Barbiturate regulation of kinetic properties of the GABAA receptor channel of mouse spinal neurones in culture

An isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS)-based candidate reference measurement procedure (RMP) for the quantification of phenobarbital in human serum and plasma

OBJECTIVES

Phenobarbital serves as an antiepileptic drug (AED) and finds application in the treatment of epilepsy either as monotherapy or adjunctive therapy. This drug exhibits various pharmacodynamic properties that account for its beneficial effects as well as potential side effects. Accurate measurement of its concentration is critical for optimizing AED therapy through appropriate dose adjustments. Therefore, our objective was to develop and validate a new reference measurement procedure (RMP) for the accurate quantification of phenobarbital levels in human serum and plasma.

METHODS

A sample preparation protocol based on protein precipitation followed by a high dilution step was established in combination with a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method using a C8 column to separate target analytes from known and unknown interferences. Assay validation and determination of measurement uncertainty were performed based on current guidelines. Selectivity and Specificity were assessed using spiked serum and plasma samples; to investigate possible matrix effects (MEs) a post-column infusion experiment and a comparison of standard line slopes was performed. Precision and accuracy were determined within a multiday precision experiment.

RESULTS

The RMP was shown to be highly selective and specific, with no evidence of matrix interferences. It can be used to quantify phenobarbital in the range of 1.92 to 72.0 μg/mL. Intermediate precision was less than 3.2 %, and repeatability coefficient of variation (CV) ranged from 1.3 to 2.0 % across all concentration levels. The relative mean bias ranged from -3.0 to -0.7 % for native serum levels, and from -2.8 to 0.8 % for Li-heparin plasma levels. The measurement uncertainties (k=1) for single measurements and target value assignment were 1.9 to 3.3 % and 0.9 to 1.6 %, respectively.

CONCLUSIONS

A novel LC-MS/MS-based candidate RMP for the quantification of phenobarbital in human serum and plasma is presented which can be used for the standardization of routine assays and the evaluation of clinically relevant samples.

New GABA-Targeting Therapies for the Treatment of Seizures and Epilepsy: I. Role of GABA as a Modulator of Seizure Activity and Recently Approved Medications Acting on the GABA System

γ-Aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter in the mammalian brain and has been found to play an important role in the pathogenesis or the expression of many neurological diseases, including epilepsy. Although GABA can act on different receptor subtypes, the component of the GABA system that is most critical to modulation of seizure activity is the GABAA-receptor-chloride (Cl-) channel complex, which controls the movement of Cl- ions across the neuronal membrane. In the mature brain, binding of GABA to GABAA receptors evokes a hyperpolarising (anticonvulsant) response, which is mediated by influx of Cl- into the cell driven by its concentration gradient between extracellular and intracellular fluid. However, in the immature brain and under certain pathological conditions, GABA can exert a paradoxical depolarising (proconvulsant) effect as a result of an efflux of chloride from high intracellular to lower extracellular Cl- levels. Extensive preclinical and clinical evidence indicates that alterations in GABAergic inhibition caused by drugs, toxins, gene defects or other disease states (including seizures themselves) play a causative or contributing role in facilitating or maintaning seizure activity. Conversely, enhancement of GABAergic transmission through pharmacological modulation of the GABA system is a major mechanism by which different antiseizure medications exert their therapeutic effect. In this article, we review the pharmacology and function of the GABA system and its perturbation in seizure disorders, and highlight how improved understanding of this system offers opportunities to develop more efficacious and better tolerated antiseizure medications. We also review the available data for the two most recently approved antiseizure medications that act, at least in part, through GABAergic mechanisms, namely cenobamate and ganaxolone. Differences in the mode of drug discovery, pharmacological profile, pharmacokinetic properties, drug-drug interaction potential, and clinical efficacy and tolerability of these agents are discussed.

Phenobarbital in Nuclear Receptor Activation: An Update

Phenobarbital (PB) is a commonly prescribed anti-epileptic drug that can also benefit newborns from hyperbilirubinemia. Being the first drug demonstrating hepatic induction of cytochrome P450 (CYP), PB has since been broadly used as a model compound to study xenobiotic-induced drug metabolism and clearance. Mechanistically, PB-mediated CYP induction is linked to a number of nuclear receptors, such as the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and estrogen receptor α, with CAR being the predominant regulator. Unlike prototypical agonistic ligands, PB-mediated activation of CAR does not involve direct binding with the receptor. Instead, dephosphorylation of threonine 38 in the DNA-binding domain of CAR was delineated as a key signaling event underlying PB-mediated indirect activation of CAR. Further studies revealed that such phosphorylation sites appear to be highly conserved among most human nuclear receptors. Interestingly, while PB is a pan-CAR activator in both animals and humans, PB activates human but not mouse PXR. The species-specific role of PB in gene regulation is a key determinant of its implication in xenobiotic metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In this review, we summarize the recent progress in our understanding of PB-provoked transactivation of nuclear receptors with a focus on CAR and PXR. SIGNIFICANCE STATEMENT: Extensive studies using PB as a research tool have significantly advanced our understanding of the molecular basis underlying nuclear receptor-mediated drug metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In particular, CAR has been established as a cell signaling-regulated nuclear receptor in addition to ligand-dependent functionality. This mini-review highlights the mechanisms by which PB transactivates CAR and PXR.

Shisa7-Dependent Regulation of GABAA Receptor Single-Channel Gating Kinetics

GABAA receptors (GABAARs) mediate the majority of fast inhibitory transmission throughout the brain. Although it is widely known that pore-forming subunits critically determine receptor function, it is unclear whether their single-channel properties are modulated by GABAAR-associated transmembrane proteins. We previously identified Shisa7 as a GABAAR auxiliary subunit that modulates the trafficking, pharmacology, and deactivation properties of these receptors. However, whether Shisa7 also regulates GABAAR single-channel properties has yet to be determined. Here, we performed single-channel recordings of α2β3γ2L GABAARs cotransfected with Shisa7 in HEK293T cells and found that while Shisa7 does not change channel slope conductance, it reduced the frequency of receptor openings. Importantly, Shisa7 modulates GABAAR gating by decreasing the duration and open probability within bursts. Through kinetic analysis of individual dwell time components, activation modeling, and macroscopic simulations, we demonstrate that Shisa7 accelerates GABAAR deactivation by governing the time spent between close and open states during gating. Together, our data provide a mechanistic basis for how Shisa7 controls GABAAR gating and reveal for the first time that GABAAR single-channel properties can be modulated by an auxiliary subunit. These findings shed light on processes that shape the temporal dynamics of GABAergic transmission.SIGNIFICANCE STATEMENT Although GABAA receptor (GABAAR) single-channel properties are largely determined by pore-forming subunits, it remains unknown whether they are also controlled by GABAAR-associated transmembrane proteins. Here, we show that Shisa7, a recently identified GABAAR auxiliary subunit, modulates GABAAR activation by altering single-channel burst kinetics. These results reveal that Shisa7 primarily decreases the duration and open probability of receptor burst activity during gating, leading to accelerated GABAAR deactivation. These experiments are the first to assess the gating properties of GABAARs in the presence of an auxiliary subunit and provides a kinetic basis for how Shisa7 modifies temporal attributes of GABAergic transmission at the single-channel level.

