Olfaction in depressive disorders: Issues and perspectives

International audience; Research on sensorial interactions with psychiatric diseases and particularly with the depressive syndrome has mainly focused on visual or auditory processes and much less on olfaction. The depressive illness is one of the most frequent psychiatric diagnoses in the community, with approximately one in five women and one in eight men experiencing a major depressive episode during their lifetime. Although genetic, epigenetic, neuroanatomical, neurochemical, neuroendocrinological and neuroimmunological changes can be detected during depression, the etiology of depression remains partly unclear. The current explanatory models are based on two main factors, i.e. pharmacological dysfunctions and stress effects. In this way and because of strong connections between olfactory pathways and cerebral areas implied in mood regulation and emotions (i.e. the limbic system and prefrontal areas), the interactions between olfaction and depression could constitute a relevant way of research at three different levels. First, olfactory dysfunction observed in depression could serve the diagnosis and contribute to a better understanding of mechanisms implied in thymic pathologies. Published papers show a decrease of olfactory sensitivity in major depression which does not occur in bipolar or saisonal depression. Second, it has been shown that olfactory deficits could induce depressive symptoms. In this context, an animal model (olfactory bulbectomized rat) reinforces the hypothesis of the important role of olfaction in depression based on neuroanatomical and neurochemical observations. Third, several publications have demonstrated that odors can positively impact the depressive mood. Thus, a remediation by odors in depression appears to be a promising way. From several decades, the olfaction/depression interactions have been covered by a broad literature. Thus, the present review will not propose an exhaustive examination but aims to point out the most recently published papers and highlight their contributions to the understanding of olfactory processes implied in depression; Il existe une abondante littérature, depuis plusieurs décennies, sur les liens entre olfaction et dépression. La revue de la littérature proposée ici n'a donc pas vocation à être exhaustive sur les travaux publiés mais vise plutôt à mettre en exergue les études les plus récentes et leurs apports à la compréhension des mécanismes olfactifs dans la dépression. En effet, étant donné l'existence de connexions étroites entre voies olfactives et aires cérébrales impliquées dans la régulation de l'humeur et des émotions (notamment au niveau du système limbique et des aires préfrontales), l'olfaction constitue une voie de recherche intéressante et novatrice à de nombreux égards. En premier lieu, l'étude des troubles olfactifs occurrents dans la dépression peut aider au diagnostic et surtout à la compréhension des mécanismes sous-jacents aux troubles thymiques. Les travaux publiés révèlent que l'épisode dépressif caractérisé est associé à une réduction de la sensibilité olfactive, ce qui n'est pas retrouvé dans la dépression bipolaire et la dépression saisonnière. En second lieu, il a été montré que des déficits de perception des odeurs pouvaient être à l'origine de symptômes dépressifs. Les corrélats neuro-anatomiques et neurochimiques plaident assez clairement pour un effet causal de la perte olfactive sur les troubles de l'humeur en général et, dans ce contexte, un modèle animal (rat bulbectomisé) conforte l'hypothèse du rôle non négligeable de l'olfaction dans les troubles dépressifs. En troisième lieu, plusieurs travaux tendent à prouver que les odeurs peuvent potentiellement avoir un impact sur l'amélioration des états dépressifs. Une remédiation par l'utilisation d'odeurs dans les troubles dépressifs et anxieux est une voie de recherche prometteuse, notamment du fait de l'impact sur le fonctionnement neurochimique de la dépression qui semble démontré chez l'animal. Research on sensorial interactions with psychiatric diseases and particularly with the depressive syndrome has mainly focused on visual or auditory processes and much less on olfaction. The depressive illness is one of the most frequent psychiatric diagnoses in the community, with approximately one in five women and one in eight men experiencing a major depressive episode during their lifetime. Although genetic, epigenetic, neuroanatomical, neurochemical, neuroendocrinological and neuroimmunological changes can be detected during depression, the etiology of depression remains partly unclear. The current explanatory models are based on two main factors, i.e. pharmacological dysfunctions and stress effects. In this way and because of strong connections between olfactory pathways and cerebral areas implied in mood regulation and emotions (i.e. the limbic system and prefrontal areas), the interactions between olfaction and depression could constitute a relevant way of research at three different levels. First, olfactory dysfunction observed in depression could serve the diagnosis and contribute to a better understanding of mechanisms implied in thymic pathologies. Published papers show a decrease of olfac-tory sensitivity in major depression which does not occur in bipolar or saisonal depression. Second, it has been shown that olfactory deficits could induce depressive symptoms. In this context, an animal model (olfactory bulbectomized rat) reinforces the hypothesis of the important role of olfaction in * Auteur correspondant. Adresses e-mail : gerard.brand@univ-fcomte.fr (G. Brand), benoist.schaal@u-bourgogne.fr (B. Schaal). 1 depression based on neuroanatomical and neurochemical observations. Third, several publications have demonstrated that odors can positively impact the depressive mood. Thus, a remediation by odors in depression appears to be a promising way. From several decades, the olfaction/depression interactions have been covered by a broad literature. Thus, the present review will not propose an exhaustive examination but aims to point out the most recently published papers and highlight their contributions to the understanding of olfactory processes implied in depression.