GABA agonists and antagonists

Monomeric Amyloid Peptide-induced Toxicity in Human Oligodendrocyte Cell Line and Mouse Brain Primary Mixed-glial Cell Cultures: Evidence for a Neuroprotective Effect of Neurosteroid 3α-O-allyl-allopregnanolone

Amyloid-peptide (Aβ) monomeric forms (ABM) occurring in presymptomatic Alzheimer's disease (AD) brain are thought to be devoid of neurotoxicity while the transition/aggregation of ABM into oligomers is determinant for Aβ-induced toxicity since Aβ is predominantly monomeric up to 3 µM and aggregates over this concentration. However, recent imaging and/or histopathological investigations revealed alterations of myelin in prodromal AD brain in absence of aggregated Aβ oligomers, suggesting that ABM may induce toxicity in myelin-producing cells in early AD-stages. To check this hypothesis, here we studied ABM effects on the viability of the Human oligodendrocyte cell line (HOG), a reliable oligodendrocyte model producing myelin proteins. Furthermore, to mimic closely interactions between oligodendrocytes and other glial cells regulating myelination, we investigated also ABM effects on mouse brain primary mixed-glial cell cultures. Various methods were combined to show that ABM concentrations (600 nM-1 µM), extremely lower than 3 µM, significantly decreased HOG cell and mouse brain primary mixed-glial cell survival. Interestingly, flow-cytometry studies using specific cell-type markers demonstrated that oligodendrocytes represent the most vulnerable glial cell population affected by ABM toxicity. Our work also shows that the neurosteroid 3α-O-allyl-allopregnanolone BR351 (250 and 500 nM) efficiently prevented ABM-induced HOG and brain primary glial cell toxicity. Bicuculline (50-100 nM), the GABA-A-receptor antagonist, was unable to block/reduce BR351 effect against ABM-induced HOG and primary glial cell toxicity, suggesting that BR351-evoked neuroprotection of these cells may not depend on GABA-A-receptor allosterically modulated by neurosteroids. Altogether, our results suggest that further exploration of BR351 therapeutic potential may offer interesting perspectives to develop effective neuroprotective strategies.

Social threat alters the behavioral structure of social motivation and reshapes functional brain connectivity

Traumatic social experiences redefine socially motivated behaviors to enhance safety and survival. Although many brain regions have been implicated in signaling a social threat, the mechanisms by which global neural networks regulate such motivated behaviors remain unclear. To address this issue, we first combined traditional and modern behavioral tracking techniques in mice to assess both approach and avoidance, as well as sub-second behavioral changes, during a social threat learning task. We were able to identify previously undescribed body and tail movements during social threat learning and recognition that demonstrate unique alterations into the behavioral structure of social motivation. We then utilized inter-regional correlation analysis of brain activity after a mouse recognizes a social threat to explore functional communication amongst brain regions implicated in social motivation. Broad brain activity changes were observed within the nucleus accumbens, the paraventricular thalamus, the ventromedial hypothalamus, and the nucleus of reuniens. Inter-regional correlation analysis revealed a reshaping of the functional connectivity across the brain when mice recognize a social threat. Altogether, these findings suggest that reshaping of functional brain connectivity may be necessary to alter the behavioral structure of social motivation when a social threat is encountered.

GABA production induced by imipridones is a targetable and imageable metabolic alteration in diffuse midline gliomas

Diffuse midline gliomas (DMGs) are lethal primary brain tumors in children. The imipridones ONC201 and ONC206 induce mitochondrial dysfunction and have emerged as promising therapies for DMG patients. However, efficacy as monotherapy is limited, identifying a need for strategies that enhance response. Another hurdle is the lack of biomarkers that report on drug-target engagement at an early timepoint after treatment onset. Here, using 1 H-magnetic resonance spectroscopy, which is a non-invasive method of quantifying metabolite pool sizes, we show that accumulation of ψ-aminobutyric acid (GABA) is an early metabolic biomarker that can be detected within a week of ONC206 treatment, when anatomical alterations are absent, in mice bearing orthotopic xenografts. Mechanistically, imipridones activate the mitochondrial protease ClpP and upregulate the stress-responsive transcription factor ATF4. ATF4, in turn, upregulates glutamate decarboxylase, which synthesizes GABA, and downregulates ABAT , which degrades GABA, leading to GABA accumulation in DMG cells and tumors. Functionally, GABA secreted by imipridone-treated cells acts in an autocrine manner via the GABAB receptor to induce expression of superoxide dismutase (SOD1), which mitigates imipridone-induced oxidative stress and, thereby, curbs apoptosis. Importantly, blocking autocrine GABA signaling using the clinical stage GABAB receptor antagonist SGS-742 exacerbates oxidative stress and synergistically induces apoptosis in combination with imipridones in DMG cells and orthotopic tumor xenografts. Collectively, we identify GABA as a unique metabolic adaptation to imipridones that can be leveraged for non-invasive assessment of drug-target engagement and therapy. Clinical translation of our studies has the potential to enable precision metabolic therapy and imaging for DMG patients.

One Sentence Summary

Imipridones induce GABA accumulation in diffuse midline gliomas, an effect that can be leveraged for therapy and non-invasive imaging.

The role of ligand-gated chloride channels in behavioural alterations at elevated CO2 in a cephalopod

Projected future carbon dioxide (CO2) levels in the ocean can alter marine animal behaviours. Disrupted functioning of γ-aminobutyric acid type A (GABAA) receptors (ligand-gated chloride channels) is suggested to underlie CO2-induced behavioural changes in fish. However, the mechanisms underlying behavioural changes in marine invertebrates are poorly understood. We pharmacologically tested the role of GABA-, glutamate-, acetylcholine- and dopamine-gated chloride channels in CO2-induced behavioural changes in a cephalopod, the two-toned pygmy squid (Idiosepius pygmaeus). We exposed squid to ambient (∼450 µatm) or elevated (∼1000 µatm) CO2 for 7 days. Squid were treated with sham, the GABAA receptor antagonist gabazine or the non-specific GABAA receptor antagonist picrotoxin, before measurement of conspecific-directed behaviours and activity levels upon mirror exposure. Elevated CO2 increased conspecific-directed attraction and aggression, as well as activity levels. For some CO2-affected behaviours, both gabazine and picrotoxin had a different effect at elevated compared with ambient CO2, providing robust support for the GABA hypothesis within cephalopods. In another behavioural trait, picrotoxin but not gabazine had a different effect in elevated compared with ambient CO2, providing the first pharmacological evidence, in fish and marine invertebrates, for altered functioning of ligand-gated chloride channels, other than the GABAAR, underlying CO2-induced behavioural changes. For some other behaviours, both gabazine and picrotoxin had a similar effect in elevated and ambient CO2, suggesting altered function of ligand-gated chloride channels was not responsible for these CO2-induced changes. Multiple mechanisms may be involved, which could explain the variability in the CO2 and drug treatment effects across behaviours.

