Stress activation of mesocorticolimbic dopamine neurons: effects of a glycine/NMDA receptor antagonist

Brain metabolic alterations in mice subjected to postnatal traumatic stress and in their offspring

Adverse environmental and social conditions early in life have a strong impact on health. They are major risk factors for mental diseases in adulthood and, in some cases, their effects can be transmitted across generations. The consequences of detrimental stress conditions on brain metabolism across generations are not well known. Using high-field (14.1 T) magnetic resonance spectroscopy, we investigated the neurochemical profile of adult male mice exposed to traumatic stress in early postnatal life and of their offspring, and of undisturbed control mice. We found that, relative to controls, early life stress-exposed mice have metabolic alterations consistent with neuronal dysfunction, including reduced concentration of N-acetylaspartate, glutamate and γ-aminobutyrate, in the prefrontal cortex in basal conditions. Their offspring have normal neurochemical profiles in basal conditions. Remarkably, when challenged by an acute cold swim stress, the offspring has attenuated metabolic responses in the prefrontal cortex, hippocampus and striatum. In particular, the expected stress-induced reduction in the concentration of N-acetylaspartate, a putative marker of neuronal health, was prevented in the cortex and hippocampus. These findings suggest that paternal trauma can confer beneficial brain metabolism adaptations to acute stress in the offspring.

Blunted accumbal dopamine response to cocaine following chronic social stress in female rats: exploring a link between depression and drug abuse

RATIONALE

Women have twice the risk as men to develop depression. Approximately, 24% of major depression disorder cases have comorbid disorders with substance abuse. Several central systems, including dopaminergic and serotonergic pathways, are thought to be involved in such comorbidity.

OBJECTIVES

The present study established a chronic social stress model in female rats, which produces some cardinal features of depressive-like symptoms. Further, we examined the effects of acute cocaine on dopamine (DA) and serotonin (5-HT) in the nucleus accumbens (NAc) using this model.

METHODS

Female Long-Evans rats confronted a nursing dam in its home cage for 30 min twice daily for 21 days. The non-stressed control group was handled daily throughout the experiment. During the 21 days of stress, behaviors during confrontations, weight, preference for saccharin, and estrous cycles were measured. Ten days after the last confrontation, the experimental rat was challenged with 10 mg/kg of cocaine, and levels of DA and 5-HT in the NAc were measured using in vivo microdialysis.

RESULTS

During the course of daily confrontations for 21 days, the experimental females significantly increased the duration of immobility, reduced weight gain and the preference for saccharin, and disrupted estrous cycles during the stress. Chronic social stress significantly attenuated cocaine-induced DA levels, and to some extent, attenuated a percent change of 5-HT compared to the non-stressed control group.

CONCLUSIONS

Chronic social defeat stress for 21 days induced physiological and behavioral depression-relevant deficits and blunted response of dopaminergic and to some extent, serotonergic neurons to cocaine challenge in females.

Effects of recovery from immobilization stress on striatal preprodynorphin- and kappa opioid receptor-mRNA levels of the male rat

Previously, we have reported that brain regions that are thought to be involved in motivated behavior are altered in animals undergoing repeated exposures to immobilization stress. The goal of the present study was to determine the effects of recovery from this type of stress on these same mesolimbic brain regions. For this purpose, adult male Sprague-Dawley rats were initially exposed to immobilization stress either once (2 h) or repeatedly (2 h×10 days). Rats were then either allowed to recover from the stressor for a shorter (2 days) or longer period of time (9 days) in their home cages. At the end of this recovery period, rats were euthanized and trunk blood and brains were processed for serum corticosterone (CORT) and neurochemistry, respectively. Brain mRNA levels were determined via in situ hybridization for the opioid preprodynorphin (DYN) and its cognate receptor (kappa, KOR), in striatal and accumbal subregions. A pattern of selective transcriptional activation emerged in the four resultant treatment conditions where a short recovery from either a single or repeated exposure to immobilization produced increases in KOR-mRNA levels in striatal and nucleus accumbens (Acb) subregions. Relative to controls, these differences were diminished after a longer recovery period. Interestingly, DYN-mRNA levels were unchanged after the shorter recovery period and after single or repeated immobilizations but appeared to be induced after a longer recovery period after repeated immobilizations. A relative amount of weight loss occurred after immobilization following repeated but not single exposure to stress. In addition, only those rats recovering from repeated stress exposures had higher CORT levels compared with non-immobilized controls. These results suggest that recovery from immobilization stress may alter the motivational system after as little as a single immobilization and that a possible dysphoric effect on appetitive behavior may be reflected by an altered striatal dynorphin system.

Regulation of netrin-1 receptors by amphetamine in the adult brain

Netrin-1 is a guidance cue molecule fundamental to the organization of neuronal connectivity during development. Netrin-1 and its receptors, deleted in colorectal cancer (DCC) and UNC-5 homologues (UNC-5), continue to be expressed in the adult brain, although neither their function nor the kinds of events that activate their expression are known. Two lines of evidence suggest a role for netrin-1 in amphetamine-induced dopamine plasticity in the adult. First, DCC is highly expressed by adult dopamine neurons. Second, adult mice with reduced DCC levels do not develop amphetamine-induced behavioral sensitization. To explore the role of netrin-1 in amphetamine-induced plasticity, we examined the effects of sensitizing treatment regimens of amphetamine on DCC and/or UNC-5 protein expression in the adult rat. These treatments produced striking and enduring increases in DCC and UNC-5 expression in the cell body, but not terminal regions, of the mesocorticolimbic dopamine system. Notably, neuroadaptations in the cell body region of mesocorticolimbic dopamine neurons underlie the development of sensitization to the effects of amphetamine. Furthermore, these localized amphetamine-induced changes were prevented by co-treatment with an N-methyl-d-aspartate receptor antagonist, a treatment known to block the development of amphetamine-induced sensitization of behavioral activation, dopamine release and motivated behavior. Using immunohistochemistry, we showed that both DCC and UNC-5 receptors are highly expressed by adult mesocorticolimbic dopamine neurons. These results provide the first evidence that repeated exposure to a stimulant drug such as amphetamine affects netrin-1 receptor expression in the adult brain. Taken together, our findings suggest that changes in netrin-1 receptor expression may play a role in the lasting effects of exposure to amphetamine and other stimulant drugs.

Electrochemical Nanoneedle Biosensor Based on Multiwall Carbon Nanotube

We report the fabrication and analytical functions of a biosensor based on a nanoneedle consisting of a multiwall carbon nanotube attached to the end of an etched tungsten tip. The devised electrode is the smallest needle-type biosensor reported to date. The nanoneedles prepared in this work are 30 nm in diameter and 2-3 microm in length. Dopamine and glutamate, which are physiologically important neurotransmitters, were successfully detected using these nanoneedles. Bare nanoneedles detected dopamine in the range from 100 to 1000 microM by differential pulse voltammetry, and enzyme-modified nanoneedles were able to respond to glutamate in the 100-500 microM range by potentiostatic amperometry.

