Mood disorders: regulation by metabotropic glutamate receptors

Medicinal therapies for mood disorders neither fully serve the efficacy needs of patients nor are they free of side-effect issues. Although monoamine-based therapies are the primary current treatment approaches, both preclinical and clinical findings have implicated the excitatory neurotransmitter glutamate in the pathogenesis of major depressive disorders. The present commentary focuses on the metabotropic glutamate receptors and their relationship to mood disorders. Metabotropic glutamate (mGlu) receptors regulate glutamate transmission by altering the release of neurotransmitter and/or modulating the post-synaptic responses to glutamate. Convergent biochemical, pharmacological, behavioral, and clinical data will be reviewed that establish glutamatergic neurotransmission via mGlu receptors as a biologically relevant process in the regulation of mood and that these receptors may serve as novel targets for the discovery of small molecule modulators with unique antidepressant properties. Specifically, compounds that antagonize mGlu2, mGlu3, and/or mGlu5 receptors (e.g. LY341495, MGS0039, MPEP, MTEP) exhibit biochemical effects indicative of antidepressant effects as well as in vivo activity in animal models predictive of antidepressant efficacy. Both preclinical and clinical data have previously been presented to define NMDA and AMPA receptors as important targets for the modulation of major depression. In the present review, we present a model suggesting how the interplay of glutamate at the mGlu and at the ionotropic AMPA and NMDA receptors might account for the antidepressant-like effects of glutamatergic- and monoaminergic-based drugs affecting mood in patients. The current data lead to the hypothesis that mGlu-based compounds and conventional antidepressants impact a network of interactive effects that converge upon a down regulation of NMDA receptor function and an enhancement in AMPA receptor signaling.     

The antidepressant-like effects of glutamatergic drugs ketamine and AMPA receptor potentiator LY 451646 are preserved in bdnf⁺/⁻ heterozygous null mice

Accumulating evidence suggests that biogenic amine-based antidepressants act, at least in part, via regulation of brain-derived neurotrophic factor (BDNF) signaling. Biogenic amine-based antidepressants increase BDNF synthesis and activate its signaling pathway through TrkB receptors. Moreover, the antidepressant-like effects of these molecules are abolished in BDNF deficient mice. Glutamate-based drugs, including the NMDA antagonist ketamine, and the AMPA receptor potentiator LY 451646, mimic the effects of antidepressants in preclinical tests with high predictive validity. In humans, a single intravenous dose of ketamine produces an antidepressant effect that is rapid, robust and persistent. In this study, we examined the role of BDNF in expression of the antidepressant-like effects of ketamine and an AMPA receptor potentiator (LY 451646) in the forced swim test (FST). Ketamine and LY 451646 produced antidepressant-like effects in the FST in mice at 45 min after a single injection, but no effects were observed one week after a single ketamine injection. As previously reported, the effects of imipramine in the forced swim test were blunted in heterozygous BDNF knockout (bdnf(+/-)) mice. However ketamine and LY 451646 produced similar antidepressant-like responses in wildtype and bdnf(+/-) mice. Neither ketamine nor LY 451646 significantly influenced the levels BDNF or TrkB phosphorylation in the hippocampus when assessed at 45 min or 7 days after the drug administration. These data demonstrate that under the conditions tested, neither ketamine nor the AMPA-potentiator LY 451656 activate BDNF signaling, but produce a characteristic antidepressant-like response in heterozygous bdnf(+/-) mice. These data indicate that unlike biogenic amine-based agents, BDNF signaling does not play a pivotal role in the antidepressant effects of glutamate-based compounds. This article is part of a Special Issue entitled 'Anxiety and Depression'.     

