GABAB receptor modulation of rapid inhibitory and excitatory neurotransmission from subfornical organ and other afferents to median preoptic nucleus neurons

Mechanisms of GABA-mediated inhibition of the angiotensin II-induced cytosolic Ca2+ increase in rat subfornical organ neurons

Neurons in the subfornical organ (SFO) sense both neurotransmitters and circulating humoral factors such as angiotensin II (AII) and atrial natriuretic peptide (ANP), and regulate multiple physiological functions including drinking behavior. We recently reported that AII at nanomolar concentrations induced a persistent [Ca²⁺]_i increase in acutely dissociated SFO neurons and that this effect of AII was reversibly inhibited by GABA. In the present study, we studied the inhibitory mechanism of GABA using Ca²⁺ imaging and patch-clamp electrophysiology. The AII-induced persistent [Ca²⁺]_i increase was inhibited by GABA in more than 90% of AII-responsive neurons and by other two SFO inhibitory ligands, ANP and galanin, in about 60 and 30% of neurons respectively. The inhibition by GABA was mimicked by the GABA_A and GABA_B receptor agonists muscimol and baclofen. The involvement of both GABA receptor subtypes was confirmed by reversal of the GABA-mediated inhibition only when the GABA_A and GABA_B receptors antagonists bicuculline methiodide and CGP55845 were both present. The GABA_B agonist baclofen rapidly and reversibly inhibited voltage-gated Ca²⁺ channel (VGCC) currents recorded in response to depolarizing pulses in voltage-clamp electrophysiology using Ba²⁺ as a charge carrier (I_Ba). Baclofen inhibition of I_Ba was antagonized by CGP55845, confirming GABA_B receptor involvement; was reduced by N-ethylmaleimide, suggesting downstream Gi-mediated actions; and was partially removed by a large prepulse, indicating voltage-dependency. The magnitude of I_Ba inhibition by baclofen was reduced by the application of selective blockers for N-, P/Q-, and L-type VGCCs (ω-conotoxin GVIA, ω-agatoxin IVA, and nifedipine respectively). Overall, our study indicates that GABA inhibition of the AII-induced [Ca²⁺]_i increase is mediated by both GABA_A and GABA_B receptors, and that GABA_B receptors associated with Gi proteins suppress Ca²⁺ entry through VGCCs in SFO neurons.

Ginsenoside Rb1 regulates prefrontal cortical GABAergic transmission in MPTP-treated mice

Parkinson's disease (PD) is a common neurodegenerative disease, featured by motor deficits and non-motor symptoms such as cognitive impairment, and malfunction of gamma-aminobutyric acid (GABA) mediated inhibitory transmission plays an important role in PD pathogenesis. The ginsenoside Rb1 molecule, a major constituent of the extract from the Ginseng root, has been demonstrated to ameliorate motor deficits and prevent dopaminergic neuron death in PD. However, whether Rb1 can regulate GABAergic transmission in PD-associated deficits and its underlying mechanisms are still unclear. In this study, we explored the effects of Rb1 on the GABAergic synaptic transmission in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. We demonstrated that Rb1 can bind with GABA_ARα1 and increase its expression in the SH-SY5Y cells and in the prefrontal cortex (PFC) of MPTP model in vitro and in vivo. Furthermore, Rb1 can promote prefrontal cortical GABA level and GABAergic transmission in MPTP-treated mice. We also revealed that Rb1 may suppress presynaptic GABA_BR1 to enhance GABA release and GABA_A receptor-mediated inhibitory transmission. In addition, Rb1 attenuated MPTP-induced dysfunctional gait dynamic and cognitive impairment, and this neuroprotective mechanism possibly involved regulating prefrontal cortical GABAergic transmission. Thus, Rb1 may serve as a potential drug candidate for the treatment of PD.

Persistent cytosolic Ca2+ increase induced by angiotensin II at nanomolar concentrations in acutely dissociated subfornical organ (SFO) neurons of rats

It is known that angiotensin II (AII) is sensed by subfornical organ (SFO) to induce drinking behaviors and autonomic changes. AII at picomolar concentrations have been shown to induce Ca²⁺ oscillations and increase in the amplitude and frequency of spontaneous Ca²⁺ oscillations in SFO neurons. The present study was conducted to examine effects of nanomolar concentrations of AII using the Fura-2 Ca²⁺-imaging technique in acutely dissociated SFO neurons. AII at nanomolar concentrations induced an initial [Ca²⁺]_i peak followed by a persistent [Ca²⁺]_i increase lasting for longer than 1 hour. By contrast, [Ca²⁺]_i responses to 50 mM K⁺, maximally effective concentrations of glutamate, carbachol, and vasopressin, and AII given at picomolar concentrations returned to the basal level within 20 min. The AII-induced [Ca²⁺]_i increase was blocked by the AT1 antagonist losartan. However, losartan had no effect when added during the persistent phase. The persistent phase was suppressed by extracellular Ca2+ removal, significantly inhibited by blockers of L and P/Q type Ca²⁺ channels , but unaffected by inhibition of Ca²⁺ store Ca²⁺ ATPase. The persistent phase was reversibly suppressed by GABA and inhibited by CaMK and PKC inhibitors. These results suggest that the persistent [Ca²⁺]_i increase evoked by nanomolar concentrations of AII is initiated by AT1 receptor activation and maintained by Ca²⁺ entry mechanisms in part through L and P/Q type Ca²⁺ channels, and that CaMK and PKC are involved in this process. The persistent [Ca²⁺]_i increase induced by AII at high pathophysiological levels may have a significant role in altering SFO neuronal functions.

