Expression of the excitatory opsin ChRERα can be traced longitudinally in rat and nonhuman primate brains with PET imaging

Optogenetics is a widely used technology with potential for translational research. A critical component of such applications is the ability to track the location of the transduced opsin in vivo. To address this problem, we engineered an excitatory opsin, ChRERα (hChR2(134R)-V5-ERα-LBD), that could be visualized using positron emission tomography (PET) imaging in a noninvasive, longitudinal, and quantitative manner. ChRERα consists of the prototypical excitatory opsin channelrhodopsin-2 (ChR2) and the ligand-binding domain (LBD) of the human estrogen receptor α (ERα). ChRERα showed conserved ChR2 functionality and high affinity for [18F]16α-fluoroestradiol (FES), an FDA-approved PET radiopharmaceutical. Experiments in rats demonstrated that adeno-associated virus (AAV)-mediated expression of ChRERα enables neural circuit manipulation in vivo and that ChRERα expression could be monitored using FES-PET imaging. In vivo experiments in nonhuman primates (NHPs) confirmed that ChRERα expression could be monitored at the site of AAV injection in the primary motor cortex and in long-range neuronal terminals for up to 80 weeks. The anatomical connectivity map of the primary motor cortex identified by FES-PET imaging of ChRERα expression overlapped with a functional connectivity map identified using resting state fMRI in a separate cohort of NHPs. Overall, our results demonstrate that ChRERα expression can be mapped longitudinally in the mammalian brain using FES-PET imaging and can be used for neural circuit modulation in vivo.

Effects of buprenorphine, methadone, and cariprazine on economic choice between remifentanil and food in squirrel monkeys

We recently reported an economic choice task in which squirrel monkeys chose between differing amounts of remifentanil, a fast-acting opioid, or a food reward to develop a preclinical screen for evaluating potential pharmacotherapies for opioid dependence. Herein, two known opioid addiction treatments are evaluated using this task, as well as a potential new agent, cariprazine, a dopamine D2/D3 receptor partial agonist currently used to treat bipolar disorder and schizophrenia. Preclinical rodent studies suggest this class of compounds may reduce opiate self-administration. Squirrel monkeys were pretreated daily with clinically relevant doses of each compound during the five days of treatment evaluation using the economic choice task. Shifts in drug preference were measured as changes in subjects' indifference values, where the probability of drug and milk choice are equivalent. Buprenorphine produced a significant shift in indifference value between baseline and treatment weeks, indicating a decrease in drug preference. Subjects treated with methadone and cariprazine did not show any significant shift in drug preference. Differences between the buprenorphine and methadone results likely reflect a lack of opioid dependence in the subjects. The cariprazine results suggest that it does not alter opioid reward in non-dependent primates over a five day period.

Reinforcement learning modeling reveals a reward-history-dependent strategy underlying reversal learning in squirrel monkeys

Insight into psychiatric disease and development of therapeutics relies on behavioral tasks that study similar cognitive constructs in multiple species. The reversal learning task is one popular paradigm that probes flexible behavior, aberrations of which are thought to be important in a number of disease states. Despite widespread use, there is a need for a high-throughput primate model that can bridge the genetic, anatomic, and behavioral gap between rodents and humans. Here, we trained squirrel monkeys, a promising preclinical model, on an image-guided deterministic reversal learning task. We found that squirrel monkeys exhibited two key hallmarks of behavior found in other species: integration of reward history over many trials and a side-specific bias. We adapted a reinforcement learning model and demonstrated that it could simulate squirrel monkey-like behavior, capture training-related trajectories, and provide insight into the strategies animals employed. These results validate squirrel monkeys as a model in which to study behavioral flexibility. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Deliberative Decision-Making in Macaques Removes Reward-Driven Response Vigor

Most of our daily decisions are governed by one of two systems: an impulsive system driving instantaneous decisions and a deliberative system driving thoughtful ones. The impulsive system reacts to immediately available concrete rewards. In contrast, the deliberative system reacts to more delayed rewards and/or punishments, which imposes consideration of longer-term choice consequences. Contingency management for addiction treatment is hypothesized to engage deliberative processes. Ultimately, in both decision-making situations, an action is needed to enact the decision. Whether those actions differ in implementation is an open question whose answer could inform as to whether distinct neural systems are engaged. To explore whether there is evidence of separate mechanisms between deliberated and immediate choices, we trained monkeys to perform a decision-making task where they made a choice on a touch screen between two visual cues predicting different amounts of reward. In immediate choice (IC) trials, the cues appeared at the final response locations where subjects could immediately touch the chosen cue. In deliberated choice (DC) trials, compound cues appeared orthogonally to the response locations. After a delay, allowing for decision formation, an identifying cue component was displaced to the randomly assigned response locations, permitting subjects to reach for the chosen cue. Both trial types showed an effect of cue value on cue selection time. However, only IC trials showed an effect of the competing cue on response vigor (measured by movement duration) and a reach trajectory that deviated in the direction of the competing cue, suggesting a decision reexamination process. Reward modulation of response vigor implicates dopaminergic mechanisms. In DC trials, reach trajectories revealed a commitment to the chosen choice target, and reach vigor was not modulated by the value of the competing cue. Our results suggest that choice–action dynamics are shaped by competing offers only during instantaneous, impulsive choice. After a deliberated decision, choice–action dynamics are unaffected by the alternative offer cue, demonstrating a commitment to the choice. The potential relevance to contingency management is discussed.

Strengths and challenges of longitudinal non-human primate neuroimaging

Longitudinal non-human primate neuroimaging has the potential to greatly enhance our understanding of primate brain structure and function. Here we describe its specific strengths, compared to both cross-sectional non-human primate neuroimaging and longitudinal human neuroimaging, but also its associated challenges. We elaborate on factors guiding the use of different analytical tools, subject-specific versus age-specific templates for analyses, and issues related to statistical power.

Translational PET applications for brain circuit mapping with transgenic neuromodulation tools

Transgenic neuromodulation tools have transformed the field of neuroscience over the past two decades by enabling targeted manipulation of neuronal populations and circuits with unprecedented specificity. Chemogenetic and optogenetic neuromodulation systems are among the most widely used and allow targeted control of neuronal activity through the administration of a selective compound or light, respectively. Innovative genetic targeting strategies are utilized to transduce specific cells to express transgenic receptors and opsins capable of manipulating neuronal activity. These allow mapping of neuroanatomical projection sites and link cellular manipulations with brain circuit functions and behavior. As these tools continue to expand knowledge of the nervous system in preclinical models, developing translational applications for human therapies is becoming increasingly possible. However, new strategies for implementing and monitoring transgenic tools are needed for safe and effective use in translational research and potential clinical applications. A major challenge for such applications is the need to track the location and function of chemogenetic receptors and opsins in vivo, and new developments in positron emission tomography (PET) imaging techniques offer promising solutions. The goal of this review is to summarize current research combining transgenic tools with PET for in vivo mapping and manipulation of brain circuits and to propose future directions for translational applications.

