Pavlovian Conditioning Sessions Research Articles

β2* nicotinic acetylcholine receptor subtypes mediate nicotine-induced enhancement of Pavlovian conditioned responding to an alcohol cue.

Nicotine enhances Pavlovian conditioned responses to reward-associated cues. We investigated through which nicotinic acetylcholine receptor (nAChR) subtypes nicotine acts to produce this behavioral effect to an alcohol-associated cue. Male Long-Evans rats with freely available food and water were first accustomed to drinking 15% ethanol in their home cages using an intermittent access, two-bottle choice procedure. Then the rats were given 15 Pavlovian conditioning sessions in which a 15-s audiovisual conditioned stimulus (CS) predicted the delivery of 0.2 ml of ethanol, the unconditioned stimulus (US). Each session contained 12 CS-US trials. A control group received explicitly unpaired presentations of the CS and US. We measured Pavlovian conditioned approach to the site of US delivery during presentations of the CS, accounting for pre-CS baseline activity. Before each conditioning session, rats were injected subcutaneously with nicotine (0.4 mg/kg) or saline (1 ml/kg). During nAChR antagonist test sessions, rats were first injected systemically with the β2*-selective nAChR antagonist dihydro-beta-erythroidine (DHβE; 3 mg/kg) or the α7-selective nAChR antagonist methyllycaconitine (MLA; 6 mg/kg), followed by their assigned nicotine or saline injection before assessing their conditioned response to the alcohol-associated cue. Consistent with previous reports, nicotine enhanced the Pavlovian conditioned response to the alcohol-paired cue. DHβE attenuated this enhancement, whereas MLA did not. These results suggest that nicotine acts via β2*, but not α7, nAChRs to amplify Pavlovian conditioned responding to an alcohol cue. These findings contribute to a growing literature that identifies nAChRs as potential targets for pharmacological treatment of co-morbid alcohol and tobacco use disorders.

Context controls the timing of responses to an alcohol-predictive conditioned stimulus

Context can influence the number of responses elicited by a discrete, appetitive conditioned stimulus (CS) but can context control when a CS elicits a response? To test this fundamental question, we gave male, Long-Evans rats Pavlovian conditioning sessions in which the same auditory conditioned stimulus (CS, 30 s, 15 trials/session) was presented in 2 different physical contexts on alternating days, according to a within-subjects design. In one context, called the early context, alcohol (15 % ethanol, 0.2 ml/trial) was delivered from the onset of the 5th second until the termination of the 10th second of the 30 s CS. In the second late context, alcohol was delivered from the onset of the 25th second until the termination of the 30th second of the same CS. In a comparison of the last session of training, the probability of making a conditioned response during the first four seconds of the CS was significantly higher in the early context than in the late context. This result shows that context can signal when an unconditioned stimulus occurs in relation to a CS and highlights a role for context in controlling precisely timed alcohol-seeking responses.

Optogenetic Activation of the Basolateral Amygdala Promotes Both Appetitive Conditioning and the Instrumental Pursuit of Reward Cues.

Reward-associated stimuli can both evoke conditioned responses and acquire reinforcing properties in their own right, becoming avidly pursued. Such conditioned stimuli (CS) can guide reward-seeking behavior in adaptive (e.g., locating food) and maladaptive (e.g., binge eating) ways. The basolateral amygdala (BLA) regulates conditioned responses evoked by appetitive CS, but less is known about how the BLA contributes to the instrumental pursuit of CS. Here we studied the influence of BLA neuron activity on both behavioral effects. Water-restricted male rats learned to associate a light-tone cue (CS) with water delivery into a port. During these Pavlovian conditioning sessions, we paired CS presentations with photo-stimulation of channelrhodopsin-2 (ChR2)-expressing BLA neurons. BLA photo-stimulation potentiated CS-evoked port entries during conditioning, indicating enhanced conditioned approach and appetitive conditioning. Next, new rats received Pavlovian conditioning without photo-stimulation. These rats then received instrumental conditioning sessions where they could press an inactive lever or an active lever that produced CS presentation, without water delivery. Rats pressed more on the active versus inactive lever, and pairing CS presentation with BLA-ChR2 photo-stimulation intensified responding for the CS. This suggests that BLA-ChR2 photo-stimulation enhanced CS incentive value. In a separate experiment, rats did not reliably self-administer BLA-ChR2 stimulations, suggesting that BLA neurons do not carry a primary reward signal. Last, intra-BLA infusions of d-amphetamine also intensified lever-pressing for the CS. The findings suggest that BLA-mediated activity facilitates CS control over behavior by enhancing both appetitive Pavlovian conditioning and instrumental pursuit of CS.SIGNIFICANCE STATEMENT Cues paired with rewards can guide animals to valuable resources such as food. Cues can also promote dysfunctional reward-seeking behavior, as in overeating. Reward-paired cues influence reward seeking through two major mechanisms. First, reward-paired cues evoke conditioned anticipatory behaviors to prepare for impending rewards. Second, reward-paired cues are powerful motivators and they can evoke pursuit in their own right. Here we show that increasing neural activity in the basolateral amygdala enhances both conditioned anticipatory behaviors and pursuit of reward-paired cues. The basolateral amygdala therefore facilitates cue-induced control over behavior by both increasing anticipation of impending rewards and making reward cues more attractive.

