Analysis and modification of opiate reinforcement

Descending facilitatory pathways from the rostroventromedial medulla mediate naloxone-precipitated withdrawal in morphine-dependent rats

Opioids produce analgesic effects, and extended use can produce physical dependence in both humans and animals. Dependence to opiates can be demonstrated by either termination of drug administration or through precipitation of the withdrawal syndrome by opiate antagonists. Key features of the opiate withdrawal syndrome include hyperalgesia, anxiety, and autonomic signs such as diarrhea. The rostral ventromedial medulla (RVM) plays an important role in the modulation of pain and for this reason, may influence withdrawal-induced hyperalgesia. The mechanisms that drive opiate withdrawal-induced hyperalgesia have not been elucidated. Here, rats made dependent upon morphine received naloxone to precipitate withdrawal. RVM microinjection of lidocaine, kynurenic acid (excitatory amino acid antagonist) or YM022 (CCK2 receptor antagonist) blocked withdrawal-induced hyperalgesia. Additionally, these treatments reduced both somatic and autonomic signs of naloxone-induced withdrawal. Spinal application of ondansetron, a 5HT3 receptor antagonist thought to ultimately be engaged by descending pain facilitatory drive, also blocked hyperalgesia and somatic and autonomic features of the withdrawal syndrome. These results indicate that the RVM plays a critical role in mediating components of opioid withdrawal that may contribute to opioid dependence.

PERSPECTIVE

Manipulations targeting these descending pathways from the RVM may diminish the consequences of prolonged opioid administration-induced dependence and be useful adjunct strategies in reducing the risk of opioid addiction.

Reflections on the past 40 Years of Behavioral Pharmacology Research on Problems of Drug Abuse

Behavioral pharmacology research has made important contributions to the conceptualization of addictions and to the treatment and prevention of substance abuse. It has its intellectual roots in the experimental analysis of behavior and close ties with experimental and clinical pharmacology. Because substance abusers self-administer drugs repeatedly, this presents the opportunity for learning to occur. The application of learning theory to the laboratory study of drug effects in both animals and humans has been a major contribution of behavioral pharmacology research. Principles derived from over 40 years of behavioral pharmacology research are now widely accepted in the addiction field and have contributed to the development of both pharmacological and behavioral treatments. Drug abuse prevention through the assessment of the abuse liability of new medications has also been an important applied area of research in the field. Personal reflections of three behavioral pharmacologists provide examples of important influences in the field.

Self-administration of cocaine, cannabis and heroin in the human laboratory: benefits and pitfalls

The objective of this review is to describe self-administration procedures for modeling addiction to cocaine, cannabis and heroin in the human laboratory, the benefits and pitfalls of the approach, and the methodological issues unique to each drug. In addition, the predictive validity of the model for testing treatment medications will be addressed. The results show that all three drugs of abuse are reliably and robustly self-administered by non-treatment-seeking research volunteers. In terms of pharmacotherapies, cocaine use is extraordinarily difficult to disrupt either in the laboratory or in the clinic. A range of medications has been shown to significantly decrease cocaine's subjective effects and craving without decreasing either cocaine self-administration or cocaine abuse by patients. These negative data combined with recent positive findings with modafinil suggest that self-administration procedures are an important intermediary step between pre-clinical and clinical studies. In terms of cannabis, a recent study suggests that medications that improve sleep and mood during cannabis withdrawal decrease the resumption of marijuana self-administration in abstinent volunteers. Clinical data on patients seeking treatment for their marijuana use are needed to validate these laboratory findings. Finally, in contrast to cannabis or cocaine dependence, there are three efficacious Food and Drug Administration-approved medications to treat opioid dependence, all of which decrease both heroin self-administration and subjective effects in the human laboratory. In summary, self-administration procedures provide meaningful behavioral data in a small number of individuals. These studies contribute to our understanding of the variables maintaining cocaine, marijuana and heroin intake, and are important in guiding the development of more effective drug treatment programs.

