Effects of haloperidol in a response-reinstatement model of heroin relapse

Low-dose polypharmacology targeting dopamine D1 and D3 receptors reduces cue-induced relapse to heroin seeking in rats

Chemical compounds that target dopamine (DA) D1 or D3 receptors have shown promise as potential interventions in animal models of cue-induced relapse. However, undesirable side effects or pharmacodynamic profiles have limited the advancement of new compounds in preclinical studies when administered as independent treatments. In this series of experiments, we explored the effects of coadministration of a D1-receptor partial agonist (SKF 77434) and a D3-receptor antagonist (NGB 2904) in heroin-seeking rats within a "conflict" model of abstinence and cue-induced relapse. Rats were first trained to press a lever to self-administer heroin, and drug delivery was paired contingently with cues (e.g., light and pump noise). Self-initiated abstinence was facilitated by applying electrical current to the flooring in front of the levers. Lastly, a relapse response was provoked by noncontingent presentation of conditioned cues. Prior to provocation, rats received a systemic injection of SKF 77434, NGB 2904, or a combination of both compounds to assess treatment effects on lever pressing. Results indicated that the coadministration of low (i.e., independently ineffective) doses of both compounds was more effective in reducing cue-induced relapse to heroin seeking than either compound alone, with some evidence of drug synergism. Follow-up studies indicated that this reduction was not due to motoric impairment nor enhanced sensitivity to the electrified flooring and that this treatment did not significantly affect motivation for food. Implications for the treatment of opiate use disorder and recommendations for further research are discussed.

Neurobiology of alcohol seeking behavior

Alcohol addiction is a chronic relapsing brain disease characterized by an impaired ability to stop or control alcohol use despite adverse consequences. A main challenge of addiction treatment is to prevent relapse, which occurs in more than >50% of newly abstinent patients with alcohol disorder within 3 months. In people suffering from alcohol addiction, stressful events, drug-associated cues and contexts, or re-exposure to a small amount of alcohol trigger a chain of behaviors that frequently culminates in relapse. In this review, we first present the preclinical models that were developed for the study of alcohol seeking behavior, namely the reinstatement model of alcohol relapse and compulsive alcohol seeking under a chained schedule of reinforcement. We then provide an overview of the neurobiological findings obtained using these animal models, focusing on the role of opioids systems, corticotropin-release hormone and neurokinins, followed by dopaminergic, glutamatergic, and GABAergic neurotransmissions in alcohol seeking behavior.

Novel Diels-Alder Type Adducts from Morus alba Root Bark Targeting Human Monoamine Oxidase and Dopaminergic Receptors for the Management of Neurodegenerative Diseases

In this study, we delineate the human monoamine oxidase (hMAO) inhibitory potential of natural Diels–Alder type adducts, mulberrofuran G (1), kuwanon G (2), and albanol B (3), from Morus alba root bark to characterize their role in Parkinson’s disease (PD) and depression, focusing on their ability to modulate dopaminergic receptors (D₁R, D_2LR, D₃R, and D₄R). In hMAO-A inhibition, 1–3 showed mild effects (50% inhibitory concentration (IC₅₀): 54‒114 μM). However, 1 displayed moderate inhibition of the hMAO-B isozyme (IC₅₀: 18.14 ± 1.06 μM) followed by mild inhibition by 2 (IC₅₀: 57.71 ± 2.12 μM) and 3 (IC₅₀: 90.59 ± 1.72 μM). Our kinetic study characterized the inhibition mode, and the in silico docking predicted that the moderate inhibitor 1 would have the lowest binding energy. Similarly, cell-based G protein-coupled receptors (GPCR) functional assays in vector-transfected cells expressing dopamine (DA) receptors characterized 1–3 as D₁R/D_2LR antagonists and D₃R/D₄R agonists. The half-maximum effective concentration (EC₅₀) of 1–3 on DA D₃R/D₄R was 15.13/17.19, 20.18/21.05, and 12.63/‒ µM, respectively. Similarly, 1–3 inhibited 50% of the DA response on D₁R/D_2LR by 6.13/2.41, 16.48/31.22, and 7.16/18.42 µM, respectively. A computational study revealed low binding energy for the test ligands. Interactions with residues Asp110, Val111, Tyr365, and Phe345 at the D₃R receptor and Asp115 and His414 at the D₄R receptor explain the high agonist effect. Likewise, Asp187 at D₁R and Asp114 at D_2LR play a crucial role in the antagonist effects of the ligand binding. Our overall results depict 1–3 from M. alba root bark as good inhibitors of hMAO and potent modulators of DA function as D₁R/D_2LR antagonists and D₃R/D₄R agonists. These active constituents in M. alba deserve in-depth study for their potential to manage neurodegenerative disorders (NDs), particularly PD and psychosis.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

The reinstatement model of drug relapse: recent neurobiological findings, emerging research topics, and translational research

BACKGROUND AND RATIONALE

Results from many clinical studies suggest that drug relapse and craving are often provoked by acute exposure to the self-administered drug or related drugs, drug-associated cues or contexts, or certain stressors. During the last two decades, this clinical scenario has been studied in laboratory animals by using the reinstatement model. In this model, reinstatement of drug seeking by drug priming, drug cues or contexts, or certain stressors is assessed following drug self-administration training and subsequent extinction of the drug-reinforced responding.

OBJECTIVE

In this review, we first summarize recent (2009-present) neurobiological findings from studies using the reinstatement model. We then discuss emerging research topics, including the impact of interfering with putative reconsolidation processes on cue- and context-induced reinstatement of drug seeking, and similarities and differences in mechanisms of reinstatement across drug classes. We conclude by discussing results from recent human studies that were inspired by results from rat studies using the reinstatement model.

