Stimulus control of cocaine self-administration

Intermittent access cocaine self-administration produces context-specific escalation and increased motivation

The intermittent-access (IntA) self-administration procedure has been reported to produce intensified addiction-like behavior compared to continuous-access (ContA) procedures. In a common variation of the IntA procedure, cocaine is available for 5 min at the beginning of each half hour of a 6-h session. In contrast, during ContA procedures, cocaine is available continuously throughout a session, typically lasting one or more hours. Previous studies comparing procedures have used between-subjects designs, where separate groups of rats self-administer cocaine on either IntA or ContA procedures. The present study used a within-subjects design where subjects self-administered cocaine on the IntA procedure in one context and self-administered cocaine on the continuous short-access (ShA) procedure in another context during separate sessions. Across sessions, rats escalated cocaine intake in the IntA, but not ShA, context. Following sessions eight and 11, rats were administered a progressive ratio test in each context to monitor the change in cocaine motivation. Rats obtained more cocaine infusions on the progressive ratio test in the IntA context than in the ShA context following 11 sessions. These results suggest that addiction-like behaviors following IntA self-administration may be influenced by context-specific learning factors.

Preclinical and clinical research on the discriminative stimulus effects of nicotine

Across species, nicotine can produce robust discriminative stimulus (DS) effects, as with other drugs of abuse, a feature that has been harnessed to advance our understanding on the neuropharmacological mechanisms of nicotine's actions. With the crucial role played by nicotine in supporting tobacco dependence, nicotine DS effects have presented an ideal platform to develop novel generation of smoking cessation compounds. Findings from preclinical strands of research have invigorated the field of human discrimination research to objectively assess nicotine's interoceptive stimulus effects. As such, translation studies provide proof of concept for nicotine DS research as a method to assess the subjective effects of nicotine per se, separate from non-nicotine stimuli involved in smoking. Recent clinical studies with low doses have demonstrated that perceiving nicotine's DS effects is necessary, yet not sufficient, for that dose to be reinforcing. These measures have been instrumental in developing novel strategies with regards to establishing threshold doses of nicotine contained in tobacco products, to then determine subthreshold doses that cannot be discriminated and, therefore, fail to maintain reinforcement. Findings from preclinical and clinical nicotine DS research could substantially inform public health policies aimed at regulating nicotine content of consumer products so that they minimize risks of dependency. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Cocaine-associated decision-making: Toward isolating preference

Ever-increasing evidence suggests that substance use disorder is mediated by decision-making processes, and as such, providing nondrug alternatives can shift maladaptive preferences away from drug reinforcers, such as cocaine. Of note, a recent hypothesis suggests that preference for cocaine is simply a byproduct of cocaine intake, such that the 'direct' effects of cocaine weaken the impact of non-drug alternatives while measuring choice. Conversely, existing quantitative theories of decision-making suggest preference is determined by various dimensions of concurrent reinforcers that in turn determine the relative value of available alternatives. Toward teasing apart the conflicting theories above, we developed a novel drug-choice procedure to control for reinforcer frequency and magnitude (two reinforcer dimensions well known to influence preference) that consequently controls for overall cocaine intake. As predicted by quantitative choice theory, results suggest that cocaine intake and preference are dissociable while measuring choice, with reinforcer frequency and magnitude having independent influence on the relative value of choice alternatives. Furthermore, we demonstrate that the choice procedure is sensitive to various manipulations known to alter cocaine reinforcement, all while keeping cocaine intake constant. Finally, the results point to the process of economic substitution as an important avenue of future neurobehavioral investigation toward the improvement of behavioral and pharmacological therapies for substance use disorders. Overall, the proposed choice procedure will allow for improved isolation of the neurobehavioral processes that mediate drug-associated decision-making in future studies.