Peripheral ChE Inhibition Modulates Brain Monoamines Levels and c-fos Oncogene in Mice Subjected to a Stress Situation

The present study examined, in mice, whether regional patterns of brain monoamines concentrations (DA, 5-HT and their metabolites) and expression of c-Fos protein, that may represent a prolonged functional change in neurons, could be changed after a combined exposure to stress and the peripheral cholinesterase reversible inhibitor pyridostigmine (PYR). Animals were subjected every day to a random combination of mild unescapable electric footshocks and immobilization over a 12-day period, resulting in a significant increase of glucocorticoids levels and an activation of c-fos in hippocampus, thalamus and piriform cortex. This stress protocol induced a significant increase of 5-HT levels in striatum, hippocampus and ponto mesencephalic area (PMA) but failed to induce any DA activation. When PYR (0.2 mg/kg s.c. inducing 19-35% inhibition of the plasmatic ChE activity) was administered twice a day during the last 5 days of the stress session, 5-HIAA levels and expression of c-fos oncogene were significantly increased in the most of the brain areas studied. DA levels were also enhanced in striatum/hippocampus as a result of a possible activation of mesolimbic and nigrostriatal dopamine systems. Taken together, these results suggest that a combined exposure to certain stress conditions and PYR leads, in mice, to functional changes in neurons and may affect centrally controlled functions. The mechanisms underlying these modifications and their behavioral implications remain to be further investigated.

Stress-induced increase of cortical dopamine metabolism: attenuation by a tachykinin NK1 receptor antagonist

The present study examined the potential role of tachykinin NK1 receptors in modulating immobilisation stress-induced increase of dopamine metabolism in rat medial prefrontal cortex. In agreement with previous studies, 20 min immobilisation stress significantly increased medial prefrontal cortex dopamine metabolism as reflected by the concentration of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC). Pretreatment with the high affinity, selective, tachykinin NK1 receptor antagonist (3(S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethyl amino)-2(S)-phenylpiperidine) ((S)-GR205171, 10 mg/kg, s.c.), a dose that in ex vivo binding studies extensively occupied rat brain tachykinin NK1 receptors for approximately 60 min, significantly attenuated the stress-induced increase of mesocortical DOPAC concentration without affecting cortical DOPAC levels per se. In contrast, pretreatment of animals with the less active enantiomer (R)-GR205171 (10 mg/kg, s.c.), which demonstrated negligible tachykinin NK1 receptor occupancy ex vivo, failed to affect either basal or stress-induced DOPAC concentration in medial prefrontal cortex. Furthermore, pretreatment of animals with the benzodiazepine/GABAA receptor antagonist, flumazenil (15 mg/kg, i.p.), did not affect the ability of (S)-GR205171 to attenuate the increase of medial prefrontal cortex DOPAC concentration by acute stress. Results demonstrate that the selective tachykinin NK1 receptor antagonist, (S)-GR205171, attenuated the stress-induced activation of mesocortical dopamine neurones by a mechanism independent of the benzodiazepine modulatory site of the GABAA receptor.

Effect of stress on prefrontal cortex function

Stress is the major epigenetic factor that contributes to the etiology, pathophysiology, and treatment outcome of most psychiatric disorders. Understanding the mechanisms by which stress contributes to these processes can have important implications for improving therapeutic outcome. Considering that a dysfunctional prefrontal cortex has been implicated in many psychiatric disorders, such as schizophrenia and mood disorders, delineating mechanisms by which stress affects prefrontal cortex (PFC) function is critical to our understanding of the role of stress in influencing the disease process. This paper will review recent mechanistic information about the effects of stress on dopamine and glutamate neurotransmission in the PFC.

The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition?

Psychopharmacological studies have implicated the dopaminergic innervation of the nucleus accumbens (NAC) in reward and reinforcement, in the actions of addictive drugs, and in the control of the symptoms of schizophrenia. Recent developments in in vivo dialysis, and other in vivo neurochemical techniques have permitted a more direct analysis of the behavioural correlates of increased dopamine release in rats, and have largely confirmed these findings in relation to reward, and drugs of abuse potential. However, dopamine release has also been found to be increased by many other stimuli/situations including aversive stimuli, stimuli conditioned to aversive stimuli, complex novel stimuli, and in the process of conditioning itself. These results contrast with electrophysiological data obtained in the behaving monkey, where rewarding stimuli, or stimuli predictive of reward are associated with increased firing of presumptive dopamine neurones projecting to the NAC (and indeed to the striatum), but mild aversive stimuli are not, leading to the suggestion that this system subserves a more purely reward function, or indeed that it provides a reward error signal. Further exploration of these issues will depend upon a comparison of increased dopamine cell firing and increased dopamine release, and an analysis of the behavioural effects of blocking these increases in dopamine transmission. One suggestion, deriving from work on latent inhibition, is that the significance of dopamine release by salient stimuli is to allow learning about stimuli which would otherwise be excluded on the basis of familiarity. This suggests that in addition to a role in some types of learning about salient stimuli, dopamine release in NAC may have a role in controlling the attention paid to familiar stimuli. Since it is difficult to see a connection between simple learning about rewards, and the symptoms of schizophrenia, this provides a more convincing link between the dopamine theory of schizophrenia, and the attentional difficulties held by many theorists to underlie schizophrenic symptoms.

Corticotropin-releasing factor requires CRF binding protein to potentiate NMDA receptors via CRF receptor 2 in dopamine neurons

Stress increases addictive behaviors and is a common cause of relapse. Corticotropin-releasing factor (CRF) plays a key role in the modulation of drug taking by stress. However, the mechanism by which CRF modulates neuronal activity in circuits involved in drug addiction is poorly understood. Here we show that CRF induces a potentiation of NMDAR (N-methyl-D-aspartate receptor)-mediated synaptic transmission in dopamine neurons of the ventral tegmental area (VTA). This effect involves CRF receptor 2 (CRF-R2) and activation of the phospholipase C (PLC)-protein kinase C (PKC) pathway. We also find that this potentiation requires CRF binding protein (CRF-BP). Accordingly, CRF-like peptides, which do not bind the CRF-BP with high affinity, do not potentiate NMDARs. These results provide evidence of the first specific roles for CRF-R2 and CRF-BP in the modulation of neuronal activity and suggest that NMDARs in the VTA may be a target for both drugs of abuse and stress.

Stress activation of glutamate neurotransmission in the prefrontal cortex: implications for dopamine-associated psychiatric disorders

In most psychiatric disorders, stress is the major nongenomic factor that contributes to the expression or exacerbation of acute symptoms, recurrence or relapse after a period of remission, and treatment outcome. Delineation of mechanisms by which stress contributes to these processes is fundamental to understanding the disease process and for improving outcome. In this article, evidence is reviewed to indicate that many central aspects of stress response, including activation of the hypothalmic-pituitary-adrenal (HPA) axis and dopamine neurotransmission, are modulated, and in some cases mediated, by glutamate neurotransmission in the prefrontal cortex (PFC). It is suggested that activation of glutamatergic neurotransmission in the PFC presents a common mechanism by which stress influences normal and abnormal processes that sustain affect and cognition. Although monoamines, in particular dopamine, have been considered the major culprits in the adverse effects of stress in disorders such as addiction and schizophrenia, it is likely that in a vulnerable brain with an underlying PFC pathophysiology, abnormal stress-activated monoaminergic neurotransmission is secondary to anomalies in cortical glutamate neurotransmission. Thus, understanding the contribution of glutamate-mediated processes to stress response through the use of experimental models that involve disrupted PFC function can provide insights to the fundamental pathophysiology of stress-sensitive psychiatric disorders and lead to novel strategies for treatment and prevention.

