Drug self-administration

A Comprehensive Analysis of Biopharmaceutical Products Listed in the FDA's Purple Book

Although biopharmaceuticals constitute around 10% of the drug landscape, eight of the ten top-selling products were biopharmaceuticals in 2023. This study did a comprehensive analysis of the FDA's Purple Book database. Firstly, our research uncovered market trends and provided insights into biologics distributions. According to the investigation, although biotechnology has advanced and legislative shifts have made the approval process faster, there are still challenges to overcome, such as molecular instability and formulation design. Moreover, our research comprehensively analyzed biological formulations, pointing out significant strategies regarding administration routes, dosage forms, product packaging, and excipients. In conjunction with biologics, the widespread integration of innovative delivery strategies will be implemented to confront the evolving challenges in healthcare and meet an expanding array of treatment needs.

Method of data interpretation for the determination of abuse liability in rodent self-administration studies under the FDA guidance document

All new molecular entities that enter the CNS and exert an activity in the brain must be assessed for abuse liability prior to a New Drug Application approval by the US Food and Drug Administration. One element of the screening process is the assessment of the reinforcing properties of the drug candidate using the regulatory-preferred species, the rat. We describe one method of data review from the standard rat IV SA study design that can be used to conclude the relative abuse liability of the new drug entity. While we do not claim the process as the only way to review or interpret the data, we believe the steps described highlight a process that the pharmaceutical development team can use as a starting point for a discussion during study protocol development.

Nicotine self-administration research: the legacy of Steven R. Goldberg and implications for regulation, health policy, and research

BACKGROUND AND RATIONALE

Steven R. Goldberg was a pioneering behavioral pharmacologist whose intravenous drug self-administration studies advanced the understanding of conditioned stimuli and schedules of reinforcement as determinants of pattern and persistence of drug-seeking behavior, and in particular, the importance of nicotine in tobacco use. His passing in 2014 led to invitations to contribute articles to psychopharmacology dedicated to his work.

OBJECTIVES

The objectives of this review are to summarize and put into historical perspective Goldberg's contributions to elucidate the reinforcing effects of nicotine and to summarize the implications of his research for medication development, tobacco regulation, and potential tobacco control policy options. This includes a review of intravenous nicotine self-administration research from the 1960s to 2016.

RESULTS

Goldberg's application of behavioral pharmacology methods to investigate nicotine reinforcement and the influence of schedule of reinforcement and conditioned stimuli on nicotine administration contributed to the conclusions of the US National Institute on Drug Abuse, and the Surgeon General, that nicotine met the criteria as a dependence-producing drug and cigarette smoking as a prototypic drug dependency or "addiction." Equally important, this work has been systematically extended to other species and applied to address a range of factors relevant to tobacco use, medication development, regulation, and public health policy.

CONCLUSIONS

Steven R. Goldberg was a pioneering scientist whose systematic application of the science of behavioral pharmacology advanced the understanding of tobacco and nicotine use and contributed to the scientific foundation for tobacco product regulation and potential public health tobacco control policy development.

The Reinforcing Effects of Nicotine in Humans and Nonhuman Primates: A Review of Intravenous Self-Administration Evidence and Future Directions for Research

INTRODUCTION

Cigarette smoking is largely driven by the reinforcing properties of nicotine. Intravenous (IV) self-administration procedures are the gold standard for investigating the reinforcing effects of psychoactive drugs. The goal of this review was to examine the results of published investigations of the reinforcing effects of nicotine measured using IV self-administration procedures in humans and nonhuman primates.

RESULTS

The body of literature using nonhuman primate subjects indicates nicotine functions as a positive reinforcer when available for self-administration via IV catheters. However, it can also be difficult to establish IV nicotine self-administration in nonhuman primates and sometimes supplemental strategies have been required (e.g., priming injections or food deprivation) before subjects acquire the behavior. Although the body of literature using human subjects is limited, the evidence indicates nicotine functions as a reinforcer via the IV route of administration in adult cigarette smokers. Rates of nicotine self-injection can be variable across subjects and responding is sometimes inconsistent across sessions in both humans and nonhuman primates.

