alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors modulate dopamine release in rat hippocampus and striatum

Direct enhancement of hippocampal dopamine or serotonin levels as a pharmacodynamic measure of combined antidepressant-anticonvulsant action

The neurobiological relationships between epilepsy and depression are receiving increased experimental attention. A key role for limbic monoamines in depression has been established and we recently showed the importance of hippocampal monoamines in limbic seizure control. We here studied whether anticonvulsant compounds are antidepressant and can elevate hippocampal dopamine (DA) or serotonin (5-HT) levels determined by in vivo microdialysis in rats. We used assessment of seizure severity in the focal pilocarpine model, antidepressant-like activity within the rat forced swim and the mouse tail suspension tests, and locomotor activity in an open field as behavioural tests. We studied the tricyclic antidepressant imipramine, the selective 5-HT reuptake inhibitor citalopram and the selective DA reuptake blocker GBR-12909. These compounds with combined antidepressant-anticonvulsant properties all directly enhanced extracellular hippocampal DA or 5-HT levels. Since glutamate-mediated hyperexcitability in temporal lobe regions seems to be involved in disturbed emotional behaviour, we next investigated possible antidepressant effects and hippocampal DA or 5-HT modulations exerted by selective ionotropic and metabotropic glutamate receptor ligands with anticonvulsant properties. Combined anticonvulsant-antidepressant activities of the NMDA antagonist MK-801 and the mGluR group I antagonists (AIDA, MPEP) were also associated with locally elicited increases in hippocampal DA and/or 5-HT levels. This study highlights that the hippocampus is an important site of action of combined anticonvulsant-antidepressant and monoamine enhancing effects.

Therapeutic potential of positive AMPA receptor modulators in the treatment of neuropsychiatric disorders

Drugs that potentiate the activity of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor cause a complex cascade of consequences in experimental models, ranging from enhancement of long-term potentiation to induction of neurotrophic factors. Animal studies characterising the pharmacological and behavioural effects of these substances have provided the rationale for several initial attempts to use these drugs in neuropsychiatric clinical settings. Applications in schizophrenia, Alzheimer's disease and mild cognitive impairment have been initiated. Other trials with these compounds include the treatment of Fragile X syndrome, and possible future applications may be in the field of Parkinson's disease. The literature published to date is limited mostly to small phase I or II trials, so there is no conclusive evidence for or against the use of these drugs. Substantial questions remain concerning which compounds to use, in what dose, for what condition and for how long.

Increases in [(3)H]-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor binding and mRNA expression of AMPA-sensitive glutamate receptor A (GluR-A) subunits in rats withdrawn from butorphanol

An autoradiographic study of [(3)H]-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA) receptor binding and an assessment of in situ hybridization of AMPA-sensitive glutamate receptor A (GluR-A) subunits in the rat brain were performed 7 h after withdrawal from butorphanol infusion. Animals were rendered dependent by intracerebroventricular (icv) infusion of butorphanol (26 nmol/microl/h) via osmotic minipumps for 3 d. Brain sections for binding of [(3)H]AMPA were incubated with 15 nM [(3)H]AMPA. The probes for in situ hybridization were labeled at its 3 cent end using terminal deoxynucleotidyl transferase and [(35)S]dATP. The highest degree of [(3)H]AMPA binding was shown in the hippocampus. The extent of [(3)H]AMPA binding was increased significantly in each of the brain areas examined, cortex, septum, caudate putamen, and hippocampus of rats, following withdrawal from butorphanol. The highest level of mRNA for GluR-A receptor for flop and flip subunits, was found in the dentate gyrus and in the CA3 region of the hippocampus. The amounts of mRNA for the flop form of GluR-A receptor were significantly increased in the cortex, caudate putamen, thalamus, and dentate gyrus of hippocampus of rat brain. The amounts of mRNA for the flip form of GluR-A receptor were markedly elevated in the cortex, thalamus, caudate putamen, and hippocampus. These findings suggest that increases in expression of mRNA for the GluR-A receptor and in the binding of AMPA to its receptor may play an important role during withdrawal from butorphanol dependence.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Animal models of attention-deficit hyperactivity disorder

