Excitatory Amino Acid Transmission Research Articles

Regulatory Role of Excitatory Amino Acids in Reproduction

Glutamate, the major excitatory amino acid (EAA) transmitter in the central nervous system, has been implicated as a critical mediator in brain function. Glutamate and its receptors are found in all key hypothalamic areas critically involved in reproduction. Administration of glutamate and its agonists can bring about LH release in animals with a steroid background. Antagonists of the ionotropic glutamate receptors inhibited LH release and abolished the steroid-induced and the preovulatory LH surge. Both NMDA and non-NMDA receptor antagonists can also inhibit pulsatile LH release in castrated animals. The preoptic area has been implicated as a primary site of action of NMDA, while non-NMDA agonists have been suggested to act primarily at the arcuate/median eminence level. While EAAs may act directly on GnRH neurons to enhance GnRH release, the majority of evidence suggests that an indirect mechanism, involving EAA activation of nitric oxide and/or catecholamines, plays a major role in the GnRH-releasing effects of EAAs. Furthermore, there is also some evidence that the tonic inhibitory effect of opioids on GnRH may also involve, at least in part, a suppression of glutamate. Finally, EAA stimulation of GnRH/LH release is markedly attenuated in middle-aged rats, suggesting that a defect in glutamate neurotransmission may underlie the attenuated LH surge observed in aging.

Tracheal tone and the role of ionotropic glutamate receptors in the nucleus ambiguus

Excitatory amino acid (EAA) transmission in the rostral portion of the ventrolateral medulla (RVLM) appears to be required for reflex airway constriction. The purpose of the present study was to determine whether our prior observations of right sided dominance are also reflected at the level of the RVLM, particularly at the right nucleus ambiguus (NA). Microinjection of glutamate (109 nl, 100 mM) into the right NA of anesthetized dogs produced significant increases in tracheal smooth muscle tone as assessed by pressure changes in a saline filled cuff of the endotracheal tube (Δ15±2 mm Hg). Increasing inspired CO 2 increased tracheal cuff pressure (Δ33±2 mm Hg) which was abolished by microinjections of the EAA antagonist kynurenic acid (109 nl, 100 mM) or the non-NMDA antagonist 6-nitro-7-sulfamobenzoquinoxaline-2,3-dione (NBQX, 109 nl, 10 mM) into the right NA. These results confirm that EAA receptors in the right NA are capable of modulating tracheal tone and suggest that non-NMDA receptors in the right NA are required for reflex airway constriction produced by systemic hypercapnia. Furthermore, the effects of right-sided NA microinjections in the present study, although not completely definitive, are consistent with previous studies that suggest dominant control of canine tracheal tone by the right vagus.

Neuroplasticity in mood disorders

Neuroimaging and neuropathological studies of major depressive disorder (MDD) and bipolar disorder (BD) have identified abnormalities of brain structure in areas of the prefrontal cortex, amygdala, striatum, hippocampus, parahippocampal gyrus, and raphe nucleus. These structural imaging abnormalities persist across illness episodes, and preliminary evidence suggests they may in some cases arise prior to the onset of depressive episodes in subjects at high familial risk for MDD. In other cases, the magnitude of abnormality is reportedly correlated with time spent depressed. Postmortem histopathological studies of these regions have shown abnormal reductions of synaptic markers and glial cells, and, in rare cases, reductions in neurons in MDD and BD. Many of the regions affected by these structural abnormalities show increased glucose metabolism during depressive episodes. Because the glucose metabolic signal is dominated by glutamatergic transmission, these data support other evidence that excitatory amino acid transmission is elevated in limbic-cortical-striatal-pallidal-thalamic circuits during depression. Some of the subject samples in which these metabolic abnormalities have been demonstrated were also shown to manifest abnormally elevated stressed plasma cortisol levels. The co-occurrence of increased glutamatergic transmission and Cortisol hypersecretion raises the possibility that the gray matter volumetric reductions in these depressed subjects are partly accounted for by processes homologous to the dendritic atrophy induced by chronic stress in adult rodents, which depends upon interactions between elevated glucocorticoid secretion and N-meihyl-D-aspartate (NMDA)-glutamate receptor stimulation. Some mood-stabilizing and antidepressant drugs that exert neurotrophic effects in rodents appear to reverse or attenuate the gray matter volume abnormalities in humans with mood disorders. These neurotrophic effects may be integrally related to the therapeutic effects of such agents, because the regions affected by structural abnormalities in mood disorders are known to play major roles in modulating the endocrine, autonomic, behavioral, and emotional experiential responses to stressors.