Hypotension-induced dopamine release in prefrontal cortex is mediated by local glutamatergic projections at the level of nerve terminals

In a previous study it was shown that nitroprusside-induced hypotension strongly enhances the release of dopamine (DA) in the prefrontal cortex (PFC). In the present study we have further investigated the mechanism involved in this effect. Glutamate receptor antagonists were infused into the ventral tegmental area (VTA) or PFC, while DA release was measured in the ipsilateral PFC and hypotension was applied by intravenous infusion of nitroprusside. Infusion into the VTA of neither a NMDA receptor antagonist (CPP), nor a non-NMDA antagonist (DNQX) affected the hypotension-induced increase of DA in the PFC. Intracortical infusion of CPP also failed to affect significantly, whereas local infusion of DNQX inhibited the hypotension-enhanced release of DA dose-dependently. The stimulation of DA release was relatively small in the VTA as well as in the nucleus accumbens when compared with the response in the PFC. It is concluded that DA released from mesocortical neurons can be modulated by two different mechanisms: first, by glutamate afferents to the VTA that modify the nerve-impulse flow of DA neurons; and, second, by glutamate afferents to the PFC that act at the level of the DA nerve terminals. The behaviour context (arousal or stress versus hypotension) determines which type of interaction predominates.

Stress-induced increase in extracellular dopamine in striatum: role of glutamatergic action via N-methyl-d-aspartate receptors in substantia nigra

There is considerable support for an influence of excitatory amino acids released from corticofugal neurons on dopaminergic activity in the basal ganglia. However, the relative importance of cortico-striatal and cortico-mesencephalic projections remains unclear, particularly with respect to the nigro-neostriatal pathway. We have therefore examined the influence of endogenous excitatory amino acids in substantia nigra on stress-induced dopaminergic activity in neostriatum. Microdialysis probes were implanted unilaterally into substantia nigra and ipsilateral neostriatum, and dopamine release in neostriatum was monitored by measuring changes in extracellular dopamine. In separate animals, neostriatal dopamine synthesis was assessed by measuring extracellular DOPA in the presence of 3-hydroxylbenzylhydrazine (NSD-1015; 100 microM), an inhibitor of aromatic amino acid decarboxylase. Thirty minutes of intermittent foot shock increased both dopamine release (+41%) and synthesis (+37%) in neostriatum. Infusion of 2-amino-5-phosphonovalerate (APV; 100 microM), an inhibitor of N-methyl-D-aspartate (NMDA) receptors, into substantia nigra greatly attenuated the stress-induced increase in neostriatal dopamine release, while having no effect on the apparent increase in stress-induced dopamine synthesis. These data suggest that excitatory amino acids such as glutamate act on NMDA receptors in substantia nigra to increase striatal dopamine release produced by exposure to stress, but that the increase in dopamine synthesis is mediated through a separate mechanism.

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

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

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Divergent effects of putative anxiolytics on stress-induced fos expression in the mesoprefrontal system of the rat

Previously, we reported that R(+)HA-966, a weak partial agonist for the glycine/NMDA receptor, and guanfacine, a noradrenergic alpha2 agonist, have anxiolytic-like actions on the biochemical activation of the mesoprefrontal dopamine neurons and fear-induced behaviors. Here, we examined these two putative anxiolytic agents, both with primary actions independent of GABAergic systems, for their ability to alter stress-induced Fos-like immunoreactivity in the mesoprefrontal cortex and in tyrosine hydroxylase-stained, presumed dopaminergic, neurons in the ventral tegmental area. The benzodiazepine agonist, lorazepam, and partial agonist, bretazenil, were also tested in this footshock paradigm [10 x 0.5 sec, 0.8 mA paired with a 5-sec tone]. In saline-treated rats, footshock resulted in an increase in Fos-li in the prelimbic and infralimbic cortices and tyrosine hydroxylase-labeled cells in the ventral tegmental area. Treatment with lorazepam or bretazenil prevented the stress-induced activation in Fos-li nuclei in all regions of the medial prefrontal cortex and in dopaminergic neurons in the ventral tegmental area. In contrast, the actions of the novel anxiolytic-like agents on stress-induced Fos-li were different than those observed with benzodiazepine agonists. Both putative anxiolytics, R(+)HA-966 and guanfacine, did not reduce, but significantly enhanced the stress-induced Fos-li in the prelimbic region of the medial prefrontal cortex. Additionally, treatment with R(+)HA-966 completely blocked, while guanfacine attenuated, the stress-induced increase in the number of Fos-li, TH-li cells in the ventral tegmental area. These results indicate that the putative anxiolytics, R(+)HA-966 and guanfacine, have actions on the stress-sensitive mesoprefrontal system which appear distinct from those of traditional anxiolytics.

The role of stress in the pathophysiology of the dopaminergic system

In this review, we will examine the most recent preclinical evidence in support of the fact that both acute and chronic stress may have a detrimental impact on the normal function of the dopaminergic system. In recent decades, the term stress has changed its meaning from that of a 'non-specific body response' to a 'monitoring system of internal and external cues'; that is a modality of reaction of the mammalian central nervous system (CNS) which is critical to the adaptation of the organism to its environment. Compelling results have demonstrated that the dopaminergic system is important not only for hedonic impact or reward learning but also, in a broader sense, for reactivity to perturbation in environmental conditions, for selective information processing, and for general emotional responses, which are essential functions in the ability (or failure) to cope with the external world. In this, stress directly influences several basic behaviors which are mediated by the dopaminergic system such as locomotor activity, sexual activity, appetite, and cross sensitization with drugs of abuse. Studies using rat lines which are genetically different in dopamine (DA) physiology, have shown that even small alterations in the birth procedure or early life stress events may contribute to the pathophysiology of psychiatric disorders-in particular those involving central DA dysfunction-and may cause depression or psychotic derangement in the offspring. Finally, the fact that the dopaminergic system after stress responds, preferentially, in the medial prefrontal cortex (MFC), is thought to serve, in humans, as a protection against positive psychotic symptoms, since the increased DA activity in the MFC suppresses limbic DA transmission. However, excessive MFC dopaminergic activity has a negative impact on the cognitive functions of primates, making them unable to select and process significant environmental stimuli. Thus it appears that a critical range of DA turnover is necessary for optimal cognitive functioning after stress, in the response of the CNS to ever-changing environmental demands. Molecular Psychiatry (2000) 5, 14-21.

Monitoring glutamate and ascorbate in the extracellular space of brain tissue with electrochemical microsensors

This paper describes electrochemical microsensors for the in vivo measurement of glutamate and ascorbate in the extracellular space of brain tissue. To prepare glutamate microsensors, carbon fiber microelectrodes (10 microns in diameter and 300-400 microns long) were modified with a cross-linked redox polymer film containing enzymes. The microsensors were coated with a thin Nafion film before use. The glutamate microsensors were both selective and sensitive toward glutamate, with detection limits in the low micromolar range. Physiologically relevant concentrations of several electroactive compounds found in brain tissue produced no response at the glutamate microsensors and also did not affect their glutamate response, the only exception being glutamine, for which a small response was observed in the absence, but not in the presence, of glutamate. The ascorbate microsensors were used in conjunction with cyclic voltammetry. They were sensitive and selective toward ascorbate, but did exhibit a small sensitivity toward the dopamine metabolite, dihydroxyphenylacetic acid. The in vivo measurements performed establish the ability of the glutamate microsensors to monitor the component of the basal extracellular glutamate level that is derived from the neuronal activity of brain tissue.

