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

Social isolation of adolescent male rats increases anxiety and K+ -induced dopamine release in the nucleus accumbens: Role of CRF-R1

Early life adversity can disrupt development leading to emotional and cognitive disorders. This study investigated the effects of social isolation after weaning on anxiety, body weight and locomotion, and on extracellular dopamine (DA) and glutamate (GLU) in the nucleus accumbens (NAc) and their modulation by corticotropin releasing factor receptor 1. On the day of weaning, male rats were housed singly or in groups for 10 consecutive days. Anxiety-like behaviors were assessed by an elevated plus maze (EPM) and an open field test (OF). Neurotransmitter levels were measured by in vivo microdialysis. Single-housed rats spent less time, and entered more, into the closed arms of an EPM than group-housed rats. They also spent less time in the center of an OF, weighed more and showed greater locomotion. In the NAc, no differences in CRF, or in basal extracellular DA or GLU between groups, were observed. A depolarizing stimulus increased DA release in both groups but to higher levels in isolated rats, whereas GLU increased only in single-housed rats. Blocking CRF-R1 receptors with CP-154,526 decreased DA release in single-housed but not in group-housed rats. The corticotropin releasing factor receptor type 1 receptor antagonist also decreased GLU in group-housed animals. These results show that isolating adolescent rats increases anxiety, body weight and ambulation, as well as the sensitivity of dopaminergic neurons to a depolarizing stimulus. This study provides further evidence of the detrimental effects of social isolation during early development and indicates that dysregulation of the CRF system in the NAc may contribute to the pathologies observed.

Sex and strain differences in dynamic and static properties of the mesolimbic dopamine system

Sex is a biological variable that contributes to the incidence, clinical course, and treatment outcome of brain disorders. Chief among these are disorders associated with the dopamine system. These include Parkinson's disease, ADHD, schizophrenia, and mood disorders, which show stark differences in prevalence and outcome between men and women. In order to reveal the influence of biological sex as a risk factor in these disorders, there is a critical need to collect fundamental information about basic properties of the dopamine system in males and females. In Long Evans rats, we measured dynamic and static properties related to the mesolimbic dopamine system. Static measures included assessing ventral tegmental area (VTA) dopamine cell number and volume and expression of tyrosine hydroxylase and dopamine transporter. Dynamic measures in behaving animals included assessing (1) VTA neuronal encoding during learning of a cue-action-reward instrumental task and (2) dopamine release in the nucleus accumbens in response to electrical stimulation of the VTA, vesicular depletion of dopamine, and amphetamine. We found little or no sex difference in these measures, suggesting sexual congruency in fundamental static and dynamic properties of dopamine neurons. Thus, dopamine related sex-differences are likely mediated by secondary mechanisms that flexibly influence the function of the dopamine cells and circuits. Finally, we noted that most behavioral sex differences had been reported in Sprague-Dawley rats and repeated some of the above measures in that strain. We found some sex differences in those animals highlighting the importance of considering strain differences in experimental design and result interpretation.

Development of a non-human primate model to support CNS translational research: Demonstration with D-amphetamine exposure and dopamine response

BACKGROUND

Challenges specific to the discovery and development of candidate CNS drugs have led to implementation of various in silico, in vitro and in vivo approaches to improve the odds for commercialization of novel treatments.

NEW METHOD

Advances in analytical methodology and microdialysis probe design have enabled development of a non-human primate model capable of measuring concentrations of drugs or endogenous chemicals in brain extracellular fluid (ECF) and cerebrospinal fluid (CSF). Linking these to population modeling reduces animal numbers to support predictive translational sciences in primates. Application to measure D-amphetamine exposure and dopamine response in ECF and CSF demonstrate the approach.

RESULTS

Following a 0.1 mg/kg intravenous bolus dose of D-amphetamine, a population approach was used to build a plasma compartmental-based and brain physiologic-based pharmacokinetic (PK) model linking drug concentrations in plasma to brain ECF and CSF concentrations. Dopamine was also measured in brain ECF. The PK model was used to simulate the relationship between D-amphetamine exposure and dopamine response in ECF over a wide dose range.

COMPARISONS WITH EXISTING METHODS

Ability to co-sample and measure drug and endogenous substances in blood, brain ECF and/or CSF, coupled with population modeling, provides an in vivo approach to evaluate CNS drug penetration and effect in non-human primates.

CONCLUSIONS

A method to measure drug and endogenous neurochemicals in non-human primate brain fluids is demonstrated. Its basis in non-human primates merits improved confidence regarding predictions of drug exposure and target engagement in human CNS.

Differential effects of chemogenetic inhibition of dopamine and norepinephrine neurons in the mouse 5-choice serial reaction time task

Attention-deficit/hyperactivity disorder (ADHD) is a psychiatric disorder characterized by inattention, aberrant impulsivity, and hyperactivity. Although the underlying pathophysiology of ADHD remains unclear, dopamine and norepinephrine signaling originating from the ventral tegmental area (VTA) and locus coeruleus (LC) is thought to be critically involved. In this study, we employ Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) together with the mouse 5-Choice Serial Reaction Time Task (5-CSRTT) to investigate the necessary roles of these catecholamines in ADHD-related behaviors, including attention, impulsivity, and motivation. By selective inhibition of tyrosine hydroxylase (TH)-positive VTA dopamine neurons expressing the Gi-coupled DREADD (hM4Di), we observed a marked impairment of effort-based motivation and subsequently speed and overall vigor of responding. At the highest clozapine N-oxide (CNO) dose tested (i.e. 2 mg/kg) to activate hM4Di, we detected a reduction in locomotor activity. DREADD-mediated inhibition of LC norepinephrine neurons reduced attentional performance in a variable stimulus duration test designed to increase task difficulty, specifically by increasing trials omissions, reducing mean score, and visual processing speed. These findings show that VTA dopamine and LC norepinephrine neurons differentially affect attention, impulsive and motivational control. In addition, this study highlights how molecular genetic probing of selective catecholamine circuits can provide valuable insights into the mechanisms underlying ADHD-relevant behaviors.

Dopaminergic modulation of pain signals in the medial prefrontal cortex: Challenges and perspectives

Chronic pain is a massive socieoeconomic burden and is often refractory to treatment. To devise novel therapeutic interventions, it is important to understand in detail the processing of pain signals in the brain. Recent studies have revealed shared features between the brain's reward and pain systems. Dopamine (DA) is a key neuromodulator in the mesocorticolimbic system that has been implicated not only in motivated behaviours, reinforcement learning and reward processing, but also in the pain axis. The medial prefrontal cortex (mPFC) is an important region for mediating executive functions including attention, judgement, and learning. Studies have revealed that the mPFC undergoes plasticity during the development of chronic pain. The mPFC receives dopaminergic input from the ventral tegmental area (VTA), and stimulation of these inputs has been shown to modulate the plasticity of the mPFC and anxiety and aversive behaviour. Here, we review the role of the mPFC and its dopaminergic modulation in chronic pain.

Burst activation of dopamine neurons produces prolonged post-burst availability of actively released dopamine

Both phasic and tonic modes of neurotransmission are implicated in critical functions assigned to dopamine. In learning, for example, sub-second phasic responses of ventral tegmental area (VTA) dopamine neurons to salient events serve as teaching signals, but learning is also interrupted by dopamine antagonists administered minutes after training. Our findings bridge the multiple timescales of dopamine neurotransmission by demonstrating that burst stimulation of VTA dopamine neurons produces a prolonged post-burst increase (>20 min) of extracellular dopamine in nucleus accumbens and prefrontal cortex. This elevation is not due to spillover from the stimulation surge but depends on impulse flow-mediated dopamine release. We identified Rho-mediated internalization of dopamine transporter as a mechanism responsible for prolonged availability of actively released dopamine. Thus, a critical consequence of burst activity of dopamine neurons may be post-burst sustained elevation of extracellular dopamine in terminal regions via an intracellular mechanism that promotes dopamine transporter internalization. These results demonstrate that phasic and tonic dopamine neurotransmission can be a continuum and may explain why both modes of signaling are critical for motivational and cognitive functions associated with dopamine.

