Distribution and kinetics of GABAB binding sites in rat central nervous system: a quantitative autoradiographic study

Blockade of pre- and post-synaptic GABAB receptors in the anteroventral bed nucleus of stria terminalis produces anxiolytic-like and anxiety-like effects in parkinsonian rats respectively

The anteroventral bed nucleus of stria terminalis (avBNST) is a limbic forebrain region involved in the regulation of anxiety, and expresses GABAB receptors, which are located at both pre- and post-synaptic sites. However, it is unclear how blockade of these receptors affects anxiety-like behaviors, particularly in Parkinson's disease (PD)-related anxiety. In the present study, unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta in rats induced anxiety-like behaviors, and increased GABA release and decreased glutamate release in the avBNST, as well as decreased level of dopamine (DA) in the basolateral amygdala (BLA). Intra-avBNST injection of pre-synaptic GABAB receptor antagonist CGP36216 produced anxiolytic-like effects, while the injection of post-synaptic GABAB receptor antagonist CGP35348 induced anxiety-like responses in both sham and 6-OHDA rats. Intra-avBNST injection of CGP36216 inhibited the GABAergic neurons and increased GABA/glutamate ratio in the avBNST and increased levels of DA and serotonin (5-HT) in the BLA; conversely, CGP35348 produced opposite effects on the firing activity of avBNST GABAergic neurons and levels of the neurotransmitters in the avBNST and BLA. Moreover, the doses of the antagonists producing significant behavioral effects in 6-OHDA rats were lower than those in sham rats, and the duration of action of the antagonists on the firing rate of the neurons and release of the neurotransmitters was prolonged in 6-OHDA rats. Altogether, these findings suggest that pre- and post-synaptic GABAB receptors in the avBNST are implicated in PD-related anxiety-like behaviors, and degeneration of the nigrostriatal pathway enhances functions and/or upregulates expression of these receptors.

Cerebral Metabolic Signature of Chronic Benzodiazepine Use in Nondemented Older Adults: An FDG-PET Study in the MEMENTO Cohort

OBJECTIVE

We sought to examine the association between chronic Benzodiazepine (BZD) use and brain metabolism obtained from 2-deoxy-2-fluoro-D-glucose (FDG) positron emission tomography (PET) in the MEMENTO clinical cohort of nondemented older adults with an isolated memory complaint or mild cognitive impairment at baseline.

METHODS

Our analysis focused on 3 levels: (1) the global mean brain standardized uptake value (SUVR), (2) the Alzheimer's disease (AD)-specific regions of interest (ROIs), and (3) the ratio of total SUVR on the brain and different anatomical ROIs. Cerebral metabolism was obtained from 2-deoxy-2-fluoro-D-glucose-FDG-PET and compared between chronic BZD users and nonusers using multiple linear regressions adjusted for age, sex, education, APOE ε 4 copy number, cognitive and neuropsychiatric assessments, history of major depressive episodes and antidepressant use.

RESULTS

We found that the SUVR was significantly higher in chronic BZD users (n = 192) than in nonusers (n = 1,122) in the whole brain (beta = 0.03; p = 0.038) and in the right amygdala (beta = 0.32; p = 0.012). Trends were observed for the half-lives of BZDs (short- and long-acting BZDs) (p = 0.051) and Z-drug hypnotic treatments (p = 0.060) on the SUVR of the right amygdala. We found no significant association in the other ROIs.

CONCLUSION

Our study is the first to find a greater global metabolism in chronic BZD users and a specific greater metabolism in the right amygdala. Because the acute administration of BZDs tends to reduce brain metabolism, these findings may correspond to a compensatory mechanism while the brain adapts with global metabolism upregulation, with a specific focus on the right amygdala.

Crosstalk between neurons and glia through G-protein coupled receptors: Insights from Caenorhabditis elegans

The past decades have witnessed a dogmatic shift from glia as supporting cells in the nervous system to their active roles in neurocentric functions. Neurons and glia communicate and show bidirectional responses through tripartite synapses. Studies across species indicate that neurotransmitters released by neurons are perceived by glial receptors, which allow for gliotransmitter release. These gliotransmitters can result in activation of neurons via neuronal GPCR receptors. However, studies of these molecular interactions are in their infancy. Caenorhabditis elegans has a conserved neuron-glia architectural repertoire with molecular and functional resemblance to mammals. Further, glia in C. elegans can be manipulated through ablation and mutations allowing for deciphering of glial dependent processes in vivo at single glial resolutions. Here, we will review recent findings from vertebrate and invertebrate organisms with a focus on how C. elegans can be used to advance our understanding of neuron-glia interactions through GPCRs.

A Brief History and the Significance of the GABAB Receptor

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. GABA type B (GABA_B) receptors (GABA_BRs) are the only metabotropic G protein-coupled receptors for GABA and can be found distributed not only in the central nervous system, but also in the periphery. This chapter introduces important, fundamental knowledge related to GABA_BR function and the various potential therapeutic applications of the development of novel GABA_BR-active compounds, as documented through extensive studies presented in subsequent chapters of this Current Topic in Behavioral Neurosciences volume on the role of the neurobiology of GABA_BR function. The compounds that have received increased attention in the last few years compared to GABA_BR agonists and antagonists - the positive allosteric modulators - exhibit better pharmacological profiles and fewer side effects. As we continue to unveil the mystery of GABA_BRs at the molecular and cellular levels, we further understand the significance of these receptors. Future directions should aim for developing highly selective GABA_BR compounds for treating neuropsychiatric disorders and their symptomatology.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Mechanisms and Regulation of Neuronal GABAB Receptor-Dependent Signaling

γ-Aminobutyric acid B receptors (GABA_BRs) are broadly expressed throughout the central nervous system where they play an important role in regulating neuronal excitability and synaptic transmission. GABA_BRs are G protein-coupled receptors that mediate slow and sustained inhibitory actions via modulation of several downstream effector enzymes and ion channels. GABA_BRs are obligate heterodimers that associate with diverse arrays of proteins to form modular complexes that carry out distinct physiological functions. GABA_BR-dependent signaling is fine-tuned and regulated through a multitude of mechanisms that are relevant to physiological and pathophysiological states. This review summarizes the current knowledge on GABA_BR signal transduction and discusses key factors that influence the strength and sensitivity of GABA_BR-dependent signaling in neurons.