Molecular mechanisms underlying specificity of excitotoxic signaling in neurons.

The central role of glutamate receptors in mediating excitotoxic neuronal death in stroke, epilepsy and trauma has been well established. Glutamate is the major excitatory amino acid transmitter within the CNS and it's signaling is mediated by a number of postsynaptic ionotropic and metabotropic receptors. Although calcium ions are considered key regulators of excitotoxicity, new evidence suggests that specific second messenger pathways rather than total Ca(2+) load, are responsible for mediating neuronal degeneration. Glutamate receptors are found localized at the synapse within electron dense structures known as the postsynaptic density (PSD). Localization at the PSD is mediated by binding of glutamate receptors to submembrane proteins such as actin and PDZ containing proteins. PDZ domains are conserved motifs that mediate protein-protein interactions and self-association. In addition to glutamate receptors PDZ-containing proteins bind a multitude of intracellular signal molecules including nitric oxide synthase. In this way PDZ proteins provide a mechanism for clustering glutamate receptors at the synapse together with their corresponding signal transduction proteins. PSD organization may thus facilitate the individual neurotoxic signal mechanisms downstream of receptors during glutamate overactivity. Evidence exists showing that inhibiting signals downstream of glutamate receptors, such as nitric oxide and PARP-1 can reduce excitotoxic insult. Furthermore we have shown that uncoupling the interaction between specific glutamate receptors from their PDZ proteins protects neurons against glutamate-mediated excitotoxicity. These findings have significant implications for the treatment of neurodegenerative diseases using therapeutics that specifically target intracellular protein-protein interactions.

Astrocyte Mitochondrial Mechanisms of Ischemic Brain Injury and Neuroprotection

Research on ischemic brain injury has established a central role of mitochondria in neuron death. Astrocytes are also damaged by ischemia, although the participation of mitochondria in their injury is ill defined. As astrocytes are responsible for neuronal metabolic and trophic support, astrocyte dysfunction will compromise postischemic neuronal survival. Ischemic alterations to astrocyte energy metabolism and the uptake and metabolism of the excitatory amino acid transmitter glutamate may be particularly important. Despite the significance of ischemic astrocyte injury, little is known of the mechanisms responsible for astrocyte death and dysfunction. This review focuses on differences between astrocyte and neuronal metabolism and mitochondrial function, and on neuronal-glial interactions. The potential for astrocyte mitochondria to serve as targets of neuroprotective interventions is also discussed.

Kainic acid does not affect CA3 hippocampal region pyramidal cells in hypothyroid rats

Thyroid hormones exert a crucial role on trophic events of the central nervous system during development, adulthood, and ageing. The deficiency of thyroid hormones could also produce a deficiency in neurotransmission in the hippocampal region. Kainic acid (KA) has become an important tool for studying functions related to excitatory amino acid transmission in mammals. Its neurotoxic effects on the pyramidal neurons of the CA3 hippocampal region are well known. We have examined the neurotoxicity of KA on these cells in hypothyroid rats. The hypothyroid state was induced by administration of methimazole. After 4 weeks of treatment, KA was administered once intraperitoneally at doses of 0, 1, 2.5, and 5 mg/kg to the hypothyroid group, and 0 and 5 mg/kg to the euthyroid group. In the euthyroid group, KA reduced the neuronal density in the CA3 hippocampal region, and in the hypothyroid rats with no administration of KA, the neuronal density of the CA3 hippocampal region is reduced also. Administering KA in hypothyroid rats did not reduce the number of CA3 pyramidal cells.

Post-treatment, but not pre-treatment, with the selective cyclooxygenase-2 inhibitor celecoxib markedly enhances functional recovery from kainic acid-induced neurodegeneration

We have investigated the role of inflammation in the excitotoxicity induced by overstimulation of glutamate receptors using kainic acid, an important tool for studying functions related to excitatory amino acid transmission and for producing neuronal death, especially in areas CA1 and CA3 of the hippocampus. We hypothesised that by inhibiting one of the major components of the neuroinflammation response, after kainic acid injection, that there would be less inflammation and therefore a reduction in cell loss, an enhancement of cognitive function (using spatial learning and object exploration tasks) or both. We examined brain-derived neurotrophic factor levels, expecting that there would be a correlation between its level and subsequent recovery. Our results confirmed our hypothesis: the kainic acid injected-rats treated with celecoxib (after kainic injection) performed better in the spatial and non-spatial tasks than the kainic acid-treated group. However, there was not any improvement if celecoxib was given before kainic acid treatment, underlining also the importance of the production of prostaglandin at the beginning of inflammation.

