Lever-press Response Research Articles

To press or not to press? Differential receptor expression and response to novelty in rats learning an operant response for reward

Learning to perform instrumental tasks is an ability of all animals. In a population of rats, not all individuals will acquire an operant response for reward. We hypothesized that there could be a genetic explanation for differences between High Consumers (those that acquired the lever press response) and Low Consumers (lever press response is low). Additionally, we proposed that this genetic difference could produce measurable changes in response to novelty. Wistar rats were trained to lever press for a 0.2% saccharin reward and on the 10th day they were placed in a novel open field for 30 min to record locomotor activity. The prefrontal cortex and hippocampus were dissected and qPCR was used to measure mRNA expression. A significant difference ( p = .048; 2-way ANOVA) in gene expression was observed between Low and High Consumers. A principal component analysis (PCA), to cluster which genes represent this difference, identified 4 genes; 5-HT 2A and mGlu 1 in the hippocampus and AMPA GluR1 and adrenergic α 2A in the prefrontal cortex. Response to a novel open field also differed since Low Consumers displayed a higher Total Distance in comparison to High Consumers. Additionally, Low Consumers could be subdivided into Low-Lever (with lever press response only when water deprived) and Low-Non-Lever (lever press response is low throughout training). PCA with this subdivision identified an additional nine genes differing within the divisions; NMDA NR2B and GABA Aα3 in the prefrontal cortex and adrenergic α 2B and α 2A, AMPA GluR1, GluR2 and GluR3, 5-HT 1B and GABA Aα5 in the hippocampus.

Ethanol and Sucrose Seeking and Consumption Following Repeated Administration of the GABAB Agonist Baclofen in Rats

Baclofen, a GABA(B) agonist, has been found to decrease alcohol craving in humans and to nonselectively decrease ethanol intake in some rodent models. This experiment assessed the effects of repeated administration of baclofen on reinforcer seeking and consumption using the sipper tube appetitive/consummatory model of ethanol access. Subjects were divided into 2 groups and trained to make 30 lever press responses that resulted in access to either 10% ethanol or 2% sucrose in a sipper tube-drinking spout for 20 minutes. Three doses of baclofen were tested (0.3, 1.0, and 3.0 mg/kg) and each drug treatment was assessed using the following schedule: Monday, saline; Tuesday to Thursday, baclofen; and Friday, saline. The low dose of baclofen had no effect on the seeking or intake of either sucrose or ethanol, and the 1.0 mg/kg dose also had no effect on the appetitive, seeking response. However, the 1.0 mg/kg dose significantly decreased sucrose intake (from an average of 0.56 to 0.41 g/kg) and significantly increased ethanol intake (from an average of 0.77 to 1.00 g/kg). Similarly, the high dose (3.0 mg/kg) decreased sucrose intake and had a tendency to increase ethanol intake while decreasing both sucrose seeking and ethanol seeking. Overall, baclofen treatment affected reinforcer intake at doses that had no effect on reinforcer seeking, and effective doses decreased both sucrose seeking and ethanol seeking. Moreover, the effects on reinforcer intake were disparate, in that baclofen increased ethanol drinking and decreased sucrose drinking. The nonspecific effects of baclofen suggest that the GABA(B) system may be involved in general consummatory or drinking behaviors and does not appear to specifically regulate ethanol-motivated responding.

Fmr1 knockout mice are impaired in a leverpress escape/avoidance task

Fragile X syndrome (FXS) is the most common form of inherited mental retardation (MR). FXS is typically caused by a mutation of the Fmr1 gene (Verkerk et al. 1991, Cell 65, 905-914). To better understand the role of Fmr1 and its gene product fragile X mental-retardation protein (FMRP) in central nervous system function, researchers have turned to the use of animal model systems to generate an Fmr1 knockout (KO) mouse that is deficient in FMRP (Bakker et al. 1994, Cell 78, 23-33). Unfortunately, a number of studies have found no consistent, robust learning and memory impairment in the Fmr1 KO mice. We conducted a study to assess the performance of Fmr1 KO and wildtype (WT) animals in a leverpress escape/avoidance paradigm. Fmr1 KO and WT littermates were studied in four daily 1-h sessions. The Fmr1 KO mice performed fewer avoidance and total responses than WT mice. The KO animals were not simply deficient in avoidance, but a within-factor ANOVA revealed that they did not acquire the leverpress response to any appreciable degree. Observation during the sessions indicated that the Fmr1 KO animals clearly responded to the shock, eliminating an obvious sensory explanation for the deficit. The fact that other studies have found that the KO mice displayed increased exploratory and locomotor activity compared with WT controls argues against a motoric deficit. Future studies will attempt to delineate the nature of the behavioral deficit as well as attempt to rescue the response with glutamatergic or dopaminergic agents.

