Effect of Acute Stress on Hippocampal Glutamate Levels and Spectrin Proteolysis in Young and Aged Rats

Abstract: Aging in rats is associated with a loss of hippocampal neurons, which may contribute to age-related cognitive deficits. Several lines of evidence suggest that stress and glucocorticoids may contribute to age-related declines in hippocampal neuronal number. Excitatory amino acids (EAAs) have been implicated in the glucocorticoid endangerment and stress-induced morphological changes of hippocampal neurons of young rats. Previously, we have reported that acute immobilization stress can increase extracellular concentrations of the endogenous excitatory amino acid, glutamate, in the hippocampus. The present study examined the effect of an acute bout of immobilization stress on glutamate levels in the hippocampus and medial prefrontal cortex of young (3–4-month) and aged (22–24-month) Fischer 344 rats. In addition, the effect of stress on spectrin proteolysis in these two brain regions was also examined. Spectrin is a cytoskeleton protein that contributes to neuronal integrity and proteolysis of this protein has been proposed as an important component of EAA-induced neuronal death. There was no difference in basal glutamate levels between young and old rats in the hippocampus or medial prefrontal cortex. During the period of restraint stress a modest increase in glutamate levels in the hippocampus of young and aged rats was observed. After the termination of the stress procedure, hippocampal glutamate concentrations continued to rise in the aged rats, reaching a level approximately five times higher than the young rats, and remained elevated for at least 2 h after the termination of the stress. A similar pattern was also observed in the medial prefrontal cortex with an augmented post-stress-induced glutamate response observed in the aged rats. There was no increase in spectrin proteolysis in the hippocampus or medial prefrontal cortex of young or aged rats after stress or under basal nonstress conditions. The enhanced poststress glutamate response in the aged rats may contribute to the increased sensitivity of aged rats to neurotoxic insults.     

An Excitant Amino Acid Projection from the Medial Prefrontal Cortex to the Anterior Part of Nucleus Accumbens in the Rat

High-affinity uptake of neurotransmitter substrates in synaptosome-containing homogenates and tissue concentrations of amino acids were examined in subcortical areas 5-6 days after bilateral N-methyl-D-aspartate lesions confined to rat medial prefrontal cortex. D-[3H]Aspartate (32% of control) and [3H] gamma-aminobutyric acid ( [3H]GABA) (60% of control) uptakes were significantly reduced in medial prefrontal cortex, whereas [3H]choline (110% of control) uptake was unchanged, suggesting the production of axon-sparing lesions. The uptake of D-[3H]aspartate (76% of control), but not of [3H]GABA or [3H]choline, was significantly reduced in nucleus accumbens, with no concomitant reduction in amino acid concentrations. When examined in serial coronal sections, reduced D-[3H]aspartate uptake was confined to the most anterior 500 micron of nucleus accumbens (67% of contralateral sample). No significant reductions of uptake or amino acid concentrations were observed in caudate putamen or ventral tegmental area. These results suggest a role for glutamate or aspartate as neurotransmitters in projections from medial prefrontal cortex to anterior nucleus accumbens. Medial prefrontal cortex may represent the major excitatory cortical input to the nucleus accumbens.     

Extracellular levels of brain aspartate, glutamate and GABA during an inhibitory avoidance response in the rat

The extracellular levels of aspartate, glutamate and GABA were measured by microdialysis, coupled with an HPLC method, in rat prefrontal cortex (mPFC) and ventral hippocampus (VH) before and during the performance of a step-down inhibitory task. The basal levels of glutamate were about 50% higher than those of aspartate, and GABA levels were about 20-folds smaller than those of the excitatory amino acids. There were no significant differences in the basal levels of any of the three amino acids between the two brain regions. The extracellular levels of aspartate increased during acquisition and recall trials in both VH and mPFC, whereas those of glutamate increased in the VH during acquisition only. A significant increase in GABA levels was also detected during acquisition but only in the mPFC. The neuronal origin of the increased extracellular levels of aspartate, glutamate and GABA was demonstrated by administering tetrodotoxin directly into the mPFC or VH by reverse dialysis. These findings, together with previous evidence from our and other laboratories, indicate a differential release of aspartate and glutamate from excitatory neurons during the performance of behavioral responses, and therefore, distinct roles for the two excitatory amino acids should be envisaged.     

