Conditioned inhibition of cocaine seeking in rats

Relapse after intermittent access to cocaine: Discriminative cues more effectively trigger drug seeking than do conditioned cues

RATIONALE

When people with drug addiction encounter cues associated with drug use, this can trigger cravings and relapse. These cues can include conditioned stimuli (CSs) signaling drug delivery and discriminative stimuli (DSs) signaling drug availability. Compared to CS effects, DS effects are less explored in preclinical studies on cue-induced relapse.

OBJECTIVE

We compared CS and DS effects on reward seeking following abstinence from intermittent-access cocaine (or sucrose) self-administration.

METHODS

During 15-20 intermittent-access sessions, rats self-administered i.v. cocaine or sucrose pellets paired with a light-tone CS. Cocaine/sucrose was available for 5-min (signalled by a light; DS+) and unavailable for 25 min (signalled by different lighting conditions; DS-), and this cycled for 4 h/session. Following abstinence, we measured cocaine/sucrose seeking under extinction triggered by CS and DS presentation, and instrumental responding reinforced by these cues.

RESULTS

Across intermittent-access sessions, rats increased lever pressing for cocaine or sucrose during DS+ periods and decreased responding during DS- periods. On days 2 and 21 of abstinence, only presentation of the DS+ or DS+ and CS combined elicited increased cocaine/sucrose-seeking behaviour (i.e., increased active lever presses). Presenting the DS- alongside the DS+ suppressed the increased cocaine-seeking behaviour otherwise produced by the DS+ . Finally, on day 21 of abstinence, rats showed equivalent levels of lever pressing reinforced by the DS+ , CS and by the DS+ and CS combined, suggesting comparable conditioned reinforcing value.

CONCLUSIONS

After intermittent self-administration, cocaine-associated DSs and CSs acquire similar conditioned reinforcing properties, but DSs more effectively trigger increases in drug seeking.

Inhibitory and excitatory alcohol-seeking cues distinct roles in behavior, neurochemistry, and mesolimbic pathway in alcohol preferring (P) rats

Cues associated with alcohol use can readily enhance self-reported cravings for alcohol, which increases the likelihood of reusing alcohol. Understanding the neuronal mechanisms involved in alcohol-seeking behavior is important for developing strategies to treat alcohol use disorder. In all experiments, adult female alcohol-preferring (P) rats were exposed to three conditioned odor cues; CS+ associated with EtOH self-administration, CS- associated with the absence of EtOH (extinction training), and a CS0, a neutral stimulus. The data indicated that presentation of an excitatory conditioned cue (CS+) can enhance EtOH- seeking while the CS- can inhibit EtOH-seeking under multiple test conditions. Presentation of the CS+ activates a subpopulation of dopamine neurons within the interfascicular nucleus of the posterior ventral tegmental area (posterior VTA) and basolateral amygdala (BLA). Pharmacological inactivation of the BLA with GABA agonists inhibits the ability of the CS+ to enhance EtOH-seeking but does not alter context-induced EtOH-seeking or the ability of the CS- to inhibit EtOH-seeking. Presentation of the conditioned odor cues in a non-drug-paired environment indicated that presentation of the CS+ increased dopamine levels in the BLA. In contrast, presentation of the CS- decreased both glutamate and dopamine levels in the BLA. Further analysis revealed that presentation of a CS+ EtOH-associated conditioned cue activates GABA interneurons but not glutamate projection neurons. Overall, the data indicate that excitatory and inhibitory conditioned cues can contrarily alter EtOH-seeking behaviors and that different neurocircuitries are mediating these distinct cues in critical brain regions. Pharmacotherapeutics for craving should inhibit the CS+ and enhance the CS- neurocircuits.