AT1a influences GABAA-mediated inhibition through regulation of KCC2 expression

The median preoptic nucleus (MnPO) is an integrative site involved in body fluid homeostasis, cardiovascular control, thermoregulation, and sleep homeostasis. Angiotensin II (ANG II), a neuropeptide shown to have excitatory effects on MnPO neurons, is of particular interest with regard to its role in body fluid homeostasis and cardiovascular control. The present study investigated the role of angiotensin type 1a (AT1a) receptor activation on neuronal excitability in the MnPO. Male Sprague-Dawley rats were infused with an adeno-associated virus with an shRNA against the AT1a receptor or a scrambled control. In vitro loose-patch voltage-clamp recordings of spontaneous action potential activity were made from labeled MnPO neurons in response to brief focal application of ANG II or the GABAA receptor agonist muscimol. Additionally, tissue punches from MnPO were taken to asses mRNA and protein expression. AT1a receptor knockdown neurons were insensitive to ANG II and showed a marked reduction in GABAA-mediated inhibition. The reduction in GABAA-mediated inhibition was not associated with reductions in mRNA or protein expression of GABAA β-subunits. Knockdown of the AT1a receptor was associated with a reduction in the potassium-chloride cotransporter KCC2 mRNA as well as a reduction in pS940 KCC2 protein. The impaired GABAA-mediated inhibition in AT1a knockdown neurons was recovered by bath application of phospholipase C and protein kinase C activators. The following study indicates that AT1a receptor activation mediates the excitability of MnPO neurons, in part, through the regulation of KCC2. The regulation of KCC2 influences the intracellular [Cl-] and the subsequent efficacy of GABAA-mediated currents.

Involvement of median preoptic nucleus and medullary noradrenergic neurons in cardiovascular and sympathetic responses of hemorrhagic rats

The infusion of hypertonic saline solution (HSS) is known to be beneficial to the treatment of hypovolemic hemorrhage (HH). The central mechanism of HSS-induced cardiovascular and autonomic recovery of animals subjected to HH remains unclear. Hence, the present study evaluated the involvement of median preoptic nucleus (MnPO) and medullary noradrenergic neurons (A1 and A2) in HSS-induced cardiovascular and sympathetic responses in hemorrhagic rats. The wistar rats were subjected to specific lesion of noradrenergic neurons through the nanoinjections of anti-DβH-saporin into caudal ventrolateral medulla (A1 neurons) and nucleus of the solitary tract (A2 neurons). After recovery, mean arterial pressure (MAP) and renal sympathetic nervous activity were recorded. The HH was performed through blood withdrawal until a MAP of 60 mmHg was attained. In sham rats, HSS infusion (3M NaCl) reestablished MAP without change in HH-induced sympathoinhibition. The muscimol (agonist of GABAA receptor) was nanoinjected in MnPO during HH and MnPO inhibition abolished the recovery of MAP and HSS-induced sympathoinhibition. Simultaneous lesions of A1 and A2 abolished MAP restoration and sympathoinhibition after HSS infusion. These results suggest that the recovery of MAP and HSS-induced sympathoinhibition in hemorrhaged rats depend on intact neural projections from A1 and A2 to MnPO.

Median preoptic nucleus excitatory neurotransmitters in the maintenance of hypertensive state

The crucial role of the median preoptic nucleus (MnPO) in the maintenance of hydroelectrolytic balance and autonomic regulation have been highlighted. Recently, the participation of the MnPO in the control of sympathetic nerve activity was demonstrated in essential hypertension model. However, peculiarities on the neurochemical changes underlying the differential role of MnPO during hypertension remain to be clarified. Therefore, this study aimed to investigate the main excitatory pathways that modulate MnPO neurons in hypertensive rats. Spontaneously hypertensive rats (SHR) and rats submitted previously to the Goldblatt protocol (two kidneys; one clip; 2K1C) were used. Rats of both groups (250 to 350 g, n = 6) were anesthetized with urethane (1.2 g/kg,i.v.) and instrumented to record mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA). Nanoinjection (100 nl) of saline (NaCl, 150 mM), losartan (AT1 receptor antagonist; 10 mM) and kynurenic acid (glutamate receptor antagonist; 50 mM) into the MnPO were performed. In 2K1C rats, glutamatergic blockade promoted decreases in MAP and RSNA (-19.1 ± 0.9 mmHg, -21.6 ± 2.8%, p < 0.05) when compared to saline (-0.4 ± 0.6 mmHg, 0.2 ± 0.7%, p < 0.05). Angiotensinergic inhibition also reduced these parameters (-11.5 ± 1.2 mmHg, -10.5 ± 1.0%, p < 0.05) in 2K1C. In SHR, Kynurenic acid nanoinjections produced hypotension and sympathoinhibition (-21.0 ± 2.5 mmHg, -24.7 ± 2.4%, p < 0.05), as well losartan nanoinjections (-9.7 ± 1.2 mmHg; p < 0.05) and RSNA (-12.0 ± 2.4%, p < 0.05). These findings support the conclusion that a tonic excitatory neurotransmission exerted by angiotensin II, and mostly by glutamate in the MnPO could participate in the modulation of blood pressure and RSNA independent on whether hypertension is primarily neurogenic or is secondary to stenosis in renal artery.

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.

Lithium reduces the span of G protein-activated K+ (GIRK) channel inhibition in hippocampal neurons

OBJECTIVES

Lithium (Li⁺ ) is one of the most widely used treatments for bipolar disorder (BD). However, the molecular and neuronal basis of BD, as well as the mechanisms of Li⁺ actions are poorly understood. Cellular and biochemical studies identified G proteins as being among the cellular targets for Li⁺ action, while genetic studies indicated an association with the KCNJ3 gene, which encodes the G protein-activated inwardly rectifying K⁺ (GIRK) channels. GIRK channels regulate neuronal excitability by mediating the inhibitory effects of multiple neurotransmitters and contribute to the resting potassium conductance. Here, we explored the effects of therapeutic dose of Li⁺ on neuronal excitability and the role of GIRK channels in Li⁺ actions.

METHODS

Effects of Li⁺ on excitability were studied in hippocampal brain slices using whole-cell electrophysiological recordings.

RESULTS

A therapeutic dose of Li⁺ (1 mM) dually regulated the function of GIRK channels in hippocampal slices. Li⁺ hyperpolarized the resting membrane potential of hippocampal CA1 pyramidal neurons and prolonged the latency to reach the action potential threshold and peak. These effects were abolished in the presence of tertiapin, a specific GIRK channel blocker, and at doses above the therapeutic window (2 mM). In contrast, Li⁺ reduced GIRK channel opening induced by GABA_B receptor (GABA_B R) activation, causing reduced hyperpolarization of the membrane potential, attenuated reduction of input resistance, and a smaller decrease of neuronal firing.

CONCLUSIONS

A therapeutic dose of Li⁺ reduces the span of GIRK channel-mediated inhibition due to enhancement of basal GIRK currents and inhibition of GABA_B R evoked responses, providing an important link between Li⁺ action, neuronal excitability, and cellular and genetic targets of BD.