Although animals cannot be used to study complex human behaviour such as language, they do have similar basic functions. In fact, human disorders that have animal models are better understood than disorders that do not. ADHD is a heterogeneous disorder. The relatively simple nervous systems of rodent models have enabled identification of neurobiological changes that underlie certain aspects of ADHD behaviour. Several animal models of ADHD suggest that the dopaminergic system is functionally impaired. Some animal models have decreased extracellular dopamine concentrations and upregulated postsynaptic dopamine D1 receptors (DRD1) while others have increased extracellular dopamine concentrations. In the latter case, dopamine pathways are suggested to be hyperactive. However, stimulus-evoked release of dopamine is often decreased in these models, which is consistent with impaired dopamine transmission. It is possible that the behavioural characteristics of ADHD result from impaired dopamine modulation of neurotransmission in cortico-striato-thalamo-cortical circuits. There is considerable evidence to suggest that the noradrenergic system is poorly controlled by hypofunctional α₂-autoreceptors in some models, giving rise to inappropriately increased release of norepinephrine. Aspects of ADHD behaviour may result from an imbalance between increased noradrenergic and decreased dopaminergic regulation of neural circuits that involve the prefrontal cortex. Animal models of ADHD also suggest that neural circuits may be altered in the brains of children with ADHD. It is therefore of particular importance to study animal models of the disorder and not normal animals. Evidence obtained from animal models suggests that psychostimulants may not be acting on the dopamine transporter to produce the expected increase in extracellular dopamine concentration in ADHD. There is evidence to suggest that psychostimulants may decrease motor activity by increasing serotonin levels. In addition to providing unique insights into the neurobiology of ADHD, animal models are also being used to test new drugs that can be used to alleviate the symptoms of ADHD.

Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder--the spontaneously hypertensive rat

RUSSELL, V.A. Dopamine hypofunction possibly results from a defect in glutamate-stimulated release of dopamine in the nucleus accumbens shell of a rat model for attention deficit hyperactivity disorder-the spontaneously hypertensive rat. NEUROSCI. BIOBEHAV. REV.27(2003). Disturbances in glutamate, dopamine and norepinephrine function in the brain of a genetic animal model for attention-deficit hyperactivity disorder (ADHD), the spontaneously hypertensive rat (SHR), and information obtained from patients with ADHD, suggest a defect in neuronal circuits that are required for reward-guided associative learning and memory formation. Evidence derived from (i). the neuropharmacology of drugs that are effective in treating ADHD symptoms, (ii). molecular genetic and neuroimaging studies of ADHD patients, as well as (iii). the behaviour and biochemistry of animal models, suggests dysfunction of dopamine neurons. SHR have decreased stimulation-evoked release of dopamine as well as disturbances in the regulation of norepinephrine release and impaired second messenger systems, cAMP and calcium. In addition, evidence supports a selective deficit in the nucleus accumbens shell of SHR which could contribute to impaired reinforcement of appropriate behaviour.

In vitro glutamate-stimulated release of dopamine from nucleus accumbens core and shell of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) are used as a model for attention-deficit hyperactivity disorder (ADHD) since SHR display the major symptoms of ADHD (hyperactivity, impulsivity, inablity to sustain attention during behavioral tasks). We previously showed that electrical and/or K+-stimulated release of dopamine (DA) from nerve terminals in the prefrontal cortex, nucleus accumbens, and caudate-putamen of SHR was significantly lower than that of Wistar-Kyoto (WKY) control rats. The aim of the present investigation was to determine whether glutamate-stimulated release of DA from nucleus accumbens core and shell of SHR was significantly different from that of WKY. Using an in vitro superfusion technique, we showed that glutamate-stimulated release of [3H]DA from striatal slices is mediated by glutamate activation of AMPA receptors and that glutamate-stimulated release of [3H]DA from nucleus accumbens core and shell of 4-6-week-old SHR and WKY is not significantly different. Glutamate-stimulated release of [3H]DA from SHR shell is significantly lower than SHR core and there is also a tendency for glutamate-stimulated [3H]DA release from SHR shell to be lower than release from WKY shell.

