Cues paired with either rapid or slower self-administered cocaine injections acquire similar conditioned rewarding properties

Intermittent nicotine access is as effective as continuous access in promoting nicotine seeking and taking in rats

RATIONALE

Nicotine is a principal psychoactive agent in tobacco, contributing to tobacco's addictive potential. Preclinical studies on the effects of voluntary nicotine intake typically use self-administration procedures that provide continuous nicotine access during each self-administration session. However, many smokers consume cigarettes intermittently rather than continuously throughout each day. For drugs including cocaine and opioids, research in laboratory rats shows that intermittent intake can be more effective than continuous intake in producing patterns of drug use relevant to addiction.

OBJECTIVE

We determined how intermittent versus continuous nicotine self-administration influences nicotine seeking and taking behaviours.

METHODS

Female and male rats had continuous (i.e., Long Access; LgA, 6 h/day) or intermittent (IntA; 12 min ON, 60 min OFF, for 6 h/day) access to intravenous nicotine (15 µg/kg/infusion), for 12 daily sessions. We then assessed intake, responding for nicotine under a progressive ratio schedule of drug reinforcement and cue- and nicotine-induced reinstatement of drug seeking. We also estimated nicotine pharmacokinetic parameters during LgA and IntA self-administration.

RESULTS

Overall, LgA rats took twice more nicotine than did IntA rats, yielding more sustained increases in estimated brain concentrations of the drug. However, the two groups showed similar motivation to seek and take nicotine, as measured using reinstatement and progressive ratio procedures, respectively.

CONCLUSIONS

Intermittent nicotine use is just as effective as continuous use in producing addiction-relevant behaviours, despite significantly less nicotine exposure. This has implications for modeling nicotine self-administration patterns in human smokers and resulting effects on brain and behaviour.

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.

Modulation of cue value and the augmentation of heroin seeking in chronically food-restricted male rats under withdrawal

Food restriction augments drug seeking in abstinent rats. The underlying motivational mechanisms, however, remain unclear. We hypothesized that caloric restriction enhances the incentive value attributed to drug-associated cues and, in turn, augments drug seeking. Male rats were trained to lever-press for heroin, and then moved to the animal colony for a forced-abstinence period. Rats were maintained on free access to food (Sated) or subjected to 14 days of food restriction (FDR). In a series of experiments, we assessed the effect of food-restriction on the incentive value of heroin-associated cues. Tests included performance under a progressive ratio (PR) schedule of reinforcement maintained by heroin-associated cues, acquisition of a novel operant response reinforced by drug-associated cues, effect of food-restriction on operant response reinforced by neutral cues, acquisition of a novel operant response reinforced by drug-associated or neutral cues, and the effect of food-restriction on operant response reinforced by drug-associated or neutral cues, under a discrete choice procedure. Food-restriction did not change breakpoints in PR maintained by heroin-associated cues. FDR rats acquired the novel response at a greater level compared to the Sated group. Food-restriction-induced increase in novel-response rate was observed for both heroin-paired and the neutral cue. Responding for a heroin-associated cue was greater than for the neutral cue in both Sated and FDR groups. Response rate for the neutral cue, however, was greater in the FDR versus Sated group. Our findings suggest that food restriction increases the conditioned motivational properties of environmental stimuli, including, but not exclusive to, heroin-paired cues.

Metabotropic glutamate group II receptor activation in the ventrolateral dorsal striatum suppresses incentive motivation for cocaine in rats

RATIONALE

After a history of intermittent cocaine intake, rats develop patterns of drug use characteristic of substance use disorder. The dorsal striatum is involved in the increased pursuit of cocaine after intermittent drug self-administration experience. Within the dorsal striatum, chronic cocaine use changes metabotropic glutamate type II receptor (mGlu_2/3) density and function.

OBJECTIVES

We examined the extent to which activity at Glu_2/3 receptors mediates responding for cocaine after intermittent cocaine use.