Extinction of Cocaine Self-Administration Reveals Functionally and Temporally Distinct Dopaminergic Signals in the Nucleus Accumbens

While Pavlovian and operant conditioning influence drug-seeking behavior, the role of rapid dopamine signaling in modulating these processes is unknown. During self-administration of cocaine, two dopaminergic signals, measured with 100 ms resolution, occurred immediately before and after the lever press (termed pre- and post-response dopamine transients). Extinction of self-administration revealed that these two signals were functionally distinct. Pre-response transients, which could reflect the motivation to obtain the drug, did not decline during extinction. Remarkably, post-response dopamine transients attenuated as extinction progressed, suggesting that they encode the learned association between environmental cues and cocaine. A third type of dopamine transient, not time locked to overt stimuli, decreased in frequency during extinction and correlated with calculated cocaine concentrations. These results show that dopamine release transients involved in different aspects of cocaine self-administration are highly plastic--differentially governed by motivation, learned associations linked with environmental stimuli, and the pharmacological actions of cocaine.

AMPA/kainate, NMDA, and dopamine D1 receptor function in the nucleus accumbens core: A context-limited role in the encoding and consolidation of instrumental memory

Neural integration of glutamate- and dopamine-coded signals within the nucleus accumbens (NAc) is a fundamental process governing cellular plasticity underlying reward-related learning. Intra-NAc core blockade of NMDA or D1 receptors in rats impairs instrumental learning (lever-pressing for sugar pellets), but it is not known during which phase of learning (acquisition or consolidation) these receptors are recruited, nor is it known what role AMPA/kainate receptors have in these processes. Here we show that pre-trial intra-NAc core administration of the NMDA, AMPA/KA, and D1 receptor antagonists AP-5 (1 microg/0.5 microL), LY293558 (0.01 or 0.1 microg/0.5 microL), and SCH23390 (1 microg/0.5 microL), respectively, impaired acquisition of a lever-pressing response, whereas post-trial administration left memory consolidation unaffected. An analysis of the microstructure of behavior while rats were under the influence of these drugs revealed that glutamatergic and dopaminergic signals contribute differentially to critical aspects of the initial, randomly emitted behaviors that enable reinforcement learning. Thus, glutamate and dopamine receptors are activated in a time-limited fashion-only being required while the animals are actively engaged in the learning context.

‘Compulsive’ lever pressing in rats is enhanced following lesions to the orbital cortex, but not to the basolateral nucleus of the amygdala or to the dorsal medial prefrontal cortex

In a new rat model of obsessive-compulsive disorder (OCD), 'compulsive' behaviour is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We have recently found that compulsive lever pressing is increased following lesions to the rat orbital cortex, in accordance with several lines of evidence implicating the orbitofrontal cortex in the pathophysiology of OCD. In view of the functional similarities between the orbital cortex, the basolateral nucleus of the amygdala and the medial prefrontal cortex, the present study compared the effects of lesions to these three regions. The present study replicated the finding that lesions to the rat orbital cortex enhance compulsive lever pressing. In contrast, lesions to the dorsal medial prefrontal cortex and to the basolateral amygdala did not affect compulsive lever pressing. A comparison of these findings to current knowledge regarding similarities and differences in the functioning of the three regions sheds light on the mechanism by which signal attenuation induces compulsive lever pressing and on the role played by the orbital cortex in compulsive behaviour.

Prazosin, an α-1 Adrenergic Antagonist, Reduces Cocaine-Induced Reinstatement of Drug-Seeking

Norepinephrine is implicated in cocaine's behavioral effects. In this study, we tested the effect of prazosin, an alpha1-adrenergic receptor antagonist, on cocaine-induced reinstatement of drug-seeking behavior. Rats were trained to self-administer cocaine intravenously under a fixed-ratio 3 schedule of reinforcement. After behavior was established, cocaine was replaced with saline and behavior extinguished. The ability of cocaine (0, 5-20 mg/kg) alone or combined with prazosin (.3 mg/kg) to reinstate lever press responding was tested. The effects of prazosin on lever press responding for food was examined in another set of rats. Cocaine induced a dose-dependent reinstatement of drug-seeking behavior that was significantly attenuated by prazosin. This dose of prazosin did not alter lever press response rates for food. The attenuation in drug-induced reinstatement is likely not due to prazosin-induced suppression of activity. These results suggest alpha1-adrenergic mechanisms contribute to reinstatement in rats and perhaps, to relapse in addicts.