Regional specific increases of [3H]AMPA binding and mRNA expression of AMPA receptors in the brain of mu-opioid receptor knockout mice

Previous pharmacological studies have indicated the possible existence of functional interactions between opioidergic and glutamatergic neurons in the CNS. In the present study, [(3)H]AMPA binding and the expression of mRNAs encoding flip and flop variants of three subtypes of AMPA glutamate receptor GluR1-3 were examined by in situ hybridization technique in order to investigate whether there is a change in the AMPA receptor system of mice lacking the mu-opioid receptor. In the mu-opioid receptor knockout mice, [(3)H]AMPA binding was increased in the hippocampal CA1 and dentate gyrus, cortex, and caudate putamen compared with that of the wild-type animals. The expression of GluR1 flip mRNA was increased in the cortex and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR1 flop mRNA was increased in the cortex, caudate putamen, and hippocampal CA1 layer of mu-opioid receptor knockout mice. The expression of GluR2 flip mRNA was decreased in the hippocampal dentate gyrus of mu-opioid receptor knockout mice. The expression of GluR2 flop was not altered in any regions studied. The expression of GluR3 flip was increased in the cortical area and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR3 flop was increased in the cortical area, hippocampal CA3 area, and caudate putamen of mu-opioid receptor knockout mice. These results indicate that [(3)H]AMPA binding and the expression of GluR1-3 mRNA were increased in a region and subunit specific manner, and suggest that changes in the AMPA receptor system are accompanied by the absence of mu-opioid receptor gene.

Antidepressant-like actions of an AMPA receptor potentiator (LY392098)

LY392098 is a member of a novel class of biarylpropylsulfonamides that potentiates AMPA receptor-mediated responses both in vitro and in vivo. In this study, the effects of LY392098 were evaluated in two "behavioral despair" models (the forced swim and tail suspension tests) commonly used to identify clinically useful antidepressants. LY392098 reduced immobility in the forced swim test in both rats and mice, with a minimum effective dose of 0.5 mg/kg (i.p.) in both species. LY392098 (0.1-10 mg/kg, i.p.) did not affect motor activity of rats, indicating that the ability of this compound to reduce immobility in the forced swim test is unrelated to a motor stimulant action. LY392098 also reduced immobility in the tail suspension test in a dose-dependent manner, with a minimum effective dose of 5 mg/kg (i.p). A non-competitive AMPA antagonist (LY300168) blocked the activity of LY392098 in the forced swim test, but did not affect imipramine-induced reductions in immobility. Thus, AMPA receptor activation appears to be required for the antidepressant-like effect of LY392098, but not imipramine. These findings indicate that biarylpropylsulfonamides, exemplified by LY392098, may represent a novel class of antidepressants.

Presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor-mediated stimulation of glutamate and GABA release in the rat striatum in vivo: a dual-label microdialysis study

The existence of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate autoreceptors on glutamate nerve terminals in vitro has recently been demonstrated using synaptosomal and brain slice preparations. In the present study we have used a modification of a rapid dual-label intracerebral microdialysis method, previously developed by Young and co-workers(80,81) for the study of presynaptic mechanisms of neurotransmitter release, to investigate whether presynaptic AMPA receptors also play a role in the control of striatal glutamate release in vivo. For comparative purposes, the action of locally applied AMPA on striatal GABA release in vivo was also monitored. Local application of AMPA (0.01-100 microM), by reverse dialysis, into the striatum resulted in concentration-dependent increases in the Ca(2+)-dependent efflux of both [3H]L-glutamate and [14C]GABA. Maximum responses reached 142.0+/-6.5% and 166.8+/-7.7% of basal efflux for [3H]L-glutamate and [14C]GABA, respectively. No marked behavioural changes were observed at any dose of the agonist. Unexpectedly, the AMPA-evoked responses were not potentiated by the AMPA receptor desensitization inhibitors cyclothiazide (10-100microM) or aniracetam (1mM). Consistent with this finding, AMPA-stimulated [3H]L-glutamate and [14C]GABA efflux were significantly attenuated by co-perfusion with the selective, competitive AMPA receptor antagonist 6-nitro-7-sulphamoylbenzo(F)quinoxaline-2,3-dione (100microM) but not 1-(aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (100microM), a non-competitive AMPA receptor antagonist known to interact with the cyclothiazide site to control AMPA receptor function. The broad spectrum ionotropic glutamate receptor antagonist, kynurenic acid (100-1000microM) also markedly inhibited the AMPA-evoked responses in the striatum in vivo. None of the antagonists, when given alone, influenced basal efflux of [3H]L-glutamate suggesting a lack of tonic regulatory control of glutamate release via presynaptic AMPA-type autoreceptors in the rat striatum. These results demonstrate the presence of presynaptic AMPA receptors, of a novel cyclothiazide- and aniracetam-insensitive subtype, on presynaptic nerve terminals in the rat striatum in vivo, acting to enhance glutamate and GABA release. Our data support the concept of AMPA receptor heterogeneity in vivo, a finding which may facilitate the development of novel, more selective drugs for the treatment of a range of neurological disorders associated with abnormal cerebral glutamate release. The pharmacological profile of these novel presynaptic receptors is currently under investigation.