Neuroadaptations of presynaptic and postsynaptic GABAB receptor function in the paraventricular nucleus in response to chronic unpredictable stress

BACKGROUND AND PURPOSE

Chronic stress impairs GABA_A (GABA type A) receptor-mediated inhibition in the hypothalamic paraventricular nucleus (PVN). It is not clear whether GABA_B receptor function is also altered. We hypothesize that chronic stress alters GABA_B receptor function in PVN corticotrophin-releasing hormone (CRH) neurons to control hypothalamus-pituitary-adrenal axis activity.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were made of PVN-CRH neurons expressing eGFP driven by CRH promoter in brain slices from unstressed rats and rats exposed to chronic unpredictable mild stress (CUMS).

KEY RESULTS

CUMS elevated the basal circulating corticosterone levels and increased the basal firing activity of PVN-CRH neurons. Microinjection of GABA_B receptor agonist baclofen into the PVN suppressed the increased corticosterone levels in CUMS rats compared with unstressed rats. CUMS blunted the baclofen-induced inhibition on PVN-CRH neurons and outward currents in these neurons. Furthermore, CUMS reduced expression of GABA_B1 (GABA_B R1) protein in the PVN. Blocking NMDA receptors with AP5 restored the reduced baclofen-induced currents in CUMS rats but had no effect on GABA_B1 expression. Furthermore, CUMS treatment augmented the baclofen-induced decrease in the frequency of glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitor postsynaptic currents in PVN-CRH neurons. The GABA_B receptor antagonist CGP55845 increased the firing activity of PVN-CRH neurons only in CUMS-treated rats and not in unstressed rats.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that chronic stress impairs postsynaptic GABA_B receptor function but augments presynaptic GABA_B receptor function in controlling glutamatergic and GABAergic synaptic inputs in PVN-CRH neurons.

Neural Substrate Essential for Suppression of Vasopressin Secretion and Excretion of a Water Load

Suppression of vasopressin secretion to very low levels is essential for the excretion of excess water. To investigate a role for the preoptic brain region in the suppression of vasopressin secretion and the excretion of a water load, lesions were made in the vicinity of the lamina terminalis in ewes (LTX-sheep) and responses to water-loading or reduction of cerebrospinal fluid NaCl by i.c.v. isotonic mannitol solution were investigated. In normal conscious sheep, intraruminal water-loading resulted in the urine flow rate increasing and urine osmolality decreasing within 1 h, such that renal free water clearance (CH 2O ) increased from -1.02 ± 0.16 ml/min (mean ± SEM) to a maximum of +4.99 ± 0.62 ml/min at 2.5 h after water-loading (P < 0.05, n = 6). Plasma vasopressin levels fell from 0.88 ± 0.17 pg/ml to undetectable levels (< 0.4 pg/ml, n = 4). In LTX-sheep (n = 6), CH 2O did not change significantly after water-loading (-1.78 ± 0.13 to -2.03 ± 0.49 ml/min at 2.5 h after water-loading). Plasma vasopressin levels were inappropriately elevated in water-loaded LTX-sheep (n = 3). Intracerebroventricular mannitol (1 ml/h for 2 h) resulted in a water diuresis and increase in CH 2O (-1.16 ± 0.12 to +2.81 ± 0.58 ml/min, P < 0.05) after 2 h in normal sheep, and plasma vasopressin levels fell significantly from to 0.88 ± 0.23 pg/ml to < 0.4 pg/ml (P < 0.05, n = 6). However, in LTX-sheep, there was no change in CH 2O (-1.31 ± 0.14 to -1.35 ± 0.12 ml/min) or the plasma vasopressin concentration (1.47 ± 0.18 to 1.60 ± 0.44 pg/ml, not significant) with i.c.v. mannitol. The results suggest that an inhibitory pathway from the vicinity of the median preoptic nucleus to the supraoptic and hypothalamic paraventricular nuclei plays an important role in the suppression of vasopressin secretion and the excretion of excess water.

The median preoptic nucleus: front and centre for the regulation of body fluid, sodium, temperature, sleep and cardiovascular homeostasis

Located in the midline anterior wall of the third cerebral ventricle (i.e. the lamina terminalis), the median preoptic nucleus (MnPO) receives a unique set of afferent neural inputs from fore-, mid- and hindbrain. These afferent connections enable it to receive neural signals related to several important aspects of homeostasis. Included in these afferent projections are (i) neural inputs from two adjacent circumventricular organs, the subfornical organ and organum vasculosum laminae terminalis, that respond to hypertonicity, circulating angiotensin II or other humoural factors, (ii) signals from cutaneous warm and cold receptors that are relayed to MnPO, respectively, via different subnuclei in the lateral parabrachial nucleus and (iii) input from the medulla associated with baroreceptor and vagal afferents. These afferent signals reach appropriate neurones within the MnPO that enable relevant neural outputs, both excitatory and inhibitory, to be activated or inhibited. The efferent neural pathways that proceed from the MnPO terminate on (i) neuroendocrine cells in the hypothalamic supraoptic and paraventricular nuclei to regulate vasopressin release, while polysynaptic pathways from MnPO to cortical sites may drive thirst and water intake, (ii) thermoregulatory pathways to the dorsomedial hypothalamic nucleus and medullary raphé to regulate shivering, brown adipose tissue and skin vasoconstriction, (iii) parvocellular neurones in the hypothalamic paraventricular nucleus that drive autonomic pathways influencing cardiovascular function. As well, (iv) other efferent pathways from the MnPO to sites in the ventrolateral pre-optic nucleus, perifornical region of the lateral hypothalamic area and midbrain influence sleep mechanisms.

Hydration and beyond: neuropeptides as mediators of hydromineral balance, anxiety and stress-responsiveness

Challenges to body fluid homeostasis can have a profound impact on hypothalamic regulation of stress responsiveness. Deficiencies in blood volume or sodium concentration leads to the generation of neural and humoral signals relayed through the hindbrain and circumventricular organs that apprise the paraventricular nucleus of the hypothalamus (PVH) of hydromineral imbalance. Collectively, these neural and humoral signals converge onto PVH neurons, including those that express corticotrophin-releasing factor (CRF), oxytocin (OT), and vasopressin, to influence their activity and initiate compensatory responses that alleviate hydromineral imbalance. Interestingly, following exposure to perceived threats to homeostasis, select limbic brain regions mediate behavioral and physiological responses to psychogenic stressors, in part, by influencing activation of the same PVH neurons that are known to maintain body fluid homeostasis. Here, we review past and present research examining interactions between hypothalamic circuits regulating body fluid homeostasis and those mediating behavioral and physiological responses to psychogenic stress.