CONCLUSIONS

Main conclusions from the studies reviewed highlight: (1) the ventral subiculum and lateral hypothalamus as emerging brain areas important for reinstatement of drug seeking, (2) the existence of differences in brain mechanisms controlling reinstatement of drug seeking across drug classes, (3) the utility of the reinstatement model for assessing the effect of reconsolidation-related manipulations on cue-induced drug seeking, and (4) the encouraging pharmacological concordance between results from rat studies using the reinstatement model and human laboratory studies on cue- and stress-induced drug craving.

Roles of dopaminergic innervation of nucleus accumbens shell and dorsolateral caudate-putamen in cue-induced morphine seeking after prolonged abstinence and the underlying D1- and D2-like receptor mechanisms in rats

Drug-associated cues can elicit relapse to drug seeking after abstinence. Studies with extinction-reinstatement models implicate dopamine (DA) in the nucleus accumbens shell (NAshell) and dorsolateral caudate-putamen (dlCPu) in cocaine seeking. However, less is known about their roles in cue-induced opiate seeking after prolonged abstinence. Using a morphine self-administration and abstinence-relapse model, we explored the roles of NAshell and dlCPu DA and the D1/D2-like receptor mechanisms underlying morphine rewarding and/or seeking. Acquisition of morphine self-administration was examined following 6-Hydroxydopamine hydrobromide (6-OHDA) lesions of the NAshell and dlCPu. For morphine seeking, rats underwent 3 weeks' morphine self-administration followed by 3 weeks' abstinence from morphine and the training environment. Prior to testing, 6-OHDA, D1 antagonist SCH23390, or D2 antagonist eticlopride was locally injected; then rats were exposed to morphine-associated contextual and discrete cues. Results show that acquisition of morphine self-administration was inhibited by NAshell (not dlCPu) lesions, while morphine seeking was attenuated by lesions of either region, by D1 (not D2) receptor blockade in NAshell, or by blockade of either D1 or D2 receptors in dlCPu. These data indicate a critical role of dopaminergic transmission in the NAshell (via D1-like receptors) and dlCPu (via D1- and D2-like receptors) in morphine seeking after prolonged abstinence.

Modeling relapse in animals

Alcohol addiction is a chronically relapsing disorder characterized by compulsive alcohol seeking and use. Alcohol craving and long-lasting vulnerability to relapse present a great challenge for the successful treatment of alcohol addiction. Therefore, relapse prevention has emerged as a critically important area of research, with the need for effective and valid animal models of relapse. This chapter provides an overview of the repertoire of animal models of craving and relapse presently available and employed in alcoholism research. These models include conditioned reinstatement, stress-induced reinstatement, ethanol priming-induced reinstatement, conditioned place preference, Pavlovian spontaneous recovery, the alcohol deprivation effect, and seeking-taking chained schedules. Thus, a wide array of animal models is available that permit investigation of behaviors directed at obtaining access to alcohol, as well as neurobehavioral mechanisms and genetic factors that regulate these behaviors. These models also are instrumental for identifying pharmacological treatment targets and as tools for evaluating the efficacy of potential medications for the prevention of alcohol craving and relapse.

Role of the dopaminergic system in the acquisition, expression and reinstatement of MDMA-induced conditioned place preference in adolescent mice

Background

The rewarding effects of 3,4-methylenedioxy-metamphetamine (MDMA) have been demonstrated in conditioned place preference (CPP) procedures, but the involvement of the dopaminergic system in MDMA-induced CPP and reinstatement is poorly understood.

Methodology/Principal Findings

In this study, the effects of the DA D1 antagonist SCH 23390 (0.125 and 0.250 mg/kg), the DA D2 antagonist Haloperidol (0.1 and 0.2 mg/kg), the D2 antagonist Raclopride (0.3 and 0.6 mg/kg) and the dopamine release inhibitor CGS 10746B (3 and 10 mg/kg) on the acquisition, expression and reinstatement of a CPP induced by 10 mg/kg of MDMA were evaluated in adolescent mice. As expected, MDMA significantly increased the time spent in the drug-paired compartment during the post-conditioning (Post-C) test, and a priming dose of 5 mg/kg reinstated the extinguished preference. The higher doses of Haloperidol, Raclopride and CGS 10746B and both doses of SCH 23390 blocked acquisition of the MDMA-induced CPP. However, only Haloperidol blocked expression of the CPP. Reinstatement of the extinguished preference was not affected by any of the drugs studied. Analysis of brain monoamines revealed that the blockade of CPP acquisition was accompanied by an increase in DA concentration in the striatum, with a concomitant decrease in DOPAC and HVA levels. Administration of haloperidol during the Post-C test produced increases in striatal serotonin, DOPAC and HVA concentrations. In mice treated with the higher doses of haloperidol and CGS an increase in SERT concentration in the striatum was detected during acquisition of the CPP, but no changes in DAT were observed.

Conclusions/Significance

These results demonstrate that, in adolescent mice, the dopaminergic system is involved in the acquisition and expression of MDMA-induced CPP, but not in its reinstatement.

Comparison of self-administration behavior and responsiveness to drug-paired cues in rats running an alley for intravenous heroin and cocaine

RATIONALE

Evidence suggests that responsiveness to a drug-paired cue is predicted by the reinforcing magnitude of the drug during prior self-administration. It remains unclear, however, if this principle holds true when comparisons are made across drug reinforcers.