Persistent Hiccups as an Atypical Presentation of SARS-CoV-2 Infection: A Systematic Review of Case Reports

Symptoms, such as fever, dry cough, dyspnoea, and respiratory distress, are commonly described in patients infected with Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Recently, a growing number of cases pertained to persistent hiccups have been reported by SARS-CoV-2 infected patients. The aim of this systematic review was to screen the current literature and provide a summary of the reported cases of SARS-CoV-2 infected patients presenting with persistent hiccups. According to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, PubMed, Scopus, and Web of Science databases were searched from inception until October 2021. Case reports or case series that provided a separate clinical description for patients with presenting complaints of persistent hiccups before or after COVID-19 diagnosis were retrieved. The critical appraisal checklist for case reports provided by the Joanna Briggs Institute (JBI) was employed to evaluate the overall quality of the eligible studies. We identified 13 eligible studies that included 16 hospitalized COVID-19 patients who complained of persistent hiccups. The mean duration of hiccups was 4.6 days reported in 88% (14/16) patients. Hypertension was the most common comorbidity present in 50% (8/16) of patients followed by diabetes mellitus (4/16). Moreover, 44% (7/16) of patients received only one medication for managing the hiccups with metoclopramide (5/16) followed by chlorpromazine and baclofen (4/16) used as primary treatment. Equally, 44% of patients (7/16) received dexamethasone followed by azithromycin (5/16), ivermectin (4/16), and ceftriaxone (4/16) for managing the infection from SARS-CoV-2. The majority of patients (14/16) improved after initiation of treatment. Persistent hiccups are possibly a rare symptom that clinicians may expect to encounter in patients infected with SARS-CoV-2. Although there is not ample proof to propose causation, increased awareness about the diversity of presentations of SARS-CoV-2 infection could be crucial in the early recognition of the disease.

Sedative-Hypnotic Agents That Impact Gamma-Aminobutyric Acid Receptors: Focus on Flunitrazepam, Gamma-Hydroxybutyric Acid, Phenibut, and Selank

There are many nonopioid central nervous system depressant substances that share a gamma-aminobutyric acid (GABA) receptor-related mechanism of action. These sedatives-hypnotics can be indicated to treat anxiety, seizures, depression, and insomnia but are also used as substances of abuse and used to facilitate sexual assault. Barbiturates, methaqualone, and glutethimide were among the first type A GABA receptor-mediated sedative-hypnotics. Their clinical use was limited for most indications by serious adverse events and strong abuse potential but continue to be used illicitly around the world. The benzodiazepines supplanted barbiturates for most indications because they were less likely to cause severe adverse events in monotherapy. Flunitrazepam is a newer benzodiazepine that is preferentially used recreationally and to facilitate sexual assault. Flunitrazepam has greater potency and higher affinity for the type A GABA receptor than most benzodiazepines. Gamma-hydroxybutyric acid is sought illicitly for its hypnotic, euphoric and anabolic effects as well as to facilitate sexual assault. When any of these GABAergic drugs are used in high doses or with other sedative hypnotic agents, respiratory depression, coma, and death have occurred. Chronic use of these GABAergic drugs can lead to significant withdrawal syndromes. Phenibut and selank are poorly studied Russian drugs with GABAergic mechanisms that are inexplicably sold to US consumers as dietary supplements. Poison control center calls regarding phenibut have increased substantially over the past 5 years. Desired euphoriant effects account for the recreational and illicit use of many GABA-modulating agents. However, illicit use can lead to significant toxicities related to abuse, dependence, and subsequent withdrawal syndromes. Significant evaluation of developing agents with GABA properties should be conducted to determine abuse potential before public access ensues.

The Effect of Anti-seizure Medications on the Propagation of Epileptic Activity: A Review

The propagation of epileptiform events is a highly interesting phenomenon from the pathophysiological point of view, as it involves several mechanisms of recruitment of neural networks. Extensive in vivo and in vitro research has been performed, suggesting that multiple networks as well as cellular candidate mechanisms govern this process, including the co-existence of wave propagation, coupled oscillator dynamics, and more. The clinical importance of seizure propagation stems mainly from the fact that the epileptic manifestations cannot be attributed solely to the activity in the seizure focus itself, but rather to the propagation of epileptic activity to other brain structures. Propagation, especially when causing secondary generalizations, poses a risk to patients due to recurrent falls, traumatic injuries, and poor neurological outcome. Anti-seizure medications (ASMs) affect propagation in diverse ways and with different potencies. Importantly, for drug-resistant patients, targeting seizure propagation may improve the quality of life even without a major reduction in simple focal events. Motivated by the extensive impact of this phenomenon, we sought to review the literature regarding the propagation of epileptic activity and specifically the effect of commonly used ASMs on it. Based on this body of knowledge, we propose a novel classification of ASMs into three main categories: major, minor, and intermediate efficacy in reducing the propagation of epileptiform activity.

Protracted Hiccups Induced by Aripiprazole and Regressed after Administration of Gabapentin

Hiccups are sudden, repeated, and involuntary contractions of the diaphragm muscle (myoclonic contraction). It involves a reflex arc that, once activated, causes a strong contraction of the diaphragm immediately followed by the closure of the glottis translating into the classic "hic" sound. Hiccups can be short, persistent, and intractable depending on the duration. The most disabling hiccups often represent the epiphenomenon of a medical condition such as gastrointestinal and cardiovascular disorders; central nervous system (CNS) abnormalities; ear, nose, and throat (ENT) conditions or pneumological problems; metabolic/endocrine disorders; infections; and psychogenic disorders. Some drugs, such as aripiprazole, a second-generation antipsychotic, can induce the onset of variable hiccups. We describe herein the cases of three hospitalized patients who developed insistent hiccups after taking aripiprazole and who positively responded to low doses of gabapentin. It is probable that aripiprazole, prescribed at a low dosage (<7.5 mg/day), would act as a dopamine agonist by stimulating D₂ and D₃ receptors at the "hiccup center" level-located in the brain stem-thus triggering the hiccup. On the other hand, gabapentin led to a complete regression of the hiccup probably by reducing the nerve impulse transmission and modulating the diaphragmatic activity. The present case series suggests the use of low doses of gabapentin as an effective treatment for aripiprazole-induced hiccups. However, our knowledge of the neurotransmitter functioning of the hiccup reflex arc is still limited, and further research is needed to characterize the neurotransmitters involved in hiccups for potential novel therapeutic targets.

Verification and Defined Dosage of Sodium Pentobarbital for a Urodynamic Study in the Possibility of Survival Experiments in Female Rat

Objectives

To evaluate the effects of pentobarbital dosages on lower urinary tract function and to define an appropriate dosage of sodium pentobarbital that would be suitable for urodynamic studies in which recovery from anesthesia and long term survive were needed for subsequent experiment.

Methods

Twenty-four 8-week-old, female, virgin, Sprague-Dawley rats (200-250 g) were used in this study. Rats in study groups received gradient doses of pentobarbital intraperitoneally, and those in the control group received urethane intraperitoneally. External urethral sphincter electromyography (EUS-EMG) was recorded simultaneously during cystometry and leak point pressure tests. The toe-pinch reflex was used to determine the level of anesthesia.

Results

Micturition was normally induced in both the urethane group and 32 mg/kg pentobarbital group. However, in groups of 40 mg/kg or 36 mg/kg pentobarbital, micturition failed to be induced; instead, nonvoiding contractions accompanied by EUS-EMG tonic activity were observed. There were no significant differences in leak point pressure or EUS-EMG amplitude or frequency between the urethane and 32 mg/kg pentobarbital groups.

Conclusions

This study confirmed significant dose-dependent effects of pentobarbital on lower urinary tract function and 32 mg/kg pentobarbital as an appropriate dosage for recovery urodynamic testing, which enable the achievement of expected essential micturition under satisfactory anesthesia in female rats.

