1,721,013 research outputs found
Endocannabinoid-dependent plasticity at GABAergic and glutamatergic synapses in the striatum is regulated by synaptic activity
No full textLong-term depression (LTD) at striatal synapses is mediated by postsynaptic endocannabinoid (eCB) release and presynaptic cannabinoid 1 receptor (CB(1)R) activation. Previous studies have indicated that eCB mobilization at excitatory synapses might be regulated by afferent activation. To further address the role of neuronal activity in synaptic plasticity we examined changes in synaptic strength induced by the L-type calcium channel activator 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL 64176, FPL) at glutamatergic and gamma-aminobutyric acid (GABA)ergic synapses in the striatum. We found that the basic mechanisms for FPL-mediated eCB signaling are the same at glutamatergic and GABAergic synapses. FPL-induced LTD (FPL-LTD) was blocked in slices treated with the CB(1)R antagonist AM251 (2 mu m), but established depression was not reversed by AM251. FPL-LTD was temperature dependent, blocked by protein translation inhibitors and prevented by intracellular loading of the anandamide transporter inhibitor VDM11 (10 mu m) at both glutamatergic and GABAergic synapses. FPL-LTD at glutamatergic synapses required paired-pulse afferent stimulation, while FPL-LTD at GABAergic synapses could be induced even in the absence of explicit afferent activation. By evaluating tetrodotoxin-insensitive spontaneous inhibitory postsynaptic currents we found that neuronal firing is vital for eCB release and LTD induction at GABAergic synapses, but not for short-term depression induced by CB(1)R agonist. The data presented here suggest that the level of neuronal firing regulates eCB signaling by modulating release from the postsynaptic cell, as well as interacting with presynaptic mechanisms to induce LTD at both glutamatergic and GABAergic synapses in the striatum
Endocannabinoid- and mGluR5-dependent Short-Term Synaptic Depression in an Isolated Neuron/Bouton Preparation from the Hippocampal CA1 Region
No full textEndocannabinoids released from the postsynaptic neuronal membrane can activate presynaptic CB 1 receptors and inhibit neurotransmitter release. In hippocampal slices, depolarization of the CA1 pyramidal neurons elicits an endocannabinoid-mediated inhibition of gamma-aminobutyric acid release known as depolarization-induced suppression of inhibition (DSI). Using the highly reduced neuron/synaptic bouton preparation from the CA1 region of hippocampus, we have begun to examine endocannabinoid-dependent short-term depression (STD) of inhibitory synaptic transmission under well-controlled physiological and pharmacological conditions in an environment free of other cells. Application of the CB 1 synthetic agonist WIN55212-2 and endogenous cannabinoids 2-AG and anandamide produced a decrease in spontaneous inhibitory postsynaptic current (sIPSC) frequency and amplitude, indicating the presence of CB 1 receptors at synapses in this preparation. Endocannabinoid-dependent STD is different from DSI found in hippocampal slices and the neuron/bouton preparation from basolateral amygdala (BLA) since depolarization alone was not sufficient to induce suppression of sIPSCs. However, concurrent application of the metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) and postsynaptic depolarization resulted in a transient (30-50 s) decrease in sIPSC frequency and amplitude. Application of DHPG alone had no effect on sIPSCs. The depolarization/DHPG-induced STD was blocked by the CB 1 antagonist SR141716A and the mGluR5 antagonist MPEP and was sensitive to intracellular calcium concentration. Comparing the present findings with earlier work in hippocampal slices and BLA, it appears that endocannabinoid release is less robust in isolated hippocampal neurons
Enhanced sensitivity to ethanol-induced inhibition of LTP in CA1 pyramidal neurons of socially isolated C57BL/6J mice: role of neurosteroids
No full textEthanol (EtOH)–induced impairment of long-term potentiation (LTP) in the rat hippocampus is prevented by the 5α-reductase inhibitor finasteride, suggesting that this effect of EtOH is dependent on the increased local release of neurosteroids such as 3α,5α-THP that promote GABA–mediated transmission. Given that social isolation (SI) in rodents is associated with altered plasma and brain levels of such neurosteroids as well as with an enhanced neurosteroidogenic action of EtOH, we examined whether the inhibitory effect of EtOH on LTP at CA3-CA1 hippocampal excitatory synapses is altered in C57BL/6J mice subjected to SI for 6 weeks in comparison with group-housed (GH) animals. Extracellular recording of fEPSPs as well as patch-clamp analysis were performed in hippocampal slices prepared from both SI and GH mice. Consistent with previous observations, recording of fEPSPs revealed that the extent of LTP induced in the CA1 region of SI mice was