Distinct properties of murine α5 γ‐aminobutyric acid type a receptors revealed by biochemical fractionation and mass spectroscopy

γ‐Aminobutyric acid type A receptors (GABA_ARs) that contain the α5 subunit are expressed predominantly in the hippocampus, where they regulate learning and memory processes. Unlike conventional postsynaptic receptors, GABA_ARs containing the α5 subunit (α5 GABA_ARs) are localized primarily to extrasynaptic regions of neurons, where they generate a tonic inhibitory conductance. The unique characteristics of α5 GABA_ARs have been examined with pharmacological, immunostaining, and electrophysiological techniques; however, little is known about their biochemical properties. The aim of this study was to modify existing purification and enrichment techniques to isolate α5 GABA_ARs preferentially from the mouse hippocampus and to identify the α5 subunit by using tandem mass spectroscopy (MS/MS). The results showed that the detergent solubility of the α5 subunits was distinct from that of α1 and α2 subunits, and the relative distribution of the α5 subunits in Triton X‐100‐soluble fractions was correlated with that of the extracellular protein radixin but not with that of the postsynaptic protein gephyrin. Mass spectrometry identified the α5 subunit and showed that this subunit associates with multiple α, β, and γ subunits, but most frequently the β3 subunit. Thus, the α5 subunits coassemble with similar subunits as their synaptic counterparts yet have a distinct detergent solubility profile. Mass spectroscopy now offers a method for detecting and characterizing factors that confer the unique detergent solubility and possibly cellular location of α5 GABA_ARs in hippocampal neurons. © 2009 Wiley‐Liss, Inc.     

α4βδ‐GABAA receptors in dorsal hippocampal CA1 of adolescent female rats traffic to the plasma membrane of dendritic spines following voluntary exercise and contribute to protection of animals from activity‐based anorexia through localization at excitatory synapses

In hippocampal CA1 of adolescent female rodents, α4βδ‐GABA_A receptors (α4βδ‐GABA_ARs) suppress excitability of pyramidal neurons through shunting inhibition at excitatory synapses. This contributes to anxiolysis of stressed animals. Socially isolated adolescent female rats with 8 days of wheel access, the last 4 days of which entail restricted food access, have been shown to exhibit excessive exercise, choosing to run instead of eat (activity‐based anorexia [ABA]). Upregulation of α4βδ‐GABA_ARs in the dorsal hippocampal CA1 (DH), seen among some ABA animals, correlates with suppression of excessive exercise. We used electron microscopic immunocytochemistry to show that exercise alone (EX), but not food restriction alone (FR), also augments α4βδ‐GABA_AR expression at axospinous excitatory synapses of the DH (67%, P = 0.027), relative to socially isolated controls without exercise or food restriction (CON). Relative to CON, ABA animals' synaptic α4βδ‐GABA_AR elevation was modestly elevated (37%), but this level correlated strongly and negatively with individual differences in ABA vulnerability—i.e., food restriction–evoked hyperactivity (Pearson R = ?0.902, P = 0.002) and weight changes (R = 0.822, P = 0.012). These correlations were absent from FR and EX brains or ventral hippocampus of ABA brains. Comparison to CON of α4βδ‐GABA_AR location in the DH indicated that ABA induces trafficking of α4βδ‐GABA_AR from reserve pools in spine cytoplasm to excitatory synapses. Pair‐housing CON animals reduced cytoplasmic α4βδ‐GABA_AR without reducing synaptic α4βδ‐GABA_AR. Thus, exercise induces trafficking of α4βδ‐GABA_ARs to excitatory synapses, while individual differences in ABA vulnerability are linked most strongly to trafficking of α4βδ‐GABA_ARs in the reverse direction—from excitatory synapses to the reserve pool during co‐occurring food restriction. © 2017 Wiley Periodicals, Inc.     

