Modulation Of Glutamate Transmission Research Articles

Implication of system xc− in neuroinflammation during the onset and maintenance of neuropathic pain

BackgroundDespite the high prevalence of neuropathic pain, treating this neurological disease remains challenging, given the limited efficacy and numerous side effects associated with current therapies. The complexity in patient management is largely attributed to an incomplete understanding of the underlying pathological mechanisms. Central sensitization, that refers to the adaptation of the central nervous system to persistent inflammation and heightened excitatory transmission within pain pathways, stands as a significant contributor to persistent pain. Considering the role of the cystine/glutamate exchanger (also designated as system xc−) in modulating glutamate transmission and in supporting neuroinflammatory responses, we investigated the contribution of this exchanger in the development of neuropathic pain.MethodsWe examined the implication of system xc− by evaluating changes in the expression/activity of this exchanger in the dorsal spinal cord of mice after unilateral partial sciatic nerve ligation. In this surgical model of neuropathic pain, we also examined the consequence of the genetic suppression of system xc− (using mice lacking the system xc− specific subunit xCT) or its pharmacological manipulation (using the pharmacological inhibitor sulfasalazine) on the pain-associated behavioral responses. Finally, we assessed the glial activation and the inflammatory response in the spinal cord by measuring mRNA and protein levels of GFAP and selected M1 and M2 microglial markers.ResultsThe sciatic nerve lesion was found to upregulate system xc− at the spinal level. The genetic deletion of xCT attenuated both the amplitude and the duration of the pain sensitization after nerve surgery, as evidenced by reduced responses to mechanical and thermal stimuli, and this was accompanied by reduced glial activation. Consistently, pharmacological inhibition of system xc− had an analgesic effect in lesioned mice.ConclusionTogether, these observations provide evidence for a role of system xc− in the biochemical processes underlying central sensitization. We propose that the reduced hypersensitivity observed in the transgenic mice lacking xCT or in sulfasalazine-treated mice is mediated by a reduced gliosis in the lumbar spinal cord and/or a shift in microglial M1/M2 polarization towards an anti-inflammatory phenotype in the absence of system xc−. These findings suggest that drugs targeting system xc− could contribute to prevent or reduce neuropathic pain.

The use of memantine for depressive symptomatology

IntroductionDepression is one of the most prevalent and incapacitating disease in current times and depressive symptoms have important global functioning implications.The serotonergic and glutamatergic systems are involved in the pathophysiology and treatment of depression. Ketamine is an N-methyl-d-aspartic acid (NMDA) receptor antagonist that has demonstrated an important role on depressive symptoms, but its use is restricted due to its dissociative effects and other possible adverse effects.Memantine is a noncompetitive antagonist of the NMDA receptor that modulates glutamate transmission. Memantine is used for the treatment of moderate to severe Alzheimer’s disease.ObjectivesIn this review, we aim to investigate, organize and synthetize the current data about the use of memantine for depressive symptoms.MethodsOur literature research focused on some of the most significative articles published in the last decade, including meta-analysis and systematic reviews.Results Most of the relevant literature suggests that memantine may effectively reduce depressive symptoms in patients with mood disorders.The literature also supports that memantine’s glutamatergic mechanism of action could reduce apathy and treat depression comorbid with alcohol abuse.Memantine affects brain-derived neurotrophic factor(BDNF) production suggesting that glutamate assumes an essential role in the pathology and etiology of depression. Also, the relationship between depression and the NMDA receptor is further supported by the fact that people with major depressive disorder demonstrate higher glutamate levels in the brain and blood.Moreover, current studies demonstrate that treatment with memantine as adjunct to selective serotonin reuptake inhibitors (namely sertraline) manifested a favourable safety and efficacy profile in patients with major depressive disorder.ConclusionsMemantine may have a wide therapeutic use beyond its utility in neurodegenerative diseases. More studies should be performed, especially larger controlled studies of longer duration focusing on long-term safety and efficacy.Disclosure of InterestNone Declared

N-acetylcysteine attenuates accumbal core neuronal activity in response to morphine in the reinstatement of morphine CPP in morphine extinguished rats