Density of GABAB Receptors Is Reduced in Granule Cells of the Hippocampus in a Mouse Model of Alzheimer's Disease

Metabotropic γ-aminobutyric acid (GABA_B) receptors contribute to the control of network activity and information processing in hippocampal circuits by regulating neuronal excitability and synaptic transmission. The dysfunction in the dentate gyrus (DG) has been implicated in Alzheimer´s disease (AD). Given the involvement of GABA_B receptors in AD, to determine their subcellular localisation and possible alteration in granule cells of the DG in a mouse model of AD at 12 months of age, we used high-resolution immunoelectron microscopic analysis. Immunohistochemistry at the light microscopic level showed that the regional and cellular expression pattern of GABA_B1 was similar in an AD model mouse expressing mutated human amyloid precursor protein and presenilin1 (APP/PS1) and in age-matched wild type mice. High-resolution immunoelectron microscopy revealed a distance-dependent gradient of immunolabelling for GABA_B receptors, increasing from proximal to distal dendrites in both wild type and APP/PS1 mice. However, the overall density of GABA_B receptors at the neuronal surface of these postsynaptic compartments of granule cells was significantly reduced in APP/PS1 mice. Parallel to this reduction in surface receptors, we found a significant increase in GABA_B1 at cytoplasmic sites. GABA_B receptors were also detected at presynaptic sites in the molecular layer of the DG. We also found a decrease in plasma membrane GABA_B receptors in axon terminals contacting dendritic spines of granule cells, which was more pronounced in the outer than in the inner molecular layer. Altogether, our data showing post- and presynaptic reduction in surface GABA_B receptors in the DG suggest the alteration of the GABA_B-mediated modulation of excitability and synaptic transmission in granule cells, which may contribute to the cognitive dysfunctions in the APP/PS1 model of AD.

Reduction in the neuronal surface of post and presynaptic GABAB receptors in the hippocampus in a mouse model of Alzheimer's disease

The hippocampus plays key roles in learning and memory and is a main target of Alzheimer's disease (AD), which causes progressive memory impairments. Despite numerous investigations about the processes required for the normal hippocampal functions, the neurotransmitter receptors involved in the synaptic deficits by which AD disables the hippocampus are not yet characterized. By combining histoblots, western blots, immunohistochemistry and high-resolution immunoelectron microscopic methods for GABA_B receptors, this study provides a quantitative description of the expression and the subcellular localization of GABA_B1 in the hippocampus in a mouse model of AD at 1, 6 and 12 months of age. Western blots and histoblots showed that the total amount of protein and the laminar expression pattern of GABA_B1 were similar in APP/PS1 mice and in age-matched wild-type mice. In contrast, immunoelectron microscopic techniques showed that the subcellular localization of GABA_B1 subunit did not change significantly in APP/PS1 mice at 1 month of age, was significantly reduced in the stratum lacunosum-moleculare of CA1 pyramidal cells at 6 months of age and significantly reduced at the membrane surface of CA1 pyramidal cells at 12 months of age. This reduction of plasma membrane GABA_B1 was paralleled by a significant increase of the subunit at the intracellular sites. We further observed a decrease of membrane-targeted GABA_B receptors in axon terminals contacting CA1 pyramidal cells. Our data demonstrate compartment- and age-dependent reduction of plasma membrane-targeted GABA_B receptors in the CA1 region of the hippocampus, suggesting that this decrease might be enough to alter the GABA_B -mediated synaptic transmission taking place in AD.

GABA and l-theanine mixture decreases sleep latency and improves NREM sleep

CONTEXT

γ-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter and it is well established that activation of GABA_A receptors favours sleep. l-Theanine, a naturally occurring amino acid first discovered in green tea, is a well-known anti-anxiety supplement with proven relaxation benefits.

OBJECTIVE

This study investigated the potential synergistic sleep enhancement effect of GABA/l-theanine mixture.

MATERIALS AND METHODS

Pentobarbital-induced sleep test was applied to find proper concentration for sleep-promoting effect in ICR mice. Electroencephalogram (EEG) analysis was performed to investigate total sleeping time and sleep quality in normal SD rats and caffeine-induced awareness model. Real-time polymerase chain reaction (RT-PCR) was applied to investigate whether the sleep-promoting mechanism of GABA/l-theanine mixture involved transcriptional processes.

RESULTS

GABA/l-theanine mixture (100/20 mg/kg) showed a decrease in sleep latency (20.7 and 14.9%) and an increase in sleep duration (87.3 and 26.8%) compared to GABA or theanine alone. GABA/l-theanine mixture led to a significant increase in rapid eye movement (REM) (99.6%) and non-REM (NREM) (20.6%) compared to controls. The use of GABA/l-theanine mixture rather than GABA or l-theanine alone restored to normal levels sleep time and quality in the arousal animal model. The administration of GABA/l-theanine led to increased expression of GABA and the glutamate GluN1 receptor subunit.

CONCLUSIONS

GABA/l-theanine mixture has a positive synergistic effect on sleep quality and duration as compared to the GABA or l-theanine alone. The increase in GABA receptor and GluN1 expression is attributed to the potential neuromodulatory properties of GABA/l-theanine combination, which seems to affect sleep behaviour.

GABAergic cell transplants in the anterior cingulate cortex reduce neuropathic pain aversiveness

Dysfunction of inhibitory circuits in the rostral anterior cingulate cortex underlies the affective (aversive), but not the sensory-discriminative features (hypersensitivity) of the pain experience. To restore inhibitory controls, we transplanted inhibitory interneuron progenitor cells into the rostral anterior cingulate cortex in a chemotherapy-induced neuropathic pain model. The transplants integrated, exerted a GABA-A mediated inhibition of host pyramidal cells and blocked gabapentin preference (i.e. relieved ongoing pain) in a conditioned place preference paradigm. Surprisingly, pain aversiveness persisted when the transplants populated both the rostral and posterior anterior cingulate cortex. We conclude that selective and long lasting inhibition of the rostral anterior cingulate cortex, in the mouse, has a profound pain relieving effect against nerve injury-induced neuropathic pain. However, the interplay between the rostral and posterior anterior cingulate cortices must be considered when examining circuits that influence ongoing pain and pain aversiveness.

Peripheral GABA receptors regulate colonic afferent excitability and visceral nociception

KEY POINTS

While the presence of GABA receptors on primary afferents has been well described, most functional analyses have focused on the regulation of transmitter release from central terminals and/or signalling in the sensory neuron cell body. Evidence that GABA receptors are transported to peripheral terminals and that there are several sources of GABA in the colon raise the possibility that GABA signalling in the periphery may influence colonic afferent excitability. GABA_A and GABA_B are present and functional in the colon, where exogenous agonists decrease the excitability of colonic afferents and suppress visceral nociception. Endogenous GABA release within the colon is sufficient to establish the resting excitability of colonic afferents as well as the behavioural response to noxious stimulation of the colon, primarily via GABA_A receptors. Peripheral GABA receptors may serve as a viable target for the treatment of visceral pain.