CONCLUSIONS

The Family Smoking Prevention and Tobacco Control Act, enacted in 2009, gave the Food and Drug Administration regulatory authority over the manufacture, marketing, and distribution of tobacco products. Research examining the threshold reinforcing doses for initiation and maintenance of nicotine self-administration, comparisons of the reinforcing effects of nicotine in adolescent versus adult subjects, investigations of gender differences in the reinforcing effects of nicotine, and studies of the abuse liability of non-nicotine tobacco product constituents and their ability to alter the reinforcing effects of nicotine will inform potential tobacco regulatory actions.

Is the rodent maternal separation model a valid and effective model for studies on the early-life impact on ethanol consumption?

RATIONALE

Early-life events can cause long-term neurobiological and behavioural changes with a resultant effect upon reward and addiction processes that enhance risk to develop alcohol use disorders. Maternal separation (MS) is used to study the mediating mechanisms of early-life influences in rodents. In MS studies, the pups are exposed to maternal absence during the first postnatal weeks. The outcome of MS experiments exhibits considerable variation and questions have been raised about the validity of MS models.

OBJECTIVES

This short review aims to provide information about experimental conditions that are important to consider when assessing the impact of early-life environment on voluntary ethanol consumption.

RESULTS

The results from currently used MS protocols are not uniform. However, studies consistently show that longer separations of intact litters predispose for higher ethanol consumption and/or preference in adult male rats as compared to shorter periods of MS. Studies using individual pup MS paradigms, other controls, low ethanol concentrations, adult females or examining adolescent consumption reported no differences or inconsistent results.

CONCLUSIONS

There is no "a rodent MS model", there are several models and they generate different results. The compiled literature shows that MS is a model of choice for analysis of early-life effects on voluntary ethanol consumption but there are examples of MS paradigms that are not suitable. These studies emphasize the importance to carefully designed MS experiments to supply the optimal conditions to definitely test the research hypothesis and to be particulate in the interpretation of the outcome.

Prenatal nicotine exposure changes natural and drug-induced reinforcement in adolescent male rats

Clinical studies have demonstrated an increased incidence of substance misuse and obesity in adolescents whose mothers smoked during pregnancy. Although dopamine systems that mediate natural and drug-induced reinforcement have been shown in animal studies to be altered by gestational nicotine treatment, it is not clear whether there are concomitant changes in reinforcement sensitivity. To test whether prenatal nicotine exposure influences sensitivity to natural and drug rewards, timed pregnant rats were implanted with osmotic minipumps delivering saline or nicotine (3 mg/kg/day) from gestational day 4 to 18. Male offspring were tested as adolescents, on postnatal day 32, for operant responding maintained by sucrose pellets or i.v. cocaine (200 or 500 mug/kg per injection). Cocaine-induced stereotypy and c-fos mRNA expression in cortex and striatum were also examined. Complex changes in reward circuitry were observed in the offspring of nicotine-exposed dams. Nicotine-exposed adolescents did not self-administer the low dose of cocaine, but, at the higher dose, exhibited significantly greater cocaine intake and c-fos mRNA expression in nucleus accumbens than did controls. In contrast, control animals showed significantly greater drug-induced stereotypy at both cocaine doses. Operant responding maintained by sucrose was also influenced by gestational nicotine exposure. At a fixed ratio (FR) 1 schedule, although the number of pellets eaten by the two experimental groups was equivalent, more pellets were left uneaten by nicotine-exposed offspring. At FR2 and FR5 schedules, the responding maintained by sucrose pellets was lower in nicotine-exposed offspring. These findings suggest that nicotine exposure during gestation may induce changes in both natural and drug reward pathways.

The role of nicotine in smoking: a dual-reinforcement model

Models of intravenous nicotine self-administration in laboratory animals are being used to investigate the behavioral and neurobiological consequences of nicotine reinforcement, and to aid in the development of novel pharmacotherapies for smoking cessation. Central to these models is the principle of primary reinforcement, which posits that response-contingent presentation of a primary reinforcer, nicotine, engenders robust operant behavior, whereas response-independent drug delivery does not. This dictum of nicotine as a primary reinforcer has been widely used to explain why people smoke tobacco-smoking results in the rapid delivery of nicotine to the brain, setting up a cascade of neurobiological processes that strengthen subsequent smoking behavior. However, there is mounting evidence that the primary reinforcement model of nicotine self-administration fails to fully explain existing data from both the animal self-administration and human smoking literatures. We have recently proposed a "dual reinforcement" model to more fully capture the relationship between nicotine and self-administration, including smoking. Briefly, the "dual reinforcement" model posits that nicotine acts as both a primary reinforcer and a reinforcement enhancer. The latter action of nicotine had originally been uncovered by showing that a reinforcing VS, which accompanies nicotine delivery, synergizes with nicotine in the acquisition and maintenance of self-administration, and that this synergism can be reproduced by combining operant responding for the reinforcing stimulus with non-contingent (response-independent) nicotine. Thus, self-administration (and smoking) is sustained by three actions: (1) nicotine, acting as a primary reinforcer, can sustain behavior that leads to its delivery; (2) nicotine, acting as a primary reinforcer, can establish neutral environmental stimuli as conditioned reinforcers through Pavlovian associations; and (3) nicotine, acting as a reinforcement enhancer, can magnify the incentive value of accompanying stimuli, be they conditioned or unconditioned reinforcers.