Potentiating α2 subunit containing perisomatic GABAA receptors protects against seizures in a mouse model of Dravet syndrome

KEY POINTS

Dravet syndrome mice (Scn1a^+/- ) demonstrate a marked strain dependence for the severity of seizures which is correlated with GABA_A receptor α₂ subunit expression. The α₂ /α₃ subunit selective positive allosteric modulator (PAM) AZD7325 potentiates inhibitory postsynaptic currents (IPSCs) specifically in perisomatic synapses. AZD7325 demonstrates stronger effects on IPSCs in the seizure resistant mouse strain, consistent with higher α₂ subunit expression. AZD7325 demonstrates seizure protective effects in Scn1a^+/- mice without apparent sedative effects in vivo.

ABSTRACT

GABA_A receptor potentiators are commonly used for the treatment of epilepsy, but it is not clear whether targeting distinct GABA_A receptor subtypes will have disproportionate benefits over adverse effects. Here we demonstrate that the α₂ /α₃ selective positive allosteric modulator (PAM) AZD7325 preferentially potentiates hippocampal inhibitory responses at synapses proximal to the soma of CA1 neurons. The effect of AZD7325 on synaptic responses was more prominent in mice on the 129S6/SvEvTac background strain, which have been demonstrated to be seizure resistant in the model of Dravet syndrome (Scn1a^+/- ), and in which the α₂ GABA_A receptor subunits are expressed at higher levels relative to in the seizure prone C57BL/6J background strain. Consistent with this, treatment of Scn1a^+/- mice with AZD7325 elevated the temperature threshold for hyperthermia-induced seizures without apparent sedative effects. Our results in a model system indicate that selectively targeting α₂ is a potential therapeutic option for Dravet syndrome.

Spontaneous activity, singly bound states and the impact of alpha1Phe64 mutation on GABAAR gating in the novel kinetic model based on the single-channel recordings

GABA_A receptor is the primary mediator of inhibition in the adult mammalian brain. Our recent studies revealed that a classic gating scheme for GABA_AR needed to be updated with an intermediate step (flipping) and that the α₁Phe64 mutation at the GABA binding site affects this transition. However, description of flipping at the single-channel level remains incomplete. In particular, its role in singly-bound and spontaneous activity remains unknown. We have performed thus single-channel recordings over wide range of agonist concentration for wild-type α₁β₂γ_2L receptors and α₁Phe64 mutants. For WT receptors we observed relatively frequent brief spontaneous openings which were also present at low [GABA]. However, closed times distributions for spontaneous activity and at low [GABA] were clearly different indicating that a proportion of short-lived openings were due to liganded, most likely singly bound receptors. Increasing [GABA] resulted in prolongation of bursts and increased occurrence of bursts with long openings and short closures. Mutations of α₁Phe64 residue dramatically affected the open and closed time distributions at high and saturating [GABA], especially in the case of cysteine mutants. However, this mutation weakly affected spontaneous or singly bound activity. Model fitting of our single-channel data led us to propose a novel and, to our knowledge, most complete GABA_AR kinetic model in which flipping occurs in singly and doubly bound states. However, spontaneous activity did not reveal involvement of flipping. Moreover, we report that α₁Phe64 mutation affects not only the flipping but also the opening/closing transitions indicating its generalized impact on the receptor gating.

Cognitive and Behavioral Comorbidities: An Unwanted Effect of Antiepileptic Drugs in Children

Epilepsy is one of the most common neurological disorders and, despite optimally chosen and dosed antiepileptic drugs (AEDs), approximately 20%-30% of patients will continue to have seizures. Behavior and cognition are negatively impacted by seizures, but AEDs are also a major contributor to behavioral and cognitive deficits. However, the cognitive and behavioral effect of AEDs in children is insufficiently emphasized in the literature. This review summarizes the cognitive and behavioral effects of AEDs in the pediatric population with the objective of helping pediatricians and pediatric neurologists to select the AEDs with the best profile for their individual patient's needs.

The Emergence of Second-Generation Lethal Injection Protocols: A Brief History and Review

The history of capital punishment in the United States is long and controversial. In many cases, lethal injection has brought medical personnel, ethically and professionally charged with preserving life, into the arena of assisting the state in taking life. U.S. Supreme Court decisions, including Baze v. Rees (2008) and Glossip v. Gross (2015), have evaluated and condoned lethal injection protocols. Despite the judicial validation of some midazolam-containing protocols, controversy exists about the level of unconsciousness provided due to the ceiling effects of the drug. Drug shortages, induced in part by manufacturers under pressure by death penalty opponents and governments opposed to capital punishment, have forced states to sometimes use creative means to obtain medications for use in lethal injection, even proposing to allow inmates to supply their own drugs for use in execution. Others have resorted to using compounding pharmacies and enacting tougher execution secrecy laws to protect the identities of those involved in the process. Professional organizations representing health care team members, including nursing, medicine, and pharmacy, among others, have roundly denounced the medicalization of capital punishment. Legal challenges continue to mount at all levels, leading to an uncertain future for lethal injection.

Diverse effects of stimulus history in waking mouse auditory cortex

Responses to auditory stimuli are often strongly influenced by recent stimulus history. For example, in a paradigm called forward suppression, brief sounds can suppress the perception of, and the neural responses to, a subsequent sound, with the magnitude of this suppression depending on both the spectral and temporal distances between the sounds. As a step towards understanding the mechanisms that generate these adaptive representations in awake animals, we quantitatively characterize responses to two-tone sequences in the auditory cortex of waking mice. We find that cortical responses in a forward suppression paradigm are more diverse in waking mice than previously appreciated, that these responses vary between cells with different firing characteristics and waveform shapes, but that the variability in these responses is not substantially related to cortical depth or columnar location. Moreover, responses to the first tone in the sequence are often not linearly related to the suppression of the second tone response, suggesting that spike-frequency adaptation of cortical cells is not a large contributor to forward suppression or its variability. Instead, we use a simple multilayered model to show that cell-to-cell differences in the balance of intracortical inhibition and excitation will naturally produce such a diversity of forward interactions. We propose that diverse inhibitory connectivity allows the cortex to encode spectro-temporally fluctuating stimuli in multiple parallel ways.NEW & NOTEWORTHY Behavioral and neural responses to auditory stimuli are profoundly influenced by recent sounds, yet how this occurs is not known. Here, the authors show in the auditory cortex of awake mice that the quality of history-dependent effects is diverse and related to cell type, response latency, firing rates, and receptive field bandwidth. In a cortical model, differences in excitatory-inhibitory balance can produce this diversity, providing the cortex with multiple ways of representing temporally complex information.

Matched Behavioral and Neural Adaptations for Low Sound Level Echolocation in a Gleaning Bat, Antrozous pallidus

In active sensing, animals make motor adjustments to match sensory inputs to specialized neural circuitry. Here, we describe an active sensing system for sound level processing. The pallid bat uses downward frequency-modulated (FM) sweeps as echolocation calls for general orientation and obstacle avoidance. The bat's auditory cortex contains a region selective for these FM sweeps (FM sweep-selective region, FMSR). We show that the vast majority of FMSR neurons are sensitive and strongly selective for relatively low levels (30-60 dB SPL). Behavioral testing shows that when a flying bat approaches a target, it reduces output call levels to keep echo levels between ∼30 and 55 dB SPL. Thus, the pallid bat behaviorally matches echo levels to an optimized neural representation of sound levels. FMSR neurons are more selective for sound levels of FM sweeps than tones, suggesting that across-frequency integration enhances level tuning. Level-dependent timing of high-frequency sideband inhibition in the receptive field shapes increased level selectivity for FM sweeps. Together with previous studies, these data indicate that the same receptive field properties shape multiple filters (sweep direction, rate, and level) for FM sweeps, a sound common in multiple vocalizations, including human speech. The matched behavioral and neural adaptations for low-intensity echolocation in the pallid bat will facilitate foraging with reduced probability of acoustic detection by prey.