Evaluation for roles of neurosteroids in modulating forebrain mechanisms controlling vasopressin secretion and related phenomena in conscious rats

Anteroventral third ventricular region (AV3V) that regulates autonomic functions through a GABAergic mechanism possesses neuroactive steroid (NS)-synthesizing ability. Although NS can exert effects by acting on a certain type of GABAA-receptor (R), it is not clear whether NS may operate to modulate AV3V GABAergic activity for controlling autonomic functions. This study aimed to investigate the issue. AV3V infusion with a GABAA antagonist bicuculline increased plasma vasopressin (AVP), glucose, blood pressure (BP), and heart rate in rats. These events were abolished by preinjecting its agonist muscimol, whereas the infusion with allopregnanolone, a NS capable of potentiating GABAA-R function, affected none of the variables in the absence or presence of such bicuculline actions. Similarly, AV3V infusion with pregnanolone sulfate, a NS capable of antagonizing GABAA-R, produced no effect on those variables. AV3V infusion with muscimol was effective in inhibiting the responses of plasma AVP or glucose, or BP to an osmotic loading or bleeding. However, AV3V infusion with aminoglutethimide, a NS synthesis inhibitor, did not affect any of the variables in the absence or presence of those stimuli. These results suggest that NS may not cause acute effects on the AV3V GABAergic mechanism involved in regulating AVP release and other autonomic function.

GABA transporter subtype 1 and GABA transporter subtype 3 modulate glutamatergic transmission via activation of presynaptic GABA(B) receptors in the rat globus pallidus

The intra-pallidal application of γ-aminobutyric acid (GABA) transporter subtype 1 (GAT-1) or GABA transporter subtype 3 (GAT-3) transporter blockers [1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-(S)-3-piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABA(B) heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole-cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABA(A) receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABA(B) receptor blockade by the intra-cellular application of N-(2,6-dimethylphenylcarbamoylmethyl)-triethylammonium bromide (OX314), bath application of SKF 89976A (10 μM) or SNAP 5114 (10 μM) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid, a GABA(B) receptor antagonist. The paired-pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABA(B) heteroreceptors following GAT-1 or GAT-3 blockade. In conclusion, our findings demonstrate that presynaptic GABA(B) heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

Insulin-producing cells in the brain of adult Drosophila are regulated by the serotonin 5-HT1A receptor

Insulin signaling regulates lifespan, reproduction, metabolic homeostasis, and resistance to stress in the adult organism. In Drosophila, there are seven insulin-like peptides (DILP1-7). Three of these (DILP2, 3 and 5) are produced in median neurosecretory cells of the brain, designated IPCs. Previous work has suggested that production or release of DILPs in IPCs can be regulated by a factor secreted from the fat body as well as by neuronal GABA or short neuropeptide F. There is also evidence that serotonergic neurons may regulate IPCs. Here, we investigated mechanisms by which serotonin may regulate the IPCs. We show that the IPCs in adult flies express the 5-HT(1A), but not the 5-HT(1B) or 5-HT(7) receptors, and that processes of serotonergic neurons impinge on the IPC branches. Knockdown of 5-HT(1A) in IPCs by targeted RNA interference (RNAi) leads to increased sensitivity to heat, prolonged recovery after cold knockdown and decreased resistance to starvation. Lipid metabolism is also affected, but no effect on growth was seen. Furthermore, we show that DILP2-immunolevels in IPCs increase after 5-HT(1A) knockdown; this is accentuated by starvation. Heterozygous 5-HT(1A) mutant flies display the same phenotype in all assays, as seen after targeted 5-HT(1A) RNAi, and flies fed the 5-HT(1A) antagonist WAY100635 display reduced lifespan at starvation. Our findings suggest that serotonin acts on brain IPCs via the 5-HT(1A) receptor, thereby affecting their activity and probably insulin signaling. Thus, we have identified a second inhibitory pathway regulating IPC activity in the Drosophila brain.

Changes in vasopressin release and autonomic function induced by manipulating forebrain GABAergic signaling under euvolemia and hypovolemia in conscious rats

The anteroventral third ventricular region (AV3V) is a pivotal area for osmotic responses and integration of autonomic functions. The purpose of this study was to investigate whether the gamma-aminobutyric acid (GABA)-ergic activity in the AV3V may be involved in the regulation of arginine vasopressin (AVP) secretion and related phenomena under the conditions with or without hypovolemia. Experiments were performed in conscious rats. We found that AV3V infusion with the GABA(A) receptor antagonist bicuculline in euvolemic rats caused prompt increases in plasma AVP, osmolality, glucose, arterial pressure and heart rate. The effects of the bicuculline infusion were abolished by prior infusion of a GABA(A) receptor agonist, muscimol. When repeated twice with a 10-min interval, removal of systemic blood (10 mL/kg body weight) lowered arterial pressure and enhanced plasma AVP, osmolality, glucose and angiotensin II. Muscimol infusion in the AV3V, but not in the cerebral ventricle, inhibited the responses of plasma AVP and glucose, despite having no effect in a sham hemorrhagic state. The inhibition of the AVP response by the muscimol infusion was also verified in rats given a combined stimulus of bleeding plus an osmotic load. In contrast, AV3V infusion with the GABA(B) receptor agonist baclofen tended to intensify the hemorrhagic responses of plasma AVP and glucose, despite its potency to prevent the hemorrhagic fall in arterial pressure. These results, taken together with our previous data, suggest that hypovolemic stimuli, like hyperosmotic stimuli, may promote AVP secretion by causing the inhibition of AV3V GABA(A)-ergic activity responsible for potentiation of glutamatergic activity.