Effects of repeated cocaine on medial prefrontal cortical GABAB receptor modulation of neurotransmission in the mesocorticolimbic dopamine system

Increased excitatory output from medial prefrontal cortex is an important component in the development of cocaine sensitization. Activation of GABAergic systems in the prefrontal cortex can decrease glutamatergic activity. A recent study suggested that sensitization might be associated with a decrease in GABAB receptor responsiveness in the medial prefrontal cortex. Therefore, the present study examined whether repeated exposure to cocaine-modified neurochemical changes in the mesocorticolimbic dopamine system induced by infusion of baclofen into the medial prefrontal cortex. In vivo microdialysis studies were conducted to monitor dopamine, glutamate and GABA levels in the medial prefrontal cortex and glutamate levels in the ipsilateral nucleus accumbens and ventral tegmental area during the infusion of baclofen into medial prefrontal cortex. Baclofen minimally affected glutamate levels in the medial prefrontal cortex, nucleus accumbens or ventral tegmental area of control animals, but dose-dependently increased glutamate levels in each of these regions in animals sensitized to cocaine. This effect was not the result of changes in GABAB receptor-mediated modulation of dopamine or GABA in the medial prefrontal cortex. The data suggest that alterations in GABAB receptor modulation of medial prefrontal cortical excitatory output may play an important role in the development of sensitization to cocaine.     

Changes in regional levels of putative neurotransmitter amino acids in brain under unilateral forebrain ischemia

The levels of the neurotransmitter amino acids glutamate, aspartate, and GABA were determined in different brain regions during ischemia and post-ischemic recirculation periods using the unilateral carotid artery occlusion model of stroke in gerbils. The levels of glutamate, aspartate and GABA in ischemic hemisphere were increased significantly by 10 min of ischemia and later declined with time. Reperfusion for 30 min following 10 min. of ischemia further enhanced the levels of glutamate and aspartate. Increase in GABA levels were found during early periods of reperfusion. Regional variations in the changes of amino acids' levels were noticed following ischemia. Hippocampus showed the highest increase in glutamate levels followed by striatum and cerebral cortex. Aspartate levels in striatum and hippocampus increased during 10 min ischemia (46% and 30%) and recirculation (70% and 79%), whereas in cerebral cortex the levels were doubled only during recirculation. Ischemia induced elevations of GABA levels were observed in cerebral cortex (68%) and in hippocampus (30%), and the levels were normalized during recirculation. No changes in GABA levels were found in striatum. It is suggested that the large increase in the levels of excitatory neurotransmitter amino acids in brain regions specially in hippocampus during ischemia and recirculation may be one of the causal factors for ischemic brain damage.     

Medial-frontal cortex hypometabolism in chronic phencyclidine exposed rats assessed by high resolution magic angle spin 11.7 T proton magnetic resonance spectroscopy

BackgroundProton magnetic resonance spectroscopy (¹H-MRS) clinical studies of patients with schizophrenia document prefrontal N-acetylaspartate (NAA) reductions, suggesting an effect of the disease or of antipsychotic medications. We studied in the rat the effect of prolonged exposure to a low-dose of the NMDA glutamate receptor antagonist phencyclidine (PCP) on levels of NAA, glutamate and glutamine in several brain regions where metabolite reductions have been reported in chronically medicated patients with schizophrenia.MethodsTwo groups of ten rats each were treated with PCP (2.58 mg/kg/day) or vehicle and were sacrificed after 1 month treatment. Concentrations of neurochemicals were determined with high resolution magic angle (HR-MAS) ¹H-MRS at 11.7 T in ex vivo punch biopsies from the medial frontal and cingulate cortex, striatum, nucleus accumbens, amygdala and ventral hippocampus.ResultsPCP treatment reduced NAA, glutamate, glycine, aspartate, creatine, lactate and GABA in medial frontal cortex. In the nucleus accumbens, PCP reduced levels of NAA, aspartate and glycine; similarly aspartate and glycine were reduced in the striatum. Finally the amygdala and hippocampus had elevations in glutamine and choline, respectively.ConclusionsLow-dose PCP in rats models prefrontal NAA and glutamate reductions documented in chronically-ill schizophrenia patients. Chronic glutamate NMDA receptor blockade in rats replicates an endophenotype in schizophrenia and may contribute to the prefrontal hypometabolic state in schizophrenia.     