Risk of punishment influences discrete and coordinated encoding of reward-guided actions by prefrontal cortex and VTA neurons

Actions motivated by rewards are often associated with risk of punishment. Little is known about the neural representation of punishment risk during reward-seeking behavior. We modeled this circumstance in rats by designing a task where actions were consistently rewarded but probabilistically punished. Spike activity and local field potentials were recorded during task performance simultaneously from VTA and mPFC, two reciprocally connected regions implicated in reward-seeking and aversive behaviors. At the single unit level, we found that ensembles of putative dopamine and non-dopamine VTA neurons and mPFC neurons encode the relationship between action and punishment. At the network level, we found that coherent theta oscillations synchronize VTA and mPFC in a bottom-up direction, effectively phase-modulating the neuronal spike activity in the two regions during punishment-free actions. This synchrony declined as a function of punishment probability, suggesting that during reward-seeking actions, risk of punishment diminishes VTA-driven neural synchrony between the two regions.

Ethanol Dependence Abolishes Monoamine and GIRK (Kir3) Channel Inhibition of Orbitofrontal Cortex Excitability

Alcohol abuse disorders are associated with dysfunction of frontal cortical areas including the orbitofrontal cortex (OFC). The OFC is extensively innervated by monoamines, and drugs that target monoamine receptors have been used to treat a number of neuropsychiatric diseases, including alcoholism. However, little is known regarding how monoamines affect OFC neuron excitability or whether this modulation is altered by chronic exposure to ethanol. In this study, we examined the effect of dopamine, norepinephrine, and serotonin on lOFC neuronal excitability in naive mice and in those exposed to chronic intermittent ethanol (CIE) treatment. All three monoamines decreased current-evoked spike firing of lOFC neurons and this action required G_iα-coupled D2, α2-adrenergic, and 5HT_1A receptors, respectively. Inhibition of firing by dopamine or the D2 agonist quinpirole, but not norepinephrine or serotonin, was prevented by the GABA_A receptor antagonist picrotoxin. GABA-mediated tonic current was enhanced by dopamine or the D1 agonist SKF81297 but not quinpirole, whereas the amplitude of spontaneous IPSCs was increased by quinpirole but not dopamine. Spiking was also inhibited by the direct GIRK channel activator ML297, whereas blocking these channels with barium increased firing and eliminated the inhibitory actions of monoamines. In the presence of ML297 or the G-protein blocker GDP-β-S, DA induced a further decrease in spike firing, suggesting the involvement of a non-GIRK channel mechanism. In neurons from CIE-treated mice, spike frequency was nearly doubled and inhibition of firing by monoamines or ML297 was lost. These effects occurred in the absence of significant changes in expression of G_i/o or GIRK channel proteins. Together, these findings show that monoamines are important modulators of lOFC excitability and suggest that disruption of this process could contribute to various deficits associated with alcohol dependence.

Chemogenetic Activation of Midbrain Dopamine Neurons Affects Attention, but not Impulsivity, in the Five-Choice Serial Reaction Time Task in Rats

Attentional impairments and exaggerated impulsivity are key features of psychiatric disorders, such as attention-deficit/hyperactivity disorder, schizophrenia, and addiction. These deficits in attentional performance and impulsive behaviors have been associated with aberrant dopamine (DA) signaling, but it remains unknown whether these deficits result from enhanced DA neuronal activity in the midbrain. Here, we took a novel approach by testing the impact of chemogenetically activating DA neurons in the ventral tegmental area (VTA) or substantia nigra pars compacta (SNc) on attention and impulsivity in the five-choice serial reaction time task (5-CSRTT) in rats. We found that activation of DA neurons in both the VTA and SNc impaired attention by increasing trial omissions. In addition, SNc DA neuron activation decreased attentional accuracy. Surprisingly, enhanced DA neuron activity did not affect impulsive action in this task. These results show that enhanced midbrain DA neuronal activity induces deficits in attentional performance, but not impulsivity. Furthermore, DA neurons in the VTA and SNc have different roles in regulating attention. These findings contribute to our understanding of the neural substrates underlying attention deficits and impulsivity, and provide valuable insights to improve treatment of these symptoms.

Amphetamine elevates phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the rat forebrain via activating dopamine D1 and D2 receptors

Psychostimulants have an impact on protein synthesis, although underlying molecular mechanisms are unclear. Eukaryotic initiation factor 2α-subunit (eIF2α) is a key player in initiation of protein translation and is regulated by phosphorylation. While this factor is sensitive to changing synaptic input and is critical for synaptic plasticity, its sensitivity to stimulants is poorly understood. Here we systematically characterized responses of eIF2α to a systemic administration of the stimulant amphetamine (AMPH) in dopamine responsive regions of adult rat brains. Intraperitoneal injection of AMPH at 5mg/kg increased eIF2α phosphorylation at serine 51 in the striatum. This increase was transient. In the medial prefrontal cortex (mPFC), AMPH induced a relatively delayed phosphorylation of the factor. Pretreatment with a dopamine D1 receptor antagonist SCH23390 blocked the AMPH-stimulated eIF2α phosphorylation in both the striatum and mPFC. Similarly, a dopamine D2 receptor antagonist eticlopride reduced the effect of AMPH in the two regions. Two antagonists alone did not alter basal eIF2α phosphorylation. AMPH and two antagonists did not change the amount of total eIF2α proteins in both regions. These results demonstrate the sensitivity of eIF2α to stimulant exposure. AMPH possesses the ability to stimulate eIF2α phosphorylation in striatal and mPFC neurons in vivo in a D1 and D2 receptor-dependent manner.

Impact of anxiety on prefrontal cortex encoding of cognitive flexibility

Anxiety often is studied as a stand-alone construct in laboratory models. But in the context of coping with real-life anxiety, its negative impacts extend beyond aversive feelings and involve disruptions in ongoing goal-directed behaviors and cognitive functioning. Critical examples of cognitive constructs affected by anxiety are cognitive flexibility and decision making. In particular, anxiety impedes the ability to shift flexibly between strategies in response to changes in task demands, as well as the ability to maintain a strategy in the presence of distractors. The brain region most critically involved in behavioral flexibility is the prefrontal cortex (PFC), but little is known about how anxiety impacts PFC encoding of internal and external events that are critical for flexible behavior. Here we review animal and human neurophysiological and neuroimaging studies implicating PFC neural processing in anxiety-induced deficits in cognitive flexibility. We then suggest experimental and analytical approaches for future studies to gain a better mechanistic understanding of impaired cognitive inflexibility in anxiety and related disorders.

Amphetamine self-administration and dopamine function: assessment of gene × environment interactions in Lewis and Fischer 344 rats

RATIONALE

Previous research suggests both genetic and environmental influences on substance abuse vulnerability.

OBJECTIVES

The current work sought to investigate the interaction of genes and environment on the acquisition of amphetamine self-administration as well as amphetamine-stimulated dopamine (DA) release in nucleus accumbens shell using in vivo microdialysis.

METHODS

Inbred Lewis (LEW) and Fischer (F344) rat strains were raised in either an enriched condition (EC), social condition (SC), or isolated condition (IC). Acquisition of amphetamine self-administration (0.1 mg/kg/infusion) was determined across an incrementing daily fixed ratio (FR) schedule. In a separate cohort of rats, extracellular DA and the metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in the nucleus accumbens shell following an acute amphetamine injection (1 mg/kg).

RESULTS

"Addiction-prone" LEW rats had greater acquisition of amphetamine self-administration on a FR1 schedule compared to "addiction-resistant" F344 rats when raised in the SC environment. These genetic differences were negated in both the EC and IC environments, with enrichment buffering against self-administration and isolation enhancing self-administration in both strains. On a FR5 schedule, the isolation-induced increase in amphetamine self-administration was greater in F344 than LEW rats. While no group differences were obtained in extracellular DA, gene × environment differences were obtained in extracellular levels of the metabolite DOPAC. In IC rats only, LEW rats showed attenuation in the amphetamine-induced decrease in DOPAC compared to F344 rats. IC LEW rats also had an attenuated DOPAC response to amphetamine compared to EC LEW rats.

CONCLUSIONS

The current results demonstrate gene × environment interactions in amphetamine self-administration and amphetamine-induced changes in extracellular DOPAC in nucleus accumbens (NAc) shell. However, the behavioral and neurochemical differences were not related directly, indicating that mechanisms independent of DA metabolism in NAc shell likely mediate the gene × environment effects in amphetamine self-administration.

Differential responses of two related neurosteroids to methylphenidate based on ADHD subtype and the presence of depressive symptomatology

RATIONALE

Attention deficit with hyperactivity disorder is a neurodevelopmental disorder associated with alterations in the prefrontal cortex via dopaminergic and noradrenergic neurotransmission. Neurosteroids (e.g. allopregnanolone and dehydroepiandrosterone) modulate the release of multiple neurotransmitters.