Metabotropic glutamate 2/3 receptors as drug targets.

Metabotropic glutamate receptors are a family of class III G-protein-coupled receptors comprising eight members (mGluR1-8), which are an attractive target in the central nervous system because of the widespread use of glutamate as the principal excitatory amino acid transmitter. The unique pharmacology of class III G-protein coupled receptors, their forebrain localization in key limbic-related cortical/thalamic/striatal/amygdaloid circuits, and the promise of subtle modulation of glutamatergic neurotransmission make these receptors intriguing targets for a wide variety of neuropsychiatric disorders.

Excitotoxicity and the putative involvement of excitatory amino acids in neurodegenerative diseases

Excitatory amino acids (EAA) represent major brain neurotransmitters. They are present in numerous neuronal systems and thus are involved in almost all aspects of normal and pathological brain activity. Changes in EAA transmission have been associated with the functional impairments characterizing major neurological disorders, including epilepsy and schizophrenia. There is also a suspicion that EAA systems underlie the neuronal death associated not only with acute CNS insults, such as in ischemia or post-traumatic lesions, but also with neurodegenerative diseases such as ALS, Huntington's disease and Parkinson's disease. The neurotoxicity of EAA, referred to as excitotoxicity, is presumably mediated primarily through an excess of EAA synaptic receptor stimulation. Indeed, overstimulation of the ionotropic NMDA or AMPA/kainate receptor subtypes has been shown to produce an intense membrane depolarisation and further a massive increase in intracellular calcium leading to cell damage. The extreme diversity and specific pattern of expression of EAA receptor subunits could account for the differential vulnerability of certain brain areas to such excitotoxic processes. In addition, it is now believed that besides abnormalities in receptor functioning or in release processes, alterations in EAA transmission may result from dysfunction of the EAA uptake system, which represents the mechanism for EAA removal from the synapse. From the five transporter proteins cloned, termed EAAT1-5, the primarily glial transporters EAAT1 and EAAT2 have been shown to mediate the bulk of EAA uptake in the brain and it has then been suggested that they play a major role in the prevention of excitotoxic processes. In this respect, the degeneration of motor neurons in ALS has been associated with altered expression or inactivation of EAAT2. Moreover, recent evidence has been provided that pharmacological alteration of glutamate transport can also induce astrocyte degeneration, as observed in neurodegenerative insults, but through a mechanism independent of stimulation of EAA receptors. Thus, one can obviously consider that these EAATs can represent a key target for further development of new neuroprotective agents.

Characterisation of sevoflurane effects on spinal somato-motor nociceptive and non-nociceptive transmission in neonatal rat spinal cord: an electrophysiological study in vitro

Sevoflurane is the latest halogenated ether introduced in clinical anaesthesia, and its effects at the spinal level are not fully characterised. The rat hemisected spinal cord preparation was used to test the effects of sevoflurane on spinal nociceptive and non-nociceptive synaptic transmission as well as on excitations produced by application of glutamate-receptor agonists. Sevoflurane was dissolved in artificial cerebrospinal fluid (ACSF) with a specific vaporiser, and its final concentration was assessed with gas chromatography. Sevoflurane reduced the mono-synaptic reflex (EC 50≈219 μM) and the slow components of the dorsal root–ventral root potentials (EC 50≈72 μM) elicited by single dorsal root stimulation as well as the cumulative depolarisation (CD) elicited by repetitive stimulation (EC 50≈98 μM). AMPA- and NMDA-induced depolarisations were also reduced by sevoflurane (respective EC 50s were 206 and 127 μM). Inhibition of NMDA-induced depolarisation was TTX resistant. However, complete blockade of NMDA receptors with d-AP5 did not prevent further reduction of the CD by sevoflurane. All the effects reported were concentration-dependent and reversible. We conclude that sevoflurane applied at clinically relevant concentrations induces a strong depression of nociceptive and non-nociceptive spinal systems, which may be partly mediated by interfering with excitatory amino acid transmission.