A role for AMPA receptors in mood disorders

Major antidepressant agents increase synaptic levels of monoamines. Although the monoamine hypothesis of depression remains a cornerstone of our understanding of the pathophysiology of depression, emerging data has suggested that the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtype of glutamate receptor may also play a pivotal role in depression. Positive allosteric modulators of AMPA receptors increase brain levels of brain-derived neurotrophic factor (BDNF) that impacts the viability and generation of neurons in key brain structures. AMPA receptor potentiators are active in rodent models predictive of antidepressant efficacy. The mechanisms by which AMPA receptor potentiators produce these biological effects, however, are uncertain. Current evidence points to an antidepressant mechanism that is independent of monoaminergic facilitation that is driven by neurogenesis, a process facilitated by increased BDNF expression. However, alternative hypotheses need to be considered given uncertainties in the relationship between BDNF increases and the effects of conventional antidepressant medications. Electrophysiological and protein conformational data indicate that structural variants of AMPA receptor potentiators can differentially modulate AMPA receptor-mediated currents, although the manner in which this impacts antidepressant efficacy is yet to be understood. Conventional antidepressants such as fluoxetine positively modulate AMPA receptors. This potentiation is engendered by specific phosphorylation pathways activated through the dopamine- and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32). Other novel compounds with antidepressant-like effects in rodents may also produce their in vivo effects through potentiation of AMPA receptors. Thus, AMPA receptor potentiation might be a general mechanism through which the clinical outcome of antidepressant efficacy is achieved.     

Effect of brain-derived neurotrophic factor on activity-regulated cytoskeleton-associated protein gene expression in primary frontal cortical neurons. Comparison with NMDA and AMPA

The effect of brain-derived neurotrophic factor (BDNF) on activity-regulated cytoskeleton-associated protein (Arc) mRNA levels in primary neuronal cultures of rat frontal cortex was characterized pharmacologically and compared to the effect on expression of c-fos, bdnf, neuritin, cox-2 as examples of other immediate early genes. BDNF induced a very strong increase (around 100 fold) in Arc mRNA and the maximal effect seen at 25 ng/ml. The effect was dose-dependent with EC50 around 1.6 ng/ml. The time profile revealed a significant effect after 25 min. BDNF also increased levels of c-Fos, neuritin and BDNF mRNA, but not COX-2 mRNA. The pharmacological profile of NMDA and AMPA-induced arc gene expression in frontal cortical neurons was compared to BDNF. NMDA and AMPA increased Arc mRNA but their maximal effect did not exceed 20-fold. The effect of AMPA was completely blocked by the NMDA receptor antagonist MK-801. Further, the relative amount of Arc mRNA compared to c-Fos mRNA was higher for BDNF, equal for NMDA and lower for AMPA. These results demonstrate BDNF to be a highly potent and efficient inducer of arc gene expression in vitro, emphasizing the role of this growth factor in synaptic plasticity in the frontal cortex.     

Behavioral and serotonergic consequences of decreasing or increasing hippocampus brain-derived neurotrophic factor protein levels in mice

Antidepressants such as Selective Serotonin Reuptake Inhibitors (SSRI) act as indirect agonists of serotonin (5-HT) receptors. Although these drugs produce a rapid blockade of serotonin transporters (SERTs) in vitro, several weeks of treatment are necessary to observe clinical benefits. This paradox has not been solved yet. Recent studies have identified modifications of intracellular signaling proteins and target genes that could contribute to antidepressant-like activity of SSRI (e.g., increases in neurogenesis and BDNF protein levels), and may explain, at least in part, their long delay of action. Although these data suggest a positive regulation of 5-HT on the expression of the gene coding for BDNF, the reciprocal effects of BDNF on brain 5-HT neurotransmission remains poorly documented. To study the impact of BDNF on serotonergic activity, a dual experimental strategy was used to analyze neurochemical and behavioral consequences of its decrease (strategy 1) or increase (strategy 2) in the brain of adult male mice. (1) In heterozygous BDNF+/- mice in which brain BDNF protein levels were decreased by half, an enhancement of basal extracellular 5-HT levels (5-HText) that induced a down-regulation of SERT, i.e., a decrease in its capacity to reuptake 5-HT, was found in the hippocampus. In addition, the SSRI, paroxetine, failed to increase hippocampal 5-HText in BDNF+/- mice, while it produces robust effects in wild-type littermates. Thus, BDNF+/- mice can be viewed as an animal model of genetic resistance to serotonergic antidepressant drugs. (2) In wild-type BDNF+/+ mice, the effects of intra-hippocampal (vHi) injection of BDNF (100 ng) in combination with a SSRI was examined by using intracerebral microdialysis and behavioral paradigms that predict an antidepressant- and anxiolytic-like activity of a molecule [the forced swim test (FST) and the open field paradigm (OF) respectively]. BDNF induced a rapid and transient increase in paroxetine response on 5-HText in the adult hippocampus, which was correlated with a potentiation of its antidepressant-like activity in the FST. The effects of BDNF were selectively blocked by K252a, an antagonist of its high-affinity TrkB receptor. Such a correlation between neurochemical and behavioral effects of [BDNF+SSRI] co-administration suggests that its antidepressant-like activity is linked to the activation of 5-HT neurotransmission in the adult hippocampus. BDNF also had a facilitatory effect on anxiety-like behavior in the OF test, and paroxetine prevented this anxiogenesis. What was the mechanism by which BDNF exerted these latter effects? Surprisingly, by using zero net flux method of quantitative microdialysis in vivo, we found that an intra-hippocampal BDNF injection in wild-type mice decreased the functional activity of SERT as observed in BDNF+/- mice. However, the decreased capacity of SERT to reuptake 5-HT was not associated to an increase in basal 5-HText in the hippocampus of WT mice. Interestingly, using in situ hybridization experiments indicated that TrkB receptor mRNA was expressed in the hippocampus and dorsal raphe nucleus in adult mice suggesting that the neurochemical and behavioral effects of intra-hippocampal BDNF injection can mobilize both pre- and post-synaptic elements of the brain 5-HT neurotransmission. Taken together, these set of experiments unveiled a relative opposition of neurochemical and behavioral responses following either a decrease (in BDNF+/- mutant mice) or an increase in brain BDNF levels (bilateral intra-hippocampal injection) in adult mice. In view of developing new antidepressant drug strategy, a poly-therapy combining BDNF with a chronic SSRI treatment could thus improve the efficacy of current medications.     

