Reward Mechanisms in Normal and Pathological Behavior –The Dopamine Link as a Target for Therapeutic Intervention Michael A. Bozarth, Ph.D. Addiction Research Unit Department of Psychology University at Buffalo Buffalo, NY 14260-4110 www.AddictionScience.net The “Program” • Concept of addiction • Dopamine involvement in psychomotor stimulant and opiate addiction – development of a model reward system • Role of organismic variables in addiction – predisposition to addiction – enabling addiction to mildly psychoactive substances • Commentary on the dopamine link as a therapeutic target What is Addiction? • Addiction is a behavioral syndrome where drug procurement and use seem to dominate the individual’s motivation and where the normal constraints on the individual’s behavior are generally ineffective (e.g., self-perceived “loss of control”) – motivational toxicity may be a defining characteristic – physical dependence is neither a necessary nor a sufficient condition Continuum of Drug Use Experimental Drug Use Casual Drug Use Intensive Drug Use Compulsive Drug Use Addiction Motivational Strength Circumstantial Drug Use Motivational Toxicity From Bozarth (1990); terms described on the continuum were suggested by Jaffe (1975). Addiction as a Motivational Problem • Addiction is best understood by considering the drug’s impact on the individual’s motivation • Experimental psychology and behavioral neuroscience provide the methods for analysis • Model building provides the heuristics – models as tentative ideational testing devices – importance of convergent operations Dynamic Feature of the Motivational Hierarchy "group" "mate" food H un ge r M at e Se xu al rn al So c ia l Dr u g Choice Behavior and the Notion of “Self-Control” Response Selection food sex Behavior social drug Motivational Toxicity • Motivational toxicity describes a disruption of the motivational hierarchy. This is manifest as – increased motivational efficacy of the drug – decreased motivational efficacy of natural rewards • Motivational toxicity produces the intense motivational focusing characteristic of addiction and the apparent “enslavement” inherent in the etymology of this term Progressive Focusing of Motivational Energy on Drug week-5 week-3 H un ge r M at e Se So xu cia al rn l al Dr u week-1 g Motivational Toxicity Producing a Self-Perceived “Loss of Control” Response Selection food sex Behavior social drug Progress in Understanding the Biological Basis of Drug Addiction • Delineation of the effects of addictive drugs on brain systems mediating reward and motivation (1980s-1990s) • Current focus – Extension of experimental findings to human studies – Exploration of CNS neuroadaptive effects produced by addictive drugs – Identification of factors that facilitate the development of an addiction • Clinical applications (2001 and beyond) Evidence that Psychomotor Stimulant Reward Involves an Action in the Nucleus Accumbens Effect Investigator(s) BSR/NAS amphetamine microinjections IVSA/NAS dopamine-depleting lesions Broekkamp et al., 1975 Lyness et al., 1979 Roberts et al., 1977, 1980 Roberts & Koob, 1982 Zito et al., 1985 Hoebel et al., 1983 White et al., this volume Spyraki et al., 1982 IVSA/VTA dopamine-depleting lesions IVSA/NAS kainic acid lesions ICSA/NAS amphetamine CPP/NAS amphetamine microinjections CPP/NAS dopamine-depleting lesions From Bozarth, 1987. Evidence that Opiate Reward Involves an Action in the Ventral Tegmental Area Effect Investigator(s) BSR/VTA morphine microinjections Broekkamp et al., 1976 Broekkamp et al., 1979 BSR/VTA dopamine-depleting lesions Hand & Franklin, 1985 IVSA/VTA dopamine-depleting lesions Bozarth & Wise, 1986 IVSA/VTA narcotic antagonist microinjections Britt & Wise, 1983 IVSA/VTA morphine reinstatement of responding Stewart, 1984 Stewart & de Wit, this volume ICSA/VTA fentynal van Ree & De Wied, 1980 ICSA/VTA morphine Bozarth & Wise, 1981, 1982 CPP/VTA morphine Bozarth & Wise, 1982 Phillips & LePiane, 1980 CPP/VTA opioid peptide Phillips & LePiane, 1982 CPP/VTA enkephalinase inhibitor Glimcher et al., 1984 CPP/VTA dopamine-depleting lesions Spyraki et al., 1983 From Bozarth, 1987. Evidence for a Common Reward Substrate: Convergent Operations-1975-1985 • No fewer than 9 independent studies had established the role of the nucleus accumbens dopamine terminal field in psychomotor stimulant reward • No fewer than 15 independent studies had established the role of the ventral tegmental area (origin of the A10 dopamine system) in opiate reward • Replication and extension of these findings continued throughout the next decade providing strong corroborating evidence for the proposed model Scientific Model Building: Heuristics & Convergent Operations Models are not required to fit all of the data but only to be the best fit for most of data. IVSA BSR CPP ICSA Old models are replaced when better models become available. Convergent operations are critically important. Brain Reward Circuitry From Bozarth, 1987. Psychomotor Stimulant Link in Brain Reward Circuitry From NIDA “Mind Over Matter,” 2000. From Bozarth, 1987. Opiate Link in Brain Reward Circuitry From Bozarth, 1987. From Scientific American Medicine Online, 2000. Dopamine & Reward • The reward model does not propose that dopamine is exclusively involved in reward nor that reward from these drugs comes entirely from this dopamine system • The reward model does suggest that any event that activates this system can produce a significant (but not necessarily addictive) rewarding effect – at least two pharmacologically distinct classes of addictive drugs derive a major part of their rewarding effects by their actions on this brain reward system Possible Action of Other Substances on Brain Reward