The basolateral amygdala is necessary for the encoding and the expression of odor memory.

Conditioned odor avoidance (COA) results from the association between a novel odor and a delayed visceral illness. The present experiments investigated the role of the basolateral amygdala (BLA) in acquisition and retrieval of COA memory. To address this, we used the GABA(A) agonist muscimol to temporarily inactivate the BLA during COA acquisition or expression. BLA inactivation before odor-malaise pairing greatly impaired COA tested 3 d later. In contrast, muscimol microinfusion between odor and malaise spared retention. Moreover, inactivation of the BLA before pre-exposure to the odor prevented latent inhibition of COA. This suggests that neural activity in the BLA is essential for the formation of odor representation. BLA inactivation before the retrieval test also blocked COA memory expression when performed either 3 d (recent memory) or 28 d (remote memory) after acquisition. This effect was transitory as muscimol-treated animals were not different from controls during the subsequent extinction tests. Moreover, muscimol infusion in the BLA neither affected olfactory perception nor avoidance behavior, and it did not induce a state-dependent learning. Altogether, these findings suggest that neural activity in the BLA is required for the encoding and the retrieval of odor memory. Moreover, the BLA seems to play a permanent role in the expression of COA.

Differential dynamics of amino acid release in the amygdala and olfactory cortex during odor fear acquisition as revealed with simultaneous high temporal resolution microdialysis.

Although the amygdala seems to be essential to the formation and storage of fear memories, it might store only some aspects of the aversive event and facilitate the storage of more specific sensory aspects in cortical areas. We addressed the time course of amygdala and cortical activation in the context of odor fear conditioning in rats. Using high temporal resolution (1-min sampling) intracerebral microdialysis, we investigated the dynamics of glutamate and GABA fluctuations simultaneously in basolateral amygdala (BLA) and posterior piriform cortex (pPCx) during the course of the acquisition session, which consisted of six odor (conditioned stimulus)-footshock (unconditioned stimulus) pairings. In BLA, we observed a transient increase in amino acid concentrations following the first odor-shock pairing, after which concentrations returned to baseline levels or slightly below. In pPCx, transient increases were seen after each pairing and were also observed after the last odor-shock pairing, corresponding to the predicted times of anticipated trials. Furthermore, we observed that for the first pairing, the increase in BLA occurred earlier than the increase in pPCx. These data suggest that the amygdala is engaged early during acquisition and precedes the activation of the olfactory cortex, which is maintained until the end of the session. In addition, our data raise the challenging idea that the olfactory cortex might store certain aspects of fear conditioning related to the timing of the associations.

Critical role of insular cortex in taste but not odour aversion memory.

Conditioned odour aversion (COA) and conditioned taste aversion (CTA) result from the association of a novel odour or a novel taste with delayed visceral illness. The insular cortex (IC) is crucial for CTA memory, and the present experiments sought to determine whether the IC is required for the formation and the retrieval of COA memory as it is for CTA. We first demonstrated that ingested odour is as effective as taste for single-trial aversion learning in rats conditioned in their home cage. COA, like CTA, tolerates long intervals between the ingested stimuli and the illness and is long-lasting. Transient inactivation of the IC during acquisition spared COA whereas it greatly impaired CTA. Similarly, blockade of protein synthesis in IC did not affect COA but prevented CTA consolidation. Moreover, IC inactivation before retrieval tests did not interfere with COA memory expression when performed either 2 days (recent memory) or 36 days after acquisition (remote memory). Similar IC inactivation impaired the retrieval of either recent or remote CTA memory. Altogether these findings indicate that the IC is not necessary for aversive odour memory whereas it is essential for acquisition, consolidation and retrieval of aversive taste memory. We propose that the chemosensory stimulations modulate IC recruitment during the formation and the retrieval of food aversive memory.

Long-term effects of infant learning on adult conditioned odor aversion are determined by the last preweaning experience.

We recently showed that odorizing mother's nipples from birth to weaning attenuated adult conditioned odor aversion (COA). The present study evaluated whether shorter durations of preweaning olfactory experiences could induce similar long-term effects. We first showed that late preweaning odorization (PN13-PN25) impaired adult COA similarly to odorization from birth to weaning (PN0-PN25) whereas early odorization (PN0-PN12) had no effect on adult COA. As early odorization was followed by an odorless suckling period, we evaluated whether this odorless suckling could have interfered with early associative learning. We therefore weaned the animals either immediately after early odorization or 7 days later. Early odorization (PN0-PN18) followed by late odorless suckling (PN19-PN25) had no effect on adult COA. However, pups with early odorization (PN0-PN18) but without late odorless suckling (weaned at PN18) showed attenuated COA. These results support the hypothesis that interference between early and late preweaning experiences with the mother determines the long-term impact on adult COA.

Neonatal odor-shock conditioning alters the neural network involved in odor fear learning at adulthood.