Inactivation of the infralimbic cortex decreases discriminative stimulus-controlled relapse to cocaine seeking in rats

Persistent susceptibility to cue-induced relapse is a cardinal feature of addiction. Discriminative stimuli (DSs) are one type of drug-associated cue that signal drug availability (DS+) or unavailability (DS-) and control drug seeking prior to relapse. We previously established a trial-based procedure in rats to isolate DSs from context, conditioned stimuli, and other drug-associated cues during cocaine self-administration and demonstrated DS-controlled cocaine seeking up to 300 abstinence days. The behavioral and neural mechanisms underlying trial-based DS-control of drug seeking have rarely been investigated. Here we show that following discrimination training in our trial-based procedure, the DS+ and DS- independently control the expression and suppression of cocaine seeking during abstinence. Using microinjections of GABA_A + GABA_B receptor agonists (muscimol + baclofen) in medial prefrontal cortex, we report that infralimbic, but not prelimbic, subregion of medial prefrontal cortex is critical to persistent DS-controlled relapse to cocaine seeking after prolonged abstinence, but not DS-guided discriminated cocaine seeking or DS-controlled cocaine self-admininstration. Finally, using ex vivo whole-cell recordings from pyramidal neurons in the medial prefrontal cortex, we demonstrate that the disruption of DS-controlled cocaine seeking following infralimbic cortex microinjections of muscimol+baclofen is likely a result of suppression of synaptic transmission in the region via a presynaptic mechanism of action.

Anti-relapse neurons in the infralimbic cortex of rats drive relapse-suppression by drug omission cues

Drug addiction is a chronic relapsing disorder of compulsive drug use. Studies of the neurobehavioral factors that promote drug relapse have yet to produce an effective treatment. Here we take a different approach and examine the factors that suppress-rather than promote-relapse. Adapting Pavlovian procedures to suppress operant drug response, we determined the anti-relapse action of environmental cues that signal drug omission (unavailability) in rats. Under laboratory conditions linked to compulsive drug use and heightened relapse risk, drug omission cues suppressed three major modes of relapse-promotion (drug-predictive cues, stress, and drug exposure) for cocaine and alcohol. This relapse-suppression is, in part, driven by omission cue-reactive neurons, which constitute small subsets of glutamatergic and GABAergic cells, in the infralimbic cortex. Future studies of such neural activity-based cellular units (neuronal ensembles/memory engram cells) for relapse-suppression can be used to identify alternate targets for addiction medicine through functional characterization of anti-relapse mechanisms.

Conditioned stimuli affect ethanol-seeking by female alcohol-preferring (P) rats: the role of repeated-deprivations, cue-pretreatment, and cue-temporal intervals

RATIONALE

Evidence indicates that drug-paired stimuli can evoke drug-craving leading to drug-seeking and repeated relapse periods can influence drug-seeking behaviors.

OBJECTIVES

The present study examined (1) the effect of an interaction between repeated deprivation cycles and excitatory conditioning stimuli (CS+) on ethanol (EtOH)-seeking; (2) the effects of EtOH-paired cue-exposure in a non-drug-paired environment on subsequent conditioning in a drug-paired environment; and (3) the temporal effects of conditioned cues on subsequent EtOH-seeking.

METHODS

Adult female alcohol-preferring (P) rats were exposed to three conditioned odor cues; CS+ associated with EtOH self-administration, CS- associated with the absence of EtOH (extinction training), and a neutral stimulus (CS0) presented in a neutral non-drug-paired environment. The rats underwent four deprivation cycles or were non-deprived, following extinction they were maintained in a home cage for an EtOH-free period, and then exposed to no cue, CS+, CS-, or CS0 to assess the effect of the conditioned cues on EtOH-seeking behavior.

RESULTS

Repeated deprivations enhanced and prolonged the duration of CS+ effects on EtOH-seeking. Presentation of the CS- in a non-drug-paired environment blocked the ability of a CS+ to enhance EtOH-seeking in a drug-paired environment. Presentation of the CS+ or CS- in a non-drug-paired environment 2 or 4 h earlier significantly altered EtOH-seeking.

CONCLUSION

Results indicated an interaction between repeated deprivation cycles and CS+ resulted in a potentiation of CS+ evoked EtOH-seeking. In addition, a CS- may have therapeutic potential by providing prophylactic protection against relapse behavior in the presence of cues in the drug-using environment.