Chronic clozapine, but not haloperidol, alters the response of mesoprefrontal dopamine neurons to stress and clozapine challenges in rats

Previously, we demonstrated that serotonin-lesioned rats had an enhanced mesoprefrontal dopaminergic response to restraint stress. This study attempted to extend our knowledge regarding this serotonin/dopamine interaction by seeing if suppression of serotonin metabolism by chronic administration of the atypical antipsychotic, clozapine, would have similar effects. Both typical and atypical neuroleptics require chronic administration in humans before antipsychotic activity is seen. Rats treated for 21 days with clozapine or haloperidol, a typical antipsychotic without significant binding affinity for serotonergic receptors, showed lowered basal dopamine metabolism in the medial prefrontal cortex, the nucleus accumbens, and the striatum, as expected. Basal serotonin metabolism in the prefrontal cortex was also lowered by clozapine treatment, but not haloperidol. One of two challenges were given to chronically treated rats: 30 min of restraint stress or an acute challenge of clozapine. When corrected for baseline differences, both challenges significantly elevated dopamine metabolism in the prefrontal cortex of the clozapine group more than the saline or haloperidol groups. No hyperresponsiveness was seen with serotonin metabolism in the prefrontal cortex or either dopamine or serotonin metabolism in the nucleus accumbens in clozapine-treated, challenged rats. Additionally, this augmentation of the dopaminergic stress response was not seen with a single, acute administration of clozapine. The significance of the clozapine-induced hyperresponsiveness of the mesoprefrontal dopamine system is discussed with regard to clinical efficacy of clozapine.

The role of mesoprefrontal dopamine neurons in the acquisition and expression of conditioned fear in the rat

The mesoprefrontal dopamine neurons are sensitive to physical, pharmacological and psychological stressors. In this report, the role of these neurons in the response to classical fear conditioning was investigated. 6-Hydroxydopamine lesions to the medial prefrontal cortex reduced dopamine levels to about 13% of controls but did not alter behavior during the acquisition of fear conditioning. As expected, conditioned fear increased dopamine metabolism (3,4-dihydroxyphenylacetic acid/dopamine ratio) in the nucleus accumbens in sham-lesion rats. The medial prefrontal 6-hydroxydopamine lesions did not alter this effect. During the expression, however, lesioned rats demonstrated a delayed extinction of the conditioned response without an overall increase in the initial conditioned response. This effect was consistent in rats receiving 6-hydroxydopamine lesions before or after the acquisition period. The calculated rates of extinction showed that the 6-hydroxydopamine lesioned rats had a reduced rate of extinction, but not acquisition, of fear conditioning. The results presented in this manuscript indicate that the mesoprefrontal dopamine neurons are involved in co-ordinating the normal extinction of a fear response but do not alter the acquisition of fearful behaviors. These data are consistent with the conclusion that the mesoprefrontal dopamine neurons are involved in maintaining the animal's response adaptability with regards to stress-related changes in the external environment.

An antisense oligonucleotide reverses the footshock-induced expression of fos in the rat medial prefrontal cortex and the subsequent expression of conditioned fear-induced immobility

The immediate-early genes, including c-fos, have been proposed to be involved in learning and memory. In this report, we examine stress-induced Fos-like immunoreactivity (Fos-li) in subregions of the prefrontal cortex during a conditioned fear paradigm. During the acquisition phase, the rats were conditioned to fear a formerly neutral tone by pairing the tone with a mild footshock. The rats were then tested for fearful behavior by reexposure to the tone without additional footshock. During acquisition, Fos-li was increased in the medial prefrontal cortex (infralimbic and prelimbic) but not the anterior cingulate and M1 motor cortex. However, during the extinction phase, no significant increase in Fos-li was observed in any region. These findings indicate that acquisition, but not extinction, of conditioned fear is associated with an increase in Fos-li in subregions of the medial prefrontal cortex. In other animals, an antisense oligonucleotide directed against the c-fos mRNA was injected into the infralimbic/prelimbic cortex 12 or 72 hr before the acquisition session. Antisense treatment given 12, but not 72, hr earlier suppressed Fos production without altering behavior during the acquisition session. Three days after the acquisition session, rats were tested for fearful behavior as before. The antisense oligonucleotide blockade of Fos production during acquisition was associated with a significantly less fearful response during the extinction session. These results support a role for Fos in the medial prefrontal cortex during the acquisition of aversive learning.

Effects of handling on extracellular levels of glutamate and other amino acids in various areas of the brain measured by microdialysis

Upon a physiological and pharmacological challenge, the responsiveness of extracellular glutamate levels in the prefrontal cortex, ventral tegmental area and locus coeruleus were studied using microdialysis. A 10-min handling period was used as a mild stressful stimulus. In all three brain areas, handling induced an immediate and short-lasting increase in glutamate levels, but the responses were highly variable. Only in the ventral tegmental area and the locus coeruleus, but not in the prefrontal cortex, the increases were significantly different from basal values. In rats with relatively low basal glutamate levels, both in the ventral tegmental area and locus coeruleus, handling had a more pronounced effect on glutamate levels than in rats with high basal levels, although in some rats with relatively low basal levels of glutamate, handling had hardly any effect. Potassium stimulation also induced variable responses in all three brain areas. Again, relatively low basal glutamate levels were more responsive to the stimulation than higher basal values, although there appeared to be a lower limit. These data suggest that relatively high basal levels contain sources of glutamate that mask the neuronal pool of glutamate and are therefore less responsive to physiological or pharmacological stimulation. However, this interpretation was questioned by the findings that basal levels and handling-induced increases in glutamate levels were found to be (partly) TTX-independent. As carrier-mediated release as a possible non-exocytotic release mechanism has only been described in vivo under pathological conditions, it seems plausible to ascribe TTX-independent glutamate increases to aspecific, non-neuronal processes. This interpretation was further supported by the observation that in all three brain areas, other amino acids, i.e., aspartate, taurine, glutamine, serine, alanine and glycine also increased upon handling in a very similar way as glutamate did. Thus, these results question a direct correlation between stimulated extracellular glutamate levels induced by handling and measured by microdialysis and glutamatergic neurotransmission.

