-to-dorsal DG (MPP-DG) synapse, which can be rescued by enhancing cholinergic activation with the acetylcholine esterase blocker, physostigmine. Importantly, NPYergic transmission is required for this rescue of LTP. Moreover, exogenous NPY application alone is sufficient to recover LTP induction in aged male mice, even in the absence of the cholinergic stimulator. Together, our results suggest that in male mice NPYergic neurotransmission is a critical factor for maintaining dorsal DG LTP during aging.

Bidirectional plasticity of GABAergic tonic inhibition in hippocampal somatostatin- and parvalbumin-containing interneurons

GABA_A receptors present in extrasynaptic areas mediate tonic inhibition in hippocampal neurons regulating the performance of neural networks. In this study, we investigated the effect of NMDA-induced plasticity on tonic inhibition in somatostatin- and parvalbumin-containing interneurons. Using pharmacological methods and transgenic mice (SST-Cre/PV-Cre x Ai14), we induced the plasticity of GABAergic transmission in somatostatin- and parvalbumin-containing interneurons by a brief (3 min) application of NMDA. In the whole-cell patch-clamp configuration, we measured tonic currents enhanced by specific agonists (etomidate or gaboxadol). Furthermore, in both the control and NMDA-treated groups, we examined to what extent these changes depend on the regulation of distinct subtypes of GABA_A receptors. Tonic conductance in the somatostatin-containing (SST+) interneurons is enhanced after NMDA application, and the observed effect is associated with an increased content of α5-containing GABA_ARs. Both fast-spiking and non-fast-spiking parvalbumin-positive (PV+) cells showed a reduction of tonic inhibition after plasticity induction. This effect was accompanied in both PV+ interneuron types by a strongly reduced proportion of δ-subunit-containing GABA_ARs and a relatively small increase in currents mediated by α5-containing GABA_ARs. Both somatostatin- and parvalbumin-containing interneurons show cell type-dependent and opposite sign plasticity of tonic inhibition. The underlying mechanisms depend on the cell-specific balance of plastic changes in the contents of α5 and δ subunit-containing GABA_ARs.

Negative Allosteric Modulation of Gamma-Aminobutyric Acid A Receptors at α5 Subunit-Containing Benzodiazepine Sites Reverses Stress-Induced Anhedonia and Weakened Synaptic Function in Mice

BACKGROUND

Abnormal reward processing, typically anhedonia, is a hallmark of human depression and is accompanied by altered functional connectivity in reward circuits. Negative allosteric modulators of GABA_A (gamma-aminobutyric acid A) receptors (GABA-NAMs) have rapid antidepressant-like properties in rodents and exert few adverse effects, but molecular targets underlying their behavioral and synaptic effects remain undetermined. We hypothesized that GABA-NAMs act at the benzodiazepine site of GABA_A receptors containing α5 subunits to increase gamma oscillatory activity, strengthen synapses in reward circuits, and reverse anhedonia.

METHODS

Anhedonia was induced by chronic stress in male mice and assayed by preferences for sucrose and female urine (n = 5-7 mice/group). Hippocampal slices were then prepared for electrophysiological recording (n = 1-6 slices/mouse, 4-6 mice/group). Electroencephalography power was quantified in response to GABA-NAM and ketamine administration (n = 7-9 mice/group).

RESULTS

Chronic stress reduced sucrose and female urine preferences and hippocampal temporoammonic-CA1 synaptic strength. A peripheral injection of the GABA-NAM MRK-016 restored hedonic behavior and AMPA-to-NMDA ratios in wild-type mice. These actions were prevented by pretreatment with the benzodiazepine site antagonist flumazenil. MRK-016 administration increased gamma power over the prefrontal cortex in wild-type mice but not α5 knockout mice, whereas ketamine promoted gamma power in both genotypes. Hedonic behavior and AMPA-to-NMDA ratios were only restored by MRK-016 in stressed wild-type mice but not α5 knockout mice.

CONCLUSIONS

α5-Selective GABA-NAMs exert rapid anti-anhedonic actions and restore the strength of synapses in reward regions by acting at the benzodiazepine site of α5-containing GABA_A receptors. These results encourage human studies using GABA-NAMs to treat depression by providing readily translatable measures of target engagement.

