Adaptive Responding to Stimulus-Outcome Associations Requires Noradrenergic Transmission in the Medial Prefrontal Cortex.

A dynamic environment, such as the one we inhabit, requires organisms to continuously update their knowledge of the setting. While the prefrontal cortex is recognized for its pivotal role in regulating such adaptive behavior, the specific contribution of each prefrontal area remains elusive. In the current work, we investigated the direct involvement of two major prefrontal subregions, the medial prefrontal cortex (mPFC, A32D + A32V) and the orbitofrontal cortex (OFC, VO + LO), in updating pavlovian stimulus-outcome (S-O) associations following contingency degradation in male rats. Specifically, animals had to learn that a particular cue, previously fully predicting the delivery of a specific reward, was no longer a reliable predictor. First, we found that chemogenetic inhibition of mPFC, but not of OFC, neurons altered the rats' ability to adaptively respond to degraded and non-degraded cues. Next, given the growing evidence pointing at noradrenaline (NA) as a main neuromodulator of adaptive behavior, we decided to investigate the possible involvement of NA projections to the two subregions in this higher-order cognitive process. Employing a pair of novel retrograde vectors, we traced NA projections from the locus ceruleus (LC) to both structures and observed an equivalent yet relatively segregated amount of inputs. Then, we showed that chemogenetic inhibition of NA projections to the mPFC, but not to the OFC, also impaired the rats' ability to adaptively respond to the degradation procedure. Altogether, our findings provide important evidence of functional parcellation within the prefrontal cortex and point at mPFC NA as key for updating pavlovian S-O associations.

Regulation of reinforcement learning parameters captures long-term changes in rat behaviour.

In uncertain environments in which resources fluctuate continuously, animals must permanently decide whether to stabilise learning and exploit what they currently believe to be their best option, or instead explore potential alternatives and learn fast from new observations. While such a trade-off has been extensively studied in pretrained animals facing non-stationary decision-making tasks, it is yet unknown how they progressively tune it while learning the task structure during pretraining. Here, we compared the ability of different computational models to account for long-term changes in the behaviour of 24 rats while they learned to choose a rewarded lever in a three-armed bandit task across 24 days of pretraining. We found that the day-by-day evolution of rat performance and win-shift tendency revealed a progressive stabilisation of the way they regulated reinforcement learning parameters. We successfully captured these behavioural adaptations using a meta-learning model in which either the learning rate or the inverse temperature was controlled by the average reward rate.

Chemogenetic silencing of hippocampus and amygdala reveals a double dissociation in periadolescent obesogenic diet-induced memory alterations.

In addition to numerous metabolic comorbidities, obesity is associated with several adverse neurobiological outcomes, especially learning and memory alterations. Obesity prevalence is rising dramatically in youth and is persisting in adulthood. This is especially worrying since adolescence is a crucial period for the maturation of certain brain regions playing a central role in memory processes such as the hippocampus and the amygdala. We previously showed that periadolescent, but not adult, exposure to obesogenic high-fat diet (HFD) had opposite effects on hippocampus- and amygdala-dependent memory, impairing the former and enhancing the latter. However, the causal role of these two brain regions in periadolescent HFD-induced memory alterations remains unclear. Here, we first showed that periadolescent HFD induced long-term, but not short-term, object recognition memory deficits, specifically when rats were exposed to a novel context. Using chemogenetic approaches to inhibit targeted brain regions, we then demonstrated that recognition memory deficits are dependent on the activity of the ventral hippocampus, but not the basolateral amygdala. On the contrary, the HFD- induced enhancement of conditioned odor aversion specifically requires amygdala activity. Taken together, these findings suggest that HFD consumption throughout adolescence impairs long-term object recognition memory through alterations of ventral hippocampal activity during memory acquisition. Moreover, these results further highlight the bidirectional effects of adolescent HFD on hippocampal and amygdala functions.

Manipulating the revision of reward value during the intertrial interval increases sign tracking and dopamine release.