Differential actions of dizocilpine (MK-801) on the mesolimbic and mesocortical dopamine systems: role of neuronal activity

The significance of impulse activity in the dopamine neurons of the ventral tegmental area for the dopamine release evoked by systemic administration of the psychotomimetic drug dizocilpine (MK-801) was investigated. Dual probe microdialysis was utilized in freely moving rats implanted with one probe in the ventral tegmental area and a second ipsilateral probe in either the nucleus accumbens or the medial prefrontal cortex. Dialysates were analyzed with high-performance liquid chromatography with electrochemical detection for dopamine. The ventral tegmental area was perfused with the sodium channel blocker tetrodotoxin (1 microM) or vehicle (perfusion solution). A total of 2 h after the onset of tetrodotoxin perfusion of the ventral tegmental area, MK-801 (0.1 mg/kg) was injected subcutaneously. Tetrodotoxin perfusion of the ventral tegmental area significantly reduced dialysate levels of dopamine both in the nucleus accumbens and the medial prefrontal cortex to approximately 30% of baseline. When given alone, MK-801 caused a significant, i.e. 50%, increase in extracellular dopamine levels in the nucleus accumbens, and an even larger increase in the medial prefrontal cortex, i.e. 150%. Tetrodotoxin perfusion of the ventral tegmental area completely blocked the systemic MK-801 induced increase in extracellular concentrations of dopamine in the nucleus accumbens. However, the MK-801-evoked increase in dopamine levels in the medial prefrontal cortex was not significantly affected. Thus, the present results allow the conclusion that basal dopamine output in mesolimbic and mesocortical dopamine nerve terminal regions is predominantly dependent on nerve impulses generated in the ventral tegmental area. Moreover, also the MK-801 evoked dopamine release in the mesolimbic projection is almost entirely dependent on the impulse activity of the dopamine neurons, in agreement with our previous results. However, the MK-801 evoked dopamine release in the mesocortical projection is, in contrast, largely independent of the nerve impulse activity in the dopamine cells. The dysfunctions of mesolimbic and mesocortical dopamine neurons induced by systemic administration of non-competitive NMDA receptor antagonists may have direct bearing on the neurobiology of psychotic states, in particular as regards the generation of emotional and cognitive impairments.

NMDA receptors mediate lasting increases in anxiety-like behavior produced by the stress of predator exposure--implications for anxiety associated with posttraumatic stress disorder

It has been proposed that NMDA-dependent long-term potentiation (LTP) of limbic system circuits controlling defensive behavior underlies stressor-induced lasting increases in anxiety-like behavior (ALB). Findings in cats given the stress-inducing beta-carboline, FG-7142, support this hypothesis. An animal model of lasting affective change following traumatic stress has recently been developed. In this model, lasting increases in anxiety-like behavior (ALB) assessed in the elevated plus maze are produced by a single 5-min exposure of a rat to a cat. Rats become more anxious in the plus maze for up to 3 weeks after the exposure. The present study demonstrates that blockade of NMDA receptors in rats with MK-801, AP7, or CPP, given 30 min prior to exposure to a cat, prevents the increase in ALB assessed 1 week later. MK-801 or AP7, given 30 min after exposure to a cat, do not prevent the increase in ALB seen 1 week later, however. MK-801, but not CPP or AP7, promotes approaches to cats during exposure. This "fearlessness" may reflect some anxiolytic action of MK-801. Approach to cats following injection of MK-801 was eliminated by prior injection of Prazosin. Prazosin did not interfere with the block of increases in ALB following cat exposure, however. These findings are consistent with the view that NMDA receptors are involved in initiation, but not maintenance of neural changes mediating lasting increases in anxiety following severe stress. The significance of these findings for PTSD are discussed.

Alterations in excitatory amino acid-mediated regulation of midbrain dopaminergic neurones induced by chronic psychostimulant administration and stress: relevance to behavioural sensitization and drug addiction

Repeated, intermittent administration of the psychostimulants d-amphetamine and cocaine, as well as other drugs of abuse, leads to an enduring augmentation of certain behavioural responses (e.g. locomotor activity) produced by these drugs. This behavioural sensitization has been the subject of considerable interest due to its potential relevance to drug addiction. Repeated administration of d-amphetamine also leads to an enhancement in the ability of electrical stimulation of the prefrontal cortex to induce burst firing in midbrain dopaminergic (DA) neurones. This hyper-responsiveness probably reflects a potentiation of transmission at excitatory amino acid (EAA)ergic synapses on DA neurones. In addition, we have previously reported that selective activation of mineralocorticoid receptors (MRs) by corticosterone leads to a potentiation of EAA-induced burst firing in midbrain DA neurones, an effect antagonized by glucocorticoid receptor (GR) activation. In this review article, we propose a model describing how drugs of abuse and stress alter EAA function at the level of DA cells in the ventral tegmental area (VTA), which can result in a long-lasting impact on behaviour. D-amphetamine produces a transitory increase in EAA-mediated transmission at the level of DA cells in the VTA, which triggers a more long-lasting change in EAAergic function resembling hippocampal long-term potentiation. Dopaminergic burst events are likely to be a critical link between enhanced EAAergic activity in afferents synapsing on DA neurones and plasticity at these synapses, by increasing calcium transport into the cell, which is known to be an important factor in synaptic plasticity. Selective MR occupation by corticosterone in the VTA facilitates the development of this plasticity. However, we hypothesize that during stress, GR-occupation also activates EAAergic afferents to DA neurones in a manner similar to that following psychostimulants. Under these circumstances, GR-occupation acts via circuitry external to the VTA, which may include the hippocampus. Thus, potentiation of EAAergic synapses on DA neurones in the VTA may represent a final common pathway by which two divserse means (psychostimulants and stress) achieve the same end (sensitization). Alterations in EAA-mediated transmission at the level of DA cells not only plays a critical role in the induction of behavioural sensitization, but probably continues to produce abnormal DA cell responses in the drug-free situation.

Involvement of gamma-aminobutyric acid neurotransmission in phencyclidine-induced dopamine release in the medial prefrontal cortex

The present study was designed to examine the possible involvement of gamma-aminobutyric acid (GABA) neurotransmission in the mechanism of phencyclidine (1-(1-phenylcyclohexyl)piperidine; PCP)-induced dopamine release in the medial prefrontal cortex, using in vivo microdialysis in awake, freely moving rats. Local perfusion via the dialysis probe into the medial prefrontal cortex with PCP (100 and 500 microM) and dizocilpine ((+)-5-methyl-10,11-dihydroxy-5-H-dibenzo(a,d)cyclo-heptan-5,10-im ine; MK-801, 10 and 50 microM), a selective non-competitive NMDA receptor antagonist, was found to increase extracellular dopamine levels. Co-perfusion with NMDA (1 mM) or the GABAA receptor agonist muscimol (50 microM) attenuated the effects of PCP (500 microM) and MK-801 (50 microM) on extracellular dopamine levels. The dopamine reuptake inhibitor nomifensine (50 microM) also produced an increase in extracellular dopamine levels in the medial prefrontal cortex, but this effect was not affected by co-perfusion with muscimol (50 microM). On the other hand, local perfusion with PCP (100 and 500 microM) and MK-801 (10 and 50 microM), but not nomifensine (50 microM), reduced extracellular GABA levels in the medial prefrontal cortex. Co-perfusion with NMDA (1 mM) reduced the effects of PCP (500 microM) and MK-801 (50 microM) on extracellular GABA levels. These results suggest that PCP may facilitate dopamine release in the medial prefrontal cortex, at least in part, by the inhibition of GABA release via the antagonism of NMDA receptors.