Stress-Induced Dopamine Release in the Neostriatum: Evaluation of the Role of Action Potentials in Nigrostriatal Dopamine Neurons or Local Initiation by Endogenous Excitatory Amino Acids

Abstract: It has been hypothesized that excitatory amino acids can initiate dopamine release in neostriatum. We examined whether the increase in extracellular dopamine in neostriatum produced by acute stress reflects presynaptic initiation of dopamine release by endogenous excitatory amino acids. Thirty minutes of intermittent tail-shock stress significantly elevated extracellular concentrations of dopamine, glutamate, aspartate, and γ-aminobutyric acid in neostriatum of freely moving rats as measured with in vivo microdialysis. Local infusion of the N -methyl- d -aspartate receptor antagonist 2-amino-5-phosphonovaler-ate or the non- N -methyl- d -aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione via the dialysis probe did not attenuate the stress-induced increase in extra cellular dopamine. In fact, the increase was prolonged in rats treated with specific excitatory amino acid receptor antagonists. Infusion of tetrodotoxin into medial forebrain bundle increased extra cellular glutamate and aspartate in neostriatum yet reduced basal dopamine in extra cellular fluid to below the limit of detection of the assay and eliminated the stress-induced increase in extra cellular dopamine. These findings fail to support the hypothesis that the stress-induced increase in extra cellular dopamine in neostriatum is initiated locally by excitatory amino acids. Rather, the effects of stress on extra cellular dopamine seem to be determined by impulse propagation in dopamine neurons.     

Acute Effects of Typical and Atypical Antipsychotic Drugs on the Release of Dopamine from Prefrontal Cortex, Nucleus Accumbens, and Striatum of the Rat: An In Vivo Microdialysis Study

In vivo microdialysis has been used to study the acute effects of antipsychotic drugs on the extracellular level of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. (-)-Sulpiride (20, 50, and 100 mg/kg i.v.) and haloperidol (0.1 and 0.5 mg/kg i.v.) enhanced the outflow of dopamine in the striatum and nucleus accumbens. In the medial prefrontal cortex, (-)-sulpiride at all doses tested did not significantly affect the extracellular level of dopamine. The effect of haloperidol was also attenuated in the medial prefrontal cortex; 0.1 mg/kg did not increase the outflow of dopamine and the effect of 0.5 mg/kg haloperidol was of shorter duration in the prefrontal cortex than that observed in striatum and nucleus accumbens. The atypical antipsychotic drug clozapine (5 and 10 mg/kg) increased the extracellular concentration of dopamine in all three regions. In contrast to the effects of sulpiride and haloperidol, that of clozapine in the medial prefrontal cortex was profound. These data suggest that different classes of antipsychotic drugs may have distinct effects on the release of dopamine from the nigrostriatal, mesolimbic, and mesocortical terminals.     

Excitatory and inhibitory amino acid changes in ischemic brain regions in spontaneously hypertensive rats

Excitatory (glutamate, aspartate) or inhibitory amino acids (-aminobutyric acid: GABA, taurine) and glutamine contents were examined in acutely induced cerebral ischemia in spontaneously hypertensive rats. At 20 min ischemia most of these amino acids remained unchanged, but glutamine significantly decreased by 14% in the CA3 hippocampal subfield. At 60 min ischemia glutamate significantly decreased by 14% in the CA3, aspartate by 17–26% in the CA3, cingulate cortex, septum and striatum. In contrast, GABA significantly increased by 48–106% in the cortices (frontal, parietal and cingulate), striatum and nucleus accumbens, but insignificantly in hippocampal subrïelds. Likewise, taurine increased in the parietal cortex and nucleus accumbens. Glutamine showed heterogeneous changes (increase in the nucleus accumbens and decrease in the CA3). Amino acid levels change during ischemia, but their changes are varied in each area, implying that different reaction of amino acids may explain the selective vulnerability to cerebral ischemia.     