Hippocampal Inhibitory Synapsis Deficits Induced by α5-Containing GABAA Receptors Mediate Chronic Neuropathic Pain-Related Cognitive Impairment

Chronic neuropathic pain often leads to cognitive impairment, but the exact mechanism remains unclear. Gamma-aminobutyric acid A receptors (GABA_ARs) are the major inhibitory receptors in the brain, of which the α5-containing GABA_ARs (GABA_ARs-α5) are implicated in a range of neuropsychiatric disorders with cognitive deficits. However, whether GABA_ARs-α5 are involved in chronic neuropathic pain-related cognitive impairment remains unknown. In this study, the rats with chronic neuropathic pain induced by right sciatic nerve ligation injury (SNI) exhibited cognitive impairment with declined spontaneous alternation in Y-maze test and discrimination index in novel object recognition test. The GABA_ARs-α5 expressing on parvalbumin and somatostatin interneurons increased remarkably in hippocampus, resulting in decreased mean frequency of spontaneous inhibitory postsynaptic currents in hippocampal pyramidal neurons. Significantly, antagonizing the GABA_ARs-α5 by L655708 rescued weakened inhibitory synaptic transmission and cognitive impairment induced by chronic neuropathic pain. Taken together, these data suggest that the GABA_ARs-α5 play a crucial role in chronic neuropathic pain-induced cognitive impairment by weakening inhibitory synaptic transmission, which may provide insights into the pharmacologic treatment of chronic neuropathic pain-related cognitive impairment.

Midazolam at low nanomolar concentrations affects long-term potentiation and synaptic transmission predominantly via the α1-GABAA receptor subunit in mice

BACKGROUND

Midazolam amplifies synaptic inhibition via different GABAA receptor subtypes defined by the presence of α1, α2, α3 or α5-subunits in the channel complex. Midazolam blocks long-term potentiation and produces postoperative amnesia. The aims of this study were to identify the GABAA receptor subtypes targeted by midazolam responsible for affecting CA1-long-term potentiation and synaptic inhibition in neocortical neurons.

METHODS

The effects of midazolam on hippocampal CA1-long-term potentiation were studied in acutely prepared brain slices of male and female mice. Positive allosteric modulation on GABAA receptor-mediated miniature inhibitory postsynaptic currents was investigated in organotypic slice cultures of the mouse neocortex. In both experiments, wild-type mice and GABAA receptor knock-in mouse lines were compared in which α1-, α5-, α1/2/3-, α1/3/5- and α2/3/5-GABAA receptor subtypes had been rendered benzodiazepine-insensitive.

RESULTS

Midazolam 10nM completely blocked long-term potentiation (midazolam mean±SD 98±11%, n=14/8 (slices/mice) vs. control 156±19%, n=20/12; p<0.001). Experiments in slices of α1-, α5-, α1/2/3-, α1/3/5- and α2/3/5-knock-in mice revealed a dominant role for the α1-GABAA receptor subtype in the long-term potentiation suppressing effect.In slices from wild-type mice, midazolam increased (mean±SD) charge transfer of miniature synaptic events concentration-dependently, 50nM: 172±71% (n=10/6) vs. 500nM: 236±54% (n=6/6), p=0.041. In α2/3/5-knock-in mice, charge transfer of miniature synaptic events did not further enhance when applying 500nM midazolam, 50nM: 171±62% (n=8/6) vs. 500nM: 175±62% (n=6/6), p=0.454) indicating two different binding affinities for midazolam to α2/3/5- and α1-subunits.

CONCLUSIONS

These results demonstrate a predominant role of α1-GABAA receptors in the actions of midazolam at low nanomolar concentrations. At higher concentrations, midazolam also enhances other GABAA receptor subtypes. α1-GABAA receptors may already contribute at sedative doses to the phenomenon of postoperative amnesia that has been reported after midazolam administration.

HC. Relevance of Cortical and Hippocampal Interneuron Functional Diversity to General Anesthetic Mechanisms: A Narrative Review

General anesthetics disrupt brain processes involved in consciousness by altering synaptic patterns of excitation and inhibition. In the cerebral cortex and hippocampus, GABAergic inhibition is largely mediated by inhibitory interneurons, a heterogeneous group of specialized neuronal subtypes that form characteristic microcircuits with excitatory neurons. Distinct interneuron subtypes regulate specific excitatory neuron networks during normal behavior, but how these interneuron subtypes are affected by general anesthetics is unclear. This narrative review summarizes current principles of the synaptic architecture of cortical and interneuron subtypes, their contributions to different forms of inhibition, and their roles in distinct neuronal microcircuits. The molecular and cellular targets in these circuits that are sensitive to anesthetics are reviewed in the context of how anesthetics impact interneuron function in a subtype-specific manner. The implications of this functional interneuron diversity for mechanisms of anesthesia are discussed, as are their implications for anesthetic-induced changes in neural plasticity and overall brain function.