The infralimbic and prelimbic cortices contribute to the inhibitory control of cocaine-seeking behavior during a discriminative stimulus task in rats

The infralimbic and prelimbic (IL and PL, respectively) regions of the medial prefrontal cortex regulate the control of drug-seeking behavior. However, their roles in cocaine seeking in a discriminative stimulus (DS)-based self-administration task are unclear. To address this issue, male Sprague Dawley rats were trained on a DS task in which, on a trial-by-trial basis, a DS+ indicated that a lever press would produce a cocaine infusion, whereas a distinct DS- indicated that a lever press would produce nothing. IL and PL inactivation via GABA receptor activation decreased performance accuracy and disinhibited behavioral responding on DS- trials, resulting in greater lever pressing during the DS- presentation. This was accompanied by a decrease in cocaine infusions obtained, a finding confirmed in a subsequent experiment using a standard FR1 cocaine self-administration paradigm. We repeated the DS study using a food reward and found that inactivation of each region decreased performance accuracy but had no effect on the total number of food pellets earned. Additional experiments with the cocaine DS task found that dopamine receptor blockade in the IL, but not PL, reduced performance accuracy and disinhibited behavioral responding on DS- trials, whereas AMPA receptor blockade in the IL and PL had no effect on performance accuracy. These findings strongly suggest that, in a DS-based self-administration task in which rats must actively decide whether to engage in lever pressing (DS+) or withhold lever pressing (DS-) on a trial-by-trial basis, both the IL and PL contribute to the inhibitory control of drug-seeking behavior.

Neural systems mediating the inhibition of cocaine-seeking behaviors

Over the past decades, research has targeted the neurobiology regulating cocaine-seeking behaviors, largely in the hopes of identifying potential targets for the treatment of cocaine addiction. Although much of this work has focused on those systems driving cocaine seeking, recently, studies examining the inhibition of cocaine-related behaviors have made significant progress in uncovering the neural systems that attenuate cocaine seeking. Such systems include the infralimbic cortex, nucleus accumbens shell, and hypothalamus. Research in this field has focused largely on the infralimbic cortex, as activity in this region appears to attenuate cocaine seeking during reinstatement and contribute to extinction learning. However, an overarching theory of function for this region that includes its role in other types of reward seeking and learning remains to be determined. Furthermore, the precise relationship between other regions involved in attenuating cocaine-seeking behavior and the infralimbic cortex remains unclear. Recent advances in the use of viral vectors combined with optogenetics, chemogenetics, and other approaches have greatly affected our capacity to investigate those systems inhibiting behavior dependent on cocaine-associated memories. This review will present current understanding regarding the neurobiology underlying the inhibition of such behaviors, especially focusing on the extinction of such memories, and explore how viral-vector targeting of specific brain circuits has begun to alter, and will continue to enrich, our knowledge regarding this issue.

Cocaine Administration and Its Withdrawal Enhance the Expression of Genes Encoding Histone-Modifying Enzymes and Histone Acetylation in the Rat Prefrontal Cortex

Chronic exposure to cocaine, craving, and relapse are attributed to long-lasting changes in gene expression arising through epigenetic and transcriptional mechanisms. Although several brain regions are involved in these processes, the prefrontal cortex seems to play a crucial role not only in motivation and decision-making but also in extinction and seeking behavior. In this study, we applied cocaine self-administration and extinction training procedures in rats with a yoked triad to determine differentially expressed genes in prefrontal cortex. Microarray analysis showed significant upregulation of several genes encoding histone modification enzymes during early extinction training. Subsequent real-time PCR testing of these genes following cocaine self-administration or early (third day) and late (tenth day) extinction revealed elevated levels of their transcripts. Interestingly, we found the enrichment of Brd1 messenger RNA in rats self-administering cocaine that lasted until extinction training during cocaine withdrawal with concomitant increased acetylation of H3K9 and H4K8. However, despite elevated levels of methyl- and demethyltransferase-encoded transcripts, no changes in global di- and tri-methylation of histone H3 at lysine 4, 9, 27, and 79 were observed. Surprisingly, at the end of extinction training (10 days of cocaine withdrawal), most of the analyzed genes in the rats actively and passively administering cocaine returned to the control level. Together, the alterations identified in the rat prefrontal cortex may suggest enhanced chromatin remodeling and transcriptional activity induced by early cocaine abstinence; however, to know whether they are beneficial or not for the extinction of drug-seeking behavior, further in vivo evaluation is required.