The role of afferents to the ventral tegmental area in the handling stress-induced increase in the release of dopamine in the medial prefrontal cortex: a dual-probe microdialysis study in the rat brain

This study was aimed to identify the neuronal pathways that mediate the handling stress induced increase in the release of dopamine in the medial prefrontal cortex (mPFC) of the rat brain. For that purpose a microdialysis probe was implanted in the ventral tegmental area (VTA) and a second probe was placed in the ipsilateral mPFC. Receptor specific compounds acting on GABA(A) (20 microM muscimol), GABA(B) (50 microM baclofen), acetylcholine (100 microM atropine, 100 microM mecamylamine), NMDA (30, 100 and 300 microM CPP; 300 microM AP-5, 1 mM (+)-HA-966) and non-NMDA receptors (500 microM CNQX) were infused into the VTA by retrograde dialysis, whereas extracellular dopamine was recorded in the ipsilateral mPFC. Intrategmental infusion of muscimol, baclofen, CPP, AP-5, (+)-HA-966 and CNQX decreased extracellular dopamine in the ipsilateral mPFC; atropine and mecamylamine were without effect on the basal values. During infusion of the various compounds rats were gently handled for 15 min. The infusions of muscimol, atropine, mecamylamine and (+)-HA-966 did not modify the handling stress induced increase in extracellular dopamine in the mPFC. However, during intrategmental infusion of baclofen, CPP, AP-5 and CNQX the handling stress induced increase in extracellular dopamine (expressed as % of controls) in the mPFC was suppressed. These results indicate that a glutamatergic projection to the VTA, acting via both NMDA and non-NMDA-glutamate receptors, play a major role in the handling stress-induced increase in dopamine release in the mPFC. In addition the results suggest a certain role for GABAergic neurones, acting via GABA(B) receptors, in the handling response.

NMDA receptors in nucleus accumbens modulate stress-induced dopamine release in nucleus accumbens and ventral tegmental area

Converging evidence suggests that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated locally by an excitatory amino acid (EAA)-containing input possibly originating in medial prefrontal cortex (PFC). In the present study, we examined the effects of intra-NAcc administration of EAA receptor antagonists on stress-induced increases of NAcc DA levels and of dendritically released DA in the ventral tegmental area (VTA). Local injection of the NMDA receptor antagonist-AP-5 (0.05, 0.5, and 5.0 nmoles)-dose-dependently potentiated increases in NAcc DA levels elicited by 15 min of restraint stress. In contrast, local application of equivalent doses of the kainate/AMPA receptor antagonist-DNQX-failed to alter the NAcc DA stress response reliably. In a separate experiment, we found that intra-NAcc injection of AP-5 also potentiated stress-induced increases in VTA DA levels. These results indicate that EAAs acting at NMDA receptors in NAcc can modulate stress-induced DA release in this region. Our data indicate, however, that this action exerts an inhibitory influence on the NAcc DA stress response, suggesting that the relevant population of NMDA receptors are not located on NAcc DA terminals. The fact that intra-NAcc AP-5 injections also potentiated the DA stress response in VTA suggests instead an action mediated by NMDA receptors located on NAcc neurons that feedback, directly or indirectly, to cell bodies of the mesocorticolimbic DA system.

L-701,324, a glycine/NMDA receptor antagonist, blocks the increase of cortical dopamine metabolism by stress and DMCM

Dopamine metabolism, as reflected by the concentration of dihydroxyphenylacetic acid (DOPAC), in the medial prefrontal cortex was significantly increased following 30 min immobilisation stress or systemic administration of the benzodiazepine/GABA(A) receptor inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). The response to stress was attenuated by pretreatment of rats with the benzodiazepine/GABA(A) receptor agonists diazepam and zolpidem. Furthermore, pretreatment with R-(+)-3-amino-1-hydroxypyrrolid-2-one (R-(+)-HA-966), a low efficacy partial agonist, and 7-chloro-4-hydroxy-3(3-phenoxy) phenylquinolin-2-(H)-one (L-701,324) a novel, high affinity, full antagonist at the glycine/NMDA receptor attenuated the response to both stress and DMCM. These results demonstrate that antagonists at the glycine/NMDA receptor complex are comparable with benzodiazepine/GABA(A) receptor agonists in their ability to prevent activation of the mesocortical dopamine system by stress and GABA(A) receptor inverse agonists. Results are discussed in relation to the interaction between glycine/NMDA receptor antagonists, the mesocorticolimbic dopamine system and stress related disorders.

Phencyclidine increases forebrain monoamine metabolism in rats and monkeys: modulation by the isomers of HA966

The noncompetitive NMDA receptor antagonist phencyclidine (PCP) has psychotomimetic properties in humans and activates the frontal cortical dopamine innervation in rats, findings that have contributed to a hyperdopaminergic hypothesis of schizophrenia. In the present studies, the effects of the enantiomers of 3-amino-1-hydroxypyrrolid-2-one (HA966) on PCP-induced changes in monoamine metabolism in the forebrain of rats and monkeys were examined, because HA966 has been shown previously to attenuate stress- or drug-induced activation of dopamine systems. In rats, PCP (10 mg/kg, i.p.) potently activated dopamine (DA) turnover in the medial prefrontal cortex (PFC) and nucleus accumbens. Serotonin utilization was also increased in PFC. Pretreatment with either R-(+)HA966 (15 mg/kg, i.p.) or S-(-)HA966 (3 mg/kg, i.p.) partially blocked PCP-induced increases in PFC DA turnover, whereas neither enantiomer altered the effect of PCP on DA turnover in the nucleus accumbens or the PCP-induced increases in serotonin turnover in PFC. PCP (0.3 mg/kg, i.m.) exerted regionally selective effects on the dopaminergic and serotonergic innervation of the monkey frontal cortex, effects blocked by pretreatment with S-(-)HA966 (3 mg/kg, i. m.). Importantly, these data demonstrate that in the primate, PCP has potent effects on dopamine transmission in the frontal cortex, a brain region thought to be dysfunctional in schizophrenia. In addition, a role for S-(-)HA966 as a modulator of cortical monoamine transmission in primates is posited.

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.

Responses of rat substantia nigra dopamine-containing neurones to (-)-HA-966 in vitro

1. Extracellular single unit recording techniques were used to compare the effects of (-)-3-amino-1-hydroxypyrrolidin-2-one ((-)-HA-966) and (+/-)-baclofen on the activity of dopamine-containing neurones in 300 microns slices of rat substantia nigra. Electrophysiological data were compared with the outcome of in vitro binding experiments designed to assess the affinity of (-)-HA-966 for gamma-aminobutyric acid (GABAB) receptors. 2. Bath application of (-)-HA-966 produced a concentration-dependent inhibition of dopaminergic neuronal firing (EC50 = 444.0 microM; 95% confidence interval: 277.6 microM - 710.1 microM, n = 27) which was fully reversible upon washout from the recording chamber. Although similar effects were observed in response to (+/-)-baclofen, the direct-acting GABAB receptor agonist proved to be considerably more potent than (-)-HA-966 (EC50 = 0.54 microM; 95% confidence interval: 0.44 microM - 0.66 microM, n = 29) in vitro. 3. Low concentrations of chloral hydrate (10 microM) were without effect on the basal firing rate of nigral dopaminergic neurones but significantly increased the inhibitory effects produced by concomitant application of (-)-HA-966. 4. The inhibitory effects of (-)-HA-966 were completely reversed in the presence of the GABAB receptor antagonists, CGP-35348 (100 microM) and 2-hydroxysaclofen (500 microM). Bath application of CGP-35348 alone increased basal firing rate. However, the magnitude of the excitation (9.2 +/- 0.3%) was not sufficient to account for the ability of the antagonist to reverse fully the inhibitory effects of (-)-HA-966. 5. (-)-HA-966 (0.1-1.0 mM) produced a concentration-dependent displacement of [3H]-GABA from synaptic membranes in the presence of isoguvacine (40 microM). However, the affinity of the drug for GABAB binding sites was significantly less than that of GABA (0.0005 potency ratio) and showed no apparent stereoselectivity. 6. These results indicate that while (-)-HA-966 appears to act as a direct GABAB receptor agonist in vitro, its affinity for this receptor site is substantially less than that of GABA or baclofen and unlikely to account for the depressant actions of this drug which occur at levels approximately ten fold lower in vivo.

