The D1 dopamine receptor antagonist SCH 23390 increases cocaine self-administration in the rat

Multi-timescale analysis of midbrain dopamine neuronal firing activities

Midbrain dopamine (DA) neurons exhibit spiking and bursting patterns under physiological conditions. Based on the data on electrophysiological recordings, Yu et al. developed a 13-dimensional mathematical model to capture the detailed characteristics of the DA neuronal firing activities. We use the fitting method to simplify the original model into a 4-dimensional model. Then, the spiking-to-bursting transition is detected from a simple and robust mathematical condition. Physiologically, this condition is a balance of the restorative and the regenerative ion channels at resting potential. Geometrically, this condition imposes a transcritical bifurcation. Moreover, we combine singularity theory and singular perturbation methods to capture the geometry of three-timescale firing attractors in a universal unfolding of a cusp singularity. In particular, the planar description of the corresponding firing patterns can generate the corresponding firing attractors. This analysis provides a new idea for understanding the firing activities of the DA neuron and the specific mechanisms for the switching and dynamic regulation among different patterns.

Differential effects of acute and chronic antagonist and an irreversible antagonist treatment on cocaine self-administration behavior in rats

According to pharmacological theory, the magnitude of an agonist-induced response is related to the number of receptors occupied. If there is a receptor reserve, when the number of receptors is altered the fractional occupancy required to maintain this set number of receptors will change. Therefore, any change in dopamine receptor number will result in a change in the concentration of cocaine required to induce the satiety response. Rats that self-administered cocaine were treated with the irreversible monoamine receptor antagonist, EEDQ, or were infused continuously for 14 days with the D₁-like antagonist, SCH23390, treatments known to decrease or increase, respectively, the number of dopamine receptors with a concomitant decrease or increase in response to dopaminergic agonists. The rate of cocaine maintained self-administration increased or decreased in rats treated with EEDQ or withdrawn from chronic SCH23390 infusion, respectively. After EEDQ treatment, the effect ratio of a single dose of SCH23390 or eticlopride were unchanged, indicating that the same SCH23390- and eticlopride-sensitive receptor populations (presumably dopamine) mediated the accelerated cocaine self-administration. Changing the receptor reserve is a key determinant of the rate of cocaine self-administration because the resulting increased or decreased concentration of cocaine results in an accelerated or decelerated rate of cocaine elimination as dictated by first-order kinetics.

Clinical neuropharmacology of cocaine reinforcement: A narrative review of human laboratory self-administration studies

Cocaine use is an unrelenting public health concern. To inform intervention and prevention efforts, it is crucial to develop an understanding of the clinical neuropharmacology of the reinforcing effects of cocaine. The purpose of this review is to evaluate and synthesize human laboratory studies that assess pharmacological manipulations of cocaine self-administration. Forty-one peer-reviewed, human cocaine self-administration studies in which participants received a pretreatment drug were assessed. The pharmacological action and treatment regimen for all drugs reviewed were considered. Drugs that increase extracellular dopamine tend to have the most consistent effects on cocaine self-administration. The ability of nondopaminergic drugs to impact cocaine reinforcement might be related to their downstream effects on dopamine, though it is difficult to draw conclusions because pharmacologically selective compounds are not widely available for human testing. The ability of acute versus chronic drug treatment to differentially affect human cocaine self-administration was not determined because buprenorphine was the only pretreatment drug that was assessed under both acute and chronic dosing regimens. Future research directly comparing acute and chronic drug treatment and/or comparing drugs with different mechanisms of action, is needed to make more conclusive determinations about the clinical neuropharmacology of cocaine reinforcement.

Dopamine neurons gate the intersection of cocaine use, decision making, and impulsivity

Gambling and substance use disorders are highly comorbid. Both clinical populations are impulsive and exhibit risky decision-making. Drug-associated cues have long been known to facilitate habitual drug-seeking, and the salient audiovisual cues embedded within modern gambling products may likewise encourage problem gambling. The dopamine neurons of the ventral tegmental area (VTA) are exquisitely sensitive to drugs of abuse, uncertain rewards, and reward-paired cues and may therefore be the common neural substrate mediating synergistic features of both disorders. To test this hypothesis, we first gained specific inhibitory control over VTA dopamine neurons by transducing a floxed inhibitory DREADD (AAV5-hSyn-DIO-hM4D(Gi)-mCherry) in rats expressing Cre recombinase in tyrosine hydroxylase neurons. We then trained rats in our cued rat gambling task (crGT), inhibiting dopamine neurons throughout task acquisition and performance, before allowing them to self-administer cocaine in the same diurnal period as crGT sessions. The trajectories of addiction differ in women and men, and the dopamine system may differ functionally across the sexes; therefore, we used male and female rats here. We found that inhibition of VTA dopamine neurons decreased cue-induced risky choice and reduced motor impulsivity in males, but surprisingly, enhanced risky decision making in females. Inhibiting VTA dopamine neurons also prevented cocaine-induced changes in decision making in both sexes, but nevertheless drove all animals to consume more cocaine. These findings show that chronic dampening of dopamine signalling can have both protective and deleterious effects on addiction-relevant behaviours, depending on biological sex and dependent variable of interest.

Effects of dopamine receptor antagonism and amphetamine-induced psychomotor sensitization on sign- and goal-tracking after extended training

The dopamine system is important for incentive salience attribution, where motivational value is assigned to conditioned cues that predict appetitive reinforcers. However, the role of dopamine in this process may change with extended training. We tested the effects of dopamine D1-like and D2-like receptor antagonism on the expression of sign-tracking and goal-tracking conditioned responses following extended Pavlovian conditioned approach (PCA) training. We also tested if amphetamine-induced psychomotor sensitization accelerates the enhanced acquisition of sign-tracking that is observed with extended training. In experiment 1, 24 male Long-Evans rats received 20 PCA sessions in which one lever (CS+, 10 s) predicted 0.2 ml sucrose (10 %, w/v) delivery and the other lever (CS-) did not. SCH-23390 (D1-like antagonist) or eticlopride (D2-like antagonist) were administered before non-reinforced behavioural tests at doses of 0, 0.01, and 0.1 mg/kg (s.c.). In experiment 2, rats received vehicle or 2 mg/kg amphetamine (i.p.) for 7 days (n = 12/group). Ten days later, they received 16 PCA training sessions. Both doses of SCH-23390 reduced sign- and goal-tracking, but also reduced locomotor behaviour. A low dose of eticlopride (0.01 mg/kg) selectively reduced goal-tracking, without affecting sign-tracking or locomotor behaviour. Amphetamine produced psychomotor sensitization, and this did not affect the acquisition of sign- or goal-tracking. Following extended PCA training, dopamine D2-like receptor activity is required for the expression of goal-tracking but not sign-tracking. Psychomotor sensitization to amphetamine did not impact incentive salience attribution; however, more selective manipulations of the dopamine system may be needed.

Pharmacological inhibition of glycogen synthase kinase 3 increases operant alcohol self-administration in a manner associated with altered pGSK-3β, protein interacting with C kinase and GluA2 protein expression in the reward pathway of male C57BL/6J mice

Glycogen synthase kinase 3 (GSK-3) is a constitutively active serine-threonine kinase that regulates numerous signaling pathways and has been implicated in neurodegenerative and neuropsychiatric diseases. Alcohol exposure increases GSK-3β (ser9) phosphorylation (pGSK-3β); however, few studies have investigated whether GSK-3 regulates the positive reinforcing effects of alcohol, which drive repetitive drug use. To address this goal, male C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule of sweetened alcohol or sucrose-only reinforcement in operant conditioning chambers. The GSK-3 inhibitor CHIR 99021 (0-10 mg/kg, i.p.) was injected 45 minutes prior to self-administration sessions. After completion of the self-administration dose-effect curve, potential locomotor effects of the GSK-3 inhibitor were assessed. To determine molecular efficacy, CHIR 99021 (10 mg/kg, i.p.) was evaluated on pGSK-3β, GSK-3β, protein interacting with C kinase (PICK1), and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA2 subunit protein expression in amygdala, nucleus accumbens (NAcb), and frontal cortex. Results showed that CHIR 99021 (10 mg/kg) dose-dependently increased alcohol reinforced responding with no effect on sucrose self-administration or locomotor activity. CHIR 99021 (10 mg/kg) significantly decreased pGSK-3β expression in all brain regions tested, reduced PICK1 and increased GluA2 total expression only in the NAcb. We conclude that GSK-3 inhibition increased the reinforcing effects of alcohol in mice. This was associated with reduced pGSK-3β and PICK1, and increased GluA2 expression. Given prior results showing that AMPA receptor activity regulates alcohol self-administration, we propose that signaling through the GSK-3/PICK1/GluA2 molecular pathway drives the positive reinforcing effects of the drug, which are required for abuse liability.