Recent computational models of sign tracking (ST) and goal tracking (GT) have accounted for observations that dopamine (DA) is not necessary for all forms of learning and have provided a set of predictions to further their validity. Among these, a central prediction is that manipulating the intertrial interval (ITI) during autoshaping should change the relative ST-GT proportion as well as DA phasic responses. Here, we tested these predictions and found that lengthening the ITI increased ST, i.e., behavioral engagement with conditioned stimuli (CS) and cue-induced phasic DA release. Importantly, DA release was also present at the time of reward delivery, even after learning, and DA release was correlated with time spent in the food cup during the ITI. During conditioning with shorter ITIs, GT was prominent (i.e., engagement with food cup), and DA release responded to the CS while being absent at the time of reward delivery after learning. Hence, shorter ITIs restored the classical DA reward prediction error (RPE) pattern. These results validate the computational hypotheses, opening new perspectives on the understanding of individual differences in Pavlovian conditioning and DA signaling.

Insular and Ventrolateral Orbitofrontal Cortices Differentially Contribute to Goal-Directed Behavior in Rodents.

The medial prefrontal cortex (mPFC) has long been considered a critical site in action control. However, recent evidence indicates that the contribution of cortical areas to goal-directed behavior likely extends beyond mPFC. Here, we examine the function of both insular (IC) and ventrolateral orbitofrontal (vlOFC) cortices in action-dependent learning. We used chemogenetics to study the consequences of IC or vlOFC inhibition on acquisition and performance of instrumental actions using the outcome devaluation task. Rats first learned to associate actions with desirable outcomes. Then, one of these outcomes was devalued and we assessed the rats' choice between the 2 actions. Typically, rats will bias their selection towards the action that delivers the still valued outcome. We show that chemogenetic-induced inhibition of IC during choice abolishes goal-directed control whereas inhibition during instrumental acquisition is without effect. IC is therefore necessary for action selection based on current outcome value. By contrast, vlOFC inhibition during acquisition or the choice test impaired goal-directed behavior but only following a shift in the instrumental contingencies. Our results provide clear evidence that vlOFC plays a critical role in action-dependent learning, which challenges the popular idea that this region of OFC is exclusively involved in stimulus-dependent behaviors.

The effect of high-fat diet consumption on appetitive instrumental behavior in rats.

Evidence now indicates that the chronic consumption of high-calorie foods, such as a high-fat diet (HFD), is associated with impaired control over food-seeking, yet the extent of this alteration is not fully understood. Using different reinforcement schedules, we evaluated whether HFD intake from weaning to adulthood modifies instrumental responding and induces a shift from goal-directed actions to habitual responding. We first observed reduced instrumental performance and motivation for a food reward in HFD-fed rats trained under schedules of reinforcement that facilitate habitual responding [Random Interval (RI)]. However, this deficit was alleviated if rats trained under RI were subsequently trained with reinforcement schedules that promote goal-directed strategies [Random Ratio (RR)]. Using an outcome devaluation procedure, we then demonstrated that consumption of a HFD promoted habitual behavior in rats trained under RI but not RR schedules. Finally, extended HFD exposure did not interfere with the ability of RR training to overcome impaired RI instrumental performance and to favor goal-directed behavior. These results indicate that chronic consumption of a HFD changes the co-ordination of goal-directed actions and habits and that alteration of food-seeking may be reversed under particular behavioral conditions.

Flexible Use of Predictive Cues beyond the Orbitofrontal Cortex: Role of the Submedius Thalamic Nucleus.

The orbitofrontal cortex (OFC) is known to play a crucial role in learning the consequences of specific events. However, the contribution of OFC thalamic inputs to these processes is largely unknown. Using a tract-tracing approach, we first demonstrated that the submedius nucleus (Sub) shares extensive reciprocal connections with the OFC. We then compared the effects of excitotoxic lesions of the Sub or the OFC on the ability of rats to use outcome identity to direct responding. We found that neither OFC nor Sub lesions interfered with the basic differential outcomes effect. However, more specific tests revealed that OFC rats, but not Sub rats, were disproportionally relying on the outcome, rather than on the discriminative stimulus, to guide behavior, which is consistent with the view that the OFC integrates information about predictive cues. In subsequent experiments using a Pavlovian contingency degradation procedure, we found that both OFC and Sub lesions produced a severe deficit in the ability to update Pavlovian associations. Altogether, the submedius therefore appears as a functionally relevant thalamic component in a circuit dedicated to the integration of predictive cues to guide behavior, previously conceived as essentially dependent on orbitofrontal functions. Significance statement: In the present study, we identify a largely unknown thalamic region, the submedius nucleus, as a new functionally relevant component in a circuit supporting the flexible use of predictive cues. Such abilities were previously conceived as largely dependent on the orbitofrontal cortex. Interestingly, this echoes recent findings in the field showing, in research involving an instrumental setup, an additional involvement of another thalamic nuclei, the parafascicular nucleus, when correct responding requires an element of flexibility (Bradfield et al., 2013a). Therefore, the present contribution supports the emerging view that limbic thalamic nuclei may contribute critically to adaptive responding when an element of flexibility is required after the establishment of initial learning.