ABSTRACT

It is well established that GABA receptors at the central terminals of primary afferent fibres regulate afferent input to the superficial dorsal horn. However, the extent to which peripheral GABA signalling may also regulate afferent input remains to be determined. The colon was used to explore this issue because of the numerous endogenous sources of GABA that have been described in this tissue. The influence of GABA signalling on colonic afferent excitability was assessed in an ex vivo mouse colorectum pelvic nerve preparation where test compounds were applied to the receptive field. The visceromotor response (VMR) evoked by noxious colorectal distension was used to assess the impact of GABA signalling on visceral nociception, where test compounds were applied directly to the colon. Application of either GABA_A or GABA_B receptor agonists attenuated the colonic afferent response to colon stretch. Conversely, GABA_A and GABA_B receptor antagonists increased the stretch response. However, while the noxious distension-induced VMR was attenuated in the presence of GABA_A and GABA_B receptor agonists, the VMR was only consistently increased by GABA_A receptor antagonists. These results suggest that GABA receptors are present and functional in the peripheral terminals of colonic afferents and activation of these receptors via endogenous GABA release contributes to the establishment of colonic afferent excitability and visceral nociception. These results suggest that increasing peripheral GABA receptor signalling could be used to treat visceral pain.

Structural biology of GABAB receptor

Metabotropic GABAB receptor is a G protein-coupled receptor (GPCR) that mediates slow and prolonged inhibitory neurotransmission in the brain. It functions as a constitutive heterodimer composed of the GABAB1 and GABAB2 subunits. Each subunit contains three domains; the extracellular Venus flytrap module, seven-helix transmembrane region and cytoplasmic tail. In recent years, the three-dimensional structures of GABAB receptor extracellular and intracellular domains have been elucidated. These structures reveal the molecular basis of ligand recognition, receptor heterodimerization and receptor activation. Here we provide a brief review of the GABAB receptor structures, with an emphasis on describing the different ligand-bound states of the receptor. We will also compare these with the known structures of related GPCRs to shed light on the molecular mechanisms of activation and regulation in the GABAB system, as well as GPCR dimers in general. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Synthesis and Evaluation of a New 18F-Labeled Radiotracer for Studying the GABAB Receptor in the Mouse Brain

New GABA_B agonists, fluoropyridyl ether analogues of baclofen, have been synthesized as potential PET radiotracers. The compound with highest inhibition binding affinity as well as greatest agonist response, ( R)-4-amino-3-(4-chloro-3-((2-fluoropyridin-4-yl)methoxy)phenyl)butanoic acid (1b), was radiolabeled with ¹⁸F with good radiochemical yield, high radiochemical purity, and high molar radioactivity. The regional brain distribution of the radiolabeled ( R)-4-amino-3-(4-chloro-3-((2-[¹⁸F]fluoropyridin-4-yl)methoxy)phenyl)butanoic acid, [¹⁸F]1b, was studied in CD-1 male mice. The study demonstrated that [¹⁸F]1b enters the mouse brain (1% ID/g tissue). The accumulation of [¹⁸F]1b in the mouse brain was inhibited (35%) by preinjection of GABA_B agonist 1a, suggesting that the radiotracer brain uptake is partially mediated by GABA_B receptors. The presented data demonstrate a feasibility of imaging of GABA_B receptors in rodents and justify further development of GABA_B PET tracers with improved specific binding and greater blood-brain barrier permeability.

The Gamma-Aminobutyric Acid B Receptor in Depression and Reward

The metabotropic gamma-aminobutyric acid B (GABA_B) receptor was the first described obligate G protein-coupled receptor heterodimer and continues to set the stage for discoveries in G protein-coupled receptor signaling complexity. In this review, dedicated to the life and work of Athina Markou, we explore the role of GABA_B receptors in depression, reward, and the convergence of these domains in anhedonia, a shared symptom of major depressive disorder and withdrawal from drugs of abuse. GABA_B receptor expression and function are enhanced by antidepressants and reduced in animal models of depression. Generally, GABA_B receptor antagonists are antidepressant-like and agonists are pro-depressive. Exceptions to this rule likely reflect the differential influence of GABA_B1 isoforms in depression-related behavior and neurobiology, including the anhedonic effects of social stress. A wealth of data implicate GABA_B receptors in the rewarding effects of drugs of abuse. We focus on nicotine as an example. GABA_B receptor activation attenuates, and deactivation enhances, nicotine reward and associated neurobiological changes. In nicotine withdrawal, however, GABA_B receptor agonists, antagonists, and positive allosteric modulators enhance anhedonia, perhaps owing to differential effects of GABA_B1 isoforms on the dopaminergic system. Nicotine cue-induced reinstatement is more reliably attenuated by GABA_B receptor activation. Separation of desirable and undesirable side effects of agonists is achievable with positive allosteric modulators, which are poised to enter clinical studies for drug abuse. GABA_B1 isoforms are key to understanding the neurobiology of anhedonia, whereas allosteric modulators may offer a mechanism for targeting specific brain regions and processes associated with reward and depression.

Synthesis of C-14 labeled GABAA α2/α3 selective partial agonists and the investigation of late-occurring and long-circulating metabolites of GABAA receptor modulator AZD7325

Anxiolytic activity has been associated with GABAA α2 and α3 subunits. Several target compounds were identified and required in C-14 labeled form to enable a better understanding of their drug metabolism and pharmacokinetic properties. AZD7325 is a selective GABAA α2 and α3 receptor modulator intended for the treatment of anxiety through oral administration. A great number of AZD7325 metabolites were observed across species in vivo, whose identification was aided by [14 C]AZD7325. An interesting metabolic cyclization and aromatization pathway leading to the tricyclic core of M9 and the oxidative pathways to M10 and M42 are presented.