Chemogenetic Inhibition of Dopamine D1-expressing Neurons in the Dorsal Striatum does not alter Methamphetamine Intake in either Male or Female Long Evans Rats

The biochemical and molecular substrates of methamphetamine (METH) use disorder remain to be elucidated. In rodents, increased METH intake is associated with increased expression of dopamine D1 receptors (D1R) in the dorsal striatum. The present study assessed potential effects of inhibiting striatal D1R activity on METH self-administration (SA) by rats. We microinjected Cre-activated adeno-associated viruses to deliver the inhibitory DREADD construct, hM4D (Gi) - mCherry, into neurons that expressed Cre-recombinase (D1-expressing neurons) in the dorsal striatum of male and female transgenic Long Evans rats (Drd1a-iCre#3). Two weeks later, we trained rats to self-administer METH. Once this behavior was acquired, intraperitoneal injections of clozapine-N-Oxide (CNO) or its vehicle (sterile water) were given to rats before each METH SA session to determine the effect of DREADD-mediated inhibition on METH intake. After the end of the experiments, histology was performed to confirm DREADD delivery into the dorsal striatum. There were no significant effects of the inhibitory DREADD on METH SA by male or female rats. Post-mortem histological assessment revealed DREADD expression in the dorsal striatum. Our results suggest that inhibition of D1R in the dorsal striatum does not suppress METH SA. It remains to be determined if activating D1R-expressing neurons might have differential behavioral effects. Future studies will also assess if impacting D1R activity in other brain regions might influence METH SA.

Lateral septum inhibition reduces motivation for cocaine: Reversal by diazepam

The lateral septum (LS) is a brain region implicated in motivation, addiction, anxiety, and affect. We recently found that LS is necessary for cocaine-seeking behaviors including conditioned place preference and reinstatement of extinguished drug seeking, which involve LS input to limbic regions including ventral tegmental area (VTA) and orexin neurons in hypothalamus. Here, we microinjected baclofen-muscimol (B-M) in LS prior to testing in a behavioral economics (BE) paradigm. We found that intra-LS B-M decreased motivation (increased demand elasticity; α) for cocaine, but did not change consumption at low effort (Q₀ ). We also compared the effects of LS inhibition with the effects of treatment with the benzodiazepine diazepam, which has been shown to facilitate reward pathways and disinhibit VTA dopamine neurons. Pretreatment with diazepam blocked the effects of LS inhibition and restored cocaine demand to that following vehicle treatment. These changes in cocaine demand after LS inhibition or diazepam were not due to effects on anxiety, as both manipulations produced similar effects on anxiety measures but opposing effects on drug taking. Collectively, these studies point to LS as a critical region driving motivation for cocaine, likely through its interactions with the mesolimbic dopamine system.

From Synapse to Function: A Perspective on the Role of Neuroproteomics in Elucidating Mechanisms of Drug Addiction

Drug addiction is a complex disorder driven by dysregulation in molecular signaling across several different brain regions. Limited therapeutic options currently exist for treating drug addiction and related psychiatric disorders in clinical populations, largely due to our incomplete understanding of the molecular pathways that influence addiction pathology. Recent work provides strong evidence that addiction-related behaviors emerge from the convergence of many subtle changes in molecular signaling networks that include neuropeptides (neuropeptidome), protein-protein interactions (interactome) and post-translational modifications such as protein phosphorylation (phosphoproteome). Advancements in mass spectrometry methodology are well positioned to identify these novel molecular underpinnings of addiction and further translate these findings into druggable targets for therapeutic development. In this review, we provide a general perspective of the utility of novel mass spectrometry-based approaches for addressing critical questions in addiction neuroscience, highlighting recent innovative studies that exemplify how functional assessments of the neuroproteome can provide insight into the mechanisms of drug addiction.

Exercise Reduces Dopamine D1R and Increases D2R in Rats: Implications for Addiction

INTRODUCTION

Exercise has been shown to be effective for preventing and treating substance abuse in both clinical and preclinical studies. Less is known, however, regarding the underlying neurobiological mechanisms driving these changes in drug-seeking behavior. One possibility is that exercise may alter the mesolimbic dopamine pathway in such a way that makes drugs of abuse less salient and/or rewarding.

METHODS

To examine possible exercise-induced changes in dopamine signaling, male and female Lewis rats were split into exercise and sedentary groups at 8 wk of age. Exercise rats were run on a treadmill at 10 m·min, 5 d·wk, for 6 wk, whereas sedentary rats remained in their home cage. Rats were killed after the 6 wk of treatment, and their brains were used for in vitro autoradiography using [H]SCH 23,390, [H]Spiperone, and [H]WIN55,428 ligands to quantify dopamine type 1-like receptor (D1R)-like, dopamine type 2-like receptor (D2R)-like, and dopamine transporter binding, respectively.

RESULTS

Exercised rats had 18% and 21% lower D1R-like binding levels compared to sedentary rats within the olfactory tubercle and nucleus accumbens shell, respectively. In addition, male and female exercise rats showed greater D2R-like binding levels within the dorsomedial caudate putamen (30%), ventrolateral caudate putamen (24%), and ventromedial caudate putamen (27%), as well as the olfactory tubercle (19%). Greater D2R-like binding in the nucleus accumbens core (24%) and shell (25%) of exercised rats compared with sedentary rats approached significance. No effects were found for dopamine transporter binding.

CONCLUSIONS

These findings support the hypothesis that aerobic exercise results in changes in the mesolimbic pathway that could mediate exercise-induced attenuation of drug-seeking behavior.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

Preweaning iron deficiency increases non-contingent responding during cocaine self-administration in rats

Iron deficiency (ID) is the most prevalent single-nutrient deficiency worldwide. There is evidence that ID early in development (preweaning in rat) causes irreversible neurologic, behavioral, and motor development deficits. Many of these effects have been attributed to damage to dopamine systems, including ID-induced changes in transporter and receptor numbers in the striatum and nucleus accumbens. These mesolimbic dopaminergic neurons are, in part, responsible for mediating reward and thus play a key role in addiction. However, there has been relatively little investigation into the behavioral effects of ID on drug addiction. In 2002, we found that rats made ID from weaning (postnatal day 21) and throughout the experiment acquired cocaine self-administration significantly more slowly than controls and failed to increase responding when the dose of the drug was decreased. In the present study, we assessed addiction for self-administered cocaine in rats with a history of preweaning ID only during postnatal days 4 through 21, and iron replete thereafter. The results showed that while ID did not affect the number of cocaine infusions or the overall addiction-like behavior score, ID rats scored higher on a measure of continued responding for drug than did iron replete controls. This increase in responding, however, was less goal-directed as ID rats also responded more quickly to the non-rewarded manipulandum than did control rats. Thus, while ID early in infancy did not significantly increase addiction-like behaviors for cocaine in this small study, the pattern of data suggests a possible underlying learning or performance impairment. Future studies will be needed to elucidate the exact neuro-behavioral deficits that lead to the increase in indiscriminate responding for drug in rats with a history of perinatal ID.

The Nociceptin Receptor as an Emerging Molecular Target for Cocaine Addiction

Cocaine addiction is a global public health and socioeconomic issue that requires pharmacological and cognitive therapies. Currently there are no FDA-approved medications to treat cocaine addiction. However, in preclinical studies, interventions ranging from herbal medicine to deep-brain stimulation have shown promise for the therapy of cocaine addiction. Recent developments in molecular biology, pharmacology, and medicinal chemistry have enabled scientists to identify novel molecular targets along the pathways involved in drug addiction. In 1994, a receptor that showed a great deal of homology to the traditional opioid receptors was characterized. However, endogenous and exogenous opioids failed to bind to this receptor, which led scientists to name it opioid receptor-like receptor, now referred to as the nociceptin receptor. The endogenous ligand of NOPr was identified a year later and named orphanin FQ/nociceptin. Nociceptin and NOPr are widely distributed throughout the CNS and are involved in many physiological responses, such as food intake, nociceptive processing, neurotransmitter release, etc. Furthermore, exogenous nociceptin has been shown to regulate the activity of mesolimbic dopaminergic neurons, glutamate, and opioid systems, and the stress circuit. Importantly, exogenous nociceptin has been shown to reduce the rewarding and addictive actions of a number of drugs of abuse, such as psychostimulants, alcohol, and opioids. This paper reviews the existing literature on the role of endogenous nociceptin in the rewarding and addictive actions of cocaine. The effect of exogenous nociceptin on these processes is also reviewed. Furthermore, the effects of novel small-molecule NOPr ligands on these actions of cocaine are discussed. Overall, a review of the literature suggests that NOPr could be an emerging target for cocaine addiction pharmacotherapy.