Numerous studies have suggested that N-acetylcysteine (NAC), has the potential to suppress drug craving in people with substance use disorder and reduce drug-seeking behaviors in animals. The nucleus accumbens (NAc) plays a crucial role in the brain's reward system, with the nucleus accumbens core (NAcore) specifically implicated in compulsive drug seeking and relapse. In this study, we aimed to explore the impact of subchronic NAC administration during the extinction period and acute NAC administration on the electrical activity of NAcore neurons in response to a priming dose of morphine in rats subjected to extinction from morphine-induced place preference (CPP).We conducted single-unit recordings in anesthetized rats on the reinstatement day, following the establishment of morphine-induced conditioned place preference (7 mg/kg, s.c., 3 days), and subsequent drug-free extinction. In the subchronically NAC-treated groups, rats received daily injections of either NAC (50 mg/kg; i.p.) or saline during the extinction period. On the reinstatement day, we recorded the spontaneous activity of NAcore neurons for 15 min, administered a priming dose of morphine, and continued recording for an additional 45 min. While morphine excited most recorded neurons in saline-treated rats, it failed to alter firing rates in NAC-treated rats that had received NAC during the extinction period. For acutely NAC-treated animals, we recorded the baseline activity of NAcore neurons for 10 min before administering a single injection of either NAC (50 mg/kg; i.p.) or saline in rats with no treatment during the extinction. Following 30 min of recording and a priming dose of morphine (1 mg/kg, s.c.), the recording continued for an additional 30 min. The firing activity of NAcore neurons did not show significant changes after morphine or NAC injection. In conclusion, our findings emphasize that daily NAC administration during the extinction period significantly attenuates the morphine-induced increase in firing rates of NAcore neurons during the reinstatement of morphine CPP. However, acute NAC injection does not produce the same effect. These results suggest that modulating glutamate transmission through daily NAC during extinction may effectively inhibit the morphine place preference following the excitatory effects of morphine on NAcore neurons.

Roles of corticotropin-releasing factor signaling in the lateral habenula in anxiety-like and alcohol drinking behaviors in male rats

Corticotropin-releasing factor (CRF) signaling in the mesocorticolimbic system is known to modulate anxiety-like behavior and alcohol consumption, behaviors that also have been associated with the hyper-glutamatergic state of the lateral habenula (LHb) neurons in rats. However, the role of CRF signaling in the LHb on the glutamate transmission, anxiety-like behaviors and alcohol consumption is unknown. Here, we used male rats that had been consuming alcohol for three months to address this gap in the literature. First, using electrophysiological techniques, we evaluated CRF's effects on the glutamate transmission in LHb neurons in brain slices. CRF facilitated glutamate transmission. The facilitation was greater in neurons of alcohol-withdrawing rats than in those of naïve rats. The facilitation was mimicked by the activation of CRF receptor 1 (CRF1R) but attenuated by the activation of CRF receptor 2 (CRF2R). This facilitation was mediated by upregulating CRF1R-protein kinase A signaling. Conversely, protein kinase C blockade attenuated CRF's facilitation in neurons of naïve rats but promoted it in neurons of alcohol-withdrawing rats. Next, using site-direct pharmacology, we evaluated the role of CRF signaling in the LHb on anxiety-like behaviors and alcohol consumption. Intra-LHb inhibition of CRF1R or activation of CRF2R ameliorated the anxiety-like behaviors in alcohol-withdrawing rats and reduced their alcohol intake when drinking was resumed. These observations provide the first direct behavioral pharmacological and cellular evidence that CRF signaling in the LHb modulates glutamate transmission, anxiety-like behaviors and alcohol consumption, and that adaptation occurs in CRF signaling in the LHb after chronic alcohol consumption.

Glutamatergic and Serotonergic Modulation of Rat Medial and Lateral Orbitofrontal Cortex in Visual Serial Reversal Learning.