significantly reduced compared with that in GH animals. EtOH (40 mM) inhibited LTP in slices from SI mice but not in those from GH mice, and this effect of EtOH was abolished by co-application of 1 µM finasteride. Current-clamp analysis of CA1 pyramidal neurons revealed a decrease in action potential frequency and an increase in the intensity of injected current required to evoke the first action potential in SI mice compared with GH mice, indicative of a decrease in neuronal excitability associated with SI. Together, our data suggest that SI results in reduced levels of neuronal excitability and synaptic plasticity in the hippocampus. Furthermore, the increased sensitivity to the neurosteroidogenic effect of EtOH associated with SI likely accounts for the greater inhibitory effect of EtOH on LTP in SI mice. The increase in EtOH sensitivity induced by SI may be important for the changes in the effects of EtOH on anxiety and on learning and memory associated with the prolonged stress attributable to social isolation
Neurosteroids, GABA(A) receptors, and ethanol dependence
No full textChanges in the expression of type A receptors for gamma-aminobutyric acid (GABA) represent one of the mechanisms implicated in the development of tolerance to and dependence on ethanol. The impact of such changes on the function and pharmacological sensitivity of GABA(A) receptors (GABA(A)Rs) has remained unclear, however. Certain behavioral and electrophysiological actions of ethanol are mediated by an increase in the concentration of neuroactive steroids in the brain that results from stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. Such steroids include potent modulators of GABA(A)R function.
We have investigated the effect of ethanol exposure and withdrawal on subunit expression and receptor function evaluated by subunit selective compounds, as well as the effects of short-term exposure to ethanol on both neurosteroid synthesis and GABA(A)R function, in isolated neurons and brain tissue.
Chronic treatment with and subsequent withdrawal from ethanol alter the expression of genes for specific GABA(A)R subunits in cultured rat neurons, and these changes are associated with alterations in receptor function and pharmacological sensitivity to neurosteroids, zaleplon, and flumazenil. Acute ethanol exposure increases the amount of 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone) in hippocampal slices by a local action independent of the activity of the HPA axis. This effect of ethanol was associated with an increased amplitude of GABA(A)R-mediated miniature inhibitory postsynaptic currents recorded from CA1 pyramidal neurons in such slices.
Chronic ethanol exposure elicits changes in the subunit composition of GABA(A)Rs, which, in turn, likely contribute to changes in receptor function associated with the altered pharmacological and behavioral sensitivity characteristic of ethanol tolerance and dependence. Ethanol may also modulate GABA(A)R function by increasing the de novo synthesis of neurosteroids in the brain in a manner independent of the HPA axis. This latter mechanism may play an important role in the central effects of ethanol
Antidepressant imipramine induces human astrocytes to differentiate into cells with neuronal phenotype
No full textSeveral recent studies have expanded our conception of the role of astrocytes in neurogenesis, proposing that these cells may contribute to this phenomenon not only as a source of trophic substances, but also as stem cells themselves. We recently observed in vitro that human mature astrocytes can be induced to differentiate into cells with a neuronal phenotype. Antidepressant drugs have been shown to increase neurogenesis in the adult rodent hippocampus. In order to better understand the role of astroglia in antidepressant-induced neurogenesis, primary astrocyte cultures were treated with the antidepressant imipramine. Cell morphology was rapidly modified by treatment. In fact, whereas untreated astrocytes showed large, flat morphology, after a few hours of treatment cells exhibited a round-shaped cell body with long, thin processes. The expression of neuronal markers was analysed by immunocytochemistry, Western Blot and RT-PCR at different treatment times. Results showed an increase in neuronal markers such as neurofilament and neuron-specific enolase (NSE), whereas glial fibrillary, acidic protein (CWAP) and nestin expression were not significantly modified by treatment. Similar results were obtained with fluoxetine and venlafaxine. Hest mRNA significantly increased after 2 h of treatment, suggesting involvement of this transcription factor in this process. These results confirm the role of astrocytes in neurogenesis and suggest that these cells may represent one of the targets of antidepressant
Antihypertensive and neuroprotective effects of catestatin in spontaneously hypertensive rats: interaction with GABAergic transmission in amygdala and brainstem.