Influence of glutamate and GABA transport on brain excitatory/inhibitory balance

An optimally functional brain requires both excitatory and inhibitory inputs that are regulated and balanced. A perturbation in the excitatory/inhibitory balance-as is the case in some neurological disorders/diseases (e.g. traumatic brain injury Alzheimer's disease, stroke, epilepsy and substance abuse) and disorders of development (e.g. schizophrenia, Rhett syndrome and autism spectrum disorder)-leads to dysfunctional signaling, which can result in impaired cognitive and motor function, if not frank neuronal injury. At the cellular level, transmission of glutamate and GABA, the principle excitatory and inhibitory neurotransmitters in the central nervous system control excitatory/inhibitory balance. Herein, we review the synthesis, release, and signaling of GABA and glutamate followed by a focused discussion on the importance of their transport systems to the maintenance of excitatory/inhibitory balance.

Fast Compensatory Functional Network Changes Caused by Reversible Inactivation of Monkey Parietal Cortex

The brain has a remarkable capacity to recover after lesions. However, little is known about compensatory neural adaptations at the systems level. We addressed this question by investigating behavioral and (correlated) functional changes throughout the cortex that are induced by focal, reversible inactivations. Specifically, monkeys performed a demanding covert spatial attention task while the lateral intraparietal area (LIP) was inactivated with muscimol and whole-brain fMRI activity was recorded. The inactivation caused LIP-specific decreases in task-related fMRI activity. In addition, these local effects triggered large-scale network changes. Unlike most studies in which animals were mainly passive relative to the stimuli, we observed heterogeneous effects with more profound muscimol-induced increases of task-related fMRI activity in areas connected to LIP, especially FEF. Furthermore, in areas such as FEF and V4, muscimol-induced changes in fMRI activity correlated with changes in behavioral performance. Notably, the activity changes in remote areas did not correlate with the decreased activity at the site of the inactivation, suggesting that such changes arise via neuronal mechanisms lying in the intact portion of the functional task network, with FEF a likely key player. The excitation-inhibition dynamics unmasking existing excitatory connections across the functional network might initiate these rapid adaptive changes.

Allosteric GABAA Receptor Modulators-A Review on the Most Recent Heterocyclic Chemotypes and Their Synthetic Accessibility

GABAA receptor modulators are structurally almost as diverse as their target protein. A plethora of heterocyclic scaffolds has been described as modulating this extremely important receptor family. Some made it into clinical trials and, even on the market, some were dismissed. This review focuses on the synthetic accessibility and potential for library synthesis of GABAA receptor modulators containing at least one heterocyclic scaffold, which were disclosed within the last 10 years.

Reversible Inactivation of Different Millimeter-Scale Regions of Primate IT Results in Different Patterns of Core Object Recognition Deficits

Extensive research suggests that the inferior temporal (IT) population supports visual object recognition behavior. However, causal evidence for this hypothesis has been equivocal, particularly beyond the specific case of face-selective subregions of IT. Here, we directly tested this hypothesis by pharmacologically inactivating individual, millimeter-scale subregions of IT while monkeys performed several core object recognition subtasks, interleaved trial-by trial. First, we observed that IT inactivation resulted in reliable contralateral-biased subtask-selective behavioral deficits. Moreover, inactivating different IT subregions resulted in different patterns of subtask deficits, predicted by each subregion's neuronal object discriminability. Finally, the similarity between different inactivation effects was tightly related to the anatomical distance between corresponding inactivation sites. Taken together, these results provide direct evidence that the IT cortex causally supports general core object recognition and that the underlying IT coding dimensions are topographically organized.

Serotonin2B receptors in the rat dorsal raphe nucleus exert a GABA-mediated tonic inhibitory control on serotonin neurons

The central serotonin_2B receptor (5-HT_2BR) is a well-established modulator of dopamine (DA) neuron activity in the rodent brain. Recent studies in rats have shown that the effect of 5-HT_2BR antagonists on accumbal and medial prefrontal cortex (mPFC) DA outflow results from a primary action in the dorsal raphe nucleus (DRN), where they activate 5-HT neurons innervating the mPFC. Although the mechanisms underlying this interaction remain largely unknown, data in the literature suggest the involvement of DRN GABAergic interneurons in the control of 5-HT activity. The present study examined this hypothesis using in vivo (intracerebral microdialysis) and in vitro (immunohistochemistry coupled to reverse transcription-polymerase chain reaction) experimental approaches in rats. Intraperitoneal (0.16 mg/kg) or intra-DRN (1 μM) administration of the selective 5-HT_2BR antagonist RS 127445 increased 5-HT outflow in both the DRN and the mPFC, these effects being prevented by the intra-DRN perfusion of the GABA_A antagonist bicuculline (100 μM), as well as by the subcutaneous (0.16 mg/kg) or the intra-DRN (0.1 μM) administration of the selective 5-HT_1AR antagonist WAY 100635. The increase in DRN 5-HT outflow induced by the intra-DRN administration of the selective 5-HT reuptake inhibitor citalopram (0.1 μM) was potentiated by the intra-DRN administration (0.5 μM) of RS 127445 only in the absence of bicuculline perfusion. Finally, in vitro experiments revealed the presence of the 5-HT_2BR mRNA on DRN GABAergic interneurons. Altogether, these results show that, in the rat DRN, 5-HT_2BRs are located on GABAergic interneurons, and exert a tonic inhibitory control on 5-HT neurons innervating the mPFC.

Acid-base physiology, neurobiology and behaviour in relation to CO2-induced ocean acidification

Experimental exposure to ocean and freshwater acidification affects the behaviour of multiple aquatic organisms in laboratory tests. One proposed cause involves an imbalance in plasma chloride and bicarbonate ion concentrations as a result of acid-base regulation, causing the reversal of ionic fluxes through GABA_A receptors, which leads to altered neuronal function. This model is exclusively based on differential effects of the GABA_A receptor antagonist gabazine on control animals and those exposed to elevated CO₂ However, direct measurements of actual chloride and bicarbonate concentrations in neurons and their extracellular fluids and of GABA_A receptor properties in aquatic organisms are largely lacking. Similarly, very little is known about potential compensatory mechanisms, and about alternative mechanisms that might lead to ocean acidification-induced behavioural changes. This article reviews the current knowledge on acid-base physiology, neurobiology, pharmacology and behaviour in relation to marine CO₂-induced acidification, and identifies important topics for future research that will help us to understand the potential effects of predicted levels of aquatic acidification on organisms.

Role of the vestibular nuclear complex in facilitating the jaw-opening reflex following stimulation of the red nucleus

According to our previous studies, stimulation of the red nucleus (RN) facilitates the low-threshold afferent-evoked jaw-opening reflex (L-JOR). It has been reported that the RN projects to the superior (SVN), lateral (LVN) and inferior vestibular (IVN) nuclei. The SVN and the LVN have reciprocal intrinsic connections with the medial vestibular nucleus (MVN). Our previous study demonstrated that stimulation of the vestibular nuclear complex (VN) modulates the L-JOR. These facts suggest that RN-induced facilitation of the L-JOR is mediated via the VN. In the present work we investigated whether electrically induced lesions of the VN, or microinjection of muscimol into the VN, affects RN-induced facilitation of the L-JOR. The L-JOR was evoked by electrical stimulation of the inferior alveolar nerve. The stimulus intensity was 1.2 times the evocation threshold. Lesions of the MVN or the LVN or the SVN, and the muscimol injection into the MVN or the LVN or the SVN, reduced the RN-induced facilitation of the L-JOR. Conversely, lesions of the IVN, and the muscimol injection into the IVN, increased the RN-induced facilitation of the L-JOR. These results suggest that the RN-induced facilitation of the L-JOR is mediated by a relay in the VN.