Adapting behavior to a dynamic environment requires both steadiness when the environment is stable and behavioral flexibility in response to changes. Much evidence suggests that cognitive flexibility, which can be operationalized in reversal learning tasks, is mediated by cortico-striatal circuitries, with the orbitofrontal cortex (OFC) playing a prominent role. The OFC is a functionally heterogeneous region, and we have previously reported differential roles of lateral (lOFC) and medial (mOFC) regions in a touchscreen serial visual reversal learning task for rats using pharmacological inactivation. Here, we investigated the effects of pharmacological overactivation of these regions using a glutamate transporter 1 (GLT-1) inhibitor, dihydrokainate (DHK), which increases extracellular glutamate by blocking its reuptake. We also tested the impact of antagonism of the serotonin 2A receptor (5-HT2AR), which modulates glutamate action, in the mOFC and lOFC on the same task. Overactivation induced by DHK produced dissociable effects in the mOFC and lOFC, with more prominent effects in the mOFC, specifically improving performance in the early, perseveration phase. Intra-lOFC DHK increased the number of omitted responses without affecting errors. In contrast, blocking the 5-HT2AR in the lOFC impaired reversal learning overall, while mOFC 5-HT2AR blockade had no effect. These results further support dissociable roles of the rodent mOFC and lOFC in deterministic visual reversal learning and indicate that modulating glutamate transmission through blocking the GLT-1 and the 5-HT2AR have different roles in these two structures.

Effects of exercise on mGluR-mediated glutamatergic transmission in the striatum of hemiparkinsonian rats

Hyperexcitability in the corticostriatal glutamatergic pathway may have a pivotal role in the pathogenesis of Parkinson’s disease (PD). Metabotropic glutamate receptors (mGluRs) modulate glutamate transmission by both pre- and postsynaptic mechanisms, making them attractive targets for modifying pathological changes in the corticostriatal pathway. Exercise reportedly alleviates motor dysfunction and induced neuroplasticity in glutamatergic transmission. Here, the mGluR-mediated plasticity mechanism underlying behavioral improvement by exercise intervention was investigated. The experimental models were prepared by 6-hydroxydopamine injection into the right medial forebrain bundle. The models were evaluated with the apomorphine-induced rotation test. Starting 2 weeks postoperatively, exercise intervention was applied to the PD + Ex group for 4 weeks. The exercise-intervention effects on locomotor behavior, glutamate levels, and mGluR (mGluR2/3 and mGluR5) expression in hemiparkinsonian rats were investigated. The results showed that hemiparkinsonian rats have a significant increase in extracellular glutamate levels in the lesioned-lateral striatum. MGluR2/3 protein expression was reduced while mGluR5 protein expression was increased in the striatum. Notably, treadmill exercise markedly reversed these abnormal changes in the corticostriatal glutamate system and promoted motor performance in PD rats. These findings suggest that mGluR-mediated glutamatergic transmission in the corticostriatal pathway may serve as an attractive target for exercise-induced neuroplasticity in hemiparkinsonian rats.

Lateral hypothalamic orexin glucose-inhibited neurons may regulate reward-based feeding by modulating glutamate transmission in the ventral tegmental area

Glucose inhibits ∼60% of lateral hypothalamic (LH) orexin neurons. Fasting increases the activation of LH orexin glucose-inhibited (GI) neurons in low glucose. Increases in spontaneous glutamate excitatory postsynaptic currents (sEPSCs) onto putative VTA DA neurons in low glucose are orexin dependent (Sheng et al., 2014). VTA DA neurons modulate reward-based feeding. We tested the hypothesis that increased activation of LH orexin-GI neurons in low glucose increases glutamate signaling onto VTA DA neurons and contributes to reward-based feeding in food restricted animals. N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) currents on putative VTA DA neurons were measured using whole cell voltage clamp recording in horizontal brain slices containing the LH and VTA. Decreased glucose increased the NMDA receptor current for at least one hour after returning glucose to basal levels (P < 0.05; N = 8). The increased current was blocked by an orexin 1 receptor antagonist (P < 0.05; N = 5). Low glucose caused a similar persistent enhancement of AMPA receptor currents (P < 0.05; N = 7). An overnight fast increased the AMPA/NMDA receptor current ratio, an in vivo index of glutamate plasticity, on putative VTA DA neurons. Conditioned place preference (CPP) for palatable food was measured during LH dialysis with glucose. CPP score was negatively correlated with increasing LH glucose (P < 0.05; N = 20). These data suggest that increased activation of LH orexin-GI neurons in low glucose after weight loss, leads to enhanced glutamate signaling on VTA DA neurons, increases the drive to eat rewarding food, and may contribute to weight regain.

