Nathan M. Holmes

Pavlovian to instrumental transfer: a neurobehavioural perspective.

Pavlovian-to-instrumental transfer (PIT) is a key concept in developing our understanding of cue-controlled behaviours. Here we have reviewed the literature on behavioural and neurobiological factors that influence PIT. Meta-analyses of the data for individual groups in PIT studies revealed that PIT is related to both the order and amounts of instrumental and Pavlovian training, and that it is critically determined by competition between instrumental and Pavlovian responses. We directly addressed the role of response competition in PIT in two experiments which showed that extensive Pavlovian conditioning produced more Pavlovian magazine visits and weaker PIT than moderate Pavlovian conditioning (Experiment 1); and that PIT lost after extensive Pavlovian conditioning was restored by Pavlovian extinction training (Experiment 2). These findings confirm that response competition is indeed an important determinant of PIT. This has significant implications for lesion and inactivation studies that assess the neurobiological substrates of PIT, as well as attempts to demonstrate PIT in the drug self-administration paradigm where the effect is yet to be reliably shown.

Extended exposure to a palatable cafeteria diet alters gene expression in brain regions implicated in reward, and withdrawal from this diet alters gene expression in brain regions associated with stress

Like people, rodents exposed to energy-rich foods over-eat and become overweight. Removal of this diet activates stress systems, which may explain why people have difficulty dieting. We exposed rats to energy-rich foods in order to identify changes in the brain induced by that diet and by its removal. Sprague Dawley rats were fed lab-chow or an energy-rich cafeteria diet (plus chow). Following 6 or 15 weeks, half of each group was switched to the opposing diet. Rats were culled 48-h later. We measured fat mass, plasma hormones, and assessed brains for mRNA expression of several genes. Cafeteria-fed rats consumed more kilojoules, weighed more and had elevated leptin (plus reduced CORT at 15 weeks) relative to chow-fed rats. Fifteen weeks of cafeteria diet suppressed μ-opioid and CB1 receptor mRNA in the VTA, but elevated amygdala GR, and 6 weeks of cafeteria diet reduced BDNF, compared to chow-fed rats. Rats switched to the cafeteria diet ate similar amounts as rats maintained on the diet, and switching to cafeteria diet after 15 weeks reduced amygdala GR expression. Rats switched to chow ate less than rats maintained on chow, and switching to chow following 15 weeks of cafeteria diet increased hypothalamic CRH mRNA. Therefore, 15 weeks of cafeteria diet produced changes in brain regions implicated in reward processes. Switching these rats to chow activated the HPA axis, while switching chow-fed rats to the cafeteria diet decreased GR expression in the amygdala, a region associated with stress. These findings have implications for dieting in humans.

Altered feeding patterns in rats exposed to a palatable cafeteria diet: increased snacking and its implications for development of obesity.

Background Rats prefer energy-rich foods over chow and eat them to excess. The pattern of eating elicited by this diet is unknown. We used the behavioral satiety sequence to classify an eating bout as a meal or snack and compared the eating patterns of rats fed an energy rich cafeteria diet or chow. Methods Eight week old male Sprague Dawley rats were exposed to lab chow or an energy-rich cafeteria diet (plus chow) for 16 weeks. After 5, 10 and 15 weeks, home-cage overnight feeding behavior was recorded. Eating followed by grooming then resting or sleeping was classified as a meal; whereas eating not followed by the full sequence was classified as a snack. Numbers of meals and snacks, their duration, and waiting times between feeding bouts were compared between the two conditions. Results Cafeteria-fed rats ate more protein, fat and carbohydrate, consistently ingesting double the energy of chow-fed rats, and were significantly heavier by week 4. Cafeteria-fed rats tended to take multiple snacks between meals and ate fewer meals than chow-fed rats. They also ate more snacks at 5 weeks, were less effective at compensating for snacking by reducing meals, and the number of snacks in the majority of the cafeteria-fed rats was positively related to terminal body weights. Conclusions Exposure to a palatable diet had long-term effects on feeding patterns. Rats became overweight because they initially ate more frequently and ultimately ate more of foods with higher energy density. The early increased snacking in young cafeteria-fed rats may represent the establishment of eating habits that promote weight gain.

