GABA in Paraventricular Nucleus Regulates Adipose Afferent Reflex in Rats

Background

Chemical stimulation of white adipose tissue (WAT) induces adipose afferent reflex (AAR), and thereby causes a general sympathetic activation. Paraventricular nucleus (PVN) is important in control of sympathetic outflow. This study was designed to investigate the role of γ-aminobutyric acid (GABA) in PVN in regulating the AAR.

Methodology/Principal Findings

Experiments were carried out in anesthetized rats. Renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) were continuously recorded. AAR was evaluated by the RSNA and MAP responses to electrical stimulation of the right epididymal WAT (eWAT) afferent nerve. Electrical stimulation of eWAT afferent nerve increase RSNA. Bilateral microinjection of the GABA_A receptor agonist isoguvacine or the GABA_B receptor agonist baclofen attenuated the AAR. The effect of isoguvacine on the AAR was greater than that of baclofen. The GABA_A receptor antagonist gabazine enhanced the AAR, while the GABA_B receptor antagonist CGP-35348 had no significant effect on the AAR. Bilateral PVN microinjection of vigabatrin, a selective GABA-transaminase inhibitor, to increase endogenous GABA levels in the PVN abolished the AAR. The inhibitory effect of vigabatrin on the AAR was attenuated by the pretreatment with gabazine or CGP-35348. Pretreatment with combined gabazine and CGP-35348 abolished the effects of vigabatrin.

Conclusions

Activation of GABA_A or GABA_B receptors in the PVN inhibits the AAR. Blockade of GABA_A receptors in the PVN enhances the AAR. Endogenous GABA in the PVN plays an important role in regulating the AAR.     

“Green odor” inhalation by stressed rat dams reduces behavioral and neuroendocrine signs of prenatal stress in the offspring

Chronic maternal stress during pregnancy results in the “prenatally stressed” offspring displaying behavioral and neuroendocrine alterations that persist into adulthood. We investigated how inhalation of green odor (a mixture of equal amounts of trans-2-hexenal and cis-3-hexenol) by stressed dams might alter certain indices of prenatal stress in their offspring. These indices were depression-like behavior (increased immobility time in the forced-swim test) and acute restraint stress-induced changes in hypothalamo-pituitary-adrenocortical (HPA) axis activity [plasma corticosterone (CORT) and ACTH levels and the number of Fos-immunoreactive cells in the hypothalamic paraventricular nucleus (an index of neuronal activity)]. Pregnant rats were exposed to restraint stress for 60 min/day for 10 days (gestational days 10-19). The prenatally stressed offspring exhibited significant increases in depression-like behavior and in restraint stress-induced ACTH, CORT, and Fos responses, unless their dam had been exposed to green odor. The behavioral effect of the odor was also seen in offspring that were fostered by unstressed dams. The results obtained in the dams themselves were as follows. In vehicle-exposed stressed dams, but not in green odor-exposed ones, total body and adrenal weights were significantly decreased or increased, respectively. Depression-like behavior was not observed in the vehicle-exposed stressed dams themselves. Green odor inhalation prevented the impairment of maternal behavior induced by restraint stress. Thus, exposure of dams to stress may affect both the fetal brain and fetal HPA axis, and also maternal behavior, leading to altered behavioral and neuroendocrine responses in the offspring. Such effects may be prevented by the stressed dams inhaling green odor.     

Stimulation of TM3 Leydig cell proliferation via GABA<Subscript>A</Subscript> receptors: A new role for testicular GABA

