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Fabien Ducrocq will defend his PhD entitled "Vulnerability of the nucleus accumbens neuronal network to developmental n-3 PUFA deficiency: consequences on the reward and motivation system" on tuesday, the 18 of december, 2018 in the amphithéâtre Broca Nouvelle Aquitaine.

His jury is composed of:

President: Dr Etienne Coutureau, Directeur de recherche, CNRS, Bordeaux, France           

Rapporteur: Pr Nathalie Ginovart, Professeure, Université de Genève, Suisse                         

Rapporteur: Dr Emmanuel Valjent, Chargé de recherche, CNRS, Montpellier, France             

Examinator: Dr Jérôme Baufreton, Directeur de recherche, CNRS, Bordeaux, France            

Examinator: Dr Elodie Fino, Chargée de recherche, CNRS, Grenoble, France                

Invited: Dr Serge Luquet, Directeur de recherche, CNRS, Paris, France                      

Thesis supervisor: Dr Pierre Trifilieff, Chargé de recherche, INRA, Bordeaux, France             

Abstract:

Various, though distinct psychiatric disorders, such as Schizophrenia, bipolar disorder or major depression are associated with a dysfunction of the reward system linked to an alteration of dopamine transmission. Furthermore, these pathologies are also accompanied by changes in lipid metabolism and in particular a decrease in the brain content of docosahexaenoic acid (DHA), the main n-3 polyunsaturated fatty acid (PUFA) in the nervous system. However, despite that n-3 PUFA supplementation seems to improve or prevent some psychiatric symptoms, these results are still controversial and the implication of brain lipid composition in the etiology of psychiatric endophenotypes has been overlooked. The aim of this study was to investigate a potential causal link between n-3 PUFA deficiency and common neurobiological and behavioral endophenotypes of psychiatric disorders. In particular, the hypothesis was that n-3 PUFA deficiency could lead to dysfunctions of mesolimbic dopamine transmission and associated behaviors. 

Using operant conditioning tasks in mice, we showed that developmental n-3 PUFA deficiency leads to a motivational deficit at adulthood, that is partially reversed by n-3 PUFA supplementation starting at birth, but not at weaning. This motivational deficit was associated with an alteration of dopaminergic transmission as revealed by the reduced sensitivity to the psychostimulant amphetamine. More precisely, we showed that n-3 PUFA deficiency leads to alterations in electrophysiological properties of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), main actors in motivational processes. Indeed, MSNs from the direct pathway (dMSNs), that express dopaminergic D1 receptors, displayed a decrease in neuronal excitability in parallel with an increase of inhibitory input onto these neurons. These alterations were reversed by the dopaminergic D2 receptor (D2R) agonist quinpirole. These data led us to hypothesize that the decreased dMSN excitability induced by n-3 PUFA deficiency could result from an increase of the inhibitory input of MSNs from the indirect pathway (iMSNs that expresses D2R), called lateral inhibition. Accordingly, using a transgenic approach that allows the expression of the fatty acid desaturase FAT1 in a cre-dependent manner, we showed that rescuing appropriate PUFA levels in D2R-expressing neurons selectively (including iMSNs), was sufficient to reverse alterations in electrophysiological properties of MSNs induced by n-3 PUFA deficiency. Moreover, the selective expression of FAT1 in D2-expressing neurons – but not in D1-expressing neurons – reversed the motivational deficit observed in n-3 PUFA deficient mice.

We demonstrated the existence of a causal link between modifications in PUFA levels in a discrete neuronal population and behavioral alterations. Overall, this study suggests that altered PUFA levels, observed in some psychiatric disorders, could directly participate in the development of symptoms such as avolition or apathy.