Presynaptic AMPA and kainate receptors increase the size of GABAergic terminals and enhance GABA release

In the developing cerebellum, NMDA receptors promote the maturation of axonal terminals of inhibitory interneurons. We compared the effects of AMPA/kainate receptor agonists in cultured cerebellar cells from GAD65-eGFP mice. Both AMPA and kainate augmented granule cell survival without affecting interneurons. The action of kainate was blocked by an AMPA but not by a NMDA receptor antagonist, suggesting AMPA receptor involvement. AMPA and kainate increased the size of the GABAergic terminals and the action of kainate was insensitive to NMDA blockers. Whole cell recordings in granule neurons revealed that chronic treatment for 5 days with kainate as well as NMDA decreased AMPA receptor expression while interneuronal kainate receptors were depressed by kainate treatment. Acute kainate application increased mIPSCs frequency in both granule neurons and interneurons and this effect was only partially blocked by an AMPA receptor antagonist. In contrast to what was reported for NMDA, chronic treatment with kainate induced a significant decrease of the basal mIPSCs frequency but increased the acute action of kainate on mIPSCs. Direct recordings from presynaptic GABAergic terminals suggest that AMPA and kainate receptors are present in developing GABAergic terminals and their activation affects the size of GABAergic terminals and spontaneous GABA release.     

Cleft-type cyclophanes confer neuroprotection against excitatory neurotoxicity in vitro and in vivo through inhibition of NMDA receptors

The cleft-type cyclophanes (ACCn, DNCn and TsDCn) were found to strongly inhibit macroscopic currents at heteromeric NMDA receptors (NR1/NR2) but not AMPA receptors expressed in Xenopus oocytes at voltage-clamp recording. The inhibition by cleft-type cyclophanes was voltage-dependent, because the inhibition was larger at -100 mV than at -20 mV. Mutations at NR1 N650, located in the vestibule of the channel pore, reduced the inhibition by DNCn and TsDCn, suggesting that the residue (N650) interacts with these cleft-type cyclophanes. Cell toxicity of TsDCn on SH-SY5Y cells was slightly weaker than that of memantine. The neuroprotective effects of cleft-type cyclophanes against cell damage caused by NMDA were investigated in cultured rat hippocampal neurons. Addition of 10 microM DNCn or TsDCn into the medium ablated the neurotoxicity induced by NMDA, and a similar effect was also observed with memantine. The neuroprotective effects of cleft-type cyclophanes were then assayed on NMDA-induced seizures in mice. Intracerebroventricular injection of TsDCn (5 mg/mouse) decreased the seizure induced by intraperitoneal injection of NMDA (115 mg/kg) in mice. The results demonstrate that these cleft-type cyclophanes interact directly with the extracellular mouth of the NMDA channel pore and exhibit neuroprotective effects on NMDA-induced excitatory toxicity in primary cultured neurons and mice.     