Metabotropic glutamate receptors in median preoptic neurons modulate neuronal excitability and glutamatergic and GABAergic inputs from the subfornical organ

Cardiovascular and behavioral responses to circulating angiotensin require intact connectivity along the upper lamina terminalis joining the subfornical organ (SFO) with the median preoptic nucleus (MnPO). In the present study on MnPO neurons, we used whole cell patch-clamp recording techniques in brain slice preparations to evaluate the influence of metabotropic glutamate receptor (mGluR) agonists on modulating their intrinsic excitability and SFO-evoked glutamatergic and GABAergic postsynaptic currents. In 22/36 cells, bath application of a mGluR group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced a TTX-resistant inward current coupled with decrease in a membrane K(+) conductance but also a possible increase in a nonselective cationic conductance. By contrast, 27/49 cells responded to a mGluR group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) with a TTX-resistant outward current and increase in membrane conductance that reversed around -95 mV, suggesting opening of K(+) channels. None of 19 cells responded to the mGluR group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4). Agonists for all mGluR groups suppressed SFO-evoked excitatory postsynaptic currents and significantly increased paired-pulse ratios, implying a presynaptic mechanism. Only the mGluR group II agonist significantly reduced SFO-evoked inhibitory postsynaptic currents and caused an increase in paired-pulse ratios. These results suggest a complexity of pre- and postsynaptic mGluRs are available to modulate rapid neurotransmission along the upper lamina terminalis from SFO to MnPO.

Endogenous angiotensin II facilitates GABAergic neurotransmission afferent to the Na+-responsive neurons of the rat median preoptic nucleus

The median preoptic nucleus (MnPO) is densely innervated by efferent projections from the subfornical organ (SFO) and, therefore, is an important relay for the peripheral chemosensory and humoral information (osmolality and serum levels ANG II). In this context, controlling the excitability of MnPO neuronal populations is a major determinant of body fluid homeostasis and cardiovascular regulation. Using a brain slice preparation and patch-clamp recordings, our study sought to determine whether endogenous ANG II modulates the strength of the SFO-derived GABAergic inputs to the MnPO. Our results showed that the amplitude of the inhibitory postsynaptic currents (IPSCs) were progressively reduced by 44 ± 2.3% by (Sar1, Ile8)-ANG II, a competitive ANG type 1 receptor (AT1R) antagonist. Similarly, losartan, a nonpeptidergic AT1R antagonist decreased the IPSC amplitude by 40.4 ± 5.6%. The facilitating effect of endogenous ANG II on the GABAergic input to the MnPO was not attributed to a change in GABA release probability and was mimicked by exogenous ANG II, which potentiated the amplitude of the muscimol-activated GABAA/Cl− current by 53.1 ± 11.4%. These results demonstrate a postsynaptic locus of action of ANG II. Further analysis reveals that ANG II did not affect the reversal potential of the synaptic inhibitory response, thus privileging a cross talk between postsynaptic AT1 and GABAA receptors. Interestingly, facilitation of GABAergic neurotransmission by endogenous ANG II was specific to neurons responding to changes in the ambient Na+ level. This finding, combined with the ANG II-mediated depolarization of non-Na+-responsive neurons reveals the dual actions of ANG II to modulate the excitability of MnPO neurons.

The physiological role of pre- and postsynaptic GABA(B) receptors in membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus

In the inferior colliculus (IC), GABAergic inhibition mediated by GABA(A) receptors has been shown to play a significant role in regulating physiological responses, but little is known about the physiological role of GABA(B) receptors in IC neurons. In the present study, we used whole-cell patch clamp recording in vitro to investigate the effects of activation of GABA(B) receptors on membrane excitability and synaptic transmission of neurons in the rat's dorsal cortex of the inferior colliculus (ICD). Repetitive stimulation of GABAergic inputs to ICD neurons at high frequencies could elicit a slow and long-lasting postsynaptic response, which was reversibly abolished by the GABA(B) receptor antagonist, CGP 35348. The results suggest that postsynaptic GABA(B) receptors can directly mediate inhibitory synaptic transmission in ICD. The role of postsynaptic GABA(B) receptors in regulation of membrane excitability was further investigated by application of the GABA(B) receptor agonist, baclofen. Baclofen hyperpolarized the cell, reduced the membrane input resistance and firing rate, increased the threshold for generating action potentials (APs), and decreased the amplitude of the AP and its associated after-hyperpolarization. The Ca2+-mediated rebound depolarization following hyperpolarization and the depolarization hump at the beginning of membrane depolarization were also suppressed by baclofen. In voltage clamp experiments, baclofen induced inward rectifying K+ current and reduced low- and high-threshold Ca2+ currents, which may account for the suppression of membrane excitability by postsynaptic GABA(B) receptors. Application of baclofen also reduced excitatory synaptic responses mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and inhibitory synaptic responses mediated by GABA(A) receptors. Baclofen increased the ratios of 2nd/1st excitatory and inhibitory postsynaptic currents to paired-pulse stimulation of the synaptic inputs. These results suggest that fast glutamatergic and GABAergic synaptic transmission in ICD can be modulated by presynaptic GABA(B) receptors.

Differential effects of GABAB autoreceptor activation on ethanol potentiation of local and lateral paracapsular GABAergic synapses in the rat basolateral amygdala

Many studies have demonstrated that GABAergic inhibition within the basolateral amygdala (BLA) plays an integral role in the regulation of anxiety, an important behavioral component in the etiology of alcoholism. Although ethanol has recently been shown to enhance BLA GABAergic inhibition via two distinct populations of inhibitory cells, local and lateral paracapsular (lpcs) interneurons, little is known about the mechanisms underlying ethanol potentiation of these two inhibitory pathways. Ethanol is known to enhance GABAergic inhibition in many brain regions via a complex array of pre- and postsynaptic mechanisms. In addition, ethanol's presynaptic effects are often subject to GABA(B) autoreceptor (GABA(B)-R) modulation. Therefore, in this study, we characterized GABA(B)-R function and modulation of ethanol actions at local and lpcs GABAergic synapses. At local synapses, we found significant paired-pulse depression (PPD, 250 ms inter-pulse interval) which was abated by SCH-50911 (GABA(B)-R antagonist). No significant PPD was detected at lpcs synapses, but SCH-50911 significantly potentiated lpcs-evoked IPSCs. Baclofen (GABA(B)-R agonist) had similar depressant effects on local- and lpcs-evoked IPSCs, however baclofen pretreatment only reduced ethanol potentiation at local synapses. Ethanol also significantly enhanced the frequency of spontaneous and miniature IPSCs, and these effects were also sensitive to GABA(B)-R modulators. Collectively, these data suggest that stimulus-independent inhibitory responses recorded from BLA principal neurons primarily reflect the activity of local GABAergic interneurons and provide additional evidence that ethanol potentiates local BLA inhibitory synapses primarily via a presynaptic enhancement of GABA release that is tightly regulated by GABA(B)-Rs. In contrast, ethanol potentiation of lpcs GABAergic synapses is not sensitive to GABA(B)-R activation and does not appear to involve increased presynaptic GABA release.