Explaining the escalation of drug use in substance dependence: models and appropriate animal laboratory tests

Escalation of drug use, a hallmark of drug dependence, has traditionally been interpreted as reflecting the development of tolerance to the drug's effects. However, on the basis of animal behavioral data, several groups have recently proposed alternative explanations, i.e. that such an escalation of drug use might not be based on (1) tolerance, but rather be indicative of (2) sensitization to the drug's reinforcing effect, (3) reward allostasis, (4) an increase in the incentive salience of drug-associated stimuli, (5) an increase in the reinforcing strength of the drug reinforcer relative to alternative reinforcers, or (6) habit formation. From the pharmacological perspective, models 1-3 allow predictions about the change in the shape of drug dose-effect curves that are based on mathematically defined models governing receptor-ligand interaction and signal transduction. These predictions are tested in the present review, which also describes the other currently championed models for drug use escalation and other components of apparent 'reinforcement' (in its original meaning, like 'tolerance' or 'sensitization', a purely descriptive term). It evaluates the animal experimental approaches employed to support or prove the existence of each of the models and reinforcement components, and recapitulates the clinical evidence, which strongly suggests that escalation of drug use is predominantly based on an increase in the frequency of intoxication events rather than an increase in the dose taken at each intoxication event. Two apparent discrepancies in animal experiments are that (a) sensitization to overall reinforcement has been found more often for psychostimulants than for opioids, and that (b) tolerance to the reinforcing and other effects has been observed more often for opioids than for cocaine. These discrepancies are resolved by the finding that cocaine levels seem to be more tightly regulated at submaximum reinforcing levels than opioid levels are. Consequently, animals self-administering opioids are more likely to expose themselves to higher above-threshold doses than animals self-administering psychostimulants, rendering the development of tolerance to opioids more likely than tolerance to psychostimulants. The review concludes by making suggestions on how to improve the current behavioral experimental approaches.

How research in behavioral pharmacology informs behavioral science

Behavioral pharmacology is a maturing science that has made significant contributions to the study of drug effects on behavior, especially in the domain of drug-behavior interactions. Less appreciated is that research in behavioral pharmacology can have, and has had, implications for the experimental analysis of behavior, especially its conceptualizations and theory. In this article, I outline three general strategies in behavioral pharmacology research that have been employed to increase understanding of behavioral processes. Examples are provided of the general characteristics of the strategies and of implications of previous research for behavior theory. Behavior analysis will advance as its theories are challenged.

The lack of A2A adenosine receptors diminishes the reinforcing efficacy of cocaine

Adenosine is an endogenous purine nucleoside, which acts as a neuromodulator in the central nervous system. A2A adenosine and D2 dopamine receptors are colocalized in the same neurons in discrete brain areas, and the dopaminergic transmission plays a crucial role in the addictive properties of drugs of abuse, such as cocaine. In the present study, we have investigated the specific role of A2A adenosine receptors in cocaine-induced behavioral responses related to its addictive properties. For this purpose, we have evaluated the acute locomotor effects produced by cocaine and the development of locomotor sensitization by repeated cocaine administration. In addition, we have also examined cocaine acute rewarding properties using the conditioned place preference. Finally, we used the intravenous drug self-administration paradigm to investigate the acquisition of an operant response maintained by cocaine self-administration and the reinforcing efficacy of the drug in these knockout animals. Acute cocaine induced a similar increase of locomotor activity in mice lacking A2A adenosine receptors and wild-type littermates. Cocaine-induced locomotor sensitization and conditioned place preference were also maintained in A2A knockout mice. Nevertheless, these knockout mice showed a lower rate of cocaine self-administration than wild-type mice in both fixed ratio 1 and 3 schedules of reinforcement. Moreover, a reduction in the maximal effort to obtain a cocaine infusion was found in A2A knockout mice under a progressive ratio schedule. In addition, a vertical shift of the cocaine dose-response curve was observed in mice lacking A2A adenosine receptors in comparison with wild-type littermates. Our study demonstrates that A2A adenosine receptors play an important role in cocaine addictive properties, and these receptors seem to be required to develop the addictive effects of this drug.