Effects of active anti-methamphetamine vaccination on intravenous self-administration in rats

BACKGROUND

d-Methamphetamine (METH) addiction is a serious public health concern for which successful treatment remains elusive. Immunopharmacotherapy has been shown to attenuate locomotor and thermoregulatory effects of METH. The current study investigated whether active vaccination against METH could alter intravenous METH self-administration in rats.

METHODS

Male Sprague-Dawley rats (Experiment 1: N=24; Experiment 2: N=18) were vaccinated with either a control keyhole-limpet hemocyanin conjugate vaccine (KLH) or a candidate anti-METH vaccine (MH6-KLH) or. Effects of vaccination on the acquisition of METH self-administration under two dose conditions (0.05, 0.1mg/kg/inf) and post-acquisition dose-substitution (0, 0.01, 0.05, 0.20mg/kg/inf, Experiment 1; 0.01, 0.05, 0.10, 0.15mg/kg/inf, Experiment 2) during steady-state responding were investigated. Plasma METH concentrations were determined 30min after an acute challenge dose of 3.2mg/kg METH.

RESULTS

Active vaccination inhibited the acquisition of METH self-administration under the 0.1mg/kg/inf dose condition, with 66% of the MH6-KLH-vaccinated rats compared to 100% of the controls reaching criteria, and produced transient and dose-dependent effects on self-administration during the maintenance phase. Under the 0.05mg/kg/inf dose condition, MH6-KLH-vaccinated rats initially self-administered more METH than controls, but then self-administration decreased across the acquisition phase relative to controls; a subsequent dose-response assessment confirmed that MH6-KLH-vaccinated rats failed to acquire METH self-administration. Finally, plasma METH concentrations were higher in MH6-KLH-vaccinated rats compared to controls after an acute METH challenge, and these were positively correlated with antibody titers.

CONCLUSIONS

These data demonstrate that active immunopharmacotherapy for METH attenuates the acquisition of METH self-administration.

Impairment of acquisition of intravenous cocaine self-administration by RNA-interference of dopamine D1-receptors in the nucleus accumbens shell

Microdialysis during i.v. drug self-administration (SA) have implicated nucleus accumbens (NAc) shell DA in cocaine and heroin reinforcement. However, this correlative evidence has not been yet substantiated by experimental evidence obtained by studying the effect of selective manipulation of NAc shell DA transmission on cocaine and heroin SA. In order to investigate this issue, DA D1a receptor (D1aR) expression was impaired in the NAc shell and core by locally infusing lentiviral vectors (LV) expressing specific D1aR-siRNAs (LV-siRNAs). Control rats were infused in the same areas with LV expressing GFP. Fifteen days later, rats were trained to acquire i.v. cocaine or heroin self-administration (SA). At the end of behavioral experiments, in order to evaluate the effect of LV-siRNA on D1aR expression, rats were challenged with amphetamine and the brains were processed for immunohistochemical detection of c-Fos and D1aR. Control rats acquired i.v. cocaine and heroin SA. Infusion of LV-siRNAs in the medial NAc shell reduced D1aR density and the number of c-Fos positive nuclei in the NAc shell, while sparing the core, and prevented the acquisition of cocaine, but not heroin SA. In turn, LV-siRNAs infusion in the core reduced D1aR density and the number of c-Fos positive nuclei in the same area, while sparing the shell, and failed to affect acquisition of cocaine. The differential effect of LV impairment of NAc shell D1aR on cocaine and heroin SA indicates that NAc shell DA acting on D1aR specifically mediates cocaine reinforcement.

Alcohol alters the activation of ERK1/2, a functional regulator of binge alcohol drinking in adult C57BL/6J mice

BACKGROUND

Binge alcohol drinking is a particularly risky pattern of alcohol consumption that often precedes alcohol dependence and addiction. The transition from binge alcohol drinking to alcohol addiction likely involves mechanisms of synaptic plasticity and learning in the brain. The mitogen-activated protein kinase (MAPK) signaling cascades have been shown to be involved in learning and memory, as well as the response to drugs of abuse, but their role in binge alcohol drinking remains unclear. The present experiments were designed to determine the effects of acute alcohol on extracellular signaling-related kinases (ERK1/2) expression and activity and to determine whether ERK1/2 activity functionally regulates binge-like alcohol drinking.

METHODS

Adult male C57BL/6J mice were injected with ethanol (EtOH) (3.0 mg/kg, intraperitoneally) 10, 30, or 90 minutes prior to brain tissue collection. Next, mice that were brought to freely consume unsweetened EtOH in a binge-like access procedure were pretreated with the MEK1/2 inhibitor SL327 or the p38 MAPK inhibitor SB239063.

RESULTS

Acute EtOH increased pERK1/2 immunoreactivity relative to vehicle in brain regions known to be involved in drug reward and addiction, including the central amygdala and prefrontal cortex. However, EtOH decreased pERK1/2 immunoreactivity relative to vehicle in the nucleus accumbens core. SB239063 pretreatment significantly decreased EtOH consumption only at doses that also produced nonspecific locomotor effects. SL327 pretreatment significantly increased EtOH, but not sucrose, consumption without inducing generalized locomotor effects.

CONCLUSIONS

These findings indicate that ERK1/2 MAPK signaling regulates binge-like alcohol drinking. As alcohol increased pERK1/2 immunoreactivity relative to vehicle in brain regions known to regulate drug self-administration, SL327 may have blocked this direct pharmacological effect of alcohol and thereby inhibited the termination of binge-like drinking.

Dopamine D1 receptor blockade impairs alcohol seeking without reducing dorsal striatal activation to cues of alcohol availability

INTRODUCTION

Alcohol-associated cues activate both ventral and dorsal striatum in functional brain imaging studies of heavy drinkers. In rodents, alcohol-associated cues induce changes in neuronal firing frequencies and increase dopamine release in ventral striatum, but the impact of alcohol-associated cues on neuronal activity in dorsal striatum is unclear. We previously reported phasic changes in action potential frequency in the dorsomedial and dorsolateral striatum after cues that signaled alcohol availability, prompting approach behavior.

METHODS

We investigated the hypothesis that dopamine transmission modulates these phasic firing changes. Rats were trained to self-administer alcohol, and neuronal activity was monitored with extracellular electrophysiology during "anticipatory" cues that signaled the start of the operant session. Sessions were preceded by systemic administration of the D1-type dopamine receptor antagonist SCH23390 (0, 10, and 20 μg/kg).

RESULTS

SCH23390 significantly decreased firing rates during the 60 s prior to cue onset without reducing phasic excitations immediately following the cues. While neuronal activation to cues might be expected to initiate behavioral responses, in this study alcohol seeking was reduced despite the presence of dorsal striatal excitations to alcohol cues.