OBJECTIVE

This study aims to determine the baseline concentrations and daily variations in allopregnanolone and dehydroepiandrosterone in children with attention deficit hyperactivity disorder (ADHD) and to determine the effect of chronic administration of methylphenidate on clinical symptoms and on the concentrations of these two neurosteroids.

METHODS

We included 148 children aged 5 to 14 years, subdivided into two groups: ADHD group (n = 107, with a diagnosis of ADHD (DSM-IV-TR criteria), further classified in subtypes by an "attention deficit and hyperactivity scale" and subgroups by the "Children's Depression Inventory") and a control group (n = 41). The clinical workup included blood samples that were drawn at 20:00 and 09:00 hours, at inclusion in both groups, and after 4.61 ± 2.29 months of treatment only in the ADHD group, for measurements for allopregnanolone and dehydroepiandrosterone. Factorial analysis, adjusted for age and gender, was performed by using Stata 12.0.

RESULTS

Methylphenidate induced the doubling of allopregnanolone levels in the predominantly inattentive ADHD patients without depressive symptoms (27.26 ± 12.90 vs. 12.67 ± 6.22 ng/ml, morning values). Although without statistical differences, baseline dehydroepiandrosterone levels were higher and slightly increased after methylphenidate in the ADHD subtype with depressive symptoms (7.74 ± 11.46 vs. 6.18 ± 5.99 ng/ml, in the morning), opposite to the lower baseline levels, and further decrease after methylphenidate in the inattentive subtype with depressive symptoms.

CONCLUSIONS

Different neurosteroids may have different baseline concentrations and differential responses to methylphenidate treatment as a function of ADHD subtype and subgroup. These differential responses may be a clinical marker of ADHD subtype and/or co-morbidities.

Amphetamine-induced release of dopamine in primate prefrontal cortex and striatum: striking differences in magnitude and timecourse

The psychostimulant amphetamine (AMPH) is frequently used to increase catecholamine levels in attention disorders and positron emission tomography imaging studies. Despite the fact that most radiotracers for positron emission tomography studies are characterized in non-human primates (NHPs), data on regional differences of the effect of AMPH in NHPs are very limited. This study examined the impact of AMPH on extracellular dopamine (DA) levels in the medial prefrontal cortex and the caudate of NHPs using microdialysis. In addition to differences in magnitude, we observed striking differences in the temporal profile of extracellular DA levels between these regions that can likely be attributed to differences in the regulation of dopamine uptake and biosynthesis. The present data suggest that cortical DA levels may remain elevated longer than in the caudate which may contribute to the clinical profile of the actions of AMPH. Using microdialysis probes implanted in the cortex and caudate region of non-human primate brains, we observed in vivo differences in the magnitude and temporal profile of extracellular dopamine levels in response to intravenous amphetamine administration.

The attenuation of dysfunctional emotional processing with stimulant medication: an fMRI study of adolescents with ADHD

Functional neuroimaging studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the neural correlates of cognitive control. However, for many youths with ADHD, emotional lability is an important clinical feature of the disorder. We aimed to identify the neural substrates associated with emotional lability that were distinct from impairments in cognitive control and to assess the effects that stimulants have on those substrates. We used functional magnetic resonance imaging (fMRI) to assess neural activity in adolescents with (N=15) and without (N=15) ADHD while they performed cognitive and emotional versions of the Stroop task that engage cognitive control and emotional processing, respectively. The participants with ADHD were scanned both on and off stimulant medication in a counterbalanced fashion. Controlling for differences in cognitive control, we found that during the emotional Stroop task, adolescents with ADHD as compared with controls demonstrated atypical activity in the medial prefrontal cortex (mPFC). Stimulants attenuated activity in the mPFC to levels comparable with controls.

Dissociation of prefrontal cortex and nucleus accumbens dopaminergic systems in conditional learning in rats

There is converging evidence that the prefrontal and mesolimbic dopaminergic (DAergic) systems are involved in the performance of a variety of tasks that require the use of contextual, or task-setting, information to select an appropriate response from a number of candidate responses. Performance on tasks of this nature are impaired in schizophrenia and in rats exposed to psychotomimetics; impairments that are often attenuated by administration of dopamine (DA) antagonists. Rats were trained on either a complex instrumental discrimination task, that required the use of task-setting cues, or a simple discrimination task that did not. Following training, microdialysis probes were implanted unilaterally in either the medial prefrontal cortex (mPFC) or nucleus accumbens (NAc) and samples were collected in freely moving animals during a behavioural test session. In Experiment 1, we found no difference in levels of DA in the mPFC of rats while they were performing the two discrimination tasks. Rats that performed the complex task did, however, show significantly higher mPFC DA levels relative to rats in the simple discrimination condition following the end of the behavioural test session. In Experiment 2, rats performing the conditional discrimination showed lower levels of DA in the NAc compared to the simple discrimination group both during the test session and after it. These results provide direct evidence that conditional discrimination tasks engage frontal and mesolimbic DAergic systems and are consistent with the proposal that regulation of fronto-striatal DA is involved in aspects of cognitive control that are known to be impaired in individuals with schizophrenia.

The high affinity dopamine uptake inhibitor, JHW 007, blocks cocaine-induced reward, locomotor stimulation and sensitization

The discovery and evaluation of high affinity dopamine transport inhibitors with low abuse liability is an important step toward the development of efficacious medications for cocaine addiction. We examined in mice the behavioural effects of (N-(n-butyl)-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane) (JHW 007), a benztropine (BZT) analogue that blocks dopamine uptake, and assessed its potential to influence the actions of cocaine in clinically-relevant models of cocaine addiction. In the conditioned place preference (CPP) paradigm, JHW 007 exposure did not produce place conditioning within an ample dose range but effectively blocked the CPP induced by cocaine administration. Similarly, in the CPP apparatus JHW 007 treatment failed to stimulate locomotor activity at any dose but dose-dependently suppressed the hyperactivity evoked by cocaine treatment. In locomotor sensitization assays performed in the open field, JHW 007 did not produce sensitized locomotor behaviour when given alone, but it prevented the sensitized component of the locomotor response elicited by subchronic (8-day) cocaine exposure. In the elevated plus maze (EPM), acute treatment with JHW 007, cocaine and combinations of the BZT analogue and cocaine produced an anxiogenic-like profile. Re-test in the EPM following subchronic (8-day) exposure enhanced the anxiogenic-like effect of the same drug treatments. The present findings indicate that JHW 007 exposure counteracts some critical behavioural correlates of cocaine treatment, including conditioned reward, locomotor stimulation and sensitization, and lend support to the further development of BZT analogues as potential replacement medications in cocaine addiction.

Amphetamine alters Ras-guanine nucleotide-releasing factor expression in the rat striatum in vivo

Ras-guanine nucleotide-releasing factors (Ras-GRFs) are densely expressed in neurons of the mammalian brain. As a Ras-specific activator predominantly concentrated at synaptic sites, Ras-GRFs activate the Ras-mitogen-activated protein kinase (Ras-MAPK) cascade in response to changing synaptic inputs, thereby modifying a variety of cellular and synaptic activities. While the Ras-MAPK cascade in the limbic reward circuit is well-known to be sensitive to dopamine inputs, the sensitivity of its upstream activator (Ras-GRFs) to dopamine remains to be investigated. In this study, the response of Ras-GRFs in their protein expression to dopamine stimulation was evaluated in the rat striatum in vivo. A single systemic injection of the psychostimulant amphetamine produced an increase in Ras-GRF1 protein levels in both the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum. The increase in Ras-GRF1 proteins was dose-dependent. The reliable increase was seen 2.5h after drug injection and returned to normal levels by 6h. In contrast to Ras-GRF1, protein levels of Ras-GRF2 in the striatum were not altered by amphetamine. In addition to the striatum, the medial prefrontal cortex is another forebrain site where amphetamine induced a parallel increase in Ras-GRF1 but not Ras-GRF2. No significant change in Ras-GRF1/2 proteins was observed in the hippocampus. These data demonstrate that Ras-GRF1 is a susceptible and selective target of amphetamine in striatal and cortical neurons. Its protein expression is subject to the modulation by acute exposure of amphetamine.