Current perspectives on the development of non-biogenic amine-based antidepressants

Compounds that inhibit the re-uptake and/or metabolism of biogenic amines (i.e. serotonin, norepinephrine, and dopamine) have been used to treat depression for more than 40 years. Selective re-uptake inhibitors, currently the most widely prescribed class of biogenic amine-based agents, are certainly safe and relatively easy to use, but do not exhibit either a faster onset of action or greater efficacy than their predecessors. An approach to overcome the limitations that may be inherent to these 'conventional' therapies is to circumvent the monoaminergic synapse. In this review, two potential antidepressant strategies are discussed that may converge with intracellular pathways impacted by chronic treatment with biogenic amine-based agents. Drugs emerging from these strategies may offer significant advantages over currently used antidepressants.

Mediation of ionotropic glutamate receptors in domoic acid-induced striatal dopamine release in rats

Our objective was to characterize the mechanism of action of intrastriatal infusion of domoic acid on extracellular dopamine levels, using in vivo dialysis in conscious and freely moving rats. The local infusion of domoic acid (500 microM) caused an increase (567.9+/-142.5%, versus basal) in dopamine extracellular levels associated with a decrease in its metabolites: dihydroxyphenylacetate (DOPAC) and homovanillate (HVA) (47.3+/-4.4% and 33.8+/-4.2%, respectively, compared to basal). Infusion of the amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX; 200 microM) reversed the effect of domoic acid infusion on striatal dopamine levels. However, the infusion of the selective non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801; 50 microM), did not change significantly the effect of domoic acid on dopamine extracellular levels. In conclusion, based on results with a microdialysis technique, we suggest that domoic acid may act through AMPA/kainate receptors in striatum.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

Nitric oxide-mediated regulation of dopamine release in the hippocampus in vivo

Infusion of N-methyl-D-aspartate (NMDA) into the hippocampus of freely moving rats produced a concentration-dependent decrease in the extracellular levels of dopamine, an effect which was reversed by D-2-amino-5-phosphonovaleric acid (D-AP5). To determine the involvement of nitric oxide (NO) in this response, two nitric oxide synthase (NOS) inhibitors, N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), were examined for their ability to modify both basal and NMDA-inhibited dopamine release. When infused alone both NOS inhibitors elicited an increase in extracellular dopamine concentration, moreover, when administered prior to the application of NMDA, the agonist failed to elicit a decrease in dopamine levels. Infusion of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) over a 30 min period caused either an increase or a decrease in dopamine release depending upon the concentration used. At the lower concentration (0.5 mM) SNAP promoted dopamine release whilst at the higher concentration (5 mM), the donor elicited a long lasting reduction in basal dopamine levels. The effect of the lower concentration of SNAP was reversed by the prior application of D-AP5, but that of the higher concentration was unaffected by the antagonist.

In vivo release of dopamine and its metabolites from rat striatum in response to domoic acid

The microdialysis technique was used to examine the effect of the neurotoxin domoate, an analog of glutamic acid, on striatal dopamine activity. Our results show that the intracerebral administration of different concentrations of domoate (100 and 500 microM) produced increases in the extracellular levels of dopamine associated to decreases in the extracellular levels of its metabolites dihydroxyphenylacetate and homovanillate from rat striatum. These changes seem to be related according to a time sequence, indicating a possible effect on the metabolism of dopamine. Changes were also observed in locomotor activity (cycling behavior, sniffing around and chewing) in rats during the domoate infusion. The physiological mechanism by which domoate increased dopamine release remains to be worked out.