CONCLUSIONS

These data suggest that D1 receptor antagonism reduces basal firing rates in the dorsal striatum and modulates the ability of neuronal activation to "anticipatory" cues to initiate alcohol seeking in rats with an extensive history of alcohol self-administration.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Responding for a conditioned reinforcer, and its enhancement by nicotine, is blocked by dopamine receptor antagonists and a 5-HT(2C) receptor agonist but not by a 5-HT(2A) receptor antagonist

An aspect of nicotine reinforcement that may contribute to tobacco addiction is the effect of nicotine to enhance the motivational properties of reward-associated cues, or conditioned stimuli (CSs). Several studies have now shown that nicotine enhances responding for a stimulus that has been paired with a natural reinforcer. This effect of nicotine to enhance responding for a conditioned reinforcer is likely due to nicotine-induced enhancements in mesolimbic dopaminergic activity, but this has not been directly assessed. In this study, we assessed roles for dopamine (DA) D1 or D2 receptors, and two serotonin (5-HT) receptor subtypes known to modulate DA activity, the 5-HT2C or 5-HT2A subtypes, on nicotine-enhanced responding for a conditioned reinforcer. Water-restricted rats were exposed to Pavlovian conditioning sessions, where a CS was paired with water delivery. Then, in a second phase, animals were required to perform a novel, lever-pressing response for presentations of the CS as a conditioned reinforcer. Nicotine (0.4 mg/kg) enhanced responding for the conditioned reinforcer. To examine potential roles for dopamine (DA) and serotonin (5-HT) receptors in this effect, separate groups of animals were used to assess the impact of administering the D1 receptor antagonist SCH 23390, D2 receptor antagonist eticlopride, 5-HT2C receptor agonist Ro 60-0175, or 5-HT2A receptor antagonist M100907 on nicotine-enhanced responding for conditioned reinforcement. SCH 23390, eticlopride, and Ro 60-0175 all reduced responding for conditioned reinforcement, and the ability of nicotine to enhance this effect. M100907 did not alter this behavior. Together, these studies indicate that DA D1 and D2 receptors, but not 5-HT2A receptors, contribute to the effect of nicotine to enhance responding for a conditioned reinforcer. This effect can also be modulated by 5-HT2C receptor activation.

Maintained cocaine self-administration is determined by quantal responses: implications for the measurement of antagonist potency

The change in frequency of cocaine self-administration as a function of the unit dose is widely assumed to represent a graded pharmacodynamic response. Alternatively, a pharmacological theory states that during maintained self-administration, a quantal response occurs at a minimum maintained cocaine concentration (satiety threshold). Rats self-administered cocaine at unit doses spanning an 8-fold range from 0.75 to 6 µmol/kg. Despite an approximately 7-fold difference in the interinjection intervals, there were no differences in the plasma cocaine concentration at the time of lever press across this range of unit doses, consistent with the satiety threshold representing an equiactive cocaine concentration. Because self-administration always occurs when cocaine concentrations decline back to the satiety threshold, this behavior represents a process of automatic back titration of equiactive agonist concentrations. Therefore, the lower frequency of self-administration at higher unit doses is caused by an increase in the duration of the cocaine-induced satiety response, and the graded dose-frequency relationship is due to cocaine pharmacokinetics. After the interinjection intervals at a particular unit dose were stable, rats were injected with the competitive D₁-like dopamine receptor antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390; 15 nmol/kg intravenously) and the session continued. At all cocaine unit doses, SCH23390 accelerated self-administration with a concomitant increase in the calculated satiety threshold, and these equiactive cocaine concentration ratios were independent of the cocaine unit dose. Therefore, the measurement of antagonist potency requires only a single unit dose of cocaine, selected on the basis of convenience, and using multiple cocaine unit doses is redundant.

The ClockΔ19 mutation in mice fails to alter the primary and secondary reinforcing properties of nicotine

BACKGROUND

Clock genes have been demonstrated to play a role in behavioral responses to a variety of drugs of abuse, including cocaine, amphetamine, morphine, and ethanol. However, no studies to date have examined the role of Clock genes on nicotine-mediated behaviors. We examined the involvement of Clock, one of several Clock genes, on the effects of nicotine by examining mice with the ClockΔ19 mutation in behaviors commonly used to assess drug effects in rodents.

METHODS

We first measured the locomotor effects of nicotine in mutants and wild type mice in response to repeated nicotine injections (0.175 mg/kg, IP). To assess the secondary properties of nicotine, we measured the ability of nicotine (0.175 mg/kg, IP) to induce a conditioned place preference. Finally, we measured the primary reinforcing properties of nicotine at two doses (0.01 and 0.03 mg/kg/infusion, IV) using the self-administration paradigm.

RESULTS

Mutant mice demonstrated no difference in magnitude of the sensitized response to nicotine as compared to wild-type controls. In the conditioned place preference paradigm, mutant and wild-type mice demonstrated a similar preference for a nicotine-paired environment. And finally, mutant and wild-type mice demonstrated a similar acquisition of nicotine self-administration, as indicated by the number of responses on a nicotine-paired lever and the number of nicotine reinforcers achieved during sessions.

CONCLUSIONS

The ClockΔ19 mutation appears to have no effect on the reinforcing properties of nicotine, in contrast to its demonstrated role in cocaine reinforcement. Further studies are needed to determine the effect of other Clock genes on nicotine reinforcement.

Role of insular cortex D₁ and D₂ dopamine receptors in nicotine self-administration in rats

The insular cortex has been associated with the processing of rewarding stimuli and with the neural bases of drug addiction. Ischemic damage to the insula has been associated with decreased desire to smoke cigarettes. Which component of insular function is involved in the neural basis of cigarette smoking is not clear. Dopamine systems are crucial for the reinforcing value of addictive drugs. The DA projection from the ventral tegmental area to the nucleus accumbens (NAc) has been shown to be a vital pathway for the primary reinforcement caused by taking a variety of abused drugs. In the current set of studies, the roles of D₁ and D₂ receptors in the insular cortex in the self-administration of nicotine by rats were assessed. Adult female Sprague-Dawley rats were fitted with jugular catheters and given access to self-administer nicotine. Bilateral local infusion cannulae were implanted into the agranular insular cortex to locally administer D₁ and D₂ antagonists (SCH-23390 and haloperidol). Acute local infusions of the D₁ antagonist SCH-23390 into the insula (1-2 μg/side) significantly decreased nicotine self-administration by more than 50%. Repeated infusions of SCH-23390 into the agranular insula caused continuing decreases in nicotine self-administration without signs of tolerance. In contrast, local infusions of the D₂ antagonist haloperidol 0.5-2 μg/side did not have any discernable effect on nicotine self-administration. These studies show the importance of DA D₁ systems in the insula for nicotine reward.

Prior access to a sweet is more protective against cocaine self-administration in female rats than in male rats

It is well established that female rats are more sensitive than male rats to the reinforcing effects of cocaine (Lynch, 2008 [42] for review). We hypothesized that greater preference for cocaine would support greater avoidance of a cocaine-paired taste cue in female vs. male rats. Moreover, at least in male rats, greater avoidance of the taste cue is associated with greater cocaine self-administration (Grigson and Twining, 2002 [3]). Thus, we anticipated that female rats would not only demonstrate greater avoidance of the drug-paired taste cue, but greater drug-taking as well. We tested these hypotheses by examining avoidance of a saccharin cue in male and female rats following several pairings with self-administered saline or cocaine (0.16, 0.33, or 0.66 mg/infusion). Contrary to expectations, the results showed that female rats exhibited less avoidance of the cocaine-associated saccharin cue than male rats and self-administered less, rather than more, cocaine, Thus, while female rats reportedly take more drug than male rats when the drug is presented in the absence of an alternative reward, they take less drug than male rats when the opportunity to self-administer cocaine is preceded by access to a palatable sweet. Females, then, may not simply be more sensitive to the rewarding properties of drug, but also to the reinforcing properties of natural rewards and this increase in sensitivity to sweets may serve to protect against drug-taking behavior.

Trace amine-associated receptor 1 partial agonism reveals novel paradigm for neuropsychiatric therapeutics

BACKGROUND

Trace amines, compounds structurally related to classical biogenic amines, represent endogenous ligands of the trace amine-associated receptor 1 (TAAR1). Because trace amines also influence the activity of other targets, selective ligands are needed for the elucidation of TAAR1 function. Here we report on the identification and characterization of the first selective and potent TAAR1 partial agonist.

METHODS

The TAAR1 partial agonist RO5203648 was evaluated for its binding affinity and functional activity at rodent and primate TAAR1 receptors stably expressed in HEK293 cells, for its physicochemical and pharmacokinetic properties, for its effects on the firing frequency of monoaminergic neurons ex vivo, and for its properties in vivo with genetic and pharmacological models of central nervous system disorders.

RESULTS

RO5203648 showed high affinity and potency at TAAR1, high selectivity versus other targets, and favorable pharmacokinetic properties. In mouse brain slices, RO5203648 increased the firing frequency of dopaminergic and serotonergic neurons in the ventral tegmental area and the dorsal raphe nucleus, respectively. In various behavioral paradigms in rodents and monkeys, RO5203648 demonstrated clear antipsychotic- and antidepressant-like activities as well as potential anxiolytic-like properties. Furthermore, it attenuated drug-taking behavior and was highly effective in promoting attention, cognitive performance, and wakefulness.