Contribution of genes in the GABAergic pathway to bipolar disorder and its executive function deficit in the Chinese Han population

In this study, we investigated the association between bipolar I disorder (BDI) and between cognitive deficits therein and SNPs in GABAergic receptor genes. The sample comprised 477 patients with BDI and 438 healthy controls, with three neurocognitive tests being administered in 123 patients and 164 controls. For three SNPs, rs505474, rs1398175, and rs4868029 in the GABRA2, GABRA4, and GABRP genes, respectively, their allele frequencies were significantly different between patients and controls (Bonferroni-adjusted p = values 3.84 × 10^-4 , 9.92 × 10^-3 , and 1.22 × 10^-2 , respectively). Four haplotypes were significantly associated with BDI (TA and AG for rs3815762 and rs4868029 in GABRP, GG for rs11636988 and rs8024256 in GABRB3 and GAGG for rs2197414, rs4921195, rs13188991, and rs11956731 in GABRA6, with p values of 0.0038, 0.044, 0.0176, and 0.0267, respectively, on 10,000 permutations). Furthermore, the SNP (rs2912585) within 250 kb upstream of the GABRB3 gene displayed a strong association with the Tower of Hanoi (TOH) executive time in the patient group (p = 2.844 × 10^-6 ). One other SNP (rs754661), which is located at the intronic region of the same gene, was associated with the global trait of the executive function and post hoc analysis showed significant SNP by group effect (p = 0.0094). Our study supports previous findings that GABA_A receptor genes are associated with bipolar disorder; it also suggests that the GABA_A genes, especially the GABRB3 gene, might play a role in the executive function deficit in bipolar disorder, although future replication with a larger sample size is needed.

A Review of the Potential Mechanisms of Action of Baclofen in Alcohol Use Disorder

Baclofen, a GABA-B receptor agonist, is a promising treatment for alcohol use disorder (AUD). Its mechanism of action in this condition is unknown. GABA-B receptors interact with many biological systems potentially involved in AUD, including transduction pathways and neurotransmitter systems. Preclinical studies have shown that GABA-B receptors are involved in memory storage and retrieval, reward, motivation, mood and anxiety; neuroimaging studies in humans show that baclofen produces region-specific alterations in cerebral activity; GABA-B receptor activation may have neuroprotective effects; baclofen also has anti-inflammatory properties that may be of interest in the context of addiction. However, none of these biological effects fully explain the mechanism of action of baclofen in AUD. Data from clinical studies have provided a certain number of elements which may be useful for the comprehension of its mechanism of action: baclofen typically induces a state of indifference toward alcohol; the effective dose of baclofen in AUD is extremely variable from one patient to another; higher treatment doses correlate with the severity of the addiction; many of the side effects of baclofen resemble those of alcohol, raising the possibility that baclofen acts as a substitution drug; usually, however, there is no tolerance to the effects of baclofen during long-term AUD treatment. In the present article, the biological effects of baclofen are reviewed in the light of its clinical effects in AUD, assuming that, in many instances, clinical effects can be reliable indicators of underlying biological processes. In conclusion, it is proposed that baclofen may suppress the Pavlovian association between cues and rewards through an action in a critical part of the dopaminergic network (the amygdala), thereby normalizing the functional connectivity in the reward network. It is also proposed that this action of baclofen is made possible by the fact that baclofen and alcohol act on similar brain systems in certain regions of the brain.

GABAB receptors in neocortical and hippocampal pyramidal neurons are coupled to different potassium channels

Classically, GABA_B receptors are thought to regulate neuronal excitability via G-protein-coupled inwardly rectifying potassium (GIRK) channels. Recent data, however, indicate that GABA_B receptors can also activate two-pore domain potassium channels. Here, we investigate which potassium channels are coupled to GABA_B receptors in rat neocortical layer 5 and hippocampal CA1 pyramidal neurons. Bath application of the non-specific GIRK channel blocker barium (200 μm) abolished outward currents evoked by GABA_B receptors in CA1 pyramidal, but only partially blocked GABA_B responses in layer 5 neurons. Layer 5 and CA1 pyramidal neurons also showed differential sensitivity to tertiapin-Q, a specific GIRK channel blocker. Tertiapin-Q partially blocked GABA_B responses in CA1 pyramidal neurons, but was ineffective in blocking GABA_B responses in neocortical layer 5 neurons. Consistent with the idea that GABA_B receptors are coupled to two-pore domain potassium channels, the non-specific blockers quinidine and bupivacaine partially blocked GABA_B responses in both layer 5 and CA1 neurons. Finally, we show that lowering external pH, as occurs in hypoxia, blocks the component of GABA_B responses mediated by two-pore domain potassium channels in neocortical layer 5 pyramidal neurons, while at the same time revealing a GIRK channel component. These data indicate that GABA_B receptors in neocortical layer 5 and hippocampal CA1 pyramidal neurons are coupled to different channels, with this coupling pH dependent on neocortical layer 5 pyramidal neurons. This pH dependency may act to maintain constant levels of GABA_B inhibition during hypoxia by enhancing GIRK channel function following a reduction in two-pore domain potassium channel activity.

Differential association of GABAB receptors with their effector ion channels in Purkinje cells

Metabotropic GABA_B receptors mediate slow inhibitory effects presynaptically and postsynaptically through the modulation of different effector signalling pathways. Here, we analysed the distribution of GABA_B receptors using highly sensitive SDS-digested freeze-fracture replica labelling in mouse cerebellar Purkinje cells. Immunoreactivity for GABA_B1 was observed on presynaptic and, more abundantly, on postsynaptic compartments, showing both scattered and clustered distribution patterns. Quantitative analysis of immunoparticles revealed a somato-dendritic gradient, with the density of immunoparticles increasing 26-fold from somata to dendritic spines. To understand the spatial relationship of GABA_B receptors with two key effector ion channels, the G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channel and the voltage-dependent Ca²⁺ channel, biochemical and immunohistochemical approaches were performed. Co-immunoprecipitation analysis demonstrated that GABA_B receptors co-assembled with GIRK and Ca_V2.1 channels in the cerebellum. Using double-labelling immunoelectron microscopic techniques, co-clustering between GABA_B1 and GIRK2 was detected in dendritic spines, whereas they were mainly segregated in the dendritic shafts. In contrast, co-clustering of GABA_B1 and Ca_V2.1 was detected in dendritic shafts but not spines. Presynaptically, although no significant co-clustering of GABA_B1 and GIRK2 or Ca_V2.1 channels was detected, inter-cluster distance for GABA_B1 and GIRK2 was significantly smaller in the active zone than in the dendritic shafts, and that for GABA_B1 and Ca_V2.1 was significantly smaller in the active zone than in the dendritic shafts and spines. Thus, GABA_B receptors are associated with GIRK and Ca_V2.1 channels in different subcellular compartments. These data provide a better framework for understanding the different roles played by GABA_B receptors and their effector ion channels in the cerebellar network.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Association of myelinated primary afferents impairment with mechanical allodynia in diabetic peripheral neuropathy: an experimental study in rats

To investigate the mechanisms underlying the efficacy of surgical treatment for painful diabetic peripheral neuropathy. Rats were initially divided into 3 groups (I, control rats, II, streptozotocin-induced diabetic rats, III, streptozotocin-induced diabetic rats with latex tube encircling the sciatic nerve without compression). When mechanical allodynia (MA) became stable in the third week, one third of group III rats were sacrificed and the remainder were further divided into subgroups depending on whether the latex tube was removed. Except for some rats in group III, all rats were sacrificed in the fifth week. Morphometric analysis of nerve fibers was performed. Expression level of GABA_B receptor protein in spinal dorsal horn was determined. Changes of GABA_B receptor within areas of primary afferents central terminal were identified. Chronic nerve compression caused by the interaction of diabetic nerves swelling and the encircling latex tube increased the incidence of MA in diabetic rats, and nerve decompression could ameliorate MA. In diabetic rats with MA, demyelination of myelinated fibers was noted and reduction of GABA_B receptor was mainly detected in the area of myelinated afferent central terminals. MA in DPN should be partially attributed to compression impairment of myelinated afferents, supporting the rationale for surgical decompression.