The antidepressant tianeptine persistently modulates glutamate receptor currents of the hippocampal CA3 commissural associational synapse in chronically stressed rats.

Recent hypotheses on the action of antidepressants imply a modulation of excitatory amino acid transmission. Here, the effects of long-term antidepressant application in rats with the drug tianeptine were examined at hippocampal CA3 commissural associational (c/a) glutamate receptor ion channels, employing the whole-cell patch-clamp technique. The drug's impact was tested by subjecting rats to daily restraint stress for three weeks in combination with tianeptine treatment (10 mg/kg/day). Whereas stress increased the deactivation time-constant and amplitude of the N-methyl-d-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs), it did not affect the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor-mediated EPSCs. Concomitant pharmacological treatment of stressed animals with tianeptine resulted in a normalized scaling of the amplitude ratio of NMDA receptor to AMPA/kainate receptor-mediated currents and prevented the stress-induced attenuation of NMDA-EPSCs deactivation. Both paired-pulse-facilitation and frequency-dependent plasticity remained unchanged. Both in control and stressed animals, however, tianeptine treatment strengthened the slope of the input-output relation of EPSCs. The latter was mimicked by exposing hippocampal slices in vitro with 10 micro m tianeptine, which rapidly increased the amplitudes of NMDA- and AMPA/kainate EPSCs. The enhancement of EPSCs could be blocked by the intracellular presence of the kinase inhibitor staurosporine (1 micro m), suggesting the involvement of a postsynaptic phosphorylation cascade rather then presynaptic release mechanisms at CA3 c/a synapses. These results indicate that tianeptine targets the phosphorylation-state of glutamate receptors at the CA3 c/a synapse. This novel signal transduction mechanism for tianeptine may provide a mechanistic resolution for its neuroprotective properties and, moreover, a pharmacological trajectory for its memory enhancing and/or antidepressant activity.

NAALADase (GCP II) inhibition as a novel therapeutic target for neuropathic pain, diabetic neuropathy, and ALS

NAALADase (N‐acetylated‐α‐linked‐acidic dipeptidase; also termed glutamate carboxypeptidase II) is an enzyme that catalyzes hydrolysis of the abundant brain peptide N‐acetyl‐aspartyl‐glutamate (NAAG) to N acetyl‐aspartate (NAA) and glutamate. We have synthesized many potent and selective NAALADase inhibitors from distinct chemical classes. In rodents, selective inhibitors have been shown to increase NAAG and decrease glutamate extracellularly and provide therapeutic benefits in animal models of neurological diseases where excess glutamate neurotransmission has been implicated. Specifically, NAALADase inhibitors exhibit neuroprotection in rats following transient middle cerebral artery occlusion, attenuate neuropathic pain following chronic constrictive injury, prevent nerve conduction velocity deficits and sural nerve degeneration following chronic diabetes in STZ and BB/W rats, and delay clinical symptoms and mortality in the mouse SOD transgenic model of ALS. These biological effects are likely mediated by increases in extracellular NAAG and decreases in extracellular glutamate as a consequence of NAALADase inhibition. NAALADase inhibition may be useful in neurological disorders in which excessive excitatory amino acid transmission is pathogenic. Also, NAALADase inhibition may represent a novel glutamate regulating strategy devoid of the side‐effects that have hampered development of postsynaptic glutamate receptor antagonists. A lead NAALADase inhibitor termed GPI 5693 is currently completing Phase I clinical testing.

P2Y 1 receptor activation inhibits NMDA receptor-channels in layer V pyramidal neurons of the rat prefrontal and parietal cortex