The role of glutamatergic modulation in the mechanism of action of ketamine, a prototype rapid-acting antidepressant drug.

Over the past decade, ketamine has been one of the most commonly studied potential antidepressants. This is because it produces a spectacularly rapid and persistent therapeutic effect in people suffering from severe treatment-resistant depression (TRD), for which classical drugs are ineffective. Similar efficacy was demonstrated for scopolamine, a drug belonging to a completely different pharmacological group. This interesting coincidence piqued the interest of psychopharmacologists and prompted them to search for a possible common mechanism of these rapid acting antidepressant drugs (RAADs). A thorough explanation of this mechanism is also important because each of these substances induces serious side effects. Knowing the mechanism responsible for the therapeutic efficacy of RAADs could lead to minimizing, or even avoiding certain undesirable effects. This review provides an overview of the mechanism of action of a prototype RAAD, ketamine, in animal models, with a particular focus on the roles of NMDA receptors, AMPA receptors, synaptogenesis, and modulation of glutamate transmission by other modulators of this system, such as mGlu receptor ligands. Recently studied roles for ketamine enantiomers and metabolites in its rapid antidepressant effect are also considered. Finally, the results of multiple clinical trials are reported and discussed in relation to basic research. This review concludes that success in introducing novel therapeutic RAADs will depend on better cooperation and integration of neuroscience research and clinical practice.

CRF modulates glutamate transmission in the central amygdala of naïve and ethanol-dependent rats

Corticotropin-releasing factor (CRF) signaling in the central nucleus of the amygdala (CeA) is hypothesized to drive the development of alcohol dependence, as it regulates ethanol intake and several anxiogenic behaviors linked to withdrawal. Excitatory glutamatergic neurotransmission contributes to alcohol reinforcement, tolerance and dependence. Therefore, in this study we used in vitro slice electrophysiology to investigate the effects of CRF and its receptor subtype (CRF1 and CRF2) antagonists on both evoked and spontaneous action potential-independent glutamatergic transmission in the CeA of naive and ethanol-dependent Sprague-Dawley rats. We found that CRF (25–200 nM) concentration-dependently diminished evoked compound excitatory postsynaptic potentials (EPSPs), but increased miniature excitatory postsynaptic current (mEPSC) frequencies similarly in CeA neurons of both naïve and ethanol-dependent rats, indicating reduced evoked glutamatergic responses and enhanced vesicular glutamate release, respectively. This CRF-induced vesicular glutamate release was prevented by the CRF1/2 antagonist (Astressin B) and the CRF1 antagonist (R121919), but not by the CRF2 antagonist (Astressin 2B). Similarly, CRF's effects on evoked glutamatergic responses were completely blocked by CRF1 antagonism, but only slightly decreased in the presence of the CRF2 antagonist. Moreover, CRF1 antagonism reveals a tonic facilitation of vesicular glutamate, whereas the CRF2 antagonism revealed a tonic inhibition of vesicular glutamate release. Collectively our data show that CRF primarily acts at presynaptic CRF1 to produce opposite effects on CeA evoked and spontaneous glutamate release and that the CRF system modulates CeA glutamatergic synapses throughout the development of alcohol dependence.

Prophylactic versus Therapeutic Fingolimod: Restoration of Presynaptic Defects in Mice Suffering from Experimental Autoimmune Encephalomyelitis.