The role of human drug self-administration procedures in the development of medications

The purpose of this review is to illustrate the utility and value of employing human self-administration procedures in medication development, including abuse liability assessments of novel medications and evaluation of potential pharmacotherapies for substance use disorders. Traditionally, human abuse liability testing has relied primarily on subjective reports describing drug action by use of questionnaires; similarly, drug interactions between putative treatment agents and the drugs of abuse have relied on these measures. Subjective reports are highly valued because they provide qualitative and quantitative information about the characteristics of central and peripheral pharmacodynamic effects as well as safety and tolerability. However, self-administration procedures directly examine the behavior of interest-that is, drug taking. The present paper (1) reviews the most commonly used human self-administration procedures, (2) discusses the concordance of subjective reports and self-administration within the context of medications development for substance use disorders, focusing primarily on illustrative examples from development efforts with opioid and cocaine dependence, and (3) explores the utility of applying self-administration procedures to assess the abuse liability of novel compounds, including "abuse-deterrent" formulations (ADFs). The review will focus on opioid and cocaine dependence because a rich database from both clinical laboratory and clinical trial research exists for these two drug classes. The data reviewed suggest that drug-induced changes in self-administration and subjective effects are not always concordant. Therefore, assessment of self-administration in combination with subjective effects provides a more comprehensive picture that may have improved predictive validity for translating to the clinical setting.

Improving detoxification outcomes from methadone maintenance treatment: the interrelationship of affective states and protracted withdrawal

Changing paradigms of health care and increasing knowledge of opioid addiction continue to influence the attitudes of the health profession toward methadone maintenance treatment (MMT). This approach has again reached the forefront of attention because of its significant role in reducing HIV transmission associated with intravenous drug use. In spite of the well-documented benefits of MMT, patients continue to detoxify from methadone for a variety of reasons both overt and covert. Variable outcomes and generally poor long-term abstinence rates results. At present uncertainty still exists surrounding who should attempt detoxification, when or how this should be done, or whether in fact detoxification should be attempted at all. This article describes and examines factors that are positively and negatively associated with detoxification outcome. Evidence for the interrelationship of two notably significant determinants, affective states and protracted withdrawal, are presented and discussed as they relate to theories of relapse and outcome.

Psychopathology, craving, and mood during heroin acquisition: an experimental study

Six detoxified addict volunteers were allowed to self-administer intravenous heroin on an essentially self-determined schedule. Two periods of heroin acquisition were compared: an unmodified cycle in which patients could become intoxicated and a later cycle in which the effects of heroin were blocked with a narcotic antagonist. In the unblocked condition, patients initially experienced an increase in positive mood, but with chronic administration there was a significant rise in psychopathology and the development of a generalized dysphoric state. Similar changes did not occur when the same patients took heroin while blocked with a narcotic antagonist. Drug craving rose dramatically when "unblocked" heroin was available, but gradually fell during methadone detoxification. Following treatment with a narcotic antagonist, the presence of heroin failed to elicit any sustained rise in craving and drug taking was dramatically reduced.

Aftercare on narcotic antagonists: prospects and problems

This study examines the problems and prospects of aftercare of the multiple treatment failure opiate addict who is being treated with narcotic antagonists (primarily naltrexone). At the time of this paper, 21 patients had been admitted to the Drug Addiction Research Project at McLean Hospital, with a retention rate of 42%. "Completers," "noncompleters," and "dropouts" could not be differentiated on the basis of demographic or psychiatric variables, thus making it impossible to define the population most suited for antagonist treatment. The degree of carry-over into the first month in the community, for those who successfully completed the inpatient segment of the program, disappeared over time. The nonreinforcing (lack of euphoric effect) properties of the antagonists and the resultant high level of motivation necessary for the continuation of its use by the patient make this form of treatment suitable for only a small percentage of the "hard core" addict population.