Prefrontal cortical cell firing during maintenance, extinction, and reinstatement of goal‐directed behavior for natural reward

The prefrontal cortex (PFC) is important for higher cognitive functioning and the processing of reward-related information. Here, electrophysiological recording procedures were used to examine cell firing in the PFC in rats (n = 12) during water reinforcement sessions consisting of three phases. In phase one (maintenance), animals pressed a lever (fixed ratio 1) for water reinforcement (0.05 ml/press) paired with an auditory stimulus. Of 62 neurons recorded during maintenance, 39 (63%) exhibited one of three types of patterned discharges relative to the reinforced response for water. Specifically, PFC neurons exhibited increases in firing rate within seconds preceding the response (type PR; n = 9 cells) or increases (type RFe; n = 16 cells) or decreases (type RFi; n = 14 cells) in firing rate immediately following response completion. The remaining neurons did not alter their firing profiles relative to the reinforced response (type nonphasic cells; n = 23 cells). In phase two (extinction), lever press responses had no programmed consequences (i.e., water reinforcement and the auditory stimulus were not presented). After 30 min of no responding, phase three (reinstatement) began, during which each lever press response was again associated with water reinforcement paired with the stimulus. Results indicate differential effects of extinction/reinstatement on cell firing rates and patterns dependent on cell type. These findings are discussed with respect to the adaptive nature of PFC activity during goal-directed behaviors for "natural" rewards, and are considered relative to prior studies that examined nucleus accumbens cell firing during a similar behavioral task.

The neurosteroid pregnenolone sulfate neutralized the learning impairment induced by intrahippocampal nicotine in alcohol-drinking rats

The effects of intrahippocampal administration of nicotine and the neurosteroids pregnenolone sulfate and allopregnanolone on acquiring the lever-press response and extinction in a Skinner box were examined using voluntary alcohol-drinking rats. A free-choice drinking procedure that implies early availability of the alcoholic solution (10% ethanol v/v+3% glucose w/v in distilled water) was used. Alcohol and control rats were deprived of food and assigned at random to six groups. Each group received two consecutive intrahippocampal (dorsal CA1) injections immediately after 1-h of drinking ethanol and before the free lever-press response shaping and extinction session. The groups were: saline–saline; saline–pregnenolone sulfate (5 ng, 24μM); saline–allopregnanolone (0.2μg, 1.26μM); nicotine (4.6μg, 20mM)–saline; nicotine–pregnenolone sulfate; nicotine–allopregnanolone. Blood alcohol concentrations were assessed the day before conditioning. The combination of the oral self-administration of ethanol and the intrahippocampal injection of nicotine deteriorated the ability to acquire the lever-press response. This effect was neutralized by intrahippocampal pregnenolone sulfate (negative modulator of the GABA A receptor complex), and it was not affected by intrahippocampal allopregnanolone (positive GABA receptor complex A modulator). Pregnenolone sulfate and allopregnanolone had no effects per se on lever-press acquisition, neither in alcohol-drinking rats nor in controls. Alcohol consumption facilitated operant extinction just as anxiolytics that act as positive modulators of the GABA receptor complex A receptors do, possibly reducing the anxiety or aversion related to non-reinforcement. This effect was increased by intrahippocampal nicotine.

