Decrease in basal dopamine levels in the nucleus accumbens shell during daily drug-seeking behaviour in rats

Nucleus accumbens sub-regions experience distinct dopamine release responses following acute and chronic morphine exposure

It is well established that dopamine neurons of the ventral tegmental area (VTA) play a critical role in reward and aversion as well as pathologies including drug dependence and addiction. The distinct effects of acute and chronic opioid exposure have been previously characterized at VTA synapses. Recent work suggests that distinct VTA projections that target the medial and lateral shell of the nucleus accumbens (NAc), may play opposing roles in modulating behavior. It is possible that these two anatomically and functionally distinct pathways also have disparate roles in opioid reward, tolerance, and withdrawal in the brain. In this study we monitored dopamine release in the medial or lateral shell of the NAc of male mice during a week-long morphine treatment paradigm. We measured dopamine release in response to an intravenous morphine injection both acutely and following a week of repeated morphine. We also measured dopamine in response to a naloxone injection both prior to and following repeated morphine treatment. Morphine induced a transient increase in dopamine in the medial NAc shell that was much larger than the slower rise observed in the lateral shell. Surprisingly, chronic morphine treatment induced a sensitization of the medial dopamine response to morphine that opposed a diminished response observed in the saline-treated control group. This study expands on our current understanding of the medial NAc shell as hub of opioid-induced dopamine fluctuation. It also highlights the need for future opioid studies to appreciate the heterogeneity of dopamine neurons.

Significance Statement

The social and economic consequences of the opioid epidemic are tragic and far-reaching. Yet, opioids are indisputably necessary in clinical settings where they remain the most useful treatment for severe pain. To combat this crisis, we must improve our understanding of opioid function in the brain, particularly the neural mechanisms that underlie opioid dependence and addictive behaviors. This study uses fiber photometry to examine dopamine changes that occur in response to repeated morphine, and morphine withdrawal, at multiple stages of a longitudinal opioid-dependence paradigm. We reveal key differences in how dopamine levels respond to opioid administration in distinct sub-regions of the ventral striatum and lay a foundation for future opioid research that appreciates our contemporary understanding of the dopamine system.

Ephedrine and cocaine cause developmental neurotoxicity and abnormal behavior in zebrafish

Ephedrine (EPH) and cocaine (COC) are illegal stimulant drugs, and have been frequently detected in aquatic environments. EPH and COC have negative effects on the nervous system and cause abnormal behaviors in mammals and fish at high concentrations, but their mechanisms of neurotoxicity remain unclear in larvae fish at low concentrations. To address this issue, zebrafish embryos were exposed to EPH and COC for 14 days post-fertilization (dpf) at 10, 100, and 1000 ng L-1. The bioaccumulation, development, behavior, cell neurotransmitter levels and apoptosis were detected to investigate the developmental neurotoxicity (DNT) of EPH and COC. The results showed that EPH decreased heart rate, while COC increased heart rate. EPH caused cell apoptosis in the brain by AO staining. In addition, behavior analysis indicated that EPH and COC affected spontaneous movement, touch-response, swimming activity and anxiety-like behaviors. EPH and COC altered the levels of the neurotransmitters dopamine (DA) and γ-aminobutyric acid (GABA) with changes of the transcription of genes related to the DA and GABA pathways. These findings indicated that EPH and COC had noticeable DNT in the early stage of zebrafish at environmentally relevant concentrations.

Altered Accumbal Dopamine Terminal Dynamics Following Chronic Heroin Self-Administration

Administration of heroin results in the engagement of multiple brain regions and the rewarding and addictive effects are mediated, at least partially, through activation of the mesolimbic dopamine system. However, less is known about dopamine system function following chronic exposure to heroin. Withdrawal from chronic heroin exposure is likely to drive a state of low dopamine in the nucleus accumbens (NAc), as previously observed during withdrawal from other drug classes. Thus, we aimed to investigate alterations in NAc dopamine terminal function following chronic heroin self-administration to identify a mechanism for dopaminergic adaptations. Adult male Long Evans rats were trained to self-administer heroin (0.05 mg/kg/inf, IV) and then placed on a long access (FR1, 6-h, unlimited inf, 0.05 mg/kg/inf) protocol to induce escalation of intake. Following heroin self-administration, rats had decreased basal extracellular levels of dopamine and blunted dopamine response following a heroin challenge (0.1 mg/kg/inf, IV) in the NAc compared to saline controls. FSCV revealed that heroin-exposed rats exhibited reduced stimulated dopamine release during tonic-like, single-pulse stimulations, but increased phasic-like dopamine release during multi-pulse stimulation trains (5 pulses, 5–100 Hz) in addition to an altered dynamic range of release stimulation intensities when compared to controls. Further, we found that presynaptic D3 autoreceptor and kappa-opioid receptor agonist responsivity were increased following heroin self-administration. These results reveal a marked low dopamine state following heroin exposure and suggest the combination of altered dopamine release dynamics may contribute to increased heroin seeking.

Biphasic reward effects are characteristic of both lorcaserin and drugs of abuse: implications for treatment of substance use disorders

Lorcaserin is a modestly selective agonist for 2C serotonin receptors (5-HT2CR). Despite early promising data, it recently failed to facilitate cocaine abstinence in patients and has been compared with dopamine antagonist medications (antipsychotics). Here, we review the effects of both classes on drug reinforcement. In addition to not being effective treatments for cocaine use disorder, both dopamine antagonists and lorcaserin can have biphasic effects on dopamine and reward behavior. Lower doses can cause enhanced drug taking with higher doses causing reductions. This biphasic pattern is shared with certain stimulants, opioids, and sedative-hypnotics, as well as compounds without abuse potential that include agonists for muscarinic and melatonin receptors. Additional factors associated with decreased drug taking include intermittent dosing for dopamine antagonists and use of progressive-ratio schedules for lorcaserin. Clinically relevant doses of lorcaserin were much lower than those that inhibited cocaine-reinforced behavior and can also augment this same behavior in different species. Diminished drug-reinforced behavior only occurred in animals after higher doses that are not suitable for use in patients. In conclusion, drugs of abuse and related compounds often act as biphasic modifiers of reward behavior, especially when evaluated over a broad range of doses. This property may reflect the underlying physiology of the reward system, allowing homeostatic influences on behavior.

Astrocytes in cocaine addiction and beyond

Drug addiction remains a key biomedical challenge facing current neuroscience research. In addition to neural mechanisms, the focus of the vast majority of studies to date, astrocytes have been increasingly recognized as an "accomplice." According to the tripartite synapse model, astrocytes critically regulate nearby pre- and postsynaptic neuronal substrates to craft experience-dependent synaptic plasticity, including synapse formation and elimination. Astrocytes within brain regions that are implicated in drug addiction exhibit dynamic changes in activity upon exposure to cocaine and subsequently undergo adaptive changes themselves during chronic drug exposure. Recent results have identified several key astrocytic signaling pathways that are involved in cocaine-induced synaptic and circuit adaptations. In this review, we provide a brief overview of the role of astrocytes in regulating synaptic transmission and neuronal function, and discuss how cocaine influences these astrocyte-mediated mechanisms to induce persistent synaptic and circuit alterations that promote cocaine seeking and relapse. We also consider the therapeutic potential of targeting astrocytic substrates to ameliorate drug-induced neuroplasticity for behavioral benefits. While primarily focusing on cocaine-induced astrocytic responses, we also include brief discussion of other drugs of abuse where data are available.