Subcellular and subsynaptic distribution of the NR1 subunit of the NMDA receptor in the neostriatum and globus pallidus of the rat: co-localization at synapses with the GluR2/3 subunit of the AMPA receptor

Glutamatergic neurotransmission in the neostriatum and the globus pallidus is mediated through NMDA-type as well as other glutamate receptors and is critical in the expression of basal ganglia function. In order to characterize the cellular, subcellular and subsynaptic localization of NMDA receptors in the neostriatum and globus pallidus, multiple immunocytochemical techniques were applied using antibodies that recognize the NR1 subunit of the NMDA receptor. In order to determine the spatial relationship between NMDA receptors and AMPA receptors, double labelling was performed with the NR1 antibodies and an antibody that recognizes the GluR2 and 3 subunits of the AMPA receptor. In the neostriatum all neurons with characteristics of spiny projection neurons, some interneurons and many dendrites and spines were immunoreactive for NR1. In the globus pallidus most perikarya and many dendritic processes were immunopositive. Immunogold methods revealed that most NR1 labelling is associated with asymmetrical synapses and, like the labelling for GluR2/3, is evenly spread across the synapse. Double immunolabelling revealed that in neostriatum, over 80% of NR1-positive axospinous synapses are also positive for GluR2/3. In the globus pallidus most NR1-positive synapses are positive for GluR2/3. In both regions many synapses labelled only for GluR2/3 were also detected. These results, together with previous data, suggest that NMDA and AMPA receptor subunits are expressed by the same neurons in the neostriatum and globus pallidus and that NMDA and AMPA receptors are, at least in part, colocalized at individual asymmetrical synapses. The synaptic responses to glutamate in these regions are thus likely be mediated by both AMPA and NMDA receptors at the level of individual synapses.

Reciprocal dopamine-glutamate modulation of release in the basal ganglia

Dopaminergic and glutamatergic transmissions have long been known to interact at multiple levels in the basal ganglia to modulate motor and cognitive functions. One important aspect of their interactions is represented by the reciprocal modulation of release. This topic has been the object of interest since the late 70's, particularly in the striatum and in midbrain dopaminergic areas (substantia nigra and ventral tegmental area). Analysis of glutamate-dopamine interactions in the control of each other's release is complicated by the fact that both glutamate and dopamine act on multiple receptor subtypes which can exert different effects. Therefore, glutamatergic modulation of dopamine release has been reviewed by analyzing the effects of glutamatergic selective receptor agonists and antagonists in the striatum (both motor and limbic portions) and in midbrain dopaminergic areas, as revealed by in vitro (slices, cell cultures, synaptosomes) and in vivo (push-pull, microdialysis and voltammetry techniques) experimental approaches. The same approach has been followed for dopaminergic modulation of glutamate release. The facilitatory nature of glutamate modulating both presynaptic and dendritic dopamine release has clearly emerged from in vitro studies. However, evidence is presented that, at least in the striatum and in the nucleus accumbens of awake rats, glutamate-mediated inhibitory effects may also occur. In vitro and in vivo experiments in the striatum and midbrain dopaminergic areas mainly depict dopamine as an inhibitory modulator of glutamate release. However, in vivo studies reporting dopamine D1 receptor mediated facilitatory effects are also considered. Therefore, the general notion that glutamate and dopamine act oppositely to regulate each other's release, is only partly supported by the available data. Conversely, the nature of the interaction between the two neurotransmitters seems to vary depending on the experimental approach, the brain area considered and the subtype of receptor involved.

Pharmacological evidence for GABAergic and glutamatergic involvement in the convulsant and behavioral effects of glutaric acid

The effect of intrastriatal administration of glutaric acid (GTR), a metabolite that accumulates in glutaric acidemia type I (GA-I), on the behavior of adult male rats was investigated. After cannula placing, rats received unilateral intrastriatal injections of GTR buffered to pH 7.4 with NaOH or NaCl. GTR induced rotational behavior toward the contralateral side of injection and clonic convulsions in a dose-dependent manner. Rotational behavior was prevented by intrastriatal preadministration of DNQX and muscimol, but not by the preadministration of MK-801. Convulsions were prevented by intrastriatal preinjection of muscimol. This study provides evidence for a participation of glutamatergic non-NMDA and GABAergic mechanisms in the GTR-induced behavioral alterations. These findings may be of value in understanding the physiopathology of the neurological dysfunction in glutaric acidemia.

