The Emergence of a Stable Neuronal Ensemble from a Wider Pool of Activated Neurons in the Dorsal Medial Prefrontal Cortex during Appetitive Learning in Mice.

Animals selectively respond to environmental cues associated with food reward to optimize nutrient intake. Such appetitive conditioned stimulus-unconditioned stimulus (CS-US) associations are thought to be encoded in select, stable neuronal populations or neuronal ensembles, which undergo physiological modifications during appetitive conditioning. These ensembles in the medial prefrontal cortex (mPFC) control well-established, cue-evoked food seeking, but the mechanisms involved in the genesis of these ensembles are unclear. Here, we used male Fos-GFP mice that express green fluorescent protein (GFP) in recently behaviorally activated neurons, to reveal how dorsal mPFC neurons are recruited and modified to encode CS-US memory representations using an appetitive conditioning task. In the initial conditioning session, animals did not exhibit discriminated, cue-selective food seeking, but did so in later sessions indicating that a CS-US association was established. Using microprism-based in vivo 2-Photon imaging, we revealed that only a minority of neurons activated during the initial session was consistently activated throughout subsequent conditioning sessions and during cue-evoked memory recall. Notably, using ex vivo electrophysiology, we found that neurons activated following the initial session exhibited transient hyperexcitability. Chemogenetically enhancing the excitability of these neurons throughout subsequent conditioning sessions interfered with the development of reliable cue-selective food seeking, indicated by persistent, nondiscriminated performance. We demonstrate how appetitive learning consistently activates a subset of neurons to form a stable neuronal ensemble during the formation of a CS-US association. This ensemble may arise from a pool of hyperexcitable neurons activated during the initial conditioning session.SIGNIFICANCE STATEMENT Appetitive conditioning endows cues associated with food with the ability to guide food-seeking, through the formation of a food-cue association. Neuronal ensembles in the mPFC control established cue-evoked food-seeking. However, how neurons undergo physiological modifications and become part of an ensemble during conditioning remain unclear. We found that only a minority of dorsal mPFC neurons activated on the initial conditioning session became consistently activated during conditioning and memory recall. These initially activated neurons were also transiently hyperexcitable. We demonstrate the following: (1) how stable neuronal ensemble formation in the dorsal mPFC underlies appetitive conditioning; and (2) how this ensemble may arise from hyperexcitable neurons activated before the establishment of cue-evoked food seeking.

Acute, but not longer-term, exposure to environmental enrichment attenuates Pavlovian cue-evoked conditioned approach and Fos expression in the prefrontal cortex in mice.

Exposure to environmental enrichment can modify the impact of motivationally relevant stimuli. For instance, previous studies in rats have found that even a brief, acute (~1 day), but not chronic, exposure to environmentally enriched (EE) housing attenuates instrumental lever pressing for sucrose-associated cues in a conditioned reinforcement setup. Moreover, acute EE reduces corticoaccumbens activity, as measured by decreases in expression of the neuronal activity marker "Fos." Currently, it is not known whether acute EE also reduces sucrose seeking and corticoaccumbens activity elicited by non-contingent or "forced" exposure to sucrose cues, which more closely resembles cue exposure encountered in daily life. We therefore measured the effects of acute/intermittent (1 day or 6 day of EE prior to test day) versus chronic (EE throughout conditioning lasting until test day) EE on the ability of a Pavlovian sucrose cue to elicit sucrose seeking (conditioned approach) and Fos expression in the medial prefrontal cortex (mPFC), orbitofrontal cortex (OFC), and nucleus accumbens (NAc) in mice. One day, but not 6 day or chronic EE , reduced sucrose seeking and Fos in the deep layers of the dorsal mPFC. By contrast, 1 day, 6 day, and chronic EE all reduced Fos in the shallow layers of the OFC. None of the EE manipulations modulated NAc Fos expression. We reveal how EE reduces behavioral reactivity to sucrose cues by reducing activity in select prefrontal cortical brain areas. Our work further demonstrates the robustness of EE in its ability to modulate various forms of reward-seeking across species.