CONCLUSIONS

With the first potent and selective TAAR1 partial agonist, RO5203648, we show that TAAR1 is implicated in a broad range of relevant physiological, behavioral, and cognitive neuropsychiatric dimensions. Collectively, these data uncover important neuromodulatory roles for TAAR1 and suggest that agonists at this receptor might have therapeutic potential in one or more neuropsychiatric domains.

Seeking-taking chain schedules of cocaine and sucrose self-administration: effects of reward size, reward omission, and α-flupenthixol

RATIONALE

In heterogeneous seeking-taking (ST) chain schedules of self-administration, seeking rewards and taking rewards are distinct actions, giving animals explicit control over their intake of the reward. However, the neurobehavioral characteristics of ST chain schedules are relatively unexplored.

OBJECTIVES

This study was made to evaluate two variants of ST chain schedules of self-administration to measure seeking and taking of sucrose and cocaine in rats.

METHODS

Rats had to respond on one lever (seeking lever) under a random interval (RI) or under a progressive ratio (PR) schedule, to gain access to a second lever (taking lever), responding on which under a fixed-ratio 1 (FR-1) schedule of reinforcement delivered the reward. We assessed the effects of reward size, reward omission, and administration of the dopamine receptor antagonist α-flupenthixol. The effects of α-flupenthixol on responding for cocaine or sucrose under an FR-1 schedule of reinforcement were also assessed.

RESULTS

Cocaine seeking under both schedules was reduced by decreasing reward size, reward omission, and α-flupenthixol treatment. Cocaine taking was decreased by α-flupenthixol treatment and reward omission, but not by altering reward size. Sucrose seeking was not affected by reward size, but was reduced by α-flupenthixol and reward omission. Sucrose taking was diminished by reward omission only. α-Flupenthixol increased cocaine but not sucrose intake under an FR-1 schedule of reinforcement.

CONCLUSIONS

Both ST(PR) and ST(RI) schedules can be used to assess seeking and taking of sucrose and cocaine. Dopaminergic neurotransmission mediates the positive subjective properties of cocaine but not sucrose and the motivational properties of both sucrose and cocaine.

Therapeutic-like properties of a dopamine uptake inhibitor in animal models of amphetamine addiction

N-substituted benztropine (BZT) analogs are molecules that display high affinity for the dopamine transporter (DAT), therapeutic-like effects in animal models of cocaine abuse, and psychopharmacological characteristics consistent with those of a substitute medication for cocaine addiction. Since amphetamine (Amph) and cocaine share mechanisms of action at the DAT, we evaluated the effectiveness of a BZT analog in animal models of Amph addiction. We tested in mice and rats the effects of the BZT derivative, 3α-[bis(4-fluorophenyl)methoxy]-tropane (AHN-1055), on Amph-induced conditioned place preference (CPP), locomotor activity, sensitization, self-administration and ΔFosB accumulation in the nucleus accumbens (NAc). The results showed that AHN-1055 did not produce rewarding, stimulant, or sensitized locomotor effects in mice when administered alone but it readily blocked the rewarding, stimulant, and sensitizing effects of repeated Amph exposure. Furthermore, in mice undergoing conditioning in the CPP paradigm, the BZT analog prevented the accumulation of ΔFosB protein induced in the NAc shell region by Amph treatment. Notably, treatment with AHN-1055 dose-dependently reduced Amph self-administration in rats with a steady history of voluntary Amph intake. These results provide a straightforward demonstration that a BZT derivative with binding affinity for DAT exhibits high efficacy in animal models of Amph abuse, suggesting that the novel generation of BZT analogs could have wider therapeutic applications in stimulant-spectrum disorders than those previously recognized.

Intraventricular administration of neuropeptide S has reward-like effects

Neuropeptide S (NPS) is an endogenous brain peptide produced by neurons located in the lower brainstem, and functional studies suggest that NPS has arousing effects. Because its receptors are found in reward-associated regions throughout the brain, we evaluated whether intraventricular NPS injections elicit reward-related effects in rats. Rats increased lever presses that led to intraventricular administration of NPS (0.34-34 pmol per infusion) in a dose dependent manner, with a cue-assisted procedure. Cue-assisted self-administration of NPS was decreased by systemic administration of the dopamine receptor antagonist SCH 23390 (0.025 mg/kg, i.p.) or the hypocretin-1 (orexin-1) receptor antagonist SB 334867 (20 mg/kg, i.p.). In addition, intraventricular NPS injections (1000 pmol) induced conditioned place preference, whereas a lower dose (100 pmol) of NPS induced conditioned place aversion. Finally, NPS injections (100-1000 pmol) acutely facilitated locomotor activity, whereas repeated NPS injections did not lead to locomotor sensitization. Our data suggest that intraventricular NPS injections have reward-like effects in that NPS weakly facilitates seeking and induces positive reinforcement. These effects may depend on intact dopamine and hypocretin systems.

Is slow-onset long-acting monoamine transport blockade to cocaine as methadone is to heroin? Implication for anti-addiction medications

The success of methadone in treating opiate addiction has suggested that long-acting agonist therapies may be similarly useful for treating cocaine addiction. Here, we examined this hypothesis, using the slow-onset long-acting monoamine reuptake inhibitor 31,345, a trans-aminotetralin analog, in a variety of addiction-related animal models, and compared it with methadone's effects on heroin's actions in the same animal models. Systemic administration of 31,345 produced long-lasting enhancement of electrical brain-stimulation reward (BSR) and extracellular nucleus accumbens (NAc) dopamine (DA). Pretreatment with 31,345 augmented cocaine-enhanced BSR, prolonged cocaine-enhanced NAc DA, and produced a long-term (24-48  h) reduction in cocaine self-administration rate without obvious extinction pattern, suggesting an additive effect of 31,345 with cocaine. In contrast, methadone pretreatment not only dose-dependently inhibited heroin self-administration with an extinction pattern but also dose-dependently inhibited heroin-enhanced BSR and NAc DA, suggesting functional antagonism by methadone of heroin's actions. In addition, 31,345 appears to possess significant abuse liability, as it produces dose-dependent enhancement of BSR and NAc DA, maintains a low rate of self-administration behavior, and dose-dependently reinstates drug-seeking behavior. In contrast, methadone only partially maintains self-administration with an extinction pattern, and fails to induce reinstatement of drug-seeking behavior. These findings suggest that 31,345 is a cocaine-like slow-onset long-acting monoamine transporter inhibitor that may act as an agonist therapy for cocaine addiction. However, its pattern of action appears to be significantly different from that of methadone. Ideal agonist substitutes for cocaine should fully emulate methadone's actions, that is, functionally antagonizing cocaine's action while blocking monoamine transporters to augment synaptic DA.

Using the self-administration of apomorphine and cocaine to measure the pharmacodynamic potencies and pharmacokinetics of competitive dopamine receptor antagonists

Competitive dopamine receptor antagonists accelerate psychomotor stimulant self-administration. According to pharmacological theory of competitive antagonism antagonists raise the equiactive agonist concentration. In the self-administration paradigm this is assumed to be the satiety threshold or C(min). The magnitude of the proportional increase in satiety threshold (agonist concentration ratio) as a function of antagonist dose should reflect the antagonist pharmacodynamic potency. The time course of this effect should reflect the rate of change of antagonist occupancy of receptors and, therefore, antagonist concentration, i.e. pharmacokinetics. Rats self-administered apomorphine or cocaine at a stable rate and were then injected i.v. with one of four competitive D₁-like or D₂-like dopamine receptor antagonists and the session continued. The agonist concentrations at the time of each self-administration (satiety thresholds) were calculated during the session. The antagonists accelerated self-administration of both agonists with a concomitant increase in the calculated satiety thresholds. The maximum agonist concentration ratio was proportional to the dose of antagonist. The time courses of the changes in agonist concentration ratio were independent of the agonist and of the dose of antagonist. Schild analysis of the maximum agonist concentration ratio as a function of the antagonist dose allowed apparent pA₂ (or K(dose)) to be measured. Antagonist K(dose) values should provide a quantitative basis for receptor identification in behavioral pharmacology. The assay system may also measure the pharmacokinetics of antagonist elimination from the brain. Agonist self-administration represents a sensitive in vivo pharmacological assay system that provides information useful for pharmacokinetic/pharmacodynamic modeling of antagonist effects.

