Flow regime in a restored wetland determines trophic links and species composition in the aquatic macroinvertebrate community

In a restored wetland (South of Spain), where different flow regimes control water exchange with the adjacent Guadalquivir estuary, the native Palaemon varians coexists with an exotic counterpart species Palaemon macrodactylus. This controlled m\acrocosm offers an excellent opportunity to investigate how the effects of water management, through different flow regimes, and the presence of a non-native species affect the aquatic community and the trophic niche (by gut contents and C-N isotopic composition) of the native shrimp Palaemon varians. We found that increased water exchange rate (5% day(-1) in mixed ponds vs. 0.1% day(-1) in extensive ponds) modified the aquatic community of this wetland; while extensive ponds are dominated by isopods and amphipods with low presence of P. macrodactylus, mixed ponds presented high biomass of mysids, corixids, copepods and both shrimp species. An estuarine origin of nutrients and primary production might explain seasonal and spatial differences found among ponds of this wetland. A combined analysis of gut contents and isotopic composition of the native and the exotic species showed that: (1) native P. varians is mainly omnivorous (2) while the non-native P. macrodactylus is more zooplanktivorous and (3) a dietary overlap occurred when both species coexist at mixed ponds where a higher water exchange and high abundance of mysids and copepods diversifies the native species' diet. Thus differences in the trophic ecology of both species are clearly explained by water management. This experimental study is a valuable tool for integrated management between river basin and wetlands since it allows quantification of wetland community changes in response to the flow regime.

Microdeletion 15q13.3: a locus with incomplete penetrance for autism, mental retardation, and psychiatric disorders

BACKGROUND

Microdeletions within chromosome 15q13.3 are associated both with a recently recognised syndrome of mental retardation, seizures, and dysmorphic features, and with schizophrenia.

METHODS AND RESULTS

Based on routine diagnostic testing of approximately 8200 samples using array comparative genomic hybridisation, we identified 20 individuals (14 children and six parents in 12 families) with microdeletions of 15q13.3. Phenotypes in the children included developmental delay, mental retardation, or borderline IQ in most and autistic spectrum disorder (6/14), speech delay, aggressiveness, attention deficit hyperactivity disorder, and other behavioural problems. Both parents were available in seven families, and the deletion was de novo in one, inherited from an apparently normal parent in four, and inherited from a parent with learning disability and bipolar disorder in two families. Of the 14 children, six in five families were adopted, and DNA was available for only one of these 10 biological parents; the deletion was very likely inherited for one of these families with two affected children. Among the unavailable parents, two mothers were described as having mental retardation, another mother as having "mental illness", and one father as having schizophrenia. We hypothesise that some of the unavailable parents have the deletion.

CONCLUSIONS

The occurrence of increased adoption, frequent autism, bipolar disorder, and lack of penetrance are noteworthy findings in individuals with deletion 15q13.3. A high rate of adoption may be related to the presence of the deletion in biological parents. Unconfirmed histories of antisocial behaviours in unavailable biological parents raise the concern that future research may show that deletion 15q13.3 is associated with such behaviours.

Effect of clozapine, haloperidol, or M100907 on phencyclidine-activated glutamate efflux in the prefrontal cortex

BACKGROUND

The increase in glutamate efflux in the prefrontal cortex by the psychotomimetic drugs phencyclidine (PCP) and ketamine may produce the dopaminergic and some of the behavioral effects of these drugs. Here, we examined whether antipsychotic drugs influence this increase.

METHODS

The effect of haloperidol, clozapine or the 5-HT(2A) antagonist, M100907, on PCP-induced increase in cortical glutamate efflux was examined by microdialysis. Because previous studies had suggested that M100907 attenuates some behavioral effects of PCP, we also examined the effect of M100907 on PCP-induced cortical and accumbal dopamine activation while making concomitant measures of locomotion and stereotypy.

RESULTS

Haloperidol, clozapine or M100907 did not significantly block hyperglutamatergic effects of PCP. M100907 was ineffective in inhibiting the dopaminergic and motoric effects of PCP.

CONCLUSIONS

These results contrast previous findings with glutamatergic drugs, such as AMPA antagonists or group II metabotropic glutamate agonists, that blocked glutamatergic and motoric effects of PCP. Thus, the PCP glutamate activation model lacks predictive validity for conventional antipsychotics; however, this model may be useful for design of novel classes of drugs that target those symptoms of schizophrenia that are not generally treated with monoamine-based antipsychotics.

Stimulation of prefrontal cortex at physiologically relevant frequencies inhibits dopamine release in the nucleus accumbens

The prefrontal cortex (PFC) is thought to provide an excitatory influence on the output of mesoaccumbens dopamine neurons. The evidence for this influence primarily arises from findings in the rat that chemical or high-intensity and high-frequency (60-200 Hz) electrical stimulations of PFC increase burst activity of midbrain dopamine neurons, and augment terminal release of dopamine in the nucleus accumbens. However, PFC neurons in animals that are engaged in PFC-dependent cognitive tasks increase their firing frequency from a baseline of 1-3 Hz to 7-10 Hz, suggesting that the commonly used high-frequency stimulation parameters of the PFC may not be relevant to the behavioral states that are associated with PFC activation. We investigated the influence of PFC activation at lower physiologically relevant frequencies on the release of dopamine in the nucleus accumbens. Using rapid (5-min) microdialysis measures of extracellular dopamine in the nucleus accumbens, we found that although PFC stimulation at 60 Hz produces the expected increases in accumbal dopamine release, the same amplitude of PFC stimulation at 10 Hz significantly decreased these levels. These results indicate that activation of PFC, at frequencies that are associated with increased cognitive demand on this region, inhibits the mesoaccumbens dopamine system.