Bud extracts from Tilia tomentosa Moench inhibit hippocampal neuronal firing through GABAA and benzodiazepine receptors activation

ETHNOPHARMACOLOGICAL RELEVANCE

Tilia tomentosa Moench bud extracts (TTBEs) is used in traditional medicine for centuries as sedative compound. Different plants belonging to the Tilia genus have shown their efficacy in the treatment of anxiety but still little is known about the mechanism of action of their bud extracts.

AIM OF THE STUDY

To evaluate the action of TTBEs as anxiolytic and sedative compound on in vitro hippocampal neurons.

MATERIAL AND METHODS

The anxiolytic effect of TTBEs was assayed by testing the effects of these compounds on GABAA receptor-activated chloride current of hippocampal neurons by means of the patch-clamp technique and microelectrode-arrays (MEAs).

RESULTS

TTBEs acutely administered on mouse hippocampal neurons, activated a chloride current comparable to that measured in the presence of GABA (100 µM). Bicuculline (100 µM) and picrotoxin (100 µM) blocked about 90% of this current, while the remaining 10% was blocked by adding the benzodiazepine (BDZ) antagonist flumazenil (30 µM). Flumazenil alone blocked nearly 60% of the TTBEs activated current, suggesting that TTBEs binds to both GABAA and BDZ receptor sites. Application of high-doses of TTBEs on spontaneous active hippocampal neurons grown for 3 weeks on MEAs blocked the synchronous activity of these neurons. The effects were mimicked by GABA and prevented by picrotoxin (100µM) and flumazenil (30 µM). At minimal doses, TTBEs reduced the frequency of synchronized bursts and increased the cross-correlation index of synchronized neuronal firing.

CONCLUSIONS

Our data suggest that TTBEs mimics GABA and BDZ agonists by targeting hippocampal GABAergic synapses and inhibiting network excitability by increasing the strength of inhibitory synaptic outputs. Our results contribute toward the validation of TTBEs as effective sedative and anxiolytic compound.

Advantages of an antagonist: bicuculline and other GABA antagonists

The convulsant alkaloid bicuculline continues to be investigated more than 40 years after the first publication of its action as an antagonist of receptors for the inhibitory neurotransmitter GABA. This historical perspective highlights key aspects of the discovery of bicuculline as a GABA antagonist and the sustained interest in this and other GABA antagonists. The exciting advances in the molecular biology, pharmacology and physiology of GABA receptors provide a continuing stimulus for the discovery of new antagonists with increasing selectivity for the myriad of GABA receptor subclasses. Interesting GABA antagonists not structurally related to bicuculline include gabazine, salicylidene salicylhydrazide, RU5135 and 4-(3-biphenyl-5-(4-piperidyl)-3-isoxazole. Bicuculline became the benchmark antagonist for what became known as GABAA receptors, but not all ionotropic GABA receptors are susceptible to bicuculline. In addition, not all GABAA receptor antagonists are convulsants. Thus there are still surprises in store as the study of GABA receptors evolves.

The development of acamprosate as a treatment against alcohol relapse

INTRODUCTION

Globally, alcohol abuse and dependence are significant contributors to chronic disease and injury and are responsible for nearly 4% of all deaths annually. Acamprosate (Campral), one of only three pharmacological treatments approved for the treatment of alcohol dependence, has shown mixed efficacy in clinical trials in maintaining abstinence of detoxified alcoholics since studies began in the 1980s. Yielding inconsistent results, these studies have prompted skepticism.

AREAS COVERED

Herein, the authors review the preclinical studies which have assessed the efficacy of acamprosate in various animal models of alcohol dependence and discuss the disparate findings from the major clinical trials. Moreover, the authors discuss the major limitations of these preclinical and clinical studies and offer explanations for the often-contradictory findings. The article also looks at the importance of the calcium moiety that accompanies the salt form of acamprosate and its relevance to its activity.

EXPERT OPINION

The recent discovery that large doses of calcium largely duplicate the effects of acamprosate in animal models has introduced a serious challenge to the widely held functional association between this drug and the glutamate neurotransmission system. Future research on acamprosate or newer pharmacotherapeutics should consider assessing plasma and/or brain levels of calcium as a correlate or mediating factor in anti-relapse efficacy. Further, preclinical research on acamprosate has thus far lacked animal models of chemical dependence on alcohol, and the testing of rodents with histories of alcohol intoxication and withdrawal is suggested.

Role of the red nucleus in suppressing the jaw-opening reflex following stimulation of the raphe magnus nucleus

In a previous study, we found that electrical and chemical stimulation of the red nucleus (RN) suppressed the high-threshold afferent-evoked jaw-opening reflex (JOR). It has been reported that the RN receives bilaterally projection fibers from the raphe magnus nucleus (RMg), and that stimulation of the RMg inhibits the tooth pulp-evoked nociceptive JOR. These facts imply that RMg-induced inhibition of the JOR could be mediated via the RN. The present study first examines whether stimulation of the RMg suppresses the high-threshold afferent-evoked JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was 4.0 (high-threshold) times the threshold. Conditioning electrical stimulation of the RMg significantly suppressed the JOR. A further study then examined whether electrically induced lesions of the RN or microinjection of muscimol into the RN affects RMg-induced suppression of the JOR. Electrically induced lesions of the bilateral RN and microinjection of muscimol into the bilateral RN both reduced the RMg-induced suppression of the JOR. These results suggest that RMg-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the RN.

CO2-induced ocean acidification increases anxiety in rockfish via alteration of GABAA receptor functioning

The average surface pH of the ocean is dropping at a rapid rate due to the dissolution of anthropogenic CO2, raising concerns for marine life. Additionally, some coastal areas periodically experience upwelling of CO2-enriched water with reduced pH. Previous research has demonstrated ocean acidification (OA)-induced changes in behavioural and sensory systems including olfaction, which is due to altered function of neural gamma-aminobutyric acid type A (GABAA) receptors. Here, we used a camera-based tracking software system to examine whether OA-dependent changes in GABAA receptors affect anxiety in juvenile Californian rockfish (Sebastes diploproa). Anxiety was estimated using behavioural tests that measure light/dark preference (scototaxis) and proximity to an object. After one week in OA conditions projected for the next century in the California shore (1125 ± 100 µatm, pH 7.75), anxiety was significantly increased relative to controls (483 ± 40 µatm CO2, pH 8.1). The GABAA-receptor agonist muscimol, but not the antagonist gabazine, caused a significant increase in anxiety consistent with altered Cl(-) flux in OA-exposed fish. OA-exposed fish remained more anxious even after 7 days back in control seawater; however, they resumed their normal behaviour by day 12. These results show that OA could severely alter rockfish behaviour; however, this effect is reversible.