No full textThe chromogranin A-derived peptide catestatin (CST) exerts sympathoexcitatory and hypertensive effects when microinjected into the rostral ventrolateral medulla (RVLM: excitatory output); it exhibits sympathoinhibitory and antihypertensive effects when microinjected into the caudal ventrolateral medulla (CVLM: inhibitory output) of vagotomized normotensive rats. Here, continuous infusion of CST into the central amygdalar nucleus (CeA) of spontaneously hypertensive rats (SHRs) for 15 days resulted in a marked decrease of blood pressure (BP) in 6-month- (by 37 mm Hg) and 9-month- (by 65 mm Hg)old rats. Whole-cell patch-clamp recordings on pyramidal CeA neurons revealed that CST increased both spontaneous inhibitory postsynaptic current (sIPSC) amplitude plus frequency, along with reductions of sIPSC rise time and decay time. Inhibition of GABAA receptors (GABAARs) by bicuculline completely abolished CST-induced sIPSC, corroborating that CST signals occur through this major neuroreceptor complex. Hypertension is a major risk factor for cerebrovascular diseases, leading to vascular dementia and neurodegeneration. We found a marked neurodegeneration in the amygdala and brainstem of 9-month-old SHRs, while CST and the GABAAR agonist Muscimol provided significant neuroprotection. Enhanced phosphorylation of Akt and ERK accounted for these neuroprotective effects through anti-inflammatory and anti-apoptotic activities. Overall our results point to CST exerting potent antihypertensive and neuroprotective effects plausibly via a GABAergic output, which constitute a novel therapeutic measure to correct defects in blood flow control in disorders such as stroke and Alzheimer's disease
Isoniazid-induced reduction in GABAergic neurotransmission alters the function of the cerebellar cortical circuit
No full textThe cerebellar cortex contributes to the control of movement, coordination, and certain cognitive functions. The cerebellar network is composed of five different types of neurons that are wired together in a repetitive module. Given that four of these five neurons synthesize and release GABA, this inhibitory neurotransmitter plays a central role in regulation of the excitability and correct functioning of the cerebellar cortex. We have now used isoniazid, an inhibitor of glutamic acid decarboxylase, the enzyme responsible for the synthesis of GABA, to evaluate the contribution of GABAergic transmission in different types of cerebellar cortical neurons to the functioning of the cerebellar circuit. Parasagittal cerebellar slices were prepared from 28- to 40-day-old male rats and were subjected to patch-clamp recording in the voltage- or current-clamp mode. Exposure of the tissue slices to isoniazid (10 mM) resulted in a decrease in the level of GABAergic transmission in Purkinje cells and a consequent increase in the firing rate of spontaneous action potentials that was apparent after 40 min. In granule neurons, isoniazid reduced both tonic and phasic GABAergic currents and thereby altered the flow of information across the cerebellar cortex. Our data support the notion that the amount of GABA at the synaptic level is a major determinant of the excitability of the cerebellar cortex, and they suggest that isoniazid may be a useful tool with which to study the function of the cerebellar network. (C) 2008 IBRO. Published by Elsevier Ltd. All rights reserved
Brain steroidogenesis mediates ethanol modulation of GABAA receptor activity in rat hippocampus
No full textAn interaction with the GABA type A (GABA(A)) receptor has long been recognized as one of the main neurochemical mechanisms underlying many of the pharmacological actions of ethanol. However, more recent data have suggested that certain behavioral and electrophysiological actions of ethanol are mediated by an increase in brain concentration of neuroactive steroids that results from stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. Neuroactive steroids such as 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha, 5alpha-THP) are, in fact, potent and efficacious endogenous positive modulators of GABAA receptor function. Because neurosteroids can be synthesized de novo in the brain, we have investigated whether ethanol might affect both neurosteroid synthesis and GABA(A) receptor function in isolated rat hippocampal tissue. Here, we show that ethanol increases the concentration of 3alpha, 5alpha-THP as well as the amplitude of GABA(A) receptor-mediated