Nucleus accumbens core lesions retard instrumental learning and performance with delayed reinforcement in the rat

BackgroundDelays between actions and their outcomes severely hinder reinforcement learning systems, but little is known of the neural mechanism by which animals overcome this problem and bridge such delays. The nucleus accumbens core (AcbC), part of the ventral striatum, is required for normal preference for a large, delayed reward over a small, immediate reward (self-controlled choice) in rats, but the reason for this is unclear. We investigated the role of the AcbC in learning a free-operant instrumental response using delayed reinforcement, performance of a previously-learned response for delayed reinforcement, and assessment of the relative magnitudes of two different rewards.ResultsGroups of rats with excitotoxic or sham lesions of the AcbC acquired an instrumental response with different delays (0, 10, or 20 s) between the lever-press response and reinforcer delivery. A second (inactive) lever was also present, but responding on it was never reinforced. As expected, the delays retarded learning in normal rats. AcbC lesions did not hinder learning in the absence of delays, but AcbC-lesioned rats were impaired in learning when there was a delay, relative to sham-operated controls. All groups eventually acquired the response and discriminated the active lever from the inactive lever to some degree. Rats were subsequently trained to discriminate reinforcers of different magnitudes. AcbC-lesioned rats were more sensitive to differences in reinforcer magnitude than sham-operated controls, suggesting that the deficit in self-controlled choice previously observed in such rats was a consequence of reduced preference for delayed rewards relative to immediate rewards, not of reduced preference for large rewards relative to small rewards. AcbC lesions also impaired the performance of a previously-learned instrumental response in a delay-dependent fashion.ConclusionsThese results demonstrate that the AcbC contributes to instrumental learning and performance by bridging delays between subjects' actions and the ensuing outcomes that reinforce behaviour.

Role of the orbital cortex and of the serotonergic system in a rat model of obsessive compulsive disorder

The serotonergic system and the orbitofrontal cortex have been consistently implicated in the pathophysiology of obsessive compulsive disorder. Yet, the relations between these two systems and the ways they interact in producing obsessions and compulsions are poorly understood. The present study tested the hypothesis that pathology of the orbitofrontal cortex leads to a dysregulation of the serotonergic system which is manifested in compulsive behavior, using a new rat model of this disorder. In the model, ‘compulsive’ behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We found that lesion to the rat orbital cortex led to a selective increase in compulsive lever-pressing that was prevented by the serotonin re-uptake inhibitor, paroxetine, and was paralleled by an increase in the density of the striatal serotonin transporter, assessed using high affinity [ 3H]imipramine binding. These results suggest that the serotonergic system is involved in orbital lesion-induced compulsivity, and provide a possible account for the observed association between obsessions and compulsions and dysfunction of the orbitofrontal cortex and of the serotonergic system in obsessive compulsive disorder.

Response acquisition with immediate and delayed conditioned reinforcement

Groups comprising eight rats initially were exposed to response-independent water deliveries, then to conditions under which a lever-press response raised an empty dipper immediately or after a resetting delay of 15, 30, or 45 s. When their performance was compared to that of control animals using a 90% confidence level, six rats in the immediate-reinforcement group met the primary criterion for response acquisition during a single 6-h session; 4, 4, and 3 did so in the 15, 30, and 45 s delay groups, respectively. Similar evidence of acquisition was obtained when a 95% confidence level was used. With a 99% confidence level, however, evidence of acquisition was not compelling. Although these data appear to provide the first demonstration of response acquisition in the absence of handshaping or autoshaping under conditions where the putative reinforcer is both conditioned and delayed, they also demonstrate that whether response acquisition occurs depends, in part, on how it is defined.

Nicotine Attenuates Relapse to Methamphetamine‐Seeking Behavior (Craving) in Rats

This study clarifies the modulating action of the nicotinic cholinergic system on reinstatement of methamphetamine (MAP)-seeking behavior (craving) using an intravenous, self-administration paradigm in rats. After self-administration of MAP for 10 days, replacing MAP with saline solution (MAP withdrawal) gradually decreased lever-pressing responses. On the sixth day of MAP withdrawal, MAP (1.0 mg/kg, i.p.)-priming injection significantly increased lever-pressing responses (reinstatement of MAP-seeking behavior). This MAP-seeking behavior was attenuated by repeated nicotine administration for 5 days during MAP withdrawal, and this attenuating effect was antagonized by the nicotinic antagonist mecamylamine. These results suggest that the appearance of MAP-seeking behavior may be due to inactivation of the nicotinic cholinergic neuron. Furthermore, it is suggested that nicotinic activating agents may be useful in preventing relapse to drug abuse.