Fingolimod, the first oral, disease-modifying therapy for MS, has been recently proposed to modulate glutamate transmission in the central nervous system (CNS) of mice suffering from Experimental Autoimmune Encephalomyelitis (EAE) and in MS patients. Our study aims at investigating whether oral fingolimod recovers presynaptic defects that occur at different stages of disease in the CNS of EAE mice. In vivo prophylactic (0.3 mg/kg for 14 days, from the 7th day post immunization, d.p.i, the drug dissolved in the drinking water) fingolimod significantly reduced the clinical symptoms and the anxiety-related behaviour in EAE mice. Spinal cord inflammation, demyelination and glial cell activation are markers of EAE progression. These signs were ameliorated following oral fingolimod administration. Glutamate exocytosis was shown to be impaired in cortical and spinal cord terminals isolated from EAE mice at 21 ± 1 d.p.i., while GABA alteration emerged only at the spinal cord level. Prophylactic fingolimod recovered these presynaptic defects, restoring altered glutamate and GABA release efficiency. The beneficial effect occurred in a dose-dependent, region-specific manner, since lower (0.1–0.03 mg/kg) doses restored, although to a different extent, synaptic defects in cortical but not spinal cord terminals. A delayed reduction of glutamate, but not of GABA, exocytosis was observed in hippocampal terminals of EAE mice at 35 d.p.i. Therapeutic (0.3 mg/kg, from 21 d.p.i. for 14 days) fingolimod restored glutamate exocytosis in the cortex and in the hippocampus of EAE mice at 35 ± 1 d.p.i. but not in the spinal cord, where also GABAergic defects remained unmodified. These results improve our knowledge of the molecular events accounting for the beneficial effects elicited by fingolimod in demyelinating disorders.

In vitro pharmacological and rat pharmacokinetic characterization of LY3020371, a potent and selective mGlu2/3 receptor antagonist

Metabotropic glutamate 2/3 (mGlu2/3) receptors are of considerable interest owing to their role in modulating glutamate transmission via presynaptic, postsynaptic and glial mechanisms. As part of our ongoing efforts to identify novel ligands for these receptors, we have discovered (1S,2R,3S,4S,5R,6R)-2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid; (LY3020371), a potent and selective orthosteric mGlu2/3 receptor antagonist. In this account, we characterize the effects of LY3020371 in membranes and cells expressing human recombinant mGlu receptor subtypes as well as in native rodent and human brain tissue preparations, providing important translational information for this molecule. In membranes from cells expressing recombinant human mGlu2 and mGlu3 receptor subtypes, LY3020371.HCl competitively displaced binding of the mGlu2/3 agonist ligand [3H]-459477 with high affinity (hmGlu2 Ki = 5.26 nM; hmGlu3 Ki = 2.50 nM). In cells expressing hmGlu2 receptors, LY3020371.HCl potently blocked mGlu2/3 agonist (DCG-IV)-inhibited, forskolin-stimulated cAMP formation (IC50 = 16.2 nM), an effect that was similarly observed in hmGlu3-expressing cells (IC50 = 6.21 nM). Evaluation of LY3020371 in cells expressing the other human mGlu receptor subtypes revealed high mGlu2/3 receptor selectivity. In rat native tissue assays, LY3020371 demonstrated effective displacement of [3H]-459477 from frontal cortical membranes (Ki = 33 nM), and functional antagonist activity in cortical synaptosomes measuring both the reversal of agonist-suppressed second messenger production (IC50 = 29 nM) and agonist-inhibited, K+-evoked glutamate release (IC50 = 86 nM). Antagonism was fully recapitulated in both primary cultured cortical neurons where LY3020371 blocked agonist-suppressed spontaneous Ca2+ oscillations (IC50 = 34 nM) and in an intact hippocampal slice preparation (IC50 = 46 nM). Functional antagonist activity was similarly demonstrated in synaptosomes prepared from epileptic human cortical or hippocampal tissues, suggesting a translation of the mGlu2/3 antagonist pharmacology from rat to human. Intravenous dosing of LY3020371 in rats led to cerebrospinal fluid drug levels that are expected to effectively block mGlu2/3 receptors in vivo. Taken together, these results establish LY3020371 as an important new pharmacological tool for studying mGlu2/3 receptors in vitro and in vivo.This article is part of the Special Issue entitled ‘Metabotropic Glutamate Receptors, 5 years on’.

Metabotropic Glutamate Receptor 7 (mGluR7) as a Target for the Treatment of Psychostimulant Dependence.