In the 1st part of this study, monosynaptic excitatory postsynaptic potentials (EPSPs) in layer V of the rat prefrontal cortex (PFC) were evoked by electrical stimulation of layer I. Recordings with intracellular sharp, microelectrodes showed a concentration-dependent inhibition of the EPSP by adenosine 5′- O-(2-thiodiphosphate) (ADP-β- S). Pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), when given alone depressed the EPSP and in addition antagonized the effect of ADP-β- S. Exclusion of the N-methyl- d-aspartate (NMDA) component of the EPSP by d(·)-amino-5-phosphonopentanoic acid (AP-5) abolished the ADP-β- S-induced depression. The pressure-application of both NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) caused reproducible depolarizations. ADP-β- S inhibited the effect of NMDA, but did not alter that of AMPA. PPADS was also under these conditions antagonistic with ADP-β- S. In the 2nd part of the study, NMDA-induced currents were measured by whole-cell patch-clamp pipettes. ADP-β- S caused a concentration-dependent inhibition of the responses to NMDA. PPADS alone did not alter the NMDA-currents but again antagonized the action of ADP-β- S; 2′-deoxy- N 6-methyladenosine-3′,5′-diphosphate (MRS 2179) also abolished the NMDA effect. The ADP-β- S-induced inhibition persisted in the presence of tetrodotoxin (TTX) or guanosine 5′- O-(3-thiodiphosphate) (GDP-β- S) applied to the external medium and the pipette solution, respectively. The 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) moderately decreased the ADP-β- S effect. The inhibitory function of ADP-β- S on EPSPs and the interaction with PPADS was observed also in layer V pyramidal neurons of the parietal somatosensory cortex. In conclusion, metabotropic P2Y 1 receptors appear to exert a new modulatory influence on fast excitatory amino acid transmission in the cerebral cortex.

Ultrastructural localization of neuropeptide Y Y1 receptors in the rat medial nucleus tractus solitarius: relationships with neuropeptide Y or catecholamine neurons.

Neuropeptide Y (NPY) Y1 receptor (Y1-R) agonists influence cardiovascular regulation. These actions may involve NPY- and catecholamine-containing neurons in the medial nucleus of the solitary tract (mNTS), at the level of the area postrema. The cellular sites through which Y1-R agonists may interact with NPY and catecholamines in the mNTS, however, are not known. To determine potential sites of action for Y1-R agonists, and their relationship to NPY or catecholamines in the mNTS, we used electron microscopic immunocytochemistry for the detection of sequence-specific antipeptide antisera against Y1-R alone or in combination with antisera against NPY or the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Analyses were conducted in the rat mNTS, at the level of the area postrema. Y1-R was found mainly in small unmyelinated axons and axon terminals but also in some somata and dendrites as well as a small number of glia. Within axon terminals, labeling for Y1-R was often present on dense core vesicles and small synaptic vesicles as well as extrasynaptic areas of the plasmalemma. Some Y1-R-labeled terminals also contained NPY or TH, suggesting that agonists of Y1-R may influence the release of NPY or catecholamines in the mNTS. In addition, Y1-R was found in dendrites that received asymmetric excitatory-type synapses from unlabeled axon terminals. Some of these dendrites contained NPY or TH, which indicates that Y1-R may be targeted for functional activation within NPY- or catecholamine-expressing neurons in the mNTS. These results demonstrate that Y1-R is a presynaptic receptor in NPY- or catecholamine-containing axon terminals within the mNTS as well as a postsynaptic receptor on NPY- or catecholamine-containing neurons that are contacted by axon terminals that likely contain excitatory amino acid transmitters. Agonists of Y1-R in the mNTS may thus affect cardiovascular regulation by modulating NPY, catecholamine, and excitatory amino acid transmission.

Laminar properties of 4-aminopyridine-induced synchronous network activities in rat neocortex

We examined the effects of 4-aminopyridine (4-AP) on isolated horizontal (superficial, middle and deep) rat neocortical slices in order to study laminar synchronous network behavior directly. Application of 4-AP induced spontaneous synchronized activity in all of these types of slices. In middle and deep layer slices the activities were similar to those of coronal slices, consisting of periodic short- and long-duration discharges. In superficial slices distinct spontaneous rhythmic multiphasic burst discharges were induced. Ionotropic glutamate receptor antagonists blocked the 4-AP-induced synchronous activities in middle and deep layer slices, but those in superficial slices persisted. The GABA A receptor antagonist picrotoxin suppressed this spontaneous synchronous activity resistant to 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (a NMDA receptor antagonist) and 6-cyano-7-nitroquinoxaline-2,3-dione (a non-NMDA receptor antagonist), in superficial slices, leaving small, slow spontaneous events. In superficial slices with intact excitatory amino acid transmission, picrotoxin attenuated the 4-AP-induced spontaneous synchronous discharges, even in this highly convulsant environment. By contrast, conventional coronal slices showed robust spontaneous epileptiform discharges under these circumstances. In intact coronal slices focal 4-AP application in superficial layers induced spontaneous inhibitory GABAergic events, while delivery into deep layers led to epileptiform discharges. From these results we conclude that: (1) 4-AP-induced population discharges are driven by glutamatergic transmission in middle and deep layer horizontal slices, and by GABAergic transmission in superficial layers; (2) only superficial layers are capable of supporting synchronized GABAergic activity independent of excitatory amino acid transmission; (3) superficial layers do not sustain epileptiform activity in the absence of deep layer neurons; and (4) synchronized superficial networks can inhibit deep layer neuronal activity.

