Effects of extended access to high versus low cocaine doses on self-administration, cocaine-induced reinstatement and brain mRNA levels in rats

Frequency of recent cocaine and alcohol use affects drug craving and associated responses to stress and drug-related cues

RATIONALE

Stress is known to increase drug craving, associated physiological arousal and risk of relapse in drug dependent individuals. However, it is unclear whether these responses are altered by recent frequency of drug use. The current study examined whether frequency of cocaine and alcohol abuse alters drug craving and associated arousal with laboratory exposure to stress and to drug related cues.

METHODS

Fifty-four recently abstinent treatment-seeking cocaine abusers who were part of a study on stress and drug craving were categorized into high- and low-frequency users on the basis of their recent cocaine use. The high use cocaine group also consumed significantly more alcohol than the low use cocaine group. Participants were exposed to a brief 5-min guided imagery procedure that involved imagining a recent personal stressful situation, a personal drug-related situation and a neutral-relaxing situation, one imagery session on separate days presented in random order. Subjective (craving and anxiety), cardiovascular (heart rate, systolic blood pressure (SBP) and diastolic blood pressure (DBP)) and biochemical (adrenocorticotropic hormone (ACTH), cortisol, prolactin) measures were assessed.

RESULTS

High-frequency abusers demonstrated a significantly greater drug craving, anxiety and associated cardiovascular and hypothalamic-pituitary-adrenal (HPA) response to both stress and drug-cue exposure as compared to low-frequency abusers.

CONCLUSIONS

Increased frequency of recent cocaine and alcohol use is associated with an enhanced stress and cue-induced drug craving and arousal response that appears to be similar to the effects of cocaine, and one that may increase the vulnerability to drug-seeking behavior and relapse in drug dependent individuals.

Sex differences in the escalation of oral phencyclidine (PCP) self-administration under FR and PR schedules in rhesus monkeys

RATIONALE

Studies with male rats indicate that long access (LgA) vs short access (ShA) to i.v. cocaine and heroin self-administration leads to an escalation of drug intake and a subsequent upward shift of the dose-response function.

OBJECTIVE

The purpose of this experiment was to extend these results to male and female rhesus monkeys and oral phencyclidine (PCP) self-administration under fixed-ratio (FR) and progressive-ratio (PR) schedules.

METHODS

Adult rhesus monkeys (seven females and nine males) orally self-administered PCP (0.25 mg/ml) and water under concurrent FR 16 FR 16 schedules during daily ShA 3-h sessions. Since females weighed less than males, each liquid delivery (0.6 ml) represented a higher unit dose mg/kg for females than males, but drug concentration mg/ml remained constant. Concurrent PR PR schedules were then used to obtain a concentration-response function (0.125, 0.25, 0.5, and 1.0 mg/ml). Next, PCP and water were available during LgA 6-h sessions under concurrent FR 16 FR 16 schedules for 21 days. The monkeys were then retested under the concurrent FR 16 FR 16 and PR PR conditions during ShA sessions.

RESULTS

Under the initial ShA concurrent FR 16 FR 16 schedules, females and males did not differ on PCP deliveries or intake (mg/kg); however, during LgA, males and females had more PCP deliveries compared with ShA. During LgA, males exceeded females in PCP deliveries, but females were higher than males in mg/kg PCP intake. Also, PCP (but not water) deliveries and mg/kg PCP intake significantly increased from the first 3 days to the last 3 days of the 21-day LgA period in both males and females. The subsequent ShA FR 16 FR 16 performance did not differ by sex, but it was significantly elevated above the first ShA period in both sexes. The concentration-response function for PCP break point under the PR PR schedules and PCP intake (mg/kg) were significantly shifted upward during the second (vs first) ShA period, and females' mg/kg intake significantly exceeded males'.

CONCLUSIONS

Male and female rhesus monkeys both showed escalation of PCP self-administration during LgA to PCP and during ShA that occurred after (vs before) LgA. Both showed vertical upward shifts in the concentration x intake (mg/kg) function under the PR schedule, and females exceeded males on this measure. These findings with PCP and monkeys are consistent with vertical upward shifts of cocaine dose-response functions in previous escalation studies in male rats and reports of sex differences (F>M) during several other phases of drug abuse.

Transition to drug addiction: a negative reinforcement model based on an allostatic decrease in reward function

RATIONALE

The transition from initial drug use to drug addiction has been proposed to result from an allostatic decrease in reward function driven by an overactivation of brain antireward processes.

OBJECTIVES

How decreased reward function explains compulsive drug use is not entirely clear at present, and is still a subject for debate.

METHODS

We present a quantitative model of cocaine self-administration that integrates pharmacokinetic, pharmacodynamic, and motivational factors to address this question. The model assumes that reward system responsivity is a homeostatically regulated process where the desired level of responsivity (called the reward set point) is initially different from the baseline level. The reduction or correction of this difference or error in reward function would drive cocaine self-administration.