Although few medications have been approved by the U.S. Food and Drug Administration (FDA) to assist people to quit tobacco smoking, there are no FDA-approved medications to treat dependence on other psychostimulant drugs, such as cocaine. The motivation to maintain psychostimulant drug seeking and self-administration involves alterations in glutamatergic neurotransmission. Thus, medications that modulate glutamate transmission may be effective treatments for psychostimulant dependence. One presynaptic inhibitory glutamate receptor that critically regulates glutamate transmission is the metabotropic glutamate 7 receptor (mGluR7). This review summarizes nonhuman experimental animal data that indicate a critical role for mGluR7 in drug-taking and drug-seeking behaviors for the psychostimulants cocaine and nicotine. AMN082, the only commercially available allosteric receptor agonist, has been used to investigate the role of mGluR7 in psychostimulant dependence. Systemic administration or microinjection of AMN082 into brain sites within the mesocorticolimbic system decreased self-administration and reinstatement of both cocaine and nicotine seeking. In vivo microdialysis results indicated that a nucleus accumbens-ventral pallidum γ-aminobutyric acid-ergic mechanism may underlie AMN082-induced antagonism of the reinforcing effects of cocaine, whereas a glutamate mGlu2/3 receptor mechanism underlies the AMN082-induced blockade of cocaine seeking. These findings indicate an important role for mGluR7 in mesolimbic areas in modulating the reinforcing effects of psychostimulant drugs, such as nicotine and cocaine, and the conditioned behaviors associated with drugs of abuse. Thus, selective mGluR7 agonists or positive allosteric modulators may have the potential to treat psychostimulant dependence.

Hippocampal-Dependent Antidepressant Action of the H3 Receptor Antagonist Clobenpropit in a Rat Model of Depression.

Background:Histamine is a modulatory neurotransmitter regulating neuronal activity. Antidepressant drugs target modulatory neurotransmitters, thus ultimately regulating glutamatergic transmission and plasticity. Histamine H3 receptor (H3R) antagonists have both pro-cognitive and antidepressant effects; however, the mechanism by which they modulate glutamate transmission is not clear. We measured the effects of the H3R antagonist clobenpropit in the Flinders Sensitive Line (FSL), a rat model of depression with impaired memory and altered glutamatergic transmission.Methods:Behavioral tests included the forced swim test, memory tasks (passive avoidance, novel object recognition tests), and anxiety-related paradigms (novelty suppressed feeding, social interaction, light/dark box tests). Hippocampal protein levels were detected by Western blot. Hippocampal plasticity was studied by in slice field recording of CA3-CA1 long-term synaptic potentiation (LTP), and glutamatergic transmission by whole-cell patch clamp recording of excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons.Results:Clobenpropit, administered systemically or directly into the hippocampus, decreased immobility during the forced swim test; systemic injections reversed memory deficits and increased hippocampal GluN2A protein levels. FSL rats displayed anxiety-related behaviors not affected by clobenpropit treatment. Clobenpropit enhanced hippocampal plasticity, but did not affect EPSCs. H1R and H2R antagonists prevented the clobenpropit-induced increase in LTP and, injected locally into the hippocampus, blocked clobenpropit’s effect in the forced swim test.Conclusions:Clobenpropit’s antidepressant effects and the enhanced synaptic plasticity require hippocampal H1R and H2R activation, suggesting that clobenpropit acts through disinhibition of histamine release. Clobenpropit reverses memory deficits and increases hippocampal GluN2A expression without modifying anxiety-related phenotypes or EPSCs in CA1 pyramidal neurons.