METHODS

Male (n = 11) and female (n = 10) Wistar rats self-administered 0.25 mg/kg/infusion cocaine during 10 daily intermittent access (IntA) sessions (5 min ON/25 min OFF, for 5 h/session). We then examined the effects of microinjections of the mGlu_2/3 receptor agonist LY379268 (0, 1, and 3 µg/hemisphere) into the ventrolateral part of the dorsal striatum on cocaine self-administration under a progressive ratio schedule of reinforcement.

RESULTS

Across 10 IntA sessions, the sexes showed similar levels of cocaine intake. In females only, locomotion significantly increased over sessions, suggesting that female rats developed psychomotor sensitization to self-administered cocaine. After 10 IntA sessions, intra-dorsal striatum LY379268 significantly reduced breakpoints achieved for cocaine, active lever presses, and cocaine infusions earned under progressive ratio. LY379268 had no effects on locomotion or inactive lever presses, indicating no motor effects.

CONCLUSIONS

These results suggest that mGlu_2/3 receptor activation in the ventrolateral dorsal striatum suppresses incentive motivation for cocaine, and this holds promise for new treatments to manage substance use disorder.

Essential role of P-glycoprotein in the mechanism of action of oliceridine

Mu opioid receptor (MOR) agonists comprise the most effective analgesics, but their therapeutic utility is limited by adverse effects. One approach for limiting such effects has been to develop "biased" MOR agonists that show preference for activating G protein over β-Arrestin signaling. However, the notion of biased agonism has been challenged by recent studies. Oliceridine (Olinvyk®, TRV-130, OLC) is a selective MOR agonist approved by the FDA in 2020 for pain management in controlled clinical settings. Oliceridine purportedly demonstrates diminished adverse effects compared to morphine or other MOR agonists, a profile attributed to its biased agonism. However, recent studies suggest that oliceridine does not display biased agonism but instead weak intrinsic efficacy for G protein and β-Arrestin activation. Nevertheless, these insights have been derived from in vitro studies. To better understand oliceridine's in vivo efficacy profile, we performed a comprehensive assessment of its in vitro and in vivo pharmacology using both cultured cells and rodents. In vitro, oliceridine displayed high MOR affinity and weak intrinsic efficacy. In vivo, oliceridine showed impaired brain penetrance and rapid clearance, effects we attributed to its interaction with the P-glycoprotein (P-gp) efflux transporter. Moreover, we found that P-gp was essential for oliceridine's in vivo efficacy and adverse effect profiles. Taken together with prior studies, our results suggest that oliceridine's in vivo efficacy and adverse effect profiles are not attributed solely to its weak intrinsic efficacy or biased agonism but, to a large extent, its interaction with P-gp as well.

D-amphetamine maintenance therapy reduces cocaine use in female rats

RATIONALE

D-amphetamine maintenance therapy is a promising strategy to reduce drug use in cocaine use disorder (addiction). In both male rats and human cocaine users, d-amphetamine treatment reduces cocaine-taking and -seeking. However, this has not been examined systematically in female animals, even though cocaine addiction afflicts both sexes, and the sexes can differ in their response to cocaine.

OBJECTIVES

We determined how d-amphetamine maintenance therapy during cocaine self-administration influences cocaine use in female rats.

METHODS

In experiment 1, two groups of female rats received 14 intermittent access (IntA) cocaine self-administration sessions. One group received concomitant d-amphetamine maintenance treatment (COC + A rats; 5 mg/kg/day, via minipump), the other group did not (COC rats). After discontinuing d-amphetamine treatment, we measured responding for cocaine under a progressive ratio schedule, responding under extinction, and cocaine-primed reinstatement of drug-seeking. In experiment 2, we assessed the effects of d-amphetamine maintenance on these measures in already IntA cocaine-experienced rats. Thus, rats first received 14 IntA cocaine self-administration sessions without d-amphetamine. They then received 14 more IntA sessions, now either with (COC/COC + A rats) or without (COC/COC rats) concomitant d-amphetamine treatment.

RESULTS

In both experiments, d-amphetamine treatment did not significantly influence ongoing cocaine self-administration behaviour. After d-amphetamine treatment cessation, cocaine-primed reinstatement of cocaine-seeking was also unchanged. However, after d-amphetamine treatment cessation, rats responded less for cocaine both under progressive ratio and extinction conditions.