Long-Term Cocaine Self-administration Produces Structural Brain Changes That Correlate With Altered Cognition

BACKGROUND

An enduring question from cross-sectional clinical studies is whether the structural and functional differences often observed between cocaine users and healthy control subjects result from a history of drug use or instead reflect preexisting differences. To assess causality from drug exposure, true predrug baseline imaging and neurocognitive assessments are needed.

METHODS

We addressed this fundamental question of causality using longitudinal anatomical magnetic resonance imaging and neurocognitive assessments in rhesus macaques. Cognitive tasks employed were stimulus reversal learning as a measure of cognitive flexibility/inhibitory control and delayed match to sample as a measure of visual working memory. Time points examined were before and following 12 months of chronic cocaine (n = 8) or water (n = 6) self-administration. A magnetic resonance imaging-only time point was also obtained following 2 years of forced abstinence.

RESULTS

We identified localized patterns of gray matter density (GMD) changes that were largely concordant with cross-sectional clinical studies. These included decreases in orbitofrontal cortex, insula, amygdala, and temporal cortex. There was also a prominent increase in GMD in the caudate putamen. GMD decreases were significantly correlated with cognitive impairments across individuals only in select cortical regions. Following abstinence, changes in GMD in some regions, including the orbitofrontal cortex, insula, and amygdala, were persistent and thus may play an important role in risk of relapse following extended abstinence.

CONCLUSIONS

Cocaine use is causal in producing regional changes in GMD, and those changes appear to drive cognitive impairments.

Tonic exploration governs both flexibility and lapses

In many cognitive tasks, lapses (spontaneous errors) are tacitly dismissed as the result of nuisance processes like sensorimotor noise, fatigue, or disengagement. However, some lapses could also be caused by exploratory noise: randomness in behavior that facilitates learning in changing environments. If so, then strategic processes would need only up-regulate (rather than generate) exploration to adapt to a changing environment. This view predicts that more frequent lapses should be associated with greater flexibility because these behaviors share a common cause. Here, we report that when rhesus macaques performed a set-shifting task, lapse rates were negatively correlated with perseverative error frequency across sessions, consistent with a common basis in exploration. The results could not be explained by local failures to learn. Furthermore, chronic exposure to cocaine, which is known to impair cognitive flexibility, did increase perseverative errors, but, surprisingly, also improved overall set-shifting task performance by reducing lapse rates. We reconcile these results with a state-switching model in which cocaine decreases exploration by deepening attractor basins corresponding to rule states. These results support the idea that exploratory noise contributes to lapses, affecting rule-based decision-making even when it has no strategic value, and suggest that one key mechanism for regulating exploration may be the depth of rule states.

Astrocytic Mechanisms Involving Kynurenic Acid Control Δ9-Tetrahydrocannabinol-Induced Increases in Glutamate Release in Brain Reward-Processing Areas

The reinforcing effects of Δ⁹-tetrahydrocannabinol (THC) in rats and monkeys, and the reinforcement-related dopamine-releasing effects of THC in rats, can be attenuated by increasing endogenous levels of kynurenic acid (KYNA) through systemic administration of the kynurenine 3-monooxygenase inhibitor, Ro 61-8048. KYNA is a negative allosteric modulator of α7 nicotinic acetylcholine receptors (α7nAChRs) and is synthesized and released by astroglia, which express functional α7nAChRs and cannabinoid CB1 receptors (CB1Rs). Here, we tested whether these presumed KYNA autoreceptors (α7nAChRs) and CB1Rs regulate glutamate release. We used in vivo microdialysis and electrophysiology in rats, RNAscope in situ hybridization in brain slices, and primary culture of rat cortical astrocytes. Acute systemic administration of THC increased extracellular levels of glutamate in the nucleus accumbens shell (NAcS), ventral tegmental area (VTA), and medial prefrontal cortex (mPFC). THC also reduced extracellular levels of KYNA in the NAcS. These THC effects were prevented by administration of Ro 61-8048 or the CB1R antagonist, rimonabant. THC increased the firing activity of glutamatergic pyramidal neurons projecting from the mPFC to the NAcS or to the VTA in vivo. These effects were averted by pretreatment with Ro 61-8048. In vitro, THC elicited glutamate release from cortical astrocytes (on which we demonstrated co-localization of the CB1Rs and α7nAChR mRNAs), and this effect was prevented by KYNA and rimonabant. These results suggest a key role of astrocytes in interactions between the endocannabinoid system, kynurenine pathway, and glutamatergic neurotransmission, with ramifications for the pathophysiology and treatment of psychiatric and neurodegenerative diseases.

Aged Chinese-origin rhesus macaques infected with SIV develop marked viremia in absence of clinical disease, inflammation or cognitive impairment

Background

Damage to the central nervous system during HIV infection can lead to variable neurobehavioral dysfunction termed HIV-associated neurocognitive disorders (HAND). There is no clear consensus regarding the neuropathological or cellular basis of HAND. We sought to study the potential contribution of aging to the pathogenesis of HAND. Aged (range = 14.7–24.8 year) rhesus macaques of Chinese origin (RM-Ch) (n = 23) were trained to perform cognitive tasks. Macaques were then divided into four groups to assess the impact of SIVmac251 infection (n = 12) and combined antiretroviral therapy (CART) (5 infected; 5 mock-infected) on the execution of these tasks.

Results

Aged SIV-infected RM-Ch demonstrated significant plasma viremia and modest CSF viral loads but showed few clinical signs, no elevations of systemic temperature, and no changes in activity levels, platelet counts or weight. Concentrations of biomarkers of acute and chronic inflammation such as soluble CD14, CXCL10, IL-6 and TNF-α are known to be elevated following SIV infection of young adult macaques of several species, but concentrations of these biomarkers did not shift after SIV infection in aged RM-Ch and remained similar to mock-infected macaques. Neither acute nor chronic SIV infection or CART had a significant impact on accuracy, speed or percent completion in a sensorimotor test.

Conclusions

Viremia in the absence of a chronic elevated inflammatory response seen in some aged RM-Ch is reminiscent of SIV infection in natural disease resistant hosts. The absence of cognitive impairment during SIV infection in aged RM-Ch might be in part attributed to diminishment of some facets of the immunological response. Additional study encompassing species and age differences is necessary to substantiate this hypothesis.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0400-y) contains supplementary material, which is available to authorized users.

Altered activity-based sleep measures in rhesus monkeys following cocaine self-administration and abstinence

BACKGROUND

Impairments in sleep and cognitive function have been observed in patients with substance abuse disorders and may be potential factors contributing to drug relapse. In addition, sleep disruption may itself contribute to cognitive deficits. In the present study we examined the impact of prolonged cocaine self-administration and abstinence on actigraphy-based measures of night-time activity in rhesus macaques as an inferential measure of sleep, and determined whether sleep-efficiency correlated with cognitive impairments in the same subjects on drug free days.

METHODS

Actigraphy data was obtained from a group of rhesus macaques intravenously self-administering cocaine (n=6) and a control group (n=5). Periods were evaluated during which the mean cumulative doses of cocaine were 3.0+0.0 and 4.5+0.2mg/kg/day for 4days (Tuesday-Thursday) each week.