Alteration of the Centromedial Amygdala Glutamatergic Synapses by the BDNF Val66Met Polymorphism

Fear expression is mediated by an activation of the centromedial amygdala (CEm), the major output nucleus of the amygdaloid complex. Consistently, fear extinction is associated with an increased synaptic inhibition as well as a suppression of the excitability of the CEm neurons. However, little is known about the role of CEm glutamatergic synapses in fear regulation and anxiety-like behaviors. The BDNF Val66Met, a single-nucleotide polymorphism in the human BDNF gene, impairs fear extinction and leads to anxiety-like symptoms. To determine whether the BDNF Val66Met polymorphism affects the CEm excitatory synapses, we examined basal glutamatergic synaptic transmission and plasticity in the CEm neurons of BDNF Val66Met knock-in (BDNF^Met/Met) mice. The BDNF Val66Met single-nucleotide polymorphism exerted an opposite effect on non-NMDA and NMDA receptor transmission with a potentiation of the former and a suppression of the latter. In addition, the decay time of NMDA currents was decreased in BDNF^Met/Met mice, suggesting a modification of NMDA receptor subunit composition. Unlike the wild-type mice that exhibited a potentiation of non-NMDA receptor transmission following fear conditioning and a depotentiation upon fear extinction, BDNF^Met/Met mice failed to show this experience-dependent synaptic plasticity in the CEm neurons. Our results suggest that the elevated non-NMDA receptor transmission, the suppression of NMDA receptor transmission, and an impairment of synaptic plasticity in the CEm neurons might contribute to the fear extinction deficit and increased anxiety-like symptoms in BDNF Val66Met carriers.     

Desensitization of AMPA receptors and AMPA-NMDA receptor interaction: an in vivo cyclic GMP microdialysis study in rat cerebellum.

1. Desensitization is an important characteristic of glutamate receptors of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type. 2. Stimulation of N-methyl-D-aspartate (NMDA) or AMPA receptors in cerebellum results in increased production of cyclic GMP. We have investigated AMPA receptor desensitization in vivo by monitoring extracellular cyclic GMP during intracerebellar microdialysis in conscious unrestrained adult rats. 3. Local infusion of AMPA (10 to 100 microM) caused dose-related elevations of cyclic GMP levels. The effect of AMPA was prevented by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) and by the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NOARG). 4. In the absence of AMPA, DNQX lowered the basal levels of cyclic GMP whereas the NMDA receptor channel antagonist dizocilpine (MK-801) was ineffective. 5. Cyclothiazide, a blocker of AMPA receptor desensitization, potentiated the cyclic GMP response to exogenous AMPA. Moreover, cyclothiazide (100-300 microM) produced on its own dose-dependent elevations of extracellular cyclic GMP. The cyclothiazide-induced response was prevented not only by DNQX but also by MK-801. 6. While the cyclic GMP response elicited by AMPA was totally insensitive to MK-801, the response produced by AMPA (10 microM) plus cyclothiazide (30 microM) was strongly attenuated by the NMDA receptor antagonist (30 microM). 7. The results suggest that (a) AMPA receptors linked to the NO-cyclic GMP pathway in the cerebellum can undergo desensitization in vivo during exposure to exogenous AMPA; cyclothiazide inhibits such desensitization; (b) AMPA receptors (but not NMDA receptors) are 'tonically' activated and kept in a partly desensitized state by endogenous glutamate; (c) if cyclothiazide is present, activation of AMPA receptors may permit endogenous activation of NMDA receptors.     

Effects of intracerebroventricular NMDA and non-NMDA receptor agonists or antagonists on general anesthesia of propofol in mice