Dopaminergic innervation of the amygdala is highly responsive to stress

The amygdala has been implicated in the neuronal sequelae of stress, although little is known about the neurochemical mechanisms underlying amygdala transmission. In vivo microdialysis was employed to measure extracellular levels of dopamine in the basolateral nucleus of the amygdala in awake rats. Once it was established that impulse-dependent release of dopamine could be measured reliably in the amygdala, the effect of stress, induced by mild handling, on amygdala dopamine release was compared with that in three other dopamine-innervated regions, the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. The magnitude of increase in dopamine in response to the handling stimulus was significantly greater in the amygdala than in the nucleus accumbens and prefrontal cortex. This increase was maximal during the application of stress and diminished after the cessation of stress. In contrast, the increases in extracellular dopamine levels in other regions, in particular the nucleus accumbens, were prolonged, reaching maximal values after the cessation of stress. These results suggest that dopaminergic innervation of the amygdala may be more responsive to stress than that of other dopamine-innervated regions of the limbic system, including the prefrontal cortex, and implicate amygdalar dopamine in normal and pathophysiological processes subserving an organism's response to stress.     

Extracellular Glutamate Is Increased in Thalamus During Thiamine Deficiency-Induced Lesions and Is Blocked by MK-801

The current study measured extracellular fluid (ECF) levels of excitatory amino acids before and during the onset of thiamine deficiency-induced pathologic lesions. Male Sprague-Dawley rats were treated with daily pyrithiamine (0.25 mg/kg i.p.) and a thiamine-deficient diet (PTD). Microdialysates were simultaneously collected from probes inserted acutely via guide cannulae into right paracentral and ventrolateral nuclei of thalamus and left hippocampus of PTD and pair-fed controls. Hourly samples were collected from unanesthetized and freely moving animals. Basal levels obtained at a prelesion stage (day 12 of PTD treatment) were unchanged from levels in pairfed controls. In samples collected 4–5 h after onset of seizures (day 14 of PTD), the levels of glutamate were elevated an average 640% of basal levels in medial thalamus and 200% in hippocampus. Glutamine levels declined, taurine and glycine were elevated, and aspartate, GABA, and alanine were unchanged during this period. Within 7 h after seizure onset glutamine was undetectable in both areas, whereas glutamate had declined to ～200% in thalamus and 70% in hippocampus. No significant change in glutamate, aspartate, or other amino acids was observed in dialysates collected from probes located in undamaged dorsal-lateral regions of thalamus. Number of neurons within ventrolateral nucleus of thalamus was significantly greater in PTD animals in which the probe was dialyzed compared with nondialyzed, suggesting that removal of excitatory amino acids was protective. No significant pathologic damage was evident in hippocampus. Pretreatment with MK-801 completely blocked the rise of ECF glutamate and significantly reduced the pathologic damage within thalamus of PTD rats and produced a significant decrease in ECF glutamate in control rats.     

Differential Effects of Forced Locomotion, Tail-Pinch, Immobilization, and Methyl-β-Carboline Carboxylate on Extracellular 3,4-Dihydroxyphenylacetic Acid Levels in the Rat Striatum, Nucleus Accumbens, and Prefrontal Cortex: An In Vivo Voltammetric Study

In vivo voltammetry with carbon fiber electrodes was used to assess extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels in striatum, nucleus accumbens, and anteromedial prefrontal cortex of freely moving rats subjected to altered motor activity or anxiogenic stimuli. Forced locomotion on a rotarod for 40 min caused an increase in extracellular DOPAC levels in the striatum and to a lesser extent in the nucleus accumbens but not in the prefrontal cortex. Subcutaneous injection of the anxiogenic agent methyl-beta-carboline carboxylate (10 mg/kg) increased extracellular DOPAC levels to a similar extent in prefrontal cortex and nucleus accumbens. Immobilization for 4 min augmented dopamine (DA) metabolism preferentially in the nucleus accumbens and to a lesser extent in the prefrontal cortex. Tail-pinch caused a selective activation of DA metabolism in the nucleus accumbens. None of these stimuli altered extracellular striatal DOPAC levels. These results confirm the involvement of dopaminergic systems projecting to the striatum and nucleus accumbens in motor function and suggest that mesolimbic and mesocortical dopaminergic systems can be specifically activated by certain kinds of anxiogenic stimuli; the relative activation of either of these latter systems could depend primarily on the nature (sensory modality, intensity) of the acute stressor.     