Feeding-evoked dopamine release in the nucleus, accumbens: regulation by glutamatergic mechanisms

The extent to which glutamate receptors in the nucleus accumbens and ventral tegmental area regulate feeding-evoked increases in dopamine release in the nucleus accumbens was determined using in vivo brain microdialysis in the rat. In some animals a second dialysis probe was implanted in the ventral tegmental area ipsilateral to the nucleus accumbens probe. The feeding protocol involved access to standard rat chow after 18 h of food deprivation. Under these conditions rats began eating approximately 30 s after the introduction of food and consumed 7-8 g, resulting in a 50% increase in dopamine release. Application of the glutamate receptor antagonist kynurenate (1 mM) in the nucleus accumbens potentiated the feeding-evoked increase in dopamine release by 80%. Application of the metabotropic glutamate receptor agonist trans-1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid (100 microM) in the nucleus accumbens blocked the feeding-evoked increase in dopamine release. Application of a combination of the ionotropic glutamate receptor antagonists 2-amino-5-phosphopentanoic acid (200 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (50 microM) through the dialysis probe in the ventral tegmental area reduced basal dopamine output in the nucleus accumbens by 20% and markedly attenuated (by 70%) the effect of feeding on dopamine release. None of the treatments affected the latency to eat or the volume of food consumed. These results indicate that glutamatergic afferents to the ventral tegmental area mediate feeding-induced increases in dopamine release in the nucleus accumbens. In contrast, at physiological concentrations, glutamate in the nucleus accumbens appears to decrease dopamine release via actions on ionotropic and metabotropic receptors.

Dopamine and spatial working memory in rats and monkeys: pharmacological reversal of stress-induced impairment

The anxiogenic benzodiazepine inverse agonist FG7142 increases dopamine turnover in rodent prefrontal cortex but not in other dopamine terminal field areas. FG7142-induced increases in prefrontal cortical dopamine receptor stimulation impair prefrontal-dependent, but not nonprefrontal-dependent, cognitive tasks in rats and monkeys. The degree of impairment correlates with levels of prefrontal cortical dopamine turnover in rats and can be blocked in rats and monkeys with dopamine receptor antagonists, suggesting that increased dopamine turnover is directly related to the cognitive deficits. The current study examined nondopaminergic drug effects on FG7142-perturbed biochemistry and cognition. Both the noradrenergic alpha-2 agonist clonidine and the glycine/NMDA antagonist (+)HA966 prevented the FG7142-induced increase in dopamine turnover in rodent prefrontal cortex. Infusion of (+)HA966 into the ventral tegmental area (VTA) also blocked this increase in dopamine turnover, indicating that critical modulatory effects of (+)HA966 on FG7142-induced changes in dopamine turnover are occurring at the level of mesoprefrontal dopamine neuron cell bodies. Systemic (+)HA966 and clonidine, but not propranolol or D-cycloserine, prevented FG7142-associated spatial working memory deficits in rats and monkeys. These results support the idea of a critical range of dopamine turnover for optimal prefrontal cortical cognitive functioning, with excessive dopamine turnover leading to cognitive impairment. These studies also provide evidence for the regulation of prefrontal cortical dopamine turnover and cognition by multiple neurotransmitter systems and suggest that the VTA is an important regulatory site for these effects.

Involvement of NMDA receptor in the regulation of plasma interleukin-6 levels in mice

MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclopepten-5,10-imine maleate), a non-competitive NMDA receptor antagonist (0.01-1 micrograms), injected intracerebroventricularly (i.c.v.) dose dependently increased the baseline levels of plasma interleukin-6 in mice. In the 1-h immobilization-stressed animals, MK-801 (1 micrograms) administered i.c.v. produced an additive increase of plasma interleukin-6. NMDA (N-methyl-D-aspartate) (3, 10 ng) administered i.c.v. attenuated dose dependently the 1-h immobilization stress-induced rise in plasma interleukin-6 level. Neither 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) (0.01-0.5 micrograms) nor alpha-methyl-4-carboxyphenylglycine (MCPG) (1-20 micrograms), antagonists of non-NMDA and metabotropic glutamate receptors, respectively, i.c.v. administered, affected the basal and stress-induced plasma interleukin-6 levels. These data indicate that NMDA receptors may be involved in the suppressive regulation of the plasma interleukin-6 levels.

The glycine/NMDA receptor antagonist HA-966 impairs visual recognition memory in rhesus monkeys

Recent studies have shown that strychnine-insensitive glycine binding sites positively modulate the N-methyl-D-asparate (NMDA) subclass of glutamate receptors, which are important in neural pathways involved in cognitive function. We examined the effect of (+/-)-3-amino-1-hydroxy-2-pyrrolidone (HA-966), a highly specific antagonist of this glycine modulatory site on the NMDA receptor, on visual recognition memory in four rhesus monkeys performing a computer-automated version of delayed nonmatching-to-sample (DNMS) with a list length of 20 trial-unique graphic symbols. In addition, the effect of HA-966 was compared with that of (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (dizocilpine; MK-801), a noncompetitive NMDA channel blocker. Administration of HA-966 (0.1-10 mg/kg, i.m.) 30 min before testing impaired DNMS performance dose-dependently, starting at doses of 3.2 mg/kg; the memory deficity following the highest dose (10 mg/kg) was associated with prolonged response latencies. Similar impairments in recognition memory were observed following treatment with MK-801, though at much lower doses (3.2-32 micrograms/kg) than those at which HA-966 was effective. Administration of low doses of HA-966 (1 mg/kg) and MK-801 (10 micrograms/kg), each of which had no significant effect on performance when given alone, also failed to impair performance when given concurrently. Combined administration of both drugs, each at amnesia-producing doses (3.2 mg/kg of HA-966 plus 32 micrograms/kg of MK-801), markedly impaired performance in an additive, not a synergistic, manner. From these results, we propose that the recognition memory impairment observed in our monkeys following HA-966 administration is via an action on the glycine modulatory site of the NMDA receptor complex.

Neuroanatomical localization of the effects of (+)-HA966 on locomotor activity after cocaine injections to the nucleus accumbens of rats

Current research has shown that manipulation at the glycine site of the NMDA receptor by various compounds can modulate NMDA receptor function and its behavioural consequences. The present study was conducted in order to neuroanatomically localize the effects of (+)-HA966 upon the acute and chronic effects of cocaine. Locomotor activity of Sprague-Dawley rats pretreated with (+)-HA966 (60 micrograms) into the nucleus accumbens five min prior to receiving cocaine (20 mg/kg i.p.) was significantly less than locomotor activity of the vehicle pretreated rats. Repeated treatment with cocaine resulted in sensitization to the locomotor stimulant effects of cocaine. Co-administration of (+)-HA966 with cocaine prevented the sensitization to a subsequent cocaine challenge, in which activity was comparable to levels of naive individuals given an acute injection of cocaine. These results suggest that the glycine site on NMDA receptors in the nucleus accumbens plays an important role in both the acute and sensitized responses to cocaine on locomotor activity.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

Tyrosine enhances behavioral and mesocorticolimbic dopaminergic responses to aversive conditioning