Effects of haloperidol on the behavioral, subjective, cognitive, motor, and neuroendocrine effects of Delta-9-tetrahydrocannabinol in humans

INTRODUCTION

Cannabinoids produce a spectrum of effects in humans including euphoria, cognitive impairments, psychotomimetic effects, and perceptual alterations. The extent to which dopaminergic systems contribute to the effects of Delta-9-tetrahydrocannabinol (Delta-9-THC) remains unclear. This study evaluated whether pretreatment with a dopamine receptor antagonist altered the effects of Delta-9-THC in humans.

MATERIALS AND METHODS

In a 2-test-day double-blind study, 28 subjects including healthy subjects (n = 17) and frequent users of cannabis (n = 11) were administered active (0.057 mg/kg) or placebo oral haloperidol in random order followed 90 and 215 min later by fixed order intravenous administration of placebo (vehicle) and active (0.0286 mg/kg) Delta-9-THC, respectively.

RESULTS

Consistent with previous reports, intravenous Delta-9-THC produced psychotomimetic effects, perceptual alterations, and subjective effects including "high." Delta-9-THC also impaired verbal recall and attention. Haloperidol pretreatment did not reduce any of the behavioral effects of Delta-9-THC. Haloperidol worsened the immediate free and delayed free and cued recall deficits produced by Delta-9-THC. Haloperidol and Delta-9-THC worsened distractibility and vigilance. Neither drug impaired performance on a motor screening task, the Stockings of Cambridge task, or the delayed match to sample task. Frequent users had lower baseline plasma prolactin levels and blunted Delta-9-THC induced memory impairments.

CONCLUSIONS

The deleterious effects of haloperidol pretreatment on the cognitive effects of Delta-9-THC are consistent with the preclinical literature in suggesting crosstalk between DAergic and CBergic systems. However, it is unlikely that DA D(2) receptor mechanisms play a major role in mediating the psychotomimetic and perceptual altering effects of Delta-9-THC. Further investigation is warranted to understand the basis of the psychotomimetic effects of Delta-9-THC and to better understand the crosstalk between DAergic and CBergic systems.

Medial prefrontal cortex infusion of alpha-flupenthixol attenuates systemic d-amphetamine-induced disruption of conditional discrimination performance in rats

RATIONALE

It has been argued that tasks that necessitate the use of context in the service of goal-directed behaviour are disrupted in both schizophrenic patients and in animal analogues by dopamine (DA) manipulation with the prefrontal cortex being implicated.

OBJECTIVES

To determine the effects on conditional discrimination performance of direct infusion of the DA D(1)/D(2) receptor antagonist alpha-flupenthixol into the medial prefrontal cortex (mPFC) and of its reversal potential on d-amphetamine-induced task disruption.

MATERIALS AND METHODS

Conditional discrimination performance in which rats learn to respond on an appropriate lever, conditional upon specific auditory stimuli, was acquired and later tested under the above drug treatment protocol in extinction.

RESULTS

Conditional discrimination performance was unaffected by bilateral intra-mPFC alpha-flupenthixol at doses of 12, 24 or 36 microg/microl. A dose of D-amphetamine (1.5 mg/kg) shown previously to disrupt conditional discrimination performance was attenuated by direct PFC infusion of alpha-flupenthixol at doses of 24 and 36 but not 12 microg/microl per site.

CONCLUSIONS

These results show that conditional discrimination performance is at least in part mediated by prefrontal DA as local PFC DA antagonism attenuates task performance disruption by the indirect DA agonist d-amphetamine further implicating the role of dysfunctional forebrain DA in cognitive deficits evident in schizophrenia.

Clozapine, SCH 23390 and alpha-flupenthixol but not haloperidol attenuate acute phencyclidine-induced disruption of conditional discrimination performance

RATIONALE

Forebrain dopamine (DA) manipulation has recently been shown to selectively disrupt a conditional discrimination task whose design parameters approximate tasks repeatedly shown to be impaired in schizophrenia.

OBJECTIVE

To investigate the reversal potential of the D(1)/D(2) receptor antagonist alpha-flupenthixol, the selective D(1) antagonist SCH 23390, the typical antipsychotic haloperidol and the atypical antipsychotic clozapine on acute phencyclidine (PCP)-induced disruption of a conditional discrimination task dependent on the ability to use task-setting cues that inform goal-directed performance.

MATERIALS AND METHODS

Rats learned a conditional discrimination task where reinforcement was contingent on an appropriate lever press during a specific auditory stimulus.

RESULTS

PCP disrupted task performance at 1.5 mg/kg, attenuated correct lever pressing at 2.5 mg/kg and abolished overall responding at 5 mg/kg (experiment 1). Pavlovian-instrumental transfer task results (experiment 2) showed that 1.5 and 2.5 mg/kg PCP had no disruptive effects on basic sensory, motor or motivational processes; however, such deficits were evident in 5-mg/kg-treated animals. PCP (1.5 mg/kg) disruption of conditional discrimination was attenuated by acute pretreatment with clozapine, SCH 23390 and alpha-flupenthixol; however, pretreatment with haloperidol did not attenuate task disruption.

CONCLUSION

The predictive validity of the conditional discrimination model is enhanced as the selective task disruption by the preeminent psychotomimetic PCP is reversed by clozapine (known to ameliorate cognitive deficits in schizophrenia) and the role of DA D(1) receptors as mediators of tasks that require conditional relationships is discussed.

Measurement of methylphenidate-induced change in extrastriatal dopamine concentration using [11C]FLB 457 PET

[(11)C]FLB 457 is a very high-affinity radiotracer that allows the measurement of dopamine D(2/3) receptor availability in regions of the brain where densities are very low, such as the cerebral cortex. It is not known if [(11)C]FLB 457 binding is sensitive to the concentration of endogenous dopamine in humans in a manner analogous to [(11)C]raclopride and [(123)I]IBZM in the striatum. To test this possibility, extrastriatal [(11)C]FLB 457 binding was measured at baseline and after the oral administration of 40 to 60 mg of the psychostimulant methylphenidate (MP) in 12 healthy volunteers using positron emission tomography (PET) in a balanced-order, double-blind design. The dynamic PET data were quantified using a two-tissue compartment model with a metabolite-corrected arterial plasma input function. Two volunteers were excluded because of excessive head movement. In the remainder, MP caused significant reductions in the volume of distribution (VD) in temporal and frontal cortical regions and thalamus, suggesting that [(11)C]FLB 457 binding is sensitive to endogenous dopamine concentration. Moreover, the change in [(11)C]FLB 457 binding after MP correlated with the dose of MP (in mg/kg body weight) in all regions assessed. We conclude that MP in doses within the therapeutic range for the treatment of attention deficit hyperactivity disorder causes increases in dopamine concentrations in extrastriatal regions and that [(11)C]FLB 457 PET may be a useful tool for the assessment of change in dopamine concentration in these areas in humans.

Dopamine activates Nrf2-regulated neuroprotective pathways in astrocytes and meningeal cells

The transcription factor Nrf2 controls inducible expression of multiple antioxidant/detoxification genes. We previously found that Nrf2-/- mice have increased sensitivity to in vivo mitochondrial stress and ischemia. Although Nrf2 regulated these forms of neuronal toxicity, it was unclear which injury-triggered signal(s) led to Nrf2 activation in vivo. In this study, we use primary cultures to test the hypothesis that excessive dopamine release can act as an endogenous Nrf2-inducing signal. We cultured two cell types that show increased Nrf2 activity during ischemia in vivo, astrocytes and meningeal cells. Cultures were infected with an adenovirus reporter of Nrf2 transcriptional activity. Dopamine-induced Nrf2 activity in both cell types by generating oxidative stressors, H2O2 and dopamine-quinones. Nrf2 activation in meningeal cells was significantly higher than astrocytes. The effect of dopamine was blocked by antioxidants, and by over-expression of either dominant-negative Nrf2 or Keap1. Nrf2 induction was specific to oxidative stress caused by catecholaminergic neurotransmitters as epinephrine also induced Nrf2, but the monoamine serotonin had no significant effect. These in vitro results suggest Nrf2 activity in astrocytes and meningeal cells link the neurotoxic actions of dopamine to neuroprotective pathways that may potentially modulate ischemic injury and neurodegeneration.