A novel approach to treating opioid use disorders: Dual agonists of glucagon-like peptide-1 receptors and neuropeptide Y2 receptors

The widespread misuse of opioids and opioid use disorder (OUD) together constitute a major public health crisis in the United States. The greatest challenge for successfully treating OUD is preventing relapse. Unfortunately, there are few FDA-approved medications to treat OUD and, while effective, these pharmacotherapies are limited by high relapse rates. Thus, there is a critical need for conceptually new approaches to developing novel medications to treat OUD. Here, we review an emerging preclinical literature that suggests that glucagon-like peptide-1 receptor (GLP-1R) agonists could be re-purposed for treating OUD. Potential limitations of this approach are also discussed along with an alternative strategy that involves simultaneously targeting and activating GLP-1Rs and neuropeptide Y2 receptors (Y2Rs) in the brain using a novel monomeric dual agonist peptide. Recent studies indicate that this combinatorial pharmacotherapy approach attenuates voluntary fentanyl taking and seeking in rats without producing adverse effects associated with GLP-1R agonist monotherapy alone. While future studies are required to comprehensively determine the behavioral effects of GLP-1R agonists and dual agonists of GLP-1Rs and Y2Rs in rodent models of OUD, these provocative preclinical findings highlight a potential new GLP-1R-based approach to preventing relapse in humans with OUD.

Monoamine and genome-wide DNA methylation investigation in behavioral addiction

Behavioral addiction (BA) is characterized by repeated, impulsive and compulsive seeking of specific behaviors, even with consequent negative outcomes. In drug addiction, alterations in biological mechanisms, such as monoamines and epigenetic processes, have been suggested, whereas whether such mechanisms are also altered in BA remains unknown. In this preliminary study with a small sample size, we investigated monoamine concentrations and genome-wide DNA methylation in blood samples from BA patients and control (CT) subjects. Higher dopamine (DA) metabolites and the ratio between DA and its metabolites were observed in the BA group than in the CT group, suggesting increased DA turnover in BA. In the methylation assay, 186 hyper- or hypomethylated CpGs were identified in the BA group compared to the CT group, of which 64 CpGs were further identified to correlate with methylation status in brain tissues with database search. Genes identified with hyper- or hypomethylation were not directly associated with DA transmission, but with cell membrane trafficking and the immune system. Some of the genes were also associated with psychiatric disorders, such as drug addiction, schizophrenia, and autism spectrum disorder. These results suggest that BA may involve alterations in epigenetic regulation of the genes associated with synaptic transmission, including that of monoamines, and neurodevelopment.

Activation of GLP-1 receptors attenuates oxycodone taking and seeking without compromising the antinociceptive effects of oxycodone in rats

Despite the effectiveness of current medications to treat opioid use disorder, there is still a high rate of relapse following detoxification. Thus, there is critical need for innovative studies aimed at identifying novel neurobiological mechanisms that could be targeted to treat opioid use disorder. A growing body of preclinical evidence indicates that glucagon-like peptide-1 (GLP-1) receptor agonists reduce drug reinforcement. However, the efficacy of GLP-1 receptor agonists in attenuating opioid-mediated behaviors has not been thoroughly investigated. Using recently established models of opioid-taking and -seeking behaviors, we showed that systemic administration of the GLP-1 receptor agonist exendin-4 reduced oxycodone self-administration and the reinstatement of oxycodone-seeking behavior in rats. We also identified behaviorally selective doses of exendin-4 that reduced opioid-taking and -seeking behaviors and did not produce adverse feeding effects in oxycodone-experienced rats. To identify a central site of action, we showed that systemic exendin-4 penetrated the brain and bound putative GLP-1 receptors on dopamine D1 receptor- and dopamine D2 receptor-expressing medium spiny neurons in the nucleus accumbens shell. Consistent with our systemic studies, infusions of exendin-4 directly into the accumbens shell attenuated oxycodone self-administration and the reinstatement of oxycodone-seeking behavior without affecting ad libitum food intake. Finally, exendin-4 did not alter the analgesic effects of oxycodone, suggesting that activation of GLP-1 receptors attenuated opioid reinforcement without reducing the thermal antinociceptive effects of oxycodone. Taken together, these findings suggest that GLP-1 receptors could serve as potential molecular targets for pharmacotherapies aimed at reducing opioid use disorder.

Extracellular dopamine, acetylcholine, and activation of dopamine D1 and D2 receptors after selective breeding for cocaine self-administration in rats

RATIONALE

The low self-administration (LS)/Kgras (LS) and high self-administration (HS)/Kgras (HS) rat lines were generated by selective breeding for low- and high-intravenous cocaine self-administration, respectively, from a common outbred Wistar stock (Crl:WI). This trait has remained stable after 13 generations of breeding.

OBJECTIVE

The objective of the present study is to compare cocaine preference, neurotransmitter release, and dopamine receptor activation in LS and HS rats.

METHODS

Levels of dopamine, acetylcholine, and cocaine were measured in the nucleus accumbens (NA) shell of HS and LS rats by tandem mass spectrometry of microdialysates. Cocaine-induced locomotor activity and conditioned-place preference were compared between LS and HS rats.

RESULTS

HS rats displayed greater conditioned-place preference scores compared to LS and reduced basal extracellular concentrations of dopamine and acetylcholine. However, patterns of neurotransmitter release did not differ between strains. Low-dose cocaine increased locomotor activity in LS rats, but not in HS animals, while high-dose cocaine augmented activity only in HS rats. Either dose of cocaine increased immunoreactivity for c-Fos in the NA shell of both strains, with greater elevations observed in HS rats. Activation identified by cells expressing both c-Fos and dopamine receptors was generally greater in the HS strain, with a similar pattern for both D1 and D2 dopamine receptors.

CONCLUSIONS

Diminished levels of dopamine and acetylcholine in the NA shell, with enhanced cocaine-induced expression of D1 and D2 receptors, are associated with greater rewarding effects of cocaine in HS rats and an altered dose-effect relationship for cocaine-induced locomotor activity.

Effects of the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI) on cocaine versus food choice and extended-access cocaine intake in rhesus monkeys

The dynorphin/kappa opioid receptor (KOR) system has been implicated as one potential neurobiological modulator of the abuse-related effects of cocaine and as a potential target for medications development. This study determined effects of the KOR antagonist nor-binaltorphimine (nor-BNI) on cocaine self-administration under a novel procedure that featured two daily components: (1) a 2-hour 'choice' component (9:00-11:00 am) when monkeys could choose between food pellets and cocaine injections (0-0.1 mg/kg per injection, intravenous) and (2) a 20-hour 'extended-access' component (noon to 8:00 am) when cocaine (0.1 mg/kg per injection) was available under a fixed-ratio schedule to promote high daily cocaine intakes. Rhesus monkeys (n = 4) were given 14 days of exposure to the choice + extended-access procedure then treated with nor-BNI (3.2 or 10.0 mg/kg, intramuscular), and cocaine choice and extended-access cocaine intake were evaluated for an additional 14 days. Consistent with previous studies, cocaine maintained both a dose-dependent increase in cocaine choice during choice components and a high level of cocaine intake during extended-access components. Neither 3.2 nor 10 mg/kg nor-BNI significantly altered cocaine choice or extended-access cocaine intake. In two additional monkeys, nor-BNI also had no effect on cocaine choice or extended-access cocaine intake when it was administered at the beginning of exposure to the extended-access components. Overall, these results do not support a major role for the dynorphin/KOR system in modulating cocaine self-administration under these conditions in non-human primates nor do they support the clinical utility of KOR antagonists as a pharmacotherapeutic strategy for cocaine addiction.