Role of the lateral reticular nucleus in suppressing the jaw-opening reflex following stimulation of the red nucleus

We found in a previous study that stimulation of the red nucleus (RN) facilitated the low-threshold afferent-evoked jaw-opening reflex (JOR) and suppressed the high-threshold afferent-evoked JOR. It has been reported that the RN projections to the contralateral lateral reticular nucleus (LRt), and stimulation of the LRt inhibits the nociceptive JOR. These facts suggest that RN-induced modulation of the JOR is mediated via the LRt. We investigated whether electrically induced lesions of the LRt, or microinjection of muscimol into the LRt, affects RN-induced modulation of the JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was either 1.2 (low-threshold) or 4.0 (high-threshold) times the threshold. Electrically induced lesion of the LRt and microinjection of muscimol into the LRt reduced the RN-induced suppression of the high-threshold afferent-evoked JOR, but did not affect the RN-induced facilitation of the low-threshold afferent-evoked JOR. These results suggest that the RN-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the contralateral LRt.

Activation of the medial prefrontal cortex by escapable stress is necessary for protection against subsequent inescapable stress-induced potentiation of morphine conditioned place preference

Stress can be a predisposing factor in the development of psychiatric disorders. However, not all individuals develop psychiatric disorders following a traumatic event. An attempt to understand these individual differences has led to a focus on factors that produce resistance. Interestingly, in rats, an experience with escapable tailshock (ES) before inescapable tailshock (IS) prevents the typical anxiety-like behavioral outcomes of IS. This type of resistance has been termed 'behavioral immunization', and it depends on activation of the medial prefrontal cortex (mPFC) during ES. However, one outcome of IS that is not anxiety-related is potentiation of morphine conditioned place preference (CPP). The present experiments investigated whether prior ES would block IS-induced potentiation of morphine CPP. Rats received either ES, IS or homecage control treatment on day 1 and then either IS or homecage control treatment on day 2. Twenty-four hours following day 2, rats underwent morphine conditioning, and CPP was subsequently assessed. In a second experiment, rats received ES 3, 14 or 56 days prior to IS to determine the duration of behavioral immunization. In a final experiment, rats were microinjected with the GABA(A) agonist muscimol (50 ng/0.5 μL) or saline in the mPFC before day 1 of stress. Prior ES blocked IS-induced potentiation of morphine CPP. This immunizing effect of ES lasted for at least 56 days. Additionally, intra-mPFC muscimol during ES prevented behavioral immunization. These results suggest that prior experience with ES activates the mPFC and produces long-lasting neural alterations that block subsequent IS-induced potentiation of morphine CPP.

Cortical NMDA receptor expression in human chronic alcoholism: influence of the TaqIA allele of ANKK1

Real-time RT-PCR normalized to GAPDH was used to assay N-methyl-D-aspartate (NMDA) receptor NR1, NR2A and NR2B subunit mRNA in human autopsy cortex tissue from chronic alcoholics with and without comorbid cirrhosis of the liver and matched controls. Subunit expression was influenced by the subject's genotype. The TaqIA polymorphism selectively modulated NMDA receptor mean transcript expression in cirrhotic-alcoholic superior frontal cortex, in diametrically opposite ways in male and female subjects. Genetic make-up may differentially influence vulnerability to brain damage by altering the excitation: inhibition balance, particularly in alcoholics with comorbid cirrhosis of the liver. The TaqIA polymorphism occurs within the poorly characterised ankyrin-repeat containing kinase 1 (ANKK1) gene. Using PCR, ANKK1 mRNA transcript was detected in inferior temporal, occipital, superior frontal and primary motor cortex of control human brain. ANKK1 expression may mediate the influence of the TaqIA polymorphism on phenotype.

The medial prefrontal cortex regulates the differential expression of morphine-conditioned place preference following a single exposure to controllable or uncontrollable stress

Experiential factors, such as stress, are major determinants of vulnerability to drug addiction and relapse. The behavioral controllability of the stressor is a major determinant of how exposure to a stressor impacts addictive processes. Recent evidence suggests that controllable stressors, such as escapable shock (ES), activate ventral regions of the medial prefrontal cortex (mPFCv), whereas physically identical, but uncontrollable stress (inescapable shock, IS) does not. This activation is critical to the blunting effect that control has on neurochemical and behavioral sequelae of stress. Our laboratory has previously shown that IS, but not ES, potentiates morphine-conditioned place preference (CPP). However, the role of the mPFCv in this phenomenon is unknown. The present experiments investigated the role of the mPFCv during ES and IS in determining the effects of the stressor on subsequent morphine-CPP. Intra-mPFCv microinjection of the GABA(A) agonist muscimol 1 h before ES led ES to potentiate morphine-CPP, as does IS. Conversely, the potentiation of morphine-CPP normally observed in IS rats was blocked by intra-mPFCv microinjection of the GABA(A) antagonist picrotoxin 1 h before IS. These results suggest that during stress, activation of the mPFCv prevents subsequent potentiation of morphine-CPP, whereas inactivation of the mPFCv during stress does not. Thus, activation of the mPFCv during a stress experience is both necessary and sufficient to block the impact of stress on morphine-CPP, and control over stress blunts stress-induced potentiation of morphine effects by activating the mPFCv.

Effect of reversible inactivation of superior colliculus on head movements

Because of limitations in the oculomotor range, many gaze shifts must be accomplished using coordinated movements of the eyes and head. Stimulation and recording data have implicated the primate superior colliculus (SC) in the control of these gaze shifts. The precise role of this structure in head movement control, however, is not known. The present study uses reversible inactivation to gain insight into the role of this structure in the control of head movements, including those that accompany gaze shifts and those that occur in the absence of a change in gaze. Forty-five lidocaine injections were made in two monkeys that had been trained on a series of behavioral tasks that dissociate movements of the eyes and head. Reversible inactivation resulted in clear impairments in the animals' ability to perform gaze shifts, manifested by increased reaction times, lower peak velocities, and increased durations. In contrast, comparable effects were not found for head movements (with or without gaze shifts) with the exception of a very small increase in reaction times of head movements associated with gaze shifts. Eye-head coordination was clearly affected by the injections with gaze onset occurring relatively later with respect to head onset. Following the injections, the head contributed slightly more to the gaze shift. These results suggest that head movements (with and without gaze shifts) can be controlled by pathways that do not involve SC.

The role of the amygdala and olfaction in unconditioned fear in developing rats

Early in ontogeny, young rats must be able to detect dangerous stimuli and to exhibit appropriate defensive behaviors. Different nuclei of the amygdala mediate unconditioned and conditioned fear responses to threat in adult rats. The aim of this study was to determine the role of the amygdala in unlearned fear behavior in young rats. When exposed to an unfamiliar adult male rat, preweaning rat pups freeze, with peak levels on postnatal day 14 and declining levels on day 18. Pups were made anosmic to block olfactory input to the amygdala, and amygdala activation was assessed by quantifying the neuronal marker c-fos. Anosmic pups did not freeze in the presence of the male rat and had decreased c-fos expression in the medial amygdala on day 14 and in the medial and lateral amygdala on day 18. However, the decrease in freezing between days 14 and 18 was not associated with a decrease in c-fos expression in the medial amygdala. The medial and lateral amygdala were then inactivated by local muscimol infusion on day 14. Muscimol infusion into the medial amygdala decreased freezing to the male rat but not to a loud noise, whereas infusion into the lateral amygdala blocked freezing to a loud noise but not to the male. These findings indicate that different nuclei of the amygdala process sensory information of different modalities, mediate unconditioned freezing, and may be involved in developmental changes in the fear response in young rats.