Amygdala regulation of nucleus accumbens dopamine output is governed by the prefrontal cortex

A dynamic interaction between the prefrontal cortex (PFC), amygdala, and nucleus accumbens (NAc) may be fundamental to regulation of goal-directed behavior by affective and cognitive processes. This study demonstrates that a mechanism for this triadic relationship is an inhibitory control by prefrontal cortex on accumbal dopamine release during amygdala activation. In freely moving rats, microstimulation of basolateral amygdala at intensities that produced mild behavioral activation produced an expected rapid increase in glutamate efflux in the prefrontal cortex and the nucleus accumbens shell region of the ventral striatum. However, during the stimulation, dopamine release increased only in the prefrontal cortex, not in the nucleus accumbens. An increase in accumbal dopamine release was observed during the stimulation if glutamate activation in the prefrontal cortex was inhibited at either presynaptic or postsynaptic levels. Some behaviors expressed during the stimulation were intensified in animals in which prefrontal cortex glutamate activation was blocked. In addition, these animals continued to express stimulus-induced behaviors after the termination of stimulation, whereas normal poststimulus behaviors such as ambulation and grooming were not displayed as frequently. Considering that dopamine neurotransmission in the nucleus accumbens is thought to play an integral role in goal-directed motor behavior, these findings suggest that the prefrontal cortex influences the behavioral impact of amygdala activation via a concomitant active suppression of accumbal dopamine release. Absence of this cortical influence appears to result in an aberrant pattern of behavioral expression in response to amygdala activation, including behavioral perseveration after stimulus termination.

Distinct contributions of glutamate and dopamine receptors to temporal aspects of rodent working memory using a clinically relevant task

RATIONALE

Understanding the mechanistic basis of working memory, the capacity to hold representation "on line," is important for delineating the processes involved in higher cognitive functions and the pathophysiology of thought disorders.

OBJECTIVES

We compared the contribution of glutamate and dopamine receptor subtypes to temporal aspects of working memory using a modified rodent spatial working memory task that incorporates important elements of clinical working memory tasks.

METHODS

A discrete paired-trial variable-delay T-maze task was used. Initial characterization studies indicated that performance on this task is stable at seconds-long retention intervals, is sensitive to retention interval and proactive interference, and is dependent on the integrity of the medial prefrontal cortex.

RESULTS

Consistent with clinical findings, low dose amphetamine (0.25 mg/kg) produced a delay-dependent improvement in performance, while higher doses impaired performance at all retention intervals. D1 receptor blockade produced the predicted dose- and delay-dependent impairment. D2 receptor blockade had no effect. Activation of metabotropic glutamate 2/3 (mGluR2/3) receptors, which in the prefrontal cortex inhibits the slow asynchronous phase of glutamate release, also produced a delay-dependent impairment. Low doses of an AMPA/kainate antagonist had effects similar to the mGluR2/3 agonist. In contrast, NMDA receptor antagonist-induced impairment was memory load-insensitive, resulting in chance-level performance at all retention intervals.

CONCLUSIONS

These findings suggest that activation of NMDA receptors is necessary for the formation of mnemonic encoding while modulatory components involving slow asynchronous release of glutamate and phasic release of dopamine contribute to the active maintenance of information during the delay period.

Target-specific glutamatergic regulation of dopamine neurons in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area (VTA) are thought to play a critical role in affective, motivational, and cognitive functioning. There are fundamental target-specific differences in the functional characteristics of subsets of these neurons. For example, DA afferents to the prefrontal cortex (PFC) have a higher firing and transmitter turnover rate and are more responsive to some pharmacological and environmental stimuli than DA projections to the nucleus accumbens (NAc). These functional differences may be attributed in part to differences in tonic regulation by glutamate. The present study provides evidence for this mechanism: In freely moving animals, blockade of basal glutamatergic activity in the VTA by the selective alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate antagonist LY293558 produced an increase in DA release in the NAc while significantly decreasing DA release in the PFC. These data support an AMPA receptor-mediated tonic inhibitory regulation of mesoaccumbens neurons and a tonic excitatory regulation of mesoprefrontal DA neurons. This differential regulation may result in target-specific effects on the basal output of DA neurons and on the regulatory influence of voltage-gated NMDA receptors in response to phasic activation by behaviorally relevant stimuli.

Tactile stimulation activates dopamine release in the lateral septum

Little is known about the functional properties of the dopamine innervation of the lateral septum. In this study, the feasibility of using microdialysis to assess action-potential mediated release of dopamine in the lateral septum was established. A mild stressor, in the form of handling, significantly increased septal dopamine levels, implicating a role for dopamine in sensory-related processing associated with the septal complex.

Glutamate and post-traumatic stress disorder: toward a psychobiology of dissociation

Dissociative cognitive and perceptual alterations commonly occur at the time of traumatization and as an enduring feature of post-traumatic stress disorder (PTSD). After stress exposure, dissociative symptoms are a predictor of the development of PTSD. Recent preclinical data suggest that stress stimulates the cortico-limbic release of glutamate. The glutamate that is released during stress in animal models influences behavior, induces a variety of changes in neural plasticity that may have long-lasting effects on brain function and behavior, and contributes to neural toxicity. Antagonist of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor also stimulates transient cortico-limbic glutamate release in animals. Further, some of the effects of NMDA antagonists in animals are blocked by drugs that attenuate glutamate release. Clinical studies suggest that NMDA antagonists may transiently stimulate glutamate release and produce symptoms resembling dissociative states in humans. A recent study suggests that a drug that reduces glutamate release also attenuates the perceptual effects of the NMDA antagonist, ketamine, in humans. Because of the possible contributions of hyperglutamatergic states to the acute and long-lasting consequences of traumatic stress exposure, the therapeutic and neuroprotective potential of drugs that attenuate glutamate release should be explored in traumatized individuals with dissociative symptoms.

Clozapine preferentially increases dopamine release in the rhesus monkey prefrontal cortex compared with the caudate nucleus

Despite substantial differences between species in the organization and elaboration of the cortical dopamine innervation, little is known about the pharmacological response of cortical or striatal sites to antipsychotic medications in nonhuman primates. To examine this issue, rhesus monkeys were chronically implanted with guide cannulae directed at the principal sulcus, medial prefrontal cortex, premotor cortex, and caudate nucleus. Alterations in dopamine release in these discrete brain regions were measured in response to administration of clozapine or haloperidol. Clozapine produced significant and long-lasting increases in dopamine release in the principal sulcus, and to a lesser extent, in the caudate nucleus. Haloperidol did not produce a consistent effect on dopamine release in the principal sulcus, although it increased dopamine release in the caudate. Clozapine's preferential augmentation of dopamine release in the dorsolateral prefrontal cortex supports the idea that clozapine exerts its therapeutic effects in part by increasing cortical dopamine neurotransmission.