The α5GABAA receptor modulates the induction of long‐term potentiation at ventral but not dorsal CA1 hippocampal synapses

The hippocampal synapses display conspicuous ability for long‐term plasticity which is thought to underlie learning and memory. Growing evidence shows that this ability differs along the long axis of the hippocampus, with the ventral CA1 hippocampal synapses displaying remarkably lower ability for long‐term potentiation (LTP) compared with their dorsal counterpart when activated with high‐frequency stimulation. Here, we show that low frequency, 10 Hz stimulation induced LTP more reliably in dorsal than in ventral CA1 field. Blockade of alpha5 subunit‐containing GABA_A receptors eliminated the difference between dorsal and ventral hippocampus. We propose that α5GABA_A receptor‐mediated activity plays a crucial role in regulating the threshold for induction of LTP especially at the ventral CA1 hippocampal synapses. This might have important implications for the functional specialization along the hippocampus. Synapse, 2014. © 2014 Wiley Periodicals, Inc.     

Direct binding of GABAA receptor β2 and β3 subunits to gephyrin

GABAergic transmission is essential to brain function, and a large repertoire of GABA type A receptor (GABA_AR) subunits is at a neuron's disposition to serve this function. The glycine receptor (GlyR)‐associated protein gephyrin has been shown to be essential for the clustering of a subset of GABA_AR. Despite recent progress in the field of gephyrin‐dependent mechanisms of postsynaptic GABA_AR stabilisation, the role of gephyrin in synaptic GABA_AR localisation has remained a complex matter with many open questions. Here, we analysed comparatively the interaction of purified rat gephyrin and mouse brain gephyrin with the large cytoplasmic loops of GABA_AR α1, α2, β2 and β3 subunits. Binding affinities were determined using surface plasmon resonance spectroscopy, and showed an ~ 20‐fold lower affinity of the β2 loop to gephyrin as compared to the GlyR β loop–gephyrin interaction. We also probed in vivo binding in primary cortical neurons by the well‐established use of chimaeras of GlyR α1 that harbour respective gephyrin‐binding motifs derived from the different GABA_AR subunits. These studies identify a novel gephyrin‐binding motif in GABA_AR β2 and β3 large cytoplasmic loops.     

Adolescent‐onset GABAAα1 silencing regulates reward‐related decision making

The GABA_A receptor mediates fast, inhibitory signaling, and cortical expression of the α1 subunit increases during postnatal development. Certain pathological stimuli such as stressors or prenatal cocaine exposure can interfere with this process, but causal relationships between GABA_Aα1 deficiency and complex behavioral outcomes remain unconfirmed. We chronically reduced GABA_Aα1 expression selectively in the medial prefrontal cortex (prelimbic subregion) of mice using viral‐mediated gene silencing of Gabra1. Adolescent‐onset Gabra1 knockdown delayed the acquisition of a cocaine‐reinforced instrumental response but spared cocaine seeking in extinction and in a cue‐induced reinstatement procedure. To determine whether response acquisition deficits could be associated with impairments in action–outcome associative learning and memory, we next assessed behavioral sensitivity to instrumental contingency degradation. In this case, the predictive relationship between familiar actions and their outcomes is violated. Adolescent‐onset knockdown, although not adult‐onset knockdown, delayed the expression of goal‐directed response strategies in this task, resulting instead in inflexible habit‐like modes of response. Thus, the maturation of medial prefrontal cortex GABA_Aα1 systems during adolescence appears necessary for goal‐directed reward‐related decision making in adulthood. These findings are discussed in the light of evidence that prolonged Gabra1 deficiency may impair synaptic plasticity.     

Chronic ethanol administration alters the modulatory effect of 5α-pregnan-3α-ol-20-one on the binding characteristics of various radioligands of GABAA receptors