Behavioral and Neural Substrates of Habit Formation in Rats Intravenously Self-Administering Nicotine

Tobacco addiction involves a transition from occasional, voluntary smoking towards habitual behavior that becomes increasingly resistant to quitting. The development of smoking habits may reflect a loss of behavioral control that can be modeled in rats. This study investigated the behavioral and neural substrates of habit formation in rats exposed to either brief (10 days) or extended (47 days) intravenous (IV) nicotine self-administration training. Following training, the first cohort of rats were exposed to a nicotine devaluation treatment, which involved repeated pairings of IV nicotine with lithium injection. They were then tested for sensitivity of responding to nicotine devaluation under extinction and reinstatement conditions. The second cohort of rats received equivalent self-administration training followed by processing of brain tissue for c-Fos immunohistochemistry. After brief training, devaluation suppressed nicotine-seeking during tests of extinction and reinstatement, confirming that responding is initially sensitive to current nicotine value, and therefore, goal directed. In contrast, after extended training, devaluation had no effect on extinction or reinstatement of responding, indicating that responding had become habitual. Complementary neuroanalysis revealed that extended nicotine self-administration was associated with increased c-Fos expression in brain regions implicated in habitual control of reward seeking, including activation of the dorsolateral striatum and substantia nigra pars compacta. These findings provide evidence of direct devaluation of an IV drug reward, that nicotine self-administration is initially goal-directed but becomes habitual with extended training, and that this behavioral transition involves activation of brain areas associated with the nigrostriatal system.

Oxytocin signaling in basolateral and central amygdala nuclei differentially regulates the acquisition, expression, and extinction of context-conditioned fear in rats

The present study investigated how oxytocin (OT) signaling in the central (CeA) and basolateral (BLA) amygdala affects acquisition, expression, and extinction of context-conditioned fear (freezing) in rats. In the first set of experiments, acquisition of fear to a shocked context was impaired by a preconditioning infusion of synthetic OT into the CeA (Experiment 1) or BLA (Experiment 2). In the second set of experiments, expression of context fear was enhanced by a pre- or post-extinction CeA infusion of synthetic OT (Experiments 3-6) or a selective OT receptor agonist, TGOT (Experiment 4). This enhancement of fear was blocked by coadministration of an OT receptor antagonist, OTA (Experiment 5) and context fear was suppressed by administration of the antagonist alone (Experiment 6). In the third set of experiments, expression of context fear was suppressed, not enhanced, by a preextinction BLA infusion of synthetic OT or a selective OT receptor agonist, TGOT (Experiments 7 and 8). This suppression of fear was blocked by coadministration of the OT receptor antagonist, OTA (Experiment 8). Taken together, these findings show that the involvement of the CeA and BLA in expression and extinction of context-conditioned fear is dissociable, and imply a critical role for oxytocin signaling in amygdala-based regulation of aversive learning.

The Basolateral Amygdala Is Critical for Learning about Neutral Stimuli in the Presence of Danger, and the Perirhinal Cortex Is Critical in the Absence of Danger

The perirhinal cortex (PRh) and basolateral amygdala (BLA) appear to mediate distinct aspects of learning and memory. Here, we used rats to investigate the involvement of the PRh and BLA in acquisition and extinction of associations between two different environmental stimuli (e.g., a tone and a light) in higher-order conditioning. When both stimuli were neutral, infusion of the GABAA, muscimol, or the NMDA receptor (NMDAR) antagonist ifenprodil into the PRh impaired associative formation. However, when one stimulus was neutral and the other was a learned danger signal, acquisition and extinction of the association between them was unaffected by manipulations targeting the PRh. Temporary inactivation of the BLA had the opposite effect: formation and extinction of an association between two stimuli was spared when both stimuli were neutral, but impaired when one stimulus was a learned danger signal. Subsequent experiments showed that the experience of fear per se shifts processing of an association between neutral stimuli from the PRh to the BLA. When training was conducted in a dangerous environment, formation and extinction of an association between neutral stimuli was impaired by BLA inactivation or NMDAR blockade in this region, but was unaffected by PRh inactivation. These double dissociations in the roles of the PRh and BLA in learning under different stimulus and environmental conditions imply that fear-induced activation of the amygdala changes how the brain processes sensory stimuli. Harmless stimuli are treated as potentially harmful, resulting in a shift from cortical to subcortical processing in the BLA.

Extinction of reinstated or ABC renewed fear responses renders them resistant to subsequent ABA renewal.