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.

Glutamatergic regulation of basal and stimulus-activated dopamine release in the prefrontal cortex

The present study was undertaken to determine whether basal and stimulus-activated dopamine release in the prefrontal cortex (PFC) is regulated by glutamatergic afferents to the PFC or the ventral tegmental area (VTA), the primary source of dopamine neurons that innervate the rodent PFC. In awake rats, blockade of NMDA or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors in the VTA, or blockade of AMPA receptors in the PFC, profoundly reduced dopamine release in the PFC, suggesting that the basal output of dopamine neurons projecting to the PFC is under a tonic excitatory control of NMDA and AMPA receptors in the VTA, and AMPA receptors in the PFC. Consistent with previous reports, blockade of cortical NMDA receptors increased dopamine release, suggesting that NMDA receptors in the PFC exert a tonic inhibitory control on dopamine release. Blockade of NMDA or AMPA receptors in the VTA as well as blockade of AMPA receptors in the PFC reduced the dopaminergic response to mild handling, suggesting that activation of glutamate neurotransmission also regulates stimulus-induced increase of dopamine release in the PFC. In the context of brain disorders that may involve cortical dopamine dysfunction, the present findings suggest that abnormal basal or stimulus-activated dopamine neurotransmission in the PFC may be secondary to glutamatergic dysregulation.

Reversal of phencyclidine effects by a group II metabotropic glutamate receptor agonist in rats

Glutamatergic abnormalities have been associated with several psychiatric disorders, including schizophrenia and addiction. Group II metabotropic glutamate receptors were targeted to normalize glutamatergic disruptions associated with an animal model of schizophrenia, the phencyclidine model. An agonist of this group of receptors, at a dose that was without effects on spontaneous activity and corticolimbic dopamine neurotransmission, attenuated the disruptive effects of phencyclidine on working memory, stereotypy, locomotion, and cortical glutamate efflux. This behavioral reversal occurred in spite of sustained dopamine hyperactivity. Thus, targeting this group of receptors may present a nondopaminergic therapeutic strategy for treatment of psychiatric disorders.

Corticolimbic dopamine neurotransmission is temporally dissociated from the cognitive and locomotor effects of phencyclidine

The behavioral syndrome produced by phencyclidine (PCP) and its analog ketamine represents a pharmacological model for some aspects of schizophrenia. Despite the multifaceted properties of these drugs, the main mechanism for their psychotomimetic and cognitive-impairing effects has been thought heretofore to involve the corticolimbic dopamine system. The present study examined the temporal relationship between alterations in corticolimbic dopamine and glutamate neurotransmission and two dopamine-dependent behavioral effects of PCP in the rodent that have relevance to the clinical phenomenology, namely, impairment of working memory, which is used to model the frontal lobe deficits associated with schizophrenia, and hyperlocomotion, which is used as a predictor of the propensity of a drug to elicit or exacerbate psychosis. PCP increased dopamine and glutamate efflux in the prefrontal cortex and nucleus accumbens, as measured by microdialysis. The increase in dopamine in both regions remained elevated well above baseline 2.5 hr after the injection, at which time the experiment was terminated. However, locomotor activity returned to baseline in <2 hr after injection. Furthermore, impaired performance in a discrete trial delayed alternation task, a rodent working memory task, was only evident up to 60 min after PCP injection; animals tested 80 min after injection, when cortical dopamine release was elevated at 300% of baseline, did not exhibit impaired performance. These findings indicate that activation of dopamine neurotransmission is not sufficient to sustain PCP-induced locomotion and impairment of working memory. Thus, effects of PCP, including a glutamatergic hyperstimulation, may be necessary to account for the psychotomimetic and cognitive-impairing effects of this drug.

The NMDA antagonist model for schizophrenia: promise and pitfalls

Drug models have been extensively used to study the pathophysiology of schizophrenia. While they provide good insight into the neurobiology of this disorder, they have several shortcomings, which if known, help in the interpretation of results. In this paper we will discuss these shortcomings in general, and in relation to the N-methyl D-aspartate antagonist model for schizophrenia. This model has recently received a great deal of attention since both phencyclidine and the structurally related drug ketamine, produce symptoms that extend beyond psychosis per se to include other symptoms associated with schizophrenia. In fact, subanesthetic doses of ketamine in healthy individuals produce not only paranoia and perceptual alterations but also thought disorder, negative symptoms, cognitive deficits, as well as impairment on a number of electrophysiologic tests known to be abnormal in schizophrenia. These effects of ketamine will be discussed with a particular emphasis on implications for the pathophysiology and therapeutics of this disorder.

Regulation of striatal dopamine release by metabotropic glutamate receptors

In vivo microdialysis in conscious rats was used to assess the effect of metabotropic glutamate receptor stimulation on striatal dopamine release. Local application of the metabotropic glutamate agonist (+/-)-trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), via a microdialysis probe, produced a concentration-dependent response: infusion of 50 microM ACPD did not produce a significant effect on extracellular dopamine levels, while application of 100 microM or 500 microM ACPD increased dopamine release by approximately 50% or 100%, respectively. To examine the contribution of impulse flow and multisynaptic mechanisms to the ACPD-induced increase in dopamine release, 500 microM ACPD were coapplied with 2 microM tetrodotoxin (TTX). An increase in extracellular dopamine levels was observed after the application of 500 microM ACPD, despite the presence of TTX. To further study the actions of metabotropic glutamate receptor-stimulation on terminal release characteristics of dopamine, the effect of ACPD on 40 mM K+-stimulated dopamine release was investigated. It was found that application ofACPD reduces dopamine release in response to K+ stimulation. These data suggest that during basal conditions, metabotropic glutamate receptor activation facilitates striatal dopamine release, possibly through presynaptic, impulse-independent mechanisms. However, during conditions of hyperstimulation, activation of metabotropic receptors, in contrast to ionotropic receptors, reduces excess dopamine release.