Differential stimulus control of drug-seeking: multimodal reinstatement

In animal models of substance-use disorder, individuals that repeatedly self-administer drugs of abuse have long-lasting neuronal adaptations that do not occur ostensibly in control animals only exposed to natural reinforcers (e.g. food). Because any treatment for substance-use disorder will be given to individuals with drug-taking histories, adequate dissociation of the specific neurobehavioral mechanisms underlying drug reinforcement, natural reinforcement and their associated cue effects requires an experimental model that exposes individuals to both reinforcer conditions, along with their associated stimuli. Furthermore, contingent stimuli that reinforce drug seeking through second-order relationships may produce reinstatement of drug seeking through different neurobehavioral means than non-contingent exposure to stimuli that signal the availability of a drug reinforcer, effectively producing different modes of stimulus-induced reinstatement. Toward experimental isolation of the relationships mentioned, herein, we used a within-session multiple schedule of reinforcement containing both discriminative (S^D ) and conditioned (CS) stimuli to study stimulus control of drug-taking and food-taking behavior, along with how these functionally distinct cues may differentially reinstate drug-seeking and food-seeking behavior within a single animal. We demonstrate specific stimulus control over drug and food taking; furthermore, we demonstrate that the same stimulus (i.e. cue light) operating as an S^D or CS produced differential reinstatement of drug-taking and food-taking behavior. The results suggest that contingent CSs and non-contingent S^D s produce reinstatement through different neurobehavioral processes and, within-session multiple schedules, can be used to study different modes of specific stimulus control over drug and food seeking in a single animal with both drug-taking and food-taking history.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Essential values of cocaine and non-drug alternatives predict the choice between them

This study investigated the relationship between reinforcer value and choice between cocaine and two non-drug alternative reinforcers in rats. The essential value (EV, a behavioral economic measure based on elasticity of demand) of intravenous cocaine and food (Experiment 1) or saccharin (Experiment 2) was determined in the first phase of each experiment. Food had higher EV than cocaine, whereas the EVs of cocaine and saccharin did not differ. In the second phase of each experiment, rats were allowed to make mutually exclusive choices between cocaine and the non-drug alternative reinforcer. The main findings were that the EV of cocaine was a positive predictor of cocaine preference and the EV of food or saccharin was a negative predictor of cocaine preference. An analysis of within-session patterns of choice behavior revealed sequential dependencies, whereby rats were more likely to choose cocaine on a particular trial after having chosen the non-drug alternative on previous trials. When the time between choices was increased, these sequential dependencies disappeared. The results of these experiments are consistent with the suggestion that addiction-like behavior involves both overvaluation of drug reinforcers and undervaluation of non-drug reinforcers.

Cocaine cues retain silent traces of an excitatory history after conversion into conditioned inhibitors: 'the ghost in the addict'

The present experiment investigated the extent to which the A+/AB- conditioned inhibition procedure could counteract an excitatory drug-related conditioning history. In two groups of rats, a light stimulus was established as a signal for the absence of cocaine. For the History group, the light had previously been a discriminative stimulus (S) that occasioned cocaine self-administration and could thus be classified as a cocaine excitor. In comparison, the No-History group first encountered the light during conditioned inhibition training. During conditioned inhibition training, both groups self-administered cocaine during tone as well as during click Ss, whereas drug seeking was eliminated in click-plus-light, wherein cocaine was not available (A+/AB-). Drug seeking was essentially eliminated in both groups. Nevertheless, on a summation test the light reduced cocaine seeking occasioned by the tone S by 95% in the No-History group, but by less than 50% in the History group. This summation test result showed that the effects of a drug-related history persisted even after the light was converted into an effective conditioned inhibitor on the training baseline through the powerful A+/AB- procedure. Future research should seek procedures that produce even stronger conditioned inhibition that eliminates such residual 'silent' drug excitation, the 'ghost in the addict'.

Continuous Aerobic Training in Individualized Intensity Avoids Spontaneous Physical Activity Decline and Improves MCT1 Expression in Oxidative Muscle of Swimming Rats

Although aerobic training has been shown to affect the lactate transport of skeletal muscle, there is no information concerning the effect of continuous aerobic training on spontaneous physical activity (SPA). Because every movement in daily life (i.e., SPA) is generated by skeletal muscle, we think that it is possible that an improvement of SPA could affect the physiological properties of muscle with regard to lactate transport. The aim of this study was to evaluate the effect of 12 weeks of continuous aerobic training in individualized intensity on SPA of rats and their gene expressions of monocarboxylate transporters (MCT) 1 and 4 in soleus (oxidative) and white gastrocnemius (glycolytic) muscles. We also analyzed the effect of continuous aerobic training on aerobic and anaerobic parameters using the lactate minimum test (LMT). Sixty-day-old rats were randomly divided into three groups: a baseline group in which rats were evaluated prior to initiation of the study; a control group (Co) in which rats were kept without any treatment during 12 weeks; and a chronic exercise group (Tr) in which rats swam for 40 min/day, 5 days/week at 80% of anaerobic threshold during 12 weeks. After the experimental period, SPA of rats was measured using a gravimetric method. Rats had their expression of MCTs determined by RT-PCR analysis. In essence, aerobic training is effective in maintaining SPA, but did not prevent the decline of aerobic capacity and anaerobic performance, leading us to propose that the decline of SPA is not fully attributed to a deterioration of physical properties. Changes in SPA were concomitant with changes in MCT1 expression in the soleus muscle of trained rats, suggestive of an additional adaptive response toward increased lactate clearance. This result is in line with our observation showing a better equilibrium on lactate production-remotion during the continuous exercise (LMT). We propose an approach to combat the decline of SPA of rats in their home cages. This new finding is worth for scientists who work with animal models to study the protective effects of exercise.