The neurotransmitter gamma-aminobutyric acid (GABA) and subtypes of GABA receptors were recently identified in adult testes. Since adult Leydig cells possess both the GABA biosynthetic enzyme glutamate decarboxylase (GAD), as well as GABA_A and GABA_B receptors, it is possible that GABA may act as auto-/paracrine molecule to regulate Leydig cell function. The present study was aimed to examine effects of GABA, which may include trophic action. This assumption is based on reports pinpointing GABA as regulator of proliferation and differentiation of developing neurons via GABA_A receptors. Assuming such a role for the developing testis, we studied whether GABA synthesis and GABA receptors are already present in the postnatal testis, where fetal Leydig cells and, to a much greater extend, cells of the adult Leydig cell lineage proliferate. Immunohistochemistry, RT-PCR, Western blotting and a radioactive enzymatic GAD assay evidenced that fetal Leydig cells of five-six days old rats possess active GAD protein, and that both fetal Leydig cells and cells of the adult Leydig cell lineage possess GABA_A receptor subunits. TM3 cells, a proliferating mouse Leydig cell line, which we showed to possess GABA_A receptor subunits by RT-PCR, served to study effects of GABA on proliferation. Using a colorimetric proliferation assay and Western Blotting for proliferating cell nuclear antigen (PCNA) we demonstrated that GABA or the GABA_A agonist isoguvacine significantly increased TM3 cell number and PCNA content in TM3 cells. These effects were blocked by the GABA_A antagonist bicuculline, implying a role for GABA_A receptors. In conclusion, GABA increases proliferation of TM3 Leydig cells via GABA_A receptor activation and proliferating Leydig cells in the postnatal rodent testis bear a GABAergic system. Thus testicular GABA may play an as yet unrecognized role in the development of Leydig cells during the differentiation of the testicular interstitial compartment.     

Transient gastric irritation in the neonatal rats leads to changes in hypothalamic CRF expression, depression- and anxiety-like behavior as adults

Aims

A disturbance of the brain-gut axis is a prominent feature in functional bowel disorders (such as irritable bowel syndrome and functional dyspepsia) and psychological abnormalities are often implicated in their pathogenesis. We hypothesized that psychological morbidity in these conditions may result from gastrointestinal problems, rather than causing them.

Methods

Functional dyspepsia was induced by neonatal gastric irritation in male rats. 10-day old male Sprague-Dawley rats received 0.1% iodoacetamide (IA) or vehicle by oral gavage for 6 days. At 8–10 weeks of age, rats were tested with sucrose preference and forced-swimming tests to examine depression-like behavior. Elevated plus maze, open field and light-dark box tests were used to test anxiety-like behaviors. ACTH and corticosterone responses to a minor stressor, saline injection, and hypothalamic CRF expression were also measured.

Results

Behavioral tests revealed changes of anxiety- and depression-like behaviors in IA-treated, but not control rats. As compared with controls, hypothalamic and amygdaloid CRF immunoreactivity, basal levels of plasma corticosterone and stress-induced ACTH were significantly higher in IA-treated rats. Gastric sensory ablation with resiniferatoxin had no effect on behaviors but treatment with CRF type 1 receptor antagonist, antalarmin, reversed the depression-like behavior in IA-treated rats

Conclusions

The present results suggest that transient gastric irritation in the neonatal period can induce a long lasting increase in depression- and anxiety-like behaviors, increased expression of CRF in the hypothalamus, and an increased sensitivity of HPA axis to stress. The depression-like behavior may be mediated by the CRF1 receptor. These findings have significant implications for the pathogenesis of psychological co-morbidity in patients with functional bowel disorders.     

Gamma-aminobutyric acid (GABA) receptor mediates suanzaorentang, a traditional Chinese herb remedy, -induced sleep alteration

Summary The sedative-hypnotic medications, including benzodiazepines and non-benzodiazepines, are the most common treatments for insomnia. However, concerns regarding patterns of inappropriate use, dependence and adverse effects have led to caution in prescribing those sedative-hypnotic medications. On the other hand, a traditional Chinese herb remedy, suanzaorentang, has been efficiently and widely used in clinic for insomnia relief without severe side effects in Asia. Although suanzaorentang has been reported to improve sleep disruption in insomniac patients, its mechanism is still unclear. The present study was designed to elucidate the effects of oral administration of suanzaorentang on physiological sleep-wake architectures and its underlying mechanism in rats. We found that oral administration of suanzaorentang at the beginning of the dark onset dose-dependently increased non-rapid eye movement sleep (NREMS) during the dark period, but had no significant effect on rapid eye movement sleep (REMS). Our results also indicated that intracerebroventricular (ICV) administration of γ-aminobutyric acid (GABA) receptor type A antagonist, bicuculline, significantly blocked suanzaorentang-induced enhancement in NREMS during the dark period, but GABA_B receptor antagonist, 2-hydroxysaclofen had no effect. These results implicated that this traditional Chinese herb remedy, suanzaorentang increases spontaneous sleep activity and its effects may be mediated through the GABA_A receptors, but not GABA_B receptors.     