Progression of cellular adaptations in medial prefrontal and orbitofrontal cortex in response to repeated amphetamine

Recent theories on addiction implicate adaptive changes in prefrontal cortex (PFC) neurons in reinforcing and psychotomimetic properties of psychostimulants, yet little is known about how neuronal responses to these drugs change over time. Here we describe electrophysiological evidence for a progressive and sustained change in the response of PFC neurons to amphetamine during repeated exposure. In spontaneously behaving rats and in rats engaged in an instrumental responding task, we followed the activity of medial PFC (mPFC) and orbitofrontal cortex (OFC) neurons during daily exposure to amphetamine and after a post-withdrawal challenge. Repeated amphetamine increased the number of responsive neurons and the magnitude of responses and modified spontaneous burst patterns. These changes were apparent after a few exposures to amphetamine, were amplified after withdrawal, and were region specific in that repeated amphetamine increasingly produced inhibitory responses in mPFC and excitatory responses in OFC. In behaviorally engaged animals, the gradual enhancement in mPFC inhibition and OFC overactivation correlated with a progressive impairment of instrumental responding. Furthermore, these changes were evident predominately in neurons that displayed phasic responses during task-related events. These rapid-onset and sustained cellular adaptations suggest that even limited exposure to psychostimulants may reduce the influence of mPFC neurons on behavior while at the same time exaggerating information encoded by OFC neurons.

Monoamine transporters: from genes to behavior

Modulation of fast neurotransmission by monoamines is critically involved in numerous physiological functions and pathological conditions. Plasma membrane monoamine transporters provide one of the most efficient mechanisms controlling functional extracellular monoamine concentrations. These transporters for dopamine (DAT), serotonin (SERT), and norepinephrine (NET), which are expressed selectively on the corresponding neurons, are established targets of many psychostimulants, antidepressants, and neurotoxins. Recently, genetic animal models with targeted disruption of these transporters have become available. These mice have provided opportunities to investigate the functional importance of transporters in homeostatic control of monoaminergic transmission and to evaluate, in an in vivo model system, their roles in physiology and pathology. The use of these mice as test subjects has been helpful in resolving several important issues on specificity and mechanisms of action of certain pharmacological agents. In the present review, we summarize recent advances in understanding the physiology and pharmacology of monoamine transporters gained in mice with targeted genetic deletion of DAT, SERT, and NET.

Metabotropic glutamate subtype 5 receptors modulate locomotor activity and sensorimotor gating in rodents

Use-dependent N-methyl-d-aspartate receptor (NMDAR) antagonists produce behaviors in human volunteers that resemble schizophrenia and exacerbate those behaviors in schizophrenic patients, suggesting that hypofunction of NMDAR-mediated neuronal circuitry may be involved in the etiology of clinical schizophrenia. Activation of the metabotropic glutamate receptor subtype 5 (mGluR5) enhances NMDAR-mediated currents in vitro. Thus, activation of mGluR5 could potentiate hypofunctional NMDARs in neuronal circuitry relevant to schizophrenia. To further elucidate the role of mGluR5, the present study examined the effects of mGluR5 antagonist administration, with and without coadministration of the use-dependent NMDAR antagonist phencyclidine (PCP), on locomotor activity and prepulse inhibition (PPI) of the acoustic startle response in rodents. We further examined PPI in mGluR5 knockout mice. Finally, we examined PPI after administration of the mGluR5 agonist 2-chloro-5-hydroxyphenylglycine (CHPG) alone and in combination with amphetamine. The data indicate that the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine has no effect on locomotor activity or PPI by itself but does potentiate both PCP-induced locomotor activity and disruption of PPI. We further found that mGluR5 knockout mice display consistent deficits in PPI relative to their wild-type controls. Finally, the data indicate that CHPG has no effect on PPI by itself, but ameliorates amphetamine-induced disruption of PPI. Collectively, these data suggest that mGlu5 receptors play a modulatory role on rodent PPI and locomotor behaviors and are consistent with the hypothesis that mGlu5 agonist/potentiators may represent a novel approach for antipsychotic drug development.

Inhibition of NMDA-induced striatal dopamine release and behavioral activation by the neuroactive steroid 3alpha-hydroxy-5beta-pregnan-20-one hemisuccinate

Our laboratory has previously shown that the synthetic neuroactive steroid 3alpha-hydroxy-5beta-pregnan-20-one hemisuccinate (3alpha5betaHS) is a negative modulator of NMDA receptors in vitro. Similarly, 3alpha5betaHS exhibits rapid sedative, analgesic, anticonvulsive, and neuroprotective effects in vivo. Here we report a study designed to investigate whether a negatively charged neuroactive steroid, 3alpha5betaHS, modulates the action of NMDA receptors in vivo. Our results indicate that peripherally administered 3alpha5betaHS enters the CNS and inhibits NMDA-mediated motor activity and dopamine release in the rat striatum. The increase in motor activity induced by intrastriatal microinjection of NMDA was blocked by the systemic administration of 3alpha5betaHS and the NMDA-induced increase in extracellular dopamine in the striatum was also attenuated by both systemically administered and intrastriatally administered (by in vivo microdialysis) 3alpha5betaHS. These data indicate that 3alpha5betaHS acts through striatal NMDA receptors in vivo. When taken together, these results suggest that neuroactive steroids may prove to be effective in the treatment of neurological and psychiatric disorders involving over-stimulation of NMDA receptors in the mesotelencephalic dopamine system.

Behavioural sensitization and enhanced dopamine response in the nucleus accumbens after intravenous cocaine self-administration in mice

The behavioural effects of cocaine are enhanced in animals with a prior history of repeated cocaine administration. This phenomenon, referred to as sensitization, is also associated with an increase in cocaine-evoked extracellular dopamine levels in the nucleus accumbens. Behavioural and neurochemical sensitization has been demonstrated in rats with a prior history of cocaine self-administration and in those that had received experimenter-administered cocaine. Although it is clear that the repeated non-contingent administration also results in behavioural sensitization in the mouse, the issue of whether behavioural and neurochemical sensitization also occur in this species following intravenous cocaine self-administration has not been assessed. The present study used the technique of in vivo microdialysis in conjunction with operant self-administration to characterize cocaine-evoked locomotor activity and dopamine levels in the nucleus accumbens in mice with a prior history of intravenous cocaine self-administration or those that had received yoked infusions of cocaine. Mice that had received contingent or non-contingent infusions of cocaine exhibited an enhanced behavioural response to cocaine and increased cocaine-evoked dopamine levels in the nucleus accumbens. There was no difference between groups in the magnitude of this effect. Prior exposure to cocaine did not modify baseline dopamine levels in the nucleus accumbens. These data demonstrate that mice with previous cocaine self-administration experience show an enhanced behavioural and dopamine response to cocaine in the nucleus accumbens. Furthermore, control over cocaine infusion does not significantly alter the magnitude of the sensitized behavioural and presynaptic dopamine responses observed in response to a challenge dose of cocaine.

Dopamine in the nucleus accumbens: cellular actions, drug- and behavior-associated fluctuations, and a possible role in an organism's adaptive activity

This review expounds the idea that the analysis of dopamine (DA) action on target cells under behaviorally relevant conditions and behavior-related changes in DA activity can offer new information to clarify the functional significance of mesocorticolimbic DA. In contrast to the traditional association of DA with certain behavioral processes and mechanisms (activation, arousal, conditioning, motivation, reinforcement, sensorimotor integration, etc.), evaluation of DA activity during well-controlled behaviors established by different reinforcers can provide important clues for determining the role of DA in the development and regulation of goal-directed behavior. This review summarizes the results of our microiontophoretic studies of striatal neurons in awake, unrestrained rats, particularly the action of DA on spontaneously active and glutamate (GLU)-stimulated cells, the pattern of DA-GLU interaction, and the role of tonic DA release in regulating the activity and afferent responsiveness of these units. We present the results of our iontophoretic studies of ventral tegmental area (VTA) neurons in freely moving animals suggesting the complexity and limitations in their identification as DA- and non-DA cells under behaviorally relevant conditions. We also consider technical and methodological problems related to electrophysiological and electrochemical evaluation of DA transmission in behaving animals. Finally, we discuss parallels and differences in the activity of presumed DA VTA neurons and changes of nucleus accumbens DA-dependent electrochemical signal during heroin self-administration (SA) behavior.