Extinction of cue-evoked food-seeking recruits a GABAergic interneuron ensemble in the dorsal medial prefrontal cortex of mice.

Animals must quickly adapt food-seeking strategies to locate nutrient sources in dynamically changing environments. Learned associations between food and environmental cues that predict its availability promote food-seeking behaviors. However, when such cues cease to predict food availability, animals undergo "extinction" learning, resulting in the inhibition of food-seeking responses. Repeatedly activated sets of neurons, or "neuronal ensembles," in the dorsal medial prefrontal cortex (dmPFC) are recruited following appetitive conditioning and undergo physiological adaptations thought to encode cue-reward associations. However, little is known about how the recruitment and intrinsic excitability of such dmPFC ensembles are modulated by extinction learning. Here, we used in vivo 2-Photon imaging in male Fos-GFP mice that express green fluorescent protein (GFP) in recently behaviorally activated neurons to determine the recruitment of activated pyramidal and GABAergic interneuron dmPFC ensembles during extinction. During extinction, we revealed a persistent activation of a subset of interneurons which emerged from a wider population of interneurons activated during the initial extinction session. This activation pattern was not observed in pyramidal cells, and extinction learning did not modulate the excitability properties of activated pyramidal cells. Moreover, extinction learning reduced the likelihood of reactivation of pyramidal cells activated during the initial extinction session. Our findings illuminate novel neuronal activation patterns in the dmPFC underlying extinction of food-seeking, and in particular, highlight an important role for interneuron ensembles in this inhibitory form of learning.

Changes in Appetitive Associative Strength Modulates Nucleus Accumbens, But Not Orbitofrontal Cortex Neuronal Ensemble Excitability.

Cues that predict the availability of food rewards influence motivational states and elicit food-seeking behaviors. If a cue no longer predicts food availability, then animals may adapt accordingly by inhibiting food-seeking responses. Sparsely activated sets of neurons, coined "neuronal ensembles," have been shown to encode the strength of reward-cue associations. Although alterations in intrinsic excitability have been shown to underlie many learning and memory processes, little is known about these properties specifically on cue-activated neuronal ensembles. We examined the activation patterns of cue-activated orbitofrontal cortex (OFC) and nucleus accumbens (NAc) shell ensembles using wild-type and Fos-GFP mice, which express green fluorescent protein (GFP) in activated neurons, after appetitive conditioning with sucrose and extinction learning. We also investigated the neuronal excitability of recently activated, GFP+ neurons in these brain areas using whole-cell electrophysiology in brain slices. Exposure to a sucrose cue elicited activation of neurons in both the NAc shell and OFC. In the NAc shell, but not the OFC, these activated GFP+ neurons were more excitable than surrounding GFP- neurons. After extinction, the number of neurons activated in both areas was reduced and activated ensembles in neither area exhibited altered excitability. These data suggest that learning-induced alterations in the intrinsic excitability of neuronal ensembles is regulated dynamically across different brain areas. Furthermore, we show that changes in associative strength modulate the excitability profile of activated ensembles in the NAc shell.SIGNIFICANCE STATEMENT Sparsely distributed sets of neurons called "neuronal ensembles" encode learned associations about food and cues predictive of its availability. Widespread changes in neuronal excitability have been observed in limbic brain areas after associative learning, but little is known about the excitability changes that occur specifically on neuronal ensembles that encode appetitive associations. Here, we reveal that sucrose cue exposure recruited a more excitable ensemble in the nucleus accumbens, but not orbitofrontal cortex, compared with their surrounding neurons. This excitability difference was not observed when the cue's salience was diminished after extinction learning. These novel data provide evidence that the intrinsic excitability of appetitive memory-encoding ensembles is regulated differentially across brain areas and adapts dynamically to changes in associative strength.

The Continuing Challenges of Studying Parallel Behaviours in Humans and Animal Models.