Oral Conditioned Cues Can Enhance or Inhibit Ethanol (EtOH)-Seeking and EtOH-Relapse Drinking by Alcohol-Preferring (P) Rats

BACKGROUND

Conditioned cues can elicit drug-seeking in both humans and rodents. The majority of preclinical research has employed excitatory conditioned cues (stimuli present throughout the availability of a reinforcer), but oral consumption of alcohol is similar to a conditional stimuli (presence of stimuli is paired with the delivery of the reinforcer) approach. The current experiments attempted to determine the effects of conditional stimuli (both excitatory and inhibitory) on the expression of context-induced ethanol (EtOH)-seeking.

METHODS

Alcohol-preferring (P) rats self-administered EtOH and water in standard 2-lever operant chambers. A flavor was added to the EtOH solution (CS+) during the EtOH self-administration sessions. After 10 weeks, rats underwent extinction training (7 sessions), followed by a 2-week home cage period. Another flavor was present during extinction (CS-). Rats were exposed to a third flavor in a non-drug-paired environment (CS(0)). EtOH-seeking was assessed in the presence of no cue, CS+, CS-, or CS(0) in the dipper previously associated with EtOH self-administration (no EtOH available). Rats were maintained a week in their home cage before being returned to the operant chambers with access to EtOH (flavored with no cue, CS+, CS-, or CS(0)).

RESULTS

The results indicated that the presence of the CS+ enhanced EtOH-seeking, while the presence of the CS- suppressed EtOH-seeking. Similarly, adding the CS- flavor to 15% EtOH reduced responding for EtOH while the CS+ enhanced responding for EtOH during relapse testing.

CONCLUSIONS

Overall, the data indicate that conditional stimuli are effective at altering both EtOH-seeking behavior and EtOH-relapse drinking.

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'.

Translation of associative learning models into extinction reminders delivered via mobile phones during cue exposure interventions for substance use

Despite experimental findings and some treatment research supporting the use of cues as a means to induce and extinguish cravings, interventions using cue exposure have not been well integrated into contemporary substance abuse treatments. A primary problem with exposure-based interventions for addiction is that after learning not to use substances in the presence of addiction cues inside the clinic (i.e., extinction), stimuli in the naturalistic setting outside the clinic may continue to elicit craving, drug use, or other maladaptive conditioned responses. For exposure-based substance use interventions to be efficacious, new approaches are needed that can prevent relapse by directly generalizing learning from the therapeutic setting into naturalistic settings associated with a high risk for relapse. Basic research suggests that extinction reminders (ERs) can be paired with the context of learning new and more adaptive conditioned responses to substance abuse cues in exposure therapies for addiction. Using mobile phones and automated dialing and data collection software, ERs can be delivered in everyday high-risk settings to inhibit conditioned responses to substance-use-related stimuli. In this review, we describe how associative learning mechanisms (e.g., conditioned inhibition) can inform how ERs are conceptualized, learned, and implemented to prevent substance use when delivered via mobile phones. This approach, exposure with portable reminders of extinction, is introduced as an adjunctive intervention that uses brief automated ERs between clinic visits when individuals are in high-risk settings for drug use.

Discriminative inhibitory control of cocaine seeking involves the prelimbic prefrontal cortex

BACKGROUND

Recent neuroimaging studies have shown that people with cocaine addiction retain some degree of control over drug craving that correlates with neural activity in the lateral prefrontal cortex (PFC). Here, we report similar findings in a rat model of inhibitory control of cocaine seeking.

METHODS

Rats actively responding for cocaine were trained to stop responding when presented with a discriminative stimulus that signaled lack of reinforcement. Rats were then tested for inhibitory control of cocaine seeking in novel behavioral contexts and in circumstances when cocaine seeking is particularly intense (e.g., following drug priming). The role of neuronal activity in different subregions of the PFC was assessed using local pharmacologic inactivation and c-Fos immunohistochemistry.

RESULTS

Rats progressively acquired the ability to stop cocaine seeking, even during drug intoxication and after a long history of cocaine self-administration. Inhibitory control of cocaine seeking was flexible, sufficiently strong to block cocaine-primed reinstatement, and selectively depended on increased neuronal activity within the prelimbic PFC, which is considered the rodent functional homolog of the human lateral PFC.