Complex interactions between nicotine and nonpharmacological stimuli reveal multiple roles for nicotine in reinforcement

RATIONALE

Although considerable progress has been made, we do not yet fully understand the behavioral and neurobiological basis of nicotine reinforcement, and without this knowledge, treatment strategies aimed at reducing smoking remain deficient.

OBJECTIVES

This review describes an original perspective on nicotine reinforcement, which arises from substantial evidence of complex interactions between nicotine and nonpharmacological stimuli. We hypothesize that nicotine reinforcement derives from at least two sources: (1) primary reinforcement, an action that requires response-dependent drug administration and is capable of conveying secondary reinforcing effects on associated stimuli, and (2) the reinforcement-enhancing effect of nicotine, which directly enhances behavior maintained by salient nonnicotine stimuli and does not require a contingent relationship between drug administration and reinforced operant responding. Although novel for nicotine, this hypothesis has origins in an extensive literature on the reinforcing effects of psychostimulants. Empirical support for this hypothesis, based largely on animal models of reinforcement, will be presented.

CONCLUSIONS

Animal models of drug reinforcement have evolved to reflect our growing awareness of the multidimensional nature of drug dependence in humans. Investigating the interaction between nicotine and nonpharmacological stimuli within the context of the drug self-administration paradigm in rats has generated new insights into the paradox of how nicotine, an apparently weak primary reinforcer, can sustain the robust behavior observed in self-administration and in smoking. The hypothesis presented in this paper--that nicotine acts as both a primary reinforcer and an enhancer of other nonnicotine reinforcers--provides important direction for future investigations into the neurobiology of nicotine reinforcement and treatments for smoking cessation.

The reinforcing properties of alcohol are mediated by GABA(A1) receptors in the ventral pallidum

It has been hypothesized that alcohol addiction is mediated, at least in part, by specific gamma-aminobutyric acid(A) (GABA(A)) receptors within the ventral pallidum (VP). Among the potential GABA(A) receptor isoforms regulating alcohol-seeking behaviors within the VP, the GABA(A) alpha1 receptor subtype (GABA(A1)) appears pre-eminent. In the present study, we developed beta-carboline-3-carboxylate-t-butyl ester (betaCCt), a mixed agonist-antagonist benzodiazepine (BDZ) site ligand, with binding selectivity at the A1 receptor to explore the functional role of VP(A1) receptors in the euphoric properties of alcohol. The in vivo actions of betaCCt were then determined following microinfusion into the VP, a novel alcohol reward substrate that primarily expresses the A1 receptor. In two selectively bred rodent models of chronic alcohol drinking (HAD-1, P rats), bilateral microinfusion of betaCCt (0.5-40 microg) produced marked reductions in alcohol-reinforced behaviors. Further, VP infusions of betaCCt exhibited both neuroanatomical and reinforcer specificity. Thus, no effects on alcohol-reinforced behaviors were observed following infusion in the nucleus accumbens (NACC)/caudate putamen (CPu), or on response maintained by saccharin. Parenteral-administered betaCCt (1-40 mg/kg) was equally effective and selective in reducing alcohol-reinforced behaviors in P and HAD-1 rats. Additional tests of locomotor activity revealed that betaCCt reversed the locomotor sedation produced by both chlordiazepoxide (10 mg/kg) and EtOH (1.25 g/kg), but was devoid of intrinsic effects when given alone. Studies in recombinant receptors expressed in Xenopus oocytes revealed that betaCCt acted as a low-efficacy partial agonist at alpha3beta3gamma2 and alpha4beta3gamma2 receptors and as a low-efficacy inverse agonist at alpha1beta3gamma2, alpha2beta3gamma2, and alpha5beta3gamma2 receptors. The present study indicates that betaCCt is capable of antagonizing the reinforcing and the sedative properties of alcohol. These anti-alcohol properties of betaCCt are primarily mediated via the GABA(A1) receptor. betaCCt may represent a prototype of a pharmacotherapeutic agent to effectively reduce alcohol drinking behavior in human alcoholics.