Compound stimulus presentation and the norepinephrine reuptake inhibitor atomoxetine enhance long-term extinction of cocaine-seeking behavior

Drug abstinence is frequently compromised when addicted individuals are re-exposed to environmental stimuli previously associated with drug use. Research with human addicts and in animal models has demonstrated that extinction learning (non-reinforced cue-exposure) can reduce the capacity of such stimuli to induce relapse, yet extinction therapies have limited long-term success under real-world conditions (Bouton, 2002; O'Brien, 2008). We hypothesized that enhancing extinction would reduce the later ability of drug-predictive cues to precipitate drug-seeking behavior. We, therefore, tested whether compound stimulus presentation and pharmacological treatments that augment noradrenergic activity (atomoxetine; norepinephrine reuptake inhibitor) during extinction training would facilitate the extinction of drug-seeking behaviors, thus reducing relapse. Rats were trained that the presentation of a discrete cue signaled that a lever press response would result in cocaine reinforcement. Rats were subsequently extinguished and spontaneous recovery of drug-seeking behavior following presentation of previously drug-predictive cues was tested 4 weeks later. We find that compound stimulus presentations or pharmacologically increasing noradrenergic activity during extinction training results in less future recovery of responding, whereas propranolol treatment reduced the benefit seen with compound stimulus presentation. These data may have important implications for understanding the biological basis of extinction learning, as well as for improving the outcome of extinction-based therapies.

Behavioral momentum of cocaine self-administration: effects of frequency of reinforcement on resistance to extinction

Persistent drug seeking is a defining property of substance abuse and is generally thought to depend, in part, on exposure to drug-associated contexts. Behavioral momentum theory provides a set of methods and a theoretical framework for understanding how stimulus contexts contribute to the persistence of operant behavior. Earlier research has extended behavioral momentum theory to alcohol self-administration, but not to intravenous drug self-administration. This experiment extended behavioral momentum theory to cocaine self-administration by examining the effects of frequency of cocaine reinforcement in a context on resistance to extinction. Rats self-administered 0.32 mg/kg infusions of cocaine in a multiple schedule of reinforcement arranging two distinct contexts. Responding in a Rich context was reinforced by cocaine infusions at a higher frequency (i.e. variable interval 120 s) and in a Lean context at a lower frequency (variable interval 360 s). After establishment of responding in the two contexts, resistance to extinction was examined. Preextinction response rates for cocaine were similar in the Rich and Lean contexts. Nonetheless, relative resistance to extinction was greater in the Rich context than in the Lean context. The difference in resistance to extinction in the two contexts was well described by a quantitative model of behavioral momentum. These results suggest that the frequency of drug reinforcement in a context contributes to the persistence of drug seeking in that context, and that behavioral momentum theory might be useful for understanding how drug-associated contexts contribute to the persistence of drug seeking.

Demand for cocaine and food over time

When reinterpreted, data from Ahmed and Koob [Ahmed, S.H., Koob, G.F., Transition from moderate to excessive drug intake: Change in hedonic set point. Science 1998; 282:298-301.] show that the reinforcing strength of cocaine, an inessential good, increases with experience. However, no such effect obtains with a homeostatically regulated good such as food. The present study evaluated whether this difference could serve to distinguish abused drugs from biologically necessary goods. In Experiment 1, five rats from Christensen, Silberberg, Hursh, Huntsberry and Riley [Christensen, C.J., Silberberg, A., Hursh, S.R., Huntsberry, M.E., Riley, A.L., Essential value of cocaine and food in rats: tests of the exponential model of demand. Psychopharmacology 2008;198(2):221-229.] earned cocaine under a Fixed-Ratio 3 schedule for 7 sessions. Thereafter, in a demand procedure identical to that in Christensen et al., demand was re-assessed by measuring consumption at Fixed Ratios between 3 and 560. In Experiment 2, five different rats from Christensen et al. had their food demand curves re-determined using an identical procedure as the first. When fit with the exponential model, the second determination of cocaine demand in Experiment 1 showed greater essential value than the first, indicating that strength increased with cocaine exposure. In Experiment 2, the re-determined food demand curves showed no change from their initial determination. These results show that the strength of cocaine, but not food, increases with increased experience. Measures of time-based changes in essential value may serve as a basis for distinguishing addictive from non-addictive reinforcers.