Adult learning and memory functions are strongly dependent on neonatal experiences. We recently showed that neonatal odor-shock learning attenuates later life odor fear conditioning and amygdala activity. In the present work we investigated whether changes observed in adults can also be observed in other structures normally involved, namely olfactory cortical areas. For this, pups were trained daily from postnatal (PN) 8 to 12 in an odor-shock paradigm, and retrained at adulthood in the same task. (14)C 2-DG autoradiographic brain mapping was used to measure training-related activation in amygdala cortical nucleus (CoA), anterior (aPCx), and posterior (pPCx) piriform cortex. In addition, field potentials induced in the three sites in response to paired-pulse stimulation of the olfactory bulb were recorded in order to assess short-term inhibition and facilitation in these structures. Attenuated adult fear learning was accompanied by a deficit in 2-DG activation in CoA and pPCx. Moreover, electrophysiological recordings revealed that, in these sites, the level of inhibition was lower than in control animals. These data indicate that early life odor-shock learning produces changes throughout structures of the adult learning circuit that are independent, at least in part, from those involved in infant learning. Moreover, these enduring effects were influenced by the contingency of the infant experience since paired odor-shock produced greater disruption of adult learning and its supporting neural pathway than unpaired presentations. These results suggest that some enduring effects of early life experience are potentiated by contingency and extend beyond brain areas involved in infant learning.

Increased brain glucocorticoid actions following social defeat in rats facilitates the long-term establishment of social subordination.

Social rank is frequently established through aggressive encounters between new conspecifics. Despite increasing evidence suggesting that social rank is critical for the well-being of both humans and animals, knowledge about the factors influencing social rank remain scarce. Stress was previously shown to affect the establishment and maintenance of social hierarchies in rats. Likewise, increasing systemic corticosterone levels post-encounter in the emerging subordinate rat facilitates the long-term establishment of social subordination. Here, we investigated whether central corticosterone actions are sufficient to mediate this effect. Our data shows that, indeed, an intracerebroventricular corticosterone injection given to the emerging subordinate rat facilitates the long-term maintenance of the subordinate rank. Next, we attempted to identify a particular brain region in which enhancement of corticosterone actions could be sufficient to exert the facilitation of a long-term maintenance in the emerging subordinate brain. However, post-encounter administration of corticosterone into the basolateral amygdala, medial amygdala, lateral septum and the nucleus accumbens, brain regions selected for their implication in social rank establishment and emotional modulation of memory, did not affect long-term social subordination. Our study highlights the involvement of intracerebral corticosterone actions on the facilitation of long-lasting subordinate behavior, likely by having a modulatory role in the neurobehavioral plasticity engaged in the shaping of social subordination.

Enduring effects of infant memories: infant odor-shock conditioning attenuates amygdala activity and adult fear conditioning.

BACKGROUND: Early life adverse experience alters adult emotional and cognitive development. Here we assess early life learning about adverse experience and its consequences on adult fear conditioning and amygdala activity. METHODS: Neonatal rats were conditioned daily from 8-12 days-old with paired odor (conditioned stimulus, CS) .5mA shock, unpaired, odor-only, or naive (no infant conditioning). In adulthood, each infant training group was divided into three adult training groups: paired, unpaired or odor-only, using either the same infant CS odor, or a novel adult CS odor without or with the infant CS present as context. Adults were cue tested for freezing (odor in novel environment), with amygdala (14)C 2-DG autoradiography and electrophysiology assessment. RESULTS: Infant paired odor-shock conditioning attenuated adult fear conditioning, but only if the same infant CS odor was used. The (14)C 2-DG activity correlated with infant paired odor-shock conditioning produced attenuated amygdala but heightened olfactory bulb activity. Electrophysiological amygdala assessment further suggests early experience causes changes in amygdala processing as revealed by increased paired-pulse facilitation in adulthood. CONCLUSIONS: This suggests some enduring effects of early life adversity (shock) are under CS control and dependent upon learning for their impact on later adult fear learning.

Involvement of CRFR1 in the Basolateral Amygdala in the Immediate Fear Extinction Deficit.

Several animal and clinical studies have highlighted the ineffectiveness of fear extinction sessions delivered shortly after trauma exposure. This phenomenon, termed the immediate extinction deficit, refers to situations in which extinction programs applied shortly after fear conditioning may result in the reduction of fear behaviors (in rodents, frequently measured as freezing responses to the conditioned cue) during extinction training, but failure to consolidate this reduction in the long term. The molecular mechanisms driving this immediate extinction resistance remain unclear. Here we present evidence for the involvement of the corticotropin releasing factor (CRF) system in the basolateral amygdala (BLA) in male Wistar rats. Intra-BLA microinfusion of the CRFR1 antagonist NBI30775 enhances extinction recall, whereas administration of the CRF agonist CRF6-33 before delayed extinction disrupts recall of extinction. We link the immediate fear extinction deficit with dephosphorylation of GluA1 glutamate receptors at Ser845 and enhanced activity of the protein phosphatase calcineurin in the BLA. Their reversal after treatment with the CRFR1 antagonist indicates their dependence on CRFR1 actions. These findings can have important implications for the improvement of therapeutic approaches to trauma, as well as furthering our understanding of the neurobiological mechanisms underlying fear-related disorders.