Nicotine enhances responding for conditioned reinforcement via α4β2 nicotinic acetylcholine receptors in the ventral tegmental area, but not the nucleus accumbens or the prefrontal cortex

Nicotine enhances the conditioned reinforcing properties of reward-paired cues. We investigated the role of the ventral tegmental area (VTA), nucleus accumbens (NAcc), and prelimbic (PrL) and infralimbic (IL) cortices in mediating this enhancement. Male Long-Evans rats were implanted with bilateral guide cannulae aimed at either the VTA, NAcc, PrL or IL cortex. Next, rats underwent 12 sessions of Pavlovian conditioning. Each session consisted of 30 trials wherein a 5 s conditioned stimulus (CS) was paired with water (0.05 ml). Tests of responding for conditioned reinforcement were conducted during which presentation of the CS, now acting as a conditioned reinforcer (CRf), was contingent upon pressing one of two levers (CRf lever). Pressing the other lever had no consequences (NCRf lever). To determine if nicotinic acetylcholine receptors (nAChRs) in the VTA, NAcc, PrL cortex, or IL cortex mediate nicotine-enhanced responding for a CRf, the α4β2 nAChR antagonist Dihydro-Beta-Erythroidine (DHβE; 10 nmol/0.5 μL) was infused into the respective areas prior to a systemic nicotine injection (0.2 mg/kg; SC). DHβE infused into the VTA, NAcc, or IL cortex, but not PrL cortex, attenuated nicotine-enhanced responding for a CRf. Next, to confirm that nAChRs in the VTA, NAcc, or IL cortex mediate this enhancement, nicotine (8, 16, or 32 nmol/0.5 μL) was infused into the respective areas. Nicotine infused into the VTA, but not NAcc or IL cortex, enhanced responding for a CRf. These findings suggest that nicotine primarily acts on α4β2 nAChRs in the VTA to potentiate the conditioned reinforcing properties of reward-related cues.

Modeling Relapse to Pavlovian Alcohol-Seeking in Rats Using Reinstatement and Spontaneous Recovery Paradigms.

Animal models are critical for studying causal explanations of relapse. Using a Pavlovian conditioning procedure with alcohol, we examined relapse after extinction triggered by either re-exposure to alcohol (reinstatement) or a delay between extinction and test (spontaneous recovery). Male, Long-Evans rats were acclimated to 15% alcohol in the home-cage using an intermittent-access 2-bottle choice procedure. Next, they received Pavlovian conditioning sessions in which an auditory-conditioned stimulus (CS; 20second white noise; 8 trials/session; variable time 240seconds) was paired with 15% alcohol (0.3ml/CS; 2.4ml/session) that was delivered into a fluid port for oral ingestion. In subsequent extinction and test sessions, CS presentations occurred as before, but without alcohol. In experiment 1, exposure to either alcohol or water in the fluid port following extinction reinstated CS-elicited port entries at test 24hours later. In a follow-up study using the same procedure (experiment 2), reinstatement was more robustly stimulated by alcohol, compared to a familiar lemon-flavored liquid. In experiment 3, systemic alcohol injections (0, 0.5, or 1.0g/kg, intraperitoneal) administered either 24hours or 15minutes before test did not reinstate CS-elicited alcohol-seeking. Importantly, enzymatic assays in experiment 4 revealed detectable levels of alcohol in the blood following oral alcohol intake or intraperitoneal injection, suggesting that a pharmacological effect was likely with either route of administration. Last, in experiment 5, a 23-day delay between extinction and test resulted in a robust spontaneous recovery of CS-elicited alcohol-seeking. The reinstatement and spontaneous recovery effects revealed herein provide evidence of viable new behavioral paradigms for testing interventions against relapse.