OBJECTIVE

The current study was therefore devised to test the hypothesis that differences in the animals' responsiveness to a cocaine- or heroin-paired cue presented during extinction would reflect differences in the patterns of prior cocaine and heroin runway self-administration.

METHODS

Rats ran a straight alley for single intravenous injections of either heroin (0.1 mg/kg/inj) or cocaine (1.0 mg/kg/inj) each paired with a distinct olfactory cue. Animals experienced 15 trials with each drug reinforcer in a counterbalanced manner. Start latencies, run times, and retreat behaviors (a form of approach-avoidance conflict) provided behavioral indices of the subjects' motivation to seek the reinforcer on each trial. Responsiveness to each drug-paired cue was assessed after 7, 14, or 21 days of non-reinforced extinction trials. Other animals underwent conditioned place preference (CPP) testing to ensure that the two drug reinforcers were capable of producing drug-cue associations.

RESULTS

While both drugs produced comparable CPPs, heroin served as a stronger incentive stimulus in the runway as evidenced by faster start and run times and fewer retreats. In contrast, cocaine- but not heroin-paired cues produced increases in drug-seeking behavior during subsequent extinction trials.

CONCLUSIONS

The subjects' responsiveness to drug-paired cues during extinction was not predicted by differences in the motivation to seek heroin versus cocaine during prior drug self-administration.

Antipsychotic agents for the treatment of substance use disorders in patients with and without comorbid psychosis

Substance dependence has serious negative consequences upon society such as increased health care costs, loss of productivity, and rising crime rates. Although there is some preliminary evidence that atypical antipsychotic agents may be effective in treating substance dependence, results have been mixed, with some studies demonstrating positive and others negative or no effect. The present study was aimed at determining whether this disparity originates from that reviewers separately discussed trials in patients with (DD) and without (SD) comorbid psychosis. Using electronic databases, we screened the relevant literature, leaving only studies that used a randomized, double-blind, placebo-controlled or case-control design that had a duration of 4 weeks or longer. A total of 43 studies were identified; of these, 23 fell into the category of DD and 20 into the category of SD. Studies in the DD category suggest that atypical antipsychotic agents, especially clozapine, may decrease substance use in individuals with alcohol and drug (mostly cannabis) use disorders. Studies in the SD category suggest that atypical antipsychotic agents may be beneficial for the treatment of alcohol dependence, at least in some subpopulations of alcoholics. They also suggest that these agents are not effective at treating stimulant dependence and may aggravate the condition in some cases.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

The runway model of drug self-administration

Behavioral scientists have employed operant runways as a means of investigating the motivational impact of incentive stimuli for the better part of the past 100 years. In this task, the speed with which a trained animal traverses a long straight alley for positive incentive stimuli, like food or water, provides a reliable index of the subject's motivation to seek those stimuli. The runway is therefore a particularly appropriate tool for investigating the drug-seeking behavior of animals working for drugs of abuse. The current review describes our laboratory's work over the past twenty years developing and implementing an operant runway model of drug self-administration. Procedures are described that methodologically dissociate the antecedent motivational processes that induce an animal to seek a drug, from the positive reinforcing consequences of actually earning the drug. Additional work is reviewed on the use of the runway method as a means of modeling the factors that often result in a "relapse" of drug self-administration after a period of abstinence (i.e., a response reinstatement test), as are runway studies that revealed the presence of opposing positive and negative consequences of self-administered cocaine. This body of work suggests that the runway method has served as a powerful behavioral tool for the study of the behavioral and neurobiological basis of drug self-administration.

Neurobiological mechanisms of the reinstatement of drug-conditioned place preference

Drug addiction is a chronic disorder characterized by a high rate of relapse following detoxification. There are two main versions of the reinstatement model that are employed to study relapse to drug abuse; one based on the operant self-administration procedure, and the other on the classical conditioned place preference procedure. In the last seven years, the use of the latter version has become more widespread, and the results obtained complement those obtained in self-administration studies. It has been observed that the conditioned place preference induced by opioids, psychostimulants, nicotine, ethanol and other drugs of abuse can be extinguished and reinstated by drug priming or exposure to stressful events. Herein, the neuroanatomical and neurochemical basis of drug priming- and stress-induced reinstatement of morphine and cocaine, together with the molecular correlates of reinstatement behavior, are reviewed. Differences between the conditioned place preference and self-administration studies are also discussed. Evidence suggests that data of reinstatement with the CPP are to be viewed with caution until more extensive analysis of operant procedures has been performed, and that further research will undoubtedly improve our understanding of the neurobiological mechanisms of relapse to drug seeking.

Motivational effect of nomifensine in the intracranial self-stimulation behavior using a runway method

Intracranial self-stimulation (ICSS) behavior is an experimental methodology to study reward and motivational effects. We have established a new paradigm to evaluate enhancing motivation by drugs in the runway method using the priming stimulation of ICSS. In the present study, we investigated the effects of nomifensine on the experimental extinction process of non-reinforcing reward and pre-trial electric priming stimulations in lateral hypothalamic self-stimulation. In this study, the experimental extinction process of the non-reinforcing reward means the experimental method of excluding reward effect in ICSS behavior. The extinction process in the runway method consisted of these 15 trials. Nomifensine, an antidepressant drug, delayed the running speed of the extinction process at doses of 5 and 10 mg/kg (i.p.) compared with the vehicle alone. This result suggests that the delay in the running speed of the extinction process promotes a motivational effect in rats. Previously, priming stimulation in the runway method was found to affect motivational function of ICSS. Therefore, our findings suggest the possible application of nomifensine for improving motivation.