RESULTS

Actigraphy-based sleep efficiency decreased during days of cocaine self-administration in a dose-dependent manner. Consistent with this observation, sleep became more fragmented. Activity-based sleep efficiency normalized during the weekend without cocaine prior to cognitive assessment on Monday. The magnitude of activity-based sleep disruption during self-administration did not correlate with the level of cognitive impairment on drug free days. With continued self-administration, the impact of cocaine on activity-based sleep efficiency declined indicating the development of tolerance.

CONCLUSIONS

Cocaine self-administration disrupted sleep efficiency in rhesus macaques as measured by actigraphy, but normalized quickly in the absence of cocaine. The cognitive impairment observed on drug free days was unlikely to be related to disruption of the nightly activity patterns on days of cocaine self-administration.

Decreased vesicular monoamine transporter type 2 availability in the striatum following chronic cocaine self-administration in nonhuman primates

BACKGROUND

Consistent with postmortem data, in a recent positron emission tomography study, we demonstrated less [(11)C]-(+)-dihydrotetrabenazine ([(11)C]DTBZ) binding to striatal vesicular monoamine transporter type 2 (VMAT2) in cocaine abusers compared with control subjects. A major limitation of these between-group comparison human studies is their inability to establish a causal relationship between cocaine abuse and lower VMAT2. Furthermore, studies in rodents that evaluated VMAT2 binding before and after cocaine self-administration do not support a reduction in VMAT2.

METHODS

To clarify these discrepant VMAT2 findings and attribute VMAT2 reduction to cocaine abuse, we imaged four rhesus monkeys with [(11)C]DTBZ positron emission tomography before and after 16 months of cocaine self-administration. [(11)C]DTBZ binding potential in the striatum was derived using the simplified reference tissue method with the occipital cortex time activity curve as an input function.

RESULTS

Chronic cocaine self-administration led to a significant (25.8 ± 7.8%) reduction in [(11)C]DTBZ binding potential.

CONCLUSIONS

In contrast to the cocaine rodent investigations that do not support alterations in VMAT2, these results in nonhuman primates clearly demonstrated a reduction in VMAT2 binding following prolonged exposure to cocaine. Lower VMAT2 implies that fewer dopamine storage vesicles are available in the presynaptic terminals for release, a likely factor contributing to decreased dopamine transmission in cocaine dependence. Future studies should attempt to clarify the clinical significance of lower VMAT2 in cocaine abusers, for example, its relationship to relapse and vulnerability to mood disorders.

Altered cerebellar and prefrontal cortex function in rhesus monkeys that previously self-administered cocaine

RATIONALE

Differences in brain function in cocaine users can occur even when frank deficits are not apparent, indicating neuroadaptive consequences of use. Using monkeys to investigate altered metabolic activity following chronic cocaine self-administration allows an assessment of altered function due to cocaine use, without confounding pre-existing differences or polysubstance use often present in clinical studies.

OBJECTIVES

To evaluate alterations in metabolic function during a working memory task in the prefrontal cortex and the cerebellum following 1 year of chronic cocaine self-administration followed by a 20 month drug-free period.

METHODS

Fluorodeoxyglucose ((18)F) PET imaging was used to evaluate changes in relative regional metabolic activity associated with a delayed match to sample working memory task. Chronic cocaine animals were compared to a control group, and region of interest analyses focused on the dorsolateral prefrontal cortex (DLPFC) and cerebellum.

RESULTS

Despite no differences in task performance, in the cocaine group, the cerebellum showed greater metabolic activity during the working memory task (relative to the control task) compared to the control group. There was also a trend toward a significant difference between the groups in DLPFC activity (p = 0.054), with the cocaine group exhibiting lower DLPFC metabolic activity during the delay task (relative to the control task) than the control group.

CONCLUSION

The results support clinical indications of increased cerebellar activity associated with chronic cocaine exposure. Consistent with evidence of functional interactions between cerebellum and prefrontal cortex, these changes may serve to compensate for potential impairments in functionality of DLPFC.

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.

Imaging dopamine transmission in the frontal cortex: a simultaneous microdialysis and [11C]FLB 457 PET study

In a recent human positron emission tomography (PET) study we demonstrated the ability to detect amphetamine-induced dopamine (DA) release in the prefrontal cortex as a reduction in the binding of the DA D(2/3) radioligand [(11)C]FLB 457. A key requirement for validating this paradigm for use in clinical studies is demonstrating that the changes in [(11)C]FLB 457 binding observed with PET following amphetamine are related to changes in dialysate DA concentration as measured with microdialysis. Microdialysis and PET experiments were performed to compare, in five rhesus monkeys, amphetamine-induced DA release and [(11)C]FLB 457 displacement in the frontal cortex after three doses of amphetamine (0.3 mg kg(-1), 0.5 mg kg(-1) and 1.0 mg kg(-1)). Amphetamine led to a significant dose-dependent increase in dialysate (0.3 mg kg(-1): 999±287%; 0.5 mg kg(-1): 1320±432%; 1.0 mg kg(-1): 2355±1026%) as measured with microdialysis and decrease in [(11)C]FLB 457 binding potential (BP(ND), 0.3 mg kg(-1): -6±6%; 0.5 mg kg(-1): -16±4%; 1.0 mg kg(-1): -24±2%) as measured with PET. The relationship between amphetamine-induced peak ΔDA and Δ[(11)C]FLB 457 BP(ND) in the frontal cortex was linear. The results of this study clearly demonstrate that the magnitude of dialysate DA release is correlated with the magnitude of the reduction in [(11)C]FLB 457 BP(ND) in the frontal cortex. The use of the [(11)C]FLB 457-amphetamine imaging paradigm in humans should allow for characterization of prefrontal cortical DA release in neuropsychiatric disorders such as schizophrenia and addiction.

Delay- and dose-dependent effects of Δ⁹-tetrahydrocannabinol administration on spatial and object working memory tasks in adolescent rhesus monkeys

Among adolescents, the perception that cannabis can cause harm has decreased and use has increased. However, in rodents, cannabinoid administration during adolescence induces working memory (WM) deficits that are more severe than if the same exposure occurs during adulthood. As both object and spatial WM mature in a protracted manner, although apparently along different trajectories, adolescent cannabis users may be more susceptible to impairments in one type of WM. Here, we evaluate the acute effects of a range of doses (30-240 μg/kg) of intravenous Δ⁹-tetrahydrocannabinol (THC) administration on the performance of spatial and object WM tasks in adolescent rhesus monkeys. Accuracy on the object WM task was not significantly affected by any dose of THC. In contrast, THC administration impaired accuracy on the spatial WM task in a delay- and dose-dependent manner. Importantly, the THC-induced spatial WM deficits were not because of motor or motivational impairments. These data support the idea that immature cognitive functions are more sensitive to the acute effects of THC.