RESULTS

Theoretical data obtained by computer simulation fit the experimental data obtained in animals self-administering cocaine (i.e., the within-session pattern of self-injections, the shape and curvature of the dose-injection function, the nonlinear relationship between drug intake and regulated drug effects). Importantly, simulation of an allostatic decrease in reward system responsivity exacerbates the initial error that drives self-administration, thereby increasing both the intake of, and the motivation for, the drug. This allostatic change manifests as a vertical shift in the dose-injection function similar to that seen in animals with escalating cocaine self-administration.

CONCLUSIONS

The present model provides a satisfactory explanation of escalated drug intake and suggests a novel negative reinforcement view of addiction based on an allostatic decrease in reward function.

Effects of extended-access self-administration and deprivation on breakpoints maintained by cocaine in rats

RATIONALE

Animal models that identify the effects of self-administration histories on subsequent patterns, levels of intake, and other aspects of reinforcement will help clarify the controlling variables of human drug use.

OBJECTIVE

Identify the effects of extended-access to cocaine and 1 or 7 days of deprivation on cocaine-maintained breakpoints on a progressive ratio (PR) schedule of reinforcement.

METHODS

Male, Sprague-Dawley rats were trained to self-administer intravenous cocaine (expt 1: 1.5 mg/kg per infusion; expt 2: 0.75 mg/kg per infusion), and then given various histories of self-administration and deprivation. Breakpoints, the number of infusions self-administered on a PR schedule, were assessed following the deprivation period.

RESULTS

Rates of cocaine intake increased when access to cocaine was extended to 6 h/day. From day 1 to day 14, daily intake increased from 92 (+/-2.5) to 101 (+/-2.8) mg/kg in expt 1, and from 55 (+/-4) to 78 (+/-2.2) mg/kg in expt 2. Total intake across this 2-week period was approximately 1260 and 970 mg/kg in expts 1 and 2. Breakpoints were not different following this escalation period. The introduction of a 7-day deprivation period failed to alter breakpoints.

CONCLUSIONS

There is dissociation between changes in rate of cocaine intake (or consumption) and breakpoints maintained on a PR schedule. Extended-access to cocaine produced increases in rate of intake without altering breakpoints. Depending on the experimental question, extended-access conditions may prove useful for studying changes in certain aspects of reinforcement, such as consumption, but not others, such as the strength of a drug as a reinforcer.

Binge self-administration and deprivation produces sensitization to the reinforcing effects of cocaine in rats

RATIONALE

Behavioral procedures that incorporate dynamic changes in drug-maintained behavior are needed to model the development of cocaine addiction in humans.

OBJECTIVES

Because sensitization may occur to some aspects of drug administration during the addiction process, the objective of the present study was to define the critical features of self-administration histories that result in subsequent increases in the reinforcing efficacy of cocaine (measured using the progressive ratio (PR) schedule).

METHODS

Animals were trained to self-administer cocaine on a fixed ratio (FR) schedule, baseline performance on a PR schedule was determined, and animals were given various histories of cocaine self-administration and drug deprivation. PR performance was reassessed following this experience.

RESULTS

Cocaine self-administration under a discrete-trials procedure (24 h/day) for 10 days, followed by a 7-day deprivation period resulted in sensitization to the reinforcing effects of cocaine as assessed by the PR schedule (increases in maximal breakpoints maintained by cocaine with no change in sensitivity at lower doses). Similar levels of daily cocaine intake on a FR schedule (typically completed within 6 h) coupled with a deprivation period failed to produce changes in breakpoint. Providing access to cocaine during the "deprivation period" by repeated testing on a PR schedule prevented the sensitization.

CONCLUSIONS

These data suggest that these self-administration-induced changes in breakpoint reflect sensitization, and show that a drug-free deprivation period is necessary, but not sufficient, to produce this increase.

Evolving perspectives on neurobiological research on the addictions: celebration of the 30th anniversary of NIDA

The roots of the Laboratory of the Biology of the Addictive Diseases are in the development of methadone maintenance for the treatment of opiate addiction. Methadone maintenance therapy continues to be one of the major effective forms of addiction pharmacotherapy and underscores the importance of biological factors in the physiology and treatment of the addictive diseases. Recent work in the Laboratory has focused on the neurobiological, neurochemical, neuroendocrine and behavioral aspects of addictive diseases (principally cocaine and the opiate addictions), using an interdisciplinary approach. The models we have focused on range from in vitro molecular biology and neuroscience, to in vivo animal models, to experiments in normal human populations and patients with specific addictive diseases, and most recently to the human molecular genetics of different addictive diseases. Two long-term corollary hypotheses have guided the Laboratory's work: (1) That the endogenous opioid peptide/receptor systems play a central role in the addictive states and therefore in their treatment. (2) That atypical responsivity to stressors (e.g., in the hypothalamic-pituitary-adrenal axis) plays a role in vulnerability and relapse to specific addictive diseases. This atypical responsivity may be drug-induced, environmentally acquired, and/or due to genetic variation.