Competitive dopamine receptor antagonists increase the equiactive cocaine concentration during self-administration

Competitive dopamine receptor antagonists increase the rate of cocaine self-administration. As the rate of self-administration at a particular unit dose is determined by the satiety threshold and the elimination half-life (t(½)) of cocaine, we investigated whether dopamine receptor antagonists altered these parameters in rats. The plasma cocaine concentration at the time of each self-administration was constant during a session demonstrating that this satiety threshold concentration represents an equiactive cocaine concentration. The plasma cocaine concentration at the time of self-administration was increased by SCH23390, consistent with pharmacological theory. In rats trained to reliably self-administer cocaine, SCH23390 had no effect on the plasma steady-state cocaine concentration produced by constant infusions of cocaine. Therefore, this antagonist had no effect on cocaine t(½) at a dose that accelerated cocaine self-administration. A constant cocaine infusion at a rate that maintained steady state concentrations above the satiety threshold stopped self-administration. SCH23390, or the D₂ dopamine receptor antagonist (-)eticlopride, reinstated self-administration in the presence of the constant cocaine infusion. This is consistent with SCH23390 and eticlopride raising the satiety threshold above the steady state level produced by the constant cocaine infusion. It is concluded that the antagonist-induced acceleration of cocaine self-administration is the result of a pharmacokinetic/pharmacodynamic interaction whereby the rate of cocaine elimination is faster at the higher concentrations, as dictated by first-order kinetics, so that cocaine levels decline more rapidly to the elevated satiety threshold. This results in the decreased interinjection intervals.

Obesity-resistant S5B rats showed greater cocaine conditioned place preference than the obesity-prone OM rats

BACKGROUND

Dopamine (DA) and the DA D2 receptor (D2R) are involved in the rewarding and conditioned responses to food and drug rewards. Osborne-Mendel (OM) rats are genetically prone and S5B/P rats are genetically resistant to obesity when fed a high-fat diet. We hypothesized that the differential sensitivity of these two rat strains to natural rewards may also be reflected in sensitivity to drugs of abuse. Therefore, we tested whether OM and S5B/P rats showed a differential preference to cocaine using conditioned place preference (CPP). To also evaluate whether there is specific involvement of the D2R in this differential conditioning sensitivity, we then tested whether the D2R agonist bromocriptine (BC) would differentially affect the effects of cocaine in the two strains.

METHODS

OM and S5B/P rats were conditioned with cocaine (5 or 10mg/kg) in one chamber and saline in another for 8days. Rats were then tested for cocaine preference. The effects of BC (0.5, 1, 5, 10, 20mg/kg) on cocaine preference were then assessed in subsequent test sessions.

RESULTS

OM rats did not show a significant preference for the cocaine-paired chamber on test day. Only the S5B/P rats showed cocaine CPP. Later treatment with only the highest dose of BC resulted in reduced cocaine CPP in S5B/P rats when treated with 5mg/kg cocaine and in OM rats treated with 10mg/kg cocaine.

CONCLUSION

Our results indicated that obesity-resistant S5B rats showed greater cocaine CPP than the obesity-prone OM rats. These findings do not support a theory of common vulnerability for reinforcer preferences (food and cocaine). However, they show that BC reduced cocaine conditioning effects supporting at least a partial regulatory role of D2R in conditioned responses to drugs.

Nicotine and cocaine self-administration using a multiple schedule of intravenous drug and sucrose reinforcement in rats

There appears to be a relatively narrow range of contingencies in which intravenous (i.v) infusions of nicotine will maintain responding in rats. The schedule of reinforcement typically used when investigating i.v. nicotine self-administration is a simple fixed-ratio (FR) schedule. This study determined if responding in rats could be established using a multiple schedule of either i.v. cocaine or nicotine and sucrose reinforcement. Following training of individual components with each reinforcer, rats were placed on an FR15 60-s timeout multiple schedule of cocaine (0.3 mg/kg/infusion) and sucrose (45 mg pellets) reinforcement or an FR5 60-s timeout multiple schedule of nicotine (0.03 mg/kg/infusion) and sucrose (45 mg pellets) reinforcement. Both cocaine and nicotine maintained significant levels of responding under the multiple schedule. Pretreatment with the dopamine D1 antagonist SCH 23390 increased cocaine-maintained responding, but not sucrose responding. Acute pretreatment with the nicotinic antagonist mecamylamine or SCH 23390 specifically decreased nicotine self-administration. Extinction of the individual nicotine and sucrose components resulted in decreases in responding in each component under extinction. These results indicate that i.v. nicotine maintains responding under a multiple schedule. This procedure may be useful when studying the specificity of drug pretreatments on nicotine self-administration.

Extracellular fluctuations of dopamine and glutamate in the nucleus accumbens core and shell associated with lever-pressing during cocaine self-administration, extinction, and yoked cocaine administration

RATIONALE

Dopamine and glutamate in the nucleus accumbens (NAS) are differentially implicated in cocaine-directed behavior.

OBJECTIVES

We sought to compare extracellular fluctuations of dopamine and glutamate in core and shell of NAS associated with operant responding during cocaine self-administration, extinction, and yoked cocaine administration.

METHODS

Rats were trained to lever-press for cocaine or saline under FR1 before undergoing microdialysis testing during cocaine self-administration, extinction, or yoked cocaine administration. Microdialysis samples were collected every 20 min and were analyzed for dopamine and glutamate with high-performance liquid chromatography.

RESULTS

Rats actively lever-pressed during cocaine self-administration and extinction. However, lever-pressing was minimal during yoked cocaine administration in both cocaine-trained and saline-trained rats. Dopamine was elevated throughout cocaine self-administration and yoked cocaine administration. The extent of cocaine-evoked dopamine was greater in shell than in core, greater in cocaine-trained than in saline-trained rats, and greater during self-administration than during yoked administration. Dopamine was also elevated in core (first 60 min) and in shell (first 40 min) during extinction. Basal concentration of glutamate, but not dopamine, was lower in cocaine-trained than in saline-trained rats. In cocaine-trained rats, glutamate was elevated during cocaine self-administration and extinction but was depressed below baseline during yoked cocaine administration. The extent and direction of glutamate fluctuation was similar between core and shell. In saline-trained rats, glutamate was not affected by yoked cocaine.

CONCLUSION

Distinct patterns of dopamine and glutamate fluctuations in core and shell appear to underlie characteristic patterns of lever-pressing associated with cocaine self-administration, extinction, and yoked cocaine administration.

A dopamine transport inhibitor with markedly low abuse liability suppresses cocaine self-administration in the rat

RATIONALE

N-substituted benztropine analogs are potent dopamine uptake inhibitors that display pharmacokinetic/dynamic properties consistent with the profile of a substitute medication for cocaine addiction.

OBJECTIVES

The purpose of the present experiments was to characterize in rats the addictive-like properties of one such analog, 3 alpha-[bis(4'-fluorophenyl)methoxy]-tropane (AHN-1055), incorporating probes of its stimulant and incentive/motivational effects and of its ability to influence cocaine self-administration.

METHODS

We used open field activity and drug self-administration assays. To examine the effects of AHN-1055 on locomotor behavior, the analog was administered alone (0, 1, 3, and 10 mg/kg intraperitoneally) and in combination with cocaine (15 mg/kg i.p.). The influence of AHN-1055 on cocaine's intake was studied by administering the analog (0, 3, and 10 mg/kg i.p.) before the start of the self-administration sessions. To compare the addictive-like properties of AHN-1055 and cocaine, progressive ratio performance and abstinence-induced context-conditioned relapse were evaluated.

RESULTS

AHN-1055 evoked robust and sustained locomotor activity when administered alone and increased cocaine-induced locomotor stimulation. Notably, the analog showed by comparison to cocaine weak reinforcing efficacy in a modified progressive ratio schedule of drug reinforcement, and contrary to cocaine, it showed no ability to promote context-conditioned relapse to drug seeking following stable self-administration and abstinence. Further, AHN-1055 treatment blocked cocaine intake dose-dependently in rats with a steady history of cocaine self-administration without reducing responding for sucrose, a natural reward.