Orexin peptides enhance median preoptic nucleus neuronal excitability via postsynaptic membrane depolarization and enhancement of glutamatergic afferents

Subpopulations of neurons in the median preoptic nucleus (MnPO) located within the lamina terminalis contribute to thermoregulatory, cardiovascular and hydromineral homeostasis, and sleep-promotion. MnPO is innervated by lateral hypothalamic neurons that synthesize and secrete the arousal-promoting and excitatory orexin (hypocretin) neuropeptides. To evaluate the hypothesis that orexins modulate the excitability of MnPO neurons, we used patch-clamp recording techniques applied in rat brain slice preparations to assess the effects of exogenously applied orexin A and orexin B peptides on their intrinsic and synaptic properties. Whole cell recordings under current-clamp mode revealed that 11/15 tested MnPO neurons responded similarly to either orexin A or B (500-1000 nM) with a slowly rising, prolonged (10-15 min) and reversible membrane depolarization. Under voltage-clamp mode, orexin applications induced a tetrodotoxin-resistant inward current of -7.2+/-1.6 pA, indicating a direct (postsynaptic) activation, with a time course similar to the observed membrane depolarization. The orexin-induced responses in 4/7 neurons were associated with a significant decrease in membrane conductance and the net orexin-induced current that reversed at -99+/-5 mV, suggesting closure of potassium channels. Orexins did not attenuate the properties of excitatory (n=4) or inhibitory (n=7) postsynaptic currents evoked by subfornical organ stimulation. By contrast, orexins applications induce a significant increase in both frequency and amplitude of spontaneous glutamatergic postsynaptic currents (5/7 cells) but had no influence on spontaneous GABAergic currents (6/6 cells). Thus, in addition to a direct postsynaptic receptor-mediated excitation, orexins can also increase the excitability of MnPO neurons via increasing their excitatory inputs, presumably through an orexin receptor-mediated excitation of local glutamatergic neurons whose axons project to MnPO neurons.

Plasticity of pre- and postsynaptic GABAB receptor function in the paraventricular nucleus in spontaneously hypertensive rats

GABA(B) receptor function is upregulated in the paraventricular nucleus (PVN) of the hypothalamus in spontaneously hypertensive rats (SHR), but it is unclear whether this upregulation occurs pre- or postsynaptically. We therefore determined pre- and postsynaptic GABA(B) receptor function in retrogradely labeled spinally projecting PVN neurons using whole cell patch-clamp recording in brain slices in SHR and Wistar-Kyoto (WKY) rats. Bath application of the GABA(B) receptor agonist baclofen significantly decreased the spontaneous firing activity of labeled PVN neurons in both SHR and WKY rats. However, the magnitude of reduction in the firing rate was significantly greater in SHR than in WKY rats. Furthermore, baclofen produced larger membrane hyperpolarization and outward currents in labeled PVN neurons in SHR than in WKY rats. The baclofen-induced current was abolished by either including G protein inhibitor GDPbetaS in the pipette solution or bath application of the GABA(B) receptor antagonist in both SHR and WKY rats. Blocking N-methyl-d-aspartic acid receptors had no significant effect on baclofen-elicited outward currents in SHR. In addition, baclofen caused significantly greater inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) in labeled PVN neurons in brain slices from SHR than WKY rats. By contrast, baclofen produced significantly less inhibition of GABAergic inhibitory postsynaptic currents (IPSCs) in labeled PVN neurons in SHR than in WKY rats. Although microinjection of the GABA(B) antagonist into the PVN increases sympathetic vasomotor tone in SHR, the GABA(B) antagonist did not affect EPSCs and IPSCs of the PVN neurons in vitro. These findings suggest that postsynaptic GABA(B) receptor function is upregulated in PVN presympathetic neurons in SHR. Whereas presynaptic GABA(B) receptor control of glutamatergic synaptic inputs is enhanced, presynaptic GABA(B) receptor control of GABAergic inputs in the PVN is attenuated in SHR. Changes in both pre- and postsynaptic GABA(B) receptors in the PVN may contribute to the control of sympathetic outflow in hypertension.

Postsynaptic mu-opioid receptor response in the median preoptic nucleus is altered by a systemic sodium challenge in rats

The median preoptic nucleus (MnPO) is an integrator site for the chemosensory and neural signals induced by a perturbation in the hydromineral balance, and it is highly involved in controlling fluid and electrolyte ingestion. Here, we hypothesize that opioid peptides, previously recognized to control ingestive behaviors, may regulate the excitability of MnPO neurons and that this regulatory action may depend on the natriuric (Na(+)) status of body fluid compartments. Our results show that activation of mu-, but not delta-, opioid receptors (OR) triggered a membrane hyperpolarization by recruiting a G-protein-regulated inward-rectifier K(+) (GIRK) conductance in 41% of the neurons tested. Interestingly, 24 h Na(+) depletion strengthened this opioid-mediated control of neuronal excitability. In Na(+)-depleted animals, the neuronal population displaying the mu-OR-induced hyperpolarization expanded to 60% (Z-test, P = 0.012), whereas Na(+) repletion restored this population to the control level (39%; Z-test, P = 0.037). Among the neurons displaying mu-OR-induced hyperpolarization, Na(+) depletion specifically increased the neuronal population responsive to variation in ambient Na(+) (from 27% to 43%; Z-test, P = 0.029). In contrast, Na(+) repletion dramatically reduced the population that was unresponsive to Na(+) (from 17% to 3%; Z-test, P = 0.031). Neither the basic properties of the neurons nor the characteristics of the mu-OR-induced response were altered by the body Na(+) challenge. Our results indicate that an episode of Na(+) depletion/Na(+) repletion modifies the organization of the opioid-sensitive network of the MnPO. Such network plasticity might be related to the avid salt ingestion triggered by repeated Na(+) depletion.

Gamma-aminobutyric acid inhibits synergistic interleukin-6 release but not transcriptional activation in astrocytoma cells

OBJECTIVE

A decline in the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) may enhance cytokine release in Alzheimer's disease (AD) resulting in neuroinflammation. We investigated the GABA-mediated suppression of the synergistic release of interleukin (IL)-6 due to interleukin 1-beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha).