A Review of Agents for Palliative Sedation/Continuous Deep Sedation: Pharmacology and Practical Applications

Continuous deep sedation at the end of life is a specific form of palliative sedation requiring a care plan that essentially places and maintains the patient in an unresponsive state because their symptoms are refractory to any other interventions. Because this application is uncommon, many providers may lack practical experience in this specialized area and resources they can access are outdated, nonspecific, and/or not comprehensive. The purpose of this review is to provide an evidence- and experience-based reference that specifically addresses those medications and regimens and their practical applications for this very narrow, but vital, aspect of hospice care. Patient goals in a hospital and hospice environments are different, so the manner in which widely used sedatives are dosed and applied can differ greatly as well. Parameters applied in end-of-life care that are based on experience and a thorough understanding of the pharmacology of those medications will differ from those applied in an intensive care unit or other medical environments. By recognizing these different goals and applying well-founded regimens geared specifically for end-of-life sedation, we can address our patients' symptoms in a more timely and efficacious manner.

Analysis of factors associated with hiccups based on the Japanese Adverse Drug Event Report database

Hiccups are occasionally experienced by most individuals. Although hiccups are not life-threatening, they may lead to a decline in quality of life. Previous studies showed that hiccups may occur as an adverse effect of certain medicines during chemotherapy. Furthermore, a male dominance in hiccups has been reported. However, due to the limited number of studies conducted on this phenomenon, debate still surrounds the few factors influencing hiccups. The present study aimed to investigate the influence of medicines and patient characteristics on hiccups using a large-sized adverse drug event report database and, specifically, the Japanese Adverse Drug Event Report (JADER) database. Cases of adverse effects associated with medications were extracted from JADER, and Fisher’s exact test was performed to assess the presence or absence of hiccups for each medication. In a multivariate analysis, we conducted a multiple logistic regression analysis using medication and patient characteristic variables exhibiting significance. We also examined the role of dexamethasone in inducing hiccups during chemotherapy. Medicines associated with hiccups included dexamethasone, levofolinate, fluorouracil, oxaliplatin, carboplatin, and irinotecan. Patient characteristics associated with hiccups included a male gender and greater height. The combination of anti-cancer agent and dexamethasone use was noted in more than 95% of patients in the dexamethasone-use group. Hiccups also occurred in patients in the anti-cancer agent-use group who did not use dexamethasone. Most of the medications that induce hiccups are used in chemotherapy. The results of the present study suggest that it is possible to predict a high risk of hiccups using patient characteristics. We confirmed that dexamethasone was the drug that has the strongest influence on the induction of hiccups. However, the influence of anti-cancer agents on the induction of hiccups cannot be denied. We consider the results of the present study to be helpful for the prevention and treatment of hiccups.

Contrasting actions of a convulsant barbiturate and its anticonvulsant enantiomer on the α1 β3 γ2L GABAA receptor account for their in vivo effects

KEY POINTS

Most barbiturates are anaesthetics but unexpectedly a few are convulsants whose mechanism of action is poorly understood. We synthesized and characterized a novel pair of chiral barbiturates that are capable of photolabelling their binding sites on GABAA receptors. In mice the S-enantiomer is a convulsant, but the R-enantiomer is an anticonvulsant. The convulsant S-enantiomer binds solely at an inhibitory site. It is both an open state inhibitor and a resting state inhibitor. Its action is pH independent, suggesting the pyrimidine ring plays little part in binding. The inhibitory site is not enantioselective because the R-enantiomer inhibits with equal affinity. In contrast, only the anticonvulsant R-enantiomer binds to the enhancing site on open channels, causing them to stay open longer. The enhancing site is enantioselective. The in vivo actions of the convulsant S-enantiomer are accounted for by its interactions with GABAA receptors.

ABSTRACT

Most barbiturates are anaesthetics but a few unexpectedly are convulsants. We recently located the anaesthetic sites on GABAA receptors (GABAA Rs) by photolabelling with an anaesthetic barbiturate. To apply the same strategy to locate the convulsant sites requires the creation and mechanistic characterization of a suitable agent. We synthesized enantiomers of a novel, photoactivable barbiturate, 1-methyl-5-propyly-5-(m-trifluoromethyldiazirinyl) phenyl barbituric acid (mTFD-MPPB). In mice, S-mTFD-MPPB acted as a convulsant, whereas R-mTFD-MPPB acted as an anticonvulsant. Using patch clamp electrophysiology and fast solution exchange on recombinant human α1 β3 γ2L GABAA Rs expressed in HEK cells, we found that S-mTFD-MPPB inhibited GABA-induced currents, whereas R-mTFD-MPPB enhanced them. S-mTFD-MPPB caused inhibition by binding to either of two inhibitory sites on open channels with bimolecular kinetics. It also inhibited closed, resting state receptors at similar concentrations, decreasing the channel opening rate and shifting the GABA concentration-response curve to the right. R-mTFD-MPPB, like most anaesthetics, enhanced receptor gating by rapidly binding to allosteric sites on open channels, initiating a rate-limiting conformation change to stabilized open channel states. These states had slower closing rates, thus shifting the GABA concentration-response curve to the left. Under conditions when most GABAA Rs were open, an inhibitory action of R-mTFD-MPPB was revealed that had a similar IC50 to that of S-mTFD-MPPB. Thus, the inhibitory sites are not enantioselective, and the convulsant action of S-mTFD-MPPB results from its negligible affinity for the enhancing, anaesthetic sites. Interactions with these two classes of barbiturate binding sites on GABAA Rs underlie the enantiomers' different pharmacological activities in mice.

Neurotransmitters in hiccups

Hiccups are the sudden involuntary contractions of the diaphragm and intercostal muscles. They are generally benign and self-limited, however, in some cases they are chronic and debilitating. There are approximately 4000 admissions for hiccups each year in the United States. The hiccup reflex arc is composed of three components: (1) an afferent limb including the phrenic, vagus, and sympathetic nerves, (2) the central processing unit in the midbrain, and (3) the efferent limb carrying motor fibers to the diaphragm and intercostal muscles. Hiccups may be idiopathic, organic, psychogenic, or medication-induced. Data obtained largely from case studies of hiccups either induced by or treated with medications have led to hypotheses on the neurotransmitters involved. The central neurotransmitters implicated in hiccups include GABA, dopamine, and serotonin, while the peripheral neurotransmitters are epinephrine, norepinephrine, acetylcholine, and histamine. Further studies are needed to characterize the nature of neurotransmitters at each anatomical level of the reflex arc to better target hiccups pharmacologically.