P.3.026 Tiagabine as an efficacious augmentation therapy to selective serotonin reuptake inhibitors in patients with generalized anxiety disorder

The serotonergic system and the orbitofrontal cortex have been consistently implicated in the pathophysiology of obsessive compulsive disorder. Yet, the relations between these two systems and the ways they interact in producing obsessions and compulsions are poorly understood. The present study tested the hypothesis that pathology of the orbitofrontal cortex leads to a dysregulation of the serotonergic system which is manifested in compulsive behavior, using a new rat model of this disorder. In the model, ‘compulsive’ behavior is induced by attenuating a signal indicating that a lever-press response was effective in producing food. We found that lesion to the rat orbital cortex led to a selective increase in compulsive lever-pressing that was prevented by the serotonin re-uptake inhibitor, paroxetine, and was paralleled by an increase in the density of the striatal serotonin transporter, assessed using high affinity [3H]imipramine binding. These results suggest that the serotonergic system is involved in orbital lesion-induced compulsivity, and provide a possible account for the observed association between obsessions and compulsions and dysfunction of the orbitofrontal cortex and of the serotonergic system in obsessive compulsive disorder.

Excitatory backward conditioning in an appetitive conditioned reinforcement preparation with rats

Four experiments were conducted to examine appetitive backward conditioning in a conditioned reinforcement preparation. In all experiments, off-line classical conditioning was conducted following lever-press training on two levers. Presentations of a sucrose solution by a liquid dipper served as an unconditioned stimulus (US) and two auditory stimuli served as conditioned stimuli (CSs); one was paired with the US in either a forward (Experiment 1a) or a backward (Experiments 1b, 2, and 3) relationship, and the other served as a control CS, which was not paired with the US. In testing, each lever-press response produced a presentation of one of the CSs instead of appetitive reinforcers. The response to a lever was facilitated, compared to the response to another lever, when the response produced the backward CS presentation as well as when it produced the forward CS presentation; that is, the backward CS served as an excitatory conditioned reinforcer.

Naltrexone attenuates cue- but not drug-induced methamphetamine seeking: a possible mechanism for the dissociation of primary and secondary reward

The present study was aimed to clarify the role of the opioid system in the reinstatement of methamphetamine (METH)-seeking behavior in METH self-administering rats. Following 12 days of self-administration of METH, the replacement of METH with saline resulted in a gradual decrease in lever press responses (extinction). Under extinction conditions, METH-priming or re-exposure to cues previously paired with METH infusion markedly increased the responses (reinstatement of drug-seeking). Naltrexone administered 30 min before re-exposure to METH-associated cues attenuated reinstatement of drug-seeking behavior. On the other hand, administration of this antagonist had no effect on the reinstatement induced by METH-priming. We discussed these findings in relation with the dissociation of primary and secondary reward, suggesting that an opioid mechanism is responsible for this dissociation. Further, these results indicate the possibility of using naltrexone as an anti-relapse agent.

Intrahippocampal nicotine in alcohol drinking rats–effects on lever-press response

We have previously shown differences on learning processes between alcohol drinking and non-alcohol drinking rats.Underlying these effects, functional differences in the septo-hippocampal pathway were hypothesized. We have performed a dose–response study for intrahippocampal nicotine (CA1) on acquisition and extinction of the lever-press response and antagonization test by co-administration of mecamylamine. Results show that the administration of nicotine in CA1 region has a detrimental dose-dependent effect on acquisition in alcohol drinkers, with a dose of 10 nM being the most disruptive. In the controls, only doses of 10 and 20 nM had detrimental effect. The effect of nicotine (10 nM) was partially (alcoholics) or fully (controls) antagonized by mecamylamine co-administration (30 nM). Summarizing, the alcohol groups showed a dose–response curve for nicotine shifted leftwards, and a partial antagonism of these effects by mecamylamine; these effects may be consequence of the functional sensitization of the nicotinic responsivity in the CA1 region which were produced by the chronic alcohol.

Endocannabinoid system modulates relapse to methamphetamine seeking: possible mediation by the arachidonic acid cascade.