The use of animal models continues to be essential for carrying out research into clinical phenomena, including addiction. However, the complexity of the clinical condition inevitably means that even the best animal models are inadequate, and this may go some way to account for the apparent failures of discoveries from animal models, including the identification of potential novel therapies, to translate to the clinic. We argue here that it is overambitious and misguided in the first place to attempt to model complex, multifacetted human disorders such as addiction in animals, and especially in rodents, and that all too frequently "validity" of such models is limited to superficial similarities, referred to as "face validity", that reflect quite different underlying phenomena and biological processes from the clinical situation. Instead, a more profitable approach is to identify (a) well-defined intermediate human behavioural phenotypes that reflect defined, limited aspects of, or contributors to, the human clinical disorder, and (b) to develop animal models that are homologous with those discrete human behavioural phenotypes in terms of psychological processes, and underlying neurobiological mechanisms. Examples of past and continuing weaknesses and suggestions for more limited approaches that may allow better homology between the test animal and human condition are made.

Incentive learning underlying cocaine-seeking requires mGluR5 receptors located on dopamine D1 receptor-expressing neurons.

Understanding the psychobiological basis of relapse remains a challenge in developing therapies for drug addiction. Relapse in cocaine addiction often occurs following exposure to environmental stimuli previously associated with drug taking. The metabotropic glutamate receptor, mGluR5, is potentially important in this respect; it plays a central role in several forms of striatal synaptic plasticity proposed to underpin associative learning and memory processes that enable drug-paired stimuli to acquire incentive motivational properties and trigger relapse. Using cell type-specific RNA interference, we have generated a novel mouse line with a selective knock-down of mGluR5 in dopamine D1 receptor-expressing neurons. Although mutant mice self-administer cocaine, we show that reinstatement of cocaine-seeking induced by a cocaine-paired stimulus is impaired. By examining different aspects of associative learning in the mutant mice, we identify deficits in specific incentive learning processes that enable a reward-paired stimulus to directly reinforce behavior and to become attractive, thus eliciting approach toward it. Our findings show that glutamate signaling through mGluR5 located on dopamine D1 receptor-expressing neurons is necessary for incentive learning processes that contribute to cue-induced reinstatement of cocaine-seeking and which may underpin relapse in drug addiction.

Deficits in sensory-specific devaluation task performance following genetic deletions of cannabinoid (CB1) receptor.

Cannabinoid CB1 receptor is abundantly expressed throughout the CNS and is implicated in numerous physiological and behavioral functions, including appetite and feeding. In the present study, wild-type and CB1 heterozygous and homozygous knockout mice were tested on an instrumental outcome-selective devaluation task to assess changes in acquired instrumental response levels for a distinct food reward following selective satiation. Deletion of CB1 receptor, as well as reduction in CB1 expression (HET), produced deficits in outcome-selective instrumental devaluation. These results identify a critical role for CB1 receptor in the ability of animals to represent, update, and/or use sensory-specific outcome representations to alter appetitive behaviors.

An open-source pipeline for analysing changes in microglial morphology.

Changes in microglial morphology are powerful indicators of the inflammatory state of the brain. Here, we provide an open-source microglia morphology analysis pipeline that first cleans and registers images of microglia, before extracting 62 parameters describing microglial morphology. It then compares control and 'inflammation' training data and uses dimensionality reduction to generate a single metric of morphological change (an 'inflammation index'). This index can then be calculated for test data to assess inflammation, as we demonstrate by investigating the effect of short-term high-fat diet consumption in heterozygous Cx3CR1-GFP mice, finding no significant effects of diet. Our pipeline represents the first open-source microglia morphology pipeline combining semi-automated image processing and dimensionality reduction. It uses free software (ImageJ and R) and can be applied to a wide variety of experimental paradigms. We anticipate it will enable others to more easily take advantage of the powerful insights microglial morphology analysis provides.

Visual cues associated with sweet taste increase short-term eating and grab attention in healthy volunteers.