Principles of drug abuse liability assessment in laboratory animals

This paper describes the rationale for use of preclinical assessments of abuse liability in laboratory animals, and then discusses "cross-cutting" methodological issues that apply to behavioral evaluations intended to contribute to an abuse liability evaluation package. Issues include use of: (1) positive and negative control conditions; (2) full dose-effect evaluations, (3) multiple dependent measures, (4) pharmacokinetic evaluations to guide choice of dose ranges, (5) a species for which good methodological and comparative data are available to aid interpretation of results, and (6) appropriate methods for the group or single-subject experimental design selected. The remainder of the paper describes basic methodology by which three core pieces of behavioral data required by the Food and Drug Administration for its use in the overall abuse liability analysis can be obtained preclinically. Reinforcing effects are assessed in study of drug self-administration; drug discrimination assesses degree of overlap of interoceptive stimulus effects with relevant comparison drugs; physical dependence potential is determined by assessing whether a withdrawal syndrome occurs after chronic drug administration. Background and methodological issues specific to each procedure are discussed. A key consideration for cross-cutting and specific methodological issues is that choices made enable confident interpretation of both positive and negative results.

Relative reinforcing effects of different oral ethanol doses in rhesus monkeys

The relative reinforcing effects of different doses of orally delivered ethanol were evaluated. Mouth-contact responding by rhesus monkeys was measured under concurrent fixed-ratio fixed-ratio schedules of liquid delivery (0.67 ml/delivery) from each of two spouts during daily 3-hr sessions. Experiment 1 examined persistence of responding with ethanol (2%, 8%, and 32% wt/vol) and water available. When fixed-ratio values from 8 to 128 were tested, the number of ethanol deliveries obtained per session decreased as the response requirement increased. The decrease in deliveries was less at higher than at lower ethanol concentrations, however. Experiment 2 examined choice between two ethanol concentrations under concurrent fixed-ratio 16 schedules (4% vs. 8%, 4% vs. 16%, 8% vs. 16%, 2% vs. 8%, 2% vs. 32%, 8% vs. 32%). Higher concentrations (16%, 32%) generally maintained more responding than concurrently available concentrations of 8% or less. An exception was the observation of a preference for 8% over 32% ethanol. When the fixed-ratio value was increased, however, the relative preference for these two doses was reversed so that 32% ethanol maintained more responding than 8% ethanol. Thus, the direction of the preference depended on the size of the response requirement. These results indicate that the reinforcing effects of ethanol increase with dose.

Effects of hippocampal damage on reward threshold and response rate during self-stimulation of the ventral tegmental area in the rat

The main purpose of this study was to explore the role of the hippocampus in motivated behavior. Rats with bilateral excitotoxic lesions of the hippocampus and controls were trained to lever press for electrical stimulation of the ventral tegmental area. Rate intensity functions were generated from an ascending and descending series of current intensities. Lesion-induced changes in sensitivity to reward were distinguished from enhancements in motor output by calculating reward thresholds and maximal response rates from the rate-intensity functions. Rats with hippocampal damage showed lower reward thresholds and higher maximal response rates than controls. These results provide further evidence of hippocampal modulation of the nucleus accumbens, suggesting that lesions of this structure enhance sensitivity to reward and increase motor output.

A novel method for self-administering addicting drugs intracerebroventricularly in a free-choice procedure

This paper describes a safe method for long term intracerebroventricular (i.c.v.) drug administration. Employing a non-preferred lever to control the self-administration the technique offers advantages over existing experimental methods. To check for any innate preference for one of two levers, male Wistar rats were allowed during the training procedure, to press two levers (L1 and L2) for one hour/day to obtain water as reinforcer for one week in a continuous reinforcement schedule (CRF). One week after surgery, during which a double-guide stainless steel cannula was inserted into both lateral ventricles, rats received 2 microliter of sterile cerebrospinal fluid (CEPH) each time they pressed one of two levers during the daily one-hour session. When a stable baseline was reached, rats were divided into three groups on the basis of their lever preference, and submitted to the testing procedure. A potent mu-opiate receptor agonist, etonitazene (0.1-0.2-1 microgram/infusion), was always associated with the non-preferred lever for each rat. When no obvious preference was shown for either lever the opiate was firstly delivered by L1(for 11 days) and then by L2 (for 20 days). The results indicate that, regardless of which lever had been preferred initially, the rats increased the pressing of only the lever associated with the opiate. The daily amount taken increased linearly and was behaviorally active. This model highlights for the first time the reinforcing properties of drugs given i.c.v. in a free-choice situation.