A metabolic biofuel cell: conversion of human leukocyte metabolic activity to electrical currents

An investigation of the electrochemical activity of human white blood cells (WBC) for biofuel cell (BFC) applications is described. WBCs isolated from whole human blood were suspended in PBS and introduced into the anode compartment of a proton exchange membrane (PEM) fuel cell. The cathode compartment contained a 50 mM potassium ferricyanide solution. Average current densities between 0.9 and 1.6 μA cm-2 and open circuit potentials (Voc) between 83 and 102 mV were obtained, which were both higher than control values. Cyclic voltammetry was used to investigate the electrochemical activity of the activated WBCs in an attempt to elucidate the mechanism of electron transfer between the cells and electrode. Voltammograms were obtained for the WBCs, including peripheral blood mononuclear cells (PBMCs - a lymphocyte-monocyte mixture isolated on a Ficoll gradient), a B lymphoblastoid cell line (BLCL), and two leukemia cell lines, namely K562 and Jurkat. An oxidation peak at about 363 mV vs. SCE for the PMA (phorbol ester) activated primary cells, with a notable absence of a reduction peak was observed. Oxidation peaks were not observed for the BLCL, K562 or Jurkat cell lines. HPLC confirmed the release of serotonin (5-HT) from the PMA activated primary cells. It is believed that serotonin, among other biochemical species released by the activated cells, contributes to the observed BFC currents.

Acquisition and baseline performance of working memory tasks by adolescent rhesus monkeys

Adolescence is a transitional stage of development characterized by protracted refinements in the neural circuits required for adult level proficiency of working memory. Because impaired working memory is a hallmark feature of several psychiatric disorders that have their onset during adolescence, model systems that can be used to assess the maturation of working memory function, and of disease-related risk factors that disrupt its development, are of particular importance. However, few studies have investigated the maturation of working memory in nonhuman primates. Thus in the present study, we adapted two working memory tests that are among the most widely used in human and adult nonhuman primates, for adolescent rhesus monkeys. Using a touch-screen apparatus, monkeys were trained on a spatial delayed-response task to assess spatial working memory and a delayed match-to-sample task to assess object working memory. The results indicate that adolescent rhesus monkeys readily and efficiently acquire the ability to perform touch-screen based, complex tests of working memory. These data establish that distinct components of adult prefrontal cortex-dependent cognitive functions can be effectively modeled and evaluated in adolescent monkeys. As such, this approach should be useful for assessing the influence of environmental risk factors on the protracted maturation of working memory in adolescent macaques.

The acute impact of ethanol on cognitive performance in rhesus macaques

Decreased cognitive control over prepotent responses has been hypothesized to contribute to ethanol-induced behavioral disinhibition. However, the effects of ethanol on specific cognitive domains associated with decision making have not been extensively studied. We examined the impact of acute ethanol administration on cognitive performance of nonhuman primates. Studies were conducted using 0.2, 0.5, and 1 g/kg intravenous ethanol in rhesus macaques performing touch screen-based tasks examining stimulus discrimination, stimulus reversal, and stimulus response performance. The impact on attentional processing was also evaluated. Ethanol reduced the accuracy of reversal performance marginally at 0.2 g/kg and significantly at 0.5 g/kg. This effect was selective given an absence of impairment on the stimulus discrimination and stimulus response tasks at these doses. Performance on stimulus discrimination was impaired at 1.0 g/kg, which prevented determination of reversal performance. Analysis of post-error response times demonstrated that error processing was impaired at both 0.2 and 0.5 g/kg. Ethanol also increased the number of omissions and delayed responses on an attentional task, suggesting more frequent attentional lapses. These data demonstrate that cognitive function mediated by specific prefrontal cortical brain regions is particularly sensitive to ethanol and suggest specific cognitive mechanisms that may underlie harmful decisions made at low doses of ethanol.

Cortical and sub-cortical effects in primate models of cocaine use: implications for addiction and the increased risk of psychiatric illness

Drug abuse is a serious risk factor for the incidence and severity of multiple psychiatric illnesses. Understanding the neurobiological consequences of repeated exposure to abused drugs can help to inform how those risks are manifested in terms of specific neurochemical mechanisms and brain networks. This review examines selective studies in non-human primates that employed a cocaine self-administration model. Neurochemical consequences of chronic exposure appear to differ from observations in rodent studies. Whereas chronic intermittent exposure in the rodent is usually associated with a dose-dependent increase in dopaminergic response to a cocaine challenge, in the rhesus monkey, high cumulative exposure was not observed to cause a sensitized dopamine response. These non-human primate observations are concordant with clinical findings in human users. The results of cue exposure studies on dopaminergic transmission are also reviewed. Direct microdialysis measurements indicate that there is not a sustained increase in dopamine associated with cocaine-linked cues. As an alternative to striatal dopaminergic mechanisms mediating cue effects, single unit studies in prefrontal cortex during self-administration in monkeys suggests the orbitofrontal and anterior cingulate cortex are strongly engaged by cocaine cues. Based on the strong clinical imaging literature on cortical and cognitive dysfunction associated with addiction, it is proposed that the strong engagement of cortical systems during repeated cocaine reinforcement results in maladaptive changes that contribute to the risks of drug use for exacerbation of other psychiatric disorders.

A touch screen based Stop Signal Response Task in rhesus monkeys for studying impulsivity associated with chronic cocaine self-administration

Among a range of cognitive deficits, human cocaine addicts display increased impulsivity and decreased performance monitoring. In order to establish an animal model that can be used to study the underlying neurobiology of these deficits associated with addiction, we have developed a touch screen based Stop Signal Response Task for rhesus monkeys. This task is essentially identical to the clinically used Stop Signal Task employed for diagnostic and research purposes. In this task, impulsivity is reflected in the amount of time needed to inhibit a response after it has been initiated, the Stop Signal Response Time (SSRT). Performance monitoring is reflected by the slowing of response times following Stop trials (Post-Stop Slowing, PSS). Herein we report on the task structure, the staged methods for training animals to perform the task, and a comparison of performance values for control and cocaine experienced animals. Relative to controls, monkeys that had self-administered cocaine, followed by 18 months abstinence, displayed increased impulsivity (increased SSRT values), and decreased performance monitoring (decreased PSS values). Our results are consistent with human data, and thereby establish an ideal animal model for studying the etiology and underlying neurobiology of cocaine-induced impulse control and performance monitoring deficits.

Evidence of temporal cortical dysfunction in rhesus monkeys following chronic cocaine self-administration

Cocaine abusers show impaired performance on cognitive tasks that engage prefrontal cortex. These deficits may contribute to impaired control and relapse in abusers. Understanding the neuronal substrates that lead to these deficits requires animal models that are relevant to the human condition. However, to date, models have mostly focused on behaviors mediated by subcortical systems. Here we evaluated the impact of long-term self-administration of cocaine in the rhesus monkey on cognitive performance. Tests included stimulus discrimination (SD)/reversal and delayed alternation tasks. The chronic cocaine animals showed marked deficits in ability to organize their behavior for maximal reward. This was demonstrated by an increased time needed to acquire SDs. Deficits were also indicated by an increased time to initially learn the delayed alternation task, and to adapt strategies for bypassing a reliance on working memory to respond accurately. Working memory per se (delay dependent performance) was not affected by chronic self-administration. This pattern of cognitive deficits suggests dysfunction that extends beyond localized prefrontal cortical areas. In particular, it appears that temporal cortical function is also compromised. This agrees with other recent clinical and preclinical findings, and suggests further study into addiction related dysfunction across more widespread cortical networks is warranted.