CONCLUSIONS

D-amphetamine treatment can both prevent and reverse increases in the motivation to take and seek cocaine in female animals.

Associative processes in addiction relapse models: A review of their Pavlovian and instrumental mechanisms, history, and terminology

Animal models of relapse to drug-seeking have borrowed heavily from associative learning approaches. In studies of relapse-like behaviour, animals learn to self-administer drugs then receive a period of extinction during which they learn to inhibit the operant response. Several triggers can produce a recovery of responding which form the basis of a variety of models. These include the passage of time (spontaneous recovery), drug availability (rapid reacquisition), extinction of an alternative response (resurgence), context change (renewal), drug priming, stress, and cues (reinstatement). In most cases, the behavioural processes driving extinction and recovery in operant drug self-administration studies are similar to those in the Pavlovian and behavioural literature, such as context effects. However, reinstatement in addiction studies have several differences with Pavlovian reinstatement, which have emerged over several decades, in experimental procedures, associative mechanisms, and terminology. Interestingly, in cue-induced reinstatement, drug-paired cues that are present during acquisition are omitted during lever extinction. The unextinguished drug-paired cue may limit the model’s translational relevance to cue exposure therapy and renders its underlying associative mechanisms ambiguous. We review major behavioural theories that explain recovery phenomena, with a particular focus on cue-induced reinstatement because it is a widely used model in addiction. We argue that cue-induced reinstatement may be explained by a combination of behavioural processes, including reacquisition of conditioned reinforcement and Pavlovian to Instrumental Transfer. While there are important differences between addiction studies and the behavioural literature in terminology and procedures, it is clear that understanding associative learning processes is essential for studying relapse.

Sex differences in cocaine self-administration behaviour under long access versus intermittent access conditions

Studies in humans suggest that women progress more rapidly from initial cocaine use to addiction. Similarly, female rats can show more incentive motivation for cocaine than male rats do. Most preclinical studies on this issue have used self-administration procedures that provide continuous cocaine access during each session ("long-access" or LgA and "short-access"). However, intermittent access (IntA) cocaine self-administration better models the intermittency of human cocaine use. Here, we compared cocaine use in female and male rats that received ten, daily 6-hour LgA or IntA sessions. Cocaine intake was greatest under LgA, and female LgA rats escalated their intake. Only IntA rats (both sexes) developed locomotor sensitization to self-administered cocaine, and sensitization was greatest in females. Five and 25 days after the last self-administration session, we quantified responding for cocaine (0.083-0.75 mg/kg/infusion) under a progressive ratio (PR) schedule, a measure of motivation for drug. Across conditions, females earned more cocaine infusions than males under the PR schedule. Across sexes, IntA rats earned more infusions than LgA rats, even though IntA rats had previously taken much less cocaine. Cumulative cocaine intake significantly predicted responding for cocaine under the PR schedule in male LgA rats only. In IntA rats, the extent of locomotor sensitization significantly predicted responding under the PR schedule. Thus, LgA might be appropriate to study sex differences in cocaine intake, whereas IntA might be best suited to study sex differences in sensitization-related neuroadaptations involved in cocaine addiction. This has implications for modelling distinct features of cocaine addiction in preclinical studies.

Taking Rapid and Intermittent Cocaine Infusions Enhances Both Incentive Motivation for the Drug and Cocaine-induced Gene Regulation in Corticostriatal Regions