The Impact of Anti-Epileptic Drugs on Growth and Bone Metabolism

Epilepsy is a common neurological disorder worldwide and anti-epileptic drugs (AEDs) are always the first choice for treatment. However, more than 50% of patients with epilepsy who take AEDs have reported bone abnormalities. Cytochrome P450 (CYP450) isoenzymes are induced by AEDs, especially the classical AEDs, such as benzodiazepines (BZDs), carbamazepine (CBZ), phenytoin (PT), phenobarbital (PB), and valproic acid (VPA). The induction of CYP450 isoenzymes may cause vitamin D deficiency, hypocalcemia, increased fracture risks, and altered bone turnover, leading to impaired bone mineral density (BMD). Newer AEDs, such as levetiracetam (LEV), oxcarbazepine (OXC), lamotrigine (LTG), topiramate (TPM), gabapentin (GP), and vigabatrin (VB) have broader spectra, and are safer and better tolerated than the classical AEDs. The effects of AEDs on bone health are controversial. This review focuses on the impact of AEDs on growth and bone metabolism and emphasizes the need for caution and timely withdrawal of these medications to avoid serious disabilities.

Association between ABCB1 genetic polymorphism and the effect on epilepsy following phenytoin treatment

The aim of the study was to analyze the effect of ABCB1 genetic polymorphisms on the efficacy of phenytoin (PHT) treatment in epilepsy patients. In total, 200 epilepsy patients who were administered PHT were divided into the responsive and pharmaco-resistance groups depending on the clinical data of PHT treatment in epilepsy patients. The serum concentration of PHT was detected by high-performance liquid chromatography (HPLC). ABCB1 polymorphisms were analyzed by the polymerase chain reaction restriction-fragment length polymorphism method. The C1236T, C3435T and G2677T/A haplotypes were reconstructed for the ABCB1 gene using SHEsis programs. One-way analysis of variance was used for data analysis. In ABCB1 C1236T, the rate of the CC genotype in pharmaco-resistance (17.5%) was higher than that of the responsive group (2.1%), while the rate of the TT genotype in pharmaco-resistance (41.6%) was lower than that of the responsive group (55.4%) (P<0.05). In ABCB1 G2677T/A, the rate of the GG genotype in pharmaco-resistance (29.6%) was higher than that of the responsive group (9.7%), while the rate of the TT genotype in pharmaco-resistance (4.6%) was lower than that of the responsive group (30.4%) (P<0.05). The rate of the TTC haploid in pharmaco-resistance (24.1%) was higher than that of the responsive group (8.8%) (P<0.05). The PHT serum concentration had no statistical significance in the patients with different genotypes. In conclusion, there was no association between ABCB1 genetic polymorphism and PHT serum concentration, although the polymorphisms affected the efficacy of PHT treatment in patients with epilepsy.

Pentobarbital modifies the lipid raft-protein interaction: A first clue about the anesthesia mechanism on NMDA and GABAA receptors

Recent studies have shown that anesthetic agents alter the physical properties of lipid rafts on model membranes. However, if this destabilization occurs in brain membranes, altering the lipid raft-protein interaction, remains unknown. We analyzed the effects produced by pentobarbital (PB) on brain plasma membranes and lipid rafts in vivo. We characterized for the first time the thermotropic behavior of plasma membranes, synaptosomes, and lipid rafts from rat brain. We found that the transition temperature from the ordered gel to disordered liquid phase of lipids is close to physiological temperature. We then studied the effect of PB on protein composition of lipid rafts. Our results show a reduction of the total protein associated to rafts, with a higher reduction of the NMDAR compared to the GABA_A receptor. Both receptors are considered the main targets of PB. In general, our results suggest that lipid rafts could be plausible mediators in anesthetic action.

Hiccup-Like Response in a Dog Anesthetized with Isoflurane

An eight-year-old, female intact Golden Retriever underwent magnetic resonance imaging (MRI) for investigation of urinary and faecal incontinence. Soon after induction of general anesthesia, tracheal intubation, and isoflurane administration, hiccup-like movements were evident. These hiccup-like movements did not respond to hyperventilation and increase of anesthetic. After having ruled out pulmonary disease, the animal was reanesthetized with a similar technique; hiccup-like movements reoccurred and did not stop after discontinuation of isoflurane and commencement of a propofol infusion. Eventually, a nondepolarizing neuromuscular blocking agent was administered to stop the hiccup-like response and allow MRI to be performed. This case report describes the pathophysiology of hiccup-like response and its management in a dog.

Emerging roles of Na⁺/H⁺ exchangers in epilepsy and developmental brain disorders

Epilepsy is a common central nervous system (CNS) disease characterized by recurrent transient neurological events occurring due to abnormally excessive or synchronous neuronal activity in the brain. The CNS is affected by systemic acid-base disorders, and epileptic seizures are sensitive indicators of underlying imbalances in cellular pH regulation. Na(+)/H(+) exchangers (NHEs) are a family of membrane transporter proteins actively involved in regulating intracellular and organellar pH by extruding H(+) in exchange for Na(+) influx. Altering NHE function significantly influences neuronal excitability and plays a role in epilepsy. This review gives an overview of pH regulatory mechanisms in the brain with a special focus on the NHE family and the relationship between epilepsy and dysfunction of NHE isoforms. We first discuss how cells translocate acids and bases across the membrane and establish pH homeostasis as a result of the concerted effort of enzymes and ion transporters. We focus on the specific roles of the NHE family by detailing how the loss of NHE1 in two NHE mutant mice results in enhanced neuronal excitability in these animals. Furthermore, we highlight new findings on the link between mutations of NHE6 and NHE9 and developmental brain disorders including epilepsy, autism, and attention deficit hyperactivity disorder (ADHD). These studies demonstrate the importance of NHE proteins in maintaining H(+) homeostasis and their intricate roles in the regulation of neuronal function. A better understanding of the mechanisms underlying NHE1, 6, and 9 dysfunctions in epilepsy formation may advance the development of new epilepsy treatment strategies.

Effects of GABAA kinetics on cortical population activity: computational studies and physiological confirmations

Voltage-sensitive dye (VSD) imaging produces an unprecedented real-time and high-resolution mesoscopic signal to measure the cortical population activity. We have previously shown that the neuronal compartments contributions to the signal are dynamic and stimulus-dependent (Chemla S, Chavane F. Neuroimage 53: 420-438, 2010). Moreover, the VSD signal can also be strongly affected by the network state, such as in anesthetized vs. awake preparations. Here, we investigated the impact of the network state, through GABAA receptors modulation, on the VSD signal using a computational approach. We therefore systematically measured the effect of the GABAA-mediated inhibitory postsynaptic potentials (IPSPs) decay time constant (τG) on our modeled VSD response to an input stimulus of increasing strength. Our simulations suggest that τG strongly modulates the dynamics of the VSD signal, affecting the amplitude, input response function, and the transient balance of excitation and inhibition. We confirmed these predictions experimentally on awake and anesthetized monkeys, comparing VSD responses to drifting gratings stimuli of various contrasts. Lastly, one in vitro study has suggested that GABAA receptors may also be directly affected by the VSDs themselves (Mennerick S, Chisari M, Shu H, Taylor A, Vasek M, Eisenman L, Zorumski C. J Neurosci 30: 2871-2879, 2010). Our modeling approach suggests that the type of modulation described in this study would actually have a negligible influence on the population response. This study highlights that functional results acquired with different techniques and network states must be compared with caution. Biophysical models are proposed here as an adequate tool to delineate the domain of VSD data interpretation.