The effects of intracerebroventricular (icv) agonists and antagonists of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors on the general anesthesia of propofol were studied. A total of 144 Kunming mice, male and female with body mass of (22±3) g, were used. Part One of the Experiment: a total of 104 Kunming mice, male and female, were randomly divided into 13 groups. Intracerebroventricular artificial cerebral fluid (aCSF) or different doses of NMDA, AMPA, MK-801 or NBQX was injected immediately after intravenously administered propofol 25 mg/kg and the recovery time following the loss of righting reflex (LORR) was recorded. Part Two of the Experiment: a total of 40 Kunming female mice were divided randomly into 5 groups and injected with icv aCSF or NMDA, AMPA, MK-801 or NBQX after intraperitoneally administered propofol 50 mg/kg. The pain threshold of the mice was then investigated by hot-plate test (HPPT). NMDA (0.05 or 0.075μg, icv) or AMPA (0.05 μg, icv) exhibited no effects on the LORR, but NMDA (0.1 μg, icv) or AMPA (0.075 or 0.1 μg, icv) prolonged the LORR significantly compared with the aCSF group (P<0.05, P<0.01). The LORR of the 2 μg MK-801 group had no changes, while those of the 4 or 8 μg MK-801 groups were prolonged significantly. The LORR of the 0.5, 2 or 4 μg NBQX groups were all prolonged significantly. NMDA 0.05 μg or AMPA 0.05 μg decreased the pain threshold slightly but did not differ in effect compared with the aCSF group; 2 μg MK-801 or 0.5 μg NBQX both increased the pain threshold significantly. Our results indicate that propofol produces general anesthesia partly through an interaction with brain NMDA and AMPA receptors in mice.     

Regulation of BDNF expression in primary neuron culture by LY392098, a novel AMPA receptor potentiator

The effects of a novel AMPA receptor potentiator (LY392098) on the expression of brain-derived neurotrophic factor (BDNF) were examined in primary neuron culture. The addition of either AMPA or LY392098 to cortical neurons elicited a time and concentration dependent increase in mRNA encoding BDNF. Moreover, co-addition of subeffective concentrations of AMPA (1 μM) and LY392098 (1 μM) resulted in dramatic increases in both BDNF mRNA (>25-fold) and protein (7-fold) levels, whilst no changes in either NT-3 or NT-4 mRNA were detected. More modest (1.5–2.5-fold) elevations in BDNF mRNA and protein expression were also produced by combinations of AMPA and LY392098 in cerebellar granule cell neurons. In contrast, AMPA and LY392098, either alone or in combination, did not elevate BDNF mRNA levels in primary astroglial cultures. Maximum elevations in BDNF mRNA and protein were produced by 6–12 h of AMPA receptor activation 1–3 h of AMPA receptor activation were required to elevate BDNF mRNA levels. AMPA receptor-mediated increases in BDNF mRNA and protein were abolished by the AMPA antagonist, NBQX, but were unaffected by the NMDA antagonist, MK-801. In cortical neuron cultures, activation of both L-type Ca⁺² channels and mitogen-activated protein (MAP) kinases contribute to AMPA receptor-mediated increases in BDNF mRNA. The ability of LY392098 to increase the expression of BDNF in primary neuron culture indicates this and related biarylpropylsulfonamides may be useful in the treatment of neuropsychiatric disorders.     

Neurotrophins induce BDNF expression through the glutamate receptor pathway in neocortical neurons

Neurotrophins jointly exert various functions in the nervous system, including neuronal differentiation, survival, and regulation of synaptic plasticity. However, the functional interactions of neurotrophins or mechanisms through which neurotrophins regulate each other are still not clear. In the present study, brain-derived neurotrophic factor (BDNF) mRNA expression is induced by neurotrophin-4/5 (NT-4/5) and by BDNF itself in neocortical neurons. K252a, a specific tyrosine kinase (Trk) inhibitor, completely suppresses BDNF- and NT-4/5-enhanced BDNF mRNA expression. NT-4/5 significantly augments BDNF protein production, which is also reversed by K252a. When neurons are incubated with neurotrophin-3 (NT-3) or nerve growth factor (NGF), there are no significant changes in BDNF mRNA or protein expression. Interestingly, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or the N-methyl-D-aspartate (NMDA) receptor blocker AP-5 completely suppresses NT-4/5-enhanced BDNF protein production, while tetrodotoxin (TTX) only suppresses NT-4/5-enhanced BDNF production by 50%. Additionally, the mitogen activated protein (MAP) kinase inhibitor PD98059 enhances BDNF-induced glutamate receptor-1 (GluR1) protein expression, but a phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 strongly reduces BDNF-induced GluR1 protein expression. Taken together, glutamate receptors are important for the regulation of BDNF expression by neurotrophins, and MAP and PI3K kinases differentially modulate AMPA receptor expression in the cortical neurons.     