METHODS

Rat C6 astrocytoma cells were treated with IL-1 beta and TNF-alpha in the absence and presence of GABA. Activation of p38, degradation of I kappaB-alpha and total cellular IL-6 were determined by Western blot analysis. IL-6 release and gene expression were measured by ELISA and RT-PCR, respectively.

RESULTS

Although p38 and nuclear factor (NF)-kappaB are essential for the synergistic release of IL-6, GABA did not affect either p38 phosphorylation or I kappaB-alpha degradation. Additionally, GABA suppressed IL-6 release but did not alter cytokine-driven synergistic increases in IL-6 gene expression. Western blot analysis revealed that co-treatments with IL-1 beta and TNF-alpha resulted in an increase in intracellular IL-6 that was prevented by GABA.

CONCLUSION

GABA-induced inhibition of IL-6 release appears to coincide with a reduction in cellular IL-6. The GABA-induced suppression of IL-6 release may include inhibition of IL-6 gene translation.

Presynaptic α-adrenoceptors in median preoptic nucleus modulate inhibitory neurotransmission from subfornical organ and organum vasculosum lamina terminalis

The median preoptic nucleus (MnPO) in the lamina terminalis receives a prominent catecholaminergic innervation from the dorsomedial and ventrolateral medulla. The present investigation used whole cell patch-clamp recordings in rat brain slice preparations to evaluate the hypothesis that presynaptic adrenoceptors could modulate GABAergic inputs to MnPO neurons. Bath applications of norepinephrine (NE; 20–50 μM) induced a prolonged and reversible suppression of inhibitory postsynaptic currents (IPSCs) and reduced paired-pulse depression evoked by stimulation in the subfornical organ and organum vasculosum lamina terminalis. These events were not correlated with any observed changes in membrane conductance arising from NE activity at postsynaptic α1- or α2-adrenoceptors. Consistent with a role for presynaptic α2-adrenoceptors, responses were selectively mimicked by an α2-adrenoceptor agonist (UK-14304) and blockable with an α2-adrenoceptor antagonist (idazoxan). Although the α1-adrenoceptor agonist cirazoline and the α1-adrenoceptor antagonist prazosin were without effect on these evoked IPSCs, NE was noted to increase (via α1-adrenoceptors) or decrease (via α2-adrenoceptors) the frequency of spontaneous and tetrodotoxin-resistant miniature IPSCs. Collectively, these observations imply that both presynaptic and postsynaptic α1- and α2-adrenoceptors in MnPO are capable of selective modulation of rapid GABAA receptor-mediated inhibitory synaptic transmission along the lamina terminalis and therefore likely to exert a prominent influence in regulating cell excitability within the MnPO.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Involvement of anteroventral third ventricular AMPA/kainate receptors in both hyperosmotic and hypovolemic AVP secretion in conscious rats

The area of the brain called the anteroventral third ventricular region (AV3V) includes three different subtypes of glutamate receptor, as well as neural circuits controlling fluid balance and cardiovascular and neuroendocrine functions. Although our previous data indicate the ability of AV3V N-methyl-d-aspartate (NMDA) and metabotropic receptors to provoke vasopressin (AVP)-releasing, pressor and hyperglycemic responses, the roles of non-NMDA receptors selective for alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate have not been elucidated to date. To address this question, the effects of intracerebral infusion with FWD or NBQX (specific agonist and antagonist for non-NMDA receptors, respectively) on plasma AVP, glucose, osmolality, electrolytes and cardiovascular parameters were examined in conscious rats in the absence or presence of an osmotic or volemic stimulus. When applied topically to AV3V structures such as the median preoptic nucleus, FWD augmented plasma AVP, osmolality, glucose and arterial pressure in a dose-associated fashion. All responses of the variables were abolished by pre-administering NBQX, which exerted no conspicuous effect on any variable except arterial pressure. It was revealed that NBQX administration in AV3V structures such as the median preoptic nucleus and the periventricular nucleus inhibited the rise of plasma AVP in response to intravenous infusion with hypertonic saline or removal of systemic blood through the femoral artery. Elevation of plasma osmolality and sodium evoked by osmotic load, and elevation of plasma osmolality, glucose and angiotensin II and decrease of arterial pressure caused by bleeding, were not significantly affected by NBQX treatment. These results suggest that AV3V non-NMDA receptors, as well as NMDA receptors, may elicit AVP-releasing, pressor and hyperglycemic actions when stimulated in the basal state, and may facilitate AVP secretion under both hyperosmotic and hypovolemic conditions, without contributing to cardiovascular, blood glucose or other responses.

Regulation of synaptic input to hypothalamic presympathetic neurons by GABA(B) receptors

The hypothalamic paraventricular (PVN) neurons projecting to the spinal cord and brainstem play an important role in the control of homeostasis and the sympathetic nervous system. Although GABA(B) receptors are present in the PVN, their function in the control of synaptic inputs to PVN presympathetic neurons is not clear. Using retrograde tracing and whole-cell patch-clamp recordings in rat brain slices, we determined the role of presynaptic GABA(B) receptors in regulation of glutamatergic and GABAergic inputs to spinally projecting PVN neurons. The GABA(B) receptor agonist baclofen (1-50 microM) dose-dependently decreased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) and inhibitory postsynaptic currents (sIPSCs). The effect of baclofen on sEPSCs and sIPSCs was completely blocked by 10 microM CGP52432, a selective GABA(B) receptor antagonist. Baclofen also significantly reduced the frequency of both miniature excitatory and miniature inhibitory postsynaptic currents (mEPSCs and mIPSCs). Furthermore, uncoupling pertussis toxin-sensitive G(i/o) proteins with N-ethylmaleimide abolished baclofen-induced inhibition of mEPSCs and mIPSCs. However, the inhibitory effect of baclofen on the frequency of mIPSCs and mEPSCs persisted in the presence of either Cd2+, a voltage-gated Ca2+ channel blocker, or 4-aminopyridine, a blocker of voltage-gated K+ channels. Our results suggest that activation of presynaptic GABA(B) receptors inhibits synaptic GABA and glutamate release to PVN presympathetic neurons. This presynaptic action of GABA(B) receptors is mediated by the N-ethylmaleimide-sensitive G(i/o) proteins, but independent of voltage-gated Ca2+ and K+ channels.

