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International audience; 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.

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. 14C 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.

International audience

National audience; Dans notre vie quotidienne, nous aimons sentir les « bonnes odeurs » et, par tradition nationale gourmande, nous cultivons volontiers « le goût des bonnes choses ». Mais, quand nous sollicitons notre odorat et notre goût, nous ne nous posons guère de questions sur le fonctionnement de ces sens, pourvu que le plaisir soit au rendez-vous. On méconnaît l'importance et la signification des signaux chimiques présents dans l'environnement et leur influence sur les comportements sociaux, sexuels et alimentaires (des comportements essentiels à la survie des individus et des espèces, depuis les insectes jusqu'à l'homme). Par ailleurs, il est assez paradoxal de voir que l'agroalimentaire et la parfumerie-cosmétique, deux secteurs importants de l'économie française dont l'activité est largement basée sur « le bon goût français », ne sont pas soutenus par un effort de recherche proportionnel à leur place économique. C'est pourquoi, près de vingt ans après la découverte des récepteurs olfactifs, consacrée par l'attribution du prix Nobel de physiologie et médecine à Linda Buck et Richard Axel, nous avons souhaité faire le bilan des apports de la biologie moléculaire et de la neurobiologie expérimentale dans le domaine des sens olfactif et gustatif. Cet ouvrage s'adresse prioritairement aux étudiants, aux chercheurs, aux médecins, mais aussi aux industriels de l'agroalimentaire et de la parfumerie. Il offre un large exposé des résultats scientifiques de ces dernières années ainsi qu'une sélection d'applications actuelles et futures.

Dans notre vie quotidienne, nous aimons sentir les « bonnes odeurs » et, par tradition nationale gourmande, nous cultivons volontiers « le goût des bonnes choses ». Mais, quand nous sollicitons notre odorat et notre goût, nous ne nous posons guère de questions sur le fonctionnement de ces sens, pourvu que le plaisir soit au rendez-vous. On méconnaît l'importance et la signification des signaux chimiques présents dans l'environnement et leur influence sur les comportements sociaux, sexuels et alimentaires (des comportements essentiels à la survie des individus et des espèces, depuis les insectes jusqu'à l'homme). Par ailleurs, il est assez paradoxal de voir que l'agroalimentaire et la parfumerie-cosmétique, deux secteurs importants de l'économie française dont l'activité est largement basée sur « le bon goût français », ne sont pas soutenus par un effort de recherche proportionnel à leur place économique. C'est pourquoi, près de vingt ans après la découverte des récepteurs olfactifs, consacrée par l'attribution du prix Nobel de physiologie et médecine à Linda Buck et Richard Axel, nous avons souhaité faire le bilan des apports de la biologie moléculaire et de la neurobiologie expérimentale dans le domaine des sens olfactif et gustatif. Cet ouvrage s'adresse prioritairement aux étudiants, aux chercheurs, aux médecins, mais aussi aux industriels de l'agroalimentaire et de la parfumerie. Il offre un large exposé des résultats scientifiques de ces dernières années ainsi qu'une sélection d'applications actuelles et futures.

International audience; The evoked potential recorded in the rat piriform cortex in response to electrical stimulation of the olfactory bulb is composed of an early component occasionally followed by a late component (60-70 ms). We previously showed that the late component occurrence was enhanced following an olfactory learning. In the present study carried out in naive rats, we investigated the precise conditions of induction of this late component, and its spatiotemporal distribution along the olfactory pathways. In the anaesthetized rat, a stimulating electrode was implanted in the olfactory bulb. Four recording electrodes were positioned, respectively, in the olfactory bulb, the anterior and posterior parts of the piriform cortex, and the entorhinal cortex. Simultaneous recording of signals evoked in the four sampled structures in response to stimulation of the olfactory bulb revealed that the late component was detected in anterior and posterior piriform cortex as well as in entorhinal cortex, but not in the olfactory bulb. The late component occurred reliably for a narrow range of low intensities of stimulation delivered at frequencies not exceeding 1 Hz. Comparison of late component amplitude and latency across the different recorded sites showed that this component appeared first and with the greatest amplitude in the posterior piriform cortex. In addition to showing a functional dissociation between anterior and posterior parts of the piriform cortex, these data suggest that the posterior piriform cortex could be the locus of generation of this late high amplitude synchronized activity, which would then propagate to the neighbouring regions.

International audience; The piriform cortex (PCx) has a potential role in storage and recall of olfactory information. This study is a first extensive investigation of the spatiotemporal distribution of activity in the PCx induced by learned sensory inputs following conditioning. In a conditioned group, rats chronically implanted with four electrodes in the olfactory bulb were trained to associate the electrical stimulation of a given bulbar electrode with a positive reinforcement, while stimulation of a different electrode predicted a negative reinforcement. In a familiarized group, rats received the same protocol of daily electrical stimulation with no associated reinforcement. At the end of the conditioning or familiarization episode, activity evoked in the PCx was optically mapped using a 144 photodiode array. In the anaesthetized rats, PCx maps were recorded in response to stimulation of each of the four bulbar electrodes using either high (0.5-1 mA) or low (0.1 mA) test current intensities. Low intensity stimulation revealed that conditioning selectively enhanced the probability of occurrence of a signal composed of a single late (56-73 ms) component which occurred almost simultaneously on a large PCx area. In the conditioned group, high intensity stimulation through either of the four electrodes revealed a potentiation of the early (17-30 ms) disynaptic component of the PCx response in the most posterior part of the PCx as well as a homogeneous increase of the late (39-52 ms) component spread over the PCx areas. These data suggest that learning induces synaptic changes at different nodes of the PCx circuitry.