The neural substrate for binaural masking level differences in the auditory cortex

The binaural masking level difference (BMLD) is a phenomenon whereby a signal that is identical at each ear (S0), masked by a noise that is identical at each ear (N0), can be made 12-15 dB more detectable by inverting the waveform of either the tone or noise at one ear (Sπ, Nπ). Single-cell responses to BMLD stimuli were measured in the primary auditory cortex of urethane-anesthetized guinea pigs. Firing rate was measured as a function of signal level of a 500 Hz pure tone masked by low-passed white noise. Responses were similar to those reported in the inferior colliculus. At low signal levels, the response was dominated by the masker. At higher signal levels, firing rate either increased or decreased. Detection thresholds for each neuron were determined using signal detection theory. Few neurons yielded measurable detection thresholds for all stimulus conditions, with a wide range in thresholds. However, across the entire population, the lowest thresholds were consistent with human psychophysical BMLDs. As in the inferior colliculus, the shape of the firing-rate versus signal-level functions depended on the neurons' selectivity for interaural time difference. Our results suggest that, in cortex, BMLD signals are detected from increases or decreases in the firing rate, consistent with predictions of cross-correlation models of binaural processing and that the psychophysical detection threshold is based on the lowest neural thresholds across the population.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Molecular mechanisms of antiseizure drug activity at GABAA receptors

The GABAA receptor (GABAAR) is a major target of antiseizure drugs (ASDs). A variety of agents that act at GABAARs s are used to terminate or prevent seizures. Many act at distinct receptor sites determined by the subunit composition of the holoreceptor. For the benzodiazepines, barbiturates, and loreclezole, actions at the GABAAR are the primary or only known mechanism of antiseizure action. For topiramate, felbamate, retigabine, losigamone and stiripentol, GABAAR modulation is one of several possible antiseizure mechanisms. Allopregnanolone, a progesterone metabolite that enhances GABAAR function, led to the development of ganaxolone. Other agents modulate GABAergic "tone" by regulating the synthesis, transport or breakdown of GABA. GABAAR efficacy is also affected by the transmembrane chloride gradient, which changes during development and in chronic epilepsy. This may provide an additional target for "GABAergic" ASDs. GABAAR subunit changes occur both acutely during status epilepticus and in chronic epilepsy, which alter both intrinsic GABAAR function and the response to GABAAR-acting ASDs. Manipulation of subunit expression patterns or novel ASDs targeting the altered receptors may provide a novel approach for seizure prevention.

GABAergic transmission in temporal lobe epilepsy: the role of neurosteroids

Modification of GABAergic inhibition is an intensely investigated hypothesis guiding research into mechanisms underlying temporal lobe epilepsy (TLE). Seizures can be initiated by blocking γ amino butyric acid type A (GABAA receptors, GABARs), which mediate fast synaptic inhibition in the brain, and controlled by drugs that enhance their function. Derivatives of steroid hormones called neurosteroids are natural substances that physiologically enhance GABAR function and suppress seizures. GABAR structure, function, expression, assembly, and pharmacological properties are changed in the hippocampus of epileptic animals. These alterations render GABARs less sensitive to neurosteroid modulation, which may contribute to seizure susceptibility. Plasticity of GABARs could play a role in periodic exacerbation of seizures experienced by women with epilepsy, commonly referred to as catamenial epilepsy.

Anesthetic state modulates excitability but not spectral tuning or neural discrimination in single auditory midbrain neurons

The majority of sensory physiology experiments have used anesthesia to facilitate the recording of neural activity. Current techniques allow researchers to study sensory function in the context of varying behavioral states. To reconcile results across multiple behavioral and anesthetic states, it is important to consider how and to what extent anesthesia plays a role in shaping neural response properties. The role of anesthesia has been the subject of much debate, but the extent to which sensory coding properties are altered by anesthesia has yet to be fully defined. In this study we asked how urethane, an anesthetic commonly used for avian and mammalian sensory physiology, affects the coding of complex communication vocalizations (songs) and simple artificial stimuli in the songbird auditory midbrain. We measured spontaneous and song-driven spike rates, spectrotemporal receptive fields, and neural discriminability from responses to songs in single auditory midbrain neurons. In the same neurons, we recorded responses to pure tone stimuli ranging in frequency and intensity. Finally, we assessed the effect of urethane on population-level representations of birdsong. Results showed that intrinsic neural excitability is significantly depressed by urethane but that spectral tuning, single neuron discriminability, and population representations of song do not differ significantly between unanesthetized and anesthetized animals.

Context dependent benzodiazepine modulation of GABA(A) receptor opening frequency

The anxiolytic, hypnotic, and anti-convulsant properties of benzodiazepines (BDZs) require modulation of distinct GABA_A receptor α-subtypes. BDZ modulation of GABA_A receptors is often described in terms of increased opening frequency, and contrasted with the increased open durations occurring with barbiturate modulation. Several studies spanning single channel, rapid kinetic, and whole cell techniques have suggested that BDZs effect this observed change in frequency through increased affinity for GABA. BDZ-sensitive αβγ isoforms exist at extrasynaptic as well as synaptic locations, where they encounter markedly different concentration and time-course of GABA exposure. Interestingly, this affinity-based mechanism (specifically, decreasing the GABA unbinding rate) is only predicted to increase opening frequency under conditions that allow the unbinding and rebinding cycles typical of prolonged exposure to low GABA concentrations, which are more likely to occur at extrasynaptic GABA_A receptors. In contrast, when rebinding is less likely, such as may occur in certain synaptic conditions, the number, but not the frequency, of channel openings increases in response to BDZ modulation. In conclusion, not only can multiple kinetic mechanisms alter channel opening frequency, but a single mechanism – increased affinity – impacts opening frequency differently under different contexts of GABA_A receptor activation.

GABAergic depolarization during early cortical development and implications for anticonvulsive therapy in neonates

Epileptic seizures rank among the most frequent neurologic symptoms during the neonatal period. Accumulating data from experimental animal studies and clinical trials in humans suggest that neonatal seizures could adversely affect normal brain development and result in long-term neurologic sequelae. Unfortunately, currently used anticonvulsive drugs are often ineffective in the neonatal period. One particularity of the immature neuronal network during neonatal development is that the neurotransmitter γ-aminobutyric acid (GABA) is mainly depolarizing, rather than hyperpolarizing as commonly observed in adults. This might, in part, explain not only the higher seizure propensity of the immature neuronal network, but also the limited anticonvulsive efficacy of GABA-enhancing drugs during early postnatal life. Accordingly, pharmacologic attenuation of GABAergic depolarization has been proposed as a strategy for neonatal seizure control. However, the underlying conjecture of a depolarizing mode of GABA action has been seriously challenged recently. In the present review, we will summarize the state of knowledge regarding GABAergic depolarization in early life and discuss how these data might impact a currently tested anticonvulsive strategy.

From elementary synaptic circuits to information processing in primary auditory cortex

A key for understanding how information is processed in the cortex is to unravel the dauntingly complex cortical neural circuitry. Recent technical innovations, in particular the in vivo whole-cell voltage-clamp recording techniques, make it possible to directly dissect the excitatory and inhibitory inputs underlying an individual cortical neuron's processing function. This method provides an essential complement to conventional approaches, with which the transfer functions of the neural system are derived by correlating neuronal spike outputs to sensory inputs. Here, we intend to introduce a potentially systematic strategy for resolving the structure of functional synaptic circuits. As complex circuits can be built upon elementary modules, the primary focus of this strategy is to identify elementary synaptic circuits and determine how these circuit units contribute to specific processing functions. This review will summarize recent studies on functional synaptic circuits in the primary auditory cortex, comment on existing experimental techniques for in vivo circuitry studies, and provide a perspective on immediate future directions.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Editing of neurotransmitter receptor and ion channel RNAs in the nervous system

The central dogma of molecular biology defines the major route for the transfer of genetic information from genomic DNA to messenger RNA to three-dimensional proteins that affect structure and function. Like alternative splicing, the post-transcriptional conversion of adenosine to inosine (A-to-I) by RNA editing can dramatically expand the diversity of the transcriptome to generate multiple, functionally distinct protein isoforms from a single genomic locus. While RNA editing has been identified in virtually all tissues, such post-transcriptional modifications have been best characterized in RNAs encoding both ligand- and voltage-gated ion channels and neurotransmitter receptors. These RNA processing events have been shown to play an important role in the function of the encoded protein products and, in several cases, have been shown to be critical for the normal development and function of the nervous system.