GABA modulates Drosophila circadian clock neurons via GABAB receptors and decreases in calcium

Circadian clocks play vital roles in the control of daily rhythms in physiology and behavior of animals. In Drosophila, analysis of the molecular and behavioral rhythm has shown that the master clock neurons are entrained by sensory inputs and are synchronized with other clock neurons. However, little is known about the neuronal circuits of the Drosophila circadian system and the neurotransmitters that act on the clock neurons. Here, we provide evidence for a new neuronal input pathway to the master clock neurons, s-LN(v)s, in Drosophila that utilizes GABA as a slow inhibitory neurotransmitter. We monitored intracellular calcium levels in dissociated larval s-LN(v)s with the calcium-sensitive dye Fura-2. GABA decreased intracellular calcium in the s-LN(v)s and blocked spontaneous oscillations in calcium levels. The duration of this response was dose-dependent between 1 nM and 100 microM. The response to GABA was blocked by a metabotropic GABA(B) receptor (GABA(B)-R) antagonist, CGP54626, but not by an ionotropic receptor antagonist, picrotoxin. The GABA(B)-R agonist, 3-APMPA, produced a response similar to GABA. An antiserum against one of the Drosophila GABA(B)-Rs (GABA(B)-R2) labeled the dendritic regions of the s-LN(v)s in both adults and larvae, as well as the dissociated s-LN(v)s. We found that some GABAergic processes terminate at the dendrites of the LN(v)s, as revealed by GABA immunostaining and a GABA-specific GAL4 line (GAD1-gal4). Our results suggest that the s-LN(v)s receive slow inhibitory GABAergic inputs that decrease intracellular calcium of these clock neurons and block their calcium cycling. This response is mediated by postsynaptic GABA(B) receptors.

Heterogeneous chloride homeostasis and GABA responses in the median preoptic nucleus of the rat

The median preoptic nucleus (MnPO) is an integrative structure of the hypothalamus receiving periphery-derived information pertinent to hydromineral and cardiovascular homeostasis. In this context, excitability of MnPO neurones is controlled by fast GABAergic, glutamatergic and angiotensinergic projection from the subfornical organ (SFO). Taking advantage of a brain slice preparation preserving synaptic connection between the SFO and the MnPO, and appropriate bicarbonate-free artificial cerebrospinal fluid (CSF), we investigated a possible implication of an active outward Cl- transport in regulating efficacy of the GABA(A) receptor-mediated inhibitory response at the SFO-MnPO synapse. When somata of the MnPO neurones was loaded with 18 mm chloride, stimulation of the SFO evoked outward inhibitory postsynaptic currents (IPSCs) in 81% of the MnPO neurones held at -60 mV. Accordingly, E(IPSC) was found 25 mV hyperpolarized from the theoretical value calculated from the Nernst equation, indicating that IPSC polarity and amplitude were driven by an active Cl- extrusion system in these neurones. E(IPSC) estimated with gramicidin-based perforated-patch recordings amounted -89.2 +/- 4.3 mV. Furosemide (100 microm), a pharmacological compound known to block the activity of the neurone-specific K(+)-Cl- cotransporter, KCC2, reversed IPSC polarity and shifted E(IPSC) towards its theoretical value. Presence of the KCC2 protein in the MnPO was further detected with immunohistochemistry, revealing a dense network of KCC2-positive intermingled fibres. In the presence of a GABA(B) receptor antagonist, high-frequency stimulation (5 Hz) of the SFO evoked a train of IPSCs or inhibitory postsynaptic potentials (IPSPs), whose amplitude was maintained throughout the sustained stimulation. Contrastingly, similar 5 Hz stimulation carried out in the presence of furosemide (50 microm) evoked IPSCs/IPSPs, whose amplitude collapsed during the high-frequency stimulation. Similar reduction in inhibitory neurotransmission was also observed in MnPO neurones lacking the functional Cl- extrusion mechanism. We conclude that a majority of MnPO neurones were characterized by a functional Cl- transporter that ensured an efficient activity-dependent Cl- transport rate, allowing sustained synaptic inhibition of these neurones. Pharmacological and anatomical data strongly suggested the involvement of KCC2, as an essential postsynaptic determinant of the inhibitory neurotransmission afferent to the MnPO, a key-structure in the physiology of the hydromineral and cardiovascular homeostasis.

Median preoptic neurones projecting to the hypothalamic paraventricular nucleus respond to osmotic, circulating Ang II and baroreceptor input in the rat

The present study sought to determine whether individual neurones of the median preoptic nucleus (MnPO) with axonal projections to the hypothalamic paraventricular nucleus (MnPO-PVN) respond to osmotic, circulating angiotensin II (Ang II), and baroreceptor stimulation. Hypertonic NaCl (0.75 or 1.5 osmol l(-1)) or Ang II (150 ng) was injected into the internal carotid artery (ICA). Baroreceptor stimulation was performed by i.v. injection of phenylephrine or sodium nitroprusside to increase or decrease arterial blood pressure, respectively. Of 65 MnPO neurones, 50 units were antidromically activated from the PVN with an average onset latency of 11.3 +/- 0.7 ms. Only 9.5% of MnPO-PVN neurones were antidromically activated from the PVN bilaterally. Type I MnPO-PVN neurones (n = 14) responded to osmotic but not Ang II stimulation. In 79% (11/14) of these type I neurones, the response was an increase in cell discharge. Type II MnPO-PVN neurones (n = 7) displayed a significant increase in cell discharge in response to ICA injection of Ang II but not hypertonic NaCl. Type III MnPO-PVN neurones (n = 16) responded to both ICA injection of hypertonic NaCl and Ang II. In 88% (14/16) of type III neurones, osmotic and Ang II stimulation each increased cell discharge. Type IV MnPO-PVN neurones (n = 13) displayed no change in cell discharge in response to ICA injection of hypertonic NaCl or Ang II. Baroreceptor stimulation altered the discharge in subpopulations of type I, II and III MnPO-PVN neurones (43-63% depending on neuronal type). Only one MnPO-PVN neurone responded solely to baroreceptor stimulation (type IV). In addition, a subset of type I, II and III neurones displayed a significant correlation with sympathetic nerve activity and/or the cardiac cycle. These findings suggest that a significant population of MnPO-PVN neurones respond to osmotic and circulating Ang II stimulation and thereby represents a neural substrate through which neurohumoral inputs are integrated within the forebrain lamina terminalis.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.