Heightened drug-seeking motivation following extended daily access to self-administered cocaine

Rats allowed extended daily access (6 h) to cocaine, consume high doses of the drug and escalate their cocaine intake over days, resembling the pattern of cocaine use seen in human addicts. The current study was designed to test whether such animals would also demonstrate the heightened motivation to seek cocaine seen in human addicts. Rats were trained to lever press for i.v. cocaine (0.25 mg/infusion) over a 5-day period of 1 h sessions. Subjects were then assigned to either a brief-access (1 h/day) or an extended-access condition for an additional 10 days. Control rats lever pressed for i.v. saline. Following the final self-administration session animals were tested for their motivation to receive cocaine in an operant runway apparatus. Extended-access animals exhibited significantly higher motivation for cocaine in the runway (where they received 1.0 mg/kg cocaine i.v. upon goal-box entry) as was evident by faster run times and less ambivalence about entering the goal box (i.e. retreat behavior) than either brief-access or control subjects. Brief and extended-access animals, tested in the Elevated Plus Maze, exhibited comparable and significant increases in anxiety following a single 1.0 mg/kg i.v. injection of cocaine, as compared to saline control animals that were challenged with i.v. saline infusion. Together, these data suggest that extended access to cocaine results in an especially high motivation for the drug that is not accounted for by reductions in the anxiogenic properties of cocaine.

Chronic heroin and cocaine abuse is associated with decreased serum concentrations of the nerve growth factor and brain-derived neurotrophic factor

Chronic cocaine and heroin users display a variety of central nervous system (CNS) dysfunctions including impaired attention, learning, memory, reaction time, cognitive flexibility, impulse control and selective processing. These findings suggest that these drugs may alter normal brain functions and possibly cause neurotoxicity. Neurotrophins are a class of proteins that serve as survival factors for CNS neurons. In particular, nerve growth factor (NGF) plays an important role in the survival and function of cholinergic neurons while brain-derived neurotrophic factor (BDNF) is involved in synaptic plasticity and in the maintenance of midbrain dopaminergic and cholinergic neurons. In the present study, we measured by enzyme-linked immunosorbent assay (ELISA) the NGF and BDNF levels in serum of three groups of subjects: heroin-dependent patients, cocaine-dependent patients and healthy volunteers. Our goal was to identify possible change in serum neurotrophins in heroin and cocaine users. BDNF was decreased in heroin users whereas NGF was decreased in both heroin and cocaine users. These findings indicate that NGF and BDNF may play a role in the neurotoxicity and addiction induced by these drugs. In view of the neurotrophin hypothesis of schizophrenia the data also suggest that reduced level of neurotrophins may increase the risk of developing psychosis in drug users.

Motivational effects of nicotine as measured in a runway model of drug self-administration

Traditional operant self-administration and conditioned place preference methods have yielded inconsistent results in studies of nicotine reinforcement thereby hindering efforts to identify the neurobiological systems underlying the drug's motivation and reinforcement. This study was designed to assess the motivation of subjects to seek nicotine using a runway self-administration procedure. Male Sprague-Dawley rats (N=67) were trained to run a straight alley for a single daily intravenous injection of nicotine (0.01-0.09 mg/kg/injection) on each of 21 consecutive trials. Run Speed (1/Run Time) served as the dependent measure for the animals' motivation to traverse the alley and enter a goal-box associated with intravenous nicotine administration. Nicotine induced an inverted U-shaped dose-response curve with the 0.03 mg/kg dose producing optimal runway performance over trials. Subjects running for doses larger or smaller than the optimal dose exhibited slower running and took longer to enter the goal-box. Thus, the runway procedure proved to be an effective methodology for reliably assessing the motivation of trained but nondrugged animals to seek intravenous nicotine.

Neurobiology of relapse to heroin and cocaine seeking: an update and clinical implications

The central problem in the treatment of cocaine and heroin addiction is high rates of relapse to drug use after periods of forced or self-imposed abstinence. Relapse can be modeled in laboratory animals a reinstatement procedure in which responding for drug is extinguished and then reinstated by acute exposure to the drug, drug cues, or stress. In this review, we first summarize data from recent (2003-2005) studies on the neural substrates involved in reinstatement of heroin and cocaine seeking. We also discuss the neural mechanisms underlying the progressive increase in cocaine seeking after withdrawal (incubation of cocaine craving). Finally, we provide an update on several novel candidate medications for relapse prevention suggested by recent preclinical studies, and we discuss the translation of findings from nonhuman laboratory studies to the clinical phenomenon of relapse.

NMDA glutamate but not dopamine antagonists blocks drug-induced reinstatement of morphine place preference

The effects of dopaminergic and glutamatergic antagonists on the drug-induced reinstatement of a previously extinguished morphine conditioned place preference (CPP) in mice were evaluated. Following extinction of a place preference induced by morphine (40 mg/kg), a non-contingent injection of the dopaminergic antagonists SCH 23390 (0.125, 0.5 mg/kg), raclopride (0.3, 1.2 mg/kg), haloperidol (0.1, 0.2 mg/kg) and the dopamine (DA) release inhibitor CGS 10746B (1, 10 mg/kg) or glutamatergic NMDA antagonists memantine (10, 20, 40 mg/kg) and MK-801 (0.1, 0.2, 0.3 mg/kg) alone or with 10 mg/kg morphine was given. Neither the dopaminergic nor the glutamatergic antagonists alone reinstated the place preference. Dopamine antagonists failed to block the morphine-induced reinstatement of place preference while memantine and MK-801 blocked it with intermediate and high doses. These results suggest that drug-induced reinstatement of place preference may be largely independent of dopamine and more closely related to glutamatergic neurotransmission.