Regulation of medial prefrontal cortex dopamine by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors

In the medial prefrontal cortex, repeated cocaine produces tolerance of the extracellular dopamine response to subsequent cocaine injection. These studies characterized the influence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors on the medial prefrontal cortex dopamine response to acute cocaine, amphetamine and potassium chloride as a first step to assess whether these receptor subtypes may be candidates for mediating dopamine tolerance after repeated cocaine. Local infusion of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) produced an approximate 40% increase in dopamine levels in the medial prefrontal cortex, while a 30 microM dose did not alter basal levels infused over a 3-h period. Thus, 30 microM CNQX was chosen for the remaining experiments, and was infused for 1 h prior to and during all in vivo treatments. Local medial prefrontal cortex infusion of the 30 microM dose blocked the small increase in dopamine levels elicited by systemic saline injection (maximum of 26%), as well as the much larger increase in response to acute cocaine injection (maximum of 340%). Local infusion of D-amphetamine (3 and 30 microM) through the probe increased dopamine to 300 and 600% of basal levels, respectively. Co-infusion of CNQX partially blocked the response for the first 40 min, but dopamine levels recovered by 60 min later. Local infusion of 100 mM potassium chloride elicited a 600% increase in dopamine levels, which was attenuated approximately 50% by CNQX co-infusion. Potassium-stimulated release of dopamine was also measured in vitro in medial prefrontal cortical and striatal tissue. By 30 s after potassium addition, dopamine levels increased to 800% above baseline in the medial prefrontal cortex, and this increase was blocked by the presence of 30 microM CNQX. In contrast, potassium-stimulated dopamine release in striatal tissue was approximately 250% above basal levels, with no effect of CNQX on dopamine release. Locomotor behavior collected during dialysis experiments demonstrated that increased activity induced by local infusion of potassium chloride was severely attenuated by co-infusion of 30 microM CNQX, while no effects of this drug were found for cocaine-elicited behavior. These results suggest a potent influence of glutamate via alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptors on extracellular dopamine in the medial prefrontal cortex, and these receptors may regulate dopamine release through a presynaptic mechanism. The findings may help elucidate the role of medial prefrontal cortex dopamine-glutamate interactions in drug abuse and stress- and drug-precipitated psychosis.

Effects of catecholamine uptake blockers in the caudate-putamen and subregions of the medial prefrontal cortex of the rat

Altered dopamine regulation in the medial prefrontal cortex has been linked to drug abuse and disorders such as schizophrenia. Heterogeneous expression of the dopamine transporter, as well as the ability of the norepinephrine transporter to clear dopamine in the prefrontal cortex, delineates two potential sites for the regulation of synaptic dopamine within the cortex. The present study used in vivo microdialysis to compare the effects of local infusions of dopamine and norepinephrine uptake blockers in the caudate putamen and two subregions of the prefrontal cortex, the anterior cingulate and prelimbic/infralimbic cortices. Results revealed that all dopamine uptake blockers produced greater increases in dopamine efflux in the caudate-putamen relative to the prefrontal cortex. In addition, amphetamine administration increased dopamine efflux to a greater degree in the prelimbic, relative to the anterior cingulate, cortex. In contrast, the increase in dopamine efflux was similar in both subregions in the presence of nomifensine and desmethylimipramine. Infusions of the selective dopamine uptake blocker GBR 12909 failed to alter dopamine efflux in any prefrontocortical subregion. These data indicate a more prominent role for the dopamine transporter in the clearance of extracellular dopamine in the caudate-putamen relative to the prefrontal cortex and an important role for NET in the clearance of dopamine in both the prelimbic and anterior cingulate subregions of the rat medial prefrontal cortex.

Functional aspects of dopamine metabolism in the putative prefrontal cortex analogue and striatum of pigeons (Columba livia)

Dopamine (DA) in mammalian associative structures, such as the prefrontal cortex (PFC), plays a prominent role in learning and memory processes, and its homeostasis differs from that of DA in the striatum, a sensorimotor region. The neostriatum caudolaterale (NCL) of birds resembles the mammalian PFC according to connectional, electrophysiological, and behavioral data. In the present study, DA regulation in the associative NCL and the striatal lobus parolfactorius (LPO) of pigeons was compared to uncover possible differences corresponding to those between mammalian PFC and striatum. Extracellular levels of DA and its metabolites (homovanillic acid [HVA], dihydroxyphenylacetic acid [DOPAC]) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were investigated by in vivo microdialysis of urethane-anesthetized pigeons under basal conditions and after systemic administration of D-amphetamine. DA was reliably determined only in LPO dialysates, and DA metabolite levels were significantly higher in LPO than in NCL. The HVA/DOPAC ratio, indicating extracellular lifetime of DA, was more than twice as high in NCL than in LPO dialysates. After amphetamine, DA increased in LPO while still being undetectable in NCL, and DA metabolites decreased in both regions. 5-HIAA slightly decreased in NCL dialysates. Amphetamine effects were delayed in NCL compared with the striatum. In conclusion, effects of amphetamine on the pigeon's ascending monoamine systems resemble those found in mammals, suggesting similar regulatory properties. The neurochemical differences between NCL and LPO parallel those between associative regions, such as PFC and dorsal striatum in mammals. They may reflect weaker regulation of extracellular DA, favoring DAergic volume transmission, in associative than striatal forebrain regions.

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.

Open field locomotion and neurotransmission in mice evaluated by principal component factor analysis-effects of housing condition, individual activity disposition and psychotropic drugs

Effects of housing condition and individual disposition on dopaminergically and GABAergically influenced open field locomotion and neurochemistry were studied in mice. Mice characterized as high active (HAM) and low active (LAM) by a running-wheel test were housed in groups or isolated for 1 day, 1 week, 3, 6, 12 or 18 weeks before an open field test was performed with saline, apomorphine (0.75 mg/kg) or diazepam (1.00 mg/kg) administration. Immediately afterwards animals were decapitated and brain sections were frozen for subsequent HPLC-analysis of dopaminergic and serotonergic transmitter metabolism. Principal component factor analysis (PCA) of locomotion variables provided three factors explaining 78.5% of total variance. Variables related to the amount of locomotion loaded highly on Factor 1 (F1-Activity), variables related to place utilization loaded highly on Factor 2 (F2-Exploration) and variables related to immobility and place preference loaded highly on Factor 3 (F3-Irritation). Apomorphine decreased F1-Activity with smaller effects in HAM and without changes in F2-Exploration and F3-Irritation independent on housing conditions. Diazepam exerted a decrease in F2-Exploration with a small increase in FI-Activity and no effects in F3-Irritation. Diazepam induced changes depended on housing conditions and were especially pronounced in isolated HAM. PCA of considerable locomotion and neurochemical data revealed interrelationships between striatal dopamine metabolism and F1-Activity, between cortical dopamine and serotonin metabolism and F2-Exploration as well as between cerebellar, hippocampal and striatal serotonin metabolism and F3-Irritation. The authors concluded that the application of PCA is a useful method to provide functionally relevant characteristics of behaviors and functionally relevant descriptions of interrrelationships between behavior and appropriate central nervous mechanisms. Furthermore the received behavioral characteristics (F1, F2, F3) of open field locomotion were sensitive to reveal housing and drug effects.

Amphetamine increases extracellular concentrations of glutamate in the prefrontal cortex of the awake rat: a microdialysis study

Using microdialysis, the effect was investigated of intracerebral infusions of different doses of amphetamine (1.25, 2.5, 5, 10, and 20 microg/microl) on the extracellular concentrations of glutamate in the medial prefrontal cortex of the rat. Amphetamine produced a dose-related increase in extracellular concentrations of glutamate. At the highest dose, amphetamine increased extracellular glutamate by 445% of baseline as well as extracellular concentrations of taurine, and reduced extracellular concentrations of glutamine. Amphetamine did not modify other amino acids such as arginine. Increases in extracellular concentrations of glutamate and taurine were independent of calcium in the perfusion medium. This is the first study showing that amphetamine produces a calcium-independent increase in extracellular concentrations of glutamate and taurine in the medial prefrontal cortex of the rat.

Re-evaluation of the role of the dopamine transporter in dopamine system homeostasis

Mice with a genetic deletion of the dopamine transporter (DAT) were used to assess its role in the function of dopamine (DA) neurons. Profound alterations in the homeostasis of the nigrostriatal DA system were induced by the absence of the DAT. Extracellular levels of DA were elevated and clearance of released DA was 300-times slower than in control mice. This was accompanied by a 20-fold decrease in tissue DA levels and a paradoxical doubling of the rate of DA synthesis. A crucial role is indicated for the DAT in maintenance of DA neuron presynaptic function, particularly in the control of storage mechanisms.