The Roles of Dopamine and α1-Adrenergic Receptors in Cocaine Preferences in Female and Male Rats

Cocaine dependence is characterized by compulsive drug taking and reduced involvement in social, occupational, or recreational activities. Unraveling the diverse mechanisms contributing to the loss-of-interest in these 'non-drug' pursuits is essential for understanding the neurobiology of addiction and could provide additional targets for treating addiction. The study objectives were to examine changes in cocaine-induced dopamine (DA) overflow in the nucleus accumbens (NAc) over the course of self-administration and determine the roles of α1- and β-adrenergic receptors (AR) in the loss-of-interest in food rewards following the development of an addicted phenotype in male and female rats. Subjects were given access to cocaine and palatable food pellets in a choice self-administration paradigm to identify 'addicted' cocaine-preferring (CP) individuals and resistant pellet-preferring (PP) individuals based on their patterns of self-administration over 7 weeks. Cocaine-induced DA overflow in the NAc was examined with microdialysis early and late during self-administration (weeks 2 and 7). Subjects were treated in counter-balanced order with propranolol (β-AR antagonist), terazosin (α1-AR antagonist), or vehicle for an additional 3 weeks of self-administration. CP rats displayed increased motivation for cocaine and attenuated motivation for pellets following the development of cocaine preferences. In females, the estrous cycle affected pellet, but not cocaine, self-administration. CP rats displayed attenuated cocaine-induced DA overflow in the NAc. Propranolol enhanced cocaine reinforcement and reduced pellet intake, whereas terazosin enhanced motivation for pellets and reversed preferences in a subset of CP rats. The implications of these results for the treatment of addiction are discussed.

Reduction of group II metabotropic glutamate receptors during development of benzodiazepine dependence

Prolonged use of benzodiazepines often leads to dependence and withdrawal syndrome. However, the cellular mechanisms underlying benzodiazepine dependence have not been fully clarified. Several investigators have shown an involvement of metabotropic glutamate receptors (mGluRs) in the pathophysiology of dependence or withdrawal. This study was performed to elucidate the role of mGluRs in benzodiazepine dependence. Withdrawal signs were precipitated in mice by flumazenil injection (25 mg/kg) after continuous subcutaneous infusion of benzodiazepines for 7 days, and the effects of several Gi-coupled receptor ligands on forskolin-stimulated cyclic AMP accumulation were examined in the cerebral cortex of mice. The mRNA expression for mGluRs was determined by RT-PCR. A single injection of flumazenil precipitated typical withdrawal signs such as tail elevation and tremor in mice treated with diazepam or alprazolam, but not quazepam. The inhibitory effect of nonselective mGluR ligands on adenylate cyclase activity was diminished in mice that showed signs of benzodiazepine withdrawal. The mRNA expression levels of mGluR2 and mGluR3 were lowered in the cerebral cortex of mice pretreated with diazepam or alprazolam. Our findings suggest that the reduction in the expression of group II mGluRs subunits may be involved in the development of benzodiazepine dependence.

Kappa-opioid receptor signaling in the striatum as a potential modulator of dopamine transmission in cocaine dependence

Cocaine addiction is accompanied by a decrease in striatal dopamine signaling, measured as a decrease in dopamine D2 receptor binding as well as blunted dopamine release in the striatum. These alterations in dopamine transmission have clinical relevance, and have been shown to correlate with cocaine-seeking behavior and response to treatment for cocaine dependence. However, the mechanisms contributing to the hypodopaminergic state in cocaine addiction remain unknown. Here we review the positron emission tomography (PET) imaging studies showing alterations in D2 receptor binding potential and dopamine transmission in cocaine abusers and their significance in cocaine-seeking behavior. Based on animal and human studies, we propose that the kappa receptor/dynorphin system, because of its impact on dopamine transmission and upregulation following cocaine exposure, could contribute to the hypodopaminergic state reported in cocaine addiction, and could thus be a relevant target for treatment development.

Presynaptic dopamine modulation by stimulant self-administration

The mesolimbic dopamine system is an essential participant in the initiation and modulation of various forms of goal-directed behavior, including drug reinforcement and addiction processes. Dopamine neurotransmission is increased by acute administration of all drugs of abuse, including the stimulants cocaine and amphetamine. Chronic exposure to these drugs via voluntary self-administration provides a model of stimulant abuse that is useful in evaluating potential behavioral and neurochemical adaptations that occur during addiction. This review describes commonly used methodologies to measure dopamine and baseline parameters of presynaptic dopamine regulation, including exocytotic release and reuptake through the dopamine transporter in the nucleus accumbens core, as well as dramatic adaptations in dopamine neurotransmission and drug sensitivity that occur with acute non-contingent and chronic, contingent self-administration of cocaine and amphetamine.

Donepezil treatment and the subjective effects of intravenous cocaine in dependent individuals

Acetylcholinesterase (AChE) inhibitors increase synaptic levels of acetylcholine (ACh) by inhibiting its breakdown. Donepezil is a reversible AChE inhibitor that is clinically available and relatively selective for inhibiting AChE but not other cholinesterases. Because AChE inhibitors have been shown to decrease the reinforcing effects of cocaine in animals, our hypothesis was that pretreatment with donepezil would attenuate the perceived value and other positive subjective effects of cocaine. We conducted a within-subject, double-blind, placebo-controlled, laboratory-based evaluation of the subjective effects produced by intravenous cocaine in human subjects receiving oral donepezil. Following three days of daily treatment with 5mg of donepezil or oral placebo, participants received intravenous placebo or cocaine (0.18 and 0.36 mg/kg). After a three-day washout period, participants were crossed over to the opposite oral treatment, which was followed by identical intravenous infusions. Donepezil was well-tolerated with only two drug-related adverse events reported that were mild and self-limiting. Treatment with donepezil increased ratings of 'any' and 'good' drug effect produced by low-dose cocaine, without modifying the response to high-dose cocaine. When collapsed across intravenous dose, treatment with donepezil decreased dysphoric effects and somatic symptoms, but did not modify the value of cocaine injections as determined by the Multiple Choice Questionnaire (MCQ). In summary, pretreatment with donepezil potentiated some measures for nonspecific and positive effects of low-dose cocaine. Across all intravenous treatments, participants receiving donepezil reported fewer somatic-dysphoric effects. Neither of these actions support the value of donepezil as a treatment for cocaine dependence.

Lower level of endogenous dopamine in patients with cocaine dependence: findings from PET imaging of D(2)/D(3) receptors following acute dopamine depletion

OBJECTIVE

Previous positron emission tomography (PET) imaging studies have demonstrated that cocaine dependence is associated with a decrease in dopamine type 2 and 3 (D(2)/D(3)) receptor binding in cocaine-dependent individuals relative to healthy comparison subjects. However, given the nature of PET imaging, it is possible that the measured decrease in radiotracer binding results from an increase in baseline dopamine levels. The purpose of this study was to measure D(2)/D(3) receptors following acute dopamine depletion in cocaine-dependent volunteers relative to healthy comparison subjects.

METHOD

Cocaine-dependent volunteers (N=15) and healthy matched comparison subjects (N=15) were scanned using PET, with the dopamine receptor radiotracer [(11)C]raclopride, at baseline and again following acute depletion of endogenous dopamine via alpha-methyl-para-tyrosine (AMPT) administration. Changes in radiotracer binding were measured in the subdivisions of the striatum (caudate, putamen, and ventral striatum) in addition to the striatum as a whole.