Modulation of the arcuate nucleus–medial preoptic nucleus lordosis regulating circuit: A role for GABAB receptors

Estradiol rapidly activates a microcircuit in the arcuate nucleus of the hypothalamus (ARH) that is needed for maximal female sexual receptivity. Membrane estrogen receptor-α complexes with and signals through the metabotropic glutamate receptor-1a stimulating NPY release within the ARH activating proopiomelanocortin (POMC) neurons. These POMC neurons project to the medial preoptic nucleus (MPN) and release β-endorphin. Estradiol treatment induces activation/internalization of MPN μ-opioid receptors (MOR) to inhibit lordosis. Estradiol membrane action modulates ARH gamma-aminobutyric acid receptor-B (GABA_B) activity. We tested the hypothesis that ARH GABA_B receptors mediate estradiol-induced MOR activation and facilitation of sexual receptivity. Double-label immunohistochemistry revealed expression of GABA_B receptors in NPY, ERα and POMC expressing ARH neurons. Approximately 70% of POMC neurons expressed GABA_B receptors. Because estradiol initially activates an inhibitory circuit and maintains activation of this circuit, the effects of blocking GABA_B receptors were evaluated before estradiol benzoate (EB) treatment and after at the time of lordosis testing. Bilateral infusions of the GABA_B receptor antagonist, CGP52432, into the ARH prior to EB treatment of ovariectomized rats prevented estradiol-induced activation/internalization of MPN MOR, and the rats remained unreceptive. However, in EB-treated rats, bilateral CGP52432 infusions 30 min before behavior testing attenuated MOR internalization and facilitated lordosis. These results indicated that GABA_B receptors were located within the lordosis-regulating ARH microcircuit and are necessary for activation and maintenance of the estradiol inhibition of lordosis behavior. Although GABA_B receptors positively influence estradiol signaling, they negatively regulate lordosis behavior since GABA_B activity maintains the estradiol-induced inhibition.     

GABAB antagonists: Resolution,absolute stereochemistry,and pharmacology of (R)- and (S)-phaclofen

Phaclofen, which is the phosphonic acid analogue of the GABA_B agonist (RS)-3-(4-chlorophenyl)-4-aminobutyric acid (baclofen), is a GABA_B antagonist. As part of our studies on the structural requirements for activation and blockade of GABA_B receptors, we have resolved phaclofen using chiral chromatographic techniques. The absolute stereochemistry of (?)-(R)-phaclofen was established by X-ray crystallographic analysis. (?)-(R)-Phaclofen was shown to inhibit the binding of [³H]-(R)-baclofen to GABA_B receptor sites on rat cerebellar membranes (IC₅₀ = 76 ± 13 μM), whereas (+)-(S)-phaclofen was inactive in this binding assay (IC₅₀ > 1000 μM). (?)-(R)-Phaclofen (200 μM) was equipotent with (RS)-phaclofen (400 μM) in antagonizing the action of baclofen in rat cerebral cortical slices, while (+)-(S)-phaclofen (200 μM) was inactive. The structural similarity of the agonist (R)-baclofen and the antagonist (?)-(R)-phaclofen suggests that these ligands interact with the GABA_B receptor sites in a similar manner. Thus, it may be concluded that the different pharmacological effects of these compounds essentially result from the different spatial and proteolytic properties of their acid groups. © 1994 Wiley-Liss, Inc.     

The GABAB receptor associates with regulators of G-protein signaling 4 protein in the mouse prefrontal cortex and hypothalamus

Regulators of G-protein signaling (RGS) proteins regulate certain G-protein-coupled receptor (GPCR)-mediated signaling pathways. The GABA_B receptor (GABA_BR) is a GPCR that plays a role in the stress response. Previous studies indicate that acute immobilization stress (AIS) decreases RGS4 in the prefrontal cortex (PFC) and hypothalamus (HY) and suggest the possibility of a signal complex composed of RGS4 and GABA_BR. Therefore, in the present study, we tested whether RGS4 associates with GABA_BR in these brain regions. We found the co-localization of RGS4 and GABA_BR subtypes in the PFC and HY using double immunohistochemistry and confirmed a direct association between GABA_B2R and RGS4 proteins using co-immunoprecipitation. Furthermore, we found that AIS decreased the amount of RGS4 bound to GABA_B2R and the number of double-positive cells. These results indicate that GABA_BR forms a signal complex with RGS4 and suggests that RGS4 is a regulator of GABA_BR. [BMB Reports 2014; 47(6): 324-329]     