Antipsychotic drug regulation of AMPA receptor affinity states and GluR1, GluR2 splice variant expression

We recently reported that chronic administration of antipsychotic drugs dramatically elevated [3H]AMPA binding, with minimal elevation of [3H]CNQX binding in rat brain. The aim of the current study was to examine the mechanism of this effect. Chronic haloperidol minimally increased the total number of binding sites (total Bmax) compared to saline-injected animals. Specifically, haloperidol dramatically increased the proportion of high-affinity-site AMPA receptors (approximately 30% increase) without inducing a significant change in the low-affinity constant. In situ hybridization for flip and flop isoforms of GluR1 and GluR2 (AMPA receptors) was not altered in a pattern or degree that compared to the changes seen in AMPA receptor binding. These findings suggest that the long-term action of antipsychotic drugs may be to regulate AMPA receptor responsiveness to agonist stimulation via posttranscriptional means, and is unlikely to be related to GluR1 or GluR2 splice variant expression. This effect may have relevance to both the therapeutic effects and side effects of antipsychotic drugs in humans.

Kainate and AMPA receptor binding in seizure-prone and seizure-resistant inbred mouse strains

Glutamate and its receptors represent the major excitatory neurotransmission system in the mammalian brain and are considered important in the pathogenesis of many neurological diseases. The present study describes saturation binding experiments performed to measure the affinity (Kd) and density (Bmax) of kainate and AMPA receptors in striatum, cortex and hippocampus from mature DBA/2J (DBA) and C57BL/6J (C57) mice. Previous studies have documented that these two strains differ significantly in seizure susceptibility, with DBA mice exhibiting greater sensitivity in various convulsant tests compared to C57 mice. Non-linear regression analysis of binding data together with Student's t-test and ANOVA revealed significantly higher densities of kainate receptors in striatum and of AMPA receptors in cortex of DBA mice. C57 mice exhibited higher striatal [3H]AMPA binding. There were no significant differences between the mouse strains in binding sites prepared from hippocampus and no differences in affinity for either receptor in any brain region studied. The results support a role for kainate and AMPA receptors in seizure sensitivity, possibly by influencing glutamate transmission in specific pathways. It is unlikely, however, that these receptors account for the generation of seizures alone but rather cooperate with other glutamatergic and non-glutamatergic neurotransmitter systems.

Cellular localization of GluR1, GluR2/3 and GluR4 glutamate receptor subunits in neurons of the rat neostriatum

Glutamate excitocytotoxicity is implied in the cause of neuronal degeneration in the neostriatum, in which the toxicity may be mediated by different families of glutamate receptors. The precise cellular localization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)-type glutamate receptor subunits (GluR1-4), one of the major family that involves in the mechanisms of glutamate excitocytotoxicity, in different populations of striatal neurons is therefore of special interest. Immunoreactivity for GluR2/3 subunits was detected in the medium-sized spiny neurons. By double labelling experiments, immunoreactivity for GluR1 and GluR4 was detected only in aspiny striatal neurons that display parvalbumin immunoreactivity, but not in the other neuron populations that display choline acetyltransferase or muscarinic m2 receptor immunoreactivity, nor neurons that display nitric oxide synthase immunoreactivity or nicotinamide adenine dinucleotide phosphate-diaphorase activity. These results indicate that GluR1 and GluR4 immunoreactivity is displayed only in the GABAergic interneurons in the neostriatum. In addition, almost all of the GluR1-immunoreactive neurons were found to display GluR4 immunoreactivity. This finding indicates for the first time that the striatal GABAergic interneurons co-express GluR1 and GluR4 subunits. The results of the present study indicate that there is a differential localization of AMPA-type glutamate receptor subunits in different populations of striatal neurons and they may have a different susceptibility to glutamate excitocytotoxicity.