Presynaptic and extrasynaptic regulation of posterior nucleus of thalamus

The posterior nucleus of thalamus (PO) is a higher-order nucleus involved in sensorimotor processing, including nociception. An important characteristic of PO is its wide range of activity profiles that vary across states of arousal, thought to underlie differences in somatosensory perception subject to attention and degree of consciousness. Furthermore, PO loses the ability to downregulate its activity level in some forms of chronic pain, suggesting that regulatory mechanisms underlying the normal modulation of PO activity may be pathologically altered. However, the mechanisms responsible for regulating such a wide dynamic range of activity are unknown. Here, we test a series of hypotheses regarding the function of several presynaptic receptors on both GABAergic and glutamatergic afferents targeting PO in mouse, using acute slice electrophysiology. We found that presynaptic GABA_B receptors are present on both GABAergic and glutamatergic terminals in PO, but only those on GABAergic terminals are tonically active. We also found that release from GABAergic terminals, but not glutamatergic terminals, is suppressed by cholinergic activation and that a subpopulation of GABAergic terminals is regulated by cannabinoids. Finally, we discovered the presence of tonic currents mediated by extrasynaptic GABA_A receptors in PO that are heterogeneously distributed across the nucleus. Thus we demonstrate that multiple regulatory mechanisms concurrently exist in PO, and we propose that regulation of inhibition, rather than excitation, is the more consequential mechanism by which PO activity can be regulated.NEW & NOTEWORTHY The posterior nucleus of thalamus (PO) is a key sensorimotor structure, whose activity is tightly regulated by inhibition from several nuclei. Maladaptive plasticity in this inhibition leads to severe pathologies, including chronic pain. We reveal here, for the first time in PO, multiple regulatory mechanisms that modulate synaptic transmission within PO. These findings may lead to targeted therapies for chronic pain and other disorders.

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

Opposing Roles of Cholinergic and GABAergic Activity in the Insular Cortex and Nucleus Basalis Magnocellularis during Novel Recognition and Familiar Taste Memory Retrieval

Acetylcholine (ACh) is thought to facilitate cortical plasticity during memory formation and its release is regulated by the nucleus basalis magnocellularis (NBM). Questions remain regarding which neuronal circuits and neurotransmitters trigger activation or suppression of cortical cholinergic activity. During novel, but not familiar, taste consumption, there is a significant increase in ACh release in the insular cortex (IC), a highly relevant structure for taste learning. Here, we evaluate how GABA inhibition modulates cholinergic transmission and its involvement during taste novelty processing and familiar taste memory retrieval. Using saccharin as a taste stimulus in a taste preference paradigm, we examined the effects of injecting the GABAA receptor agonist muscimol or the GABAA receptor antagonist bicuculline into the IC or NBM during learning or retrieval of an appetitive taste memory on taste preference in male Sprague Dawley rats. GABAA receptor agonism and antagonism had opposite effects on cortical ACh levels in novel taste presentation versus familiar taste recognition and ACh levels were associated with the propensity to acquire or retrieve a taste memory. These results indicate that the pattern of cortical cholinergic and GABAergic neuroactivity during novel taste exposure is the opposite of that which occurs during familiar taste recognition and these differing neurotransmitter system states may enable different behavioral consequences. Divergences in ACh and GABA levels may produce differential alterations in excitatory and inhibitory neural processes within the cortex during acquisition and retrieval.

SIGNIFICANCE STATEMENT

During learning and recall, several brain structures act together. This work demonstrates interactions between cortical cholinergic and GABAergic systems during taste learning and memory retrieval. We found that the neuroactivity pattern during novel taste exposure is opposite that which occurs during familiar taste recognition. GABAA receptors must be inactive during novel tasting to enable new memory formation, but must be active and inhibiting acetylcholine release in the cortex to allow memory retrieval. These findings indicate that GABA inhibition modulates cholinergic transmission and that cholinergic-GABAergic system interactions are important during the transition from novel to familiar memory.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Sodium salicylate reduces the level of GABAB receptors in the rat's inferior colliculus

Previous studies have indicated that sodium salicylate (SS) can cause hearing abnormalities through affecting the central auditory system. In order to understand central effects of the drug, we examined how a single intraperitoneal injection of the drug changed the level of subunits of the type-B γ-aminobutyric acid receptor (GABAB receptor) in the rat's inferior colliculus (IC). Immunohistochemical and western blotting experiments were conducted three hours following a drug injection, as previous studies indicated that a tinnitus-like behavior could be reliably induced in rats within this time period. Results revealed that both subunits of the receptor, GABABR1 and GABABR2, reduced their level over the entire area of the IC. Such a reduction was observed in both cell body and neuropil regions. In contrast, no changes were observed in other brain structures such as the cerebellum. Thus, a coincidence existed between a structure-specific reduction in the level of GABAB receptor subunits in the IC and the presence of a tinnitus-like behavior. This coincidence likely suggests that a reduction in the level of GABAB receptor subunits was involved in the generation of a tinnitus-like behavior and/or used by the nervous system to restore normal hearing following application of SS.

Regulation of neuronal communication by G protein-coupled receptors

Neuronal communication plays an essential role in the propagation of information in the brain and requires a precisely orchestrated connectivity between neurons. Synaptic transmission is the mechanism through which neurons communicate with each other. It is a strictly regulated process which involves membrane depolarization, the cellular exocytosis machinery, neurotransmitter release from synaptic vesicles into the synaptic cleft, and the interaction between ion channels, G protein-coupled receptors (GPCRs), and downstream effector molecules. The focus of this review is to explore the role of GPCRs and G protein-signaling in neurotransmission, to highlight the function of GPCRs, which are localized in both presynaptic and postsynaptic membrane terminals, in regulation of intrasynaptic and intersynaptic communication, and to discuss the involvement of astrocytic GPCRs in the regulation of neuronal communication.