In this study, we investigated the modulatory effect of 5α-pregnan-3α-ol-20-one, a neurosteroid, on the binding characteristics of [ ]flunitrazepam (2 nM), [ ]muscimol (5 nM), and 4 nM [ ]t-butylbicyclophosphorothionate (TBPS) in cerebral cortex, cerebellum, and hippocampus of control, ethanol-dependent, and ethanol-withdrawn rats. 5α-Pregnan-3α-ol-20-one potentiated the binding of [ ]flunitrazepam and [ ]muscimol in all the rat brain regions investigated in this study. There was a significant increase in the maximal potentiation of [ ]flunitrazepam as well as [ ]muscimol binding (E_max) in the ethanol-dependent rat cerebellum as compared to control group (p<0.025). Furthermore, 5α-pregnan-3α-ol-20-one elicited a biphasic response, i.e., it potentiated the binding of [ ]TBPS at lower concentrations (100 nM) and inhibited the binding at higher concentrations (>100 nM). There was a significant higher inhibition of [ ]TBPS binding (−E_max) by 5α-pregnan-3α-ol-20-one in the hippocampus of ethanol-dependent as well as ethanol-withdrawn rats (p<0.025). These observations suggest that the neurosteroid binding site associated with the γ-aminobutyric acid_A (GABA_A) receptors in cerebellum and hippocampus plays an important role during ethanol-dependence and ethanol-withdrawal, and some of the changes following ethanol dependence and its withdrawal may be mediated through the neurosteroid binding site.     

Roles for γ‐aminobutyric acid in the development of the paraventricular nucleus of the hypothalamus

The development of the hypothalamic paraventricular nucleus (PVN) involves several factors that work together to establish a cell group that regulates neuroendocrine functions and behaviors. Several molecular markers were noted within the developing PVN, including estrogen receptors (ER), neuronal nitric oxide synthase (nNOS), and brain‐derived neurotrophic factor (BDNF). By contrast, immunoreactive γ‐aminobutyric acid (GABA) was found in cells and fibers surrounding the PVN. Two animal models were used to test the hypothesis that GABA works through GABA_A and GABA_B receptors to influence the development of the PVN. Treatment with bicuculline to decrease GABA_A receptor signaling from embryonic day (E) 10 to E17 resulted in fewer cells containing immunoreactive (ir) ERα in the region of the PVN vs. control. GABA_BR1 receptor subunit knockout mice were used to examine the PVN at P0 without GABA_B signaling. In female but not male GABA_BR1 subunit knockout mice, the positions of cells containing ir ERα shifted from medial to lateral compared with wild‐type controls, whereas the total number of ir ERα‐containing cells was unchanged. In E17 knockout mice, ir nNOS cells and fibers were spread over a greater area. There was also a significant decrease in ir BDNF in the knockout mice in a region‐dependent manner. Changes in cell position and protein expression subsequent to disruption of GABA signaling may be due, in part, to changes in nNOS and BDNF signaling. Based on the current study, the PVN can be added as another site where GABA exerts morphogenetic actions in development. J. Comp. Neurol. 518:2710–2728, 2010. © 2010 Wiley‐Liss, Inc.     

Partial inactivation of GABAA receptors containing the α5 subunit affects the development of adult‐born dentate gyrus granule cells

Alterations of neuronal activity due to changes in GABA_A receptors (GABA_AR) mediating tonic inhibition influence different hippocampal functions. Gabra5‐null mice and α5 subunit^(H105R) knock‐in mice exhibit signs of hippocampal dysfunction, but are capable of improved performance in several learning and memory tasks. Accordingly, alleviating abnormal GABAergic tonic inhibition in the hippocampal formation by selective α5‐GABA_AR modulators represents a possible therapeutic approach for several intellectual deficit disorders. Adult neurogenesis in the dentate gyrus is an important facet of hippocampal plasticity; it is regulated by tonic GABAergic transmission, as shown by deficits in proliferation, migration and dendritic development of adult‐born neurons in Gabra4‐null mice. Here, we investigated the contribution of α5‐GABA_ARs to granule cell development, using retroviral vectors expressing eGFP for labeling precursor cells in the subgranular zone. Global α5‐GABA_AR knockout (α5‐KO) mice showed no alterations in migration and morphological development of eGFP‐positive granule cells. However, upregulation of α1 subunit‐immunoreactivity was observed in the hippocampal formation and cerebral cortex. In contrast, partial gene inactivation in α5‐heterozygous (α5‐het) mice, as well as single‐cell deletion of Gabra5 in newborn granule cells from α5‐floxed mice, caused severe alterations of migration and dendrite development. In α5‐het mice, retrovirally mediated overexpression of Cdk5 resulted in normal migration and dendritic branching, suggesting that Cdk5 cooperates with α5‐GABA_ARs to regulate neuronal development. These results show that minor imbalance of α5‐GABA_AR‐mediated transmission may have major consequences for neuronal plasticity; and call for caution upon chronic therapeutic use of negative allosteric modulators acting at these receptors.     