We clarified the modulating action of the endocannabinoid system, and its possible mediation by the arachidonic acid cascade, on the reinstatement of methamphetamine (METH)-seeking behavior, using the intravenous self-administration paradigm in rats. Following 12 days of self-administration of METH, the replacement of METH with saline resulted in a gradual decrease in lever press responses (extinction). Under extinction conditions, METH-priming or re-exposure to cues previously paired with METH infusion markedly increased the responses (reinstatement of drug-seeking). The cannabinoid CB1 receptor antagonist, SR141716A, blocked this behavior. Although the cannabinoid agonist, Delta8-tetrahydrocannabinol (THC), had no effects by itself, coadministration of the agonist and METH at small doses reinstated the drug-seeking behavior. THC attenuated the effects of the reinstatement-inducing dose of METH, but enhanced the effect of cues. Either given repeatedly during the extinction or singly, 24 h before the first METH-priming or cues challenge, THC suppressed the reinstatement. In another set of experiments, we found that diclofenac, a cyclooxygenase inhibitor, also attenuated the reinstatement induced by exposure to cues or drug-priming. These results suggest that the endocannabinoid system, through possible mediation by the arachidonic acid cascade, serves as a modulator of the reinstating effects of METH-priming and cues. Extending the current view on the treatment of drug dependence, these results indicate that endocannabinoid-activating substances as well as cyclooxygenase inhibitors may be promising as antirelapse agents.

Differential effects of delta9-tetrahydrocannabinol and methanandamide in CB1 knockout and wild-type mice.

Mice devoid of CB1 cannabinoid receptors (CB1-/- mice) provide a unique opportunity to further investigate the role of CB1 receptors in exocannabinoid and endocannabinoid effects. CB1-/- mice (N = 18) and their wild-type littermates (CB1+/+ mice; N = 12) were placed in standard mouse operant chambers and trained to lever press under a fixed ratio 10 schedule of reinforcement. When stable lever press responding under the fixed ratio 10 schedule had been established, cannabinoids and noncannabinoids were administered to both groups. CB1+/+ mice acquired the lever press response more readily than CB1-/- mice. Delta(9)-Tetrahydrocannabinol (Delta(9)-THC) decreased lever press responding in CB1+/+ mice only, whereas methanandamide, a metabolically stable endocannabinoid analog, produced similar response rate decreases in both genotypic groups. Similar to Delta(9)-THC, another endocannabinoid analog, (R)-(20-cyano-16,16-dimethyl docosa-cis-5,8,11,14-tetraeno)-1'-hydroxy-2'-propylamine (O-1812), decreased responding in CB1+/+ mice, but not in CB1-/- mice. The CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A) blocked the effects of Delta(9)-THC, but not those of methanandamide. Because methanandamide binds poorly to CB2 receptors, these results suggest possible non-CB1, non-CB2 mechanisms of action for methanandamide-induced behavioral disruption of lever press responding. Ethanol and morphine elicited greater response decreases in CB1-/- mice than in CB1+/+ mice, suggesting a possible role of CB1 receptors in the rate disruptive effects of these drugs. In contrast, diazepam did not produce between group differences, suggesting that CB1 receptors are not involved in diazepam-induced disruption of lever press responding.

Conditioned reinforcing properties of stimuli paired with self-administered cocaine, heroin or sucrose: implications for the persistence of addictive behaviour

Conditioned environmental stimuli are known to be important determinants of drug seeking. Traditional models of drug seeking under the control of conditioned stimuli have focused on the ability of conditioned reinforcers either to reinstate extinguished responding or to maintain prolonged chains of drug seeking under second-order schedules. These models have consistently suggested that it is the conditioned reinforcing, rather than other, effects of Pavlovian drug stimuli that most profoundly influence drug seeking. However, the impact of drug-associated conditioned reinforcers has not been studied directly and in isolation, not least because the instrumental seeking response is invariably the same as that which was previously reinforced with the drug itself. The purpose of the present study was, therefore, to investigate the conditioned reinforcing properties of drug-paired CSs using an acquisition of a new response procedure in which an animal learns to make a new instrumental response reinforced solely by the CS. It was found that CSs paired with either cocaine, heroin or sucrose supported the rapid acquisition of lever pressing for the CS that persisted over months of repeated, intermittent testing. Furthermore, rats did not acquire the lever press response when the CS was not paired with drug, suggesting that for this stimulus to acquire conditioned reinforcing properties, it must be predictively associated with the drug’s effect. Moreover, lever pressing for the CS could not be explained as coincidental to an over-riding Pavlovian approach response to the location of the lever, since animals also acquired discriminated lever pressing when the CS was above the opposite, inactive lever. Extinction decreased responding with conditioned reinforcement, but only when the CS–US association was devalued prior to, and not after, acquisition of the lever press response, providing evidence for the establishment of habitual CS-maintained responding that may explain the persistence of drug-seeking responses in animal models of addiction and relapse.