Juvenile obesity enhances emotional memory and amygdala plasticity through glucocorticoids.

In addition to metabolic and cardiovascular disorders, obesity is associated with adverse cognitive and emotional outcomes. Its growing prevalence during adolescence is particularly alarming since recent evidence indicates that obesity can affect hippocampal function during this developmental period. Adolescence is a decisive period for maturation of the amygdala and the hypothalamic-pituitary-adrenal (HPA) stress axis, both required for lifelong cognitive and emotional processing. However, little data are available on the impact of obesity during adolescence on amygdala function. Herein, we therefore evaluate in rats whether juvenile high-fat diet (HFD)-induced obesity alters amygdala-dependent emotional memory and whether it depends on HPA axis deregulation. Exposure to HFD from weaning to adulthood, i.e., covering adolescence, enhances long-term emotional memories as assessed by odor-malaise and tone-shock associations. Juvenile HFD also enhances emotion-induced neuronal activation of the basolateral complex of the amygdala (BLA), which correlates with protracted plasma corticosterone release. HFD exposure restricted to adulthood does not modify all these parameters, indicating adolescence is a vulnerable period to the effects of HFD-induced obesity. Finally, exaggerated emotional memory and BLA synaptic plasticity after juvenile HFD are alleviated by a glucocorticoid receptor antagonist. Altogether, our results demonstrate that juvenile HFD alters HPA axis reactivity leading to an enhancement of amygdala-dependent synaptic and memory processes. Adolescence represents a period of increased susceptibility to the effects of diet-induced obesity on amygdala function.

Microglial activation enhances associative taste memory through purinergic modulation of glutamatergic neurotransmission.

The cerebral innate immune system is able to modulate brain functioning and cognitive processes. During activation of the cerebral innate immune system, inflammatory factors produced by microglia, such as cytokines and adenosine triphosphate (ATP), have been directly linked to modulation of glutamatergic system on one hand and learning and memory functions on the other hand. However, the cellular mechanisms by which microglial activation modulates cognitive processes are still unclear. Here, we used taste memory tasks, highly dependent on glutamatergic transmission in the insular cortex, to investigate the behavioral and cellular impacts of an inflammation restricted to this cortical area in rats. We first show that intrainsular infusion of the endotoxin lipopolysaccharide induces a local inflammation and increases glutamatergic AMPA, but not NMDA, receptor expression at the synaptic level. This cortical inflammation also enhances associative, but not incidental, taste memory through increase of glutamatergic AMPA receptor trafficking. Moreover, we demonstrate that ATP, but not proinflammatory cytokines, is responsible for inflammation-induced enhancement of both associative taste memory and AMPA receptor expression in insular cortex. In conclusion, we propose that inflammation restricted to the insular cortex enhances associative taste memory through a purinergic-dependent increase of glutamatergic AMPA receptor expression at the synapse.

Long-lasting deficits in hedonic and nucleus accumbens reactivity to sweet rewards by sugar overconsumption during adolescence.

