The mouse model of CIA has been proven to be a useful animal model for RA research because it shares many immunological and pathological features with human RA [
13,
14]. In the present study it was demonstrated that there is a strong correlation between the macroscopic arthritis score and locomotion. This locomotion test, by which mice were placed in a new surrounding, strongly correlates with the open field-tests performed to study incidence and duration of certain behaviors [
15]. Corresponding results have been indicated by Inglis et al. studying hyperalgesia [
10] and Millecamps et al. studying behavior in a monoarthritic rat model [
11]. However, in these studies the correlation between locomotion and the disease severity was not tested. To evaluate possible corruption of the locomotion values by habituation, the effect of repeated measurements was tested. No differences in walking distance were detected in control mice between different days of assaying (5 tests with an interval of 3–7 days between each test, data not shown). The repeated detection of locomotion in control mice did reveal however, a mouse dependent initial locomotion. These results stress the need for determination of the initial locomotion distance for each individual mouse. These initial values were set by averaging the locomotion values detected on two separate days before the start of the experiment. The present data indicate that the quantitative detection of locomotion strongly corresponds to clinical changes as detected by the semi-quantitative detection of disease severity. Also the total mass of the skeletal muscles of the hind legs, as detected at the end of the experiment, revealed to correlate to locomotion. These results suggest a direct relation between movement and muscle mass. However, movement is not the only factor effecting the muscle mass in the CIA model. In contrast to the fact that food intake by RA patients does not differ form the intake by healthy people [
16,
17] a strong significant decreased food intake by the CIA animals has been detected (data not shown). Moreover, TNF-α which is believed to be a central mediator of muscle wasting in RA exerts a powerful influence on muscle protein turnover resulting in a net muscle protein wasting [
18,
19]. Increased serum levels of TNF-α were detected in arthritic mice at the end of the experiment (data not shown). Besides, direct effects of pro-inflammatory cytokines on muscle metabolism, they play a role in hyperalgesia resulting in decreased movement. These interactions might indicate that muscle mass might be a perfect biomarker for disease severity. However, the significant decrease in food intake hampers the correlations between muscle mass loss and arthritis score although a weak correlation between total muscle mass and arthritis score could be detected.
A MTX intervention group was included to study more detailed the applicability of locomotion detection for testing treatment effectiveness of pharmaceuticals or other disease interventions. MTX is the most frequently used DMARD. Although the precise mechanisms in the treatment of RA are not completely clear, MTX exerts a variety of pharmacological actions resulting in suppression of the disease activity and reduced joint damage [
20,
21]. In the present CIA study the effects of MTX treatment on arthritis score, locomotion and muscle mass were studied. In agreement with previous publications [
22] MTX inhibited the arthritis development in the CIA model. Moreover, treatment with MTX results in an increased locomotion. The MTX data are in agreement with the finding that the arthritis score displays an inverse correlation with locomotion. A protective effect of MTX treatment was also detected on the muscle masse of the TA, the EDL and gastrocnemius. Although the correlation between muscle masse and arthritis score was weak a clear modifying effect by MTX could be detected in the separate muscles.