A conditioned reinforcer did not help to maintain an operant conditioning in the absence of a primary reinforcer in horses

The use of conditioned reinforcers is increasingly promoted in animal training. Surprisingly, the efficiency of their use remains to be demonstrated in horses. This study aimed to determine whether an auditory signal which had previously been associated with a food reward 288 times could be used as a conditioned reinforcer to replace the primary reinforcer in an unrelated operant conditioning procedure. Fourteen horses were divided into two groups of 7: No Reinforcement (NR) and Conditioned Reinforcement (CR). All horses underwent nine sessions of Pavlovian conditioning during which the word “good” was associated with food (32 associations/session). The horses then followed five sessions of operant conditioning (30 trials/session) during which they had to touch a cone signaled by an experimenter to receive a food reward. The last day, horses underwent one test session of the operant response: no reward was given, but the word “good” was said each time a CR horse touched the cone. Nothing was said in the NR group. CR horses did not achieve more correct trials than NR horses during the test. These findings again show that the conditioned reinforcement was ineffective when used instead of the primary reinforcement to maintain conditioning.

Nicotine-induced enhancement of Pavlovian alcohol-seeking behavior in rats.

Nicotine enhances responding elicited by Pavlovian cues that predict positive outcomes. We tested the hypothesis that nicotine acting at nicotinic acetylcholine receptors (nAChRs) would augment Pavlovian alcohol-seeking. Male, Long-Evans rats with unrestricted access to food and water were acclimated to drinking 15% ethanol in their home cages and then given Pavlovian conditioning sessions in which each trial of a 15-s conditioned stimulus (CS, 12 trials/session) was paired with 0.2ml of ethanol (unconditioned stimulus, US, 2.4ml/session). Entries into a port where ethanol was delivered were used to assess conditioning. Control groups received explicitly unpaired trials of the CS and US. In experiment 1, systemic injections of saline (1ml/kg) or nicotine (0.4mg/kg, freebase) were administered before each session. In experiments 2 and 3, an identical regimen of saline or nicotine injections was administered before the start of Pavlovian conditioning sessions. All paired groups acquired conditioned port-entry responding to the CS, indicative of Pavlovian alcohol-seeking, whereas unpaired control group did not. Pre-session nicotine injections increased CS port-entries relative to saline, only in the paired group. This nicotine-induced enhancement of Pavlovian alcohol-seeking was blocked by pre-treatment with the nAChR antagonist mecamylamine. Prior exposure to nicotine did not influence the subsequent acquisition of Pavlovian alcohol-seeking. These findings highlight for the first time that nicotine acting at nAChRs augments Pavlovian alcohol-seeking, specifically in non-restricted rats. Individuals who smoke and drink may thus be particularly susceptible to alcohol cues that could trigger further drinking.

Orbitofrontal participation in sign- and goal-tracking conditioned responses: Effects of nicotine

Pavlovian conditioned stimuli can acquire incentive motivational properties, and this phenomenon can be measured in animals using Pavlovian conditioned approach behavior. Drugs of abuse can influence the expression of this behavior, and nicotine in particular exhibits incentive amplifying effects. Both conditioned approach behavior and drug abuse rely on overlapping corticolimbic circuitry. We hypothesize that the orbitofrontal cortex (OFC) regulates conditioned approach, and that one site of nicotine action is in the OFC where it reduces cortical output. To test this, we repeatedly exposed rats to 0.4 mg/kg nicotine (s.c.) during training and then pharmacologically inactivated the lateral OFC or performed in vivo electrophysiological recordings of lateral OFC neurons in the presence or absence of nicotine. In Experiment 1, animals were trained in a Pavlovian conditioning paradigm and behavior was evaluated after inactivation of the OFC by microinfusion of the GABA agonists baclofen and muscimol. In Experiment 2, we monitored phasic firing of OFC neurons during Pavlovian conditioning sessions. Nicotine reliably enhanced conditioned responding to the conditioned cue, and inactivation of the OFC reduced conditioned responding, especially the sign-tracking response. OFC neurons exhibited phasic excitations to cue presentation and during goal tracking, and nicotine acutely blunted this phasic neuronal firing. When nicotine was withheld, both conditioned responding and phasic firing in the OFC returned to the level of controls. These results suggest that the OFC is recruited for the expression of conditioned responses, and that nicotine acutely influences this behavior by reducing phasic firing in the OFC.