Most studies that examine responses to food cues use images of actual foods as stimuli. Since foods are rewarding in multiple ways, it then becomes difficult to try and partial out the role of the importance of different aspects of food reward. Here we aimed to evaluate the impact of novel visual cues specifically associated with the immediate sensory reward from a liked sweet taste. In the training phase, one visual cue (CSsweet) was associated with the experience of sweet taste (10%sucrose) and a second, control cue (CSneutral) with a neutral taste (artificial saliva) using a disguised training procedure. In Experiment 1, participants (n = 45) were given an ad libitum snack intake test 30 min post-training, either labelled with CSsweet or CSneutral. Total caloric consumption was significantly higher in the CSsweet (650 ± 47 kcal) than CSneutral (477 ± 45 kcal) condition, but ratings of liking for the snacks did not differ significantly between conditions. In Experiment 2, participants (n = 80) exhibited an overall attentional bias (22.1 ± 9.9 ms) for the CSsweet relative to CSneutral cue (assessed using a dot-probe task), however rated liking for the CSsweet did not change significantly after cue-sweet training. Likewise, measures of expected satiety for drinks labelled with CSsweet did not differ significantly from CSneutral. Overall these two experiments provide evidence that associations between neutral visual cues and the experience of a liked sweet taste leads to cue-potentiated eating in the presence of the CSsweet cue. With no evidence that cue-sweet training altered rated liking for the visual cues, and in keeping with extant literature on the dissociation of hedonic and rewarding properties of food rewards, we propose this potentiation effect to reflect increased incentive salience.

Reward sensitivity: issues of measurement, and achieving consilience between human and animal phenotypes.

Reward is a concept fundamental to discussions of drug abuse and addiction. The idea that altered sensitivity to either drug-reward, or to rewards in general, contributes to, or results from, drug-taking is a common theme in several theories of addiction. However, the concept of reward is problematic in that it is used to refer to apparently different behavioural phenomena, and even to diverse neurobiological processes (reward pathways). Whether these different phenomena are different behavioural expressions of a common underlying process is not established, and much research suggests that there may be only loose relationships among different aspects of reward. Measures of rewarding effects of drugs in humans often depend upon subjective reports. In animal studies, such insights are not available, and behavioural measures must be relied upon to infer rewarding effects of drugs or other events. In such animal studies, but also in many human methods established to objectify measures of reward, many other factors contribute to the behaviour being studied. For that reason, studying the biological (including genetic) bases of performance of tasks that ostensibly measure reward cannot provide unequivocal answers. The current overview outlines the strengths and weaknesses of current approaches that hinder the conciliation of cross-species studies of the genetics of reward sensitivity and the dysregulation of reward processes by drugs of abuse. Some suggestions are made as to how human and animal studies may be made to address more closely homologous behaviours, even if those processes are only partly able to isolate 'reward' from other factors contributing to behavioural output.

Drunk, dangerous and delusional: how legal concept-creep risks overcriminalization.

BACKGROUND: In the recent case of R v. Taj, the Court of Appeal of England and Wales upheld the conviction of a defendant who, in a psychotic delusional state, mistook his non-threatening victim to be a terrorist, violently attacking him. The law typically allows honest mistakes (even if unreasonable) as a basis for self-defence (in this case the defence of others). However, because Taj's delusions were found by the court to have been caused by voluntary alcohol consumption, special legal (prior-fault) intoxication rules were applied to block his defence; Taj was convicted and sentenced to 19 years imprisonment for attempted murder. ARGUMENT: We focus here on the simple question-what does it mean to be intoxicated? On the facts, Taj did not have drugs active in his system at the time of the attack, but the court nonetheless insisted that Taj's delusional mistake was 'attributable to intoxication'; namely, to drink- and drug-taking in the previous days and weeks. This extended conception of intoxication was questionably distinguished from psychosis induced by withdrawal. Furthermore, the court was unreceptive to evidence of a long-standing, underlying mental health disorder. We argue that the court's expanded view of intoxication is problematic, in that intoxication-induced psychosis cannot be sharply distinguished from other causes such as mental disorders; and even if it could be distinguished, it should not give rise to blame and punishment in the same way as does conduct induced by chemically active intoxicants ('drug-on-board'). CONCLUSION: The courts' expansion of the definition of intoxication is both legally and forensically problematic, introducing legal vagaries where the clinical science is already vague; and with intoxication frequently interlocking with historic intoxication and secondary or comorbid mental health conditions, the decision risks inappropriately and/or over-criminalizing defendants.