Alteration in brain-derived neurotrophic factor (BDNF) after treatment of mice with herbal mixture containing Euphoria longana,Houttuynia cordata and Dioscorea japonica

Background

Literature data indicate that brain-derived neurotrophic factor (BDNF), cyclic-AMP response element-binding protein (CREB) and phospho-CREB (pCREB) may have a place in depression. BDNF belongs to the neurotrophin family that plays an important role in proliferation, survival and differentiation of different cell populations in the mammalian nervous system. The herbal mixture used in the present study consists of Euphoria longana, Houttuynia cordata and Dioscorea japonica. The purpose of the present study was to determine the neuroprotective effect of herbal mixture. We also tested the hypothesis that administration of herbs reverses memory deficits and promotes the protein expression of BDNF in the mouse brain.

Methods

Mice were randomized into four different treatment groups (n = 10/group). Normal and stress groups received regular lab chow without stress and under stress conditions, respectively, for 3 weeks. The animals in the stress group were immobilized for 4 hours a day for 2 weeks. Different doses of herbal mixture (206 and 618 mg/kg) were administered for 3 weeks to those mice under stress conditions. Mice were analyzed by behavioral tests and immunoblotting examination in the hippocampus and cortex. An additional in vitro investigation was performed to examine whether herbs induce neurotoxicity in a human neuroblastoma cell line, SH-SY5Y cells.

Results

No significant toxicity of herbs on human neuroblastoma cells was observed. These herbs demonstrated an inductive effect on the expression of BDNF, pCREB and pAkt. For spatial working memory test, herbal mixture fed mice exhibited an increased level of spontaneous alternation (p < 0.01) compared to those in stress conditions. Moreover, herbal mixture produced highly significant (p < 0.01) reduction in the immobility time in the tail suspension test. Mice in the herbal mixture groups demonstrated lower serum corticosterone concentration than mice in the stress group (p < 0.05). Effects of the oral administration of herbal mixture on protein levels of BDNF in the hippocampi and cortices were significant.

Conclusions

Our study showed that herbal mixture administration has antidepressant effects in mice. It is proposed that adverse events such as stress and depression can modulate the expression of molecular players of cellular plasticity in the brain.     

Environmental Enrichment Ameliorates Behavioral Impairments Modeling Schizophrenia in Mice Lacking Metabotropic Glutamate Receptor 5

Schizophrenia arises from a complex interplay between genetic and environmental factors. Abnormalities in glutamatergic signaling have been proposed to underlie the emergence of symptoms, in light of various lines of evidence, including the psychotomimetic effects of NMDA receptor antagonists. Metabotropic glutamate receptor 5 (mGlu5) has also been implicated in the disorder, and has been shown to physically interact with NMDA receptors. To clarify the role of mGlu5-dependent behavioral expression by environmental factors, we assessed mGlu5 knockout (KO) mice after exposure to environmental enrichment (EE) or reared under standard conditions. The mGlu5 KO mice showed reduced prepulse inhibition (PPI), long-term memory deficits, and spontaneous locomotor hyperactivity, which were all attenuated by EE. Examining the cellular impact of genetic and environmental manipulation, we show that EE significantly increased pyramidal cell dendritic branching and BDNF protein levels in the hippocampus of wild-type mice; however, mGlu5 KO mice were resistant to these alterations, suggesting that mGlu5 is critical to these responses. A selective effect of EE on the behavioral response to the NMDA receptor antagonist MK-801 in mGlu5 KO mice was seen. MK-801-induced hyperlocomotion was further potentiated in enriched mGlu5 KO mice and treatment with MK-801 reinstated PPI disruption in EE mGlu5 KO mice only, a response that is absent under standard housing conditions. Together, these results demonstrate an important role for mGlu5 in environmental modulation of schizophrenia-related behavioral impairments. Furthermore, this role of the mGlu5 receptor is mediated by interaction with NMDA receptor function, which may inform development of novel therapeutics.     