Dopamine D3 Receptor Mediates Preadolescent Stress-Induced Adult Psychiatric Disorders

Several studies have shown that repeated stressful experiences during childhood increases the likelihood of developing depression- and anxiety-related disorders in adulthood; however, the underlying mechanisms are not well understood. We subjected drd3-EGFP and drd3-null mice to daily, two hour restraint stress episodes over a five day period during preadolescence (postnatal day 35 to 39), followed by social isolation. When these mice reached adulthood (post-natal day > 90), we assessed locomotor behavior in a novel environment, and assessed depression-related behavior in the Porsolt Forced Swim test. We also measured the expression and function of dopamine D3 receptor in limbic brain areas such as hippocampus, nucleus accumbens and amygdala in control and stressed drd3-EGFP mice in adulthood. Adult male mice subjected to restraint stress during preadolescence exhibited both anxiety- and depression-related behaviors; however, adult female mice subjected to preadolescent restraint stress exhibited only depression-related behaviors. The development of preadolescent stress-derived psychiatric disorders was blocked by D3 receptor selective antagonist, SB 277011-A, and absent in D3 receptor null mice. Adult male mice that experienced stress during preadolescence exhibited a loss of D3 receptor expression and function in the amygdala but not in hippocampus or nucleus accumbens. In contrast, adult female mice that experienced preadolescent stress exhibited increased D3 receptor expression in the nucleus accumbens but not in amygdala or hippocampus. Our results suggest that the dopamine D3 receptor is centrally involved in the etiology of adult anxiety- and depression-related behaviors that arise from repeated stressful experiences during childhood.     

Maternal glucocorticoid deficit affects hypothalamic-pituitary-adrenal function and behavior of rat offspring

Detrimental consequences of prenatal stress include increased hypothalamic-pituitary-adrenal (HPA) function, anxiety and depression-like behavior in adult offspring. To identify the role of maternal corticosterone milieu in the fetal programming of adult function, we measured these same behavioral and hormonal endpoints after maternal adrenalectomy (ADX) and replacement with normal or moderately high levels of corticosterone (CORT). Adult male and female offspring exhibited differing HPA responses to maternal ADX. In female offspring of ADX mothers, exaggerated plasma ACTH stress responses were reversed by the higher, but not the lower, dose of maternal CORT. In contrast, male offspring of both ADX and ADX dams with higher CORT replacement showed exaggerated ACTH stress responses. Hypothalamic glucocorticoid receptor (GR) expression was decreased in these latter groups, while hippocampal GR increased only in the ADX offspring. Activity of young offspring of ADX dams replaced with the higher dose of CORT decreased in the open field test of exploration/anxiety, while immobility behavior of adult offspring in the forced swim test of depression increased following maternal ADX or higher levels of CORT replacement. Interestingly, for some measures, none or moderately high CORT replacement resulted in similar deficits in this study. These findings are in accord with consequences of prenatal stress or prenatal dexamethasone exposure, suggesting that a common mechanism may underlie the effects of too low or too high maternal glucocorticoids on adult HPA function and behavior.     

Taurine release modified by GABAergic agents in hippocampal slices from adult and developing mice

Summary. In order to characterize the possible regulation of taurine release by GABAergic terminals, the effects of several agonists and antagonists of GABA receptors on the basal and K⁺-stimulated release of [³H]taurine were investigated in hippocampal slices from adult (3-month-old) and developing (7-day-old) mice using a superfusion system. Taurine release was concentration-dependently potentiated by GABA, which effect was reduced by phaclofen, saclofen and (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) at both ages, suggesting regulation by both GABA_B and GABA_C receptors. The involvement of GABA_A receptors could not be excluded since the antagonist bicuculline was able to affect both basal and K⁺-evoked taurine release. Furthermore, several GABA_B receptor effectors were able to inhibit K⁺-stimulated taurine release in the adults, while the GABA_C receptor agonists trans-4-aminocrotonic acid (TACA) and cis-4-aminocrotonic acid (CACA) potentiated this release. The potentiation of taurine release by agents acting on the three types of GABA receptors in both adult and developing hippocampus further indicates the involvement of transporters operating in an outward direction. This inference is corroborated by the moderate but significant inhibition of taurine uptake by the same compounds. Received June 28, 1999, Accepted August 31, 1999     