CPP impairs contextual learning at concentrations below those that block pyramidal neuron NMDARs and LTP in the CA1 region of the hippocampus

Drugs that block N-methyl-d-aspartate receptors (NMDARs) suppress hippocampus-dependent memory formation; they also block long-term potentiation (LTP), a cellular model of learning and memory. However, the fractional block that is required to achieve these effects is unknown. Here, we measured the dose-dependent suppression of contextual memory in vivo by systemic administration of the competitive antagonist (R,S)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP); in parallel, we measured the concentration-dependent block by CPP of NMDAR-mediated synapses and LTP of excitatory synapses in hippocampal brain slices in vitro. We found that the dose of CPP that suppresses contextual memory in vivo (EC50 = 2.3 mg/kg) corresponds to a free concentration of 53 nM. Surprisingly, applying this concentration of CPP to hippocampal brain slices had no effect on the NMDAR component of evoked field excitatory postsynaptic potentials (fEPSPNMDA), or on LTP. Rather, the IC50 for blocking the fEPSPNMDA was 434 nM, and for blocking LTP was 361 nM - both nearly an order of magnitude higher. We conclude that memory impairment produced by systemically administered CPP is not due primarily to its blockade of NMDARs on hippocampal pyramidal neurons. Rather, systemic CPP suppresses memory formation by actions elsewhere in the memory-encoding circuitry.

Hippocampal β2-GABAA receptors mediate LTP suppression by etomidate and contribute to long-lasting feedback but not feedforward inhibition of pyramidal neurons

The general anesthetic etomidate, which acts through γ-aminobutyric acid type A (GABA_A) receptors, impairs the formation of new memories under anesthesia. This study addresses the molecular and cellular mechanisms by which this occurs. Here, using a new line of genetically engineered mice carrying the GABA_A receptor (GABA_AR) β2-N265M mutation, we tested the roles of receptors that incorporate GABA_A receptor β2 versus β3 subunits to suppression of long-term potentiation (LTP), a cellular model of learning and memory. We found that brain slices from β2-N265M mice resisted etomidate suppression of LTP, indicating that the β2-GABA_ARs are an essential target in this model. As these receptors are most heavily expressed by interneurons in the hippocampus, this finding supports a role for interneuron modulation in etomidate control of synaptic plasticity. Nevertheless, β2 subunits are also expressed by pyramidal neurons, so they might also contribute. Therefore, using a previously established line of β3-N265M mice, we also examined the contributions of β2- versus β3-GABA_ARs to GABA_A,slow dendritic inhibition, because dendritic inhibition is particularly well suited to controlling synaptic plasticity. We also examined their roles in long-lasting suppression of population activity through feedforward and feedback inhibition. We found that both β2- and β3-GABA_ARs contribute to GABA_A,slow inhibition and that both β2- and β3-GABA_ARs contribute to feedback inhibition, whereas only β3-GABA_ARs contribute to feedforward inhibition. We conclude that modulation of β2-GABA_ARs is essential to etomidate suppression of LTP. Furthermore, to the extent that this occurs through GABA_ARs on pyramidal neurons, it is through modulation of feedback inhibition.NEW & NOTEWORTHY Etomidate exerts its anesthetic actions through GABA_A receptors. However, the mechanism remains unknown. Here, using a hippocampal brain slice model, we show that β2-GABA_ARs are essential to this effect. We also show that these receptors contribute to long-lasting dendritic inhibition in feedback but not feedforward inhibition of pyramidal neurons. These findings hold implications for understanding how anesthetics block memory formation and, more generally, how inhibitory circuits control learning and memory.

Induction of Inhibitory Synaptic Plasticity Enhances Tonic Current by Increasing the Content of α5-Subunit Containing GABAA Receptors in Hippocampal Pyramidal Neurons

It is known that besides synaptic inhibition, there is a persistent component of inhibitory drive mediated by tonic currents which is believed to mediate majority of the total inhibitory charge in hippocampal neurons. Tonic currents, depending on cell types, can be mediated by a variety of GABA_A receptor (GABA_AR) subtypes but in pyramidal neurons, α5-subunit containing receptors were found to be predominant. Importantly, α5-GABA_ARs were implicated in both inhibitory and excitatory synaptic plasticity as well as in a variety of cognitive tasks. In the present study, we asked whether the protocol that evokes NMDAR-dependent GABAergic inhibitory long-term potentiation (iLTP) also induces the plasticity of tonic inhibition in hippocampal pyramidal neurons. Our whole-cell patch-clamp recordings revealed that the induction of this type of iLTP is associated with a marked increase in tonic current. By using the specific inverse agonist of α5-containing GABA_ARs (L-655,709) we provide evidence that this plastic change in tonic current is correlated with an increased proportion of this type of GABA_ARs. On the contrary, the iLTP induction did not affect the tonic current potentiated by THIP, indicating that the pool of δ subunit-containing GABA_ARs receptors remains unaffected. We conclude that the α5-GABA_ARs-dependent plasticity of tonic inhibition is a novel dimension of the neuroplasticity of the inhibitory drive in the hippocampal principal neurons. Overall, α5-containing GABA_ARs emerge as key players in a variety of plasticity mechanisms operating over a large span of time and spatial scales.