A goal in addiction research is to distinguish forms of neuroplasticity that are involved in the transition to addiction from those involved in mere drug taking. Animal models of drug self-administration are essential in this context. Here, we compared in male rats two cocaine self-administration procedures that differ in the extent to which they evoke addiction-like behaviours. We measured both incentive motivation for cocaine using progressive ratio procedures, and cocaine-induced c-fos mRNA expression, a marker of neuronal activity. Rats self-administered intravenous cocaine (0.25 mg/kg/infusion) for seven daily 6-hour sessions. One group had intermittent access (IntA; 6 minutes ON, 26 min OFF × 12) to rapid infusions (delivered over 5 s). This models the temporal kinetics of human cocaine use and produces robust addiction-like behaviour. The other group had Long access (LgA) to slower infusions (90 s). This produces high levels of intake without promoting robust addiction-like behaviour. LgA-90 s rats took twice as much cocaine as IntA-5 s rats did, but IntA-5 s rats showed greater incentive motivation for the drug. Following a final self-administration session, we quantified c-fos mRNA expression in corticostriatal regions. Compared to LgA-90 s rats, IntA-5 s rats had more cocaine-induced c-fos mRNA in the orbitofrontal and prelimbic cortices and the caudate-putamen. Thus, a cocaine self-administration procedure (intermittent intake of rapid infusions) that promotes increased incentive motivation for the drug also enhances cocaine-induced gene regulation in corticostriatal regions. This suggests that increased drug-induced recruitment of these regions could contribute to the neural and behavioural plasticity underlying the transition to addiction.

Role of the orbitofrontal cortex and the dorsal striatum in incentive motivation for cocaine

Drug addiction involves increased incentive motivation for drug. Intermittent access to cocaine (IntA; 5-6 minutes ON, 25-26 minutes OFF, for 5-6 hours/session) enhances motivation to take the drug. The orbitofrontal cortex (OFC) and the dorsal striatum (DS) are part of a corticolimbic circuit that encodes incentive value and regulates reward-directed behaviour. We predicted that inactivation of the OFC, DS or both suppresses incentive motivation for cocaine after IntA experience. Male Wistar rats had IntA to cocaine (0.25 mg/kg/infusion) for 10 sessions. The rats developed a 'loading' pattern of intake, taking most of their cocaine in the first minute of each drug-available period. They also developed psychomotor sensitization to self-administered cocaine. We then measured incentive motivation for cocaine using a progressive ratio schedule of reinforcement (PR). Before some PR sessions, rats received microinfusions of a baclofen/muscimol cocktail (0.3 and 0.03 nmol/hemisphere, respectively, or saline) to temporarily inactivate the OFC or DS, or to disconnect the two regions. None of these treatments changed spontaneous locomotion in cocaine-naïve rats. However, both baclofen/muscimol and saline infusions influenced cocaine self-administration behaviour. Infusing baclofen/muscimol or saline into the OFC or into the OFC and contralateral DS decreased responding for cocaine under PR, with baclofen/muscimol and saline having similar effects, except that only OFC-DS disconnection with baclofen/muscimol slowed the pace of cocaine intake. Baclofen/muscimol or saline into the DS also reduced responding for cocaine under PR, but baclofen/muscimol was more effective. We conclude that neuronal activity in the OFC and DS might regulate incentive motivation for cocaine.

Revisiting long-access versus short-access cocaine self-administration in rats: intermittent intake promotes addiction symptoms independent of session length

In rats, continuous cocaine access during long self-administration sessions (6 versus 1-2 hours) promotes the development of behavioral symptoms of addiction. This has led to the assumption that taking large amounts of drug during extended daily bouts is necessary to develop an addiction phenotype. Recent work shows that within-session intermittent access (IntA) to cocaine produces much less drug intake than continuous-access procedures (i.e. long-access sessions) but evokes addiction symptoms more effectively. IntA-sessions are also long, typically lasting 6 hours. It is not known whether IntA-sessions must be extended to promote addiction-relevant changes in drug use over time. Here, we determined the influence of IntA-session length on patterns of cocaine use relevant to addiction. Two groups of male Wistar rats self-administered cocaine (0.25 mg/kg/injection, injected over 5 seconds) during 18 daily IntA-sessions. One group had long 6-hour sessions (Long-IntA), the other group had shorter, 2-hour sessions (Short-IntA). Only Long-IntA rats escalated their cocaine intake over sessions, but both groups developed a burst-like pattern of drug use over time and similar levels of psychomotor sensitization. The two groups also showed robust and similar levels of both responding for cocaine under a progressive ratio schedule of reinforcement and cocaine-induced reinstatement of extinguished drug-seeking behavior. In summary, long IntA-sessions lead to greater cocaine intake than shorter IntA-sessions, but the two conditions are equally effective in evoking the patterns of drug-taking and drug-seeking that define addiction. This suggests that chronic intermittent cocaine use, even during short daily bouts, is sufficient to promote addiction symptoms.