Acute and Repeated Systemic Amphetamine Administration: Effects on Extracellular Glutamate, Aspartate, and Serine Levels in Rat Ventral Tegmental Area and Nucleus Accumbens

Abstract: Recent work indicates an important role for excitatory amino acids in behavioral sensitization to amphetamine. We therefore examined, using in vivo microdialysis in awake rats, the effects of amphetamine on efflux of glutamate, aspartate, and serine in the ventral tegmental area and nucleus accumbens, brain regions important for the initiation and expression of amphetamine sensitization, respectively. Water-pretreated and amphetamine-pretreated rats were compared to determine if sensitization altered such effects. In both brain regions, Ca²⁺-dependent efflux of glutamate accounted for ∼20% of basal glutamate efflux. A challenge injection of water or 2.5 mg/kg of amphetamine did not significantly alter glutamate, aspartate, or serine efflux in the ventral tegmental area or nucleus accumbens of water- or amphetamine-pretreated rats. However, 5 mg/kg of amphetamine produced a gradual increase in glutamate efflux in both regions that did not reverse, was observed in both water- and amphetamine-pretreated rats, and was prevented by haloperidol. Although increased glutamate efflux occurred with too great a delay to mediate acute behavioral responses to amphetamine, it is possible that repeated augmentation of glutamate efflux during repeated amphetamine administration results in compensatory changes in levels of excitatory amino acid receptors in the ventral tegmental area and nucleus accumbens that contribute to development or expression of amphetamine sensitization.     

The effect of the antipsychotic drug mosapramine on the expression of Fos protein in the rat brain: comparison with haloperidol, clozapine and risperidone

In this study, we examined the effect of the acute p.o. administration of the antipsychotic drug mosapramine, as well as the antipsychotic drugs clozapine, haloperidol and risperidone, on the expression of Fos protein in the medial prefrontal cortex, nucleus accumbens and dorsolateral striatum of rat brain. The administration of mosapramine (1 or 3 mg/kg) significantly increased the number of Fos protein positive neurons in the medial prefrontal cortex, but not in the dorsolateral striatum. In addition, mosapramine (1, 3 or 10 mg/kg) produced a dose-dependent increase in the number of Fos protein positive neurons in the nucleus accumbens. The acute administration of 10 mg/kg of mosapramine significantly increased the number of Fos protein positive neurons in all brain regions. The acute administration of clozapine (30 mg/kg), similarly to mosapramine at lower doses (1 or 3 mg/kg), significantly increased the number of Fos protein positive neurons in the medial prefrontal cortex and nucleus accumbens, but not dorsolateral striatum. In contrast, haloperidol (0.3 mg/kg) significantly increased the number of Fos protein positive neurons in the nucleus accumbens and dorsolateral striatum, but not medial prefrontal cortex. The acute administration of risperidone (0.3 or 1 mg/kg) did not affect the number of Fos protein positive neurons in the medial prefrontal cortex, nucleus accumbens or dorsolateral striatum of rat brain, whereas a 3 mg/kg dose of risperidone significantly increased the number of Fos protein positive neurons in all brain regions. These results suggest that the ability of mosapramine to enhance expression of Fos protein in the medial prefrontal cortex may contribute to a clozapine-like profile with respect to actions on negative symptoms in schizophrenia. Furthermore, the lack of effect of low doses of mosapramine on Fos protein expression in the dorsolateral striatum, an area believed to play a role in movement, suggests that it may have a lower tendency to induce neurological side effects.     

Chronic treatment with a dopamine uptake blocker changes dopamine and acetylcholine but not glutamate and GABA concentrations in prefrontal cortex, striatum and nucleus accumbens of the awake rat

The present study was aimed to investigate the effects of a chronic treatment with the dopamine uptake blocker nomifensine on the in vivo extracellular concentrations of dopamine, acetylcholine, glutamate and GABA in the prefrontal cortex, striatum and nucleus accumbens. Male Wistar rats received intraperitoneal (i.p.) daily injections of nomifensine (10 mg/kg) or saline for 22 days. Microdialysis experiments were performed on days 1, 8, 15 and 22 of treatment to evaluate the effects of the injection of nomifensine or saline. Motor activity of the animals was monitored during microdialysis experiments. Injections of nomifensine increased extracellular concentration of dopamine in striatum and nucleus accumbens, but not in prefrontal cortex. Acetylcholine concentrations in striatum but not in nucleus accumbens were increased by nomifensine on days 15 and 22 of treatment. In prefrontal cortex, nomifensine increased acetylcholine levels without differences among days. No changes were found on glutamate and GABA concentrations in the three areas studied. Injections of nomifensine also increased spontaneous motor activity and stereotyped behaviour without differences among days. These results show that systemic chronic treatment with a dopamine uptake blocker produces differential effects on extracellular concentrations of dopamine and acetylcholine, but not glutamate and GABA, in different areas of the brain.     