Pavlovian influences on goal-directed behavior in mice: the role of cue-reinforcer relations.

Two experiments refined procedures to study Pavlovian influences on goal-directed behavior in mice and studied the effects of CS-US relations in Pavlovian-instrumental interactions. Independent groups of mice underwent Pavlovian training to associate either a 10-sec or 2-min auditory stimulus (CS) with reward. We next assessed the ability of the response-contingent CS presentations to reinforce novel instrumental responding (conditioned reinforcement; CRf) or the ability of noncontingent CS presentations to increase ongoing instrumental responding (Pavlovian-instrumental transfer; PIT). Whereas 10-sec training conditions produced strong CRf (and no PIT), 2-min training conditions produced robust PIT (but no CRf).

Reward Devaluation Attenuates Cue-Evoked Sucrose Seeking and Is Associated with the Elimination of Excitability Differences between Ensemble and Non-ensemble Neurons in the Nucleus Accumbens.

Animals must learn relationships between foods and the environmental cues that predict their availability for survival. Such cue-food associations are encoded in sparse sets of neurons or "neuronal ensembles" in the nucleus accumbens (NAc). For these ensemble-encoded, cue-controlled appetitive responses to remain adaptive, they must allow for their dynamic updating depending on acute changes in internal states such as physiological hunger or the perceived desirability of food. However, how these neuronal ensembles are recruited and physiologically modified following the update of such learned associations is unclear. To investigate this, we examined the effects of devaluation on ensemble plasticity at the levels of recruitment, intrinsic excitability, and synaptic physiology in sucrose-conditioned Fos-GFP mice that express green fluorescent protein (GFP) in recently activated neurons. Neuronal ensemble activation patterns and their physiology were examined using immunohistochemistry and slice electrophysiology, respectively. Reward-specific devaluation following 4 d of ad libitum sucrose consumption, but not general caloric devaluation, attenuated cue-evoked sucrose seeking. This suggests that changes in the hedonic and/or incentive value of sucrose, and not caloric need, drove this behavior. Moreover, devaluation attenuated the size of the neuronal ensemble recruited by the cue in the NAc shell. Finally, it eliminated the relative enhanced excitability of ensemble (GFP+) neurons against non-ensemble (GFP-) neurons observed under non-devalued conditions, and did not induce any ensemble-specific changes in excitatory synaptic physiology. Our findings provide new insights into neuronal ensemble mechanisms that underlie the changes in the incentive and/or hedonic impact of cues that support adaptive food seeking.

A selective role for neuronal activity regulated pentraxin in the processing of sensory-specific incentive value.

Neuronal activity regulated pentraxin (Narp) is a secreted neuronal product which clusters AMPA receptors and regulates excitatory synaptogenesis. Although Narp is selectively enriched in brain, its role in behavior is not known. As Narp is expressed prominently in limbic regions, we examined whether Narp deletion affects performance on tasks used to assess motivational consequences of food-rewarded learning. Narp knock-out (KO) mice were unimpaired in learning simple pavlovian discriminations, instrumental lever pressing, and in acquisition of at least two aspects of pavlovian incentive learning, conditioned reinforcement and pavlovian-instrumental transfer. In contrast, Narp deletion resulted in a substantial deficit in the ability to use specific outcome expectancies to modulate instrumental performance in a devaluation task. In this task, mice were trained to respond on two levers for two different rewards. After training, mice were prefed with one of the two rewards, devaluing it. Responding on both levers was then assessed in extinction. Whereas control mice showed a significant preference in responding on the lever associated with the nondevalued reward, Narp KO mice responded equally on both levers, failing to suppress responding on the lever associated with the devalued reward. Both groups consumed more of the nondevalued reward in a subsequent choice test, indicating Narp KO mice could distinguish between the rewards themselves. These data suggest Narp has a selective role in processing sensory-specific information necessary for appropriate devaluation performance, but not in general motivational effects of reward-predictive cues on performance.