Behavioral characteristics and neurobiological substrates shared by Pavlovian sign-tracking and drug abuse

Drug abuse researchers have noted striking similarities between behaviors elicited by Pavlovian sign-tracking procedures and prominent symptoms of drug abuse. In Pavlovian sign-tracking procedures, repeated paired presentations of a small object (conditioned stimulus, CS) with a reward (unconditioned stimulus, US) elicits a conditioned response (CR) that typically consists of approaching the CS, contacting the CS, and expressing consummatory responses at the CS. Sign-tracking CR performance is poorly controlled and exhibits spontaneous recovery and long-term retention, effects that resemble relapse. Sign-tracking resembles psychomotor activation, a syndrome of behavioral responses evoked by addictive drugs, and the effects of sign-tracking on corticosterone levels and activation of dopamine pathways resemble the neurobiological effects of abused drugs. Finally, the neurobiological profile of individuals susceptible to sign-tracking resembles the pathophysiological profile of vulnerability to drug abuse, and vulnerability to sign-tracking predicts vulnerability to impulsive responding and alcohol self-administration. Implications of sign-tracking for models of drug addiction are considered.

A stimulus-control account of regulated drug intake in rats

RATIONALE

Patterns of drug self-administration are often highly regular, with a consistent pause after each self-injection. This pausing might occur because the animal has learned that additional injections are not reinforcing once the drug effect has reached a certain level, possibly due to the reinforcement system reaching full capacity. Thus, interoceptive effects of the drug might function as a discriminative stimulus, signaling when additional drug will be reinforcing and when it will not.

OBJECTIVE

This hypothetical stimulus control aspect of drug self-administration was emulated using a schedule of food reinforcement.

MATERIALS AND METHODS

Rats' nose-poke responses produced food only when a cue light was present. No drug was administered at any time. However, the state of the light stimulus was determined by calculating what the whole-body drug level would have been if each response in the session had produced a drug injection. The light was only presented while this virtual drug level was below a specific threshold. A range of doses of cocaine and remifentanil were emulated using parameters based on previous self-administration experiments.

RESULTS

Response patterns were highly regular, dose-dependent, and remarkably similar to actual drug self-administration.

CONCLUSION

This similarity suggests that the emulation schedule may provide a reasonable model of the contingencies inherent in drug reinforcement. Thus, these results support a stimulus control account of regulated drug intake in which rats learn to discriminate when the level of drug effect has fallen to a point where another self-injection will be reinforcing.

Contextual renewal of cocaine seeking in rats and its attenuation by the conditioned effects of an alternative reinforcer

The present experiment investigated contextual renewal of cocaine seeking and potential methods of attenuating this renewal. Rats were first trained in one context to self-administer cocaine when a discriminative stimulus (tone) was presented. Then, the ABA Group was placed in a second context and responses in tone no longer produced cocaine (extinction). The AAA Group received this extinction in the original context. For two additional groups, responding to the tone was eliminated in a second context by additionally presenting food in tone according to a differential-reinforcement-of-other-behavior (DRO Group) schedule or independently of the rats' behavior on a fixed-time (FT Group) schedule. Renewal of responding to the tone was observed when the ABA Group was returned to the original context for a renewal test, but no renewal was observed in the AAA Group. Renewal also occurred in the DRO and FT Groups upon returning to the original context, but this renewal was significantly less than that of the ABA Group. These results suggest that response elimination techniques that are more active than simple extinction, such as pairing drug-related stimuli with alternative reinforcement, could reduce renewal of drug seeking and thereby help prevent relapse.

Stereotyped and complex motor routines expressed during cocaine self-administration: results from a 24-h binge of unlimited cocaine access in rats

RATIONALE

Cocaine binges represent the most severe form of cocaine taking due to high levels of cocaine consumed and a potential loss of self-control over cocaine taking. Experimentally, regulation of intravenous cocaine self-administration is observed during binges as revealed by nearly constant sequential inter-infusion intervals, suggesting that pharmacological and unconditioned behavioral effects of cumulative cocaine leave intact its reinforcing effect.