Activation of glutamatergic neurotransmission by ketamine: a novel step in the pathway from NMDA receptor blockade to dopaminergic and cognitive disruptions associated with the prefrontal cortex

Subanesthetic doses of ketamine, a noncompetitive NMDA receptor antagonist, impair prefrontal cortex (PFC) function in the rat and produce symptoms in humans similar to those observed in schizophrenia and dissociative states, including impaired performance of frontal lobe-sensitive tests. Several lines of evidence suggest that ketamine may impair PFC function in part by interacting with dopamine neurotransmission in this region. This study sought to determine the mechanism by which ketamine may disrupt dopaminergic neurotransmission in, and cognitive functions associated with, the PFC. A thorough dose-response study using microdialysis in conscious rats indicated that low doses of ketamine (10, 20, and 30 mg/kg) increase glutamate outflow in the PFC, suggesting that at these doses ketamine may increase glutamatergic neurotransmission in the PFC at non-NMDA glutamate receptors. An anesthetic dose of ketamine (200 mg/kg) decreased, and an intermediate dose of 50 mg/kg did not affect, glutamate levels. Ketamine, at 30 mg/kg, also increased the release of dopamine in the PFC. This increase was blocked by intra-PFC application of the AMPA/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione CNQX. Furthermore, ketamine-induced activation of dopamine release and impairment of spatial delayed alternation in the rodent, a PFC-sensitive cognitive task, was ameliorated by systemic pretreatment with AMPA/kainate receptor antagonist LY293558. These findings suggest that ketamine may disrupt dopaminergic neurotransmission in the PFC as well as cognitive functions associated with this region, in part, by increasing the release of glutamate, thereby stimulating postsynaptic non-NMDA glutamate receptors.

Temporal dynamics of glutamate efflux in the prefrontal cortex and in the hippocampus following repeated stress: effects of pretreatment with saline or diazepam

Acute stress has been associated with activation of glutamate efflux in forebrain structures. The present study sought to characterize the extracellular dynamics of glutamate in response to acute and repeated stress in the prefrontal cortex and hippocampus in rats. One-minute sampling of extracellular glutamate levels was performed during repeated tail-pinch stress. Animals were stressed three times, beginning at approximately 10.30 a.m. and continuing at 2.5-h intervals. In the prefrontal cortex, the initial 10-min tail pinch produced a robust increase in extracellular levels of glutamate. This increase was apparent immediately (i.e. 1 min) after the start of the stress procedure. The second tail pinch produced a smaller increase in glutamate levels while the third tail pinch did not significantly increase these levels. In the hippocampus, the initial stress response was smaller in magnitude than that observed in the prefrontal cortex. Furthermore, responses to subsequent tail pinches were similar to that seen following the first tail pinch. Treatment with diazepam (3 mg/kg/i.p.) 30 min before the first stress session abolished the stress response in the prefrontal cortex and hippocampus. However, in the prefrontal cortex, the second tail pinch (performed approximately 3 h after diazepam administration) produced a robust increase in glutamate efflux. In contrast, in the hippocampus of diazepam-treated rats, the second tail pinch produced a small delayed response. Pretreatment with saline resulted in non-significant responses to all three tail pinches in the prefrontal cortex. The present study suggests that: (i) stress produces a rapid increase in glutamate efflux in the prefrontal cortex and hippocampus, (ii) repeated stress reveals tolerance in the glutamatergic response in the prefrontal cortex, (iii) saline and diazepam pretreatment reduce the stress-induced efflux of glutamate in the prefrontal cortex, and (iv) exposure to diazepam may prevent the prefrontal cortex from adapting its response to the subsequent stressor. These finding are consistent with the role of the prefrontal cortex as a region which may regulate reactions to aversive stimuli.

Excitatory amino acid receptors in the ventral tegmental area regulate dopamine release in the ventral striatum

The role of excitatory amino acid (EAA) receptors located in the ventral tegmental area (VTA) in tonic and phasic regulation of dopamine release in the ventral striatum was investigated. Microdialysis in conscious rats was used to assess dopamine release primarily from the nucleus accumbens shell region of the ventral striatum while applying EAA antagonists or agonists to the VTA. Infusion of the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (25 and 100 microM) into the VTA did not affect dopamine release in the ventral striatum. In contrast, intra-VTA infusion of the NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (100 and 500 microM) dose-dependently decreased the striatal release of dopamine. Intra-VTA application of the ionotropic EAA receptor agonists NMDA and AMPA dose-dependently (10 and 100 microM) increased dopamine efflux in the ventral striatum. However, infusion of 50 or 500 microM trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), a metabotropic EAA receptor agonist, did not significantly affect these levels. These data suggest that NMDA receptors in the VTA exert a tonic excitatory influence on dopamine release in the ventral striatum. Furthermore, dopamine neurotransmission in this region may be enhanced by activation of NMDA and AMPA receptors, but not ACPD-sensitive metabotropic receptors, located in the VTA. These data further suggest that EAA regulation of dopamine release primarily occurs in the VTA as opposed to presynaptically at the terminal level.

Effect of a pharmacological stressor on glutamate efflux in the prefrontal cortex

The anxiogenic beta-carboline, FG 7142 (20 mg/kg) significantly increased glutamate efflux in the prefrontal cortex of conscious rats as assessed by microdialysis. Pretreatment with the benzodiazepine receptor agonist, diazepam (5 mg/kg), abolished this effect. These findings indicate that anxiogenic compounds produce an effect similar to physical stressors on the outflow of glutamate, and implicate the GABA/benzodiazepine receptor complex in the stress-induced activation of glutamate systems in the prefrontal cortex.