Context-specific sensitization of cocaine-induced locomotor activity and associated neuronal ensembles in rat nucleus accumbens.

Repeated cocaine administration to rats outside their home cage induces behavioral sensitization that is strongly modulated by the drug administration environment. We hypothesized that stimuli in the drug administration environment activate specific sets of striatal neurons, called neuronal ensembles, for further cocaine-enhanced activation, and that repeated activation of these neuronal ensembles underlies context-specific sensitization. In the present study, we repeatedly administered cocaine or saline to rats on alternate days in two distinct environments outside the home cage, one paired with cocaine and the other with saline. On test day, cocaine challenge injections in the cocaine-paired environment produced strongly enhanced levels of locomotor activity, while cocaine challenge injections in the saline-paired environment did not. The corresponding record of past neuronal activation in nucleus accumbens and caudate-putamen during repeated drug administration was assessed using FosB immunohistochemistry, while acute neuronal activation on test day was assessed using c-fos in situ hybridization. Although only 2% of striatal neurons were FosB labeled, 87% of these FosB-labeled neurons were co-labeled with c-fos when cocaine was injected in the cocaine-paired environment. The degree of co-labeling was significantly less following cocaine or saline challenge injections in the saline-paired environment. Furthermore, the total number of c-fos-labeled neurons was greater with either cocaine or saline challenge injections in the cocaine-paired environment than in the saline-paired environment. These findings demonstrate that the drug administration environment partly determines which striatal neuronal ensembles are activated, and to what extent, following context-specific sensitization to cocaine.

A role for alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid GluR1 phosphorylation in the modulatory effects of appetitive reward cues on goal-directed behavior.

Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor regulation has been shown to be critically involved in synaptic plasticity underlying learning and memory. This regulation occurs through trafficking of the receptor and modulation of the receptor's channel properties, both of which depend on protein phosphorylation. Using homologous recombination (knock-in) techniques we targeted two phosphorylation sites on the AMPA-GluR1 receptor: the Ser831 site, phosphorylated by calcium calmodulin-dependent protein kinase II/protein kinase C, and the Ser845 site, phosphorylated by protein kinase A. Mice with mutations that prevented phosphorylation at one or both of these sites were tested on a single-outcome Pavlovian-instrumental transfer task often used to assess the acquisition of incentive motivation by cues for food reinforcement. Mice were separately trained to associate a Pavlovian cue with food and to perform an instrumental lever-press response to earn that same reward. During a transfer test, the cue was presented while the mice were lever-pressing under extinction conditions. Whereas wild-type control mice showed substantial enhancement of lever-pressing when the cue was presented (i.e. showed Pavlovian-instrumental transfer), mice with mutations at both of these phosphorylation sites showed no evidence of such transfer. By contrast, mice with either serine site mutated alone showed normal transfer. These results suggest critical roles for GluR1 phosphorylation pathways in a form of incentive learning that can play an important part in regulating normal motivated behavior as well as maladaptive behaviors such as addiction and eating disorders.

The brain-derived neurotrophic factor receptor TrkB is critical for the acquisition but not expression of conditioned incentive value.

Stimuli paired with reward acquire incentive properties that are important for many aspects of motivated behavior, such as feeding and drug-seeking. Here we used a novel chemical-genetic strategy to determine the role of the brain-derived neurotrophic factor (BDNF) receptor TrkB, known to be critical to many aspects of neural development and plasticity, during acquisition and expression of positive incentive value by a cue paired with food. We assessed that cue's learned incentive value in a conditioned reinforcement task, in which its ability to reinforce instrumental responding later, in the absence of food itself, was examined. In TrkB (F616A) knock-in mice, TrkB kinase activity was suppressed by administering the TrkB inhibitor 1NMPP1 during the period of initial cue incentive learning only (i.e. Pavlovian training), during nose-poke conditioned reinforcement testing only, during both phases, or during neither phase. All mice acquired cue-food associations as indexed by approach responses. However, TrkB (F616A) mice that received 1NMPP1 during initial cue incentive learning failed to show conditioned reinforcement of nose-poking, regardless of their treatment in testing, whereas administration of 1NMMP1 only during the testing phase had no effect. The effects of 1NMPP1 administration were due to inhibition of TrkB(F616A), because the performance of wild-type mice was unaffected by administration of the compound during either phase. These data indicate that BDNF or NT4 signaling through TrkB receptors is required for the acquisition of positive incentive value, but is not needed for the expression of previously acquired incentive value in the reinforcement of instrumental behavior.