RESULTS

Findings revealed that cocaine-dependent volunteers exhibited lower levels of endogenous dopamine relative to comparison subjects, which was measured as an increase in [(11)C]raclopride binding following AMPT administration. The increase in [(11)C]raclopride binding in the striatum was 11.1% (SD=4.4%) in healthy comparison subjects and 5.7% (SD=5.9%) in cocaine-dependent volunteers. Similar differences were seen in the subdivisions of the striatum.

CONCLUSIONS

The decrease in striatal D(2)/D(3 )receptors associated with cocaine dependence cannot be attributed to higher levels of endogenous dopamine.

Long-lasting decreases in cocaine-reinforced behavior following treatment with the cholinesterase inhibitor tacrine in rats selectively bred for drug self-administration

Tacrine is a centrally acting, reversible cholinesterase inhibitor that increases synaptic levels of acetylcholine (ACh) and can potentiate the actions of dopamine (DA). The present study was conducted to evaluate effects of tacrine on cocaine-reinforced responding in a rat line selectively bred for high levels of drug self-administration (the HS line). HS rats self-administered different doses of cocaine under a fixed-ratio-5 (FR-5) schedule. Over a four-day period, vehicle or tacrine (1.0, 3.2, or 10 mg/kg-day) was infused when animals were maintained in home cages (21 h per day). Tacrine dose-dependently decreased cocaine self-administration. Actions of tacrine differed for self-administration that was initiated within 20 min of pretreatment (described as early sessions), and for self-administration that occurred between one and three days after administration of tacrine was discontinued (late sessions). Tacrine's potency for attenuating self-administration during late sessions was greater for cocaine- relative to food-reinforcement in HS rats, and for HS relative to outbred rats. In a subset of tacrine-treated HS rats, cocaine self-administration was persistently attenuated by more than 80% from pretreatment baseline levels over a one-week period during which no further tacrine was administered. In summary, pretreatment with tacrine can produce a long-lasting attenuation of cocaine-reinforced responding.

Cocaine abuse and sensitization of striatal dopamine transmission: a critical review of the preclinical and clinical imaging literature

Much effort has been devoted in the preclinical addiction literature to understanding the phenomenon of sensitization, an enhanced dopaminergic response in the nucleus accumbens that occurs after repeated exposure to psychostimulant drugs. Although sensitization has been reported in preclinical studies, studies of sensitization in humans measuring behavioral and physiological responses have been mixed and inconclusive. However, imaging studies with positron emission tomography (PET) and single photon emission computed tomography (SPECT) using a stimulant challenge to induce dopamine (DA) release provide a unique opportunity to probe DA transmission in cocaine dependent human subjects. In contrast to the basic science literature that predicted sensitization, three independent cohorts have shown a blunted DA response, or the opposite of sensitization, in human cocaine dependent subjects. This article reviews the methodological differences between the preclinical and clinical PET studies that have investigated DA sensitization in order to address the discrepancy between the human and animal literature.

Strain differences between Lewis and Fischer 344 rats in the modulation of dopaminergic receptors after morphine self-administration and during extinction

The Lewis (LEW) and Fischer 344 (F344) rat strains have been used as a model to study genetic vulnerability to drug addiction and they differ in their dopaminergic systems. We have studied the variation in the D1-like and D2-like receptors in distinct brain regions of LEW and F344 rats that self-administered morphine (1 mg/kg) for 15 days and also after different extinction periods (3, 7 and 15 days). Under basal conditions, binding to D1-like receptors in the olfactory tubercle and substantia nigra, and to D2-like receptors in the Pyriform cortex and hippocampal-CA1 was lower in LEW rats than in F344 rats. Conversely, the LEW rats exhibited stronger D2-like binding in the caudate-putamen. In most brain regions there was a decrease in D1-like binding in LEW rats after self-administration while the F344 animals displayed an increment. Additionally, D2 receptors of LEW rats were down-regulated after self-administration in the caudate-putamen and in the nucleus accumbens (shell and core divisions). Binding to D1-like receptors increased in both strains in the early phases of extinction, while in the later stages a differential regulation was observed between both strains. During the early phases of extinction only F344 rats showed alterations in D2-like receptor binding, however in the latter phases a specific modulation occurred in both strains. These differences in basal D1-like and D2-like receptor binding, and their differential modulation after self-administration and during extinction, may be reflected in the greater vulnerability to opiate addiction shown by LEW strain.

Behavioral and neurochemical changes induced by oxycodone differ between adolescent and adult mice

Nonmedical use of the prescription opioid analgesic oxycodone is a major problem in the United States, particularly among adolescents and young adults. This study characterized self-administration of oxycodone by adolescent and adult mice, and how this affects striatal dopamine levels. Male C57BL/6J mice (4 or 10 weeks old) were allowed to acquire oxycodone self-administration (0.25 mg/kg per infusion) for 9 days, and then tested with varying doses of oxycodone (0, 0.125, 0.25, 0.5, and 0.75 mg/kg per infusion). On completion of the self-administration study, a guide cannula was implanted into the striatum of these mice. Six days later, microdialysis was conducted on the freely moving mouse. After collection of baseline samples, oxycodone was administered i.p. (1.25, 2.5, and 5.0 mg/kg) and samples were collected for 1 h after each dose. Adult mice self-administered significantly more oxycodone across the doses tested. After 1 week, basal striatal dopamine levels were lower in mice of both ages that had self-administered oxycodone than in yoked saline controls. Oxycodone challenge increased striatal dopamine levels in a dose-dependent manner in both age groups. Of interest, the lowest dose of oxycodone led to increased striatal dopamine levels in the mice that had self-administered oxycodone during adolescence but not those that self-administered it as adults. The lower number of infusions of oxycodone self-administered by adolescent mice, and their later increased striatal dopamine in response to the lowest dose of oxycodone (not found in adults), suggest differential sensitivity to the reinforcing and neurobiological effects of oxycodone in the younger mice.

D2R DNA transfer into the nucleus accumbens attenuates cocaine self-administration in rats

Dopamine (DA) D2 receptor (D2R) agonists and antagonists can modulate self-administration behavior, conditioned place preference, and locomotor responses to cocaine. Low levels of D2R have also been observed in cocaine addicted subjects and in non human primates after chronic cocaine exposures. Prior studies had shown that D2R upregulation in the nucleus accumbens (NAc) in rodents trained to self-administer alcohol markedly attenuated alcohol preference and intake. Here we assess the effects of D2R upregulation in the NAc on cocaine intake in rats trained to self-administer cocaine. Following 2 weeks of i.v. cocaine self-administration (CSA), rats were stereotaxically treated with an adenovirus that carried the D2R gene to upregulate D2R in the NAc. D2R vector treatment resulted in a significant decrease (75%) in cocaine infusions and lever presses (70%) for cocaine. This effect lasted 6 days before cocaine consumption returned to baseline levels, which corresponds roughly to the time it takes D2R to return to baseline levels. These findings show that CSA and D2R in the NAc are negatively correlated and suggest that cocaine intake is modulated in part by D2R levels in NAc. Thus strategies aimed at increasing D2R expression in NAc may be beneficial in treating cocaine abuse and addiction.