Relative preservation of GABAA receptor β-subunit immunoreactivity in the hippocampus in mesial temporal sclerosis

The pattern of cell loss and changes in the expression of inhibitory and excitatory receptors in the subzones of the hippocampus in patients with pharmacoresistant (drug-refractory) complex partial seizures was studied. Neuronal densities were measured in the mid-hippocampus in temporal lobectomy (n = 10) and total hemispherectomy (n = 2) specimens from 12 patients with mesial temporal sclerosis (MTS), in temporal lobectomy (n = 6) and hemispherectomy (n = 5) samples from 11 epileptic patients without MTS, and in six autopsy brains from patients without neurological disease. No differences in hippocampal cell densities were found between the autopsy and the non-MTS group. The MTS group, compared with the non-MTS or the autopsy group, showed a 95% decrease in neuronal density in CA1, a 70% decrease in cornu ammonis (CA)2, an 85% decrease in CA3, and an 80% decrease in CA4. The proportion of neurons staining positively with a polyclonal antibody for the α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) glutamate receptor subunits GluR2 and GluR3 was increased to 32% in the CA1 region in the MTS group, compared with 21% in the non-MTS hippocampi. This difference was, however, just at the level of statistical significance (P = 0.054). Elsewhere in the hippocampus no significant change was observed. By contrast, the proportion of cells staining positively with a monoclonal antibody to the β-subunit of the GABA_A benzodiazepine (BZ) receptor was markedly increased in the MTS hippocampi, from 13% in non-MTS CA1 to 50% in MTS CA1 region; and from 26% in non-MTS CA3 to 63% in MTS CA3. Although previous studies (in vivo and ex vivo) have emphasized a loss of BZ receptors in temporal lobe epilepsy, this study shows that the expression of the GABA_A receptor immunopositive population appears to be increased relative to the neuronal density.     

Baicalein reduces β‐amyloid and promotes nonamyloidogenic amyloid precursor protein processing in an Alzheimer's disease transgenic mouse model

Baicalein, a flavonoid isolated from the roots of Scutellaria baicalensis, is known to modulate γ‐aminobutyric acid (GABA) type A receptors. Given prior reports demonstrating benefits of GABA_A modulation for Alzheimer's disease (AD) treatment, we wished to determine whether this agent might be beneficial for AD. CHO cells engineered to overexpress wild‐type amyloid precursor protein (APP), primary culture neuronal cells from AD mice (Tg2576) and AD mice were treated with baicalein. In the cell cultures, baicalein significantly reduced the production of β‐amyloid (Aβ) by increasing APP α‐processing. These effects were blocked by the GABA_A antagonist bicuculline. Likewise, AD mice treated daily with i.p. baicalein for 8 weeks showed enhanced APP α‐secretase processing, reduced Aβ production, and reduced AD‐like pathology together with improved cognitive performance. Our findings suggest that baicalein promotes nonamyloidogenic processing of APP, thereby reducing Aβ production and improving cognitive performance, by activating GABA_A receptors. © 2013 Wiley Periodicals, Inc.     

GABAA and GABAB receptor dysregulation in superior frontal cortex of subjects with schizophrenia and bipolar disorder

Schizophrenia and bipolar disorder are complex psychiatric disorders that affect millions of people worldwide. Evidence from gene association and postmortem studies has identified abnormalities of the gamma‐aminobutyric acid (GABA) signaling system in both disorders. Abnormal GABAergic signaling and transmission could contribute to the symptomatology of these disorders, potentially through impaired gamma oscillations which normally occur during cognitive processing. In the current study, we examined the protein expression of 14 GABA_A and two GABA_B receptor subunits in the superior frontal cortex of subjects with schizophrenia, bipolar disorder, and healthy controls. Analyses of Variance (ANOVAs) identified significant group effects for protein levels for the α1, α6, β1, β3, δ, ?, and π GABA_A receptor subunits and R1 and R2 GABA_B receptor subunits. Follow‐up t tests confirmed changes for these subunits in subjects with schizophrenia, subjects with bipolar disorder, or both groups. Alterations in stoichiometry of GABA receptor subunits could result in altered ligand binding, transmission, and pharmacology of GABA receptors in superior frontal cortex. Thus, impaired GABAergic transmission may negatively contribute to symptoms such as anxiety or panic as well as impaired learning and information processing, all of which are disrupted in schizophrenia and bipolar disorder. Taken together, these results provide additional evidence of GABAergic receptor abnormalities in these disorders.     