Bidirectional regulation of distinct memory domains by α5-subunit-containing GABAA receptors in CA1 pyramidal neurons

Reduction in the expression or function of α5-subunit-containing GABA_A receptors (α5GABA_ARs) leads to improvement in several hippocampus-dependent memory domains. However, studies thus far mostly lack anatomical specificity in terms of neuronal circuits and populations. We demonstrate that mice with a selective knockdown of α5GABA_ARs in CA1 pyramidal neurons (α5CA1KO mice) show improved spatial and trace fear-conditioning memory. Unexpectedly, α5CA1KO mice were comparable to controls in contextual fear-conditioning but showed an impairment in context discrimination, suggesting fine-tuning of activity in CA1 pyramidal cell dendrites through α5-mediated inhibition might be necessary for distinguishing highly similar contexts.

Disruption of the GABAergic system contributes to the development of perioperative neurocognitive disorders after anesthesia and surgery in aged mice

Aims

Perioperative neurocognitive disorders (PND) are associated with cognitive impairment in the preoperative or postoperative period, and neuroinflammation is thought to be the most important mechanisms especially during the postoperative period. The GABAergic system is easily disrupted by neuroinflammation. This study investigated the impact of the GABAergic system on PND after anesthesia and surgery.

Methods

An animal model of laparotomy with inhalation anesthesia in 16‐month‐old mice was addressed. Effects of the GABAergic system were assessed using biochemical analysis. Pharmacological blocking of α5GABA_ARs or P38 mitogen‐activated protein kinase (MAPK) were applied to investigate the effects of the GABAergic system.

Results

After laparotomy, the hippocampus‐dependent memory and long‐term potentiation were impaired, the levels of IL‐6, IL‐1β and TNF‐α up‐regulated in the hippocampus, the concentration of GABA decreased, and the protein levels of the surface α5GABA_ARs up‐regulated. Pharmacological blocking of α5GABA_ARs with L655,708 alleviated laparotomy induced cognitive deficits. Further studies found that the P38 MAPK signaling pathway was involved and pharmacological blocking with SB203,580 alleviated memory dysfunctions.

Conclusions

Anesthesia and surgery caused neuroinflammation in the hippocampus, which consequently disrupted the GABAergic system, increased the expressions of surface α5GABA_ARs especially through the P38 MAPK signaling pathway, and eventually led to hippocampus‐dependent memory dysfunctions.

Postacute Delivery of GABAA α5 Antagonist Promotes Postischemic Neurological Recovery and Peri-infarct Brain Remodeling

Supplemental Digital Content is available in the text.

Background and Purpose—

Poststroke, neuronal excitability is tonically reduced in peri-infarct tissue via inhibitory influences of extrasynaptic GABA_A receptors. We hypothesized that GABA_A α5 blockade by the competitive antagonist S44819 enhances postischemic neurological recovery, brain remodeling, and neuroplasticity.

Methods—

In an explorative study followed by a confirmation study, male C57Bl6/j mice were exposed to transient intraluminal middle cerebral artery occlusion. Starting 72 hours poststroke, vehicle or S44819 (3 or 10 mg/kg, BID) was delivered orally for 28 days. Neurological recovery, perilesional tissue remodeling, and contralesional pyramidal tract plasticity were evaluated for 42 days, that is, 14 days after completion of S44819 delivery.

Results—

S44819, delivered at 10 but not 3 mg/kg, persistently improved motor coordination and spatial memory in both studies. Striatal atrophy was reduced by 10 mg/kg S44819 at 42 days post-treatment onset, and neuronal long-term survival in the peri-infarct striatum was increased. Delayed neuroprotection was associated with reduced peri-infarct astrogliosis, increased peri-infarct brain capillary density, and increased neural precursor cell proliferation and differentiation in proximity to the ipsilesional subventricular zone. Contralesional pyramidal tract plasticity, evaluated by anterograde tract tracing at the level of the red nucleus, was not influenced by S44819. Concentrations of neurotrophic (brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor) and angiogenic (vascular endothelial growth factor and basic fibroblast growth factor) growth factors were elevated by 10 mg/kg S44819 in peri-infarct but not contralesional brain tissue.