Bidirectional Crosstalk between Stress-Induced Gastric Ulcer and Depression under Chronic Stress

Stress contributes to a variety of diseases and disorders such as depression and peptic ulcer. The present study aimed to investigate the correlation between stress ulcer and depression in pathogenesis and treatment by using chronic stress depression (CSD), chronic psychological stress ulcer (CPSU) and water immersion restrain stress models in rats. Our data showed that the ulcer index of the animals after CSD exposure was significantly higher than that of controls. Depression-like behaviors were observed in rat after CPSU exposure. Fluoxetine hydrochloride significantly reduced the ulcer index of rats exposed to CPSU stress, while ranitidine inhibited depression-like behavior of the animals in CSD group. The ulcer index of rats administered with mifepristone after CPSU stress was markedly reduced compared to CPSU group, although there was no significant difference in the depression-like behavior between mifepristone-treated CSD group and naive controls. We also found that the rats exposed to CPSU or CSD stress displayed a lower level of corticosterone than naive controls, however, the acute stress (AS) group showed an opposite result. Additionally, in order to study the relevance of H₂ receptors and depression, we treated the CSD group with cimetidine and famotidine respectively. The data showed that cimetidine inhibited depression-like behavior in CSD rats, and famotidine had no impact on depression. Overall our data suggested that the hypothalamic-pituitary-adrenal (HPA) axis dysfunction may be the key role in triggering depression and stress ulcer. Acid-suppressing drugs and antidepressants could be used for treatment of depression and stress ulcer respectively. The occurrence of depression might be inhibited by blocking the central H₂ receptors.     

Gestational Hypoxia Induces Sex-Differential Methylation of Crhr1 Linked to Anxiety-like Behavior

Stress during gestation increases vulnerability to disease and changes behavior in offspring. We previously reported that hypoxia and restraint during pregnancy sensitized the hypothalamic–pituitary–adrenal (HPA) axis and induced anxiety-like behavior in the adult offspring. Here, we report that gestational intermittent hypoxia (GIH) elicited a sex-dependent anxiety-like behavior in male P90 offspring and activation of corticotropin-releasing hormone (CRH) and CRH type-1 receptor (CRHR1) mRNA in the hypothalamic paraventricular nucleus (PVN) and in male E19 hypothalamus. These linked to demethylation at several specific sites of CpG island of Crhr1 promoter in P90 PVN and E19 embryo hypothalamus in GIH groups. Crhr1 DNA demethylation is more crucial in CpG island 1 than island 2 for activation of CRHR1 mRNA. DNMT3b is required for the Crhr1 DNA methylation than DNMT1 and DNMT3a in increased CRHR1 mRNA. We first address a novel hypothesis that GIH-induced male-sex-dependent demethylation at CpG sites of Crhr1 DNA in promoter triggers elevation of CRHR1 mRNA in PVN and anxiety-like behavior in adult offspring.     

STRUCTURE OF GABAB RECEPTORS IN RAT RETINA

ABSTRACT

In the search for yet unknown subtypes of GABA_B receptors, the subunit architecture of GABA_B receptors in the retina was analyzed using selective antisera. Immunopurification of the splice variants GABA_B1a and GABA_B1b demonstrated that both were associated with GABA_B2. Quantitative immunoprecipitation experiments indicated that practical the entire GABA_B receptor population in the retina consists of the receptor subtypes GABAB_1a/GABA_B2 and GABA_B1b/GABA_B2, although low levels of GABA_B1c/GABA_B2 cannot be excluded. The data rule out the existence of GABA_B receptors containing the splice variants GABA_B1d and GABA_B1e. Moreover, no evidence for homomeric GABA_B1 receptors was found. Among the splice variants, GABA_B1a is by far the predominant one in neonatal and adult retina, whereas GABA_B1b is expressed only late in postnatal development and in the adult retina. Since GABA_B1a is expressed at high levels before functional synapses are formed, this specific receptor subtype might be involved in the maturation of the retina. Finally, subcellular fractionation demonstrated that GABA_B1a, but not GABA_B1b, is present in postsynaptic densities, suggesting a differential pre- and postsynaptic localisation of both splice variants.     