Opponent process properties of self-administered cocaine

Over the past decade, data collected in our laboratory have demonstrated that self-administered cocaine produces Opponent-Process-like behavioral effects. Animals running a straight alley once each day for IV cocaine develop over trials an approach-avoidance conflict about re-entering the goal box. This conflict behavior is characterized by a stop in forward locomotion (usually at the very mouth of the goal box) followed by a turn and 'retreat' back toward the goal box. The results of a series of studies conducted over the past decade collectively suggest that the behavioral ambivalence exemplified by rats running the alley for IV cocaine stems from concurrent and opponent positive (rewarding) and negative (anxiogenic) properties of the drug--both of which are associated with the goal box. These opponent properties of cocaine have been shown to result from temporally distinct affective states. Using a conditioned place preference test, we have been able to demonstrate that while the initial immediate effects of IV cocaine are reinforcing, the state present 15 min post-injection is aversive. In our most recent work, the co-administration of IV cocaine with either oral ethanol or IV heroin was found to greatly diminish the development and occurrence of retreat behaviors in the runway. It may therefore be that the high incidence of co-abuse of cocaine with either ethanol or heroin, stems from the users' motivation to alleviate some of the negative side effects of cocaine. It would seem then that the Opponent Process Theory has provided a useful conceptual framework for the study of the behavioral consequences of self-administered cocaine including the notion that both positive and negative reinforcement mechanisms are involved in the development and maintenance of cocaine abuse.

Memantine presents different effects from MK-801 in motivational and physical signs of morphine withdrawal

Adaptive changes in neural systems due to chronic opiate exposure are related to the neural plasticity phenomenon, NMDA receptors being implicated in these processes, e.g. tolerance, dependence or withdrawal. In this work, we investigated the effect of two non-competitive NMDA antagonists, memantine and MK-801, in motivational (Conditioned Place Aversion paradigm, CPA) and physical aspects of morphine withdrawal. After the induction of morphine dependence, animals in which the CPA was studied, received memantine (5 and 10 mg/kg) or MK-801 (0.3-0.006 mg/kg) either during the acquisition (conditioning) or expression (test) phase of this procedure. Both drugs were capable of inhibiting conditioned aversion when administered in any phase. In a second experiment, the effects of these drugs were evaluated in the intensity of the physical signs of withdrawal, only memantine administration being efficient. In addition to these studies, the intensity of morphine dependence was investigated under the blockade of NMDA receptors, i.e. morphine was co-administered with memantine or MK-801. These animals did not develop CPA and present less intensity in the physical signs of morphine withdrawal. Our results support the idea that NMDA receptors are involved in the behavioural changes and therefore in the neural adaptations produced by repeated morphine administration.

Opioids, cocaine, and food change runtime distribution in a rat runway procedure

RATIONALE

The nature of the relationship between the dose of a drug of abuse and its reinforcing effect has come under close scrutiny. It is currently debated if the steep ascending part of the biphasic dose-response curve typically obtained in multiple-dosing lever-press-based operant conditioning procedures represents a satiety-driven, all-or-none response or if the response is gradual and tightly adjusted to the various doses of the reinforcer.

OBJECTIVES

Dose-response relationships of drug reinforcers (remifentanil, alfentanil, morphine, cocaine) as well as a physiological reinforcer, i.e. food (sweetened condensed milk) were investigated in a different operant conditioning paradigm, i.e. a runway procedure.

METHODS

Administration of the mu opioid receptor agonists remifentanil (0.0032-0.1 mg/kg IV), alfentanil (0.032 mg/kg IV), morphine (0.032-1 mg/kg IV), the psychostimulant cocaine (0.001-0.1 mg/kg IV) or sweetened condensed milk (diluted 1:100-1:3 in water) was made contingent upon alley running for male Sprague-Dawley rats.

RESULTS

All drug reinforcers dose-dependently decreased runtimes and, more importantly, significantly increased the percentage of runs at a certain speed (> or =10 cm/s) from an average 29% to an average 71% (water versus milk, 22% versus 83%).

CONCLUSION

Both drug- and food reinforcers produced a discontinuous, qualitative change in the rats' operant behavior rather than a simple gradual increase along a continuum, an effect that could be seen clearly only after a histogram analysis of runtime distribution.

The reinstatement model and relapse prevention: a clinical perspective

OBJECTIVES

This commentary assesses the degree to which the reinstatement model is homologous to the human experience of relapse.

RESULTS

A review of the literature suggests that the relationship is less clear than is often assumed, largely due to a lack of prospective data on the precipitants and process of relapse (especially relapse to heroin or cocaine abuse). However, reinstatement does not need to resemble relapse to have immediate clinical value; predictive validity as a medication screen would be sufficient. Whether the model has predictive validity is unknown, because, to date, very few clinical trials have tested medications that are effective in the reinstatement model, and even fewer have used designs comparable to those of reinstatement experiments. A clinical trial comparable to a reinstatement experiment would enroll participants who are already abstinent, and its main outcome measure would be propensity to undergo a specific type of relapse (e.g., relapse induced by stress or cues).

CONCLUSIONS

Until clinical and preclinical work are more comparable, criticisms of the reinstatement model's presumed shortcomings are premature.

Effects of haloperidol on cue-induced autonomic and behavioral indices of heroin reward and motivation

RATIONALE

Most theoretical conceptions of motivation presume an internal state of sympathetic nervous system activation that precedes and accompanies goal-seeking behavior.