Cocaine sensitization and dopamine mediation of cue effects in rodents, monkeys, and humans: areas of agreement, disagreement, and implications for addiction

BACKGROUND

Sensitization of mesocorticolimbic dopamine projections has been a valuable model of neurobiological adaptation to chronic exposure to cocaine and other psychostimulants.

DISCUSSIONS

In addition to providing an explanation of exaggerated responses to drugs that might explain their increased ability to serve as reinforcers, sensitization has also been incorporated into influential theories of how drug associated cues can acquire increased salience and incentive motivation. However, almost all of the work exploring behavioral and neurochemical sensitization has been conducted in rodents. Importantly, the relatively small amount of work conducted in human and nonhuman primates differs from the rodent work in some important regards. This review will examine areas of convergence and divergence between the rodent and primate literature on sensitization and the ability of drug associated environmental cues to elicit dopamine release. The implications of this comparison for expanding addiction research beyond dopaminergic mechanisms in the striatum/nucleus accumbens will be considered.

Dopaminergic responses to self-administered cocaine in Rhesus monkeys do not sensitize following high cumulative intake

Sensitization of mesolimbic dopamine (DA) systems by administration of psychostimulants has been observed repeatedly in rodents. This phenomenon has been incorporated into theories of neurobiological adaptation underlying addiction, and is believed to be a mechanism whereby drug-associated cues acquire the ability to control behaviour via a conditioned release of DA. However, we have previously demonstrated in nonhuman primates that drug cues that cause cocaine seeking do not promote a conditioned increase in DA release of sufficient endurance to be measured in 2-min samples. In addition, imaging studies in humans and nonhuman primates that have been chronically exposed to psychostimulants have not demonstrated an increase in DA release upon psychostimulant challenge. Here we report that following 32 weeks of self-administration by rhesus monkeys, no increase over time in the DA response to self-administered cocaine was observed in any striatal subregion or individual animal. These results are consistent with clinical imaging studies showing a lack of DA sensitization, and might provide a mechanism to explain our previous observation that the rodent and primate differ in neurochemical response to drug-associated cues.

Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

The ability of environmental cues associated with cocaine availability to cause relapse may result from conditioned activation of dopamine (DA) release. We examined this hypothesis in macaque monkeys by conducting microdialysis studies in animals during exposure to a cocaine predictive compound cue. In addition to studying DA release in mesolimbic and sensorimotor striatum, both DA and serotonin levels were determined in the prefrontal cortex (medial orbitofrontal and anterior cingulate). The compound cue employed visual, auditory, and olfactory components, and was salient to the animals as demonstrated by anticipatory lever pressing in the absence of cocaine. During a 10-min period of exposure prior to cocaine availability, there was no significant increase in striatal or cortical DA. The addition of a DA uptake inhibitor to the striatal perfusate to reduce the potential interference of neuronal uptake did not alter the results. In contrast to the lack of any change in striatal DA, a significant decrease in extracellular serotonin in the prefrontal cortex during the 10 min of cue exposure was observed.

Local anesthetic effects of cocaethylene and isopropylcocaine on rat peripheral nerves

Cocaethylene is a naturally occurring cocaine derivative that has been used as a tool in both clinical studies of cocaine reward and as a potential model compound for agonist substitution therapy in cocaine dependence. It is equipotent to cocaine at inhibiting dopamine uptake in-vitro and in-vivo. Because it has been reported that local anesthetic properties may influence the reinforcing effects of dopamine uptake inhibitors, we investigated the local anesthetic properties of cocaethylene as well as isopropylcocaine, another potential pharmacological tool in studies of cocaine reward and agonist substitution therapy. We compared the efficacy of nerve impulse blockade by lidocaine, cocaine, cocaethylene and isopropylcocaine using rat sciatic nerves and dorsal roots (DRs). Nerves were placed in a modified sucrose gap chamber and repetitively stimulated at high frequency. The amplitude of compound action potentials (CAPs) at the beginning and end of each stimulus train was measured before and after exposure to each compound. All compounds produced concentration-dependent and use-dependent decrements in CAP amplitude, but cocaethylene and isopropylcocaine at medium to high concentration (0.375-1.875 mM) showed a more prolonged block after washout relative to cocaine or lidocaine. Patch clamp studies on dorsal root ganglion (DRG) neurons indicated a use-dependent blockade of sodium channels. These studies provide a more complete understanding of the pharmaocology of potential agonist treatment candidates, and suggest a mechanism whereby cocaethylene produces a decreased euphoria in humans compared to cocaine.

Dose-dependent effect of ethanol on extracellular dopamine in mesolimbic striatum of awake rhesus monkeys: comparison with cocaine across individuals

RATIONALE

Dependence on both alcohol and cocaine is a widespread example of polydrug abuse/dependence. It has been hypothesized that ethanol reward is mediated via increased dopaminergic neurotransmission in mesolimbic striatum, as is the case for cocaine. However, little is known about the neurobiology of ethanol in primates, or how the effects of ethanol compare to those of cocaine across individual animals.

OBJECTIVES

To determine in animals with a history of cocaine exposure whether there is a dopaminergic impact of ethanol in non-human primates, and if so, whether the magnitude of that effect correlates with the dopaminergic effect of cocaine across individuals.

METHODS

Microdialysis studies were conducted in rhesus monkeys previously trained to self-administer cocaine. The dopaminergic impact of cocaine had been determined in those animals during cocaine self-administration sessions. Probes were placed in the ventral mesolimbic and associational central striatum. Ethanol was administered non-contingently by a slow intravenous infusion at doses of 0.5 g/kg (administered over 10 min) and 1.0 g/kg (administered over 20 min).

RESULTS

The mean dopaminergic response to ethanol in four animals (with 2-4 trials in each animal at each dose) indicated a small but significant increase in extracellular dopamine at each dose (12% above baseline at 0.5 g/kg, 22% above baseline at 1.0 g/kg). Examining the responses across individual animals indicated substantial variability, in that two of the four animals showed no increase at either dose. Across individuals, regression analysis of cocaine-induced changes in dopamine with 1.0 g/kg ethanol-induced changes indicated a positive correlation between the drug effects, with a trend in this direction observed with the 0.5-g/kg dose of ethanol.

CONCLUSIONS

These results provide support for the ability of ethanol to elevate extracellular dopamine in the mesolimbic striatum, though with a modest effect size and variability among individuals. Further, they suggest that some common mechanism influences the effects of ethanol and cocaine on dopaminergic output despite seemingly unrelated pharmacological mechanisms of action.

