Impact of Mitofusin 2 in the Nucleus Accumbens on motivated behavior and underlying neurobiological mechanisms

The nucleus accumbens (NAc) is part of the ventral striatum and plays a major role in motivation and goal-directed behaviour. Increasing evidence implicates impairments in accumbal function in anxiety and depression, two conditions that are commonly accompanied by motivational impairments. Recent research implicates mitochondria in the aetiology of psychiatric conditions. Data from our lab has identified lower mitochondrial activity in the NAc of outbred Wistar rats characterized by high anxiety (HA) compared to low anxious (LA) individuals. Cellular and functional characterization identified decreased dendritic arborization of medium spiny neurons (MSNs) as well as reduced excitatory inputs into MSNs, reduced maximal mitochondrial respiratory capacity and ATP production in the NAc of HA rats, along with reduced levels of the mitochondrial fusion protein mitofusin 2 (Mfn2). Consistently, viral-induced downregulation of the mitochondrial gene mfn2 in the D1-expressing neurons in the NAc of C57BL/6 mice mimicked the cellular and functional characteristics found in HA rats, along with increased anxiety- and depression-like behaviours. My project aims at investigating the mechanisms whereby alterations on mfn2 expression levels in NAc medium spiny neurons (MSNs) lead to behavioural disturbances. Given the role of the NAc in motivated behaviours, we focus on effort-based behaviour for both, aversively (avoidance-based) and appetitively (reward-based by operant conditioning) tasks. As to the mediating mechanisms, we will analyse how variation in NAc D1-containing MSNs (experiments including genetic manipulations in D2-MSNs will be planned to address specificity of specific results) impact: i) Circuit engagement by task performance. Relevant regions will be evaluated via c-fos mapping. A priori we plan to start focusing on a first NAc downstream target, the ventral pallidum (VP), via mini endoscope imaging. ii) Molecular determinants of dendritic atrophy and/or alterations in excitatory inputs in NAc MSNs. Ribotag and Mitotag approaches will be applied to identify molecular markers modulated by mfn2 variation in D1-expressing neurons. We will subsequently assess their role on dendritic structure and/or excitatory input modulation, and complement by behavioural analyses. Alterations in NAc synaptic inputs and outputs will be assessed by ex vivo electrophysiology coupled to optogenetics. Cre-inducible lines and/or viral vectors will be used to target NAc subregions (i.e core and shell) and cell types (i.e. D1- or 2- MSNs).In the last months, I obtained initial results from mice exposed to the forced swim test (FST) showing that mfn2 downregulation in D1-expressing neurons NAc enhances passive-coping responses (i.e., floating). This was accompanied by higher blood corticosterone levels, indicative of higher stress levels. I am currently performing ex vivo analysis of neuronal activity by cFos staining from these animals, in order to identify alterations in the engagement of relevant brain areas, including NAc, and afferent/efferent areas, such as prefrontal cortex (PFC), hippocampus (HPC), VP, ventral tegmental area (VTA), substantia nigra (SN) and the amygdala (Amy).We expect this project to offer new insights on how variation in mfn2 in specific NAc MSN subtypes affects effort-related behaviour. Our final goal is to advance progress for the identification of new targets for intervention on motivational impairments.