Varenicline Reduces Context-Induced Relapse to Alcohol-Seeking through Actions in the Nucleus Accumbens.

Varenicline, a pharmacotherapy for tobacco addiction, reduces alcohol consumption in humans and rodents. The therapeutic potential of varenicline would escalate if it also diminished conditioned responses elicited by alcohol-predictive cues, which can precipitate relapse in abstinent individuals. We investigated this application, along with the underlying neural substrates, using a robust preclinical assay in which relapse to alcohol-seeking was triggered by re-exposure to an alcohol-associated environmental context. Male, Long-Evans rats received Pavlovian conditioning sessions in which one auditory conditioned stimulus (CS+) was paired with 15% ethanol and a second conditioned stimulus (CS-) was not. Ethanol was delivered into a port for oral consumption and port entries triggered by each CS were recorded. Extinction was then conducted in a different context where the CS+ and CS- were presented without ethanol. To stimulate relapse, both cues were subsequently presented without ethanol in the prior conditioning context. Systemic varenicline (0, 0.5 or 2.5 mg/kg; intraperitoneal) blocked context-induced relapse to alcohol-seeking without affecting the ability to make a port entry. It also reduced context-induced relapse to sucrose-seeking, but only at the 2.5 mg/kg dose. Neuropharmacological studies showed that context-induced relapse to alcohol-seeking was attenuated by bilateral microinfusion of varenicline (0.3 μl/side) into the nucleus accumbens (NAc; 0 or 3.5 μg), but not the ventral tegmental area (0, 2 or 4 μg). These data show for the first time that varenicline reduces relapse triggered by contexts that predict alcohol, and suggest that nicotinic acetylcholine receptors in the NAc are critical for this effect.

Changes in the Influence of Alcohol-Paired Stimuli on Alcohol Seeking across Extended Training.

Previous work has demonstrated that goal-directed control of alcohol-seeking and other drug-related behaviors is reduced following extended self-administration and drug exposure. Here, we examined how the magnitude of stimulus influences on responding changes across similar training and drug exposure. Rats self-administered alcohol or sucrose for 2 or 8 weeks. Previous work has shown that 8 weeks, but not 2 weeks of self-administration produces habitual alcohol seeking. Next, all animals received equivalent Pavlovian conditioning sessions where a discrete stimulus predicted the delivery of alcohol or sucrose. Finally, the impact of the stimuli on ongoing instrumental responding was examined in a Pavlovian–instrumental transfer (PIT) test. While a significant PIT effect was observed following 2 weeks of either alcohol or sucrose self-administration, the magnitude of this effect was greater following 8 weeks of training. The specificity of the PIT effect appeared unchanged by extended training. While it is well established that evaluation of the outcome of responding contributes less to behavioral control following extended training and/or drug exposure, our data indicate that reward–predictive stimuli have a stronger contribution to responding after extended training. Together, these findings provide insight into the factors that control behavior after extended drug use, which will be important for developing effective methods for controlling and ideally reducing these behaviors.

Effects of sucrose concentration and water deprivation on Pavlovian conditioning and responding for conditioned reinforcement.

An appetitive Pavlovian conditioned stimulus (CS) can predict an unconditioned stimulus (US) and acquire incentive salience. We tested the hypothesis that US intensity and motivational state of the subject would influence Pavlovian learning and impact the attribution of incentive salience to an appetitive Pavlovian CS. To this end, we examined the effects of sucrose concentration and water deprivation on the acquisition of Pavlovian conditioning and responding for a conditioned reinforcer. Male Long-Evans rats (Harlan; 220-240 g) receiving 3% (3S) or 20% (20S) sucrose were either non-water deprived or given water for 1 hr per day. During Pavlovian conditioning sessions, half the rats in each concentration and deprivation condition received a 10-s CS paired with 0.2 ml of sucrose (16 trials/session; 3.2 ml/session). The remainder received unpaired CS and US presentations. Entries into a port where sucrose was delivered were recorded. Next, responding for conditioned reinforcement was tested, wherein pressing an active lever produced the CS and pressing an inactive lever had no consequences. CS-elicited port entries increased, and latency to the first CS-elicited port entry decreased across sessions in paired groups. Water deprivation augmented these effects, whereas sucrose concentration had no significant impact on behavior. Responding for conditioned reinforcement was observed in the 20S water-deprived, paired group. Thus, water deprivation can facilitate the acquisition of Pavlovian conditioning, potentially by enhancing motivational state, and a high-intensity US and a high motivational state can interact to heighten the attribution of incentive salience to an appetitive Pavlovian CS. (PsycINFO Database Record

Alcohol-Seeking Triggered by Discrete Pavlovian Cues is Invigorated by Alcohol Contexts and Mediated by Glutamate Signaling in the Basolateral Amygdala.