Dynamics of extracellular dopamine in the acute and chronic actions of cocaine

Cocaine amplifies dopaminergic neurotransmission via blockade of presynaptic neuronal uptake. This action is believed to be a crucial component of cocaine's ability to exert its reinforcing effects. This review will provide a brief overview of extracellular dopamine dynamics associated with cocaine. The acute effects of cocaine reviewed include comparison of intravenous and intraperitoneal routes of administration to better understand how fast and slow routes (e.g., crack and intranasal) differ in their pharmacokinetics and neurochemical effects and how those differences relate to differences in abuse potential. Changes in the acute effects of cocaine within a session have been examined in neurochemical studies of acute tolerance to self-administered cocaine in rhesus monkeys, and the potential impact of that tolerance to patterns of use is discussed. Between-session sensitization of the dopaminergic response to cocaine is reviewed, and data indicating this also occurs in primates have been obtained in self-administering rhesus monkeys, demonstrating neurochemical sensitization in a primate species. The important question of whether cocaine-associated environmental cues elicit conditioned increases in dopamine release has also been examined in the rhesus monkey, with results indicating that, unlike rats, nonhuman primates do not show conditioned increases in dopamine release.

NMDA antagonist effects on striatal dopamine release: microdialysis studies in awake monkeys

Brain imaging studies have suggested that the NMDA antagonist ketamine is as potent a releaser of striatal dopamine as amphetamine. This conclusion contradicts microdialysis findings in the rodent that NMDA antagonists, in contrast to amphetamine, have little or no effect on striatal dopamine release. The present study addressed two mechanisms that could account for this discrepancy: 1) whether there is a species difference, i.e., rodents vs. primates, in the responsivity of striatal dopamine to NMDA antagonists, and 2) whether rapid uptake of dopamine prevents reliable measures of synaptic dopamine release by microdialysis in response to NMDA antagonists. MRI-directed in vivo microdialysis was used to compare the effects of psychotomimetic NMDA antagonists phencyclidine (PCP), ketamine, and amphetamine on extracellular striatal dopamine levels in awake rhesus monkeys. The effect of PCP was also investigated in the presence of intrastriatally applied nomifensine, a dopamine uptake blocker. Amphetamine (0.1 or 0.4 mg/kg) produced robust and dose-dependent increases in dopamine release ranging 2-10-fold above baseline. PCP at 0.1 mg/kg had no effect and at 0.3 mg/kg produced a small 50% increase over baseline. Ketamine, at the relatively high dose of 5 mg/kg, produced only a 30% increase in dopamine release. Intrastriatal application of nomifensine did not influence the effect of PCP, suggesting that rapid uptake of dopamine is not preventing the detection of a PCP-induced increase in dopamine release. These findings suggest that in the primate, ketamine and PCP are not effective dopamine releasers, as has been suggested by previous imaging studies.

Applications of microdialysis methodology in nonhuman primates: practice and rationale

This review provides a comprehensive summary of the use of microdialysis procedures in nonhuman primates. Using these methods in primates is feasible, including both the anesthetized and awake preparations. The latter permit neurochemical sampling during complex information processing and behavioral performance. The close homology in structure and function between human and nonhuman primate brain makes using nonhuman primates particularly appealing, and the literature suggests that their use has provided instances of unique insight into brain structure and function. The continued use of these methods in areas such as the neurobiology of addiction, affective disorders, and psychosis will help in our ever-increasing understanding of the complex pathophysiologies of these disorders.

Acute and chronic dopamine dynamics in a nonhuman primate model of recreational cocaine use

Using a model of recreational cocaine consumption, we have determined in four rhesus monkeys the impact of self-administered cocaine on mesolimbic and sensorimotor striatal dopaminergic neurotransmission. The effects of cocaine repeated within a self-administration session and across multiple sessions over a 6 month period were determined by the use of fixed-ratio self-administration and microdialysis procedures. The exposure to cocaine was modest, with at most two 0.5 mg/kg infusions permitted in each weekly session. Within a cocaine self-administration session, acute tolerance to the ability of cocaine to elevate extracellular striatal dopamine was observed. Over a period of 6 months of repeated self-administration, there was a significant increase in the impact of a fixed dose on extracellular dopamine, indicating that neurochemical sensitization to the effects of self-administered cocaine occurs in primates. A pronounced dopaminergic response to noncontingent cocaine was also observed, with no increases in extracellular dopamine in response to an unexpected saline substitution, indicating that the neurochemical response to self-administered cocaine is primarily caused by direct pharmacological effects of the drug rather than by conditioning to external environmental cues. These results highlight the contrast in time-dependent changes in neurochemical responsiveness to cocaine, depending on whether within-session or between-session comparisons are made. They also demonstrate that recreational levels of cocaine consumption can result in neurochemical sensitization, an enduring change in brain function that may contribute to addiction.

Impact of self-administered cocaine and cocaine cues on extracellular dopamine in mesolimbic and sensorimotor striatum in rhesus monkeys

Studies were conducted to determine the impact of self-administered cocaine on extracellular striatal dopamine in four rhesus monkeys. The extent to which external cue conditioning contributed to the effects of cocaine and whether there is activation of striatal dopaminergic neurotransmission during drug-seeking behavior was also examined. Microdialysis measurements were made at 2 min intervals in sensorimotor (dorsolateral) and mesolimbic (central and ventromedial) striatum. A fixed-ratio schedule of reinforcement was used, with cocaine availability signaled by a visual cue. Studies examined the effects of cocaine or cocaine cues against a drug-free baseline. Large (fivefold to eightfold) increases in extracellular dopamine after a self-administered infusion of 0.5 mg/kg cocaine were quite rapid and matched the time course of reported subjective effects in human laboratory studies. To determine if conditioning to external cues contributed to the cocaine-induced increases, saline was substituted for cocaine in the infusion, leaving all other visual and auditory stimuli unchanged. No increase in extracellular dopamine in either sensorimotor or mesolimbic striatal subdivisions was observed. Extracellular dopamine during extended periods of drug-seeking behavior triggered by a visual cue was determined in both central and ventromedial striatum. This procedure also did not result in any measurable changes in extracellular dopamine. These studies demonstrate rapid and pronounced pharmacological actions of self-administered cocaine. No apparent conditioned component of those actions was associated with external environmental cues, suggesting that cues that trigger drug-seeking behavior in nonhuman primates do not cause conditioned increases in mesolimbic striatal dopamine.

Rapid induction of behavioral and neurochemical tolerance to cocaethylene, a model compound for agonist therapy of cocaine dependence

RATIONALE

Tolerance to abused drugs may impact on patterns of abuse, and in the case of agonist therapies, may be beneficial in that it reduces the reward value of a given dose of abused drug. Cocaethylene, a psychoactive metabolite resulting from concurrent alcohol and cocaine consumption, was examined because of its use in human research studies of drug reward mechanisms, and its potential as a model compound for an agonist based therapy for cocaine dependence.

OBJECTIVE

Comparisons were made between cocaine and cocaethylene in the acute development of tolerance to the neurochemical and behavioral effects of cocaine. With chronic exposure, tolerance to the behavioral effects of cocaine was examined.

METHODS

In awake rats with a microdialysis probe in the nucleus accumbens and a jugular catheter, an IV bolus/3-h infusion of cocaine or cocaethylene and a subsequent cocaine challenge was administered while extracellular dopamine and locomotion were monitored. Chronic IV treatment with cocaine, cocaethylene, and a water control was accomplished for 7 days using osmotic minipumps attached to jugular catheters. Animals were then challenged with an IV bolus of cocaine.