Allosteric Modulation of GABAA Receptors by an Anilino Enaminone in an Olfactory Center of the Mouse Brain

In an ongoing effort to identify novel drugs that can be used as neurotherapeutic compounds, we have focused on anilino enaminones as potential anticonvulsant agents. Enaminones are organic compounds containing a conjugated system of an amine, an alkene and a ketone. Here, we review the effects of a small library of anilino enaminones on neuronal activity. Our experimental approach employs an olfactory bulb brain slice preparation using whole-cell patch-clamp recording from mitral cells in the main olfactory bulb. The main olfactory bulb is a key integrative center in the olfactory pathway. Mitral cells are the principal output neurons of the main olfactory bulb, receiving olfactory receptor neuron input at their dendrites within glomeruli, and projecting glutamatergic axons through the lateral olfactory tract to the olfactory cortex. The compounds tested are known to be effective in attenuating pentylenetetrazol (PTZ) induced convulsions in rodent models. One compound in particular, KRS-5Me-4-OCF₃, evokes potent inhibition of mitral cell activity. Experiments aimed at understanding the cellular mechanism underlying the inhibitory effect revealed that KRS-5Me-4-OCF₃ shifts the concentration-response curve for GABA to the left. KRS-5Me-4-OCF₃ enhances GABA affinity and acts as a positive allosteric modulator of GABA_A receptors. Application of a benzodiazepine site antagonist blocks the effect of KRS-5Me-4-OCF₃ indicating that KRS-5Me-4-OCF₃ binds at the classical benzodiazepine site to exert its pharmacological action. This anilino enaminone KRS-5Me-4-OCF₃ emerges as a candidate for clinical use as an anticonvulsant agent in the battle against epileptic seizures.

Opposite role played by GABAA and GABAB receptors in the modulation of peristaltic activity in mouse distal colon

We investigated the role of GABA on intestinal motility using as model the murine distal colon. Effects induced by GABA receptors recruitment were examined in whole colonic segments and isolated circular muscle preparations to analyze their influence on peristaltic reflex and on spontaneous and neurally-evoked contractions. Using a modified Trendelenburg set-up, rhythmic peristaltic contractions were evoked by gradual distension of the colonic segments. Spontaneous and neurally-evoked mechanical activity of circular muscle strips were recorded in vitro as changes in isometric tension. GABA, at low concentrations (10-50 µM), potentiated peristaltic activity and the neural cholinergic contractions, whilst it, at higher concentrations (500 µM-1mM), had inhibitory effects. GABA excitatory effects were mimicked by muscimol, GABAA-receptor agonist, and prevented by bicuculline, GABAA-receptor antagonist, which per se reduced peristaltic activity and the cholinergic contractile responses. Inhibitory effects were mimicked by baclofen, GABAB-receptor agonist, and antagonized by phaclofen, GABAB-receptor antagonist and by hexamethonium, neural nicotinic receptor antagonist. Guanethidine was ineffective on GABA effects. Non-cholinergic responses were not affected by GABA agents. All drugs failed to affect the response to carbachol. Lastly, GABAC receptor agonist/antagonist had any effect on colonic motility. In conclusion, GABA in mouse distal colon is a modulator of peristaltic activity via the regulation of acetylcholine release from cholinergic neurons through interaction with GABAA or GABAB receptors. GABAA receptors are recruited at low GABA concentrations, increasing acetylcholine release and propulsive activity. At high GABA concentrations the activation of GABAB receptors overrides GABAA receptor effects, decreasing acetylcholine release and peristaltic activity.

Anticonvulsant and behavioral effects of GABA(B) receptor positive modulator CGP7930 in immature rats

Possible anticonvulsant action of GABAB receptor positive allosteric modulator CGP7930 was studied in cortical epileptic afterdischarges (ADs) in rat pups 12, 18, and 25 days old. Afterdischarges were induced by six series of stimulation of sensorimotor cortex, and CGP7930 (20 or 40 mg/kgi.p.) was administered after the first AD. In addition, the effects of CGP7930 on sensorimotor performance and behavior in open field and elevated plus maze were assessed. CGP7930 decreased duration of ADs in 12-day-old but not in older rats. Motor phenomena (movements accompanying stimulation and clonic seizures) were not changed. CGP7930 only moderately affected sensorimotor performance, altered slightly spontaneous behavior in the open field, and did not influence behavior in the elevated plus maze in terms of an adaptive form of learning or anxiety-like behavior. Marked anticonvulsant action with subtle deficits in sensorimotor performance in 12-day-old rats suggests a possible use of CGP7930 as an age-specific anticonvulsant.

Effects of the benzodiazepine GABAA α1-preferring ligand, 3-propoxy-β-carboline hydrochloride (3-PBC), on alcohol seeking and self-administration in baboons

RATIONALE

The various α subtypes of GABAA receptors have been strongly implicated in alcohol reinforcement and consumption.

OBJECTIVES

The effects of the GABAA α1-preferring ligand, 3-propoxy-β-carboline hydrochloride (3-PBC), on seeking and self-administration responses were evaluated in two groups of baboons trained under a 3-component chained schedule of reinforcement (CSR).

METHODS

Alcohol (4 % w/v; n = 5; alcohol group) or a preferred nonalcoholic beverage (n = 4; control group) was available for self-administration only in component 3 of the CSR. Responses in component 2 provided indices of motivation to drink (seeking). 3-PBC (1.0-30.0 mg/kg) and saline were administered before drinking sessions under both acute and 5-day dosing conditions.

RESULTS

Repeated, and not acute, doses of 3-PBC significantly decreased total self-administration responses (p < 0.05), volume consumed (p < 0.05), and gram per kilogram of alcohol (p < 0.05) in the alcohol group. In the control group, 5-day administration of 3-PBC significantly decreased total self-administration responses (p < 0.05) but produced nonsignificant decreases in volume consumed. Within-session pattern of drinking was characterized by a high level of drinking in the first 20 min of the session for both groups, which was significantly (p < 0.05) decreased by all doses of 3-PBC (1.0-18.0 mg/kg) only in the alcohol group. In contrast, the first drinking bout in the control group was only reduced at the highest doses of 3-PBC (10.0 and 18.0 mg/kg).

CONCLUSIONS

The results support the involvement of the GABAA α1 subtype receptor in alcohol reinforcement and consumption.