Evidence for participation of GABA(A) receptors in a rat model of secondary hyperalgesia

We investigated the involvement of endogenous gamma-aminobutyric acid (GABA) in the modulation of secondary hyperalgesia induced by intraplantar (i.pl.) injection of 5% formalin in the rat tail-flick test. Intraplantar injection of gabamimetic drugs such as gabapentin (150-600 microg/site) or phenobarbital (20-80 microg/site) reversed secondary hyperalgesia, as measured by an increase in the tail-flick latency, thus displaying a peripheral antihyperalgesic effect. Central inhibition of the secondary hyperalgesia response by gabapentin was obtained following injection of either 200 microg intrathecally (i.t.) or 50 mg intraperitoneally (i.p.). The effects induced by gabamimetics were blocked locally or centrally by prior treatment with the specific GABA(A) receptor antagonist, bicuculline (80 ng/paw or 20 ng, i.t.). These data indicate the participation of endogenous GABA in the modulation of secondary hyperalgesia, through either a peripheral and/or a central action. They also indicate that GABA(A) receptors might be involved since a specific antagonist of these receptors (bicuculline) blocked this response.

Effects of acute acamprosate and homotaurine on ethanol intake and ethanol-stimulated mesolimbic dopamine release

The purpose of the present study was to determine the acute effects of the anticraving compound acamprosate (calcium acetylhomotaurinate) and the closely related compound homotaurine on ethanol intake and ethanol-stimulated dopamine release in the nucleus accumbens. Male rats were treated with acamprosate (200 or 400 mg/kg intraperitoneally, i.p.) or homotaurine (10, 50, or 100 mg/kg i.p.) 15 min prior to access to 10% ethanol and water for 1 h in a two-bottle choice restricted access paradigm. A separate group of rats was implanted with microdialysis probes in the nucleus accumbens and given an acute injection of ethanol (1.5 g/kg i.p.) that was preceded by saline, acamprosate, or homotaurine. Acamprosate and homotaurine dose-dependently reduced ethanol intake and preference. These compounds also delayed or suppressed ethanol-stimulated increases in nucleus accumbens dopamine release, suggesting that acamprosate and homotaurine may reduce ethanol intake by interfering with the ability of ethanol to activate the mesolimbic dopamine reward system.

Microscopic kinetics and energetics distinguish GABA(A) receptor agonists from antagonists

Although agonists and competitive antagonists presumably occupy overlapping binding sites on ligand-gated channels, these interactions cannot be identical because agonists cause channel opening whereas antagonists do not. One explanation is that only agonist binding performs enough work on the receptor to cause the conformational changes that lead to gating. This idea is supported by agonist binding rates at GABA(A) and nicotinic acetylcholine receptors that are slower than expected for a diffusion-limited process, suggesting that agonist binding involves an energy-requiring event. This hypothesis predicts that competitive antagonist binding should require less activation energy than agonist binding. To test this idea, we developed a novel deconvolution-based method to compare binding and unbinding kinetics of GABA(A) receptor agonists and antagonists in outside-out patches from rat hippocampal neurons. Agonist and antagonist unbinding rates were steeply correlated with affinity. Unlike the agonists, three of the four antagonists tested had binding rates that were fast, independent of affinity, and could be accounted for by diffusion- and dehydration-limited processes. In contrast, agonist binding involved additional energy-requiring steps, consistent with the idea that channel gating is initiated by agonist-triggered movements within the ligand binding site. Antagonist binding does not appear to produce such movements, and may in fact prevent them.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

Hamster sperm glycine receptor: evidence for its presence and involvement in the acrosome reaction

Recent reports have provided evidence for the presence of amino acid neurotransmitter receptor/chloride channels in human and porcine spermatozoa and their involvement in the acrosome reaction (AR). In this work we investigated whether a glycine receptor (GlyR) was present in golden hamster sperm, and whether it had a role in the hamster AR. The neuronal GlyR agonist glycine, stimulated in a dose-dependent manner, the AR of hamster spermatozoa previously capacitated for at least 3 hr. This stimulation was completely inhibited by 50 microM (+)-bicuculline and by concentrations of strychnine as low as 10-50 nM; both agents are antagonists of neuronal GlyR when used at the concentrations reported in this study. beta-Alanine, another agonist of the neuronal GlyR, also stimulated the AR. The AR-stimulatory effect of this compound was completely abolished by 50 nM strychnine. The inhibitory effect of strychnine on the glycine-induced hamster sperm AR was completely overcome by subsequent treatment with the calcium ionophore ionomycin, demonstrating that the strychnine effect was specific for GlyR. Additional binding studies with (3)[H]-strychnine, the typical radioligand used to detect GlyR in several cells, demonstrated for the first time the presence of specific binding sites for strychnine in the hamster spermatozoa. Interestingly, binding increased during in vitro capacitation, particularly in those sperm suspensions showing high percentages of AR. Taken together these results strongly suggest the presence of a GlyR in the hamster spermatozoa, with a role in the AR when activated.

Effects of reversible inactivation of the primate mesencephalic reticular formation. I. Hypermetric goal-directed saccades

Single-neuron recording and electrical microstimulation suggest three roles for the mesencephalic reticular formation (MRF) in oculomotor control: 1) saccade triggering, 2) computation of the horizontal component of saccade amplitude (a feed-forward function), and 3) feedback of an eye velocity signal from the paramedian zone of the pontine reticular formation (PPRF) to higher structures. These ideas were tested using reversible inactivation of the MRF with pressure microinjection of muscimol, a GABA(A) agonist, in four rhesus monkeys prepared for chronic single-neuron and eye movement recording. Reversible inactivation revealed two subregions of the MRF: ventral-caudal and rostral. The ventral-caudal region, which corresponds to the central MRF, the cMRF, or nucleus subcuneiformis, is the focus of this paper and is located lateral to the oculomotor nucleus and caudal to the posterior commissure (PC). Inactivation of the cMRF produced contraversive, upward saccade hypermetria. In three of eight injections, the velocity of hypermetric saccades was too fast for a given saccade amplitude, and saccade duration was shorter. The latency for initiation of most contraversive saccades was markedly reduced. Fixation was also destabilized with the development of macrosaccadic square-wave jerks that were directed toward a contraversive goal in the hypermetric direction. Spontaneous saccades collected in total darkness were also directed toward the same orbital goal, up and to the contraversive side. Three of eight muscimol injections were associated with a shift in the initial position of the eyes. A contralateral head tilt was also observed in 5 out of 8 caudal injections. All ventral-caudal injections with head tilt showed no evidence of vertical postsaccadic drift. This suggested that the observed changes in head movement and posture resulted from inactivation of the caudal MRF and not spread of the muscimol to the interstitial nucleus of Cajal (INC). Evidence of hypermetria strongly supports the idea that the ventral-caudal MRF participates in the feedback control of saccade accuracy. However, development of goal-directed eye movements, as well as a shift in the initial position following some of the cMRF injections, suggest that this region also contributes to the generation of an estimate of target or eye position coded in craniotopic coordinates. Last, the observed reduction in contraversive saccade latency and development of macrosaccadic square-wave jerks supports a role of the MRF in saccade triggering.