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.

Benzodiazepine prevention of swim stress-induced sensitization of cortical biogenic amines: an in vivo microdialysis study

In vivo microdialysis was used to determine the effect of diazepam, flumazenil and FG-7142 upon the biogenic amine response to acute and repeated swim stress in the medial prefrontal cortex of the rat. Acute swim stress increased norepinephrine levels, although dopamine and serotonin levels remained stable. Upon re-exposure to swim stress twenty-four hours later, sustained increases (200-300% of baseline) in all three biogenic amines were detected. This enhanced response to re-stress was not seen in rats pretreated with either a benzodiazepine: agonist (diazepam, 2 mg/kg), an antagonist (flumazenil, 10 mg/kg), or an inverse agonist (FG-7142, 10 mg/kg) given prior to the first swim stress. Therefore, the sensitization of biogenic amine response to re-stress may be prevented by compounds which differ in their activity at the benzodiazepine receptor.

Neurochemical and behavioural interactions between ibogaine and nicotine in the rat

1. In vivo brain microdialysis has been employed to investigate the effects of ibogaine on nicotine-induced changes in dopamine overflow in the nucleus accumbens (NAc) of freely moving rats. The effects of the compound on locomotor responses to nicotine and behaviour in the elevated plus-maze were also examined. 2. No changes were observed in the dopamine overflow or the locomotor activity of the animals following the administration of ibogaine (40 mg kg-1, i.p.). However, ibogaine, administered 22 h earlier, significantly (P < 0.01) attenuated the increase in dopamine overflow but not the hyperlocomotion, evoked by nicotine. 3. In the elevated plus-maze test, significant reductions in the open:total runway entries in both saline-treated controls (P < 0.05) and nicotine-treated (P < 0.01) rats were obtained when the animals were tested 22 h after pretreatment with ibogaine (40 mg kg-1, i.p.). The total activity was significantly (P < 0.01) greater in the nicotine-treated rats but this response was not affected by ibogaine pretreatment. 4. Administration of ibogaine was associated with reductions in the tissue levels of 5-hydroxyindoleacetic acid (5-HIAA) in the NAc (P < 0.01) and striatum (P < 0.05) and an increase in the level of this metabolite in the medial prefrontal cortex (mPFC) (P < 0.01) while the levels of dopamine and 5-hydroxytryptamine (5-HT) in the mPFC were reduced (P < 0.05). The DOPAC/dopamine (P < 0.05) and 5-HIAA/5-HT (P < 0.01) ratios were significantly increased in the mPFC for at least 7 days after a single treatment with ibogaine. 5. Ibogaine attenuates the nicotine-induced increases in dopamine overflow in the NAc and may, therefore, inhibit the rewarding effects of this drug. However, the long lasting anxiogenesis induced by ibogaine warrant further investigation before its use could be recommended for smokers.

A central cholinergic link in the cardiovascular effects of the benzodiazepine receptor partial inverse agonist FG 7142

Previous work demonstrated that systemic administration of the benzodiazepine receptor (BZR) partial inverse agonist beta-carboline FG 7142 (FG) augments the cardiovascular response to non-signal stimuli, similar to the effects of an aversive context. Analysis of the parasympathetic and sympathetic contributions to the effects of FG prompted the hypothesis that increases in central cholinergic activity mediates the potentiation of the cardioacceleratory response by FG. Consistent with this hypothesis, the present experiments demonstrate: (a) intracerebroventricular (ICV) infusion of the cholinergic receptor agonist carbachol mimics the response-potentiating effects of FG; (b) this effect of carbachol was blocked by ICV co-administration of the muscarinic antagonist atropine; (c) ICV infusions of atropine blocked the potentiation of the cardioacceleratory response by systemically administered FG, but did not alter the basal response to the stimulus; and (d) 192 IgG-saporin-induced lesions of basal forebrain cholinergic neurons prevented the FG-induced potentiation of the cardioacceleratory response, again without altering the basal cardiac response. These data strongly support the hypothesis that the effects of FG on cardiac reactivity are mediated via an activation of central muscarinic cholinergic mechanisms.

Effects of D-amphetamine and phencyclidine on behavior and extracellular concentrations of neurotensin and dopamine in the ventral striatum and the medial prefrontal cortex of the rat

The effects of systemically administered phencyclidine (PCP; 2.5 mg/kg, s.c.) and D-amphetamine (1.5 mg/kg, s.c.) on the extracellular concentrations of neurotensin-like immunoreactivity (NT-LI) and dopamine (DA) in the ventral striatum (vSTR) and the medial prefrontal cortex (mPFC) were studied in freely moving rats using microdialysis. In separate animals, the effects of PCP and D-amphetamine on open field activity were also analyzed. PCP, but not D-amphetamine, caused a significant increase (156% over baseline) of NT-LI levels in the vSTR which was relatively short lasting, i.e., of less than 2 h duration. In contrast, both drugs significantly increased NT-LI concentrations in the mPFC by almost 100% during the same period. PCP and D-amphetamine also significantly increased extracellular levels of DA in the vSTR by 83 and 364%, respectively. However, the peak effect of PCP on DA appeared later than that of D-amphetamine, i.e., at 150 and 60 min, respectively, after drug administration. Also in the mPFC, both PCP and D-amphetamine significantly increased DA concentrations by 98 and 284%, respectively. Generally, effects on DA levels of both PCP and D-amphetamine were, in contrast to their effects on NT-LI levels, clearly more long-lasting, i.e., of 3-4 h duration. Behaviorally, D-amphetamine produced a more pronounced, general activation than PCP, with a faster onset of activation, i.e. within 30 vs 90 min after administration. However, both drugs produced long-lasting effects on the spatial organization of behavioral activity, which lasted for 3-4 h. In conclusion, the more pronounced behavioral stimulation by D-amphetamine (1.5 mg/kg, s.c.) vs PCP (2.5 mg/kg, s.c.) in the rat may largely be explained by its more potent DA-releasing effect in the brain. Initial behavioral suppression by PCP, e.g., of rearing, as well as its rather poor locomotor stimulant action in general, might relate to release of NT in the vSTR. The long-lasting, behavioral disorganization by both PCP and D-amphetamine may, however, be related to increased release of DA rather than NT in the mesolimbocortical areas.

Dizocilpine (MK-801) elicits a tetrodotoxin-sensitive increase in extracellular release of dopamine in rat medial frontal cortex

We have examined in the rat the effects of a selective non-competitive antagonist for the N-methyl-D-aspartate (NMDA) type excitatory amino acid receptor, dizocilpine (MK-801), on cortical dopamine (DA) metabolism using an in vivo dialysis technique. An acute intraperitoneal injection of MK-801 (0.4-1.25 mg/kg) dramatically increased the concentrations of dopamine, 3,4-dihydroxy-phenylacetic acid and homovanillic acid in the dialysates from the medial frontal cortex in a dose-dependent fashion. Moreover, MK-801 caused a delayed and small augmentation of the cortical extracellular release of 5-hydroxyindoleacetic acid. Continuous infusion of tetrodotoxin into the prefrontal region via the microdialysis tube completely blocked the ability of MK-801 (1.25 mg/kg, intraperitoneally) to augment the extracellular release of DA, its metabolites and the serotonin metabolite in the frontal cortex. The present results suggest that MK-801 facilitates DA release in the medial frontal cortex by increasing impulse flow in the DA neurons projecting to the cortical area adding further support to the view that the NMDA receptor may be involved in the tonic inhibition of frontal cortical DA neurons. It is also proposed that frontal serotonin neurons might be under regulation by excitatory amino acidergic transmission via the NMDA receptor.