The prefrontal cortex regulates the basal release of dopamine in the limbic striatum: an effect mediated by ventral tegmental area

The present study examined whether the prefrontal cortex (PFC) exerts a tonic control over the basal release of dopamine in the limbic striatum and whether this control is mediated by glutamatergic afferents to the dopamine cell body or terminal regions. Using intracerebral microdialysis in freely moving rats, it was demonstrated that application of tetrodotoxin in the contralateral PFC significantly decreased the release of dopamine in the medial striatum. Conversely, blockade of the tonic inhibitory GABAergic input in the PFC with bicuculline increased the release of dopamine in the medial striatum. Application of excitatory amino acid receptor antagonists into the striatum, while bicuculline was perfused in the PFC, did not affect the bicuculline-evoked dopamine increase in the striatum. However, infusion of tetrodotoxin or excitatory amino acid receptor antagonists into the ventral tegmental area, a region containing dopamine cell bodies that project to the medial striatum, blocked the stimulation of striatal dopamine release induced by infusion of bicuculline into the PFC. These data demonstrate that the basal output of dopamine terminals in the medial striatum is under a tonic excitatory control of the PFC. Furthermore, this control occurs primarily through glutamatergic projections to the dopamine cell body area rather than the terminal regions.

NMDA receptor antagonists impair prefrontal cortex function as assessed via spatial delayed alternation performance in rats: modulation by dopamine

The present study was performed to assess the role of excitatory amino acid and dopamine receptors on associative functions of the prefrontal cortex (PFC) of the rat. Spatial delayed alternation was used as a PFC-sensitive cognitive task. In addition, in vivo microdialysis was used to assess the release of dopamine in the PFC. The noncompetitive NMDA antagonists ketamine (10-30 mg/kg) and MK-801 (0.1 and 0.5 mg/kg) dose-dependently impaired the spatial delayed alternation performance compared with the saline-treated control group. Administration of the dopamine antagonists raclopride (0.1 and 0.5 mg/kg), SCH-23390 (0.1 mg/kg), or haloperidol (0.1 mg/kg) was without a significant effect. However, haloperidol and raclopride (but not SCH-23390) reversed the disruptive effect of 30 mg/kg ketamine on spatial delayed alternation performance. Microdialysis studies revealed that this dose of ketamine preferentially increased the release of dopamine in the PFC compared with the striatum. These findings indicate that attenuation of glutamatergic neurotransmission at the NMDA receptor impairs PFC-dependent cognitive functions. Furthermore, activation of dopamine neurotransmission contributes, at least in part, to this impairment.

Regulation of glutamate efflux by excitatory amino acid receptors: evidence for tonic inhibitory and phasic excitatory regulation

Several biochemical and electrophysiological studies have proposed the presence of presynaptic receptors that potentiate the release of excitatory amino acids (EAA). However, these studies have utilized exogenous EAA agonists and thus have assessed the autoregulation of EAA release during conditions of receptor hyperstimulation, and not during base line conditions. The aim of the present study was to address the question of whether there is a tonic autoregulation of base line EAA release. It was demonstrated that in the hippocampus and the striatum of freely moving rats, basal outflow of glutamate (Glu) and aspartate (Asp) are increased, in a dose-dependent manner, by local application of the antagonists of N-methyl-D-aspartate (NMDA) or non-NMDA receptors, suggesting that there is an ongoing tonic inhibition of aspartate and Glu outflow by different subtypes of EAA receptors. Subsequently, to investigate the effect EAA receptor-hyperstimulation on Glu outflow, a comprehensive study of the effect of various doses of ionotropic and metabotropic EAA agonists on the extracellular levels of Glu was performed. At high concentrations, agonists of all known subtypes of EAA receptors induced (large) increases in extracellular levels of Glu and in most cases caused behavioral stimulation and/or convulsion. This suggests that during conditions of high agonist availability, such as the massive Glu release thought to occur during pathological conditions, a positive feedback presynaptic mechanism may overcome the autoregulatory mechanism operating during base line conditions.

Glutamatergic antagonists attenuate ability of dopamine uptake blockers to increase extracellular levels of dopamine: implications for tonic influence of glutamate on dopamine release

Previous in vivo studies reporting a dose-dependent increase in extracellular dopamine (DA) levels by excitatory amino acid (EAA) antagonists have been interpreted to indicate a lack of tonic excitatory effect exerted by these amino acids on striatal DA release. Alternatively, a tonic excitatory influence on DA release may affect a small fraction of DA terminals, so that blockade of this effect does not make a great enough contribution to the extracellular fluid to be detected by microdialysis. To examine this possibility, the effect of EAA antagonists was assessed by microdialysis in the presence of DA uptake blockers. It was found that in the presence of nomifensine or cocaine, antagonists of either NMDA or AMPA/kainate receptors decreased extracellular DA levels in the striatum. These data suggest that EAAs may exert a tonic facilitatory influence on striatal DA release and/or that endogenous EAAs may potentiate the action of DA uptake blockers through mechanisms that are mediated by EAA receptors.

Recent basic findings in support of excitatory amino acid hypotheses of schizophrenia

1. Several clinical and post-mortem tissue findings have suggested a role for excitatory amino acid neuronal systems in the pathophysiology of schizophrenia. 2. These include the ability of NMDA antagonists, phencyclidine and ketamine, to cause both negative and positive symptoms in healthy subjects, and abnormalities in the densities of some types of excitatory amino acid receptors in the postmortem tissue of schizophrenic brains. 3. The present review describes recent basic findings that have examined the involvement of excitatory amino acids in the mechanism of action of antipsychotic drugs. These include studies on the functional links between glutamatergic and dopaminergic systems, effect of acute and chronic antipsychotic drug treatment on excitatory amino acid function, and stress-induced activation of excitatory amino acid release, in particular in the prefrontal cortex.

Preferential activation of cortical dopamine neurotransmission by clozapine: functional significance

Dopamine projections to the prefrontal cortex are thought to be essential for the proper functioning of this region and are proposed to be involved in negative (deficit) symptomatology of schizophrenia. Our studies in the rodent indicate that clozapine, the most effective antipsychotic drug for the treatment of negative symptoms, causes an increase in the basal output of dopamine neurons projecting to the prefrontal cortex. This finding is in contrast to the effect of clozapine in the basal ganglia and the effect of typical antipsychotic drugs such as haloperidol in the prefrontal cortex. The ability of clozapine to increase dopamine release in the prefrontal cortex and its relatively weak affinity for some types of dopamine receptors suggest that this drug may exert its therapeutic influence in part by increasing dopaminergic function in the prefrontal cortex.