Muscimol-induced inactivation of monkey frontal eye field: effects on visually and memory-guided saccades

Muscimol-induced inactivation of the monkey frontal eye field: effects on visually and memory-guided saccades. Although neurophysiological, anatomic, and imaging evidence suggest that the frontal eye field (FEF) participates in the generation of eye movements, chronic lesions of the FEF in both humans and monkeys appear to cause only minor deficits in visually guided saccade generation. Stronger effects are observed when subjects are tested in tasks with more cognitive requirements. We tested oculomotor function after acutely inactivating regions of the FEF to minimize the effects of plasticity and reallocation of function after the loss of the FEF and gain more insight into the FEF contribution to the guidance of eye movements in the intact brain. Inactivation was induced by microinjecting muscimol directly into physiologically defined sites in the FEF of three monkeys. FEF inactivation severely impaired the monkeys' performance of both visually guided and memory-guided saccades. The monkeys initiated fewer saccades to the retinotopic representation of the inactivated FEF site than to any other location in the visual field. The saccades that were initiated had longer latencies, slower velocities, and larger targeting errors than controls. These effects were present both for visually guided and for memory-guided saccades, although the memory-guided saccades were more disrupted. Initially, the effects were restricted spatially, concentrating around the retinotopic representation at the center of the inactivated site, but, during the course of several hours, these effects spread to flanking representations. Predictability of target location and motivation of the monkey also affected saccadic performance. For memory-guided saccades, increases in the time during which the monkey had to remember the spatial location of a target resulted in further decreases in the accuracy of the saccades and in smaller peak velocities, suggesting a progressive loss of the capacity to maintain a representation of target location in relation to the fovea after FEF inactivation. In addition, the monkeys frequently made premature saccades to targets in the hemifield ipsilateral to the injection site when performing the memory task, indicating a deficit in the control of fixation that could be a consequence of an imbalance between ipsilateral and contralateral FEF activity after the injection. There was also a progressive loss of fixation accuracy, and the monkeys tended to restrict spontaneous visual scanning to the ipsilateral hemifield. These results emphasize the strong role of the FEF in the intact monkey in the generation of all voluntary saccadic eye movements, as well as in the control of fixation.

Effects of pregnanolone on behavioural parameters and the responses to GABA(A) receptor antagonists in the late gestation fetal sheep

Placental progesterone metabolites may suppress fetal behaviour by interacting with GABA(A) receptors. In an initial study, the effect of 5beta-pregnan-3alpha-ol-20-one (pregnanolone) given as a bolus (2.5 and 5.0 mg) or infused at a rate of 25 mg/h was investigated in unanaesthetized, catheterized fetal sheep, 127-135 days gestation. The incidence of fetal breathing movements (FBM) and behavioural arousal activity, defined as nuchal muscle electromyographic (EMG) activity during low voltage electrocortical (LV ECoG) activity were suppressed by pregnanolone administered as a bolus, while the pregnanolone infusion produced a significant decrease in arousal and EOG activity, and an increase in the presence of HV ECoG. The effect of pregnanolone on fetal behaviour and arousal induced by the GABA(A) antagonist picrotoxin was also investigated. Picrotoxin was given as a bolus (approximately 300 microg/kg) and pregnanolone was subsequently administered as a bolus (5.0 mg), and behavioural parameters were recorded and analysed. The incidence of arousal and FBM were 1.1 +/- 1.6 min/10 min and 2.5 +/- 2.3 min/10 min, respectively, before picrotoxin treatment and increased during the 10-20 and 20-30 min epochs after picrotoxin treatment (arousal: 5.0 +/- 2.2 and 6.5 +/- 3.6 min/10 min, respectively, n = 6, P < 0.05; FBM: 7.3 +/- 3.2 and 9.3 +/- 1.2 min/10 min, respectively, n = 6, P < 0.05). The picrotoxin-induced increases in arousal and FBM were significantly suppressed (n = 6, P < 0.05) by pregnanolone treatment to 1.6 +/- 1.5 min/10 min and 4.6 +/- 2.3 min/10 min, respectively. We conclude that; (i) the GABA(A) active steroid pregnanolone suppresses basal and picrotoxin-induced fetal arousal and FBM; and (ii) steroid sensitive GABA(A) receptors may regulate fetal behaviour and breathing.

GABA(A) and GABA(B) agonists and antagonists alter the phase-shifting effects of light when microinjected into the suprachiasmatic region

GABAergic drugs have profound effects on the regulation of circadian rhythms. The present study evaluated the effects of microinjections of GABAergic drugs into the suprachiasmatic region in hamsters on phase shifts induced by light and by microinjection of a cocktail containing vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and gastrin-releasing peptide (GRP). The phase-advancing effects of light at circadian time (CT) 19 were significantly reduced by microinjection of GABA(A) or GABA(B) agonists into the SCN, but were not altered by microinjection of GABA(A) or GABA(B) antagonists. Microinjection of a GABA(B) agonist also reduced the phase-delaying effects of light at CT 13.5-14 while a GABA(B) antagonist increased the phase delays caused by light. Neither GABA(B) drug altered the phase delays produced by microinjection of a peptide cocktail containing VIP, PHI, GRP. These data indicate that changes in GABA(A) or GABA(B) activity within the SCN can alter the phase-shifting effects of light on circadian rhythms and support a role for GABA in gating photic input to the circadian clock.