GABRA2 markers moderate the subjective effects of alcohol

Individual differences in subjective responses (SRs) to alcohol are moderated by genetic variants and may be risk factors for the development of alcohol use disorders. Variation in the GABA(A) α2 receptor subunit gene (GABRA2) has been associated with alcohol dependence (AD). Therefore, we examined whether individual differences in SRs, which reflect sensitivity to the effects of alcohol, are associated with variation in GABRA2. Sixty-nine healthy subjects (21-30 years) underwent a laboratory-based within-session cumulative oral alcohol dosing procedure, achieving a mean peak blood alcohol level of 100.4 mg/dl (standard error = 2.5). Subjective assessments were obtained throughout the session, including ascending and descending limbs of the alcohol curve. We genotyped single nucleotide polymorphisms (SNPs) across the chromosome 4 region spanning GABRA2 and analyzed the effect of genotype and haplotypes on subjective responses to alcohol. Population substructure was characterized through the use of ancestry informative markers. Individual SNP analysis demonstrated that carriers of the minor alleles for SNPs rs279858, rs279844, rs279845, rs279826, rs279828 and rs279836 had lower 'Negative' alcohol effects scores than individuals homozygous for the common allele at each SNP (P = 0.0060, P = 0.0035, P = 0.0045, P = 0.0043, P = 0.0037 and P = 0.0061, respectively). Haplotype effects of block 1 showed concordant results with SNPs in this block (P = 0.0492 and P = 0.0150 for haplotypes 1 and 4, respectively). The minor alleles for several of these SNPs have previously been associated with AD. Our findings provide further evidence that variation within GABRA2 is associated with attenuated negative responses to alcohol, a known risk factor for vulnerability to alcohol use disorders.

GABAB receptor activation attenuates the stimulant but not mesolimbic dopamine response to ethanol in FAST mice

Neural processes influenced by γ-aminobutyric acid B (GABA(B)) receptors appear to contribute to acute ethanol sensitivity, including the difference between lines of mice bred for extreme sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effect of ethanol. One goal of the current study was to determine whether selection of the FAST and SLOW lines resulted in changes in GABA(B) receptor function, since the lines differ in sensitivity to the GABA(B) receptor agonist baclofen and baclofen attenuates the stimulant response to ethanol in FAST mice. A second goal was to determine whether the baclofen-induced reduction in ethanol stimulation in FAST mice is associated with an attenuation of the mesolimbic dopamine response to ethanol. In Experiment 1, the FAST and SLOW lines were found to not differ in GABA(B) receptor function (measured by baclofen-stimulated [(35)S]GTPγS binding) in whole brain or in several regional preparations, except in the striatum in one of the two replicate sets of selected lines. In Experiment 2, baclofen-induced attenuation of the locomotor stimulant response to ethanol in FAST mice was not accompanied by a reduction in dopamine levels in the nucleus accumbens, as measured by microdialysis. These data suggest that, overall, GABA(B) receptor function does not play an integral role in the genetic difference in ethanol sensitivity between the FAST and SLOW lines. Further, although GABA(B) receptors do modulate the locomotor stimulant response to ethanol in FAST mice, this effect does not appear to be due to a reduction in tonic dopamine signaling in the nucleus accumbens.

The role of GABAergic signalling in stress-induced suppression of gonadotrophin-releasing hormone pulse generator frequency in female rats

Stress exerts profound inhibitory effects on reproductive function by suppressing the pulsatile release of gonadotrophin-releasing hormone (GnRH) and therefore luteinising hormone (LH). This effect is mediated in part via the corticotrophin-releasing factor (CRF) system, although another potential mechanism is via GABAergic signalling within the medial preoptic area (mPOA) because this has known inhibitory influences on the GnRH pulse generator and shows increased activity during stress. In the present study, we investigated the role of the preoptic endogenous GABAergic system in stress-induced suppression of the GnRH pulse generator. Ovariectomised oestradiol-replaced rats were implanted with bilateral and unilateral cannulae targeting toward the mPOA and lateral cerebral ventricle, respectively; blood samples (25 μl) were taken via chronically implanted cardiac catheters every 5 min for 6 h for the measurement of LH pulses. Intra-mPOA administration of the specific GABA(A) receptor antagonist, bicuculline (0.2 pmol each side, three times at 20-min intervals) markedly attenuated the inhibitory effect of lipopolysaccharide (LPS; 25 μg/kg i.v.) but not restraint (1 h) stress on pulsatile LH secretion. By contrast, restraint but not LPS stress-induced suppression of LH pulse frequency was reversed by application of the selective GABA(B) receptor antagonist, CGP-35348, into the mPOA (1.5 nmol each side, three times at 20-min intervals). However, intra-mPOA application of either bicuculline or CGP-35348 attenuated the inhibitory effect of CRF (1 nmol i.c.v.) on the pulsatile LH secretion. These data indicate a pivotal and differential role of endogenous GABAergic signalling in the mPOA with respect to mediating psychological and immunological stress-induced suppression of the GnRH pulse generator.

Defective dentate nucleus GABA receptors in essential tremor

The development of new treatments for essential tremor, the most frequent movement disorder, is limited by a poor understanding of its pathophysiology and the relative paucity of clinicopathological studies. Here, we report a post-mortem decrease in GABA(A) (35% reduction) and GABA(B) (22-31% reduction) receptors in the dentate nucleus of the cerebellum from individuals with essential tremor, compared with controls or individuals with Parkinson's disease, as assessed by receptor-binding autoradiography. Concentrations of GABA(B) receptors in the dentate nucleus were inversely correlated with the duration of essential tremor symptoms (r(2) = 0.44, P < 0.05), suggesting that the loss of GABA(B) receptors follows the progression of the disease. In situ hybridization experiments also revealed a diminution of GABA(B(1a+b)) receptor messenger RNA in essential tremor (↓27%). In contrast, no significant changes of GABA(A) and GABA(B) receptors (protein and messenger RNA), GluN2B receptors, cytochrome oxidase-1 or GABA concentrations were detected in molecular or granular layers of the cerebellar cortex. It is proposed that a decrease in GABA receptors in the dentate nucleus results in disinhibition of cerebellar pacemaker output activity, propagating along the cerebello-thalamo-cortical pathways to generate tremors. Correction of such defective cerebellar GABAergic drive could have a therapeutic effect in essential tremor.