Conclusions—

Our data demonstrate that S44819 enhances neurological recovery and peri-infarct brain remodeling in the postacute stroke phase.

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.

Zolpidem Activation of Alpha 1-Containing GABAA Receptors Selectively Inhibits High Frequency Action Potential Firing of Cortical Neurons

Introduction: High frequency neuronal activity in the cerebral cortex can be induced by noxious stimulation during surgery, brain injury or poisoning. In this scenario, it is essential to block cortical hyperactivity to protect the brain against damage, e.g., by using drugs that act as positive allosteric modulators at GABA_A receptors. Yet, cortical neurons express multiple, functionally distinct GABA_A receptor subtypes. Currently there is a lack of knowledge which GABA_A receptor subtypes would be a good pharmacological target to reduce extensive cortical activity. Methods: Spontaneous action potential activity was monitored by performing extracellular recordings from organotypic neocortical slice cultures of wild type and GABA_AR-α1(H101R) mutant mice. Phases of high neuronal activity were characterized using peri-event time histograms. Drug effects on within-up state firing rates were quantified via Hedges' g. Results: We quantified the effects of zolpidem, a positive modulator of GABA_A receptors harboring α1-subunits, and the experimental benzodiazepine SH-053-2'F-S-CH3, which preferably acts at α2/3/5- but spares α1-subunits. Both agents decreased spontaneous action potential activity but altered the firing patterns in different ways. Zolpidem reduced action potential firing during highly active network states. This action was abolished by flumazenil, suggesting that it was mediated by benzodiazepine-sensitive GABA_A receptors. SH-053-2'F-S-CH3 also attenuated neuronal activity, but unlike zolpidem, failed to reduce high frequency firing. To confirm that zolpidem actions were indeed mediated via α1-dependent actions, it was evaluated in slices from wild type and α(H101R) knock-in mice. Inhibition of high frequency action potential firing was observed in slices from wild type but not mutant mice. Conclusion: Our results suggest that during episodes of scarce and high neuronal activity action potential firing of cortical neurons is controlled by different GABA_A receptor subtypes. Exaggerated firing of cortical neurons is reduced by positive modulation of α1-, but not α2/3/5-subunit containing GABA_A receptors.

γ-Aminobutyric Acid Type A Receptor Potentiation Inhibits Learning in a Computational Network Model

BACKGROUND

Propofol produces memory impairment at concentrations well below those abolishing consciousness. Episodic memory, mediated by the hippocampus, is most sensitive. Two potentially overlapping scenarios may explain how γ-aminobutyric acid receptor type A (GABAA) potentiation by propofol disrupts episodic memory-the first mediated by shifting the balance from excitation to inhibition while the second involves disruption of rhythmic oscillations. We use a hippocampal network model to explore these scenarios. The basis for these experiments is the proposal that the brain represents memories as groups of anatomically dispersed strongly connected neurons.

METHODS

A neuronal network with connections modified by synaptic plasticity was exposed to patterned stimuli, after which spiking output demonstrated evidence of stimulus-related neuronal group development analogous to memory formation. The effect of GABAA potentiation on this memory model was studied in 100 unique networks.

RESULTS

GABAA potentiation consistent with moderate propofol effects reduced neuronal group size formed in response to a patterned stimulus by around 70%. Concurrently, accuracy of a Bayesian classifier in identifying learned patterns in the network output was reduced. Greater potentiation led to near total failure of group formation. Theta rhythm variations had no effect on group size or classifier accuracy.

CONCLUSIONS

Memory formation is widely thought to depend on changes in neuronal connection strengths during learning that enable neuronal groups to respond with greater facility to familiar stimuli. This experiment suggests the ability to form such groups is sensitive to alteration in the balance between excitation and inhibition such as that resulting from administration of a γ-aminobutyric acid-mediated anesthetic agent.