Effects of dopamine depletions in the medial prefrontal cortex on active avoidance and escape in the rat

Dopamine systems have been implicated in the performance of avoidance behavior, and the dopaminergic innervation of medial prefrontal cortex is known to be responsive to stressful stimuli. In the present investigation, injections of 6-hydroxydopamine were used to produce moderate depletions of dopamine in the medial prefrontal cortex of rats trained to perform an active avoidance/escape task. In this task, 0.5 mA shock was presented for 5 s every 30 s, and the rat could escape shock presentation, or avoid the shock for 30 s, by pressing a lever. Depletion of dopamine in the medial prefrontal cortex did not affect total number of responses, and did not impair avoidance responding (i.e. responding when the shock was off), and in fact dopamine-depleted animals tended to make slightly more avoidance responses than control animals. Prefrontal dopamine depletions did result in a significant decrease in the number of escape responses (i.e. responding to terminate shock when the shock was on). Moreover, dopamine depletions significantly decreased response efficiency, which is an index of the reduction of shock time produced per lever pressing response. Previous work has indicated that dopamine antagonists and accumbens dopamine depletions have dramatic effects on avoidance behavior; thus, the present results indicate that prefrontal cortex dopamine depletions do not mimic the effects of interference with subcortical dopamine systems. The selective effects of dopamine depletions on escape behavior in the present study suggest that rats with medial prefrontal dopamine depletions have an impairment in the ability to respond appropriately to the direct presentation of footshock.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and neurochemical sensitization following cocaine self-administration

To determine if behavioral and neurochemical sensitization results from cocaine self-administration, rats were trained to self-administer cocaine for 20 consecutive days (26.5 +/- 2.6 mg/kg, IV/day). At 24 h or 21 days after discontinuing cocaine self-administration or yoked saline control, rats were administered an acute injection of saline IP, followed 60 min later by cocaine (15 mg/kg IP). Cocaine-induced changes in motor activity were monitored with a photocell apparatus and alterations in extracellular dopamine in the ventral striatum were measured with microdialysis. There was no difference between treatment groups in the basal level of extracellular dopamine as determined by in vitro calibration. Neither the motor stimulant response nor the increase in extracellular dopamine following an acute cocaine challenge given after 24 h of withdrawal was different between rats which self-administered cocaine and yoked saline controls. However, when the cocaine challenge was given 21 days after discontinuing cocaine self-administration both the motor response and extracellular dopamine content in the ventral straitum were significantly augmented in rats that self-administered cocaine. While no correlation was observed between the average amount of cocaine self-administered each day and the cocaine-induced alterations in extracellular dopamine at either 24 h or 21 days of withdrawl, a significant positive correlation was measured between the increase in photocell counts and the average daily cocaine administration at 21 days of withdrawl. These data show that cocaine self-administration produces an augmentation in the acute behavioral and neurochemical response to a cocaine challenge that resembles the sensitization previously demonstrated with repeated noncontingent administration.

Cardiovascular effects of the benzodiazepine receptor partial inverse agonist FG 7142 in rats

Effects of the benzodiazepine receptor (BZR) partial inverse agonist FG 7142 (FG) on basal and reactive cardiovascular measures were examined in freely moving rats. FG (8 mg/kg) modestly increased basal heart period, but had no effects on basal blood pressure. More notably, however, FG augmented the cardioacceleratory response to an auditory stimulus relative to vehicle controls. Selective blockade of sympathetic (atenolol, 1 mg/kg) or parasympathetic (scopolamine methylnitrate, 0.1 mg/kg) effects on the heart under control conditions revealed that the stimulus-evoked cardiac response originated from a concurrent (reciprocal) sympathetic activation and vagal withdrawal. Following FG pretreatment, both atenolol and scopolamine blocked the cardioacceleratory response to the auditory stimulus. Thus, although FG minimally increased basal heart period, FG significantly enhanced a reactive cardioacceleration. More importantly, these results demonstrate that the cardiovascular effects of BZR inverse agonists are more fully characterized by an assessment of both tonic and reactive cardiovascular responses.

Calcium dependency and tetrodotoxin sensitivity of neostriatal dopamine release in 5-day-old and adult rats as measured by in vivo microdialysis

The calcium dependency and tetrodotoxin sensitivity of extracellular dopamine levels were assessed by microdialysis in the neostriatum of 5-day-old rat pups and were compared with those obtained in adult rats. The removal of calcium from the dialysate reduced spontaneous levels of extracellular dopamine to 20% of normal in the 5-day-old pups and to 10% of normal in the adults. Calcium-free dialysate also decreased potassium-evoked dopamine release to approximately 20% of baseline in both ages. Furthermore, the addition of tetrodotoxin to the dialysate decreased spontaneous levels of extracellular dopamine to 10% of baseline in both ages. The effects of calcium removal and the addition of tetrodotoxin on extracellular levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid were less pronounced. The results of this study demonstrate that extracellular levels of dopamine sampled by microdialysis in rats as young as 5 days of age are both calcium dependent and tetrodotoxin sensitive; thus, they are derived from neuronal activity and not from injury caused by acute implantation of the probe. Other age-related differences support the hypothesis that dopamine release and turnover is greater in immature rats and may represent a form of compensation for incomplete dopamine nerve terminal ingrowth.

Neuronal mechanisms of the attentional dysfunctions in senile dementia and schizophrenia: two sides of the same coin?

Deficits in early stages of information processing, specifically the inability to "disattend" irrelevant stimuli and to selectively allocate processing resources (i.e., hyperattention), have been associated with the development of psychotic symptoms. Opposite deficits, i.e., the failure to attend and select stimuli, and to divide attention (i.e., hypoattention), represent a major variable in the development of dementia. The hypothesis that hyperattention and hypoattention are mediated via cortical cholinergic hyperactivity and hypoactivity, respectively, is discussed. Several lines of evidence support the role of cholinergic hyperactivity in the development of psychotic symptoms, including the therapeutic effects of anticholinergic drugs in schizophrenic patients, the psychotic effects of chronic exposure to irreversible cholinesterase inhibitors, and the worsening of psychotic symptoms as a result of the treatment with cholinomimetic compounds. The potent impairments of attentional abilities as a result of the administration of muscarinic antagonists in intact subjects, and the attentional effects of cholinomimetic compounds in demented patients are two examples of the evidence that supports the role of cholinergic hypofunction in the cognitive impairments of dementia. A neuronal model of dopamine-GABAergic modulation of cortical acetylcholine is proposed on the basis of evidence indicating that nucleus accumbens dopamine, via a GABAergic pathway to the substantia innominata of the basal forebrain, modulates cortical acetylcholine release. The available evidence confirms several predictions derived from this model, including the dopaminergic regulation of cortical acetylcholine (ACh) release, the bidirectional modulation of this release by benzodiazepine receptor (BZR) agonists and inverse agonists, and the antipsychotic effects of BZR agonists. Bidirectional deviations in the activity of cortical cholinergic inputs are hypothesized to represent a major neuronal substrate of the attentional dysfunctions associated with, or even underlying, the development of psychotic symptoms and dementia.

The involvement of nucleus accumbens dopamine in appetitive and aversive motivation

In recent years, considerable emphasis has been placed upon the putative role of nucleus accumbens dopamine systems in appetitive motivation and positive reinforcement. However, considerable evidence indicates that brain dopamine in general, and nucleus accumbens dopamine in particular, is involved in aspects of aversive motivation. Administration of dopamine antagonists or localized interference with nucleus accumbens dopamine systems has been shown to disrupt active avoidance behavior. In addition, accumbens dopamine release and metabolism is activated by a wide variety of stressful conditions. A review of the literature indicates that there are substantial similarities between the characteristics of dopaminergic involvement in appetitive and aversive motivation. There is conflicting evidence about the role of dopamine in emotion, and little evidence to suggest that the profound and consistent changes in instrumental behavior produced by interference with DA systems are due to direct dopaminergic mediation of positive affective responses such as hedonia. It is suggested that nucleus accumbens dopamine is involved in aspects of sensorimotor functions that are involved in both appetitive and aversive motivation.

The development of D2 autoreceptor-mediated modulation of K(+)-evoked dopamine release in the neostriatum

A within-subject dose-response analysis was conducted by locally perfusing increasing concentrations (0.1, 1, 10 and 100 microM) of the selective D2 agonist quinpirole via a microdialysis probe into the neostriatum of urethane-anesthetized rat pups 5, 10-11, 15-16 and 21-22 days of age and adult rats. In Expt. 1, K(+)-evoked dopamine release was significantly decreased by quinpirole relative to the vehicle control group for each age in a dose-dependent manner. The maximum effect of quinpirole was not influenced by acute tolerance or the length of the experiment (Expt. 2). Finally, the effect of quinpirole (10 microM) was blocked by the addition of the selective D2 antagonist (-)-sulpiride (100 microM) to the perfusion solution (Expt. 3). These results support and extend previous research that suggests that presynaptic D2 autoreceptors in the neostriatum are able to modulate K(+)-evoked dopamine release in vivo by postnatal day 5 in the rat.