CONCLUSIONS

Parallel evidence in both animal models and humans indicate that recruitment of prefrontal inhibitory control of drug seeking is still functional after prolonged cocaine use. Preclinical investigation of the mechanisms underlying this capacity may contribute to designing new behavioral and/or pharmacologic strategies to promote its use for the prevention of relapse in addiction.

A comparison of therapies for the treatment of drug cues: counterconditioning vs. extinction in male rats

Although extinction has been used as a treatment to reduce the power of drug cues, a better method is needed. Research with traditional reinforcers has shown that counterconditioning--pairing an appetitive cue with an aversive stimulus--can suppress cue-controlled behavior. The present experiment compared the counterconditioning and extinction of cocaine cues. Male rats were first trained to self-administer cocaine during a light cue. In the second phase, the light was paired with footshock in the counterconditioning group. The extinction group was treated similarly, except light presentations did not end in footshock. Counterconditioning suppressed cocaine seeking to a greater extent than extinction while the counterconditioning treatment was actively administered. On a subsequent stimulus compounding test, where footshock was discontinued and the light was presented simultaneously with an untreated cocaine cue (a tone), suppressive effects of counterconditioning were evident during the early portion of the test but not during later trials. Overall, results of the present experiment suggest that counterconditioning produces only temporarily suppressive effects on cue-controlled cocaine seeking. Methods for directly weakening the cue-drug association (e.g., "deepened extinction") may prove to be more useful drug cue treatments.

Deepened extinction of cocaine cues

BACKGROUND

A method for reducing the power of drug cues could help in treating drug abuse and addiction. Extinction has been used, with mixed success, in such an effort. Research with non-drug cues has shown that simultaneously presenting (compounding) those cues during extinction can enhance the effectiveness of extinction. The present study investigated whether this procedure could be used to similarly deepen the extinction of cocaine cues.

METHODS

Rats were first trained to self-administer cocaine during tone, click, and light stimuli. Then, these stimuli were subjected to extinction in an initial phase where they were presented individually. In a second extinction phase, one of the auditory stimuli (counterbalanced) was compounded with the light. The other auditory stimulus continued to be presented alone. Rats were then given a week of rest in their homecages prior to testing for spontaneous recovery of cocaine seeking.

RESULTS

The cue that was compounded with the light during the second phase of extinction training occasioned less spontaneous recovery of cocaine seeking than the cue that was always presented individually during extinction. Increasing the number of compound cue extinction sessions did not produce a greater deepened extinction effect.

CONCLUSIONS

The present study showed that simultaneously presenting already-extinguished cocaine cues during additional extinction training enhanced extinction. This extends the deepened extinction effect from non-drug cues to drug cues and further confirms predictions of error-correction learning theory. Incorporating deepened extinction into extinction-based drug abuse treatments could help to reduce the power of drug cues.

An early attentional bias to BEGIN-stimuli of the smoking ritual is accompanied with mesocorticolimbic deactivations in smokers

RATIONALE

Biased processing of drug-associated stimuli is believed to be a crucial feature of addiction. Particularly, an attentional bias seems to contribute to the disorder's maintenance. Recent studies suggest differential effects for stimuli associated with the beginning (BEGIN-smoking-stimuli) or the terminal stage of the smoking ritual (END-smoking-stimuli), with the former but not the later evoking high cue-reactivity.

OBJECTIVE

The current study investigated the neuronal network underlying an attentional bias to BEGIN-smoking-stimuli and END-smoking-stimuli in smokers and tested the hypothesis that the attentional bias is greater for BEGIN-smoking-stimuli.

METHODS

Sixteen non-deprived smokers and 16 non-smoking controls participated in an fMRI study. Drug pictures (BEGIN-smoking-stimuli, END-smoking-stimuli) and control pictures were overlaid with geometrical figures and presented for 300 ms. Subjects had to identify picture content (identification-task) or figure orientation (distraction-task). The distraction-task was intended to demonstrate attentional bias.

