Understanding the spectrum of non-motor symptoms in multiple sclerosis:insights from animal models

Multiple sclerosis is a chronic autoimmune disease of the central nervous system and is generally considered to be a non-traumatic,physically debilitating neurological disorder.In addition to experiencing motor disability,patients with multiple sclerosis also experience a variety of nonmotor symptoms,including cognitive deficits,anxiety,depression,sensory impairments,and pain.However,the pathogenesis and treatment of such non-motor symptoms in multiple scle rosis are still under research.Preclinical studies for multiple sclerosis benefit from the use of disease-appropriate animal models,including experimental autoimmune encephalomyelitis.Prior to understanding the pathophysiology and developing treatments for non-motor symptoms,it is critical to chara cterize the animal model in terms of its ability to replicate certain non-motor features of multiple sclerosis.As such,no single animal model can mimic the entire spectrum of symptoms.This review focuses on the non-motor symptoms that have been investigated in animal models of multiple sclerosis as well as possible underlying mechanisms.Further,we highlighted gaps in the literature to explain the nonmotor aspects of multiple sclerosis in expe rimental animal models,which will serve as the basis for future studies.

Changes in structural plasticity of hippocampal neurons in an animal model of multiple sclerosis

Structural plasticity is critical for the functional diversity of neurons in the brain.Experimental autoimmune encephalomyelitis(EAE)is the most commonly used model for multiple sclerosis(MS),successfully mimicking its key pathological features(inflammation,demyelination,axonal loss,and gliosis)and clinical symptoms(motor and non-motordysfunctions).Recentstudieshave demonstrated the importance of synaptic plasticity in EAE pathogenesis.In the present study,we investigated the features of behavioral alteration and hippocampal structural plasticity in EAE-affected mice in the early phase(11 days post-immunization,DPI)and chronic phase(28DPI).EAE-affected mice exhibited hippocampus-related behavioral dysfunction in the open field test during both early and chronic phases.Dendritic complexity was largely affected in the cornu ammonis 1(CA1)and CA3 apical and dentate gyrus(DG)subregions of the hippocampus during the chronic phase,while this effect was only noted in the CA1 apical subregion in the early phase.Moreover,dendritic spine density was reduced in the hippocampal CA1 and CA3 apical/basal and DG subregions in the early phase of EAE,but only reduced in the DG subregion during the chronic phase.Furthermore,mRNA levels of proinflammatory cytokines(Il1β,Tnfα,and Ifnγ)and glial cell markers(Gfap and Cd68)were significantly increased,whereas the expression of activity-regulated cytoskeletonassociated protein(ARC)was reduced during the chronic phase.Similarly,exposure to the aforementioned cytokines in primary cultures of hippocampal neurons reduced dendritic complexity and ARC expression.Primary cultures of hippocampal neurons also showed significantly reduced extracellular signal-regulated kinase(ERK)phosphorylation upon treatment with proinflammatory cytokines.Collectively,these results suggest that autoimmune neuroinflammation alters structural plasticity in the hippocampus,possibly through the ERK-ARC pathway,indicating that this alteration may be associated with hippocampal dysfunctions in EAE.

Temporal profiles of synaptic plasticity-related signals in adult mouse hippocampus with methotrexate treatment

Methotrexate, which is used to treat many malignancies and autoimmune diseases, affects brain functions including hippocampal-dependent memory function. However, the precise mechanisms underlying methotrexate-induced hippocampal dysfunction are poorly understood. To evaluate temporal changes in synaptic plasticity-related signals, the expression and activity of N-methyI-D-aspartic acid receptor 1, calcium/calmodulin-dependent protein kinase II, extracellular signal-regulated kinase 1/2, cAMP responsive element-binding protein, glutamate receptor 1, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor were examined in the hippocampi of adult C57BL/6 mice after methotrexate （40 mg/kg） intraperitoneal injection. Western blot analysis showed biphasic changes in synaptic plasticity-related signals in adult hippocampi following methotrexate treatment. N-methyI-D-aspartic acid receptor 1, calcium/calmodulin-dependent protein kinase II, and glutamate receptor 1 were acutely activated during the early phase （1 day post-injection）, while extracellular signal-regulated kinase 1/2 and cAMP responsive element-binding protein activation showed biphasic increases during the eady （1 day post-injection） and late phases （7-14 days post-injection）. Brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor expression increased significantly during the late phase （7-14 days post-injection）. Therefore, methotrexate treatment affects synaptic plasticity-related signals in the adult mouse hippocampus, suggesting that changes in synaptic plasticity-related signals may be associated with neuronal survival and plasticity-related cellular remodeling.