CONCLUSIONS

These findings demonstrate essential psychopharmacological differences between AHN-1055 and cocaine and highlight important properties of the analog as a possible pharmacotherapy in cocaine addiction.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

Genetics of dopamine receptors and drug addiction: a comprehensive review

Drug dependence is a chronic, relapsing disorder in which compulsive drug-seeking and drug-taking behaviours persist despite serious negative consequences. Addictive substances, such as opioids, ethanol, psychostimulants and nicotine, induce pleasant states or relieve distress, effects that contribute to their recreational use. Dopamine is critically involved in drug addiction processes. However, the role of the various dopaminergic receptor subtypes has been difficult to delineate. Here, we will review the information collected implicating the receptors of the D1 family (DRD1 and DRD5) and of the D2 family (DRD2, DRD3 and DRD4) in drug addiction. We will summarize the distribution of these receptors in the brain, the preclinical experiments carried out with pharmacological and transgenic approaches and the genetic studies carried out linking genetic variants of these receptors to drug addiction phenotypes. A meta-analysis of the studies carried out evaluating DRD2 and alcohol dependence is also provided, which indicates a significant association. Overall, this review indicates that different aspects of the addiction phenotype are critically influenced by dopaminergic receptors and that variants of those genes seem to influence some addiction phenotypes in humans.

Methylphenidate-induced dendritic spine formation and DeltaFosB expression in nucleus accumbens

Methylphenidate is the psychostimulant medication most commonly prescribed to treat attention deficit hyperactivity disorder (ADHD). Recent trends in the high usage of methylphenidate for both therapeutic and nontherapeutic purposes prompted us to investigate the long-term effects of exposure to the drug on neuronal adaptation. We compared the effects of chronic methylphenidate or cocaine (15 mg/kg, 14 days for both) exposure in mice on dendritic spine morphology and DeltaFosB expression in medium-sized spiny neurons (MSN) from ventral and dorsal striatum. Chronic methylphenidate increased the density of dendritic spines in MSN-D1 (MSN-expressing dopamine D1 receptors) from the core and shell of nucleus accumbens (NAcc) as well as MSN-D2 (MSN-expressing dopamine D2 receptors) from the shell of NAcc. In contrast, cocaine increased the density of spines in both populations of MSN from all regions of striatum. In general, the effect of methylphenidate on the increase of shorter spines (class 2) was less than that of cocaine. Interestingly, the methylphenidate-induced increase in the density of relatively longer spines (class 3) in the shell of NAcc was bigger than that induced by cocaine. Furthermore, methylphenidate exposure increased expression of DeltaFosB only in MSN-D1 from all areas of striatum, and surprisingly, the increase was greater than that induced by cocaine. Thus, our results show differential effects of methylphenidate and cocaine on neuronal adaptation in specific types of MSN in reward-related brain regions.

NMDA receptors regulate nicotine-enhanced brain reward function and intravenous nicotine self-administration: role of the ventral tegmental area and central nucleus of the amygdala

Nicotine is considered an important component of tobacco responsible for the smoking habit in humans. Nicotine increases glutamate-mediated transmission throughout brain reward circuitries. This action of nicotine could potentially contribute to its intrinsic rewarding and reward-enhancing properties, which motivate consumption of the drug. Here we show that the competitive N-methyl-D-aspartate (NMDA) receptor antagonist LY235959 (0.5-2.5 mg per kg) abolished nicotine-enhanced brain reward function, reflected in blockade of the lowering of intracranial self-stimulation (ICSS) thresholds usually observed after experimenter-administered (0.25 mg per kg) or intravenously self-administered (0.03 mg per kg per infusion) nicotine injections. The highest LY235959 dose (5 mg per kg) tested reversed the hedonic valence of nicotine from positive to negative, reflected in nicotine-induced elevations of ICSS thresholds. LY235959 doses that reversed nicotine-induced lowering of ICSS thresholds also markedly decreased nicotine self-administration without altering responding for food reinforcement, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor antagonist NBQX had no effects on nicotine intake. In addition, nicotine self-administration upregulated NMDA receptor subunit expression in the central nucleus of the amygdala (CeA) and ventral tegmental area (VTA), suggesting important interactions between nicotine and the NMDA receptor. Furthermore, nicotine (1 microM) increased NMDA receptor-mediated excitatory postsynaptic currents in rat CeA slices, similar to its previously described effects in the VTA. Finally, infusion of LY235959 (0.1-10 ng per side) into the CeA or VTA decreased nicotine self-administration. Taken together, these data suggest that NMDA receptors, including those in the CeA and VTA, gate the magnitude and valence of the effects of nicotine on brain reward systems, thereby regulating motivation to consume the drug.

The effect of a chimeric human/murine anti-cocaine monoclonal antibody on cocaine self-administration in rats

The predominantly human sequence anti-cocaine monoclonal antibody (mAb), 2E2, has high affinity and specificity for cocaine and antagonizes cocaine distribution to the brain in mice. To determine whether 2E2 can alter the self-administration of cocaine in rats, both cocaine-induced reinstatement (priming) of self-administration, and the rates of cocaine consumption were assessed during daily sessions. After self-administration training, the rats' cocaine priming threshold values were stable over a 2-week baseline period. Furthermore, the rates of cocaine consumption at unit doses of 0.3 and 3.0 micromol/kg were steady within sessions and stable between sessions. Then, 2E2 (120 mg/kg i.v.) or an equivalent dose of nonspecific human polyclonal IgG (control) was infused and daily sessions continued. 2E2 produced an initial, approximately 3-fold, increase in the cocaine priming threshold that declined toward baseline values over the subsequent 3 weeks, with an effect t((1/2)) of approximately 4 days. In contrast to the substantial increase in the cocaine priming threshold, 2E2 produced only modest dose-dependent increases (42 and 18%) in the cocaine consumption rates, and these also gradually declined toward baseline values. There was no significant effect of the control IgG on the priming threshold or rates of consumption of cocaine. After infusion, antibody blood concentrations declined over time, and a two-compartment pharmacokinetic model generated values for the distribution and elimination half-lives of 0.5 and 11.6 days for 2E2 and 0.4 and 6.0 days for control IgG. 2E2 had a long-lasting effect on cocaine-induced priming, which may predict its efficacy as an immunotherapy for cocaine abuse.

Contributions of dopamine D1, D2, and D3 receptor subtypes to the disruptive effects of cocaine on prepulse inhibition in mice

Deficits in prepulse inhibition (PPI) of startle, an operational measure of sensorimotor gating, are characteristics of schizophrenia and related neuropsychiatric disorders. Previous studies in mice demonstrate a contribution of dopamine (DA) D(1)-family receptors in modulating PPI and DA D(2) receptors (D2R) in mediating the PPI-disruptive effects of amphetamine. To examine further the contributions of DA receptor subtypes in PPI, we used a combined pharmacological and genetic approach. In congenic C57BL/6 J wild-type mice, we tested whether the D1R antagonist SCH23390 or the D2/3R antagonist raclopride would attenuate the effects of the indirect DA agonist cocaine (40 mg/kg). Both the D1R and D2/3R antagonists attenuated the cocaine-induced PPI deficit. We also tested the effect of cocaine on PPI in wild-type and DA D1R, D2R, or D3R knockout mice. The cocaine-induced PPI deficit was influenced differently by the three DA receptor subtypes, being absent in D1R knockout mice, partially attenuated in D2R knockout mice, and exaggerated in D3R knockout mice. Thus, the D1R is necessary for the PPI-disruptive effects of cocaine, while the D2R partially contributes to these effects. Conversely, the D3R appears to inhibit the PPI-disruptive effects of cocaine. Uncovering neural mechanisms involved in PPI will further our understanding of substrates of sensorimotor gating and could lead to better therapeutics to treat complex cognitive disorders such as schizophrenia.

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.

Developmental lead exposure alters methamphetamine self-administration in the male rat: acquisition and reinstatement

The rate of acquisition of drug self-administration and the return to drug seeking are important elements of the overall drug profile, and are essential factors in understanding risks associated with drug abuse. Experiment 1 examined the effects of perinatal (gestation/lactation) lead exposure on adult rates of acquisition of intravenous (i.v.) methamphetamine self-administration. Experiment 2 investigated the effects of perinatal lead exposure on drug-maintained responding in a reinstatement (relapse) paradigm. In Experiment 1, female rats were gavaged daily with 0 or 16 mg lead for 30 days prior to breeding with nonexposed males. Lead exposure continued through gestation and lactation and was discontinued at weaning (postnatal day [PND] 21). Male rats born to control or lead-exposed dams were tested daily as adults in an acquisition paradigm that incorporated both Pavlovian and operant components. An initial 3-h autoshaping period preceded a 3-h self-administration period. For 35 daily training sessions i.v. methamphetamine infusions [inf] (0.02 mg/kg) were paired with the extension and retraction of a lever (autoshaping), while inf occurred during self-administration only when a lever press was executed (FR-1). In Experiment 2 animals developmentally exposed to lead were trained on an FR-2 to self-administer methamphetamine (0.04 mg/kg/inf) and then placed on an extinction schedule prior to receiving intraperitoneal (i.p.) priming injections of saline, 0.50, 1.00, or 1.50 mg/kg methamphetamine. The findings from Experiment 1 showed that acquisition was delayed in rats born to lead-exposed dams gavaged daily with 16 mg lead throughout gestation and lactation when a 0.02-mg/kg/inf of methamphetamine served as the reinforcement outcome. Additional data from Experiment 2 indicated priming cues (injections of methamphetamine [i.p.]) administered after extinction were less likely to occasion a return to drug seeking (relapse) in the 16-mg group relative to the 0-mg control group. These results suggest perinatal lead exposure alters patterns of methamphetamine self-administration during the adult cycle.