Methamphetamine‐induced up‐regulation of α2/δ subunit of voltage‐gated calcium channels is regulated by DA receptors

This study was carried out to determine the roles of dopamine D1 and D2 receptors on the up‐regulation of α₂/δ subunit of voltage‐gated Ca²⁺ channels (VGCCs) induced by methamphetamine (METH). In the conditioned place preference paradigm, METH‐induced place preference suppressed with gabapentin, an antagonist for α₂/δ subunit. Under these conditions, the increase in α₂/δ subunit expression was found in the frontal cortex and limbic forebrain. In addition, the METH‐induced place preference was significantly attenuated by dopamine D1 and D2 receptor antagonists, SCH23390 and sulpiride, respectively. The expression of α₂/δ subunit protein and its mRNA was significantly enhanced in the METH‐treated cortical neurons. These increases in protein and mRNA of α₂/δ subunit were completely abolished by SCH23390 and sulpiride with simultaneous exposure to METH. These findings indicate that up‐regulation of α₂/δ subunit is regulated through the activation of dopamine D1 and D2 receptors during METH treatment. Synapse 64:822–828, 2010. © 2010 Wiley‐Liss, Inc.     

Neuroprotective effect of vitamin C against the ethanol and nicotine modulation of GABAB receptor and PKA‐α expression in prenatal rat brain

Prenatal ethanol exposure has various deleterious effects on neuronal development and can induce various defects in developing brain, resulting in fetal alcohol syndrome (FAS). γ‐Aminobutyric acid (GABA_B) receptor (R) is known to play an important role during the development of the central nervous system (CNS). Our study was designed to investigate the effect of ethanol (100 mM), nicotine (50 μM) (for 30 min and 1 h), vitamin C (vitC, 0.5 mM), ethanol plus vitC, and nicotine plus vitC on expression level of GABA_B1, GABA_B2R, and protein kinase A‐α (PKA) in prenatal rat cortical and hippocampal neurons at gestational days (GD) 17.5. The results showed that, upon ethanol and nicotine exposure, GABA_B1 and GABA_B2R protein expression increased significantly in the cortex and hippocampus for a short (30 min) and long term (1 h), whereas only GABA_B2R subunit was decreased upon nicotine exposure for a long term in the cortex. Furthermore, PKA expression in cortex and hippocampus increased with ethanol exposure during short term, whereas long‐term exposure results increased in cortex and decreased in hippocampus. Moreover, the cotreatment of vitC with ethanol and nicotine showed significantly decreased expression of GABA_B1, GABA_B2R, and PKA in cortex and hippocampus for a long‐term exposure. Mitochondrial membrane potential, Fluoro‐jade‐B, and propidium iodide staining were used to elucidate possible neurodegeneration. Our results suggest the involvement of GABA_BR and PKA in nicotine and ethanol‐mediated neurodevelopmental defects and the potential use of vitC as a effective protective agent for FAS‐related deficits. Synapse 64:467–477, 2010. © 2010 Wiley‐Liss, Inc.     