OBJECTIVES

The present study investigates the animals' physiological and behavioral response to presentation of environmental cues predictive of availability (S+) or non-availability (S-) of heroin in the goal box of a straight-arm alley.

METHODS

Animals were trained to discriminate between two olfactory cues, one predictive of the delivery of i.v. heroin (0.1 mg/kg) upon the rat's entry into the goal box of the runway (the S+) and another cue (the S-) predictive of i.v. saline upon goal-box entry. Once discriminative performance had stabilized, animals were challenged with each of four haloperidol treatments in a counterbalanced manner (0.0, 0.075, 0.15, 0.3 mg/kg i.p.). Run times and heart rates (measured via radiotelemetry) served as the dependent variables on every trial.

RESULTS

Both behavioral and physiological measures of motivation responded differentially to S+ and S- cues. Haloperidol had no effect during or immediately following S- trials, nor prior to reinforcer delivery on S+ trials. However, the behavioral and physiological consequences of heroin delivery during dopamine receptor antagonism were reliable - animals ran more slowly and showed less activation (lower heart rates) on the first S+ trial following a heroin + haloperidol experience.

CONCLUSIONS

The current data demonstrate that physiological and behavioral indices of cue-induced motivation remained intact during haloperidol challenge, while the reinforcing consequences of heroin appear to have been attenuated by dopamine receptor antagonism.

The reinstatement model of drug relapse: history, methodology and major findings

RATIONAL AND OBJECTIVES

The reinstatement model is currently used in many laboratories to investigate mechanisms underlying relapse to drug seeking. Here, we review briefly the history of the model and describe the different procedures that have been used to study the phenomenon of reinstatement of drug seeking. The results from studies using pharmacological and neuroanatomical techniques to determine the neuronal events that mediate reinstatement of heroin, cocaine and alcohol seeking by acute priming injections of drugs, drug-associated cues and environmental stressors are summarized. In addition, several issues are discussed, including (1) the concordance between the neuronal mechanisms involved in drug-induced reinstatement and those involved in drug reward and discrimination, (2) the role of drug withdrawal states and periods in reinstatement of drug seeking, (3) the role of neuronal adaptations induced by exposure to drugs in relapse, and (4) the degree to which the rat reinstatement model provides a suitable preclinical model of relapse to drug taking.

CONCLUSIONS

The data derived from studies using the reinstatement model suggest that the neuronal events that mediate drug-, cue- and stress-induced reinstatement of drug seeking are not identical, that the mechanisms underlying drug-induced reinstatement are to some degree different from those mediating drug discrimination or reward, and that the duration of the withdrawal period following cocaine and heroin self-administration has a profound effect on reinstatement induced by drug cues and stress. Finally, there appears to be a good correspondence between the events that induce reinstatement in laboratory animals and those that provoke relapse in humans.

Nucleus accumbens shell and core dopamine: differential role in behavior and addiction

Drug addiction can be conceptualized as a disturbance of behavior motivated by drug-conditioned incentives. This abnormality has been explained by Incentive-Sensitization and Allostatic-Counteradaptive theories as the result of non-associative mechanisms acting at the stage of the expression of incentive motivation and responding for drug reinforcement. Each one of these theories, however, does not account per se for two basic properties of the motivational disturbance of drug addiction: (1). focussing on drug- at the expenses of non-drug-incentives; (2). virtual irreversibility. To account for the above aspects we have proposed an associative learning hypothesis. According to this hypothesis the basic disturbance of drug addiction takes place at the stage of acquisition of motivation and in particular of Pavlovian incentive learning. Drugs share with non-drug rewards the property of stimulating dopamine (DA) transmission in the nucleus accumbens shell but this effect does not undergo habituation upon repeated drug exposure, as instead is the case of non-drug rewards. Repetitive, non-decremental stimulation of DA transmission by drugs in the nucleus accumbens septi (NAc) shell abnormally strengthens stimulus-drug associations. Thus, stimuli contingent upon drug reward acquire powerful incentive properties after a relatively limited number of predictive associations with the drug and become particularly resistant to extinction. Non-contingent occurrence of drug-conditioned incentive cues or contexts strongly facilitates and eventually reinstates drug self-administration. Repeated drug exposure also induces a process of sensitization of drug-induced stimulation of DA transmission in the NAc core. The precise significance of this adaptive change for the mechanism of drug addiction is unclear given the complexity and uncertainties surrounding the role of NAc core DA in responding but might be more directly related to instrumental performance.

Sexually conditioned incentives: attenuation of motivational impact during dopamine receptor antagonism

The motivational impact of sexually conditioned incentives was examined in two experiments. In Experiment 1, male Long-Evans rats copulated to ejaculation in the presence of one of two scents (orange or almond extract) on five separate occasions. On alternating days, subjects spent an equal amount of time in social isolation with the opposing scent. Following the 10-day conditioning regimen, subjects ran more rapidly down an operant runway toward a goalbox containing the sex-paired scent (CS+) compared to trials on which the isolation-paired scent (CS-) or no scent was provided. In Experiment 2, comparably conditioned male rats were first given a baseline runway trial with an unscented goalbox. The following day, subjects were pretreated with one of four doses of haloperidol (0.0, 0.075, 0.15, or 0.30 mg/kg i.p.) 45 min prior to being tested in the runway for their motivation to approach either the CS+ or CS- scents. Control subjects given vehicle injections performed comparably to subjects from Experiment 1, taking significantly less time to approach the CS+ than an unscented goalbox. This decrease in run latency was not observed in subjects within the 0.075 and 0.15 mg/kg haloperidol groups. Subjects in the 0.30 mg/kg haloperidol groups took significantly more time to approach both the CS+ and CS- compared to their baseline run times. These data reveal that an olfactory cue associated with sexual reward becomes a conditioned incentive capable of eliciting approach behavior, and that dopamine receptor antagonism (at moderate but not high doses) selectively attenuates this cue-induced motivation.