Narp regulates long-term aversive effects of morphine withdrawal.

Although long-lasting effects of drug withdrawal are thought to play a key role in motivating continued drug use, the mechanisms mediating this type of drug-induced plasticity are unclear. Because Narp is an immediate early gene product that is secreted at synaptic sites and binds to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, it has been implicated in mediating enduring forms of synaptic plasticity. In previous studies, the authors found that Narp is selectively induced by morphine withdrawal in the extended amygdala, a group of limbic nuclei that mediate aversive behavioral responses. Accordingly, in this study, the authors evaluate whether long-term aversive effects of morphine withdrawal are altered in Narp knockout (KO) mice. The authors found that acute physical signs of morphine withdrawal are unaffected by Narp deletion. However, Narp KO mice acquire and sustain more aversive responses to the environment conditioned with morphine withdrawal than do wild type (WT) controls. Paradoxically, Narp KO mice undergo accelerated extinction of this heightened aversive response. Taken together, these studies suggest that Narp modulates both acquisition and extinction of aversive responses to morphine withdrawal and, therefore, may regulate plasticity processes underlying drug addiction.

A necessary role for GluR1 serine 831 phosphorylation in appetitive incentive learning.

It is widely thought that regulation of post-synaptic AMPA receptors is a critical component in changes in synaptic efficacy underlying learning and memory. The regulation of AMPA receptors occurs through trafficking of the receptor and/or modulation of the receptor's channel properties and both of these processes depend on phosphorylation of the receptor. Using homologous recombination (knock-in) techniques we targeted two phosphorylation sites on the AMPA-GluR1 receptor: the CaMKII/PKC Ser 831 site and the PKA Ser 845 site. Mice with either or both of these sites mutated were then tested on an incentive learning task that assessed their ability to acquire a simple association between a cue and reward and to then use this cue as a reinforcer to guide their behavior (conditioned reinforcement). We report that, whereas WT mice showed enhanced responding for the reward-associated cue, mice with mutations of both phosphorylation sites or the Ser 831 site alone, failed to show such a conditioned reinforcement effect. By contrast, mice with only the Ser 845 site deficient showed normal CS+ reinforced responding. Thus, action at the Ser 831 phosphorylation site was necessary for normal conditioned reinforcement. Finally, the behavioral deficit was highly specific: performance on a number of other measures of motivated performance, including responding reinforced by the food itself, was unaffected by the mutations. Our findings provide novel evidence for a molecular mechanism in a form of appetitive incentive learning critical in regulating normal motivated behavior, as well as maladaptive forms such as addiction and eating disorders.

The rate of intravenous cocaine or amphetamine delivery does not influence drug-taking and drug-seeking behavior in rats.

We studied the influence of rate of intravenous infusion of cocaine or amphetamine on drug-taking and seeking behavior. First, drug-naive rats were tested for acquisition of self-administration of increasing doses of amphetamine or cocaine infused over 5 or 100 s. Second, self-administration of cocaine or amphetamine infused over 5-100 s was assessed on fixed or progressive-ratio (PR) reinforcement schedules. Finally, the ability of a single 5 or 100 s amphetamine or cocaine infusion to reinstate extinguished drug seeking was assessed. Although slower infusion rates produced a small effect on drug taking under continuous-reinforcement conditions, infusion rate did not alter drug taking on intermittent or PR reinforcement schedules, or the ability of cocaine or amphetamine to reinstate drug seeking. Taken together, our results suggest that variation in drug delivery rate over a range that we previously found alters the induction of behavioral sensitization, gene-expression and striatal dopamine activity, does not markedly alter drug-taking or seeking behavior.