Hydrogen sulfide decreases a slow rectifying potassium conductance in nucleus tractus solitarius neurons (1129.4)

Hydrogen Sulfide (H2S) is an endogenously produced gasotransmitter which has been shown to play a role within the central nervous system. Prior studies have shown that H2S has presynaptic effects on neurons of the nucleus tractus solitarius (NTS) through modulation of glutamate transmission. We have also recently demonstrated postsynaptic effects of H2S on the membrane potential of NTS neurons. The current study was undertaken to investigate the ion channels in NTS neurons modulated by H2S. Using whole cell voltage‐clamp recordings we found that H2S causes a decrease in current evoked in the depolarizing range of a slow voltage ramp indicative of modulation of the voltage activated potassium conductance IK. This conductance was found to decrease by a mean of 20.2 ± 2.5% (n=11) at the end of the ramp (10 mV) following application of 10 mM NaHS, a H2S donor, with all 11 neurons influenced in a similar manner. All cells demonstrated a partial to full recovery of this conductance after wash off of NaHS. The present work suggests that H2S controls the excitability of NTS neurons, at least in part through the modulation of the delayed rectifier potassium conductance, IK.Grant Funding Source: This work was supported by the Canadian Institutes for Health Research.

Convergence of dopamine and glutamate signaling onto striatal ERK activation in response to drugs of abuse

Despite their distinct targets, all addictive drugs commonly abused by humans evoke increases in dopamine (DA) concentration within the striatum. The main DA Guanine nucleotide binding protein couple receptors (GPCRs) expressed by medium-sized spiny neurons of the striatum are the D1R and D2R, which are positively and negatively coupled to cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling, respectively. These two DA GPCRs are largely segregated into distinct neuronal populations, where they are co-expressed with glutamate receptors in dendritic spines. Direct and indirect interactions between DA GPCRs and glutamate receptors are the molecular basis by which DA modulates glutamate transmission and controls striatal plasticity and behavior induced by drugs of abuse. A major downstream target of striatal D1R is the extracellular signal-regulated kinase (ERK) kinase pathway. ERK activation by drugs of abuse behaves as a key integrator of D1R and glutamate NMDAR signaling. Once activated, ERK can trigger chromatin remodeling and induce gene expression that permits long-term cellular alterations and drug-induced morphological and behavioral changes. Besides the classical cAMP/PKA pathway, downstream of D1R, recent evidence implicates a cAMP-independent crosstalk mechanism by which the D1R potentiates NMDAR-mediated calcium influx and ERK activation. The mounting evidence of reciprocal modulation of DA and glutamate receptors adds further intricacy to striatal synaptic signaling and is liable to prove relevant for addictive drug-induced signaling, plasticity, and behavior. Herein, we review the evidence that built our understanding of the consequences of this synergistic signaling for the actions of drugs of abuse.

Mediators of Inflammation as Targets for Chronic Pain Treatment

Mediators of Inflammation as Targets for Chronic Pain Treatment

GluN2B subunit deletion reveals key role in acute and chronic ethanol sensitivity of glutamate synapses in bed nucleus of the stria terminalis.

The bed nucleus of the stria terminalis (BNST) is a critical region for alcohol/drug-induced negative affect and stress-induced reinstatement. NMDA receptor (NMDAR)-dependent plasticity, such as long-term potentiation (LTP), has been postulated to play key roles in alcohol and drug addiction; yet, to date, little is understood regarding the mechanisms underlying LTP of the BNST, or its regulation by ethanol. Acute and chronic exposure to ethanol modulates glutamate transmission via actions on NMDARs. Despite intense investigation, tests of subunit specificity of ethanol actions on NMDARs using pharmacological approaches have produced mixed results. Thus, we use a conditional GluN2B KO mouse line to assess both basal and ethanol-dependent function of this subunit at glutamate synapses in the BNST. Deletion of GluN2B eliminated LTP, as well as actions of ethanol on NMDAR function. Further, we show that chronic ethanol exposure enhances LTP formation in the BNST. Using KO-validated pharmacological approaches with Ro25-6981 and memantine, we provide evidence suggesting that chronic ethanol exposure enhances LTP in the BNST via paradoxical extrasynaptic NMDAR involvement. These findings demonstrate that GluN2B is a key point of regulation for ethanol's actions and suggest a unique role of extrasynaptic GluN2B-containing receptors in facilitating LTP.