Beta-endorphin and drug-induced reward and reinforcement

Although drugs of abuse have different acute mechanisms of action, their brain pathways of reward exhibit common functional effects upon both acute and chronic administration. Long known for its analgesic effect, the opioid beta-endorphin is now shown to induce euphoria, and to have rewarding and reinforcing properties. In this review, we will summarize the present neurobiological and behavioral evidences that support involvement of beta-endorphin in drug-induced reward and reinforcement. Currently, evidence supports a prominent role for beta-endorphin in the reward pathways of cocaine and alcohol. The existing information indicating the importance of beta-endorphin neurotransmission in mediating the reward pathways of nicotine and THC, is thus far circumstantial. The studies described herein employed diverse techniques, such as biochemical measurements of beta-endorphin in various brain sites and plasma, and behavioral measurements, conducted following elimination (via administration of anti-beta-endorphin antibodies or using mutant mice) or augmentation (by intracerebral administration) of beta-endorphin. We suggest that the reward pathways for different addictive drugs converge to a common pathway in which beta-endorphin is a modulating element. Beta-endorphin is involved also with distress. However, reviewing the data collected so far implies a discrete role, beyond that of a stress response, for beta-endorphin in mediating the substance of abuse reward pathway. This may occur via interacting with the mesolimbic dopaminergic system and also by its interesting effects on learning and memory. The functional meaning of beta-endorphin in the process of drug-seeking behavior is discussed.

Development of an anti-cotinine vaccine to potentiate nicotine-based smoking cessation strategies

Nicotine replacement therapies (NRT) have limited success in smoking cessation. The efficacy of nicotine may be compromised by its main metabolite, cotinine. An anti-cotinine vaccine to remove this antagonism could enhance the efficacy of NRT. We show that cotinine is a weak nicotinic agonist and decreases responses to nicotine, consistent with antagonism through receptor desensitisation. trans-4-Thiol cotinine was coupled to tetanus toxoid, and rats immunised repeatedly. Vaccination raised antibodies specific for cotinine that do not recognise other metabolites or nicotine. Increased serum cotinine concentrations following nicotine administration indicate sequestration of cotinine by antibodies, encouraging further evaluation of this vaccine in behavioural models of nicotine addiction and relapse.

Active versus passive cocaine administration: differences in the neuroadaptive changes in the brain dopaminergic system

There is considerable evidence that chronic exposure to cocaine is associated with low striatal dopamine D2 receptor availability. In the present study we wished to determine whether neuroadaptive changes in densities of D2 receptors were due to direct pharmacological actions of cocaine or they reflected motivational states that were present when cocaine injection depended on active drug-seeking behavior and whether these changes were related to the actual expression of D2 mRNA. To achieve this goal we utilized a "yoked" procedure in which rats were tested simultaneously in groups of three, with only one rat actively self-administering cocaine while the other two received yoked injections of either cocaine or saline. Only passively administered cocaine produced a decrease in dopamine D2 receptor levels in the anterior and central regions of caudate/putamen, and both the shell and core of the nucleus accumbens, as measured by in vitro quantitative autoradiography. In contrast, examination of D2 receptor gene expression using in situ hybridization analysis revealed that there was an increase in D2 receptor mRNA levels in the ventral tegmental area of rats actively self-administered cocaine. We conclude that the reductions in striatal D2 receptor densities may be related to the chronic administration of cocaine per se and not to the motivated process of reinforced responding. Our results also suggest that increases in D2 receptor mRNA levels in limbic regions do not necessarily result in increased receptor densities and these changes likely reflect motivational states that were present when cocaine injection dependent on active drug self-administration.

Prior exposure to chronic stress and MDMA potentiates mesoaccumbens dopamine release mediated by the 5-HT(1B) receptor

(+) 3,4,-Methylenedioxymethamphetamine (MDMA) is an abused drug that acutely releases serotonin (5-HT) and dopamine (DA) but produces long-term damage to 5-HT terminals. MDMA-induced DA release has been shown to be dampened by 5-HT. Although stress also activates the mesolimbic DA pathway, it is unknown if chronic stress after exposure to neurotoxic doses of MDMA will augment MDMA-induced DA release in the nucleus accumbens shell (NAcc(sh)). Rats were pretreated with MDMA (10 mg/kg x 4, intraperitoneal (i.p.)). After 7 days, rats were subjected to 10 days of chronic unpredictable stress. DA release in the NAcc(sh) and 5-HT in the ventral tegmental area (VTA) were measured after a challenge injection of MDMA (5 mg/kg, i.p.). The combination of pretreatment with MDMA+stress decreased basal concentrations of 5-HT in the VTA and DA in the NAcc(sh) and enhanced MDMA-stimulated DA release in the NAcc(sh). Pretreatment with MDMA or stress alone blunted MDMA-induced 5-HT release in the VTA. The augmentation of MDMA-induced DA release in rats pretreated with MDMA+chronic stress was attenuated by perfusion of the 5-HT(1B) antagonist, GR127935 into the VTA before the MDMA challenge injection. These results suggest that prior exposure to both MDMA and stress can produce a long-term augmentation in mesolimbic DA transmission and enhanced drug abuse vulnerability that is mediated, in part, by the 5-HT(1B) receptor in the VTA.

Persistent and reversible morphine withdrawal-induced morphological changes in the nucleus accumbens

Morphine withdrawal produces a hypofunction of mesencephalic dopamine (DA) neurons which impinge upon medium spiny neurons (MSN) of the forebrain. After chronic treatment rats were either spontaneously or pharmacologically withdrawn from chronic morphine: under these two distinct conditions we studied the effects of withdrawal on spine density of MSN of the core and shell of the nucleus accumbens (NAcc) at various times (1-3-7-14 days). MSN were stained with the Golgi-Cox procedure and analyzed by a confocal laser-scanning microscope. Our analysis shows that both spontaneous and naloxone-induced withdrawal produces a long-lasting but reversible reduction in spines' density in shell MSN, as compared with core MSN. This effect is selectively localized at the level of second-order dendritic trunks and persists up to 14 days when spine density was found within control (pretreatment) values. By contrast, spine density counts of NAcc MSN from rats chronically treated with morphine, did not reveal any change over time. Collectively, the results of the present article suggest that spontaneous and pharmacologically precipitated withdrawal, but not chronic morphine, persistently but reversibly reduce spines' density under a condition of reduced mesolimbic DA transmission, and the reduction of spines' density in second-order dendritic trunks is selectively segregated in the MSN of the shell of the NAcc. Morphine withdrawal dramatically, lastingly, and reversibly reduces spine density, selectively in second-order dendritic trunks of NAcc shell MSN, thereby further impoverishing the already abated DA transmission. These results may be relevant in the most harmful consequences of drug addiction such as craving and loss of control over intake and are in line with recent views suggesting the hypodopaminergic state as a cardinal feature of drug dependence.

L-methamphetamine and selective MAO inhibitors decrease morphine-reinforced and non-reinforced behavior in rats; Insights towards selegiline's mechanism of action

Selegiline is an inhibitor of type B monoamine oxidase (MAO) with psychostimulant effects that can decrease morphine-reinforced and non-reinforced responding. The present study was undertaken to compare the effects of MAO inhibition and treatment with L-methamphetamine, the major psychostimulant metabolite of selegiline, on these behaviors. After rats acquired a stable pattern of morphine self-administration under a progressive ratio schedule, chronic treatment was initiated with vehicle, L-methamphetamine, clorgyline (a selective inhibitor of MAO-A), or rasagiline (a selective inhibitor of MAO-B); with both MAO inhibitors administered at a dose selective for one MAO isoform and a higher dose that inhibited both isoforms. Rats were evaluated for up to four cycles of opiate dependence maintained by morphine self-administration and withdrawal during which extinction responding was recorded. Most behavioral measures (92.4%) did not differ in animals evaluated during an initial and subsequent cycles of dependence and withdrawal. All active treatments attenuated non-reinforced responding during extinction. Morphine reinforcement was also decreased by each of the three active treatments, but greater and more prolonged effects were observed following inhibition of MAO-B with rasagiline. Responding during either cue- or morphine-induced reinstatement was attenuated by either clorgyline or rasagiline administered at nonselective doses, but not by either compound administered at selective dose levels. Treatment with L-methamphetamine did not produce significant effects on cue-induced reinstatement, but decreased non-reinforced responding during morphine-induced reinstatement. These findings indicate that morphine reinforcement and different non-reinforced behaviors differ greatly in their susceptibility to modification by psychostimulant treatment or MAO inhibition.