Evidence for a role of oxytocin receptors in the long-term establishment of dominance hierarchies.

Exposure to stress can affect the establishment of dominance hierarchies. In our model, a social hierarchy established by two male rats during a first encounter is not maintained 1 week later. If one of the two rats is stressed, the stressed rat becomes subordinate and the hierarchy that is formed is maintained. In this study, we investigated the changes in the expression of oxytocin (Otr) and vasopressin (V1aR) receptor genes in the medial amygdala (MeA) and the lateral septum (LS) in the hours following hierarchy establishment under both stressed and basal conditions. We found that the potentiation of a social hierarchy induced by stress is accompanied by social status- and region-specific changes in the expression of Otr mRNA in the MeA 3 h after the social encounter. At this time point, no evidence was found for the regulation of V1aR mRNA in any of the brain regions examined. Results from pharmacological experiments involving the microinfusion of a specific OTR antagonist immediately after the acquisition of a subordinate status under basal, non-stress conditions suggested a role for this receptor in the MeA on the long-term establishment of the subordinate status. Altogether, these findings highlight a role for the oxytocinergic system in the mechanisms through which stress facilitates the long-term establishment of a social hierarchy.

Adult depression-like behavior, amygdala and olfactory cortex functions are restored by odor previously paired with shock during infant's sensitive period attachment learning.

Maltreatment from the caregiver induces vulnerability to later life psychopathologies, yet attraction and comfort is sometimes provided by cues associated with early life maltreatment. We used a rat model of early life maltreatment with odor-0.5 mA shock conditioning to produce depressive-like behaviors and questioned whether stimuli associated with maltreatment would restore emotional neurobehavioral function to control levels. Pups received daily novel odor-0.5 mA shock conditioning from postnatal day 8 to 12. This procedure produces a new maternal odor that controls pups' attachment behaviors. In adulthood, either with or without the infant odor, animals received a Forced Swim Test, Sucrose Preference Test or assessment of amygdala and olfactory system functioning using field potential signal evoked by olfactory bulb paired-pulse electrical stimulation. Following neonatal odor-shock pairings, but not unpaired controls, adults without the odor present showed increased depression-like behavior in the Forced Swim Test and Sucrose Preference Test and a deficit in paired-pulse inhibition in amygdala and piriform (olfactory) cortex. All effects were brought to control levels when the infant conditioned odor was presented during behavioral and neural tests. The ability of cues associated with early life maltreatment to normalize behavior and amygdala activity suggests these cues provide adaptive value in adulthood.

Rearing with artificially scented mothers attenuates conditioned odor aversion in adulthood but not its amygdala dependency.

The aim of the present study was to investigate whether neonatal odor experience associated with the mother affects food avoidance learning and basolateral amygdala (BLA) involvement in adulthood. Odorization of mother's nipples with banana or almond solutions from birth to weaning resulted in an impairment at adulthood of conditioned odor aversion (COA). These effects were specific to the early-experienced odor since no deficit was observed for COA to a novel odor (Experiment 1). In contrast, mere exposure to an odor in the home cage instead of on mother's nipples induced no deficit in COA at adulthood (Experiment 2). Finally, transitory inactivation of the BLA during COA acquisition in adult animals impaired the normal COA of naïve animals but also the attenuated COA of rats with early odor experience on the mother (Experiment 3). These results demonstrate that neonatal odor experience associated with the mother promotes the acquisition of appetitive memories which can interfere with food avoidance learning in adulthood. They also suggest that this early experience did not modify the BLA involvement in learned aversion.

Olfactory fear conditioning induces field potential potentiation in rat olfactory cortex and amygdala.

The widely used Pavlovian fear-conditioning paradigms used for studying the neurobiology of learning and memory have mainly used auditory cues as conditioned stimuli (CS). The present work assessed the neural network involved in olfactory fear conditioning, using olfactory bulb stimulation-induced field potential signal (EFP) as a marker of plasticity in the olfactory pathway. Training consisted of a single training session including six pairings of an odor CS with a mild foot-shock unconditioned stimulus (US). Twenty-four hours later, the animals were tested for retention of the CS as assessed by the amount of freezing exhibited in the presence of the learned odor. Behavioral data showed that trained animals exhibited a significantly higher level of freezing in response to the CS than control animals. In the same animals, EFPs were recorded in parallel in the anterior piriform cortex (aPC), posterior piriform cortex (pPC), cortical nucleus of the amygdala (CoA), and basolateral nucleus of the amygdala (BLA) following electrical stimulation of the olfactory bulb. Specifically, EFPs recorded before (baseline) and after (during the retention test) training revealed that trained animals exhibited a lasting increase (present before and during presentation of the CS) in EFP amplitude in CoA, which is the first amygdaloid target of olfactory information. In addition, a transient increase was observed in pPC and BLA during presentation of the CS. These data indicate that the olfactory and auditory fear-conditioning neural networks have both similarities and differences, and suggest that the fear-related behaviors in each sensory system may have at least some distinct characteristics.