Agonist pharmacology of two Drosophila GABA receptor splice variants

1. The Drosophila melanogaster gamma-aminobutyric acid (GABA) receptor subunits, RDLac and DRC 17-1-2, form functional homo-oligomeric receptors when heterologously expressed in Xenopus laevis oocytes. The subunits differ in only 17 amino acids, principally in regions of the N-terminal domain which determine agonist pharmacology in vertebrate ionotropic neurotransmitter receptors. A range of conformationally restricted GABA analogues were tested on the two homo-oligomers and their agonists pharmacology compared with that of insect and vertebrate iontropic GABA receptors. 2. The actions of GABA, isoguvacine and isonipecotic acid on RDLac and DRC 17-1-2 homo-oligomers were compared, by use of two-electrode voltage-clamp. All three compounds were full agonists of both receptors, but were 4-6 fold less potent agonists of DRC 17-1-2 homo-oligomers than of RDLac. However, the relative potencies of these agonists on each receptor were very similar. 3. A more complete agonist profile was established for RDLac homo-oligomers. The most potent agonists of these receptors were GABA, muscimol and trans-aminocrotonic acid (TACA), which were approximately equipotent. RDLac homo-oligomers were fully activated by a range of GABA analogues, with the order of potency: GABA > ZAPA ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid) > isoguvacine > imidazole-4-acetic acid > or = isonipecotic acid > or = cis-aminocrotonic acid (CACA) > beta-alanine. 3-Aminopropane sulphonic acid (3-APS), a partial agonist of RDLac homo-oligomers, was the weakest agonist tested and 100 fold less potent than GABA. 4. SR95531, an antagonist of vertebrate GABAA receptors, competitively inhibited the GABA responses of RDLac homo-oligomers, which have previously been found to insensitive to bicuculline. However, its potency (IC50 500 microM) was much reduced when compared to GABAA receptors. 5. The agonist pharmacology of Drosophila RDLac homo-oligomers exhibits aspects of the characteristic pharmacology of certain native insect GABA receptors which distinguish them from vertebrate GABA receptors. The high potency and efficacy of isoguvacine and ZAPA distinguishes RDLac homo-oligomers from bicuculline-insensitive vertebrate GABAC receptors, while the low potency of SR95531 and 3-APS distinguishes them from GABAA receptors. The differences in the potency of agonists on RDLac and DRC 17-1-2 homo-oligomers observed in the present study may assist in identification of further molecular determinants of GABA receptor function.

GABA(A) Receptor-Related Chloride Flux of Rats Receiving Continuous Intracerebroventricular Infusion of Pentobarbital

The in vitro receptor-stimulated synaptoneurosomal chloride uptake induced by pentobarbital or muscimol was studied in the brains of rats which had been rendered tolerant to or dependent on pentobarbital. In the chronic study, rats received continuous administration of sodium pentobarbital by intracerebroventricular (i.c.v.) infusion. In rats continuously infused with pentobarbital for 6 days or in rats which had the infusion terminated for 24 h, the basal synaptoneurosomal chloride uptake was not altered in either the cerebral cortex or the cerebellum. On the other hand, pentobarbital (500 &mgr;M)-stimulated chloride uptake was significantly decreased in both cerebral cortices and cerebella of rats which received pentobarbital for 6 days as compared with the saline control groups. Twenty-four hours after termination of pentobarbital infusion, this high concentration of pentobarbital-stimulated chloride uptake remained low in both cerebral cortices and cerebella of rats which had been infused with pentobarbital for 6 days. In addition, similar results were also obtained in muscimol (2.5 &mgr;M)-stimulated chloride uptake in the cerebral cortices of pentobarbital-infused animals. However, pentobarbital infusion failed to alter muscimol-stimulated chloride uptake in the cerebellum. These results suggest that the GABA(A) receptor regulated chloride uptake is downregulated after chronic pentobarbital administration. The results also suggest that down-regulation of the GABA(A) receptor chloride channel complex takes place at different recognition sites on the complex. It further substantiates the allosteric effects of GABA and barbiturate recognition sites. Copyright 1996 S. Karger AG, Basel

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Developmental changes in the inducibility of fos-like immunoreactivity in primary embryonic spinal cord cultures

The immediate early gene (IEG) transcription factor c-fos coordinates changes in the pattern of long term gene expression and, therefore, it may be involved in mediating epigenetic control during neurodevelopment. We used pharmacological treatments mimicking various environmental and intracellular signals and assessed the inducibility of fos-like immunoreactivity (LIR) at various stages of neurodifferentiation in a primary embryonic spinal cord culture system by immunohistochemistry. Constitutive fos LIR exclusively found in neurons, was driven by the onset and extent of spontaneous electrical activity, as it was blockable by tetrodotoxin (TTX) at all developmental stages. Phorbol myristate 13 acetate (PMA) increased the number of fos-LIR cells equally effectively at all stages, but the predominant cellular localization of fos-LIR changed through ontogeny. The effect of veratridine, kainate and serum-derived factors in significantly inducing fos-LIR was restricted to the earliest developmental stage (4 days in vitro; DIV) investigated; whereas forskolin, the GABAA antagonist picrotoxin and NMDA failed to induce fos-LIR at this stage, but increased the number of fos-LIR neurons at later stages. Dihydropyridine agonists of the voltage-sensitive calcium channels (VSCC) raised the number of fos-LIR neurons and also prevented TTX-mediated down-regulation; whereas antagonists markedly reduced fos-LIR at all ages. Either type of NMDA antagonists (AP5 and MK801) and the GABAA agonist muscimol significantly reduced fos-LIR at all ages. These findings demonstrate that the inducibility of fos-LIR is substantially different in embryonic neurons than in adult ones and that inducibility by various first and second messengers is dependent on the development stage.

Brain neuromediator systems in the immune response control: pharmacological analysis of pre- and postsynaptic mechanisms

The involvement of the dopaminergic (DAergic), GABAergic and serotoninergic (5-HTergic) synaptic mechanisms in neuroimmunomodulation are considered. It is shown that the central DA- and GABAergic systems exert a stimulatory influence on the immune reactivity. Activation of postsynaptic DA receptors with apomorphine and (+)(3-hydroxyphenyl)-N-n-propylpiperidine [(+)-3-PPP] at high doses, D2 receptor activation with quinpirole, as well as DA reuptake blockade with amantadine and bupropion, resulted in an increase in the immune response. The intensity of the immune response was also increased during activation of the GABAergic system with the GABA agonist muscimol or the benzodiazepine agonists diazepam and tazepam. On the other hand, decreases in the activity of either system by different drugs inhibiting DAergic and GABAergic neurotransmission (antagonists of DA postsynaptic receptors haloperidol at 1.0 mg/kg and (-)-3-PPP at 6.8 mg/kg, agonists of DA autoreceptors apomorphine at 0.02 mg/kg, (-)- and (+)-3-PPP at 1.7 and 3.4 mg/kg, antagonist of GABA receptors bicuculline, blocker of chloride channels picrotoxin, blockers of benzodiazepine receptors Ro 15-1788 and Ro 15-3505) produce suppression of the immune response. The 5-HTergic system is involved in the inhibitory mechanism of neuroimmunomodulation. 5-HT reuptake blockade with 4-(5,6-dimethyl-R-benzophuranil)-piperidine (CGP-6085A) and sertraline led to immunosuppression.