Plasmalemmal mu-opioid receptor distribution mainly in nondopaminergic neurons in the rat ventral tegmental area.

Opiate-evoked reward and motivated behaviors reflect, in part, the enhanced release of dopamine produced by activation of the mu-opioid receptor (muOR) in the ventral tegmental area (VTA). We examined the functional sites for muOR activation and potential interactions with dopaminergic neurons within the rat VTA by using electron microscopy for the immunocytochemical localization of antipeptide antisera raised against muOR and tyrosine hydroxylase (TH), the synthesizing enzyme for catecholamines. The cellular and subcellular distribution of muOR was remarkably similar in the two major VTA subdivisions, the paranigral (VTApn) and parabrachial (VTApb) nuclei. In each region, somatodendritic profiles comprised over 50% of the labeled structures. MuOR immunolabeling was often seen at extrasynaptic/perisynaptic sites on dendritic plasma membranes, and 10% of these dendrites contained TH. MuOR-immunoreactivity was also localized to plasma membranes of axon terminals and small unmyelinated axons, none of which contained TH. The muOR-immunoreactive axon terminals formed either symmetric or asymmetric synapses that are typically associated with inhibitory and excitatory amino acid transmitters. Their targets included unlabeled (30%), muOR-labeled (25%), and TH-labeled (45%) dendrites. Our results suggest that muOR agonists in the VTA affect dopaminergic transmission mainly indirectly through changes in the postsynaptic responsivity and/or presynaptic release from neurons containing other neurotransmitters. They also indicate, however, that muOR agonists directly affect a small population of dopaminergic neurons expressing muOR on their dendrites in VTA and/or terminals in target regions.

Experimental uremia affects hypothalamic amino acid neurotransmitter milieu.

Chronic renal failure is associated with delayed puberty and hypogonadism. To investigate the mechanisms subserving the reported reduced pulsatile release of gonadotropin-releasing hormone (GnRH) in chronic renal failure, this study examined the amino acid neurotransmitter milieu in the medial preoptic area (MPOA), the hypothalamic region where the GnRH-secreting neurons reside, in 5/6-nephrectomized male rats and in ad libitum-fed or pair-fed controls. All rats were castrated and received either a testosterone or a vehicle implant to evaluate additional effects of the prevailing sex steroid milieu. Local excitatory (essential amino acids: aspartate, glutamate) and inhibitory (gamma-aminobutyric acid [GABA], taurine) amino acid transmitter outflow in the MPOA was measured by microdialysis via stereotactically implanted cannulae in the awake, freely moving rats. In addition to basal extracellular concentrations, the neurosecretory capacity was assessed by the addition of 100 mM KCl to the dialysis fluid. The mechanisms of neurosecretion were evaluated further by inhibition of vesicular release with the use of Ca(2+)-free, Mg(2+)-enriched dialysis fluid and by local perfusion with inhibitors of voltage-dependent synaptic release (1 microM tetrodotoxin) and of GABA reuptake (0.5 mM nipecotic acid). In the uremic rats, basal outflow of GABA, glutamate and aspartate, and K(+)-stimulated aspartate outflow were increased. K(+)-stimulated GABA and glutamate release was less sensitive to Ca(2+) depletion in the uremic than in the control rats. The elevated basal GABA and essential amino acid outflow in the uremic rats was due to a voltage- and Ca(2+)-independent mechanism. GABA reuptake was inhibited proportionately by nipecotic acid in uremic and pair-fed control rats. Testosterone supplementation had no independent effects on neurotransmitter outflow. In summary, the amino acid neurotransmitter milieu is altered in the MPOA of uremic rats by a nonsynaptic, nonvesicular mechanism. These abnormalities may contribute to the impaired function of the GnRH pulse generator.