The environmental context in which a discrete Pavlovian conditioned stimulus (CS) is experienced can profoundly impact conditioned responding elicited by the CS. We hypothesized that alcohol-seeking behavior elicited by a discrete CS that predicted alcohol would be influenced by context and require glutamate signaling in the basolateral amygdala (BLA). Male, Long-Evans rats were allowed to drink 15% ethanol (v/v) until consumption stabilized. Next, rats received Pavlovian conditioning sessions in which a 10 s CS (15 trials/session) was paired with ethanol (0.2 ml/CS). Entries into a port where ethanol was delivered were measured. Pavlovian conditioning occurred in a specific context (alcohol context) and was alternated with sessions in a different context (non-alcohol context) where neither the CS nor ethanol was presented. At test, the CS was presented without ethanol in the alcohol context or the non-alcohol context, following a bilateral microinfusion (0.3 μl/hemisphere) of saline or the AMPA glutamate receptor antagonist NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt) in the BLA (0, 0.3, or 1.0 μg/0.3 μl). The effect of NBQX (0, 0.3 μg/0.3 μl) in the caudate putamen (CPu) on CS responding in the non-alcohol context was also tested. The discrete alcohol CS triggered more alcohol-seeking behavior in the alcohol context than the non-alcohol context. NBQX in the BLA reduced CS responding in both contexts but had no effect in the CPu. These data indicate that AMPA glutamate receptors in the BLA are critical for alcohol-seeking elicited by a discrete CS and that behavior triggered by the CS is strongly invigorated by an alcohol context.

Pavlovian conditioning enhances resistance to disruption of dogs performing an odor discrimination.

Domestic dogs are used to aid in the detection of a variety of substances such as narcotics and explosives. Under real-world detection situations there are many variables that may disrupt the dog's performance. Prior research on behavioral momentum theory suggests that higher rates of reinforcement produce greater resistance to disruption, and that this is heavily influenced by the stimulus-reinforcer relationship. The present study tests the Pavlovian interpretation of resistance to change using dogs engaged in an odor discrimination task. Dogs were trained on two odor discriminations that alternated every six trials akin to a multiple schedule in which the reinforcement probability for a correct response was always 1. Dogs then received several sessions of either odor Pavlovian conditioning to the S+ of one odor discrimination (Pavlovian group) or explicitly unpaired exposure to the S+ of one odor discrimination (Unpaired group). The remaining odor discrimination pair for each dog always remained an unexposed control. Resistance to disruption was assessed under presession feeding, a food-odor disruptor condition, and extinction, with baseline sessions intervening between disruption conditions. Equivalent baseline detection rates were observed across experimental groups and odorant pairs. Under disruption conditions, Pavlovian conditioning led to enhanced resistance to disruption of detection performance compared to the unexposed control odor discrimination. Unpaired odor conditioning did not influence resistance to disruption. These results suggest that changes in Pavlovian contingencies are sufficient to influence resistance to change.

Inactivating the infralimbic but not prelimbic medial prefrontal cortex facilitates the extinction of appetitive Pavlovian conditioning in Long-Evans rats