IPSCs recorded from CA1 pyramidal neurons in isolated hippocampal slices. These effects are shared by the neurosteroid precursor progesterone, the peripheral benzodiazepine receptor-selective agonist CB34, and gamma-hydroxybutyrate, all of which are known to increase the formation of neuroactive steroids in plasma and in the brain. The action of ethanol on GABA(A) receptor-mediated IPSC amplitude is biphasic, consisting of a rapid, direct effect on GABA(A) receptor activity and an indirect effect that appears to be mediated by neurosteroid synthesis. Furthermore, ethanol affects GABA(A) receptor activity through a presynaptic action, an effect that is not dependent on neurosteroid formation. These observations suggest that ethanol may modulate GABA(A) receptor function through an increase in de novo neurosteroid synthesis in the brain that is independent of the HPA axis. This novel mechanism may have a crucial role in mediating specific central effects of ethanol
Comparison of the effects of zaleplon, zolpidem, and triazolam at various GABA(A) receptor subtypes
No full textThe pyrazolopyrimidine zaleplon is a hypnotic agent that acts at the benzodiazepine recognition site of GABA(A) receptors. Zaleplon, like the hypnotic agent zolpidem but unlike classical benzodiazepines, exhibits preferential affinity for type I benzodiazepine (BZ(1)/omega(1)) receptors in binding assays. The modulatory action of zaleplon at GABA(A) receptors has now been compared with those of zolpidem and the triazolobenzodiazepine triazolam. Zaleplon potentiated GABA-evoked Cl- currents in Xenopus oocytes expressing human GABA(A) receptor subunits with a potency that was higher at alpha1beta2gamma2 receptors than at alpha2- or alpha3-containing receptors. Zolpidem, but not triazolam, also exhibited selectivity for alpha1-containing receptors. However, the potency of zaleplon at these various receptors was one-third to one-half that of zolpidem. Zaleplon and zolpidem also differed in their actions at receptors containing the alpha5 or gamma3 subunit. Zaleplon, zolpidem, and triazolam exhibited similar patterns of efficacy among the different receptor subtypes. The affinities of zaleplon for [H-3]flunitrazepam or t-[S-35]butylbicyclophosphorothionate ([S-35]TBPS) binding sites in rat brain membranes were lower than those of zolpidem or triazolam. Furthermore, zaleplon, unlike zolpidem, exhibited virtually no affinity for the peripheral type of benzodiazepine receptor. (C) 2002 Elsevier Science B.V. All rights reserved
Reduced brain levels of DHEAS in hepatic coma patients: significance for increased GABAergic tone in hepatic encephalopathy
No full textIncreased neurosteroids with allosteric modulatory activity on GABA A receptors such as 3α-5α tertrahydroprogesterone; allopregnanolone (ALLO), are candidates to explain the phenomenon of "increased GABAergic tone" in hepatic encephalopathy (HE). However, it is not known how changes of other GABA A receptor modulators such as dehydroepiandrosterone sulfate (DHEAS) contribute to altered GABAergic tone in HE. Concentrations of DHEAS were measured by radioimmunoassay in frontal cortex samples obtained at autopsy from 11 cirrhotic patients who died in hepatic coma and from an equal number of controls matched for age, gender, and autopsy delay intervals free from hepatic or neurological diseases. To assess whether reduced brain DHEAS contributes to increased GABAergic tone, in vitro patch clamp recordings in rat prefrontal cortex neurons were performed. A significant reduction of DHEAS (5.81 ± 0.88 ng/g tissue) compared to control values (9.70 ± 0.79 ng/g, p < 0.01) was found. Brain levels of DHEAS in patients with liver disease who died without HE (11.43 ± 1.74 ng/g tissue), and in a patient who died in uremic coma (12.56 ng/g tissue) were within the control range. Increasing ALLO enhances GABAergic tonic currents concentration-dependently, but increasing DHEAS reduces these currents. High concentrations of DHEAS (50 μM) reduce GABAergic tonic currents in the presence of ALLO, whereas reduced concentrations of DHEAS (1 μM) further stimulate these currents. These findings demonstrate that decreased concentrations of DHEAS together with increased brain concentrations of ALLO increase GABAergic tonic currents synergistically; suggesting that reduced brain DHEAS could further increase GABAergic tone in human HE. © 2012 Elsevier Ltd
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