Adolescence is a critical period characterized by major neurobiological changes. Chronic stimulation of the reward system might constitute an important factor in vulnerability to pathological development. In spite of the dramatic increase in the consumption of sweet palatable foods during adolescence in our modern societies, the long-term consequences of such exposure on brain reward processing remain poorly understood. Here, we investigated in rats the long-lasting effects of sugar overconsumption during their adolescence on their adult reactivity to the hedonic properties of sweet rewards. Adolescent rats with continuous access to 5% sucrose solution (from postnatal day 30-46) showed escalating intake. At adulthood (post-natal day 70), using two-bottle free choice tests, sucrose-exposed rats showed lower intake than non-exposed rats suggesting decreased sensitivity to the rewarding properties of sucrose. In Experiment 1, we tested their hedonic-related orofacial reactions to intraoral infusion of tasty solutions. We showed that sucrose-exposed rats presented less hedonic reactions in response to sweet tastes leaving the reactivity to water or quinine unaltered. Hence, in Experiment 2, we observed that this hedonic deficit is associated with lower c-Fos expression levels in the nucleus accumbens, a brain region known to play a central role in hedonic processing. These findings demonstrate that a history of high sucrose intake during the critical period of adolescence induces long-lasting deficits in hedonic treatment that may contribute to reward-related disorders.

Parallel inputs from the mediodorsal thalamus to the prefrontal cortex in the rat.

There is a growing interest in determining the functional contribution of thalamic inputs to cortical functions. In the context of adaptive behaviours, identifying the precise role of the mediodorsal thalamus (MD) in particular remains difficult despite the large amount of experimental data available. A better understanding of the thalamocortical connectivity of this region may help to capture its functional role. To address this issue, this study focused exclusively on the specific connections from the MD to the prefrontal cortex (PFC) by means of direct comparisons of labelling produced by single and dual injections of retrograde tracers in the different subdivisions of the PFC in the rat. We show that at least three parallel and essentially separate thalamocortical pathways originate from the MD, as follows: projections to the dorsal (1) and the ventral (2) subdivisions of the mPFC follow a mediolateral topography at the thalamic level (i.e. medial thalamic neurons target the mPFC ventrally whereas lateral thalamic neurons project dorsally), whereas a considerable innervation to the OFC (3) includes thalamic cells projecting to both the lateral and the ventral OFC subdivisions. These observations provide new insight on the functions of the MD and suggest a specific focus on each of these pathways for future functional studies.

Acquisition of specific response-outcome associations requires NMDA receptor activation in the basolateral amygdala but not in the insular cortex.

The basolateral amygdala (BLA) and the gustatory region of the insular cortex (IC) are required for the encoding and retrieval of outcome value. Here, we examined if these regions are also necessary to learn associations between actions and their outcomes. Hungry rats were first trained to press two levers for a common outcome. Next, specific response-outcome (R-O) associations were introduced such that each response now earned a distinct food outcome. Prior to each specific R-O training session, rats received a bilateral infusion of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-APV, into either the BLA or the IC. One of the two outcomes was then devalued immediately prior to a choice test. Inhibition of NMDA receptor activity in the BLA, but not the IC, during the acquisition of specific R-O associations abolished selective devaluation. These results indicate that the BLA is critical for learning the association between actions and their specific consequences.

A time course analysis of satiety-induced instrumental outcome devaluation.

Sensory-specific satiety is commonly used in studies of decision making to selectively devalue a food reward. Devaluation is reflected in an immediate reduction in the subsequent intake of the food and in the performance of actions that gain access to that food. Despite its frequent use, the lasting effects of satiety-induced devaluation on instrumental actions are unknown. Here, we examined the time course and contextual dependency of sensory-specific satiety-induced devaluation on instrumental responding and consumption. Rats were trained to perform two instrumental actions for two distinct food rewards. Then, one of the instrumental outcomes was provided ad libitum for 1 hour in separate feeding cages and the effect of this devaluation was assessed 0, 2, or 5 hours after satiation. At a delay of 0 or 2 hours, both intake and instrumental responding were sensitive to the satiety treatment. That is, rats consumed less of the devalued outcome and responded less for the devalued outcome than for the valued outcome. By contrast, after 5 hours, rats showed sensitivity to devaluation in consumption but not in instrumental responding. Strikingly, sensitivity to devaluation was restored for the instrumental response after a 5 hour delay when devaluation was performed in the instrumental context. These results indicate that, in rats, specific satiety-induced devaluation endures and is context-independent for up to 2 hours post-satiation. At longer delays, the impact of sensory-specific satiety on instrumental responding is context-dependent, suggesting that contextual cues may be required for the value of specific outcomes to control instrumental responding.

Mediodorsal but not anterior thalamic nuclei lesions impair acquisition of a conditional discrimination task.