Lever-press responding maintained by contingent intraperitoneal administration of etonitazene in Long Evans hooded rats

Lever pressing maintained by intraperitoneal (i.p.) injections of etonitazene was established in five Long Evans hooded rats. Each training session consisted of an 8-min fixed interval (FI) during which lever pressing was maintained by food pellets delivered at the end of the interval. Food delivery was accompanied by illumination of stimulus lights in the chamber. Every 20th response during the 8 min interval also produced a brief illumination of the stimulus lights (FI 8 min (FR 20:S)). Administration of etonitazene was then introduced. Immediately following food delivery, the rat received an i.p. drug injection and was returned to the operant chamber for 30 min. During this confinement, the stimulus lights remained illuminated. This procedure resembles conditioned place preference in that an environment is paired with the effects of an investigator-delivered drug. When food pellet delivery subsequently was discontinued, responding persisted when followed by drug, but not saline, administration. Alternating blocks of sessions with administration of etonitazene (6.0-9.0 microg/kg) or saline produced corresponding increases or decreases in responding. These results indicate that etonitazene can function as a reinforcer when administered to rats by the i.p. route, and thus extend the range of conditions under which drug reinforcement can be investigated.

Nicotine and heroin augment cocaine-induced dopamine overflow in nucleus accumbens

The current public debate on nicotine concentrates on the abuse potential of nicotine per se. However, little is known about the interaction of nicotine with other drugs of well-established abuse liability such as cocaine. Indeed, cigarette smoking increases the intake of cocaine and other drugs of abuse. In order to test if these epidemiological data are reflected in a neurochemical correlate of the reinforcing effects of drugs of abuse, i.e., dopamine overflow in the nucleus accumbens, in vivo brain microdialysis was used to examine the effects of nicotine and cocaine either alone or in combination in freely moving rats. Furthermore, the effects of the nicotine + cocaine combination were compared to another drug combination of high abuse potential, i.e., heroin + cocaine ('speedball'). Both nicotine + cocaine as well as heroin + cocaine stimulated nucleus accumbens dopamine overflow in an additive manner. Repeated intermittent administration of nicotine did not significantly alter the effects of a subsequent challenge with the nicotine + cocaine combination. These data suggest that the clinical-epidemiological findings on either drug combination are reflected in a stimulatory interaction on nucleus accumbens dopamine overflow that is additive. No significant tolerance seems to develop to this effect of nicotine. These neurochemical findings support behavioral data suggesting that the reinforcing effects of cocaine and heroin are additive and predict that nicotine will enhance the reinforcing effects of cocaine.

Orally delivered methadone as a reinforcer in rhesus monkeys

Methadone usually is taken orally for drug abuse treatment in humans but oral methadone self-administration by laboratory animals has not been investigated extensively. The present study examines acquisition and maintenance of oral methadone maintained responding in four adult male rhesus monkeys. Drug solution was available from one liquid delivery system and water from a second system during daily 3-h sessions. Locations of liquids were reversed each session, and liquid (0.65 ml per delivery) was delivered according to a fixed-ratio reinforcement schedule. Initially a test for the reinforcing effects of 0.00625-0.4 mg/ml methadone solutions was carried out but a consistent preference for drug over water was not seen. To establish methadone as a reinforcer, a fading procedure was used in which responding was first maintained by solutions of methadone (0.00625-0.4 mg/ml) combined with ethanol (0.0325-2.0% w/v). Subsequently, the concentration of the ethanol in the combination was gradually reduced to zero. Methadone-maintained responding (0.4 mg/ml) persisted when ethanol was no longer present. To confirm that the drug was serving as a reinforcer, the dose was varied: (a) by changing the volume delivered while the concentration was held constant and (b) by changing the concentration of the methadone while the volume per delivery was held constant. Over a wide range of doses, deliveries of methadone solution usually exceeded deliveries of concurrently available water. Orderly relationships were observed among methadone dose, response rate, and drug intake. The study of oral self-administration of opioid drugs by nonhuman primates may be a useful strategy for the development and evaluation of new drug substitution or replacement therapies.