Circuitry Psychomotor Stimulants Caffeine Pseudoephedrine Opiates Barbiturates Nicotine Phencyclidine Ethanol? Experimental Preparation to Study Rewarding Effects of Electrical Brain Stimulation From Reid, 1987. Effect of Cocaine on Brain Reward Systems as Measured by Brain Stimulation Reward 120 % Baseline Threshold 110 100 90 80 70 60 50 saline 1.25 mg cocaine 2.5 mg cocaine 5 mg cocaine 10 mg cocaine 20 mg cocaine 40 30 20 10 0 15 30 45 60 75 90 105 120 135 150 165 180 Minutes Post Injection From Bozarth, Pudiak, & KuoLee, 1997. Dose-Response Analysis of BSR Facilitation Produced by Various Substances Relative Potencies of Various Compounds % Threshold Lowering 80 nicotine pseudoephedrine cocaine caffeine diphenhydramine tripelennamine 70 60 50 40 30 20 10 0 0.3 Note: The maximum facilitation seen at any time after injection is shown for each compound. 1 3 10 30 Dose (moles/kg) 100 300 From Bozarth, Pudiak, & KuoLee, 1997. A Comparison of the Effects of Cocaine and Mildly Psychoactive Substances on BSR % Threshold Lowering 80 70 60 50 40 30 20 10 0 ine ne li sa a oc c h p oe ud e ps e e rin ed n tin ico e aff ine c From Bozarth, Pudiak, & KuoLee, 1997. A Quantitative Comparison of the Effects of Cocaine and Nicotine on BSR % Baseline Threshold 140 120 100 80 60 40 20 0 cocaine nicotine From Bozarth, unpublished observations, 1997. A Substance’s Addiction Liability versus Addiction to a Substance • The case of nicotine is particularly problematic for models attributing addiction to a simple pharmacological activation of brain reward systems – obviously numerous cases of addiction-like tobacco use exist – nicotine’s action of “normal” brain reward systems is too weak to motivate this behavior • Organismic variables must be important in “enabling” apparent addiction to nicotine – psychological stress – abnormalities in brain reward systems Revised Role of the Mesolimbic Dopamine System in Drug Addiction • Activation provides positive reinforcement and maintains initial drug use • Neuroadaptive changes produce . . . – negative reinforcement (normalization) – enhanced positive reinforcement (sensitization) – motivational toxicity (derived process) • Simple activation of this dopamine system is not sufficient to produce addiction • Organismic variables can significantly modify the motivational impact of various rewards Importance of Neuroadaptive Effects in Drug Addiction • Drug-induced neuroadaptive effects may distinguish the normal influence this brain reward system has on behavior from the extreme control characteristic of drug addiction • Events that activate this system without producing neuroadaptive changes may lack the ability to produce an addiction in “normal” subjects • Neuroadaptive effects may be less important in individuals with pre-existing abnormalities in reward function Modulation of Drug Reward • Events known to enhance reward from psychomotor stimulants and/or opiates – exposure to some drugs • highly addictive drugs (e.g., psychomotor stimulants, opiates) • mildly psychoactive substances (e.g., caffeine? nicotine?) – psychological stress (i.e., uncontrollable) – pharmacological manipulation of dopamine systems • chronic neuroleptic treatment accompanying schizophrenia – genetic differences in brain dopamine function Dopamine Activity & Predisposition to Addiction • Which condition increases addiction risk? Too much or too little dopamine activity? • Either one! • Abnormally high or low dopaminergic activity may mimic the individual components of motivational toxicity – hyperdopaminergic activity may increase the motivational impact of drug reward – hypodopaminergic activity may diminish the motivational impact of ‘natural’ rewards Implications of Abnormal Dopamine Function • Subjects with abnormal dopaminergic function may become “addicted” to events which produce minimal neuroadaptive changes or that produce only moderate activation of this reward system – a hyperactive dopamine system would provide the preexisting condition necessary for cross-sensitization or priming-like effects – a hypoactive dopamine system would provide the preexisting condition necessary for a positive contrast-like effect The “Exposure” View of Addiction (Revisited) • Addiction is ultimately pharmacologically determined – the result of the action of certain drugs on brain reward mechanisms • Several factors can – hasten the development of an addiction – make certain individuals more likely to develop an addiction – enable addiction to some substances and events that are not considered “addictive” for the general population Some Possible Predisposing and Enabling Factors for Drug Addiction • Psychological stress • Drug exposure • Genetic abnormalities Relapse to Drug Use Motivational Toxicity Dependent Relapse Chronic hypodopaminergic activity anhedonia Transitory hyperdopaminergic activity craving The Dopamine Link as a Possible Target for Therapeutic Intervention • Abnormalities in brain dopamine function may identify individuals with a high risk for addiction • Direct manipulation of dopaminergic function is unlikely to prove successful in treating addiction – addiction probably involves neuroadaptive effects and not just simple activation of dopamine systems – modification of dopaminergic action is likely to have global effects on motivation, affect, and behavior • Systems modulating dopaminergic activity are more viable targets for therapeutic intervention Beyond Dopamine Agonist & Antagonist Treatments • Orthomolecular psychiatry – precursor loading – macronutrient manipulation • Autoreceptor regulation • Other regulatory systems – – – – – 5-HT GABA endogenous opioid peptides NMDA NO A Psychobiological Model of Drug Addiction From Bozarth, 1990.