Neurobehavioral effects of alcohol in AMPA receptor subunit (GluR1) deficient mice

Of the ionotropic glutamatergic receptors, the NMDA receptor is clearly implicated in the acute and chronic effects of ethanol; however, the role of the AMPA receptor in mediating the effects of ethanol in vivo is as yet unclear. Using mice deficient in the AMPA receptor subunit GluR1 (GluR1-/- mice), we investigated whether the AMPA receptor had a significant role in mediating the effects of ethanol. GluR1-/- mice showed greater locomotor activity in a novel environment, but by the fifth day of repeated testing their activity was the same as that of wild-type mice. In contrast to their enhanced locomotor activity, on an accelerating rotarod GluR1-/- mice performed consistently worse than wild-types. With regard to the effects of ethanol on motor responses, GluR1-/- mice did not differ significantly from wild-type mice in ethanol's sedative or incoordinating effects. However, the GluR1-/- mice were insensitive to the hypothermic effects of a hypnotic dose of ethanol in contrast to wild-types; this effect was dissociable from the hypnotic effects of ethanol. Further, tolerance to ethanol developed equally for GluR1-/- mice versus wild-type mice. In terms of alcohol drinking behavior, compared to wild-types, GluR1-/- mice differed neither in the acquisition of voluntary ethanol consumption nor in stress-induced ethanol drinking, nor in the expression of an alcohol deprivation effect (ADE) which is used as a model of relapse-like drinking behavior. In summary, although the loss of a hypothermic effect of ethanol in GluR1-/- mice indicates a critical role for the AMPA receptors in this effect, the GluR1 subunit of the AMPA receptor does not seem to play a critical role in the etiology of alcohol dependence. However, changes observed in activity patterns may be related to the putative role of AMPA receptors in attention deficit hyperactivity disorder.     

Neonatal toluene exposure alters N-methyl-D-aspartate receptor subunit expression in the hippocampus and cerebellum in juvenile rats

Recent evidence indicates that toluene is a non-competitive inhibitor of N-methyl-d-aspartate (NMDA) receptor-mediated synaptic currents. The NMDA receptor plays a major role in neuronal development and differentiation. The present study characterized the long-term effects of toluene exposure during synaptogenesis on the expression of NMDA receptor subunits (NR1, NR2A and NR2B). Neonatal rats were administered toluene (500 mg/kg, ip) daily over postnatal days (PN) 4-9. The expression of NMDA receptor subunits in rat brain was measured on PN 30. Western blot analysis demonstrated that toluene exposure significantly increased NR2A expression in the hippocampus and cerebellum. Immunohistochemical results indicated that the increased NR2A expression is mainly in hippocampal CA1-stratum oriens, CA1-stratum radiatum, CA1-lacunosm molecular, CA2- stratum oriens, and dentate gyrus-molecular layer and the cerebellar Purkinje cell layer, respectively. In contrast, the levels of NR2B in the toluene-exposed rats were decreased in the molecular layer. These results suggest that the region-specific changes in the expression of NMDA receptor subunits may be related to the neurobehavioral dysfunction following toluene exposure during synaptogenesis.     

Evidence for the presence of glutamatergic receptors in adult Schistosoma mansoni

Several studies have suggested that L-glutamate is a putative neurotransmitter in helminths. The present study investigated the presence of non-N-methyl-D-aspartate (NMDA) ionotropic receptors for glutamate in four subcellular fractions from adult male Schistosoma mansoni. Low-affinity (K(d)=221+/-80 nM) binding sites for [3H]kainic acid (KA) were detected in the heterogeneous (P(1)) fraction, which contains pieces of unbroken worm tissues, tegument, nuclei, and some vesicles. This binding was inhibited by classical glutamatergic ligands in the following order of potency: KA>L-glutamate>alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)>quisqualate congruent with 6,7-dinitroquinoline-2,3-dione (DNQX). However, neither NMDA, a selective agonist for NMDA receptors, nor DL-threo-beta-hydroxyaspartate (THA) and 1-trans-pyrollidine-2-dicarboxylic acid (PDC), inhibitors of high-affinity glutamate transporters, modified [3H]KA binding to the P(1) fraction. In addition, no specific binding for 10nM [3H]AMPA was detected in any subcellular fraction from S. mansoni. These results suggested the presence of KA receptors in adult male worms. This is supported by the evidence that direct application of 10 microM KA to whole worms produced a corkscrew-like coiling of their bodies, modifying the motility of the worms. The KA-induced response, measured as a decrease of the body area, was time-dependent and reversible. PDC was ineffective at blocking the KA effects, indicating that KA does not depend on high-affinity glutamate transporters to reach its site of action. On the other hand, DNQX, the non-NMDA antagonist, was able to partially inhibit KA-induced responses. As a whole, the present data support the presence of a glutamatergic signaling pathway in this parasite.