Cocaine during adolescence enhances dopamine in response to a natural reinforcer

The use of cocaine during adolescent development could alter the normal growth of brain regions affected by cocaine, specifically the reward system, and impact the adult mesolimbic system. However, there is scant literature aimed at determining whether animals are more vulnerable to the adverse effects of drugs during adolescence. The present study investigated whether cocaine pretreatment in either adolescence or adulthood altered the dopaminergic response to a naturally reinforcing substance in adulthood. To evaluate the responsivity of the mesolimbic system after repeated cocaine, sucrose was offered during the dialysis procedure and dialysates were collected. Regardless of age all saline pretreated rats had significant increases in sucrose-induced extracellular dopamine (DA) levels in the nucleus accumbens septi (NAcc) as compared to baseline levels. Rats pretreated with cocaine as adults also had significant increases in DA levels after sucrose. Interestingly, sucrose intake significantly enhanced DA levels in cocaine pretreated adolescent rats as compared to all other conditions. The results from the present study show that in rats pretreated with cocaine during adolescence there is an enhanced response of the dopaminergic system in animals exposed to a naturally reinforcing substance. Therefore, cocaine exposure during adolescence results in long-term functional changes in the mesolimbic pathway. Future studies need to ascertain the underlying mechanisms and their potential role in cocaine addiction.

Comparing nucleus accumbens and dorsal striatal dopamine responses to self-administered cocaine in naïve rats

Dopamine (DA) responses in the nucleus accumbens (NAcc) and dorsal striatum (DS) are commonly associated with different aspects of cocaine effects. Enhanced NAcc DA has been most convincingly linked with the positive reinforcing effects of cocaine, while DS DA is thought to mediate cocaine-induced motoric effects. Though several studies have shown NAcc DA enhancement following cocaine self-administration, very little work has examined the effects of cocaine self-administration on DS DA. In this study, DA levels in the NAcc and DS, and locomotor responses to a single self-administered cocaine injection (1.5mg/kg) were assessed in operant-trained, drug-naïve Sprague-Dawley rats. Locomotor activity, NAcc and DS DA levels increased significantly over baseline activity immediately after cocaine injection. However, while basal and cocaine-stimulated NAcc DA concentrations (nM) were significantly greater than DS DA levels, the magnitude of response was statistically comparable between brain regions. These findings indicate that, though both the NAcc and DS are importantly involved in the dopaminergic response to self-administered cocaine in drug-naïve rats, basal DA differences in dialysis data are obscured by statistical conversions to baseline percentages.

Current status of immunologic approaches to treating tobacco dependence: vaccines and nicotine-specific antibodies

In contrast to current pharmacotherapies, immunologic approaches to treating tobacco dependence target the drug itself rather than the brain. This approach involves the use of nicotine-specific antibodies that bind nicotine in serum, resulting in a decrease in nicotine distribution to the brain and an increase in nicotine's elimination half-life. This review summarizes the literature examining the effects of immunologic interventions on the pharmacokinetics and behavioral effects of nicotine in animal models, as well as recent phase I and II clinical trials in humans. Studies using various vaccines and nicotine-specific antibodies in rodents have shown that immunization can significantly reduce the behavioral effects of nicotine that are relevant to tobacco dependence (eg, nicotine self-administration). These findings provide proof of principle that immunologic interventions could have utility in the treatment of tobacco dependence. Thus far, phase I clinical trials of nicotine vaccines have not produced any serious adverse events in humans and have produced dose-dependent increases in serum antibody levels. Although preliminary data from these small trials suggest that vaccination can facilitate abstinence from tobacco use, more advance trials are needed. By acting outside the nervous system, immunologic approaches are less likely to produce the adverse side effects associated with current medications. In addition, the unique mechanism of action of immunotherapy makes it particularly suitable for combination with other pharmacological approaches. Taken together, the work completed to date provides substantial evidence that immunologic interventions could play an important role in future treatment strategies for tobacco dependence.

Morphine withdrawal-induced morphological changes in the nucleus accumbens

Morphine withdrawal produces a hypofunction of mesencephalic dopamine neurons that impinge upon medium spiny neurons (MSN) of the forebrain. After chronic treatment (from 20 to 140 mg/kg of morphine twice a day over 14 days at escalating doses) rats were withdrawn from chronic morphine spontaneously and pharmacologically. In these two distinct conditions we studied the effects of withdrawal on the morphology of MSN of the core and shell of the nucleus accumbens (Nacc). MSN were stained with the Golgi-Cox procedure and analysed by a confocal laser-scanning microscope (CLSM). Our analysis shows that, shell and core MSN differed significantly for perikarya size and spine density, and the various morphine treatments did not affect the perikarya morphometry. Both spontaneous and naloxone-induced withdrawal produced a similar reduction in spine density in MS shell neurons, as compared with MS core neurons. This effect is selectively localized at the level of second order dendritic trunks where afferents converge. By contrast, spine density counts of accumbens MSN from rats chronically treated with morphine, did not reveal any change. Collectively, the results of the present study are twofold: (i) spontaneous and pharmacologically precipitated withdrawal, but not chronic morphine per se, affects spine density of target structures of a reduced mesolimbic dopamine transmission, and (ii) the reduction of spine density in second order dendritic trunks is selectively segregated in the MSN of the shell of the Nacc. In conclusion, morphine withdrawal dramatically alters spine density, selectively in second order dendritic trunks of Nacc shell MSN, thereby further impoverishing the already abated dopamine (DA) transmission. This is in line with recent views suggesting the hypodopaminergic state as a cardinal feature of opioid dependence.

Different effects of GABAergic receptors located in the ventral tegmental area on the expression of morphine-induced conditioned place preference in rat

In the present study, an unbiased conditioned place preference paradigm was used to study the effects of intra-ventral tegmental area injections of Gama-amino-butyric acid (GABA)-A and B (GABA(A) and GABA(B)) receptor agonists and antagonists on the expression of morphine-induced conditioned place preference (CPP) in rats. Subcutaneous (s.c.) injections of morphine sulfate (5 mg/kg) induced CPP. Intra-ventral tegmental area administration of the GABA(A) receptor agonist, muscimol (6 microg/rat) reduced the expression of morphine-induced CPP. Muscimol (25 microg/rat) increased the expression of CPP induced by morphine. A reduction of the expression of morphine-induced CPP was observed on intra-ventral tegmental area injection of GABA(A) receptor antagonist bicuculline (25 microg/rat). Bicuculline (10 microg/rat) increased the expression of CPP induced by morphine. Baclofen (12 microg/rat) increased where as (19 and 25 microg/rat) reduced the expression of morphine-induced CPP. Injection of CGP38345 (10, 19, 25 and 50 microg/rat) into the ventral tegmental area significantly reduced the expression of CPP induced by morphine. It is concluded that GABA(A) and GABA(B) receptor subtypes within the ventral tegmental area may have different effects on the expression of morphine-induced CPP.