Loss of striatal cannabinoid CB1 receptor function in attention‐deficit / hyperactivity disorder mice with point‐mutation of the dopamine transporter

Abnormal dopamine (DA) transmission in the striatum plays a pivotal role in attention-deficit/hyperactivity disorder (ADHD). As striatal DA signalling modulates the endocannabinoid system (ECS), the present study was aimed at investigating cannabinoid CB1 receptor (CB1R) function in a model of ADHD obtained by triple point-mutation in the dopamine transporter (DAT) gene in mice, making them insensitive to cocaine [DAT cocaine-insensitive (DAT-CI) mice]. DAT-CI mice had a marked hyperactive phenotype, and neurophysiological recordings revealed that the sensitivity of CB1Rs controlling GABA-mediated synaptic currents [CB1Rs((GABA)) ] in the striatum was completely lost. In contrast, CB1Rs modulating glutamate transmission [CB1Rs((Glu)) ], and GABA(B) receptors were not affected in this model of ADHD. In DAT-CI mice, the blockade of CB1R((GABA)) function was complete even after cocaine or environmental manipulations activating the endogenous DA-dependent reward system, which are known to sensitize these receptors in control animals. Conversely, the hedonic property of sucrose was intact in DAT-CI mice, indicating normal sweet perception in these animals. Our results point to CB1Rs as novel molecular players in ADHD, and suggest that therapeutic strategies aimed at interfering with the ECS might prove effective in this disorder.

Preferential accumulation of amyloid-beta in presynaptic glutamatergic terminals (VGluT1 and VGluT2) in Alzheimer's disease cortex

Amyloid-beta (Aβ) is thought to play a central role in synaptic dysfunction (e.g. neurotransmitter release) and synapse loss. Glutamatergic dysfunction is involved in the pathology of Alzheimer's disease (AD) and perhaps plays a central role in age-related cognitive impairment. Yet, it is largely unknown whether Aβ accumulates in excitatory boutons. To assess the possibility that glutamatergic terminals are lost in AD patients, control and AD synaptosomes were immunolabeled for the most abundant vesicular glutamate transporters (VGluT1 and VGluT2) and quantified by flow cytometry and immunoblot methods. In post-mortem parietal cortex from aged control subjects, glutamatergic boutons are fairly abundant as approximately 40% were immunoreactive for VGluT1 (37%) and VGluT2 (39%). However, the levels of these specific markers of glutamatergic synapses were not significantly different among control and AD cases. To test the hypothesis that Aβ is associated with excitatory terminals, AD synaptosomes were double-labeled for Aβ and for VGluT1 and VGluT2, and analyzed by flow cytometry and confocal microscopy. Our study demonstrated that Aβ immunoreactivity (IR) was present in glutamatergic terminals of AD patients. Quantification of Aβ and VGluT1 in a large population of glutamatergic nerve terminals was performed by flow cytometry, showing that 42% of VGluT1 synaptosomes were immunoreactive for Aβ compared to 9% of VGluT1 synaptosomes lacking Aβ-IR. Percentage of VGluT2 synaptosomes immunoreactive for Aβ (21%) was significantly higher than VGluT2 synaptosomes lacking Aβ-IR (9%). Moreover, Aβ preferentially affects VGluT1 (42% positive) compared to VGluT2 terminals (21%). These data represent the first evidence of high levels of Aβ in excitatory boutons in AD cortex and support the hypothesis that Aβ may play a role in modulating glutamate transmission in AD terminals.

Recent Progress in the Synthesis and Characterization of Group II Metabotropic Glutamate Receptor Allosteric Modulators

Group II metabotropic glutamate (mGlu) receptors consist of the metabotropic glutamate 2 (mGlu(2)) and metabotropic glutamate 3 (mGlu(3)) receptor subtypes which modulate glutamate transmission by second messenger activation to negatively regulate the activity of adenylyl cyclase. Excessive accumulation of glutamate in the perisynaptic extracellular region triggers mGlu(2) and mGlu(3) receptors to inhibit further release of glutamate. There is growing evidence that the modulation of glutamatergic neurotransmission by small molecule modulators of Group II mGlu receptors has significant potential for the treatment of several neuropsychiatric and neurodegenerative diseases. This review provides an overview of recent progress on the synthesis and pharmacological characterization of positive and negative allosteric modulators of the Group II mGlu receptors.