Biphasic effect of ethanol on extracellular accumulation of glutamate in the hippocampus and the nucleus accumbens

The effect of systemic ethanol administration on the extracellular levels of glutamate in the hippocampus and nucleus accumbens of conscious rats was assessed using microdialysis. At 0.5 g/kg, ethanol caused a sustained increase in the levels of glutamate in both regions. Following 1.0 g/kg, response was observed in the nucleus accumbens while a trend towards a decrease in glutamate levels occurred in hippocampus. Injection of 2.0 g/kg ethanol decreased these levels in both regions. These findings suggest a relationship between the previously reported biphasic behavioral consequence of ethanol and its effect on glutamatergic neurotransmission.

Glucocorticoids mediate the stress-induced extracellular accumulation of glutamate

The hippocampal damage caused by stress has been attributed to an increased glutamatergic tone brought about by secretion of glucocorticoids. Although exposure to stress has been shown to increase the outflow of glutamate, direct involvement of glucocorticoid in this phenomenon has not been examined. The present study demonstrates that adrenalectomy attenuates the stress-induced outflow of glutamate in the hippocampus and prefrontal cortex and that glucocorticoid replacement abolishes this attenuation.

Glutamatergic control of dopamine release during stress in the rat prefrontal cortex

In vivo microdialysis was used to assess the hypothesis that the stress-induced increase in dopamine release in the prefrontal cortex is mediated by stress-activated glutamate neurotransmission in this region. Local perfusion of an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, blocked the stress-induced increase in dopamine levels, whereas an NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid, at the dose tested, was not able to alter this response significantly. These data indicate that the effect of stress on dopamine release in the prefrontal cortex is mediated locally by activation of AMPA/kainate receptors, which modulate the release of dopamine in this region.

Opiate withdrawal increases glutamate and aspartate efflux in the locus coeruleus: an in vivo microdialysis study

Electrophysiological studies suggest that an increase in excitatory amino acid release may occur in the locus coeruleus during opiate withdrawal. The present study examined directly by microdialysis in anesthetized rats the effect of naltrexone-precipitated opiate withdrawal on the efflux of excitatory amino acids in the locus coeruleus. A withdrawal-induced increase in glutamate and aspartate efflux was found when the microdialysis probe was located in the core of the locus coeruleus; no increase was seen in adjacent regions.

Preferential activation of dopamine overflow in prefrontal cortex produced by chronic clozapine treatment

The effect of chronic treatment with clozapine on extracellular dopamine levels in the rat striatum, nucleus accumbens and medial prefrontal cortex (mPFC) was examined using intracerebral microdialysis. Clozapine (20 mg/kg/day x 21 days in drinking water) increased basal dopamine release in the mPFC but had no effect in the striatum or nucleus accumbens. After chronic treatment, an acute dose of clozapine (20 mg/kg i.p.) produced large and long-lasting increases in extracellular dopamine in all three brain regions. The data suggest that chronic clozapine produces a sustained enhancement in dopaminergic tone in the mPFC.

Depolarization inactivation of dopamine neurons: terminal release characteristics

The functional consequences of chronic treatment with haloperidol (0.5 mg/kg s.c. for 21-23 days) on striatal extracellular levels of dopamine and excitatory amino acids, aspartate and glutamate, were examined using microdialysis techniques. Our studies indicate that, in both awake and anesthetized animals, chronic haloperidol treatment does not appear to change basal outflow of dopamine and its response to an exogenous antagonist (i.e., a challenge dose of haloperidol). Furthermore, in chronic haloperidol and vehicle-treated animals, extracellular dopamine levels were decreased below our limit of detection following perfusion of tetrodotoxin through the probe, or into the medial forebrain bundle, suggesting that in both groups of animals extracellular dopamine levels are neuronally derived and seemed to depend equally on impulse flow. However, some differences were observed between the vehicle and haloperidol-treated animals: the excitatory action of 30 mM K+ on extracellular dopamine levels was decreased, and extracellular levels of glutamate were significantly increased, in animals treated chronically with haloperidol. The alterations in extracellular glutamate levels suggests that events at the terminal may be involved in maintaining the "normal" extracellular dopamine levels. Furthermore, the decrease in response to stimulation by K+ suggests that chronic haloperidol treatment may decrease the responsivity of the striatal dopamine system to stimuli.

Stress preferentially increases extraneuronal levels of excitatory amino acids in the prefrontal cortex: comparison to hippocampus and basal ganglia

The technique of intracerebral microdialysis was used to assess the effect of stress on the extracellular concentrations of excitatory amino acids, glutamate and aspartate, in the rat medial prefrontal cortex, hippocampus, striatum, and nucleus accumbens. A 20-min restraint procedure led to an increase in extracellular glutamate in all regions tested. The increase in glutamate levels was significantly higher in the prefrontal cortex than that observed in other regions. With the exception of the striatum, extracellular levels of aspartate were increased in all regions. Furthermore, the increase in aspartate levels was significantly higher in prefrontal cortex compared to hippocampus and nucleus accumbens. Local perfusion of tetrodotoxin during the restraint procedure significantly decreased the stress-induced increase in extracellular excitatory amino acids. In order to ensure that the above results were not an artifact of restraint not associated with stress (e.g., decreased mobility), we also examined the effect of swimming stress on the extracellular levels of excitatory amino acids in selected regions, i.e., striatum and medial prefrontal cortex. Both regions displayed a significant increase in extracellular levels of aspartate and glutamate following 20 min of swimming in room temperature water. This study provides direct evidence that stress increases the neuronal release of excitatory amino acids in a regionally selective manner. The implications of the present findings for stress-induced catecholamine release and/or hippocampal degeneration are discussed.

