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Fibromyalgia syndrome (FMS) is a chronic pain disorder with unknown etiology. It is characterized by widespread musculoskeletal pain in different regions of the body. Furthermore, it is often accompanied by non-specific symptoms. The prevalance of FMS in the USA is 2 % for both sexes with dominance of the female sex. There are various different approaches to the etiopathogenesis of FMS. On the one hand, structural, morphological as well as metabolic changes in the skeletal muscles could induce the symptoms of FMS. On the other hand, an imbalance between pro- and anti-inflammatory cytokines, oxidative stress, mitochondrial dysfunction or an altered central pain processing could be the causes of FMS.
In the present study we used the mouse modell of FMS using intermittent cold stress (ICS). The aim of this study was to investigate how ICS affects skeletal muscles (M. gastrocnemius and M. soleus) of male and female mice. For this purpose we made use of (immun)histochemical stainings, molecular biological techniques and transmission electron microscopy for revealing changes in the morphology, capillarization and angiogenesis, content of glycogen, inflammation, atrophy, apoptosis, innervation and mitochondria of skeletal muscles. There was a 21,6 % and 33,2 % decrease in fiber cross sectional area (FCSA) in gastrocnemius/soleus and a 22,9 % and 9,4 % increase in fiber density in the male ICS mice in gastrocnemius/soleus. These changes are found especially in gastrocnemius of male ICS mice. In addition, a 12,5 % and 11,8 % decrease of the capillary contacts per fiber exists in gastrocnemius and soleus muscles of male ICS mice in comparison with the control. Furthermore, Vegfa, Vegfb, Notch1 and Notch3 expressions (RNA level) were decreased in gastrocnemius und soleus muscles of male ICS mice.
Gastrocnemius and soleus muscles of male ICS mice show an 14,7 % and 29,7 % increase of density of MuRF1+ cells and an 17,7 % and 8,9 % increase of density of Fbxo32+ cells, whereas in female ICS mice MuRF1 expression and Fbxo32 expression (RNA level) were increased in these muscles. In contrast, the densities of MuRF1+ and Fbxo32+ cells were not increased in female ICS mice.
There is evidence, that in gastrocnemius muscle of male ICS mice inflammation and/or atrophy-relevant proteins seem to be associated with the regulation of fiber density and/or FCSA.
In contrast, apoptosis and innervation seem not to be influenced by ICS.
Furthermore, there was a 17,6 % and 7,9 % decrease of density of mitochondria in gastrocnemius and soleus of male ICS mice in comparison with control. Additionally, ICS affected the morphology of mitochondria negatively in gastrocnemius and soleus muscles of male, but also in female mice. In gastrocnemius muscles there was a significant 30,7 % increase of “damaged” mitochondria in male ICS mice and a significant 22,5 % increase in female ICS mice in comparison with the control. Morover, in soleus muscles there was a significant 16,9 % increase of “damaged” mitochondria in male ICS mice and a significant 31,1 % increase in female ICS mice in comparison with the control.
In male ICS mice the size of mitochondria showed an 63,5 % and 26,8 % increase in gastrocnemius and soleus muscles in comparison with the control.
These results confirm already described findings in skeletal muscles of FMS patients. They suggest that ICS induces atrophic processes especially in gastrocnemius of male ICS mice, which leads to a decreased FCSA. The two muscle-specific E3 ubiquitin ligases MuRF1 and Fbxo32 seem to be involved in these processes. In skeletal muscles of female mice ICS-induced alterations seem to occur not at all or delayed. The decrease in capillarization of skeletal muscles in male ICS mice points to a reduced distribution of oxygen/nutritive substances to muscle fibers and to a decreased evacuation of end products of the metabolism. These changes could explain the pain and the postexertional fatigue in FMS patients. In addition to the described results, mitochondrial alterations seem to be induced by ICS especially in gastrocnemius muscle of male mice. The present results suggest that the numerical and morphological mitochondrial abnormalities lead to an impaired mitochondrial function. This could not only lead to a loss of oxidative capacity but also to an atrophy of muscle fibers.
We recommend for future research on FMS using ICS to prefer gastrocnemius muscle of male ICS mice because the ICS-induced alterations in male mice begin earlier and more clearly than in female mice. Moreover, in further research there should be an observance of the metabolism of the skeletal muscles for clearing mitochondrial disorders. Especially the concentrations of adenosine triphosphate (ATP) and phosphocreatinine as well as the concentrations of lactate and pyruvate should be investigated and quantified in skeletal muscles of ICS mice. Furthermore, there should be research on oxidative and anti-oxidative parameters, the protein expression of IL-6 and TNF as well as a potential invasion of CD68+ cells in skeletal muscle of ICS mice.
Moreover, the correlation between inflammation and/or atrophy-relevant proteins and fiber density and/or FCSA should be tested in skeletal muscles of FMS patients. Presuming appropriate clinical data, the results of this dissertation might open novel possibilities for therapeutic intervention.