Behavioral response to novelty correlates with dopamine receptor availability in striatum of Göttingen minipigs

Behavioral response to novelty in rats has been linked both to dopamine transmission in the ventral striatum, and to propensity to self-administer psychostimulant drugs. In order to probe the relationship between behavioral response to novelty and dopamine systems we have developed a behavioral model for correlation with positron emission tomography (PET) of dopamine transmission in brain of Göttingen minipigs. In the present study, we measured exploration of a novel object by recording the number of contacts, and duration of contact with a novel object, in groups of six male and six female adult minipigs. We hypothesized that these novelty scores would correlate with the amphetamine-evoked dopamine release in ventral striatum, measured 2 weeks later in a PET study of the availability of binding sites for the dopamine D2/3 antagonist [11C]raclopride. There were significant correlations between duration of contact with a novel object and the amphetamine-evoked reductions in binding potential (DeltapB) in the left ventral striatum of the 12 animals; Comparison of results by gender revealed that the correlation was driven mainly by the male group, and was not present in the female group. We interpret these results to show that propensity to explore an unfamiliar object is relatively elevated in pigs with low basal occupancy of dopamine D2/3 receptors by endogenous dopamine, and with high amphetamine-induced occupancy of released dopamine in the male pigs.

Neurobiology of addiction: treatment and public policy ramifications

In the United States, efforts to treat addiction are hampered by prejudice and a public view that treats it as a disorder of self-control, not a disease. We highlight select advances in addiction research that, if disseminated to the public, could reverse these misconceptions and facilitate changes in policy to improve treatment access and care delivery for this highly prevalent disease.

Effects of subchronic methamphetamine exposure on basal dopamine and stress-induced dopamine release in the nucleus accumbens shell of rats

RATIONALE

Subchronic administration of stimulants reduces basal dopamine (DA) concentrations and blocks stress-induced DA release in the nucleus accumbens (NA) of rats during withdrawal. However, no studies have attempted to relate early withdrawal from chronic drug exposure to stress reactivity and changes in DA transmission.

OBJECTIVES

The effects of subchronic low-dose methamphetamine (METH) administration on regional changes in dopamine transporter (DAT) and norepinephrine transporter (NET) immunoreactivity and function during early withdrawal were examined. The effects of subchronic METH on stress responsivity measured by DA release in the nucleus accumbens shell (NA SHELL) and core (NA CORE) during acute restraint stress were also examined.

METHODS

Male rats received single injections of METH (2.0 mg/kg i.p.) or saline (SAL) for 10 days and then were killed 24 h after the last injection. DAT and NET protein in NA, striatum (STR), medial prefrontal cortex (mPFC), and hippocampus were assayed by Western blot analysis. Experiment 2 measured basal extracellular DA concentrations and restraint-stress-induced DA release in vivo in the NA SHELL and CORE of SAL- and METH-pretreated rats after 24-h withdrawal. Experiment 3 examined the in vivo regulation of extracellular DA in the NA SHELL and/or CORE after local administration of GBR12909 (50 microM) or nisoxetine (100 microM; NA SHELL).

RESULTS

Subchronic METH increased DAT but not NET immunoreactivity in the NA compared to the STR and mPFC. METH reduced basal extracellular DA and blocked restraint-stress-induced DA release in the NA SHELL. DA uptake blockade increased extracellular DA more in the NA SHELL of METH rats, whereas NE uptake blockade increased basal DA concentrations to a similar extent in METH and SAL rats.

CONCLUSIONS

These results suggest that subchronic METH exposure selectively increases NA DAT and consequently reduces basal and stress-induced DA release in the NA SHELL during early withdrawal.

Reduced dopamine terminal function and insensitivity to cocaine following cocaine binge self-administration and deprivation

Despite large numbers of studies describing neuroadaptations caused by chronic cocaine exposure, there remains considerable uncertainty as to whether alterations in dopamine (DA) neurotransmission are responsible for progression into an addicted state. High-intake, 24-h access cocaine self-administration (SA, 10 days) followed by an extended (7 days), but not 1 day deprivation period produces an increased motivation to SA cocaine as measured by a progressive ratio protocol. Following binge cocaine SA and deprivation, the status of DA terminals in the nucleus accumbens (NAc) was investigated using microdialysis in freely moving rats and voltammetry in brain slices. At 1 and 7 days following binge cocaine SA, baseline extracellular DA concentrations in the NAc core were decreased by 40 and 55% of control levels, in the 1 and 7 day deprivation groups, respectively. Acute cocaine (1.5 mg/kg, i.v.) administration increased extracellular DA (350%) in the NAc core of naïve animals but failed to significantly increase DA at 1 or 7 days following binge cocaine SA. The shell of the NAc showed a similar lack of effect of cocaine. Analysis of DA terminals in brain slices showed that cocaine was markedly less effective in inhibiting DA uptake at 1 and 7 days of cocaine deprivation (max effect 40% of control). Electrically stimulated DA release was decreased at 1 day and further decreased at 7 days of deprivation (67 and 49% of control, respectively). The rate of DA uptake was increased (150% of control) following binge SA, irrespective of deprivation period. Finally, presynaptic autoreceptors were subsensitive at both time points, as measured by the ability of quinpirole, a D2-like DA receptor agonist, to inhibit DA release. Thus, the NAc was hypodopaminergic and DA terminals were less sensitive to cocaine following binge cocaine SA and deprivation.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

Effects of high-dose selegiline on morphine reinforcement and precipitated withdrawal in dependent rats

Selegiline is an irreversible inhibitor of monoamine oxidase (MAO) with psychostimulant and neuroprotective effects. Several lines of evidence suggest that treatment with selegiline at doses that exceed levels required for inhibition of MAO can produce distinct pharmacologic effects. The purpose of this study was to evaluate the effects of chronic treatment with high-dose selegiline on extinction responding, cue-induced reinstatement, morphine reinforcement and naloxone-precipitated withdrawal. After pretreatment with noncontingent morphine to establish opiate dependence, rats acquired self-administration of 3.2 mg/kg per injection of morphine under a progressive ratio schedule. Daily treatment with saline or 6.4 mg/kg per day of selegiline was then administered over extinction, reinstatement and re-acquisition of morphine self-administration. To enhance or diminish the potential for psychostimulant effects, selegiline was administered either immediately prior to (pre-session) or 1 h following (post-session) extinction, reinstatement and self-administration sessions. Pre-session selegiline decreased the number of ratios completed on days 2, 3 and 4 of extinction, and decreased morphine self-administration during all four re-acquisition sessions. When administered at the same dose level, post-session selegiline decreased responding on the fourth extinction session, and was ineffective in modifying re-acquisition of self-administration. Selegiline administered by either schedule did not modify cue-induced reinstatement. Daily treatment with 6.4 mg/kg per day of selegiline did not modify self-administration of food under a progressive ratio schedule. Acute treatment with single, 6.4 mg/kg doses of selegiline attenuated naloxone-induced increases in ptosis and global withdrawal score, but did not modify any other sign of withdrawal or global withdrawal score calculated without ratings of ptosis. In conclusion, high-dose selegiline can attenuate extinction responding and morphine-reinforced behavior, and these effects may be mediated by psychostimulant metabolites.