Actions of clozapine and haloperidol on the extracellular levels of excitatory amino acids in the prefrontal cortex and striatum of conscious rats

The technique of intracerebral microdialysis was employed to assess the effect of acute clozapine and haloperidol on the extracellular levels of aspartate and glutamate in the striatum and medial prefrontal cortex of conscious rats. Subcutaneous injection of 25 mg/kg clozapine, but not the lower dose of 15 mg/kg, led to a significant increase in both aspartate and glutamate levels over time. The maximum effect was observed two hours after injection. Haloperidol (0.5 and 1 mg/kg s.c.) did not increase the extracellular levels of aspartate and glutamate over time. At all doses tested, both drugs were without a significant effect in the striatum. These data suggest that clozapine may have selective actions on the cortical excitatory amino acid systems.

In vivo assessment of basal and drug-induced dopamine release in cortical and subcortical regions of the anesthetized primate

There is an acute interest in studying the functional characteristics of dopamine systems in the cortex of primates. In particular, the prefrontal cortical dopamine projections have received a great deal of attention. This system is essential for proper functioning of the prefrontal cortex, and dysfunction within the system may be involved in some psychiatric and neurological illnesses. In vivo assessments of cortical dopamine in the primate have been scarce. This has been due, in part, to technical difficulties associated with these studies and with quantifying the relatively low levels of dopamine found in cortical regions. In the present study, intracerebral microdialysis was utilized to assess the extracellular concentration of dopamine in cortical and subcortical areas of the pentobarbital-anesthetized rhesus monkey. Basal extracellular dopamine levels were consistently detected in the medial prefrontal cortex, premotor cortex, and caudate-putamen. The basal extracellular concentration of dopamine in the dorsolateral prefrontal cortex was reliably detected in 1 of 4 animals. Intravenous administration of amphetamine (1 mg/kg) enhanced extracellular dopamine levels in the caudate-putamen area by more than 20-fold. In cortical areas, amphetamine's effect was less profound: An increase of 400-500 percent over basal extracellular dopamine levels was observed in each region. These studies demonstrate the feasibility of microdialysis for detecting extracellular fluxes of dopamine in the cortex of nonhuman primates. They further provide direct evidence that the dopamine released within the prefrontal cortex and the premotor cortex of nonhuman primates responds to pharmacological manipulation.

Distinct actions of endogenous excitatory amino acids on the outflow of dopamine in the nucleus accumbens

Intracerebral microdialysis was utilized to assess the effect of endogenous excitatory amino acids (EAA), l-glutamate (GLU) and l-aspartate (ASP), on the extracellular levels of dopamine in the rat nucleus accumbens. Both ASP and GLU produced a release response at a concentration range of 1 to 10 mM. GLU was generally less efficacious in increasing dopamine outflow; at 5 and 10 mM, the maximum effect exerted by GLU was significantly less than that observed with ASP. The specific N-methyl-D-aspartate (NMDA) antagonist 2-amino-5-phosphonopentanoic (AP5) acid was more effective in attenuating the actions of 5 and 10 mM ASP than the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). On the other hand, the stimulatory actions of 5 and 10 mM GLU were more effectively decreased with CNQX when compared with AP5. Perfusion of tetrodotoxin before application of either GLU or ASP blocked the excitatory effect of these amino acids on dopamine overflow. These results suggest that in the nucleus accumbens, ASP and GLU may increase dopamine release through distinct mechanisms and that their stimulatory action is dependent on axonal impulse flow.

Tonic inhibition of striatal dopamine transmission: effects of benzodiazepine and GABAA receptor antagonists on extracellular dopamine levels

At present, it is unclear whether ligands which bind at the benzodiazepine/GABA receptor complex play a tonic modulatory role with regard to striatal dopamine (DA) transmission. The present study was designed to examine the effects of Ro15-1788, a benzodiazepine (BZ) receptor antagonist, and SR 95531, a GABAA receptor antagonist, on striatal extracellular DA (DA[e]) concentrations in anesthetized and awake rats using the technique of in vivo microdialysis. Local administration of Ro15-1788 resulted in a dose-dependent increase in DA[e] in both anesthetized and awake animals. The Ro15-1788-induced increase in DA[e] was blocked by coadministration of the BZ agonist diazepam, as well as GABA. Local administration of SR 95531 also resulted in a dose-dependent alteration in striatal DA levels in both anesthetized and awake animals. The SR 95531-induced increase in DA was blocked by coadministration of GABA. The results suggest that GABA may play a tonic inhibitory role with regard to striatal DA transmission.

Do endogenous excitatory amino acids influence striatal dopamine release?

In vivo microdialysis techniques were used to examine whether endogenous excitatory amino acids exert a tonic facilitatory influence on striatal dopamine release. Local application of NMDA and non-NMDA antagonists at 10 microM was without an effect on basal dopamine release while 100 microM and 1 mM of these drugs significantly enhanced the release. Our findings do not support the idea that excitatory amino acids have a tonic excitatory effect on striatal dopamine release.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Effect of L-glutamate on the release of striatal dopamine: in vivo dialysis and electrochemical studies

Microdialysis and in vivo voltammetry combined with K(+)-selective microelectrodes were utilized to study the effect of L-glutamate (GLU) on the in vivo release of dopamine (DA) from the rat striatum. Perfusion of 500 nM-5 mM GLU through the microdialysis probe was without an effect on DA outflow whereas 10 mM GLU resulted in a significant (295%) increase in the basal level of DA. This increase was blocked in the presence of 2-amino-5-phosphonopentanoic acid, an N-Methyl-D-aspartate (NMDA) receptor antagonist. Repetitive local applications of 10 mM GLU were also required to observe an increase in extracellular DA measured by in vivo voltammetry. These signals were accompanied with a massive increase in extracellular K+ and a large negative shift in the field potential resembling the ionic changes seen after the phenomenon spreading depression. These studies suggest that high concentrations of GLU are required to enhance the extracellular concentration of DA in vivo. Further, pathophysiological conditions such as spreading depression may be responsible for the observed increase in extracellular DA concentration.

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.

Utilization of microdialysis for assessing the release of mesotelencephalic dopamine following clozapine and other antipsychotic drugs

1. In vivo microdialysis was utilized to assess the effect of clozapine, haloperidol, and sulpiride on the release of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. 2. The results suggest that acute administration of various classes of antipsychotic drugs may differentially increase the extracellular concentration of dopamine in mesotelencephalic systems. Haloperidol and sulpiride were more effective in releasing dopamine from the terminals of the nigrostriatal dopamine system while clozapine had a more prefrontal effect on the mesolimbic and particularly the mesocortical dopamine systems.