Cellular, subcellular, and subsynaptic distribution of AMPA-type glutamate receptor subunits in the neostriatum of the rat

Glutamate released in the basal ganglia is involved in the expression of clinical symptoms of neurodegenerative diseases like Parkinson's or Huntington's. Neostriatal neurons are the targets of glutamatergic inputs derived from the cortex and the thalamus acting via AMPA-type as well as other glutamate receptors. To determine the location of subunits of the AMPA subclass of glutamate receptors (GluR) in the rat neostriatum, we applied multiple immunocytochemical techniques using anti-peptide antibodies against the GluR1, GluR2/3, and GluR4 subunits at both the light and electron microscopic levels. All medium spiny efferent neurons, some of which were identified as striatonigral neurons, displayed immunoreactivity for GluR1 and GluR2/3 subunits. Double immunofluorescence revealed that at least 70-90% of parvalbumin-immunopositive GABAergic interneurons were immunoreactive for each of GluR1, GluR2/3, or GluR4 subunits and that at least 40% of choline acetyltransferase-immunopositive cholinergic interneurons were immunopositive for GluR1 or GluR4 subunits. The majority of nitric oxide synthase-immunopositive neurons had no detectable immunoreactivity for any of the AMPA receptor subunits. Electron microscopic analysis confirmed the presence of immunoreactivity for GluR1 and GluR2/3 in the perikarya of spiny neurons and interneurons and GluR4 in perikarya of interneurons only. GluR1 and GluR2/3 subunits were detected in dendrites and spines. A significant population of extrasynaptic receptors was revealed by pre-embedding immunogold labeling along the plasma membranes of perikarya, dendrites, and spines. Receptors were concentrated in the postsynaptic membrane specialization of asymmetrical synapses, as revealed by the postembedding immunogold method. Quantitative analysis demonstrated that immunoreactivity for the GluR1 and GluR2/3 subunits is higher at the periphery than at the middle of the postsynaptic membrane specialization. Our results demonstrate that AMPA receptor subunits are distributed widely and heterogeneously among striatal neurons and are concentrated on the postsynaptic membrane of asymmetrical synaptic specializations, although extrasynaptic receptors are also present.

The glutamate receptor/NO/cyclic GMP pathway in the hippocampus of freely moving rats: modulation by cyclothiazide, interaction with GABA and the behavioural consequences

Monitoring of extracellular cGMP during intracerebral microdialysis in freely moving rats permits the study of the functional changes occurring in the glutamate receptor/nitric oxide (NO) synthase/guanylyl cyclase pathway and the relationship of these changes to animal behaviour. When infused into the rat hippocampus in Mg2+-free medium, cyclothiazide, a blocker of desensitization of the AMPA-preferring receptor, increased cGMP levels. The effect of cyclothiazide (300 microM) was abolished by the NO synthase inhibitor L-NARG (100 microM) or the soluble guanylyl cyclase inhibitor ODQ (100 microM). During cyclothiazide infusion the animals displayed a pre-convulsive behaviour characterized by frequent "wet dog shakes" (WDS). Neither L-NARG nor ODQ decreased the WDS episodes. Both cGMP and WDS responses elicited by cyclothiazide were prevented by blocking NMDA receptor function with the glutamate site antagonist CGS 19755 (100 microM), the channel antagonist MK-801 (30 microM) or Mg2+ ions (1 mM). The AMPA/kainate receptor antagonists DNQX (100 microM) and NBQX (100 microM) abolished the WDS episodes but could not inhibit the cyclothiazide-evoked cGMP response. DNQX or NBQX (but not MK-801) elevated, on their own, extracellular cGMP levels. The cGMP response elicited by the antagonists appears to be due to prevention of a glutamate-dependent inhibitory GABAergic tone, since infusion of bicuculline (50 microM) caused a strong cGMP response. The results suggest that (a) AMPA/kainate receptors linked to the NO/cGMP pathway in the hippocampus (but not NMDA receptors) are tonically activated and kept in a desensitized state by endogenous glutamate; (b) blockade of AMPA/kainate receptor desensitization by cyclothiazide leads to endogenous activation of NMDA receptors; (c) the hippocampal NO/cGMP system is under a GABAergic inhibitory tone driven by non-NMDA ionotropic receptors; (d) the pre-convulsive episodes observed depend on hippocampal NMDA receptor activation but not on NO and cGMP production.

Glutamatergic control over brain dopamine release in vivo and in vitro

Substantial evidence supports an important role for the excitatory neurotransmitter L-glutamate as a modulator of dopamine release in the central nervous system. All of the established glutamate receptor subtypes identified to date have been implicated in the regulation of dopamine release. It appears that glutamate can exert both facilitatory and inhibitory control over dopamine release and that this may be both phasic and tonic in nature. This regulatory role suggests that drugs acting at glutamate receptors may be potentially useful therapeutic agents in neurological disorders such as parkinsonism and schizophrenia.