Partial Disinhibition Is Required for Transition of Stimulus-Induced Sharp Wave–Ripple Complexes Into Recurrent Epileptiform Discharges in Rat Hippocampal Slices

Sharp wave–ripple complexes (SPW-Rs) in the intact rodent hippocampus are characterized by slow field potential transients superimposed by close to 200-Hz ripple oscillations. Similar events have been recorded in hippocampal slices where SPW-Rs occur spontaneously or can be induced by repeated application of high-frequency stimulation, a standard protocol for induction of long-lasting long-term potentiation. Such stimulation is reminiscent of protocols used to induce kindling epilepsy and ripple oscillations may be predictive of the epileptogenic zone in temporal lobe epilepsy. In the present study, we investigated the relation between recurrent epileptiform discharges (REDs) and SPW-Rs by studying effects of partial removal of inhibition. In particular, we compared the effects of nicotine, low-dose bicuculline methiodide (BMI), and elevated extracellular potassium concentration ([K+]o) on induced SPW-Rs. We show that nicotine dose-dependently transformed SPW-Rs into REDs. This transition was associated with reduced inhibitory conductance in CA3 pyramidal cells. Similar results were obtained from slices where the GABAergic conductance was reduced by application of low concentrations of BMI (1–2 μM). In contrast, sharp waves were diminished by phenobarbital. Elevating [K+]o from 3 to 8.5 mM did not transform SPW-Rs into REDs but significantly increased their incidence and amplitude. Under these conditions, the equilibrium potential for inhibition was shifted in depolarizing direction, whereas inhibitory conductance was significantly increased. Interestingly, the propensity of elevated [K+]o to induce seizure-like events was reduced in slices where SPW-Rs had been induced. In conclusion, recruitment of inhibitory cells during SPW-Rs may serve as a mechanism by which hyperexcitation and eventually seizure generation might be prevented.

Allosteric modulators induce distinct movements at the GABA-binding site interface of the GABA-A receptor

Benzodiazepines (BZDs) and barbiturates exert their CNS actions by binding to GABA-A receptors (GABARs). The structural mechanisms by which these drugs allosterically modulate GABAR function, to either enhance or inhibit GABA-gated current, are poorly understood. Here, we used the substituted cysteine accessibility method to examine and compare structural movements in the GABA-binding site interface triggered by a BZD positive (flurazepam), zero (flumazenil) and negative (3-carbomethoxy-4-ethyl-6,7-dimethoxy-β-carboline, DMCM) modulator as well as the barbiturate pentobarbital. Ten residues located throughout the GABA-binding site interface were individually mutated to cysteine. Wild-type and mutant α(1)β(2)γ(2) GABARs were expressed in Xenopus laevis oocytes and functionally characterized using two-electrode voltage clamp. We measured and compared the rates of modification of the introduced cysteines by sulfhydryl-reactive methanethiosulfonate (MTS) reagents in the absence and presence of BZD-site ligands and pentobarbital. Flurazepam and DMCM each accelerated the rate of reaction at α(1)R131C and slowed the rate of reaction at α(1)E122C, whereas flumazenil had no effect indicating that simple occupation of the BZD binding site is not sufficient to cause movements near these positions. Therefore, BZD-induced movements at these residues are likely associated with the ability of the BZD to modulate GABAR function (BZD efficacy). Low, modulating concentrations of pentobarbital accelerated the rate of reaction at α(1)S68C and β(2)P206C, slowed the rate of reaction at α(1)E122C and had no effect at α(1)R131C. These findings indicate that pentobarbital and BZDs induce different movements in the receptor, providing evidence that the structural mechanisms underlying their allosteric modulation of GABAR function are distinct.

Benzodiazepine modulation of GABA(A) receptor opening frequency depends on activation context: a patch clamp and simulation study

Benzodiazepines (BDZs) are GABA(A) receptor modulators with anxiolytic, hypnotic, and anticonvulsant properties. BDZs are understood to potentiate GABA(A) receptor function by increasing channel opening frequency, in contrast to barbiturates, which increase channel open duration. However, the in vitro evidence demonstrating increased opening frequency involved prolonged exposure to sub-saturating GABA concentrations, conditions most similar to those found in extrasynaptic areas. In contrast, synaptic GABA(A) receptors are transiently activated by high GABA concentrations. To determine if BDZ modulation of single-channel opening frequency would be different for BDZ-sensitive receptors activated under synaptic versus extrasynaptic conditions, a combination of patch clamp recording and kinetic modeling was used. Consistent with the original experimental findings, BDZs were found to increase receptor affinity for GABA by decreasing the unbinding rate. While this mechanism was predicted to increase opening frequency under extrasynaptic conditions, simulations predicted that the same mechanism under synaptic conditions would increase the number, but not the frequency, of single-channel openings. Thus, a single mechanism (slower GABA unbinding) can produce differential changes in opening frequency under synaptic versus extrasynaptic conditions. The functional impact of BDZs on GABA(A) receptors therefore depends upon the physiological context of receptor activation.

Gamma-aminobutyric acid (GABA) and pentobarbital induce different conformational rearrangements in the GABA A receptor alpha1 and beta2 pre-M1 regions

Gamma-aminobutyric acid (GABA) binding to GABA(A) receptors (GABA(A)Rs) triggers conformational movements in the alpha(1) and beta(2) pre-M1 regions that are associated with channel gating. At high concentrations, the barbiturate pentobarbital opens GABA(A)R channels with similar conductances as GABA, suggesting that their open state structures are alike. Little, however, is known about the structural rearrangements induced by barbiturates. Here, we examined whether pentobarbital activation triggers movements in the GABA(A)R pre-M1 regions. Alpha(1)beta(2) GABA(A)Rs containing cysteine substitutions in the pre-M1 alpha(1) (K219C, K221C) and beta(2) (K213C, K215C) subunits were expressed in Xenopus oocytes and analyzed using two-electrode voltage clamp. The cysteine substitutions had little to no effect on GABA and pentobarbital EC(50) values. Tethering chemically diverse thiol-reactive methanethiosulfonate reagents onto alpha(1)K219C and alpha(1)K221C affected GABA- and pentobarbital-activated currents differently, suggesting that the pre-M1 structural elements important for GABA and pentobarbital current activation are distinct. Moreover, pentobarbital altered the rates of cysteine modification by methanethiosulfonate reagents differently than GABA. For alpha(1)K221Cbeta(2) receptors, pentobarbital decreased the rate of cysteine modification whereas GABA had no effect. For alpha(1)beta(2)K215C receptors, pentobarbital had no effect whereas GABA increased the modification rate. The competitive GABA antagonist SR-95531 and a low, non-activating concentration of pentobarbital did not alter their modification rates, suggesting that the GABA- and pentobarbital-mediated changes in rates reflect gating movements. Overall, the data indicate that the pre-M1 region is involved in both GABA- and pentobarbital-mediated gating transitions. Pentobarbital, however, triggers different movements in this region than GABA, suggesting their activation mechanisms differ.