Expression of AMPA Receptor Flip and Flop mRNAs in the Nucleus Accumbens and Prefrontal Cortex after Neonatal Ventral Hippocampal Lesions

Many lines of evidence implicate dysfunctional excitatory amino acid (EAA) transmission in schizophrenia. The present study examined α-amino-3-hydroxy-5-methylisoxazole- 4-priopionate (AMPA) receptor expression in the prefrontal cortex (PFC) and nucleus accumbens (NAc) using a rat model of schizophrenia in which excitotoxic lesions of ventral hippocampus (VH) on postnatal day (PD) 7 lead to the postpubertal emergence of behavioral abnormalities that resemble schizophrenic symptoms. In situ hybridization histochemistry with 35S-labeled oligonucleotide probes was used to quantify mRNA levels for GluR1-3 flip and flop variants at prepubertal and postpubertal time-points (PD 35 and 60, respectively). Comparisons of PD 35 and PD 60 groups suggested that there may be changes in the relative expression of flip and flop isoforms during normal development. The most pronounced change was an apparent increase in the flip/flop ratio for GluR1 from PD 35 to PD 60 in both the PFC and the NAc. Neonatal VH lesions did not significantly alter GluR1-3 flip and flop mRNA levels in the NAc at either time-point. However, in the PFC, GluR3 flop mRNA levels were significantly decreased in lesioned rats at PD 60. AMPA receptors incorporating flop variants exhibit faster desensitization, so a shift towards flip incorporation might lead to increased neuronal excitability in the PFC, thereby influencing subcortical DA systems regulated by PFC efferents. In summary, an early developmental injury can predispose rats to abnormal development of EAA transmission in the PFC. This may contribute to the postpubertal onset of symptoms after neonatal VH lesions. [Neuropshychopharmacology 24:253–266, 2001]

Changes in striatal electroencephalography and neurochemistry induced by kainic acid seizures are modified by dopamine receptor antagonists

We investigated the involvement of striatal dopamine release in electrographic and motor seizure activity evoked by kainic acid in the guinea pig. The involvement of the dopamine receptor subtypes was studied by systemic administration of the dopamine D 1 receptor antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrochloride (SCH 23390; 0.5 mg kg −1), or the dopamine D 2 antagonist, (5-aminosulphonyl)- N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride, 30 mg kg −1). Microdialysis and high performance liquid chromatography were used to monitor changes in extracellular levels of striatal dopamine and its metabolites, glutamate, aspartate and γ-amino-butyric acid (GABA). These data were correlated with changes in the striatal and cortical electroencephalographs and clinical signs. We found that, although neither dopamine receptor antagonist inhibited behavioural seizure activity, blockade of the dopamine D 1-like receptor with SCH 23390 significantly reduced both the ‘power’ of the electrical seizure activity and the associated change in extracellular striatal concentration of glutamate, whilst increasing the extracellular striatal concentration of GABA. In contrast, blockade of the dopamine D 2-like receptor with sulpiride significantly increased the extracellular, striatal content of glutamate and the dopamine metabolites. These results confirm previous evidence in other models of chemically-evoked seizures that antagonism of the dopamine D 1 receptor tends to reduce motor and electrographic seizure activity as well as excitatory amino-acid transmitter activity, while antagonism of the dopamine D 2 receptor has relatively less apparent effect.

Comparative Studies on the Distribution of Glutamate Transporters in the Retinae of the Mongolian Gerbil and the Rat

Glutamate is the major excitatory amino acid transmitter in vertebrate retinae. Glutamate transporters therefore play an important role in the precise control of glutamate concentration in the synaptic cleft by regulating extracellular glutamate concentration. In the present study, we performed an analysis of the expressions of three glutamate transporters in gerbil retina using immunohistochemistry. In the gerbil retina, excitatory amino acid carrier 1 and glutamate transporter 1 immunoreactivity was predominant in the ganglion cells but not amacrine or bipolar cells. Glutamate/aspartate transporter (GLAST) immunoreactivity was observed in the radial gliocytes of which the dense network of fine processes was localized in the inner and outer plexiform layers. GLAST immunoreactivity was also detected in astrocytes in the nerve fibre layer. These results demonstrate that three glutamate transporters show specific distributions in the gerbil retina and suggest that the glutamate re-uptake system in the gerbil retina may be different from that of the rat.