Synthesis of the Nanomolar Photoaffinity GABAB Receptor Ligand CGP 71872 Reveals Diversity in the Tissue Distribution of GABAB Receptor Forms

A radioiodinated probe, [¹²⁵I]-CGP 71872, containing an azido group that can be photoactivated, was synthesized and used to characterize GABA_B receptors. Photoaffinity labeling experiments using crude membranes prepared from rat brain revealed two predominant ligand binding species at 130 and 100 kDa believed to represent the long (GABA_BR1a) and short (GABA_BR1b) forms of the receptor. Indeed, these ligand binding proteins were immunoprecipitated using a GABA_B receptor-specific antibody confirming the receptor specificity of the photoaffinity probe. Most convincingly, [¹²⁵I]-CGP 71872 binding was competitively inhibited in a dose-dependent manner by cold CGP 71872, GABA, saclofen, (−)-baclofen, (+)-baclofen and ( )-glutamic acid with a rank order and stereospecificity characteristic of the GABA_B receptor. Photoaffinity labeling experiments revealed that the recombinant GABA_BR2 receptor does not bind [¹²⁵I]-CGP 71872, providing surprising and direct evidence that CGP 71872 is a GABA_BR1 selective antagonist. Photoaffinity labeling experiments using rat tissues showed that both GABA_BR1a and GABA_BR1b are co-expressed in the brain, spinal cord, stomach and testis, but only the short GABA_BR1b receptor form was detected in kidney and liver whereas the long GABA_BR1a form was selectively expressed in the adrenal gland, pituitary, spleen and prostate. We report herein the synthesis and biochemical characterization of the nanomolar affinity [¹²⁵I]-CGP 71872 and CGP 71872 GABA_BR1 ligands, and differential tissue expression of the long GABA_BR1a and short GABA_BR1b receptor forms in rat and dog.     

Baclofen,a GABABR Agonist,Ameliorates Immune-Complex Mediated Acute Lung Injury by Modulating Pro-Inflammatory Mediators

Immune-complexes play an important role in the inflammatory diseases of the lung. Neutrophil activation mediates immune-complex (IC) deposition-induced acute lung injury (ALI). Components of gamma amino butyric acid (GABA) signaling, including GABA B receptor 2 (GABA_BR2), GAD65/67 and the GABA transporter, are present in the lungs and in the neutrophils. However, the role of pulmonary GABA_BR activation in the context of neutrophil-mediated ALI has not been determined. Thus, the objective of the current study was to determine whether administration of a GABA_BR agonist, baclofen would ameliorate or exacerbate ALI. We hypothesized that baclofen would regulate IC-induced ALI by preserving pulmonary GABA_BR expression. Rats were subjected to sham injury or IC-induced ALI and two hours later rats were treated intratracheally with saline or 1 mg/kg baclofen for 2 additional hours and sacrificed. ALI was assessed by vascular leakage, histology, TUNEL, and lung caspase-3 cleavage. ALI increased total protein, tumor necrosis factor α (TNF-α and interleukin-1 receptor associated protein (IL-1R AcP), in the bronchoalveolar lavage fluid (BALF). Moreover, ALI decreased lung GABA_BR2 expression, increased phospho-p38 MAPK, promoted IκB degradation and increased neutrophil influx in the lung. Administration of baclofen, after initiation of ALI, restored GABA_BR expression, which was inhibited in the presence of a GABA_BR antagonist, CGP52432. Baclofen administration activated pulmonary phospho-ERK and inhibited p38 MAPK phosphorylation and IκB degradation. Additionally, baclofen significantly inhibited pro-inflammatory TNF-α and IL-1βAcP release and promoted BAL neutrophil apoptosis. Protective effects of baclofen treatment on ALI were possibly mediated by inhibition of TNF-α- and IL-1β-mediated inflammatory signaling. Interestingly, GABA_BR2 expression was regulated in the type II pneumocytes in lung tissue sections from lung injured patients, further suggesting a physiological role for GABA_BR2 in the repair process of lung damage. GABA_BR2 agonists may play a potential therapeutic role in ALI.     