Determinants of cocaine self-administration by laboratory animals

The reinforcing effect of a drug is that effect that increases the probability that the drug will be self-administered again. Like other drug effects, a reinforcing effect is the result of an interaction between organism, drug and environment. Laboratory research using animal subjects has helped elucidate the contribution of each of these factors to the self-administration of cocaine. A substantial amount of research indicates that increased dopamine neurotransmission in the brain, particularly in mesolimbic and mesocortical regions, plays a major role in cocaine self-administration. Both indirect and direct dopamine agonists can function as positive reinforcers in animals, whereas noradrenergic and serotonergic (5-HT, 5-hydroxytryptamine) agonists have not been found to do so. In addition, evidence suggests that dopamine but not noradrenaline (norepinephrine) or serotonin antagonists can attenuate the reinforcing effect of cocaine. Environmental factors have also been shown to be critical determinants of the reinforcing effect of cocaine. The schedule of reinforcement essentially determines the rate and pattern of drug-maintained behaviour. In addition, punishing self-administration, increasing the value of alternative reinforcers that are available, and increasing the cost of cocaine have all been shown to decrease the reinforcing effect of cocaine. With regard to organismic factors, recent research has suggested that there are significant genetic determinants of cocaine consumption. Taken together these research findings in animals imply that certain individuals may be more sensitive to the reinforcing effect of cocaine but that cocaine abuse can be decreased by pharmacological or behavioural means or by a combination of the two.

The neurobiology of cocaine-induced reinforcement

Cocaine has potent pharmacological actions on a number of monoaminergic systems in the brain, including those that use noradrenaline, dopamine and serotonin as neurotransmitters. There is growing evidence that cocaine's effects on dopaminergic neurons, particularly those that make up the mesolimbic system, are closely associated with its rewarding properties. For example, low doses of dopamine receptor antagonists reliably influence cocaine self-administration, whereas noradrenaline and serotonin receptor antagonists are without consistent effects. Similarly, selective lesions of dopaminergic terminals in the nucleus accumbens, a major target of the mesolimbic dopamine projection, disrupt cocaine self-administration in a manner that is consistent with loss of cocaine-induced reward. The introduction of in vivo brain microdialysis as a tool with which to investigate the neurochemical correlates of motivated behaviour has provided new opportunities for investigating the role of dopamine in the nucleus accumbens in the acquisition and maintenance of cocaine self-administration. Although the body of literature that has been generated by this approach appears to contain some important inconsistencies, these probably reflect the use of inappropriate microdialysis conditions by some investigators. A critical review of the literature suggests that microdialysis results are generally consistent with a role for mesolimbic dopamine in cocaine-induced reward, although it does not seem to be the case that animals will work to maintain consistent increases in extracellular concentrations of dopamine in the nucleus accumbens in all experimental conditions. Elucidation of the complete neural circuitry of cocaine-induced reward remains an important priority for future research.

Computational model predicts a role for ERG current in repolarizing plateau potentials in dopamine neurons: implications for modulation of neuronal activity

Blocking the small-conductance (SK) calcium-activated potassium channel promotes burst firing in dopamine neurons both in vivo and in vitro. In vitro, the bursting is unusual in that spiking persists during the hyperpolarized trough and frequently terminates by depolarization block during the plateau. We focus on the underlying plateau potential oscillation generated in the presence of both apamin and TTX, so that action potentials are not considered. We find that although the plateau potentials are mediated by a voltage-gated Ca(2+) current, they do not depend on the accumulation of cytosolic Ca(2+), then use a computational model to test the hypothesis that the slowly voltage-activated ether-a-go-go-related gene (ERG) potassium current repolarizes the plateaus. The model, which includes a material balance on calcium, is able to reproduce the time course of both membrane potential and somatic calcium concentration, and can also mimic the induction of plateau potentials by the calcium chelator BAPTA. The principle of separation of timescales was used to gain insight into the mechanisms of oscillation and its modulation using nullclines in the phase space. The model predicts that the plateau will be elongated and ultimately result in a persistent depolarization as the ERG current is reduced. This study suggests that the ERG current may play a role in burst termination and the relief of depolarization block in vivo.

Safety, tolerability and efficacy of levodopa-carbidopa treatment for cocaine dependence: two double-blind, randomized, clinical trials

RATIONALE

The role of dopamine in cocaine abuse has been long recognized. Cocaine use can profoundly alter dopaminergic functioning through depletion of this monoamine and changes in receptor functioning. Based on these facts, levodopa (L-dopa) pharmacotherapy may be helpful in reducing or abolishing cocaine use.

OBJECTIVE

The current studies sought to evaluate the safety, tolerability and efficacy of L-dopa as a treatment for cocaine dependence.

METHODS

In Study 1, 67 cocaine-dependent subjects were randomized in a 5-week, double-blind, placebo-controlled safety trial. Subjects received either placebo, or 400 mg L-dopa plus 100 mg of the peripheral decarboxylase inhibitor, carbidopa, in a sustained-release preparation (Sinemet CR). In Study 2, 122 cocaine-dependent subjects were enrolled in a 9-week, randomized, double-blind, placebo-controlled trial to compare placebo to 400/100 mg and 800/200 mg L-dopa/carbidopa treatments. Placebo or L-dopa were administered twice daily in both studies.

RESULTS

L-dopa was well tolerated with similar retention and medication adherence rates compared to placebo. Only two side effects occurred more often in L-dopa-treated patients: nausea and dizziness. L-dopa lowered diastolic blood pressure in a dose-dependent fashion. In these trials, L-dopa had no effect on cocaine use, cocaine craving, or mood.

CONCLUSION

These two studies demonstrate the safety and tolerability of L-dopa pharmacotherapy in cocaine-dependent patients. No evidence for greater efficacy of L-dopa compared to placebo was observed. The possibility of enhancing treatment effects by combining L-dopa with other behavioral or pharmacological interventions is discussed.

Addiction-related alterations in D1 and D2 dopamine receptor behavioral responses following chronic cocaine self-administration

The cocaine-addicted phenotype can be modeled in rats based on individual differences in preferred levels of cocaine intake and a propensity for relapse in withdrawal. These cocaine-taking and -seeking behaviors are strongly but differentially regulated by postsynaptic D1 and D2 receptors in the mesolimbic dopamine system. Thus, we determined whether addiction-related differences in cocaine self-administration would be related to differential sensitivity in functional D1 and D2 receptor responses. Using a population of 40 outbred Sprague-Dawley rats trained to self-administer cocaine for 3 weeks, we found that animals with higher preferred levels of cocaine intake exhibited a vertical and rightward shift in the self-administration dose-response function, and were more resistant to extinction from cocaine self-administration, similar to phenotypic changes reported in other models of cocaine addiction. After 3 weeks of withdrawal from cocaine self-administration, high intake rats were subsensitive to the ability of the D1 agonist SKF 81297 to inhibit cocaine-seeking behavior elicited by cocaine priming, but supersensitive to cocaine seeking triggered by the D2 agonist quinpirole, when compared to low intake rats. Additionally, high intake rats developed profound increases in locomotor responses to D2 receptor challenge from early to late withdrawal times, whereas low intake rats developed increased responsiveness to D1 receptor challenge. In a second experiment, responses to the mixed D1/D2 agonist apomorphine and the NMDA glutamate receptor antagonist MK-801 failed to differ between low and high intake rats. These findings suggest that cocaine addiction is related specifically to differential alterations in functional D1 and D2 receptors and their ability to modulate cocaine-seeking behavior.