Activation of α1‐adrenoceptors enhances excitatory synaptic transmission via a pre‐ and postsynaptic protein kinase C‐dependent mechanism in the medial prefrontal cortex of rats

The physiological effects of α1‐adrenoceptors (α1‐ARs) have been examined in many brain regions. However, little is known about the mechanism of modulation on synaptic transmission by α1‐ARs in the medial prefrontal cortex (mPFC). The present study investigated how α₁‐AR activation regulates glutamatergic synaptic transmission in layer V/VI pyramidal cells of the rat mPFC. We found that the α₁‐AR agonist phenylephrine (Phe) induced a significant enhancement of the amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs). The facilitation effect of Phe on the frequency of mEPSCs involved a presynaptic protein kinase C‐dependent pathway. Phe produced a significant enhancement on the amplitude of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor (AMPA‐R)‐ and N‐methyl‐d ‐aspartic acid receptor (NMDA‐R)‐mediated evoked excitatory postsynaptic currents (eEPSCs). Phe enhanced inward currents evoked by puff application of glutamate or NMDA. The Phe‐induced facilitation of AMPA‐R‐ and NMDA‐R‐mediated eEPSCs required, in part, postsynaptic G_q, phospholipase C and PKC. These findings suggest that α₁‐AR activation facilitates excitatory synaptic transmission in mPFC pyramidal cells via both pre‐ and post‐synaptic PKC‐dependent mechanisms.     

Distribution of α1A adrenergic receptor m RNA in the rat brain visualized by in situ hybridization

Norepinephrine has been implicated in a number of physiological, behavioral, and cellular modulatory processes in the brain, and many of these modulatory effects are attributable to α₁ adrenergic receptors. At least three α₁ receptor subtypes have been identified by molecular criteria, designated α_1A, α_1B, and α_1D. The distributions of α_1B and α_1D receptor mRNA expression in rat brain have been described previously, but the cDNA for the rat α_1A receptor has only recently been cloned and characterized. In the present study, we used a radiolabelled riboprobe derived from the rat α_1A receptor cDNA to describe the distribution of α_1A message expression in the rat brain. The highest levels of α_1A adrenergic receptor mRNA expression were seen in the olfactory bulb, tenia tectae, horizontal diagonal band/magnocellular preoptic area, zona incerta, ventromedial hypothalamus, lateral mammillary nuclei, ventral dentate gyrus, piriform cortex, medial and cortical amygdala, magnocellular red nuclei, pontine nuclei, superior and lateral vestibular nuclei, brainstem reticular nuclei, and several cranial nerve motor nuclei. Dual in situ hybridization combining a radioactive riboprobe for choline acetyltransferase mRNA with a digoxigenin-labeled α_1A riboprobe in the fifth and seventh cranial nerve motor nuclei showed that the α_1A mRNA is expressed in cholinergic motor neurons. Prominent α_1A hybridization signal was also seen in the neocortex, claustrum, lateral amygdala, ventral cochlear nucleus, raphe magnus, and in the ventral horn of thoracic spinal cord. This overall pattern of expression, considered in comparison with that previously described for the other α₁ adrenergic receptor subtypes, may shed light on the different roles of the α₁ receptors in mediating the neuromodulatory effects of norepinephrine in processes such as arousal, neuroendocrine control, sensorimotor regulation, and the stress response. J. Comp. Neurol. 386:358-378, 1997. © 1997 Wiley-Liss, Inc.     

α2‐Adrenergic drugs modulate the binding of [18F]fallypride to dopamine D2/3 receptors in striatum of living mouse

Aim. To test for α₂ adrenergic modulation of dopamine D_2/3 receptor availability in striatum of living mice using the high‐affinity ligand [¹⁸F]fallypride and microPET. Methods. Groups of anesthetized mice were pretreated with saline, the α₂‐agonist clonidine (1 mg/kg), and the α₂‐antagonists RX821002 (1 mg/kg) and yohimbine (1 mg/kg). Dynamic microPET recordings lasting 120 min were then initiated upon i.v. tracer injection of [¹⁸F]fallypride. Parametric maps of [¹⁸F]fallypride binding potential (BP_ND) were calculated using the Logan method, with cerebellum serving as the reference region. Results. Mean striatal [¹⁸F]fallypride BP_ND was 10.6 ± 1.7 in the saline control animals, 8.9 ± 1.7 (?16%; P < 0.05) in the RX821002 group, 8.3 ± 2.6 (?22%; P < 0.05) in the yohimbine group and 10.3 ± 2.2 (n.s.) in the clonidine group. Conclusions. These findings are consistent with a tonic inhibition of dopamine release by α₂ adrenergic receptors, such that α₂ blockade increased the competition from endogenous dopamine at D_2/3 receptors, thus reducing the [¹⁸F]fallypride BP_ND by about 20%. Absent effects of clonidine suggest a ceiling effect in the tonic inhibition of dopamine release. This in vivo PET evidence for α₂/dopaminergic interaction may be relevant to putative actions of atypical antipsychotic medications via adrenergic receptors. Synapse, 2010. © 2010 Wiley‐Liss, Inc.     