RESULTS

With acute treatment, the IV bolus of cocaethylene at the beginning of the infusion period resulted in an initial behavioral activation equivalent to that caused by cocaine, after which there was a striking difference in that the cocaethylene group displayed a return to predrug levels of activity, while the cocaine group showed high levels of activity throughout the 3-h period. Both cocaethylene and cocaine resulted in an initial increase in the extracellular concentration of dopamine. However, after that initial increase, levels of dopamine dropped in the cocaethylene group while the cocaine group levels remained elevated. A 1-week infusion of cocaine or cocaethylene resulted in tolerance to the behavioral activating effects of a subsequent cocaine challenge.

CONCLUSIONS

These results demonstrate a rapid induction of tolerance to the behavioral and neurochemical properties of cocaethylene, resulting in a diminished behavioral response to a cocaine challenge both acutely, and after 7 days. The relevance of these data for the use of cocaethylene as a model compound for an agonist approach to therapy for cocaine dependence is discussed.

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

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

Effects of repeated amphetamine treatment on regional GABAA receptor binding

The present study examined the effects of repeated exposure to amphetamine on GABAA receptor binding in cortical and subcortical areas. The goal of the study was to determine whether changes in specific binding were related to behavioral sensitization. Animals were exposed to either saline (0.3 ml, s.c.; n=12) or d-amphetamine (2.5 mg/kg, s.c.; n=12) for 6 consecutive days and sacrificed after a 14-day withdrawal period. Differences in GABAA receptor binding in these two groups of animals were assessed using the GABAA receptor antagonist [3H]SR 95531. To verify that the preceding treatment regimen led to the development of behavioral sensitization, a separate set of animals (n=8/group) was exposed to the same regimen and challenged with d-amphetamine (2.5 mg/kg, s.c.) after the 14-day withdrawal period. As expected, preexposure to amphetamine led to the development of amphetamine sensitization. There were no differences in GABAA receptor binding in animals preexposed to saline and amphetamine in the prefrontal cortex, caudate-putamen, hypothalamus, or cerebellum. These findings do not provide support for the idea that changes in GABAA receptor binding in the medial prefrontal cortex or various subcortical areas are related to the development of behavioral sensitization.

Microdialysis and SPECT measurements of amphetamine-induced dopamine release in nonhuman primates

The competition between endogenous transmitters and radiolabeled ligands for in vivo binding to neuroreceptors might provide a method to measure endogenous transmitter release in the living human brain with noninvasive techniques such as positron emission tomography (PET) or single photon emission computerized tomography (SPECT). In this study, we validated the measure of amphetamine-induced dopamine release with SPECT in nonhuman primates. Microdialysis experiments were conducted to establish the dose-response curve of amphetamine-induced dopamine release and to document how pretreatment with the dopamine depleter alpha-methyl-para-tyrosine (alpha MPT) affects this response. SPECT experiments were performed with two iodinated benzamides, [123I]IBZM and [123I]IBF, under sustained equilibrium condition. Both radio-tracers are specific D2 antagonists, but the affinity of [123I]IBZM (KD-0.4 nM) is lower than that of [123I]IBF (KD 0.1 nM). With both tracers, we observed a prolonged reduction in binding to D2 receptors following amphetamine injection. [123I]IBZM binding to D2 receptors was more affected than [123I]IBF by high doses of amphetamine, indicating that a lower affinity increases the vulnerability of a tracer to endogenous competition. With [123I]IBZM, we observed an excellent correlation between reduction of D2 receptor binding measured with SPECT and peak dopamine release measured with microdialysis after various doses of amphetamine. Pretreatment with alpha MPT significantly reduced the effect of amphetamine on [123I]IBZM binding to D2 receptors, confirming that this effect was mediated by intrasynaptic dopamine release. Together, these results validate the use of this SPECT paradigm as a noninvasive measurement of intrasynaptic dopamine release in the living brain.

Novelty-associated locomotion: correlation with cortical and sub-cortical GABAA receptor binding

The present study was designed to determine whether variability in GABA (eta-aminobutyric acid)A receptor binding in cortical and subcortical brain regions was correlated with locomotor activity in a novel environment. Twenty four animals were rated for locomotor activity in a novel circular runway. Eight days later, locomotor activity was assessed following 1.5 mg/kg amphetamine sulfate (i.p.). After four to six days, animals were killed and samples were pooled in groups of four animals ranked according to novely locomotor score, and specific binding of the GABAA receptor antagonist [2-(3'-carboxy-2'-propyl)-3-amino-6-p-methoxy phenylpyridazinium bromide] ([3H]SR95531) was determined. Significant negative correlations were seen between specific ([3H]SR95531) binding and novelty induced locomotion in the cingulate and prefrontal cortices, and in the ventral pallidum. A near-significant negative correlation was seen in the striatum. Correlation coefficients between locomotion scores in the novel environment and specific [3H]SR95531 binding were: cingulate cortex, R = -0.91, P = 0.012; prefrontal cortex, R = -0.85, P = 0.032; ventral pallidum, R = -0.85, P = 0.030; striatum, R = -0.73, P = 0.097; and nucleus accumbens, R = -0.09, P = 0.85. The positive correlation between novelty- and amphetamine-induced locomotion was also quite high (R = 0.95, P = 0.004). These results are discussed in terms of their relevance to potential biochemical correlates of drug abuse vulnerability.

Alcohol plus cocaine: the whole is more than the sum of its parts

Cocaethylene, an active metabolite that arises through hepatic transesterification of cocaine when cocaine and ethanol are used together, shares many neurochemical and pharmacological properties with cocaine. Cocaethylene is similar to cocaine in its properties as an indirect dopamine agonist, and human subjects cannot distinguish its effects from those of cocaine. Cocaethylene, and especially its isopropyl analog, are more selective indirect dopamine agonists than cocaine, with relatively weak potency at the serotonin transporter. Cocaethylene may contribute to the manifestations and consequences of combined cocaine and ethanol use, although its relative importance remains unclear.

Effect of ethanol on extracellular 5-HT and glutamate in the nucleus accumbens and prefrontal cortex: comparison between the Lewis and Fischer 344 rat strains

The present study investigated the impact of systemic (i.p.) ethanol administration on extracellular levels of serotonin and glutamate in the prefrontal cortex and the nucleus accumbens in Lewis and Fischer 344 rat strains using in vivo microdialysis. At 1.0 g/kg, ethanol elicited a significant increase in nucleus accumbens-dialysate levels of both 5-HT (44% +/- 16, P = 0.002) and glutmate (90% +/- 43, P = 0.009) in Lewis rats. In Fischer rats, there was no increase in 5-HT (6% +/- 7: P = 0.5), and a trend toward an increase in glutamate (88% +/- 46: P = 0.1). The 0.5 and 2.0 g/kg doses did not result in any significant change in extracellular 5-HT or glutamate in the nucleus accumbens or prefrontal cortex of either strain. The basal levels of glutamate, in both brain regions, were significantly lower in Lewis than in Fischer 344 rats. The basal levels of 5-HT were also lower in the nucleus accumbens of Lewis rats. These findings suggest that enhanced sensitivity of the mesoaccumbens 5-HT or glutamate systems to ethanol and/or inherent low basal levels of 5-HT or glutamate activity may be associated with the predisposition to alcohol-drinking behavior seen in Lewis rats.