Tyrosine is a precursor in the biosynthesis of catecholamines and, when administered systemically, has been shown to enhance the in vivo rate of tyrosine hydroxylation in the medial prefrontal cortex. Additionally, exogenous tyrosine has been demonstrated to enhance the pharmacologically-induced increase in dopamine metabolism seen following administration of haloperidol or the anxiogenic B-carboline, FG-7142. In this report, we examine the effect of a physiologically relevant dose of tyrosine (25 mg/kg) on biochemical and behavioral consequences of aversive conditioning. Rats were conditioned to fear a tone by pairing it with footshock, so that when challenged with the tone alone, rats responded with immobility, defecation, and elevated dopamine metabolism in the medial prefrontal cortex and nucleus accumbens. When tyrosine was administered on the test day (tones alone), the rats displayed an even greater elevation of dopamine metabolism in the nucleus accumbens and prolonged immobility to the tone, compared to the saline/conditioned controls. Tyrosine did not alter mobility or dopamine utilization in the nucleus accumbens in nonconditioned controls. However, dopamine metabolism in the medial prefrontal cortex of nonconditioned rats treated with tyrosine was increased to levels similar to those in the conditioned groups. This may be accounted for by handling and by exposure to an unfamiliar environment necessary for nonconditioned controls. We conclude that exogenous tyrosine is able to 1) elevate stress-induced dopamine metabolism in the nucleus accumbens, 2) alter dopamine utilization in the medial prefrontal cortex of handled, nonconditioned controls, and 3) enhance fear-induced immobilization. These data suggest a role for dietary tyrosine in biochemical and behavioral responses to aversive stimuli.

Prior exposure to cocaine diminishes behavioral and biochemical responses to aversive conditioning: reversal by glycine/N-methyl-D-aspartate antagonist co-treatment

Animals will respond with stress-like behavioral and biochemical changes when exposed to a neutral stimulus that had previously been paired with a stressful stimulus. This phenomenon is generally known as aversive conditioning or conditioned fear. We tested the effect of prior exposure to cocaine on rats subjected to an aversive conditioning paradigm. Rats were given repeated doses of cocaine to develop a reverse tolerance or sensitization to the locomotor stimulant properties of cocaine. We blocked this sensitization to cocaine in one cocaine-exposed group by co-administering an antagonist of the strychinine-insensitive glycine site of the N-methyl-D-aspartate receptor complex, R-(+)-HA-966, which prevented the development of locomotor sensitization to cocaine. After about three weeks, we examined the effect of cocaine sensitization and the prevention of sensitization by R-(+)-HA-966 on aversive conditioning. Rats were exposed to 10 tones (neutral stimuli) paired with footshock (stressful stimuli) over 30 min for the conditioning session. The following day, rts were returned to the cages, received 10 tones only over 30 min and were killed. No drugs were given to any rat before either session and control rats received the tones without footshock in both sessions. Prior exposure to cocaine caused an attenuation of the behavioral effects of aversive conditioning, namely the amount of time spent immobilized and the number of fecal boli expelled. Additionally, the elevated metabolic activity of dopamine in the medial prefrontal cortex, nucleus accumbens and ventral tegmental area associated with aversive conditioning was diminished in rats pre-exposed to cocaine. The behavioral and biochemical effects of pre-exposure to cocaine were reversed in rats that receive R-(+)-HA-966 co-treatment with the five day cocaine sensitization regimen. These data suggest that prior behavioral sensitization to cocaine diminishes the stressful effect of conditioned fear and that these effects are reversed when sensitization is prevented with R-(+)-HA-966.

Effects of the enantiomers of (+/-)-HA-966 on dopamine neurons: an electrophysiological study of a chiral molecule

The present study was conducted to evaluate the effects of the resolved enantiomers of (+/-)-1-1 hydroxy-3-aminopyrrolidone-2 ((+/-)-HA-966) on the electrophysiological properties of dopamine-containing neurons in the substantia nigra of the chloral hydrate anesthetized rat. Both (+)- and (-)-HA-966 produced a dose-dependent reduction in firing rate that eventually resulted in total cessation of spontaneous neuronal activity (ID50 = 5.7 and 57.8 mg/kg i.v., respectively). The inhibitory effects of both drugs were accompanied by a marked increase in the regularity of neuronal firing and a concomitant suppression of bursting activity. Although approximately 10-fold less potent than the (-) enantiomer, the inhibitory effects of (+)-HA-966 were completely antagonized by the centrally active, GABAB receptor antagonist, CGP-35348 (300 mg/kg i.v.). These data suggest that the complementary electrophysiological effects of the enantiomers of (+/-)-HA-966 on nigral dopamine neurons are mediated through a common mechanism of action possibly involving a novel interaction with GABAB receptors.

The role of 5-HT2A receptors in antipsychotic activity

The correlation between the clinical activity of antipsychotic agents and their affinity for the D2 dopamine receptor has been the mainstay of the hypothesis that schizophrenia is due to excessive dopaminergic function. More recently, the unique clinical profile of the atypical antipsychotic clozapine has been proposed to involve actions on additional receptor systems. In particular, the high affinity of clozapine for the 5HT2A receptor subtype has been suggested to contribute to its reduced side-effect liability, greater efficacy and its activity in therapy-resistant schizophrenia. We have used the highly selective 5-HT2A antagonist MDL 100,907 to explore the contribution of 5-HT2A receptor blockade to antipsychotic activity. Biochemical, electrophysiological and behavioral studies reveal that selective 5HT2A receptor antagonists have the preclinical profile of an atypical antipsychotic. The limited clinical evidence available also suggests that compounds producing 5-HT2A receptor blockade are effective, in particular, against the negative symptoms of schizophrenia.

R-(+)-HA-966, an antagonist for the glycine/NMDA receptor, prevents locomotor sensitization to repeated cocaine exposures

Repeated administration of cocaine results in a reverse tolerance or sensitization to the locomotor stimulant properties of cocaine. In this study, we examined the effect of an antagonist for the strychinine-insensitive glycine receptor of the NMDA receptor complex, R-(+)-HA-966, on the development of locomotor sensitization to cocaine. Co-administration of R-(+)-HA-966 with repeated cocaine prevented locomotor sensitization to a subsequent challenge dose of cocaine. However, R-(+)-HA-966 (15 mg/kg i.p.) did not attenuate locomotor activation to an acute dose of cocaine (15 mg/kg). These results indicate that the glycine/NMDA receptor is involved in locomotor sensitization to repeated cocaine administration but not in the locomotor activation to the acute stimulant effects of cocaine administration.

Excitatory amino acids: implications for psychiatric disorders research

The hyperdopaminergic theory of schizophrenia may account for some types of schizophrenia, but schizophrenia with negative symptoms or resulting in a chronic state of deterioration after repeated relapses cannot be explained by this theory. This minireview first discusses the interactions between dopamine and excitatory amino acid (EAA) neurons to produce abnormal behavior. Secondly, it deals with the influence of the psychotropic drugs on EAA, such as the relationship between phencyclidine and the hypoglutamate theory, the involvement of EAA in behavioral sensitization induced by amphetamines, the interactions between antipsychotic, antidepressant and antianxiety drugs and EAA, considering the possibility of developing newer psychotropic drugs related with EAA. Finally, glutamate receptors measured in postmortem schizophrenic brains are tabulated and the bases of the hypoglutamate hypothesis are discussed.