Chronic opiate treatment enhances both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal

After chronic exposure to psychostimulants or opiates, self-administration or conditioned place preference with either class is increased (sensitized). Cross-sensitization of conditioned place preference, i.e., enhancement of psychostimulant-induced preferences after exposure to opiates, has also been described, but increases in cocaine self-administration after morphine pretreatment have not been reported. The present study evaluated effects of chronic morphine treatment on cocaine reinforcement. Opiate dependence was established in Wistar rats by administration of morphine as a constant infusion that was gradually increased to a dose of 50mg/kg per day over a 1-week period. Immediately after discontinuation of chronic morphine treatment, animals were allowed to acquire cocaine self-administration under a simple fixed-ratio schedule (FR-1), and were subsequently advanced to a progressive ratio schedule. Acquisition of cocaine self-administration under the FR-1 did not differ in saline- and morphine-pretreated animals. For cocaine self-administration under a progressive ratio schedule measured at 5 or more days after the onset of opiate withdrawal, chronic pretreatment with morphine increased the number of ratios completed, augmented final response requirements, and produced a more stable pattern of cocaine self-administration. Responding was also increased in morphine-pretreated animals during an initial extinction session. These results show that chronic opiate treatment can enhance both cocaine-reinforced and cocaine-seeking behaviors following opiate withdrawal. A similar effect may occur in human patients who discontinue methadone or other forms of replacement therapy for opiate abuse, and may contribute to relapse involving use of cocaine or other psychostimulants.

Neuroadaptive changes in the mesoaccumbens dopamine system after chronic nicotine self-administration: A microdialysis study

There is little evidence to date to indicate if mesoaccumbens dopamine function at the neurochemical level is altered during early abstinence from chronic i.v. nicotine self-administration. Thus, a quantitative microdialysis (no-net-flux) approach was used to measure basal extracellular concentrations and extraction fractions of dopamine in the nucleus accumbens (ACB) of rats that self-administered nicotine i.v. for 25 days, as well as in rats serving as yoked comparison groups (yoked nicotine and yoked saline). After 24-48 h of the final self-administration session, there was a significant reduction in basal extracellular dopamine levels in the ACB of the self-administration group compared with the yoked saline group (1.35+/-0.15 nM versus 3.70+/-0.28 nM). The basal extracellular dopamine levels in the yoked nicotine group (1.46+/-0.20 nM) were not significantly different compared with the nicotine self-administration group. The in vivo extraction fraction of dopamine, an indirect measure of dopamine uptake, was significantly increased in the nicotine self-administration (86%) and yoked nicotine (91%) groups compared with the yoked saline group (77%). In addition, a marked reduction in the elevation of extracellular dopamine levels in the ACB occurred after a nicotine challenge as measured by conventional microdialysis in the self-administration (112% of basal) and yoked nicotine (121% of basal) groups as compared with a yoked saline (154% of basal) group. The reduced basal ACB dopamine levels in the nicotine groups during early abstinence appears to be due to increased clearance, suggesting increased dopamine uptake is occurring as a result of the chronic nicotine treatment. The reduced elevation of extracellular dopamine levels in the ACB upon nicotine challenge suggests a functional desensitization or downregulation phenomenon involving acetylcholine receptors (nicotinic nAChRs). Overall, these results provide clear evidence for a neuroadaptive change that alters dopamine transmission in the ACB during abstinence from chronic i.v. nicotine exposure.

Opiate tolerance by heroin self-administration: An fMRI study in rat

Functional MRI (fMRI) was employed to determine whether repeated heroin self-administration (SA) produces tolerance or sensitization in the brain of heroin-SA rats. Twelve rats were evenly divided into saline and heroin (0.06 mg/kg, 4 hr/day) SA groups. There was a progressive increase in drug-SA behavior and daily heroin intake during the 8-9 days of heroin-SA training. Within 24 hr after the last session of daily SA, acute heroin (0.1 mg/kg) administration induced regional blood oxygen level-dependent (BOLD) signals in both groups of rats. The positive BOLD signals appeared mainly in the cortical regions, including the prefrontal cortex, cingulate, and olfactory cortex, while the negative BOLD signals were predominantly located in subcortical regions such as caudate and putamen, nucleus accumbens, thalamus, and hypothalamus. However, the number of activated voxels or BOLD-signal intensity was significantly less in heroin-SA rat in regions of prefrontal cortex, nucleus accumbens, and thalamus, etc., compared to the changes in the saline control rats. Application of gamma-vinyl GABA (100 mg/kg), an irreversible GABA-transaminase inhibitor, failed to block opiate actions in the heroin-SA rats. Together, these data suggest that repeated heroin-SA produces tolerance or desensitization of opiate actions in the rat brain, which may in turn potentiate drug SA behavior and drug intake.

Drug dependence and the endogenous opioid system

The discovery of endogenous opioids has markedly influenced the research on the biology of drug dependence. Evidence has been presented that these brain substances are self-administered by laboratory animals. This finding, among others, has led to the hypothesis that endogenous opioids are involved in reinforcing habits, including dependence on drugs of abuse. The course of drug dependence is presented as a continuum from no drug use via controlled use to an actual dependence on the drug. Specific brain opioid systems belonging to four conceptualized brain circuits are described to be involved during the different phases of the drug dependence continuum. More recent research to delineate the role of endogenous opioid systems in drug dependence has focussed on genetic research in humans and animals. Among others, the findings obtained from studies of opioid receptor and opioid peptide precursor knockout mice provided further support for a role of endogenous opioid systems in drug dependence, in agreement with previous pharmacological studies.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Effect of denervation of the locus coeruleus projections by DSP-4 treatment on [3H]-raclopride binding to dopamine D(2) receptors and D(2) receptor-G protein interaction in the rat striatum

Changes in the control of dopaminergic neurotransmission by noradrenergic locus coeruleus (LC) projections has been implicated in such disorders as depression, drug addiction, and Parkinson's disease. In the present study, the effect of DSP-4, a neurotoxin highly selective for LC projections, on D(2) receptor abundance as assessed by [3H]-raclopride binding in the striatum was studied in rats after administration in doses of 10 and 50 mg/kg either 3 days or 1 month before decapitation. Three days after DSP-4 the levels of noradrenaline in the frontal cortex were dose-dependently reduced; after 1 month, noradrenaline levels were lowered only by the higher dose. DOPAC levels were dose-dependently reduced in the frontal cortex and striatum 3 days but not 1 month after DSP-4 treatment. Cortical 5-HIAA levels were reduced 3 days but not 1 month after DSP-4. The apparent number of D(2) receptor binding sites in the striatum was higher 1 month after either dose of DSP-4. DSP-4 treatment had no effect on [3H]-raclopride binding affinity, the ability of dopamine (DA) to compete with [3H]-raclopride binding and to activate [35S]GTPgammaS binding or on the binding affinities of GDP and [35S]GTPgammaS for corresponding G proteins 1 month after administration of the neurotoxin. These data suggest that after administration of DSP-4, short-term reduction in DA and 5-HT metabolism occurs. Subsequently, an upregulation of D(2) receptor binding sites develops in the striatum even after a minor denervation of the LC projections. Thus, alterations in the LC projection systems elicit lasting adaptive changes in DA-ergic neurotransmission that can serve as a substrate for psychiatric disorders.

Denervation of the locus coeruleus projections by treatment with the selective neurotoxin DSP-4 [N (2-chloroethyl)-N-ethyl-2-bromobenzylamine] reduces dopamine release potential in the nucleus accumbens shell in conscious rats

Pretreatment with DSP-4, a neurotoxin highly selective for the locus coeruleus (LC) noradrenergic projections, 2 weeks before in vivo microdialysis in conscious rats had no effect on baseline extracellular dopamine (DA) levels in the nucleus accumbens shell, but reduced dose-dependently the dopamine response to depolarisation induced by 50 mM KCl. DA metabolism in the frontal cortex, as measured ex vivo, was increased in animals treated with a low (10 mg/kg) but not with a high dose (50 mg/kg) of DSP-4, possibly indicating an increased sensitivity to stress in these animals and thus suggesting differential regulation of DA in the forebrain by the LC lesions. The reduced DA release potential in the nucleus accumbens after DSP-4 treatment suggests that weakening of the LC input to DA nerve cells contributes to motivational deficits.