Intermittent intake of rapid cocaine injections promotes the risk of relapse and increases mesocorticolimbic BDNF levels during abstinence

Cocaine is thought to be more addictive when it reaches the brain rapidly. We predicted that variation in the speed of cocaine delivery influences the likelihood of addiction in part by determining the risk of relapse after abstinence. Under an intermittent-access schedule, rats pressed a lever for rapid (injected over 5 s) or slower (90 s) intravenous cocaine injections (0.5 mg/kg/injection). Control rats self-administered food pellets. A tone-light cue accompanied each self-administered reward. The 5s- and 90s-rats took a similar average amount of cocaine. One or 45 days after withdrawal from cocaine/forced abstinence, lever-pressing behaviour was extinguished during a 6-h session. Immediately thereafter, cue- or cocaine (10 mg/kg, i.p.)-induced reinstatement was assessed for 1 h. One or 45 days after withdrawal, only 5s-rats showed significant cocaine-induced reinstatement of reward-seeking behaviour. In both cocaine groups, cue-induced reinstatement behaviour was more pronounced after 45 days than after 1 day of withdrawal from cocaine, indicating incubation of conditioned drug craving. However, cue-induced reinstatement after extended abstinence was greatest in the 5s-rats. Brain-derived neurotrophic factor (BDNF) activity in the brain regulates reinstatement behaviour. Thus, 24 h after reinstatement tests, we measured BDNF protein concentrations in mesocorticolimbic regions. Only 5s-rats showed time-dependent increases in BDNF concentrations in the prelimbic cortex, nucleus accumbens core and ventral tegmental area after withdrawal from cocaine (day 45 > day 1). Thus, rapidly rising brain cocaine levels might facilitate addiction by evoking changes in the brain that intensify drug craving after abstinence, and these changes persist long after the last bout of cocaine use.

High and escalating levels of cocaine intake are dissociable from subsequent incentive motivation for the drug in rats

RATIONALE

Taking high and increasing amounts of cocaine is thought to be necessary for the development of addiction. Consequently, a widely used animal model of drug self-administration involves giving animals continuous drug access during long sessions (LgA), as this produces high and escalating levels of intake. However, human cocaine addicts likely use the drug with an intermittent rather than continuous pattern, producing spiking brain cocaine levels.

OBJECTIVES

Using an intermittent-access (IntA) cocaine self-administration procedure in rats, we studied the relationship between escalation of cocaine intake and later incentive motivation for the drug, as measured by responding under a progressive ratio schedule of cocaine reinforcement.

RESULTS

First, under IntA, rats escalated their cocaine use both within and between sessions. However, escalation did not predict later incentive motivation for the drug. Second, incentive motivation for cocaine was similar in IntA-rats limited to low- and non-escalating levels of drug intake (IntA-Lim) and in IntA-rats that took high and escalating levels of drug. Finally, IntA-Lim rats took much less cocaine than rats given continuous drug access during each self-administration session (LgA-rats). However, IntA-Lim rats later responded more for cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Taking large and escalating quantities of cocaine does not appear necessary to increase incentive motivation for the drug. Taking cocaine in an intermittent pattern-even in small amounts-is more effective in producing this addiction-relevant change. Thus, beyond the amount of drug taken, the temporal kinetics of drug use predict change in drug use over time.

The self-administration of rapidly delivered cocaine promotes increased motivation to take the drug: contributions of prior levels of operant responding and cocaine intake

RATIONALE

Rapid drug delivery to the brain might increase the risk for developing addiction. In rats, increasing the speed of intravenous cocaine delivery (5 vs. 90 s) increases drug intake and the subsequent motivation to self-administer cocaine. Increased motivation for cocaine could result not only from more extensive prior drug intake and operant responding for drug, but also from neuroplasticity evoked by rapid drug uptake.

OBJECTIVE

We determined the contributions of prior drug intake and operant responding to the increased motivation for cocaine evoked by rapid delivery. We also investigated the effects of cocaine delivery speed on corticostriatal expression of brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) mRNA.