Anxiety: a potential predictor of vulnerability to the initiation of ethanol self-administration in rats

Anxiolytic effects of ethanol have been proposed to be important factors in the initiation of ethanol consumption. To examine this hypothesis, drug-naive Wistar rats were tested in the elevated plus-maze to determine their initial level of anxiety. Based on their response, we separated the animals into anxious and non-anxious groups. After that, animals went through an oral ethanol self-administration procedure. Rats that were initially classified as anxious showed a significantly (P < 0.01) higher intake and preference for ethanol during the initiation phase of the voluntary drinking procedure than non-anxious animals. In another experiment, intraperitoneal (IP) injections of ethanol (0.5-1.5 g/kg) produced dose-dependent anxiolytic effects in rats when tested in the elevated plus-maze procedure. Blood ethanol levels following IP injections during the plus-maze test were similar to those reached during the oral ethanol self-administration procedure, which shows that the rats indeed drank sufficient amounts of ethanol to experience its anxiolytic effects. These findings indicate that the basal level of anxiety plays an important role in vulnerability to alcohol drinking.

Genetic differences in intravenous cocaine self-administration between C57BL/6J and DBA/2J mice

In experiment 1, two different strains of mice [C57BL/6J (B6) and DBA/2J (D2)] were allowed to nosepoke for 5 microliters intravenous (IV) infusions during 2-h daily sessions. Two nosepoke holes were available, only one of which was reinforced on an FR-3 schedule with a 10-s time-out indicated by a light inside the reinforced nosepoke hole. During the first nine sessions, infusions were saline. On subsequent sessions, mice acquired nosepoking for 0.5 mg/kg cocaine. Finally, all mice were extinguished by again receiving only saline infusions. Cocaine acted as a reinforcer in both strains. In experiment 2, different mice from the same two strains were allowed to acquire nosepoking for IV cocaine at one of three unit doses (0.5, 1.0, or 2.0 mg/kg). Although there were no effects of unit dose on rate of acquisition, B6 mice were faster in acquiring self-administration behavior than were D2 mice. Experiment 3 assessed behavior in the same mice, after acquisition had occurred. D2 mice nosepoked at a lower rate at asymptote than did B6 mice, but with a higher preference for the cocaine reinforced hole. Unit doses of cocaine were then manipulated within subjects, from 0.125 to 2.0 mg/kg per infusion. Higher doses yielded lower response rates than lower doses, both between and within subjects. Behavior in D2 mice relative to B6 mice also appeared to be shifted to the left of the dose-response curve measured within-subjects. Together, these findings indicate that although cocaine serves as a reinforcer in both strains, there are genetic differences in the pattern of cocaine self-administration between these two mouse strains.

Tolerance to the reinforcing effects of cocaine in a progressive ratio paradigm

This experiment used rats to test whether a regimen of chronic cocaine would produce tolerance to cocaine i.v. self-administration under a progressive ratio (PR) schedule of reinforcement. Under this PR schedule, an increasing number of responses was required to complete the ratio for each subsequent cocaine injection, and failure to complete the required ratio for the next injection within 1 h of the previous cocaine injection terminated the session. The number of injections taken in the session was termed the breaking point and used as the dependent variable. Rats were trained under this schedule until breaking point values were stable, after which cocaine dose-effect data were obtained: the breaking point increased as the dose of cocaine increased. Subsequently, rats were assigned to one of two groups for 7 days of chronic treatment: one group was infused with cocaine (18 mg/kg, given over 20 min once every 8 h) and the other group received 0.9% saline. Following termination of chronic treatment, cocaine dose-effect data were redetermined in both groups. Chronic cocaine treatment significantly decreased breaking point values across the entire dose-effect curve, although the effect was observed in only four of seven subjects. In contrast, chronic saline treatment produced no significant effect on the breaking point measures. Following a further 5 days of recovery from chronic treatment, cocaine dose-effect data were redetermined in both groups; these curves were essentially identical to those obtained before chronic treatments. These data support the hypothesis that tolerance occurs to the reinforcing effects of cocaine, as measured by a decrease in the breaking point, at least for a subset of animals.