Three experiments used an ABA renewal paradigm to study deepening of response loss produced by extinction of reinstated or ABC renewed fear responses. In Experiment 1, rats were trained with two stimuli, S1 and S2, in context A and extinguished to S1 in context B and S2 in context C, shocked in B but not in C, and subjected to additional extinction of S1 in B and S2 in C. Rats froze less to S1 than S2 when subsequently tested in A. In Experiments 2 and 3, following training of S1 and S2 in A, one group received extinction of S1 in B and S2 in C followed by extinction of S1 in C and S2 in B. This group froze less to S1 in A or to S2 in a novel context, D, than a group always extinguished to S1 in B and S2 in C or a group extinguished to both S1 and S2 in B and C. These results show that additional extinction of a conditioned stimulus (conditional stimulus [CS]) exhibiting either reinstatement or ABC renewal renders that CS resistant to ABA renewal. They are consistent with theories that allow a role for context in extinction learning and that use error-correction mechanisms to update this learning.

Counterconditioned fear responses exhibit greater renewal than extinguished fear responses

This series of experiments used rats to compare counterconditioning and extinction of conditioned fear responses (freezing) with respect to the effects of a context shift. In each experiment, a stimulus was paired with shock in context A, extinguished or counterconditioned through pairings with sucrose in context B, and then tested for renewal outside of context B. Counterconditioned fear responses exhibited greater ABA renewal than extinguished fear responses. This result was observed using a between-subjects design (Experiment 1) and a within-subject design in which counterconditioned and extinguished stimuli were equated in all respects other than their signaling of sucrose (Experiment 2). Counterconditioned fear responses also exhibited greater ABC renewal than extinguished fear responses (Experiment 3). This result was observed using a within-subject design in which context C was identical to context B in terms of its associative history, and when counterconditioned and extinguished CSs were tested in compounds matched for their association with both shock and sucrose (Experiment 4). These results are consistent with models which hold that context regulates expression of associations formed in counterconditioning and extinction, and allow the level of regulation to be greater following counterconditioning than extinction, as noted in previous studies.

Additional exposures reverse the latent inhibitory effects of recent and remote exposures.

We studied the learning produced by simple exposures to a stimulus. Exposures depressed orienting and subsequent conditioned freezing in rats. A remotely preexposed conditioned stimulus (CS) conditioned better and overshadowed a novel CS more than a recently preexposed CS. Additional preexposures reversed these effects: a remotely preexposed CS elicited more orienting, conditioned worse and overshadowed less than a recently preexposed CS. Exposure to a compound composed of a novel CS and a remotely preexposed CS resulted in the novel CS subsequently conditioning better than a novel CS exposed in compound with a recently preexposed CS. The results were interpreted to mean that stimulus-alone exposures produce a loss in associability which recovers across time, that this restoration deepens the loss in associability, and that this deepening is regulated by a common error term.

Varenicline impairs extinction and enhances reinstatement across repeated cycles of nicotine self-administration in rats.

Varenicline is a partial nicotine receptor agonist widely prescribed as a smoking cessation medication. Repeated (or long-term) use of varenicline has been proposed as a treatment option for tobacco addiction. However the effect of repeated varenicline use on motivation for nicotine is unknown. Here the intravenous nicotine self-administration paradigm in rats was used to model the consequences of varenicline treatment across repeated cycles of administration, extinction and reinstatement. Rats acquired nicotine self-administration across 20 days before undergoing 6 days of extinction, where each extinction session was preceded by a single injection of varenicline or saline. This was followed by a single varenicline-free nicotine-primed reinstatement test. All rats then reacquired nicotine self-administration for 10 days followed by a second cycle of extinction. Across this period, rats either received a second cycle of varenicline (VAR-VAR) or saline (SAL-SAL), or the alternative treatment (SAL-VAR, VAR-SAL), followed by a final reinstatement test. Treatment with varenicline increased responding across the first cycle of extinction, but did not affect responding in the reinstatement test. Across the second cycle, varenicline again increased responding across extinction, and critically, rats treated with varenicline across cycle 1 and saline across cycle 2 (Group VAR-SAL) exhibited more reinstatement than rats in any other group. The effect of VAR on nicotine seeking was not due to its effects on locomotor activity. Instead, the results suggest that a history of VAR can increase vulnerability to reinstatement/relapse when its treatment is discontinued. The possible mechanisms of this increased vulnerability are discussed.