RESULTS

Behavioral data revealed an attentional bias to BEGIN-smoking-stimuli but not to END-smoking-stimuli in both groups. However, only smokers showed mesocorticolimbic deactivations in the distraction-task with BEGIN-smoking-stimuli. Importantly, these deactivations were significantly stronger for BEGIN- than for END-smoking-stimuli and correlated with the attentional bias score.

CONCLUSIONS

Several explanations may account for missing group differences in behavioral data. Brain data suggest smokers using regulatory strategies in response to BEGIN-smoking-stimuli to prevent the elicitation of motivational responses interfering with distraction-task performance. These strategies could be reflected in the observed deactivations and might lead to a performance level in smokers that is similar to that of non-smokers.

Learning that a cocaine reward is smaller than expected: A test of Redish's computational model of addiction

The present experiment tested the prediction of Redish's (2004) computational model of addiction that drug reward expectation continues to grow even when the received drug reward is smaller than expected. Initially, rats were trained to press two levers, each associated with a large dose of cocaine. Then, the dose associated with one of the levers was substantially reduced. Thus, when rats first pressed the reduced-dose lever, they expected a large cocaine reward, but received a small one. On subsequent choice tests, preference for the reduced-dose lever was reduced, demonstrating that rats learned to devalue the reduced-dose lever. The finding that rats learned to lower reward expectation when they received a smaller-than-expected cocaine reward is in opposition to the hypothesis that drug reinforcers produce a perpetual and non-correctable positive prediction error that causes the learned value of drug rewards to continually grow. Instead, the present results suggest that standard error-correction learning rules apply even to drug reinforcers.

Neural responses to BEGIN- and END-stimuli of the smoking ritual in nonsmokers, nondeprived smokers, and deprived smokers

Drug-associated stimuli (cues) have a prominent role in addiction research because they are able to provoke craving and relapses. Generally, drug cues are seen as conditioned excitatory stimuli, which elicit drug seeking and usage. However, newer data suggest differential effects for smoking stimuli depending on their stage in the smoking ritual. Specifically, stimuli associated with the terminal stage of smoke consumption (END-stimuli) may evoke reactivity opposite to the reactivity evoked by stimuli associated with the beginning of smoke consumption (BEGIN-stimuli). This fMRI study compared 20 nondeprived smokers with 20 nonsmokers to unravel the influence of smoking-related pictures displaying the beginning (BEGIN-stimuli) and termination (END-stimuli) of the smoking ritual on neural activity in the addiction network. In addition, 20 deprived smokers (12 h deprivation) were investigated to explore the effects of deprivation on the processing of these stimuli. In nondeprived smokers, BEGIN-stimuli reliably activated the addiction network (for example, the ventral striatum, orbitofrontal cortex, and anterior cingulate cortex (ACC)). In contrast, END-stimuli triggered a differential pattern of activations as well as deactivations; deactivations were found in the ventral striatum and the ACC. Deprivation had no clear effect on the responses triggered by BEGIN-stimuli, but affected the reactivity to END-stimuli. Our data clearly suggest that stimuli associated with different stages of the smoking ritual trigger differential neuronal responses. While BEGIN-stimuli generally seem to activate the addiction network, END-stimuli presumably have some inhibitory properties. This new finding might add to a more differentiated understanding of cue reactivity and addiction.

Smoking stimuli from the terminal phase of cigarette consumption may not be cues for smoking in healthy smokers

BACKGROUND

Stimuli from the terminal phase of smoke or drug intake are paired with drug effect but have surprisingly low cue reactivity. Smoking terminal stimuli were compared to cues under conditions of different perceived smoke intake to probe whether (1) terminal stimuli are only weak cues, (2) any effect is an artifact of rigid test conditions, and (3) terminal stimuli have a unique function during the intake ritual.

MATERIALS AND METHODS

Nonabstinent, healthy smokers were tested in three experiments with one-session, within-subject cue reactivity tests. Smoking terminal stimuli and cues were compared using pictures depicting events after completion (END) and before start of smoke inhalation (BEGIN). Test pictures were presented alone and in combination with no-go symbols (from no-smoking signs) or with extra cues to decrease and to increase perceived smoke availability, respectively. Measured were subjective effects and affect modulation of the startle reflex.