Input-Specific Synaptic Location and Function of the α5 GABAA Receptor Subunit in the Mouse CA1 Hippocampal Neurons

Hippocampus-dependent learning processes are coordinated via a large diversity of GABAergic inhibitory mechanisms. The α5 subunit-containing GABA_A receptor (α5-GABA_AR) is abundantly expressed in the hippocampus populating primarily the extrasynaptic domain of CA1 pyramidal cells, where it mediates tonic inhibitory conductance and may cause functional deficits in synaptic plasticity and hippocampus-dependent memory. However, little is known about synaptic expression of the α5-GABA_AR and, accordingly, its location site-specific function. We examined the cell- and synapse-specific distribution of the α5-GABA_AR in the CA1 stratum oriens/alveus (O/A) using a combination of immunohistochemistry, whole-cell patch-clamp recordings and optogenetic stimulation in hippocampal slices obtained from mice of either sex. In addition, the input-specific role of the α5-GABA_AR in spatial learning and anxiety-related behavior was studied using behavioral testing and chemogenetic manipulations. We demonstrate that α5-GABA_AR is preferentially targeted to the inhibitory synapses made by the vasoactive intestinal peptide (VIP)- and calretinin-positive terminals onto dendrites of somatostatin-expressing interneurons. In contrast, synapses made by the parvalbumin-positive inhibitory inputs to O/A interneurons showed no or little α5-GABA_AR. Inhibiting the α5-GABA_AR in control mice in vivo improved spatial learning but also induced anxiety-like behavior. Inhibiting the α5-GABA_AR in mice with inactivated CA1 VIP input could still improve spatial learning and was not associated with anxiety. Together, these data indicate that the α5-GABA_AR-mediated phasic inhibition via VIP input to interneurons plays a predominant role in the regulation of anxiety while the α5-GABA_AR tonic inhibition via this subunit may control spatial learning.SIGNIFICANCE STATEMENT The α5-GABA_AR subunit exhibits high expression in the hippocampus, and regulates the induction of synaptic plasticity and the hippocampus-dependent mnemonic processes. In CA1 principal cells, this subunit occupies mostly extrasynaptic sites and mediates tonic inhibition. Here, we provide evidence that, in CA1 somatostatin-expressing interneurons, the α5-GABA_AR subunit is targeted to synapses formed by the VIP- and calretinin-expressing inputs, and plays a specific role in the regulation of anxiety-like behavior.

Sevoflurane Blocks the Induction of Long-term Potentiation When Present during, but Not When Present Only before, the High-frequency Stimulation

Background

This study tests the hypothesis that sevoflurane blocks long-term potentiation only if it is present during the high-frequency stimulation that induces long-term potentiation.

Methods

Long-term potentiation, an electrophysiologic correlate of memory, was induced by high-frequency stimulation and measured as a persistent increase in the field excitatory postsynaptic potential slope in the CA1 region.

Results

Long-term potentiation was induced in the no sevoflurane group (171 ± 58% vs. 96 ± 11%; n = 13, mean ± SD); when sevoflurane (4%) was present during the high-frequency stimulation, long-term potentiation was blocked (92 ± 22% vs. 99 ± 7%, n = 6). While sevoflurane reduced the size of the field excitatory postsynaptic potential to single test stimuli by 59 ± 17%, it did not significantly reduce the size of the field excitatory postsynaptic potentials during the 100 Hz high-frequency stimulation. If sevoflurane was removed from the artificial cerebrospinal fluid superfusing the slices 10 min before the high-frequency stimulation, then long-term potentiation was induced (185 ± 48%, n = 7); this was not different from long-term potentiation in the no sevoflurane slices (171 ± 58). Sevoflurane before, but not during, ⊖-burst stimulation, a physiologic stimulus, did not block the induction of long-term potentiation (151 ± 37% vs. 161 ± 34%, n = 7).

Conclusions

Sevoflurane blocks long-term potentiation formation if present during the high-frequency stimulation; this blockage of long-term potentiation does not persist if sevoflurane is discontinued before the high-frequency stimulation. These results may explain why short periods of insufficient sevoflurane anesthesia may lead to recall of painful or traumatic events during surgery.

New insights in the systemic and molecular underpinnings of general anesthetic actions mediated by γ-aminobutyric acid A receptors

PURPOSE OF REVIEW

The review highlights novel insights into the role of γ-aminobutyric acid A (GABAA) receptors in mediating clinically relevant actions of anesthetic agents.

RECENT FINDINGS

GABAA receptors in the hippocampus are located on glutamatergic pyramidal cells and GABAergic interneurons. Etomidate-induced inhibition of a synaptic correlate of learning and memory is caused by receptors on nonpyramidal neurons, likely on interneurons that incorporate α5 subunits. Selective enhancement of α2 subunit containing GABAA receptors in the spinal cord provides antihyperalgesia against inflammatory and neuropathic pain without causing sedation, motor impairment, and tolerance development. Inflammation, traumatic brain injury, and exposure to anesthetic agents modify the expression patterns of GABAA receptors in a subtype-specific manner. These modifications may persist for weeks. The neuroactive steroid alphaxalone causes fast-onset and short-duration anesthesia in humans. Cardiovascular and respiratory side-effects are less severe than with propofol.