METHODS

Rats self-administered cocaine (0.25 mg/kg/infusion) delivered over 5 or 90 s during short-access (1 h/session; ShA) or long-access (6 h; LgA) sessions. Motivation for cocaine was then assessed by measuring responding under a progressive ratio schedule of reinforcement. Next, BDNF and TrkB mRNA levels were measured in 5- and 90-s rats.

RESULTS

Five-second ShA and 5-s-LgA rats were more motivated for cocaine than their 90-s counterparts. This effect was dissociable from previous levels of drug intake or of operant responding for cocaine. In parallel, only rats self-administering rapid cocaine injections had altered BDNF and TrkB mRNA levels in corticostriatal regions.

CONCLUSIONS

Rapid drug delivery augments the motivation for cocaine independently of effects on the levels of drug intake or operant responding for drug. We suggest that rapid delivery might increase the motivation for drug by promoting neuroplasticity within reward pathways. This neuroplasticity could involve increased regulation of BDNF/TrkB.

The speed of cocaine delivery determines the subsequent motivation to self-administer the drug

The rapid delivery of drugs of abuse to the brain is associated with an increased likelihood and severity of addiction. Here we evaluated the hypothesis that rapidly delivered cocaine facilitates the addiction process by promoting the development of enhanced motivation for the drug. Rats lever-pressed for cocaine delivered intravenously over 5 or 90 s under fixed ratio (FR) during 6-h sessions. The motivation for cocaine was subsequently assessed using a progressive ratio (PR) schedule, where each successive drug injection cost an exponentially greater number of lever presses, until the cessation of responding. Throughout all self-administration sessions, all rats could only take one injection every 90 s. The 5-s groups self-administered more drug than the 90-s groups across the FR sessions. Under PR, animals that had chronically self-administered rapidly delivered cocaine took more cocaine across a range of doses and regardless of whether the drug was delivered over 5 or 90 s during PR testing. The speed of delivery also determined the long-term neurobiological impact of cocaine. Fourteen days following cocaine withdrawal, caudate-putamen D2 levels were decreased only in the 90-s rats, and quinpirole-mediated Gα(i/o)-protein activation was increased to a greater extent in the 90- vs 5-s rats. Thus, rapid delivery promotes the pursuit of cocaine in the face of rising costs and alters cocaine-induced changes in striatal D2 receptor number and function. As such, rapidly delivered cocaine might facilitate addiction because it more readily alters brain motivation circuits in ways that contribute to the compulsive pursuit of the drug.

Acquisition of responding with a remifentanil-associated conditioned reinforcer in the rat

RATIONALE

Drug-associated environmental stimuli may serve as conditioned reinforcers to enhance drug self-administration behaviors in humans and laboratory animals. However, it can be difficult to distinguish experimentally the conditioned reinforcing effects of a stimulus from other behavioral processes that can change rates of responding.

OBJECTIVES

To characterize the conditioned reinforcing effects of a stimulus paired with the μ-opioid agonist, remifentanil, using a new-response acquisition procedure in the rat.

METHODS

First, in Pavlovian conditioning (PAV) sessions, rats received response-independent IV injections of remifentanil and presentations of a light-noise compound stimulus. In paired PAV groups, injections and stimulus presentations always co-occurred. In random PAV control groups, injections and stimulus presentations occurred with no consistent relationship. Second, in instrumental acquisition (ACQ) sessions, all animals could respond in an active nose-poke that produced the stimulus alone or in an inactive nose-poke that had no scheduled consequences.

RESULTS

During ACQ, rats made significantly more active nose-pokes than inactive nose-pokes after paired PAV, but not after random PAV. Between groups, rats also made more active nose-pokes after paired PAV than after random PAV. After paired PAV, increased active responding was obtained under different schedules of reinforcement, persisted across multiple ACQ sessions, and depended on the number of PAV sessions conducted.

CONCLUSIONS

The remifentanil-paired stimulus served as a conditioned reinforcer for nose-poking: responding depended on both the contingency between the stimulus and remifentanil and the contingency between the nose-poke and the stimulus. Generally, new-response acquisition procedures may provide valid, flexible models for studying opioid-based conditioned reinforcement.