A critical role of TRPM2 channel in Aβ42‐induced microglial activation and generation of tumor necrosis factor‐α

Amyloid β (Aβ)‐induced neuroinflammation plays an important part in Alzheimer's disease (AD). Emerging evidence supports a role for the transient receptor potential melastatin‐related 2 (TRPM2) channel in Aβ‐induced neuroinflammation, but how Aβ induces TRPM2 channel activation and this relates to neuroinflammation remained poorly understood. We investigated the mechanisms by which Aβ₄₂ activates the TRPM2 channel in microglial cells and the relationships to microglial activation and generation of tumor necrosis factor‐α (TNF‐α), a key cytokine implicated in AD. Exposure to 10–300 nM Aβ₄₂ induced concentration‐dependent microglial activation and generation of TNF‐α that were ablated by genetically deleting (TRPM2 knockout ;TRPM2‐KO) or pharmacologically inhibiting the TRPM2 channel, revealing a critical role of this channel in Aβ₄₂‐induced microglial activation and generation of TNF‐α. Mechanistically, Aβ₄₂ activated the TRPM2 channel via stimulating generation of reactive oxygen species (ROS) and activation of poly(ADPR) polymerase‐1 (PARP‐1). Aβ₄₂‐induced generation of ROS and activation of PARP‐1 and TRPM2 channel were suppressed by inhibiting protein kinase C (PKC) and NADPH oxidases (NOX). Aβ₄₂‐induced activation of PARP‐1 and TRPM2 channel was also reduced by inhibiting PYK2 and MEK/ERK. Aβ₄₂‐induced activation of PARP‐1 was attenuated by TRPM2‐KO and moreover, the remaining PARP‐1 activity was eliminated by inhibiting PKC and NOX, but not PYK2 and MEK/ERK. Collectively, our results suggest that PKC/NOX‐mediated generation of ROS and subsequent activation of PARP‐1 play a role in Aβ₄₂‐induced TRPM2 channel activation and TRPM2‐dependent activation of the PYK2/MEK/ERK signalling pathway acts as a positive feedback to further facilitate activation of PARP‐1 and TRPM2 channel. These findings provide novel insights into the mechanisms underlying Aβ‐induced AD‐related neuroinflammation.     

Reduction of predator odor-induced anxiety in mice by the neurosteroid 3α-hydroxy-4-pregnen-20-one (3αHP)

The effects of the centrally produced allylic neurosteroid, 3α-hydroxy-4-pregnen-20-one (3αHP), on the responses of male mice to an aversive, anxiety-inducing, predator (cat) odor were examined in an odor preference test. Control untreated mice displayed an anxiogenic response to the cat odor, spending a minimal amount of time in a Y-maze in the vicinity of the cat odor. Intracerebroventricular (i.c.v.) administrations of 3αHP had an anxiolytic action, resulting in significant dose-related (0.01–1.0 μg) increases in the amount of time spent in the proximity of the cat odor. These anxiolytic effects of 3αHP were stereospecific, with the stereoisomer, 3β-hydroxy-4-pregnen-20-one (3βHP) having no significant effects on odor preferences. The analgesic, morphine, also had no significant effects on the response to cat odor indicating that the anxiolytic actions of 3αHP were unlikely to be related to any analgesic effects. The effects of 3αHP were significantly reduced by peripheral administrations of the GABA_A antagonists, bicuculline and picrotoxin, but were unaffected by either the benzodiazepine antagonist, Ro 15-1788, or the opiate antagonist, naloxone. These results indicate that the allylic neurosteroid 3αHP has anxiolytic actions involving interactions with the GABA_A receptor.