SUMMARY

Identification of the molecular and cellular substrates involved in anesthesia and insights into disease and drug-induced alterations in the expression patterns of GABAA receptors in the central nervous system are emphasizing the need for individualized anesthesia care. Introducing neuroactive steroids into clinical anesthesia is expected to reduce cardiovascular and respiratory side-effects.

Decreasing the Expression of GABAA α5 Subunit-Containing Receptors Partially Improves Cognitive, Electrophysiological, and Morphological Hippocampal Defects in the Ts65Dn Model of Down Syndrome

Trisomy 21 or Down syndrome (DS) is the most common cause of intellectual disability of a genetic origin. The Ts65Dn (TS) mouse, which is the most commonly used and best-characterized mouse model of DS, displays many of the cognitive, neuromorphological, and biochemical anomalies that are found in the human condition. One of the mechanisms that have been proposed to be responsible for the cognitive deficits in this mouse model is impaired GABA-mediated inhibition. Because of the well-known modulatory role of GABA_A α5 subunit-containing receptors in cognitive processes, these receptors are considered to be potential targets for improving the intellectual disability in DS. The chronic administration of GABA_A α5-negative allosteric modulators has been shown to be procognitive without anxiogenic or proconvulsant side effects. In the present study, we use a genetic approach to evaluate the contribution of GABA_A α5 subunit-containing receptors to the cognitive, electrophysiological, and neuromorphological deficits in TS mice. We show that reducing the expression of GABA_A α5 receptors by deleting one or two copies of the Gabra5 gene in TS mice partially ameliorated the cognitive impairments, improved long-term potentiation, enhanced neural differentiation and maturation, and normalized the density of the GABAergic synapse markers. Reducing the gene dosage of Gabra5 in TS mice did not induce motor alterations and anxiety or affect the viability of the mice. Our results provide further evidence of the role of GABA_A α5 receptor-mediated inhibition in cognitive impairment in the TS mouse model of DS.

Developmental control of spike-timing-dependent plasticity by tonic GABAergic signaling in striatum

Activity-dependent long-term potentiation (LTP) and depression (LTD) of synaptic strength underlie multiple forms of learning and memory. Spike-timing-dependent plasticity (STDP) has been described as a Hebbian synaptic learning rule that could account for experience-dependent changes in neural networks, but little is known about whether and how STDP evolves during development. We previously showed that GABAergic signaling governs STDP polarity and thus operates as a Hebbian/anti-Hebbian switch in the striatum. Although GABAergic networks are subject to important developmental maturation, it remains unclear whether STDP is developmentally shaped by GABAergic signaling. Here, we investigated whether STDP rules are developmentally regulated at corticostriatal synapses in the dorsolateral striatum. We found that striatal STDP displays unidirectional plasticity (Hebbian tLTD) in young rats (P_7-10) whereas STDP is bidirectional and anti-Hebbian in juvenile (P_20-25) and adult (P_60-90) rats. We also provide evidence that the appearance of tonic (extrasynaptic) GABAergic signaling from the juvenile stage is a crucial factor in shaping STDP rules during development, establishing bidirectional anti-Hebbian STDP in the adult striatum. Thus, developmental maturation of GABAergic signaling tightly drives the polarity of striatal plasticity.

The α5 subunit containing GABAA receptors contribute to chronic pain

It has been recently proposed that α5-subunit containing GABAA receptors (α5-GABAA receptors) that mediate tonic inhibition might be involved in pain. The purpose of this study was to investigate the contribution of α5-GABAA receptors in the loss of GABAergic inhibition and in formalin-induced, complete Freund's adjuvant (CFA)-induced and L5 and L6 spinal nerve ligation-induced long-lasting hypersensitivity. Formalin or CFA injection and L5 and L6 spinal nerve ligation produced long-lasting allodynia and hyperalgesia. Moreover, formalin injection impaired the rate-dependent depression of the Hofmann reflex. Peripheral and intrathecal pretreatment or post-treatment with the α5-GABAA receptor antagonist, L-655,708 (0.15-15 nmol), prevented and reversed, respectively, these long-lasting behaviors. Formalin injection increased α5-GABAA receptor mRNA expression in the spinal cord and dorsal root ganglia (DRG) mainly at 3 days. The α5-GABAA receptors were localized in the dorsal spinal cord and DRG colabeling with NeuN, CGRP, and IB4 which suggests their presence in peptidergic and nonpeptidergic neurons. These receptors were found mainly in small and medium sized neurons. Formalin injection enhanced α5-GABAA receptor fluorescence intensity in spinal cord and DRG at 3 and 6 days. Intrathecal administration of L-655,708 (15 nmol) prevented and reversed formalin-induced impairment of rate-dependent depression. These results suggest that α5-GABAA receptors play a role in the loss of GABAergic inhibition and contribute to long-lasting secondary allodynia and hyperalgesia.