OBJECTIVE

To elucidate factors that contribute to the patterning of cocaine self-administration behavior during binges, the current study quantifies the expression of specific motor routines that emerge from the beginning of a cocaine infusion until the next cocaine reinforcement is received, in six separate rats over 24 h of continuous access.

MATERIALS AND METHODS

During each 24-h cocaine binge, behavior was continuously monitored using a force plate actometer that provides reliable quantitation of rodent behaviors, including rotational behavior, locomotor activity, and focused stereotypy.

RESULTS

Corroborating earlier results, each rat accumulated between 4 and 11 mg/kg/h during periods of active cocaine self-administration. All rats displayed a similar narrow range of unconditioned behavioral responses to cocaine, yet each rat varied with regard to the intensity and specific predominant pattern of behavioral activation. Focused stereotyped behavior, in particular, was apparent in all rats and continued for as long as cocaine self-administration behavior remained active.

CONCLUSIONS

The current results support the hypothesis that individual differences in cocaine's pharmacodynamics may contribute to specific behaviors expressed during cocaine self-administration, but leave unresolved whether or not intense unconditioned behavior (e.g., focused stereotypy) conflicts with, or contributes to, ongoing self-administration behavior.

Cocaine self-administration under variable-dose schedules in squirrel monkeys

Squirrel monkeys self-administered cocaine under a variable-dose schedule, with the dose varied from injection to injection. As in earlier studies with rats, post-injection pauses varied as a monotonic function of dose, allowing a cocaine dose-effect curve to be obtained during each session. These curves were shifted by pretreatment with dopamine antagonists, demonstrating that this procedure may provide an efficient means of evaluating treatments that affect drug self-administration. However, drug intake eventually became "dysregulated" after extensive training (100-300 sessions), with relatively short pauses following all doses. Dose-sensitivity was restored by adding a 60-s timeout period after each injection, suggesting that dysregulation occurred because the monkeys developed a tendency to self-administer another injection before the previous injection had been adequately distributed. Finally, when the response requirement under the variable-dose schedule was increased from 1 to 10, both the post-injection pause and the rate of responding following the pause ("run rates") were found to vary with dose. The dose-dependency of run rates suggests that post-injection pauses reflect not only motivational factors, such as satiety, but also the direct effects of cocaine on leverpressing.

Conditioned inhibition of cocaine seeking in rats

Despite its potential relevance to the treatment of drug abuse, conditioned inhibition of drug seeking has not been systematically investigated before. In this study, rats could self-administer cocaine by lever pressing whenever a click or tone was present. Responding was not reinforced when a light was present. The light was presented simultaneously with the click (i.e., in an excitatory context) in 1 group, but the light was always presented alone in another group. When it was later presented in compound with the tone, the light was a highly effective conditioned inhibitor, suppressing cocaine seeking by 92% in the former group and by 74% in the latter. These results suggest ways to improve cue-oriented behavioral treatments for drug abuse.

Sign-tracking (autoshaping) in rats: a comparison of cocaine and food as unconditioned stimuli

A series of experiments was performed to determine whether sign-tracking would occur in rats with intravenous (i.v.) cocaine as the unconditioned stimulus. In Experiment 1, a retractable lever paired with food produced strong sign-tracking, but a lever paired with one of three doses of i.v. cocaine did not elicit any approach or contact behavior. Experiment 2 demonstrated that doses of cocaine that did not elicit sign-tracking would function as a positive reinforcer for a lever contact operant. In Experiment 3, an artificial consummatory response was added to make the cocaine reinforcement episode more behaviorally comparable to that occasioned by food. Although the rats readily performed this response when it was required to receive cocaine infusions, they still did not contact a lever that signaled the availability of these infusions. It appears that cocaine is different from other positive reinforcers (e.g., food, water, warmth, or intracranial stimulation) in that it will not produce sign-tracking in rats.

On the origin and preservation of cumulative record in its struggle for life as a favored term

This paper offers a case study of the origins, emergence, and evolution of the term cumulative record as the name for the means by which B. F. Skinner brought his behavior under the control of his subject matter. Our methods included on-line searches, reviews of Skinner's publications, and journal codings and counts. The results reveal that the term is not originally attributable to Skinner, but emerged earlier in ordinary language and in another discipline--education. It was not even original to Skinner in print in his own science. Still, the term was once original to him, which we address with additional analyses of his having originated and advanced it. We conclude with a discussion the constraints of our methods, suggestions for future research, and the variable appreciation of technology and terminology in science studies.