Pharmacology of GABA rho 1 and GABA alpha/beta receptors expressed in Xenopus oocytes and COS cells

1. The rho 1 protein, which we previously cloned from retina, assembles as a homooligomer that transduces the binding of gamma-aminobutyric acid (GABA) into robust chloride currents. However, its insensitivity to bicuculline, pentobarbitone and benzodiazepines, all potent agents at typical GABAA receptors, suggested that it may react atypically to other GABA agonists and antagonists. 2. cDNAs for the rho 1 and the alpha 5 beta 1 receptors for GABA were expressed as homo- and heterooligomers, respectively, in Xenopus oocytes. The selectivities of the respective receptors for various agonists were investigated using concentration-response experiments in voltage clamped cells. 3. The most potent agonists at the rho 1 receptor were trans-4-aminocrotonic acid (TACA) > GABA > muscimol; at the alpha 5 beta 1 receptor the rank order was muscimol > GABA > 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP). The most specific agonists were cis-(2-(aminomethyl)-cyclopropyl-carboxylic acid (CAMP) and THIP for the rho 1 and the alpha 5 beta 1 receptors, respectively. 4. Comparing GABA, TACA and cis-aminocrotonic acid (CACA) at rho 1 receptors expressed in COS cells gave results almost indistinguishable from those found at oocytes; the pharmacology of rho 1 seems independent of the expression system. 5. Agonists THIP, piperidine-4-sulphonic acid (P4S), and isoguvacine, whose C-C-C-N chains are constrained by rings into a folded conformation and were potent at the alpha 5 beta 1 receptor, were among the weakest at the rho 1 receptor. However CACA and CAMP, which align better with the extended than the folded conformation, were weakest at the alpha 5 beta 1 receptor but moderately potent at the pl receptor. These findings suggest that the rho l receptor recognizes agonists in the extended conformation, in contrast to GABAA receptors, which are believed to recognize agonists in the partially folded conformation.6. In contrast to the alpha 5 beta 1 receptor, gradations in maximum responses were apparent in the rho l receptor,suggesting various degrees of partial agonism. In particular, imidazole-4-acetic acid (I4AA), whose maximum response was only 3% of GABA's maximum, had an apparent Kd for activating the rho l receptor of 16 microM; but it had an apparent Kd for competitively blocking the receptor of 0.64 microM. This difference suggests that steric constraints in the activated (open channel) receptor are tighter than in the resting receptor.7. Hill coefficients approached 2 at the rho l receptor, but were closer to unity at the alpha 5 beta 1 receptor. Thus,the rho l receptor displayed higher cooperativity.8. Unlike typical GABAA receptors, the rho l receptor was insensitive to the competitive antagonists bicuculline, SR95531, securinine, and (+)-tubocurarine.

Significance of chloride channel activation in the gamma-aminobutyric acid induced growth hormone secretion in the neonatal rat pituitary

Growth hormone (GH) secretion of the neonatal pituitary is stimulated by tau-aminobutyric acid (GABA) (1,2). Since in most cases GABA is known to act by increasing postsynaptic membrane permeability to chloride ions we tested the importance of chloride channel activation in the GH stimulatory effect of GABA in the neonatal pituitary. In the absence of chloride in the superfusion medium GABA was without effect on GH secretion of the neonatal pituitaries and its effect was attenuated by chloride channel inhibitors. The effect of growth hormone releasing hormone (GHRH) on GH secretion was attenuated in the chloride-free media, but it was not affected by simultaneous administration of chloride channel blockers. The present study indicates that GH stimulatory effect of GABA in the neonatal pituitaries might involve chloride channel activation probably resulting in secondary activation of calcium channels.

Bilateral cerebral metabolic effects of pharmacological manipulation of the substantia nigra in the rat: unilateral intranigral application of the inhibitory GABAA receptor agonist muscimol

Rates of cerebral glucose utilization were measured by means of the autoradiographic 2-deoxy-D[1-14C]glucose technique in the rat brain in order to determine the metabolic effects of unilateral intranigral application of the GABAA agonist muscimol upon the substantia nigra and its targets. Intranigral injection of 1.5 microliters 0.3 M muscimol (52 micrograms total dose) induced local metabolic activation in the injected substantia nigra reticulata (by 87% as compared to the control group), and distal metabolic depressions in the nucleus accumbens, striatum, globus pallidus and subthalamic nucleus only ipsilaterally to the injected nigra. The remaining basal ganglia components, including the substantia nigra compacta and the entopeduncular nucleus were bilaterally unaffected. Among the principal efferent projections of the substantia nigra reticulata, the ventromedial and centrolateral thalamic nuclei as well as the deep layers of the superior colliculi were metabolically depressed bilaterally, whereas the ventrolateral, parafascicular and mediodorsal thalamic nuclei as well as the pedunculopontine nucleus displayed metabolic depressions ipsilateral to the muscimol-injection nigra. The ventromedial and centrolateral thalamic nuclei were depressed by 41 and 42%, respectively, in the ipsilateral side, and by 30 and 26% in the contralateral side, when compared to the respective values of the control group of rats. Furthermore, unilateral intranigral injection of 0.3 M muscimol induced metabolic depressions in reticular, intralaminar and prefrontal thalamocortical areas mostly ipsilateral to the injected nigra, as well as in limbic areas bilaterally. It is suggested that the present findings are due to a postsynaptic effect of muscimol on the nigral GABAergic cells and to a consequent metabolic depression of the basal ganglia and associated thalamocortical areas, in contrast to an earlier suggested presynaptic nigral effect of lower doses of intranigrally injected muscimol which induced metabolic activations within the same network. This suggestion is further supported by the fact that intranigrally injected substrate P19 induced similar effects to those elicited by the lower doses of intranigral muscimol and opposite to those induced at present by the higher muscimol dose. Moreover, it is further substantiated that the nigrothalamic GABAergic system is responsible for considerable transfer of information from one substantia nigra reticulata to the ipsilateral basal ganglia and associated thalamocortical components as well as to bilateral motor, intralaminar and limbic areas.

GABAA agonists as targets for drug development

1. Agents with selective actions on bicuculline-sensitive GABAA receptors have been developed by systematically restricting the conformational mobility of the GABA molecule. 2. THIP, a bicyclic isoxazole that represents GABA held in a relatively rigid and partially extended conformation, is an analgesic of potency comparable to that of morphine. THIP represents a lead compound for a novel series of analgesics acting independently of Naloxone-sensitive opiate systems. 3. ZAPA, a conformationally-restricted analogue of GABA containing an isothiouronium moiety, is a selective agonist for low affinity GABAA receptors and is a lead compound for the development of a novel series of anthelmintics. 4. (+)-TACP, a cyclopentane analogue of GABA, may activate a different subclass of GABAA receptors from THIP. 5. Pharmacological, molecular modelling and molecular biological studies provide evidence for a heterogeneity of GABAA receptors which might be exploited for drug development.

The physiology of GABAB receptors in the vertebrate retina

Writing a chapter on retinal GABAB receptors is premature, as evidenced by the paucity of citations more than two years old. Despite that, this area of retinal pharmacology has made significant strides and, although it is a story without an ending, it has had an exciting beginning. To date, the experiments indicate that horizontal cell feedback to cones is mediated, at least in part, by the GABAB receptor system which probably regulates a potassium conductance. In the inner retina, GABAB receptors are found on bipolar cells, amacrines, and ganglion cells. Here, the actions are a subtle regulation of channel conductance, but the effects are a dramatic reorganization of a fundamental coding property of the retina, namely the distinction between tonic and phasic responses to light. In both the distal and proximal retina, the GABAB receptor does not appear to work alone, but instead works in concert with the GABAA receptor. The full significance of these interactions has yet to be determined. Although the discovery of the GABAB receptor has led to the resolution of several retinal mysteries, the case is far from closed. At this juncture, what can be said is that the GABAB receptor represents a unique and ubiquitous system that reveals the power of regulating calcium and potassium conductances, as opposed to the more familiar properties of the glutamate/acetylcholine regulated cationic conductances or the GABAA/glycine controlled chloride channels.