Characterization of dopamine release in the rat medial prefrontal cortex as assessed by in vivo microdialysis: comparison to the striatum

Using the technique of perfusion microdialysis combined with a small-bore liquid chromatography system we have measured the basal and drug-induced fluxes of extracellular dopamine in the medial prefrontal cortex of chloral hydrate-anesthetized rats and have compared our findings in the cortex to that observed in the striatum. The results were as follows. (1) At a flow rate of 2 microliter/min, the basal level of dopamine in the medial prefrontal cortex was 0.28 +/- 0.1 (n = 32) fmol/microliter perfusate, which was nearly an order of magnitude less than that obtained from the striatum. (2) alpha-Methyl-para-tyrosine (150 mg/kg i.v.) significantly decreased the extracellular levels of striatal and cortical dopamine. The magnitude and duration of the responses were similar in both regions. (3) Local perfusion with 30 mM K+ had a more profound effect on dopamine release in the striatum than in the medial prefrontal cortex. The K(+)-induced release in both regions was significantly attenuated in the absence of Ca2+. (4) The anxiogenic beta carboline FG 7142 (15 mg/kg, i.p.) enhanced the release of cortical dopamine by about 50% while it was without an effect in the striatum. (5) Amphetamine (1 mg/kg, i.v.) significantly elevated, while reserpine (5 mg/kg, i.p.) rapidly attenuated, the dopamine level in the medial prefrontal cortex. These studies demonstrate that perfusion microdialysis, in conjunction with small-bore liquid chromatography with electrochemical detection, can be used to measure the basal release of dopamine in the rat medial prefrontal cortex and that the dopamine release process in this region, as has been shown in the striatum, is sensitive to stimulation conditions and pharmacological manipulations.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionic composition of microdialysis perfusing solution alters the pharmacological responsiveness and basal outflow of striatal dopamine

While using the technique of in vivo microdialysis, we have assessed the effect of the ionic composition of the perfusing solution on extracellular dopamine levels during resting conditions and following a pharmacological manipulation. Our results indicate that perfusion with solutions containing the ionic composition of commercially available Ringer's solution, which mimic the ionic composition of plasma as opposed to brain extracellular fluid, alters the turnover rate and basal release of dopamine. Moreover, perfusion with solutions containing higher calcium levels, i.e., 3.4 mM, than the amount we have determined to be present in the extracellular fluid of striatum (1.2 mM) alters the pharmacological responsiveness of the nigrostriatal dopamine system to synthesis inhibition.

Differential effect of cocaine on extracellular dopamine levels in rat medial prefrontal cortex and nucleus accumbens: comparison to amphetamine

The technique of in vivo microdialysis was used to measure the extracellular levels of dopamine in the nucleus accumbens septi and medial prefrontal cortex of chloral-hydrate-anaesthetized rats following systemic administration of cocaine and amphetamine. Intravenous injection of cocaine increased the extracellular levels of dopamine in the medial prefrontal cortex and the nucleus accumbens septi in a dose-dependent manner. However, the magnitude of increase was significantly greater in nucleus accumbens than in medial prefrontal cortex. In comparison to cocaine, amphetamine increased the extracellular levels of dopamine in the nucleus accumbens and medial prefrontal cortex to the same degree. Based on the relatively small increase of extracellular dopamine levels in medial prefrontal cortex by cocaine, it is postulated that dopaminergic innervation of other structures besides medial prefrontal cortex may be involved in maintenance of cocaine self-administration.

Electrocoating carbon fiber microelectrodes with Nafion improves selectivity for electroactive neurotransmitters

A method which improves carbon fiber microelectrode selectivity for cationic amines by electrocoating the fiber with a thin film of the ionic polymer, Nafion, is described. The selectivity and response speed of these electrodes for the detection of electroactive cationic and anionic species found in brain extracellular fluid was evaluated using differential pulse voltammetry and chronoamperometry and compared to uncoated fibers. Carbon fiber microelectrodes electrocoated with Nafion are highly sensitive to cationic amines such as dopamine and serotonin and have minimal sensitivity to anions such as ascorbic acid and uric acid at physiological concentrations.

Temporal relationship between neurotransmitter release and ion flux during spreading depression and anoxia

Brain ion homeostasis is severely perturbed during spreading depression of Leao and during anoxia. The ionic composition of the extracellular space changes abruptly and approaches the intracellular concentrations owing to an increase in cell permeability. In spreading depression, synchronous transmitter efflux caused by a depolarization of the presynaptic terminals has been implicated as a possible mechanism that would explain the concomitant movement of ions. Anoxia, having many features in common with spreading depression, may follow the same mechanism. We have measured the concentrations of extracellular potassium with ion-selective microelectrodes and dopamine by in vivo voltammetry with carbon fiber microelectrodes during spreading depression and anoxia to compare the temporal relationship between the release of dopamine and ion movements in the striatum. There is a pronounced release of dopamine during both spreading depression and anoxia. In spreading depression, the sharp increase of potassium concentration that follows an initial smaller and slower increase of potassium is accompanied by the release of dopamine. In anoxia, the dopamine release clearly precedes the fast rise of extracellular potassium concentration. We conclude that in striatum, there is a pronounced dopamine release during spreading depression and anoxia, but that the relationships between ionic changes and transmitter release for these two phenomena are different and probably reflect different mechanisms.

Regional differences in resting extracellular potassium levels of rat brain

The extracellular potassium concentration in the cerebral cortex of the mammalian brain has been repeatedly reported to be approximately 3.0 mM. We have made detailed measurements with potassium-selective microelectrodes and have found significantly lower extracellular potassium concentrations in unstimulated rat brain caudate and thalamus (1.9-2.5 mM) when compared to cortex and cerebral spinal fluid (3.0-3.5 mM). These regional differences may be caused by variations in spontaneous activity of neurons, regional permeability differences in endothelial cells of brain capillaries to potassium, or caused by variations in uptake by glia.