The limbic thalamus is a heterogeneous structure with distinctive cortical connectivity. A recent review suggests that the mediodorsal thalamic nucleus (MD), unlike the anterior thalamic nuclei (ATN), may be involved in selecting relevant information in tasks relying on executive functions. We compared the effects of excitotoxic lesions of the MD or the ATN on the acquisition of a simple conditional discrimination in rats. When required to choose from two levers according to auditory or visual cues, ATN rats and sham-lesioned rats performed to the same levels and displayed similar acquisition curves. Under the same conditions, MD rats' acquisition of the task was markedly delayed. This group nevertheless attained nearly normal performances after more extensive training. Furthermore, all rats learned reversal of the original discrimination at the same rate. These results highlight functional specialization within the limbic thalamus and support the notion that MD contributes to the identification of relevant dimensions in conditional tasks during the initial stages of acquisition.

Differential role of insular cortex muscarinic and NMDA receptors in one-trial appetitive taste learning.

Our current understanding of the neurobiology of taste learning and memory has been greatly facilitated by the use of a reliable behavioural model, conditioned taste aversion (CTA). This model has revealed that the insular cortex (IC), specifically muscarinic and N-methyl-d-aspartate (NMDA) receptor activation in the IC, is critical for the formation of aversive taste memories. In contrast, current models of appetitive taste learning are less adequate, relying on the use of neophobic tastes (attenuation of neophobia) or on the integration of appetitive and aversive taste memories (latent inhibition of CTA). While these models have implicated IC muscarinic receptors, the involvement of NMDA receptors in the IC remains unclear. Here, we examined the role of both muscarinic and NMDA receptors in appetitive taste learning using a simple paradigm that is independent of neophobic and aversive components. First, we demonstrated that a single exposure to a novel taste, saccharin 0.1%, is sufficient to promote an appetitive taste memory as revealed by an increase in saccharin consumption during the second presentation. This increase was blocked by bilateral infusion in the IC of the muscarinic receptor antagonist, scopolamine. In contrast, infusion of the NMDA receptor antagonist, AP5, did not block appetitive taste learning but did abolish CTA. Therefore, common and distinct molecular substrates within the IC mediate appetitive versus aversive learning about the same taste.

Obesogenic diet induces circuit-specific memory deficits in mice.

Obesity is associated with neurocognitive dysfunction, including memory deficits. This is particularly worrisome when obesity occurs during adolescence, a maturational period for brain structures critical for cognition. In rodent models, we recently reported that memory impairments induced by obesogenic high-fat diet (HFD) intake during the periadolescent period can be reversed by chemogenetic manipulation of the ventral hippocampus (vHPC). Here, we used an intersectional viral approach in HFD-fed male mice to chemogenetically inactivate specific vHPC efferent pathways to nucleus accumbens (NAc) or medial prefrontal cortex (mPFC) during memory tasks. We first demonstrated that HFD enhanced activation of both pathways after training and that our chemogenetic approach was effective in normalizing this activation. Inactivation of the vHPC-NAc pathway rescued HFD-induced deficits in recognition but not location memory. Conversely, inactivation of the vHPC-mPFC pathway restored location but not recognition memory impairments produced by HFD. Either pathway manipulation did not affect exploration or anxiety-like behaviour. These findings suggest that HFD intake throughout adolescence impairs different types of memory through overactivation of specific hippocampal efferent pathways and that targeting these overactive pathways has therapeutic potential.

Parallel maturation of goal-directed behavior and dopaminergic systems during adolescence.

Adolescence is a crucial developmental period characterized by specific behaviors reflecting the immaturity of decision-making abilities. However, the maturation of precise cognitive processes and their neurobiological correlates at this period remain poorly understood. Here, we investigate whether a differential developmental time course of dopamine (DA) pathways during late adolescence could explain the emergence of particular executive and motivational components of goal-directed behavior. First, using a contingency degradation protocol, we demonstrate that adolescent rats display a specific deficit when the causal relationship between their actions and their consequences is changed. When the rats become adults, this deficit disappears. In contrast, actions of adolescents remain sensitive to outcome devaluation or to the influence of a pavlovian-conditioned stimulus. This aspect of cognitive maturation parallels a delayed development of the DA system, especially the mesocortical pathway involved in action adaptation to rule changes. Unlike in striatal and nucleus accumbens regions, DA fibers and DA tissue content continue to increase in the medial prefrontal cortex from juvenile to adult age. Moreover, a sustained overexpression of DA receptors is observed in the prefrontal region until the end of adolescence. These findings highlight the relationship between the emergence of specific cognitive processes, in particular the adaptation to changes in action consequences, and the delayed maturation of the mesocortical DA pathway. Similar developmental processes in humans could contribute to the adolescent vulnerability to the emergence of several psychiatric disorders characterized by decision-making deficits.