Renewal of drug seeking by contextual cues after prolonged extinction in rats

Contextual stimuli associated with drug exposure can modulate various effects of drugs, but little is known about their role in relapse to drug seeking. Using a renewal procedure, the authors report that drug-associated contextual stimuli play a critical role in relapse to drug-seeking previously maintained by a heroin-cocaine mixture (speedball). Rats were trained to self-administer speedball, after which drug-reinforced behavior was extinguished over 20 days in the self-administration context or in a different context. On the test day, rats exposed to the drug-associated context, after extinction in a different context, reliably renewed drug seeking. The authors suggest that the renewal procedure can be used to study mechanisms underlying relapse to drug seeking elicited by drug-associated contextual stimuli.

Dopamine antagonism attenuates the unconditioned incentive value of estrous female cues

The role of dopaminergic transmission in the incentive-motivational processes involved in the generation of male sexual behavior was examined. Three groups of sexually naïve Long-Evans male rats traversed a straight alley for one of three goalbox targets: an empty goalbox, a nonestrous female, or an estrous female. A Plexiglas partition within the goalbox allowed for the perception of visual, auditory, and olfactory cues, but prevented physical contact. Baseline run times revealed that subjects returned to the goalbox significantly faster for an estrous female than for a nonestrous female, replicating our earlier work on the inherent incentive value of primary female cues. When subjects were then pretreated with the dopamine receptor antagonist, haloperidol (0.0, 0.075, or 0.15 mg/kg), they expressed decreased sexual motivation as reflected by increased run times for estrous female targets. Subjects' run times for the empty goalbox condition were unaffected by haloperidol, suggesting that the drug did not reliably impair motoric capacity. Results support the contention that central dopaminergic systems are involved in the regulation of the positive, unconditioned incentive value of estrous female cues.

Dissociation in conditioned dopamine release in the nucleus accumbens core and shell in response to cocaine cues and during cocaine-seeking behavior in rats

The dopaminergic innervation of the nucleus accumbens is generally agreed to mediate the primary reinforcing and locomotor effects of psychostimulants, but there is less consensus on conditioned dopamine (DA) release during drug-seeking behavior. We investigated the neurochemical correlates of drug-seeking behavior under the control of a drug-associated cue [a light conditioned stimulus (CS+)] and to noncontingent presentations of the CS+ in the core and shell subregions of the nucleus accumbens. Rats self-administered cocaine under a continuous reinforcement schedule in which a response on one of two identical levers led to an intravenous cocaine infusion (0.25 mg/infusion) and a 20 sec light CS+. Response requirements for cocaine and the CS+ were then progressively increased until stable responding was established under a second-order schedule of reinforcement. During microdialysis, rats were presented noncontingently with a set of 10 sec CS+ and neutral tone stimuli (CS-) before and after a 90 min period during which they responded for cocaine under a second-order schedule. Results showed the following: (1) nucleus accumbens DA increased in both the core and shell during intravenous cocaine self-administration; (2) noncontingent presentations of a cocaine-associated CS+ led to increased DA release selectively in the nucleus accumbens core; and (3) extracellular DA levels were unaltered in both core and shell during a protracted period of drug-seeking behavior under the control of the same cocaine-associated cue. These results indicate that the mesolimbic dopamine system is activated after exposure to drug-associated stimuli under specific conditions.

Haloperidol challenge during copulation prevents subsequent increase in male sexual motivation

Male rats manifest an increase in sexual motivation following sexual experience. The current experiment was devised to investigate the role of dopamine in this process by assessing whether sexual behavior occurring in the presence of the dopamine receptor antagonist, haloperidol, would continue to alter the subjects' subsequent sexual motivation. Four groups of male Long-Evans rats (total N=34) traversed an operant runway once per day for one of two goalbox targets: a nonestrous or estrous female. Following establishment of baseline run times (10 trials), all males received one ejaculation with a receptive female in a separate testing environment. Subjects were pretreated with vehicle or one of three doses of haloperidol (0.05, 0.075, 0.10 mg/kg) 45 min prior to being paired with the receptive female. All subjects successfully achieved ejaculation under these conditions. Subjects were then re-tested within the runway for their motivation to approach the two types of female targets (10 trials). Vehicle-treated subjects expressed the expected increase in sexual motivation following sexual experience, while haloperidol treatment dose-dependently attenuated this effect. Subjects that received the highest haloperidol dose subsequently manifested increased run times and intra-runway "retreat" behaviors, suggesting that female cues may have become associated with an aversive sexual experience. These results are consistent with the view that dopamine systems play a role in the rewarding or reinforcing consequences of male sexual behavior.

Stress-induced relapse to heroin and cocaine seeking in rats: a review

Studies in humans suggest that exposure to stress increases the probability of relapse to drug use, but until recently there has been no animal model to study the mechanisms that mediate this effect. We have developed a reinstatement procedure that allows us to study the effect of stress on relapse to drug seeking in rats. Using this procedure, we have shown that exposure to intermittent footshock stress reliably reinstates heroin and cocaine seeking after prolonged drug-free periods. In the present paper, we summarize results from several studies on stress-induced reinstatement of heroin and cocaine seeking in rats. We first assess the degree to which the phenomenon of stress-induced relapse generalizes to other stressors, to behaviors controlled by other drugs of abuse, and to behaviors controlled by non-drug reinforcers. We then review evidence from studies concerned with the neurotransmitters, the brain sites, and the neural systems involved in stress-induced reinstatement of drug seeking. Finally, we consider the mechanisms that might underlie stress-induced relapse to drug seeking and the possible implications of the findings for the treatment of relapse to drug use in humans.