Alleviation by GABA<Subscript>B</Subscript> Receptors of Neurotoxicity Mediated by Mitochondrial Permeability Transition Pore in Cultured Murine Cortical Neurons Exposed to <Emphasis Type="Italic">N</Emphasis>-Methyl-<Emphasis Type="SmallCaps">d</Emphasis>-aspartate

Mitochondrial permeability transition pore (PTP) is supposed to at least in part participate in molecular mechanisms underlying the neurotoxicity seen after overactivation of N-methyl-d-aspartate (NMDA) receptor (NMDAR) in neurons. In this study, we have evaluated whether activation of GABA_B receptor (GABA_BR), which is linked to membrane G protein-coupled inwardly-rectifying K⁺ ion channels (GIRKs), leads to protection of the NMDA-induced neurotoxicity in a manner relevant to mitochondrial membrane depolarization in cultured embryonic mouse cortical neurons. The cationic fluorescent dye 3,3′-dipropylthiacarbocyanine was used for determination of mitochondrial membrane potential. The PTP opener salicylic acid induced a fluorescence increase with a vitality decrease in a manner sensitive to the PTP inhibitor ciclosporin, while ciclosporin alone was effective in significantly preventing both fluorescence increase and viability decrease by NMDA as seen with an NMDAR antagonist. The NMDA-induced fluorescence increase and viability decrease were similarly prevented by pretreatment with the GABA_BR agonist baclofen, but not by the GABA_AR agonist muscimol, in a fashion sensitive to a GABA_BR antagonist. Moreover, the GIRK inhibitor tertiapin canceled the inhibition by baclofen of the NMDA-induced fluorescence increase. These results suggest that GABA_BR rather than GABA_AR is protective against the NMDA-induced neurotoxicity mediated by mitochondrial PTP through a mechanism relevant to opening of membrane GIRKs in neurons.     

Differential Regulation of GABAB Receptor Trafficking by Different Modes of N-methyl-d-aspartate (NMDA) Receptor Signaling

Inhibitory GABA_B receptors (GABA_BRs) can down-regulate most excitatory synapses in the CNS by reducing postsynaptic excitability. Functional GABA_BRs are heterodimers of GABA_B1 and GABA_B2 subunits and here we show that the trafficking and surface expression of GABA_BRs is differentially regulated by synaptic or pathophysiological activation of NMDA receptors (NMDARs). Activation of synaptic NMDARs using a chemLTP protocol increases GABA_BR recycling and surface expression. In contrast, excitotoxic global activation of synaptic and extrasynaptic NMDARs by bath application of NMDA causes the loss of surface GABA_BRs. Intriguingly, exposing neurons to extreme metabolic stress using oxygen/glucose deprivation (OGD) increases GABA_B1 but decreases GABA_B2 surface expression. The increase in surface GABA_B1 involves enhanced recycling and is blocked by the NMDAR antagonist AP5. The decrease in surface GABA_B2 is also blocked by AP5 and by inhibiting degradation pathways. These results indicate that NMDAR activity is critical in GABA_BR trafficking and function and that the individual subunits can be separately controlled to regulate neuronal responsiveness and survival.     

Neuropeptide Y-induced feeding is dependent on GABAA receptors in neonatal chicks

In mammals and birds, neuropeptide Y (NPY) and gamma-aminobutyric acid (GABA) are found in brain areas known to be involved in the control of ingestive behavior and act to increase voluntary food intake. In rats, significant evidence suggest a functional and behavioral interaction between NPY and GABA mediated transmission in various brain regions, including the arcuate and paraventricular nuclei of the hypothalamus which can be important in the regulation of feeding behavior. In the present study, the effect of intracerebroventricular (ICV) administration of NPY and GABA receptor antagonists on food intake was examined in neonatal chicks. The ICV injection of NPY strongly stimulated food intake while co-administration of NPY and picrotoxin, a GABA_A antagonist, (but not CGP54626, a GABA_B antagonist) weakened food intake induced by NPY. These results suggest that central NPY stimulates food intake in neonatal chicks by interaction with the GABAergic system via GABA_A receptors.