Contrasting Effects of the γ-Aminobutyric Acid Type A Receptor β3 Subunit N265M Mutation on Loss of Righting Reflexes Induced by Etomidate and the Novel Anesthetic Barbiturate R-mTFD-MPAB

BACKGROUND

Previous studies have shown that etomidate modulates γ-aminobutyric acid type A receptors by binding at the β-α subunit interface within the transmembrane domain of receptors that incorporate β2 or β3 subunits. Introducing an asparagine-to-methionine (N265M) mutation at position 265 of the β3 subunit, which sits within the etomidate-binding site, attenuates the hypnotic effect of etomidate in vivo. It was reported recently that the photoactivatable barbiturate R-mTFD-MPAB also acts on γ-aminobutyric acid type A receptors primarily by binding to a homologous site at the γ-β interface. Given this difference in drug-binding sites established by the in vitro experiments, we hypothesized that the β3-N265M-mutant mice would not be resistant to the anesthetic effects of R-mTFD-MPAB in vivo, whereas the same mutant mice would be resistant to the anesthetic effects of R-etomidate.

METHODS

We measured the effects of IV injection of etomidate and R-mTFD-MPAB on loss and recovery of righting reflex in wild-type mice and in mice carrying the β3-N265M mutation.

RESULTS

Etomidate-induced hypnosis, as measured by the duration of loss of righting reflex, was attenuated in the N265M knock-in mice, confirming prior results. By contrast, recovery of balance and coordinated movement, as measured by the ability to maintain all 4 paws on the ground, was unaffected by the mutation. Neither hypnosis nor impairment of coordinated movement produced by the barbiturate R-mTFD-MPAB was affected by the mutation.

CONCLUSIONS

The findings confirmed our hypothesis that mutating the etomidate-binding site would not alter the response to the barbiturate R-mTFD-MPAB. Furthermore, we confirmed previous studies indicating that etomidate-induced hypnosis is mediated in part by β3-containing receptors. We also extended previous findings by showing that etomidate-impaired balance and coordinated movement are not mediated by β3-containing receptors, thus implicating β2-containing receptors in this end point.

The neurophysiological effects of single-dose theophylline in patients with chronic stroke: A double-blind, placebo-controlled, randomized cross-over study

BACKGROUND

Reducing inhibitory neurotransmission with pharmacological agents is a potential approach for augmenting plasticity after stroke. Previous work in healthy subjects showed diminished intracortical inhibition after administration of theophylline.

OBJECTIVE

We assessed the effect of single-dose theophylline on intracortical and interhemispheric inhibition in patients with chronic stroke, in a double-blind, placebo-controlled, cross-over study.

METHODS

Eighteen subjects were randomly administered 300 mg of extended-release theophylline or placebo. Immediately and 5 hours following administration, transcranial magnetic stimulation was used to assess bihemispheric resting motor threshold, short-interval intracortical inhibition, long-interval intracortical inhibition, and interhemispheric inhibition. Adverse effects on cardiovascular, neurological, and motor performance outcomes were also surveilled. Change between morning and afternoon sessions were compared across conditions. One week later, patients underwent the same assessments after crossing over to the opposite experimental condition. Subjects and investigators were blinded to the experimental condition during data acquisition and analysis.

RESULTS

For both hemispheres, changes in intracortical or interhemispheric neurophysiology were comparable under theophylline and placebo conditions. Theophylline induced no adverse neurological, cardiovascular, or motor performance effects. For both conditions and hemipsheres, the baseline level of inhibition inversely correlated with its change between sessions: less baseline inhibition (i.e. disinhibition) was associated with a strengthening in inhibition over the day, and vice versa.

CONCLUSION

A single dose of theophylline is well-tolerated by patients with chronic stroke, but does not alter cortical excitability. The inverse relationship between baseline inhibition and its change suggests the existence of a homeostatic process. The lack of effect on cortical inhibition may be related to an insufficiently long exposure to theophylline, or to differential responsiveness of disinhibited neural circuitry in patients with stroke.