Heroin-specific stimuli reinstate operant heroin-seeking behavior in rats after prolonged extinction

The clinical literature suggests that exposure to environmental stimuli previously associated with heroin availability may precipitate relapse. However, experimental studies elucidating the significance of learned associations between drug availability and reinstatement of heroin-seeking behavior in the rat are still scarce. To examine the role of environmental stimuli in reinstatement of heroin-seeking behavior, rats were trained to associate discriminative stimuli (DS+) with intravenous heroin availability vs. nonreward [i.e., availability of intravenous saline (DS-)]. The animals then were subjected to extinction training during which the discriminative stimuli were not presented, and lever pressing did not result in drug or saline infusion. The resistance to extinction varied greatly among animals (2.5-11.4 weeks). When the discriminative stimuli were reintroduced, the DS+ reinstated responding while the DS- did not. The average number of responses for heroin during the reinstatement trial (12.8) paralleled the average responding for heroin during discrimination training (12.6), suggesting that the associations between environmental stimuli and drug availability are long-lasting and powerful motivators of drug-seeking behavior.

Sexual motivation in the male rat: the role of primary incentives and copulatory experience

Two experiments explored the motivational impact of primary incentive female cues on the operant behavior of sexually naive and experienced male rats. In the first experiment, a straight-arm runway was used to assess the subjects' motivation to approach a goalbox containing either male or female "targets." Twelve sexually naive Long-Evans males ran for: (1) an empty goalbox; (2) a male conspecific; (3) an ovariectomized (OVX) female; (4) an OVX female given estradiol; (5) or an OVX female treated with estradiol and progesterone. A perforated Plexiglas partition in the goalbox prevented the subject males from physically interacting with the targets, although olfactory, visual, and auditory cues were accessible. We hypothesized that subjects would manifest shorter run times (reflecting greater motivation) when the goalbox contained a receptive/proceptive female as opposed to a nonreceptive female target. Subjects' run times were ordered depending on the nature of the target (from slowest to fastest): empty goalbox, male conspecific, OVX female, OVX + estradiol female, and OVX + estradiol + progesterone female. As predicted, subjects ran significantly faster for a receptive/proceptive female than for a nonreceptive female, indicating that sexually naive males are inherently motivated by female precopulatory cues. In the second experiment, 30 sexually naive male subjects ran for a goalbox containing either a nonestrous (OVX) or an estrous (OVX + estradiol + progesterone) female. Following six trials, 10 males were allowed one intromission with a receptive female, 10 males experienced one ejaculation, and 10 remained sexually naive. Only those males having experienced an ejaculation subsequently decreased their run times for both nonestrous and estrous females, indicating that sexual reinforcement produced by ejaculation, but not intromission, further enhances the motivational impact of female incentive cues.

Drug addiction as dopamine-dependent associative learning disorder

Natural rewards preferentially stimulate dopamine transmission in the nucleus accumbens shell. This effect undergoes adaptive changes (one-trial habituation, inhibition by appetitive stimuli) that are consistent with a role of nucleus accumbens shell dopamine in associative reward-related learning. Experimental studies with a variety of paradigms confirm this role. A role in associative stimulus-reward learning can provide an explanation for the extinction-like impairment of primary reinforcement that led Wise to propose the 'anhedonia hypothesis'. Addictive drugs share with natural rewards the property of stimulating dopamine transmission preferentially in the nucleus accumbens shell. This response, however, in contrast to that to natural rewards, is not subjected to one-trial habituation. Resistance to habituation allows drugs to activate dopamine transmission in the shell non-decrementally upon repeated self-administration. It is hypothesized that this process abnormally strengthens stimulus-drug associations thus resulting in the attribution of excessive motivational value to discrete stimuli or contexts predictive of drug availability. Addiction is therefore the expression of the excessive control over behaviour acquired by drug-related stimuli as a result of abnormal associative learning following repeated stimulation of dopamine transmission in the nucleus accumbens shell.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Haloperidol differentially affects reinforcement and motivational processes in rats running an alley for intravenous heroin

The role of drug-paired environmental stimuli in opiate self-administration was investigated by exposing animals to discrete cues that were predictive of the availability or unavailability of heroin reinforcement. Rats were trained to traverse a straight arm runway for a reinforcement consisting of a single 0.1 mg/kg intravenous infusion of heroin delivered upon entrance to the goal box. On each trial, one of two discriminative olfactory stimuli (orange and almond) was used: one which signaled the availability of heroin in the goal box (S+), and one which signaled its absence (S-). The effect of dopamine (DA) receptor antagonism on reinforcement and motivational processes was investigated by pretreating subjects with 0.0, 0.15 or 0.30 mg/kg of the DA receptor antagonist drug, haloperidol. Haloperidol had no effect on operant runway performance (i.e. goal time) in any condition. However, 24 h later, on the first post-treatment trial, those haloperidol animals that received heroin in the goal box on the previous trial (i.e. the S+ condition) ran reliably more slowly than subjects that received vehicle on the previous S+ trial. These results suggest that haloperidol does not affect the motivational properties of stimuli which predict the availability of heroin, while it does diminish the reinforcing effects of actually receiving heroin.