Serotonin-mediated increase in prefrontal cortex dopamine release: pharmacological characterization

Interactions between serotonin (5-HT) and dopamine (DA) neuronal systems in the prefrontal cortex (PFC) may be important in the pathophysiology of cognitive disorders such as schizophrenia. We have examined the effect of 5-HT, applied locally through a microdialysis probe, on extracellular DA in the PFC, and compared the response to that observed in the striatum. 5-HT in concentrations of 1 to 10 microM increased extracellular DA dose-dependently to a greater extent in the PFC than in the striatum. The PFC response was pharmacologically characterized to determine the 5-HT receptor subtype mediating the increase in DA levels. The coperfusion of selective 5-HT2A and 5-HT3 antagonists MDL 100,907 ((R-(+)-(2,3-dimethoxyphenyl)-1-[2(4-flourophenylethyl)]-4- piperidine-methanol) and MDL 72222 (3-tropanyl-3,5-dichlorobenzoate), respectively, with 5-HT failed to significantly attenuate the 5-HT induced increase of extracellular DA. Furthermore, the local application of the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl aminopropane did not yield an increase in extracellular DA. On the other hand, coperfusion of the selective 5-HT1B/1D antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1 ,2,4-oxadiazol-3-yl)-[1,1-biphenyl]-4-carboxamide)) with 5-HT completely blocked the effect of 5-HT alone. Infusion of the selective 5-HT1B agonists CP 93,129 (3-(1,2,5,6-tetrahydro-4-pyridyl)pyrrolo[3,2-b]pyrid-5-one) and CP 94,253 (3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrolo[3,2-b]pyridine) resulted in a significant increase in extracellular DA and the effect of CP 93,129 was attenuated by coperfusion of GR 127935. The results obtained demonstrate a functional interaction between DA and 5-HT pathways in the PFC, with evidence of potential mediation by the 5-HT1B receptor subtype.

Effect of ethanol on extracellular dopamine in nucleus accumbens: comparison between Lewis and Fischer 344 rat strains

The purpose of this study was to examine the differential effects of intraperitoneal ethanol on the mesoaccumbens dopamine (DA) system in Fischer 344 and Lewis rat strains, utilizing microdialysis in awake animals. At the lowest dose tested (0.5 g/kg), there were no changes in extracellular DA in the nucleus accumbens in either strain. There was a differential response to the intermediate dose of 1 g/kg ethanol, with an 84% increase in extracellular DA in the Fischer, but no change in Lewis rats. The highest dose administered (2 g/kg) did not induce significant increases in DA in either strain. These data demonstrate that the mesoaccumbens DA systems of Fischer and Lewis rat strains differ in their susceptibility to activation by ethanol, and suggest that the higher alcohol preference of Lewis rats is not associated with an enhanced DAergic response to acute experimental administration of ethanol.

The role of serotonin in the actions of psychostimulants: molecular and pharmacological analyses

Cocaine is a highly abused psychostimulant which is a local anesthetic and inhibitor of the reuptake of dopamine (DA), serotonin (5-HT) and norepinephrine (NE). This manuscript details a brief summary and the primary conclusions of several presentations geared to present recent pharmacological analyses of the interaction of cocaine with 5-HT systems. These data illustrate the complexity of actions for cocaine in the brain and emphasize that, to fully understand the mechanisms which underlie its potent behavioural effects, the impact of this drug on 5-HT function as well as the interactions between 5-HT and the function of DA mesolimbic pathways must be considered.

Cocaine and cocaethylene: effects on extracellular dopamine in the primate

Cocaine and cocaethylene (a psychoactive metabolite of concurrent cocaine and ethanol consumption) were studied in the anesthetized vervet monkey. The ability of each to elevate extracellular DA in the caudate nucleus was assessed using microdialysis probes acutely lowered through chronic guide cannulae. Blood samples were also collected to determine plasma levels of the two drugs. Doses of 1.5 mumol/kg cocaine (equivalent to 0.5 mg/kg cocaine-HCl) and cocaethylene were administered intravenously. Microdialysis and blood samples were collected at 5-min intervals immediately following drug administration. Both drugs caused a maximal four-fold increase in extracellular DA during the 5- to 10-min period following drug administration. This is the first report of cocaine (and cocaethylene) induced alterations in extracellular DA in primates. The abilities of cocaine and cocaethylene to produce euphoria are being compared in ongoing clinical research studies. The potential use of these results for interpreting the neurochemical basis of any differences in those studies is discussed.

Excitatory amino acid antagonists attenuate the effects of cocaine on extracellular dopamine in the nucleus accumbens

The effect of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate excitatory amino acid antagonists on systemic cocaine-induced increases in extracellular dopamine was examined. The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor subtype antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovalerate and dizocilpine, were infused via a microdialysis probe placed in the nucleus accumbens. The local infusion of 2-amino-5-phosphonovalerate (500 microM), dizocilpine (50 microM) and CNQX (100 microM), started 80 min before cocaine injection, significantly inhibited the cocaine-induced increase in extracellular dopamine. The CNQX blockade was dose-dependent with respect to both CNQX concentration infused and dose of cocaine administered. Simultaneous infusion of the two antagonists (500 microM 2-amino-5-phosphonovalerate and 100 microM CNQX) did not lead to further reductions in the effects of cocaine when compared to either antagonist alone. Our results suggest that both N-methyl-Daspartate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate subtypes of excitatory amino acid antagonist receptors may contribute to the stimulatory effect of cocaine on extracellular dopamine in the nucleus accumbens.

Differences in bioavailability between cocaine and cocaethylene and their implications for drug-reward studies

Cocaethylene, a psychoactive metabolite resulting from combined ethanol/cocaine consumption, is of interest because its psychostimulant properties may partially underlie combined cocaine/ethanol use, and because it has the potential for use as a probe of drug reward mechanisms due to its enhanced selectivity at monoamine uptake sites compared to cocaine. To determine the relative systemic bioavailabilities of cocaine and cocaethylene, sequential plasma samples were obtained from awake rats following drug administration. Following intravenous administration of 3 mumol/kg (molar equivalent of 1 mg/kg cocaine-HCl), both drugs achieved similar time courses and areas under the plasma concentration versus time curve. In contrast, intraperitoneal administration of 44 mumol/kg (molar equivalent of 15 mg/kg cocaine HCl) showed peak plasma levels, and the area under the plasma concentration vs time curve for cocaine to be approximately twice that for cocaethylene. Comparison of dose corrected areas under the curve of the two routes of administration for each drug indicated that relative systemic bioavailability of cocaethylene following intraperitoneal administration is only 58% that of cocaine. In addition, the elimination of both cocaine and cocaethylene was found to be slower following intraperitoneal administration compared to the intravenous route. The implications of these results are discussed with respect to the relative potency of these two compounds, as inferred from behavioral, drug reward, and lethality studies. Also, the differences noted will need to be taken into account when making mechanistic interpretations from comparative drug reward studies.