RESULTS

END stimuli relative to BEGIN stimuli evoked less subjective craving and pleasure but more arousal. A no-go stimulus, which reduced reports of intention to smoke, reduced the reactivity to BEGIN but only marginally affected responses to END stimuli. This was confirmed with different sets of test pictures and using tests with the startle response. An extra cue did not affect reactivity to a BEGIN stimulus but increased craving and pleasure to the END stimulus, although not to the level of BEGIN stimuli alone.

CONCLUSIONS

This first systematic study of terminal stimuli found their effects to be robust and have test generality. They are probably not weak cues but evoke reactivity, which may oppose reactivity of cues. They may signal poor availability of drug. Methodological, clinical, and theoretical implications were noted.

DIFFERENTIAL EFFECTS OF A FOOD-BASED CONDITIONED INHIBITOR ON FOOD- OR COCAINE-SEEKING BEHAVIOR

The present experiment compared the effects of a food-based conditioned inhibitor on food seeking vs. cocaine seeking behavior. In two groups of rats, the A+/AB- Pavlovian conditioned inhibition procedure was used to create a conditioned inhibitor for food. Then, for one group of rats (Food-Food Group), a click stimulus was established as an operant discriminative stimulus (S(D)) for food-reinforced lever pressing. In the other group (Food-Cocaine Group), the click was established as an S(D) for cocaine self-administration. In testing, the putative inhibitor for food was simultaneously presented with the click for the first time in both groups. In the Food-Food Group, the food-based inhibitor suppressed responding occasioned by the click significantly more than did a neutral control stimulus. In contrast, in the Food-Cocaine Group, there was no difference in the amount of suppression produced by the food-based inhibitor and the control stimulus. These results suggest that the effects of food-based Pavlovian conditioned inhibitors are specific for food-motivated behavior and do not easily transfer to cocaine-motivated behavior.

A comparison of the effects of discriminative and Pavlovian inhibitors and excitors on instrumental responding

In two experiments, the effects of Pavlovian or discriminative conditioned inhibitors on operant responding were investigated in rats. Experiment 1 found that a Pavlovian conditioned inhibitor for food suppressed food-reinforced lever pressing more than a non-differentially trained control stimulus did. Experiment 2 demonstrated that an operant discriminative inhibitor produced greater suppression of lever pressing than a Pavlovian conditioned inhibitor. Experiment 2 also found that compounding an operant discriminative stimulus (SD) for food-reinforced responding with another SD for food-reinforced responding resulted in more additive summation than when an SD was compounded with a Pavlovian conditioned excitor for food. The results of these experiments support two-factor theories that postulate that incentive and response discriminative processes summate algebraically when the processes are inhibitory or excitatory.

Differential involvement of dopamine receptors in conditioned suppression induced by cocaine

Cocaine-paired stimuli can suppress food-reinforced operant behavior in rats, providing an animal model of conditioned drug effects. To study the neuropharmacological basis of this phenomenon, we examined the effects of various dopamine receptor antagonists on the acquisition and expression of cocaine-induced conditioned suppression in rats. Superimposed on an ongoing baseline of food-reinforced operant responding, a stimulus was paired with response-independent cocaine (3.0 mg/kg, i.v.) during each of 8 training sessions. To study acquisition, independent groups of rats were given saline, the dopamine D(1)-like receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390) (0.001-0.03 mg/kg, i.p.), or the dopamine D(2)-like receptor antagonist eticlopride (0.001-0.03 mg/kg, i.p.) prior to each training session. To study expression, independent groups of rats were trained first, then given saline, SCH 23390, eticlopride, or N-[4-(4-(2-methoxyphenyl)piperazinyl)butyl]-2-naphthamide (BP 897) (a dopamine D(3) partial receptor agonist; 0.1-1.0 mg/kg, i.p.) before test sessions in which the stimulus was presented without cocaine. Pre-treatment with either SCH 23390 or eticlopride during acquisition reduced the direct suppressant effects of cocaine, but conditioning was blocked only in rats that were treated with SCH 23390 during acquisition training. Expression of conditioning was attenuated only by eticlopride. Thus, dopamine at least partially mediates both the acquisition and expression of cocaine-induced conditioned suppression, with activation of dopamine D(1)- and D(2)-like receptors underlying these respective processes.

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.