Effect of common anesthetics on dendritic properties in layer 5 neocortical pyramidal neurons

Understanding the impact of active dendritic properties on network activity in vivo has so far been restricted to studies in anesthetized animals. However, to date no study has been made to determine the direct effect of the anesthetics themselves on dendritic properties. Here, we investigated the effects of three types of anesthetics commonly used for animal experiments (urethane, pentobarbital and ketamine/xylazine). We investigated the generation of calcium spikes, the propagation of action potentials (APs) along the apical dendrite and the somatic firing properties in the presence of anesthetics in vitro using dual somatodendritic whole cell recordings. Calcium spikes were evoked with dendritic current injection and high-frequency trains of APs at the soma. Surprisingly, we found that the direct actions of anesthetics on calcium spikes were very different. Two anesthetics (urethane and pentobarbital) suppressed dendritic calcium spikes in vitro, whereas a mixture of ketamine and xylazine enhanced them. Propagation of spikes along the dendrite was not significantly affected by any of the anesthetics but there were various changes in somatic firing properties that were highly dependent on the anesthetic. Last, we examined the effects of anesthetics on calcium spike initiation and duration in vivo using high-frequency trains of APs generated at the cell body. We found the same anesthetic-dependent direct effects in addition to an overall reduction in dendritic excitability in anesthetized rats with all three anesthetics compared with the slice preparation.

At clinically relevant concentrations the anaesthetic/amnesic thiopental but not the anticonvulsant phenobarbital interferes with hippocampal sharp wave-ripple complexes

Background

Many sedative agents, including anesthetics, produce explicit memory impairment by largely unknown mechanisms. Sharp-wave ripple (SPW-R) complexes are network activity thought to represent the neuronal substrate for information transfer from the hippocampal to neocortical circuits, contributing to the explicit memory consolidation. In this study we examined and compared the actions of two barbiturates with distinct amnesic actions, the general anesthetic thiopental and the anticonvulsant phenobarbital, on in vitro SPW-R activity.

Results

Using an in vitro model of SPW-R activity we found that thiopental (50–200 μM) significantly and concentration-dependently reduced the incidence of SPW-R events (it increased the inter-event period by 70–430 %). At the concentration of 25 μM, which clinically produces mild sedation and explicit memory impairment, thiopental significantly reduced the quantity of ripple oscillation (it reduced the number of ripples and the duration of ripple episodes by 20 ± 5%, n = 12, P < 0.01), and suppressed the rhythmicity of SPWs by 43 ± 15% (n = 6, P < 0.05). The drug disrupted the synchrony of SPWs within the CA1 region at 50 μM (by 19 ± 12%; n = 5, P < 0.05). Similar effects of thiopental were observed at higher concentrations. Thiopental did not affect the frequency of ripple oscillation at any of the concentrations tested (10–200 μM). Furthermore, the drug significantly prolonged single SPWs at concentrations ≥50 μM (it increased the half-width and the duration of SPWs by 35–90 %). Thiopental did not affect evoked excitatory synaptic potentials and its results on SPW-R complexes were also observed under blockade of NMDA receptors. Phenobarbital significantly accelerated SPWs at 50 and 100 μM whereas it reduced their rate at 200 and 400 μM. Furthermore, it significantly prolonged SPWs, reduced their synchrony and reduced the quantity of ripples only at the clinically very high concentration of 400 μM, reported to affect memory.

Conclusion

We hypothesize that thiopental, by interfering with SPW-R activity, through enhancement of the GABA_A receptor-mediated transmission, affects memory processes which involve hippocampal circuit activation. The quantity but not the frequency of ripple oscillation was affected by the drug.

Novel antagonists of the thioesterase domain of human fatty acid synthase

Fatty acid synthase (FAS) is up-regulated in a wide range of cancers and has been recently identified as a potential therapeutic target. Indeed, previous research has shown that inhibition of FAS with active site-modifying agents can block tumor cell proliferation, elicit tumor cell death, and prevent tumor growth in animal models. Here, we use a high-throughput fluorogenic screen and identify a novel pharmacophore, 5-(furan-2-ylmethylene) pyrimidine-2,4,6-trione, which inhibits the thioesterase domain of FAS. The novel antagonists are competitive inhibitors of the thioesterase domain, inhibit de novo fatty acid synthesis, and elicit FAS-dependent tumor cell death. This set of novel FAS antagonists provides an important pharmacologic lead for further development of anticancer therapeutics.

Transformation of temporal properties between auditory midbrain and cortex in the awake Mongolian gerbil

The neural representation of meaningful stimulus features is thought to rely on precise discharge characteristics of the auditory cortex. Precisely timed onset spikes putatively carry the majority of stimulus-related information in auditory cortical neurons but make a small contribution to stimulus representation in the auditory midbrain. Because these conclusions derive primarily from anesthetized preparations, we reexamined temporal coding properties of single neurons in the awake gerbil inferior colliculus (IC) and compared them with primary auditory cortex (AI). Surprisingly, AI neurons displayed a reduction of temporal precision compared with those in the IC. Furthermore, this hierarchical transition from high to low temporal fidelity was observed for both static and dynamic stimuli. Because most of the data that support temporal precision were obtained under anesthesia, we also reexamined response properties of IC and AI neurons under these conditions. Our results show that anesthesia has profound effects on the trial-to-trial variability and reliability of discharge and significantly improves the temporal precision of AI neurons to both tones and amplitude-modulated stimuli. In contrast, IC temporal properties are only mildly affected by anesthesia. These results underscore the pitfalls of using anesthetized preparations to study temporal coding. Our findings in awake animals reveal that AI neurons combine faster adaptation kinetics and a longer temporal window than evident in IC to represent ongoing acoustic stimuli.

The role of mood stabilisers in the treatment of the depressive facet of bipolar disorders

It was previously shown that available mood stabilisers are used to treat bipolar depression. As part of the natural course of illness, patients with bipolar disorder often suffer from episodes of depression more frequently and for longer durations than mania. A major challenge in the treatment of bipolar depression is the tendency for antidepressant medications, particularly tricyclic antidepressants, to precipitate episodes of mania, or to increase cycle frequency or symptom intensity. Thus, exploring the utility of mood stabilisers as monotherapy for bipolar depression is important. The aim of this review it to collate data involving the effects of some mood stabilisers like lithium, carbamazepine, valproate and lamotrigine in depressive aspects of bipolar disorder, but as well using an animal model of depression, to understand their mechanism of action.

Mechanisms of action of antiepileptic drugs

The management of seizures in the patient with epilepsy relies heavily on antiepileptic drug (AED) therapy. Fortunately, for a large percentage of patients, AEDs provide excellent seizure control at doses that do not adversely affect normal function. At the molecular level, the majority of AEDs are thought to modify excitatory and inhibitory neurotransmission through effects on voltage-gated ion channels (e.g., sodium and calcium) and gamma-aminobutyric acid (GABA)(A) receptors, respectively. In addition to these effects, two of the "second-generation" AEDs have been found to limit glutamate-mediated excitatory neurotransmission (i.e., felbamate and topiramate). Not surprisingly, those AEDs with broad spectrum clinical activity are often found to exert an action at more than one molecular target. Emerging evidence suggests that receptor and voltage-gated subunits are modified by chronic seizures. Thus, attempts to understand the relationship between target and effect continue to provide important information about the neuropathology of the epileptic network and to facilitate the development of novel therapies for the treatment of refractory epilepsy.