Changes in the Extracellular Concentrations of Amino Acids in the Rat Striatum During Transient Focal Cerebral Ischemia

Abstract: Although considerable evidence supports a role for amino acids in transient global cerebral ischemia and permanent focal cerebral ischemia, effects of transient focal cerebral ischemia on the extracellular concentrations of amino acids have not been reported. Accordingly, our study was undertaken to examine the patterns of changes of extracellular glutamate, aspartate, GABA, taurine, glutamine, alanine, and phosphoethanolamine in the striatum of transient focal cerebral ischemia, as evidence to support their pathogenic roles. Focal ischemia was induced using the middle cerebral artery occlusion model, with no need for craniotomy. Microdialysis was used to sample the brain's extracellular space before, during, and after the ischemic period. One hour of middle cerebral artery occlusion followed by recirculation caused neuronal damage that was common in the frontoparietal cortex and the lateral segment of the caudate nucleus. During 1 h of ischemia, the largest increase occurred for GABA and moderate increases were observed for taurine, glutamate, and aspartate. Alanine, which is a nonneuroactive amino acid, increased little. After recirculation, the levels of glutamate and aspartate reverted to normal baseline values right after reperfusion. Despite these rapid normalizations, neuronal damage occurred. Therefore, uptake of excitatory amino acids can still be restored after 1 h of middle cerebral artery occlusion, and tissue damage occurs even though high extracellular levels of glutamate are not maintained.     

Stress-Induced Sensitization of Dopamine and Norepinephrine Efflux in Medial Prefrontal Cortex of the Rat

Abstract: We examined whether prior exposure to chronic cold (17–28 days, 5°C) alters basal or stress-evoked (30-min tail shock) catecholamine release in medial prefrontal cortex, nucleus accumbens, and striatum, using in vivo microdialysis. Basal norepinephrine (NE) concentrations in medial prefrontal cortex did not differ between chronically cold-exposed rats and naive control rats (2.7 ± 0.3 vs. 2.5 ± 0.2 pg/20 µl, respectively). Basal dopamine (DA) efflux in any of the brain regions was not significantly different between chronically cold-exposed rats and naive rats. However, a trend for lower basal DA efflux in the cold-exposed relative to naive rats was observed in medial prefrontal cortex (1.5 ± 0.2 vs. 2.2 ± 0.3 pg/20 µl, respectively), nucleus accumbens (3.7 ± 0.8 vs. 5.4 ± 0.9 pg/20 µl, respectively), and striatum (4.4 ± 0.5 vs. 7.2 ± 1.5 pg/20 µl, respectively). In medial prefrontal cortex of rats previously exposed to cold, tail shock elicited a greater increase from baseline in both DA and NE efflux relative to that measured in naive rats (DA, 2.3 ± 0.3 vs. 1.2 ± 0.1 pg, respectively; NE, 3.8 ± 0.4 vs. 1.4 ± 0.2 pg, respectively). However, in nucleus accumbens or striatum of rats previously exposed to cold, the stress-induced increase in DA efflux was not significantly different from that of naive rats (nucleus accumbens, 1.8 ± 0.7 vs. 1.5 ± 0.3 pg, respectively; striatum, 1.9 ± 0.4 vs. 2.6 ± 0.7 pg, respectively). Thus, both cortical NE projections and cortically projecting DA neurons sensitize after chronic exposure to cold. In contrast, subcortical DA projections do not sensitize under these conditions.     