The infralimbic medial prefrontal cortex (IL) has been posited as a common node in distinct neural circuits that mediate the extinction of appetitive and aversive conditioning. However, appetitive extinction is typically assessed using instrumental conditioning procedures, whereas the extinction of aversive conditioning is customarily studied using Pavlovian assays. The role of the IL in the extinction of appetitive Pavlovian conditioning remains underexplored. We investigated the involvement of the IL and prelimbic medial prefrontal cortex (PrL) in appetitive extinction in Pavlovian and instrumental conditioning assays in male, Long-Evans rats. Following acquisition, a gamma-aminobutyric acid agonist solution (0.03nmol muscimol; 0.3nmol baclofen; 0.3μl/side) was bilaterally microinfused into the IL or PrL to pharmacologically inactivate each region before the first extinction session. Compared to saline, PrL inactivation did not affect the acquisition of extinction or the recall of extinction memory 24-h later. IL inactivation caused a more rapid extinction of Pavlovian conditioning, but had no effect on the extinction of instrumental conditioning or extinction recall. IL inactivation during a Pavlovian conditioning session in which conditioned stimulus (CS) trials were paired with sucrose did not affect CS-elicited behaviour, but increased responding during intervals that did not contain the CS. The same manipulation did not impact lever pressing for sucrose. These findings suggest that the IL may normally maintain Pavlovian conditioned responding when an anticipated appetitive CS is unexpectedly withheld, and that this region has distinct roles in the expression of Pavlovian conditioning when an appetitive unconditioned stimulus is either presented or omitted.

The incentive amplifying effects of nicotine are reduced by selective and non-selective dopamine antagonists in rats

Nicotine is a psychomotor stimulant with ‘reinforcement enhancing’ effects — the actions of nicotine in the brain increase responding for non-nicotine rewards. We hypothesized that this latter effect of nicotine depends on increased incentive properties of anticipatory cues; consistent with this hypothesis, multiple laboratories have reported that nicotine increases sign tracking, i.e. approach to a conditioned stimulus (CS), in Pavlovian conditioned-approach tasks. Incentive motivation and sign tracking are mediated by mesolimbic dopamine (DA) transmission and nicotine facilitates mesolimbic DA release. Therefore, we hypothesized that the incentive-promoting effects of nicotine would be impaired by DA antagonists. To test this hypothesis, separate groups of rats were injected with nicotine (0.4mg/kg base) or saline prior to Pavlovian conditioning sessions in which a CS (30s illumination of a light or presentation of a lever) was immediately followed by a sweet reward delivered in an adjacent location. Both saline and nicotine pretreated rats exhibited similar levels of conditioned approach to the reward location (goal tracking), but nicotine pretreatment significantly increased approach to the CS (sign tracking), regardless of type (lever or light). The DAD1 antagonist SCH-23390 and the DAD2/3 antagonist eticlopride reduced conditioned approach in all rats, but specifically reduced goal tracking in the saline pretreated rats and sign tracking in the nicotine pretreated rats. The non-selective DA antagonist flupenthixol reduced sign-tracking in nicotine rats at all doses tested; however, only the highest dose of flupenthixol reduced goal tracking in both nicotine and saline groups. The reductions in conditioned approach behavior, especially those by SCH-23390, were dissociated from simple motor suppressant effects of the antagonists. These experiments are the first to investigate the effects of dopaminergic drugs on the facilitation of sign-tracking engendered by nicotine and they implicate dopaminergic systems both in conditioned approach as well as the incentive-promoting effects of nicotine.

Responding for a conditioned reinforcer, and its enhancement by nicotine, is blocked by dopamine receptor antagonists and a 5-HT2C receptor agonist but not by a 5-HT2A receptor antagonist

An aspect of nicotine reinforcement that may contribute to tobacco addiction is the effect of nicotine to enhance the motivational properties of reward-associated cues, or conditioned stimuli (CSs). Several studies have now shown that nicotine enhances responding for a stimulus that has been paired with a natural reinforcer. This effect of nicotine to enhance responding for a conditioned reinforcer is likely due to nicotine-induced enhancements in mesolimbic dopaminergic activity, but this has not been directly assessed. In this study, we assessed roles for dopamine (DA) D1 or D2 receptors, and two serotonin (5-HT) receptor subtypes known to modulate DA activity, the 5-HT2C or 5-HT2A subtypes, on nicotine-enhanced responding for a conditioned reinforcer. Water-restricted rats were exposed to Pavlovian conditioning sessions, where a CS was paired with water delivery. Then, in a second phase, animals were required to perform a novel, lever-pressing response for presentations of the CS as a conditioned reinforcer. Nicotine (0.4mg/kg) enhanced responding for the conditioned reinforcer. To examine potential roles for dopamine (DA) and serotonin (5-HT) receptors in this effect, separate groups of animals were used to assess the impact of administering the D1 receptor antagonist SCH 23390, D2 receptor antagonist eticlopride, 5-HT2C receptor agonist Ro 60-0175, or 5-HT2A receptor antagonist M100907 on nicotine-enhanced responding for conditioned reinforcement. SCH 23390, eticlopride, and Ro 60-0175 all reduced responding for conditioned reinforcement, and the ability of nicotine to enhance this effect. M100907 did not alter this behavior. Together, these studies indicate that DA D1 and D2 receptors, but not 5-HT2A receptors, contribute to the effect of nicotine to enhance responding for a conditioned reinforcer. This effect can also be modulated by 5-HT2C receptor activation.

