Multiple sclerosis (MS) is a disease of the central nervous system (CNS), where demyelination and inflammation lead to axonal degeneration and plaque formation (Reich et al., 2018). MS patients suffer from variable symptoms, and mood changes are often expressed in MS patients who suffer particularly from depression and anxiety (Boeschoten et al., 2017). Monoamines including 5-hydroxytryptamine (5-HT or serotonin), dopamine, and norepinephrine are primary contributors to anxiety and depressive disorders (Shao and Zhu, 2020; Pourhamzeh et al., 2022). Research on MS pathogenesis has mainly focused on the interplay components between the immune and the nervous systems, where monoamines including 5-HT are crucial for such interactions due to their high presence within the periphery and CNS, and have the capability of binding to receptors of both neuronal- and immune cells (Carandini et al., 2021; Melnikov et al., 2020). Among monoamines, we focused specifically on 5-HT because its role in modulating both neuronal and immune responses in cuprizone (CPZ)-induced demyelination remains relatively underexplored, which offers a novel perspective on the serotonergic contribution to MS pathophysiology.

5-HT is produced in several compartments of 5-HT neurons of the dorsal, median, and other raphe nuclei located primarily in the brainstem and in their projections located in distal dendrites as well as in varicosities and terminals throughout the brain (Muzerelle et al., 2016). In MS animal models, 5-HT metabolism is altered due to changes in the 5-HT degradation enzyme monoamine oxidase (MAO-A), and increased brain production of monoamines CPZ-intoxicated mice (Kesterson and Carlton, 1971; Chang et al., 2017). Moreover, pharmacological inhibition of the MAO-A enzyme delayed the onset and severity of symptoms in the experimental autoimmune encephalomyelitis (EAE) MS model, and MAO-A inhibition resulted in the improvement of the depression-like symptoms and normalization of central 5-HT levels (Benson et al., 2013; Musgrave et al., 2011). Treatment of EAE mice with a selective 5-HT reuptake inhibitor (SSRI) such as fluvoxamine causes symptomatic relief and neuroprotection (Ghareghani et al. 2017). Furthermore, treatment with antagonists for 5-HT2A (Mihai et al., 2021) or 5-HT3 receptors (Aminian et al., 2013) alleviated symptoms in CPZ-intoxicated mice, and markedly suppressed demyelination and clinical symptoms in EAE mice respectively. Based on these studies, the involvement of monoamines, and particularly 5-HT, seems to be plausible in MS pathophysiology and behavior consequences of MS.

We hypothesize that CPZ exposure induces mood changes that could be related to altered brain 5-HT system components. The present study investigates depressive and anxiety using behavioral tests and evaluates the level of the 5-HT system components (5-HT and its transporters SERT) using immunohistochemistry (IHC) in CPZ-treated mice and controls within the primary somatosensory (S1Tr) sensory cortex (Cx) and the associative cortex; the retrosplenial granular cortex (RSG), known to be involved in mood states (Harel et al., 2016; Kropf et al., 2019).