A role for anterior thalamic nuclei in affective cognition: interaction with environmental conditions.

Damage to anterior thalamic nuclei (ATN) is a well-known cause of diencephalic pathology that produces a range of cognitive deficits reminiscent of a hippocampal syndrome. Anatomical connections of the ATN also extend to cerebral areas that support affective cognition. Enriched environments promote recovery of declarative/relational memory after ATN lesions and are known to downregulate emotional behaviors. Hence, the performance of standard-housed and enriched ATN rats in a range of behavioral tasks engaging affective cognition was compared. ATN rats exhibited reduced anxiety responses in the elevated plus maze, increased activity and reduced corticosterone responses when exploring an open field, and delayed acquisition of a conditioned contextual fear response. ATN rats also exhibited reduced c-Fos and phosphorylated cAMP response element-binding protein (pCREB) immunoreactivity in the hippocampal formation and the amygdala after completion of the contextual fear test. Marked c-Fos hypoactivity and reduced pCREB levels were also evident in the granular retrosplenial cortex and, to a lesser extent, in the anterior cingulate cortex. Unlike standard-housed ATN rats, enriched ATN rats expressed virtually no fear of the conditioned context. These results show that the ATN regulate affective cognition and that damage to this region may produce markedly different behavioral effects as a function of environmental housing conditions.

Inhibition of noradrenergic signalling in rodent orbitofrontal cortex impairs the updating of goal-directed actions.

In a constantly changing environment, organisms must track the current relationship between actions and their specific consequences and use this information to guide decision-making. Such goal-directed behaviour relies on circuits involving cortical and subcortical structures. Notably, a functional heterogeneity exists within the medial prefrontal, insular, and orbitofrontal cortices (OFC) in rodents. The role of the latter in goal-directed behaviour has been debated, but recent data indicate that the ventral and lateral subregions of the OFC are needed to integrate changes in the relationships between actions and their outcomes. Neuromodulatory agents are also crucial components of prefrontal functions and behavioural flexibility might depend upon the noradrenergic modulation of the prefrontal cortex. Therefore, we assessed whether noradrenergic innervation of the OFC plays a role in updating action-outcome relationships in male rats. We used an identity-based reversal task and found that depletion or chemogenetic silencing of noradrenergic inputs within the OFC rendered rats unable to associate new outcomes with previously acquired actions. Silencing of noradrenergic inputs in the prelimbic cortex or depletion of dopaminergic inputs in the OFC did not reproduce this deficit. Together, our results suggest that noradrenergic projections to the OFC are required to update goal-directed actions.

The mediodorsal thalamus supports adaptive responding based on stimulus-outcome associations.

The ability to engage into flexible behaviors is crucial in dynamic environments. We recently showed that in addition to the well described role of the orbitofrontal cortex (OFC), its thalamic input from the submedius thalamic nucleus (Sub) also contributes to adaptive responding during Pavlovian degradation. In the present study, we examined the role of the mediodorsal thalamus (MD) which is the other main thalamic input to the OFC. To this end, we assessed the effect of both pre- and post-training MD lesions in rats performing a Pavlovian contingency degradation task. Pre-training lesions mildly impeded the establishment of stimulus-outcome associations during the initial training of Pavlovian conditioning without interfering with Pavlovian degradation training when the sensory feedback provided by the outcome rewards were available to animals. However, we found that both pre- and post-training MD lesions produced a selective impairment during a test conducted under extinction conditions, during which only current mental representation could guide behavior. Altogether, these data suggest a role for the MD in the successful encoding and representation of Pavlovian associations.