GABAB receptor-positive modulators: brain region-dependent effects

This study examined the positive modulatory properties of 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) at γ-aminobutyric acid B (GABA(B)) receptors in different brain regions. Using quantitative autoradiography, we measured GABA(B) receptor-stimulated binding of guanosine 5'-O-(3-[³⁵S]thiotriphosphate) ([³⁵S]GTPγS) to G proteins in medial prefrontal cortex (mPFC), hippocampus, and cerebellum. CGP7930 and rac-BHFF enhanced baclofen-stimulated [³⁵S]GTPγS binding similarly in mPFC and hippocampus, but were more effective in cerebellum. CGP7930 (100 μM) increased [³⁵S]GTPγS binding stimulated by baclofen (30 μM) from 29 to 241% above basal in mPFC and from 13 to 1530% above basal in cerebellum. Likewise, rac-BHFF (10 μM) increased baclofen-stimulated [³⁵S]GTPγS binding more in cerebellum (from 13 to 1778% above basal) than in mPFC (from 29 to 514% above basal). rac-BHFF (10 μM) in combination with γ-hydroxybutyrate (20 mM) increased [³⁵S]GTPγS binding in cerebellum but not in mPFC. rac-BHFF also enhanced the effects of 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348). Consistent with its partial agonist properties, CGP35348 stimulated [³⁵S]GTPγS binding in mPFC when given alone (to 18% above basal), but less extensively than baclofen (140% above basal), and antagonized baclofen when given together. CGP35348 (1 mM) in combination with rac-BHFF (100 μM) produced an increase in [³⁵S]GTPγS binding that was larger in cerebellum (from 61 to 1260% above basal) than in mPFC (from 18 to 118% above basal). Taken together, the results show that GABA(B) receptor-positive modulators enhance [³⁵S]GTPγS binding stimulated by GABA(B) receptor agonists in a brain region-dependent manner. This regionally selective enhancement is further evidence of pharmacologically distinct GABA(B) receptor populations, possibly allowing for more selective therapeutic targeting of the GABA(B) system.

The level and distribution of the GABA(B)R2 receptor subunit in the rat's central auditory system

The GABA(B) receptor is important for the function of auditory neurons. We used Western blotting and immunohistochemical methods to examine the level and localization of GABA(B)R2, a required subunit of a functional GABA(B) receptor, in the rat's central auditory system. Results revealed that this subunit was expressed throughout the auditory system with the level being high in the layers I-V of the auditory cortex, medial geniculate nucleus, dorsomedial and lateral parts of the inferior colliculus, and the molecular and fusiform cell layers of the dorsal cochlear nucleus. Labeled cell bodies were found in all the areas showing immunoreactivity. Neuropil labeling was strong in areas with high overall levels of immunoreactivity. Regional distributions of the receptor subunit revealed clear boundaries of some auditory subnuclei including the dorsal and ventral cochlear nuclei and the lateral superior olivary nucleus. Differences in immunoreactivity were found between the central nucleus and the dorsal cortex of the inferior colliculus and between the dorsal and ventral parts of the ventral nucleus of the lateral lemniscus, although no clear boundaries were observed. No differences in immunoreactivity were found between the core and the belt areas of the auditory cortex and among the subdivisions of the medial geniculate nucleus. The regional distribution of the receptor subunit in auditory structures is consistent with inputs to these structures and the cellular localization of the receptor in auditory neurons supports the contribution of the GABA(B) receptor to synaptic responses in these neurons.

A substituted anilino enaminone acts as a novel positive allosteric modulator of GABA(A) receptors in the mouse brain

A small library of anilino enaminones was analyzed for potential anticonvulsant agents. We examined the effects of three anilino enaminones on neuronal activity of output neurons, mitral cells (MC), in an olfactory bulb brain slice preparation using whole-cell patch-clamp recording. These compounds are known to be effective in attenuating pentylenetetrazol-induced convulsions. Among the three compounds tested, 5-methyl-3-(4-trifluoromethoxy-phenylamino)-cyclohex-2-enone (KRS-5Me-4-OCF₃) showed potent inhibition of MC activity with an EC₅₀ of 24.5 μM. It hyperpolarized the membrane potential of MCs accompanied by suppression of spontaneous firing. Neither ionotropic glutamate receptor blockers nor a GABA(B) receptor blocker prevented the KRS-5Me-4-OCF(3)-evoked inhibitory effects. In the presence of GABA(A) receptor antagonists, KRS-5Me-4-OCF(3) completely failed to evoke inhibition of MC spiking activity, suggesting that KRS-5Me-4-OCF₃-induced inhibition may be mediated by direct action on GABA(A) receptors or indirect action through the elevation of tissue GABA levels. Neither vigabatrin (a selective GABA-T inhibitor) nor 1,2,5,6-tetrahydro-1-[2-[[(diphenylmethylene)amino]oxy]ethyl]-3-pyridinecarboxylic acid hydrochloride (NNC-711) (a selective inhibitor of GABA uptake by GABA transporter 1) eliminated the effect of KRS-5ME-4-OCF₃ on neuronal excitability, indicating that the inhibitory effect of the enaminone resulted from direct activation of GABA(A) receptors. The concentration-response curves for GABA are left-shifted by KRS-5Me-4-OCF₃, demonstrating that KRS-5Me-4-OCF₃ enhanced GABA affinity and acted as a positive allosteric modulator of GABA(A) receptors. The effect of KRS-5Me-4-OCF₃ was blocked by applying a benzodiazepine site antagonist, suggesting that KRS-5Me-4-OCF₃ binds at the classic benzodiazepine site to exert its pharmacological action. The results suggest clinical use of enaminones as anticonvulsants in seizures and as a potential anxiolytic in mental disorders.

Changes in NMDA receptor contribution to synaptic transmission in the brain in a rat model of glaucoma

In the age-related, blinding disease glaucoma, retinal ganglion cells (RGCs) degenerate, possibly affecting glutamatergic retinofugal transmission to the brain. The superior colliculus (SC) is a major central target of retinofugal axons in the rodent, a much used disease model. We investigated the contribution of NMDA-type glutamate receptors to retinocollicular transmission in a rat glaucoma model, using a SC brain slice preparation to determine the sensitivity of synaptic responses to the NMDAR antagonist D-AP5. At 32 weeks after induction of experimental glaucoma, but not earlier, there was an increase in NMDAR contribution to SC synaptic responses in slices receiving input from glaucomatous eyes. This suggests that there are changes in NMDAR function after RGC degeneration in experimental glaucoma, which may represent functional SC compensation through plasticity via NMDARs. This has implications for studies carried out using rodent glaucoma models, especially those evaluating potential treatment strategies, as it suggests that functional changes in the central visual system need to be considered in addition to those in the eye. Furthermore, the data underline the need for early therapeutic intervention in order to pre-empt subsequent central functional changes.