Effects of pramipexole on the acquisition of responding with opioid-conditioned reinforcement in the rat.

Dopamine D3 receptor-preferring ligands may be able to modify the conditioned reinforcing effects of drug-associated stimuli. In evaluating the effects of these compounds, it is important to clarify the extent to which responding depends on (1) conditioned reinforcement vs. other behavioral mechanisms and (2) dopamine D3 vs. D2 receptor activity. To use behaviorally stringent new-response acquisition procedures to characterize the effects of the D3-preferring agonist, pramipexole, on the conditioned reinforcing effects of a stimulus paired with the opioid agonist, remifentanil. First, in Pavlovian conditioning (PAV) sessions, rats received response-independent IV injections of remifentanil and presentations of a light-noise stimulus. In separate groups, injections and stimuli either always co-occurred ("paired PAV") or occurred with no consistent relationship ("random PAV" control). Next, in instrumental acquisition (ACQ) sessions, all animals could respond in two nose-poke manipulanda: an active nose-poke, which produced the stimulus alone, or an inactive nose-poke. Pramipexole was injected SC prior to ACQ sessions with or without pretreatments of the D3-preferring antagonist, SB-277011A, or the D2-preferring antagonist, L-741,626. After paired PAV, but not random PAV, rats acquired nose-poke responding during ACQ (i.e., active > inactive). Pramipexole dose-dependently increased active responding without changing inactive responding. Pramipexole-induced increases in responding were blocked by pretreatment with L-741,626, but not SB-277011A. Pramipexole specifically enhanced remifentanil-conditioned reinforcement: active responding was selectively increased only after the stimulus was paired with remifentanil. Although pramipexole is D3-preferring, the antagonist effects obtained presently suggest an important role for the D2 receptor in opioid-conditioned reinforcement.

Differential recruitment of distinct amygdalar nuclei across appetitive associative learning

The amygdala is important for reward-associated learning, but how distinct cell groups within this heterogeneous structure are recruited during appetitive learning is unclear. Here we used Fos induction to map the functional amygdalar circuitry recruited during early and late training sessions of Pavlovian appetitive conditioning. We found that a number of distinct amygdalar nuclei were differentially recruited by tone-food pairings during the early and late stages of training, suggesting evidence of learning-induced plasticity. Notably, these selectively activated nuclei belong to dissociable subsystems that are well placed to simultaneously inform cortical (cognitive) processing and behavioral control during tone-food learning.

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

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. 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. 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. 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. 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.

Nicotine-induced enhancement of responding for conditioned reinforcement in rats: Role of prior nicotine exposure and α4β2 nicotinic receptors

Stimuli associated with nicotine can become motivationally significant and may play a role in tobacco dependence. Previous work indicates that nicotine enhances responding for a conditioned reinforcer (CR). These studies examined the effects of prior exposure to nicotine on responding for a CR, persistence of this response, and the role of α4β2 or α7 nicotinic receptor subtypes. Water deprived rats were given 13 Pavlovian conditioning sessions where a light/tone conditioned stimulus (CS) was paired with the delivery of water. Then, rats were presented with two levers: one delivered the CS (now a CR), the other was inactive. Experiments examined the effect of nicotine administered prior to Pavlovian conditioning sessions on approach behavior during CS presentations, operant responding for CR in the presence and absence of nicotine, and the persistence of responding for CR. The effects of nicotinic acetylcholine receptor (nAChR) antagonism with mecamylamine and α4β2 or α7 nAChR antagonism with dihydro-beta-erythroidine (DHβE) or methyllycaconitine (MLA) on nicotine-enhanced responding for CR were examined. Nicotine enhanced approach behavior during CS presentations and potentiated operant responding for CR, an effect sensitized as a result of nicotine exposure during conditioning. Responding for CR and its potentiation by nicotine was stable over multiple tests. Enhanced responding for the CR induced by nicotine was blocked by mecamylamine and DHβE, but not MLA. Nicotine enhances Pavlovian discriminated approach and shows sensitized nicotine-induced enhancements in responding for CR, an effect depending on α4β2 nAChRs.