In Vivo Assessment of Dopamine Uptake in Rat Medial Prefrontal Cortex: Comparison with Dorsal Striatum and Nucleus Accumbens

Abstract: In vivo electrochemistry was used to characterize dopamine clearance in the medial prefrontal cortex and to compare it with clearance in the dorsal striatum and nucleus accumbens. When calibrated amounts of dopamine were pressure-ejected into the cortex from micropipettes adjacent to the recording electrodes, transient and reproducible dopamine signals were detected. The local application of the selective uptake inhibitors GBR-12909, desipramine, and fluoxetine before the application of dopamine indicated that at the lower recording depths examined (2.5–5.0 mm below the brain surface), locally applied dopamine was cleared from the extracellular space primarily by the dopamine transporter. The norepinephrine transporter played a greater role at the more superficial recording sites (0.5–2.25 mm below the brain surface). To compare clearance of dopamine in the medial prefrontal cortex (deeper sites only), striatum, and nucleus accumbens, varying amounts of dopamine were locally applied in all three regions of individual animals. The signals recorded from the cortex were of greater amplitude and longer time course than those recorded from the striatum or accumbens (per picomole of dopamine applied), indicating less efficient dopamine uptake in the medial prefrontal cortex. The fewer number of transporters in the medial prefrontal cortex may be responsible, in part, for this difference, although other factors may also be involved. These results are consistent with the hypothesis that regulation of dopaminergic function is unique in the medial prefrontal cortex.     

Differential Changes of Extracellular Aspartate and Glutamate in the Striatum of Domestic Chicken Evoked by High Potassium or Distress: An In Vivo Microdialysis Study

It has long been proposed that L: -aspartate (Asp) is an excitatory neurotransmitter similar to L: -glutamate (Glu) but with distinct signaling properties. The presence of Asp in excitatory synapses of the medial striatum/nucleus accumbens of domestic chicks suggests that Asp plays a role of neurotransmitter also in the avian brain. Neurotransmitters are released from the presynaptic bouton mostly by Ca(2+) dependent exocytosis. We used in vivo microdialysis to monitor the simultaneous changes of the extracellular levels of Asp and Glu in the medial striatum of young post-hatch domestic chicks. Microdialysis samples were collected from freely moving birds at 5 min intervals and analysed off-line using capillary electrophoresis. Event-related elevations of extracellular Glu and Asp concentrations in response to handling stress and to high KCl (50 mM) were observed. Increase of Glu and Asp on handling stress was 200 and 250 %, whereas on KCl stimulation the values were 300 and 1,000 %, respectively, if stress was applied before high KCl, and 150 and 200 %, respectively, in the absence of stress. In most cases, the amino acids showed correlated changes, Asp concentrations being consistently smaller at resting but exceeding Glu during stimulation. Using Ca(2+) free medium, the KCl triggered elevation of Glu was reduced. When KCl stimulation was combined with tetrodotoxin infusion, there was no significant elevation in Asp or in Glu suggesting that most of the extracellular excitatory amino acids were released by synaptic mechanisms. The results support the suggestion that Asp is co-released with Glu and may play a signaling role (as distinct from that of glutamate) in the striatum of birds.     

Repeated exposure to cocaine alters medial prefrontal cortex dopamine D₂-like receptor modulation of glutamate and dopamine neurotransmission within the mesocorticolimbic system

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Previous studies have demonstrated that sensitization to cocaine is associated with a decrease in dopamine D? receptor function in the medial prefrontal cortex. The present report tested the hypothesis that reduced medial prefrontal cortex D? receptor function as a result of repeated cocaine exposure results in augmented excitatory transmission to the nucleus accumbens and ventral tegmental area, possibly as a partial result of enhanced inhibition of local dopamine release. Dual probe microdialysis experiments were conducted in male Sprague-Dawley rats 1, 7 or 30 days following the last of four daily injections of saline (1.0 mL/kg) or cocaine (15 mg/kg). Infusion of quinpirole (0.01, 1.0 and 100 μM), a D?-like receptor agonist, into the medial prefrontal cortex produced a dose-dependent decrease in cortical, nucleus accumbens and ventral tegmental area extracellular glutamate levels in control but not sensitized animals. Quinpirole also reduced basal dopamine levels in the medial prefrontal cortex in sensitized animals following 1 day of withdrawal from cocaine. Following 30 days of withdrawal, quinpirole also reduced dopamine levels in sensitized animals relative to saline controls, but not relative to baseline levels. These findings indicate that the expression of sensitization to cocaine is associated with altered modulation of mesocorticolimbic glutamatergic transmission at the level of the medial prefrontal cortex.