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Work-related repetitive strain injury (RSI), one of the work-related musculoskeletal disorders, is the most commonly reported occupational illness, yet the pathophysiological mechanisms are not yet clear. Using our unique RSI animal model, pathophysiological tissue responses can be examined simultaneously with behavioral responses that serve as indicators of sensorimotor function. Studies in humans and with this animal model have shown that prolonged performance of repetitive tasks leads to declines in grip strength and increased of serum pro-inflammatory cytokines, both valuable biomarkers of underlying tissue injuries. Identification of biomarkers would help to elucidate the time course of the inflammatory stage of these disorders and disease progression. Sickness responses/behaviors, normal responses and behaviors occurring as a consequence of infection or illness, are responsible for re-organizing perceptions and actions to enable individuals to respond appropriately to the infection, injury and other illnesses. These behaviors can include malaise, fatigue, increased irritability and social withdrawal, behaviors that can be beneficial when ill for avoiding others and enhancing wellness. However, sickness behaviors can also include depression, sleep disorders and an enhanced perception of illness. Although recent studies have shown that circulating pro-inflammatory cytokines in serum may trigger sickness behaviors, only a few studies have examined if there is an association between work-related musculoskeletal disorders and subsets of sickness behavior; no one to date has elucidated any plausible biological cause of this association. Also, the combined effects of both aging and performance of repetitive tasks needs more evaluation since several epidemiological studies have shown a relationship between advancing age and susceptibility to musculoskeletal disorders. Therefore, using female Sprague-Dawley rats, the aims of this dissertation project are to: 1) To determine if a systemic inflammatory response is maintained long-term in rats exposed to a moderate demand lever-pulling task with high repetition, low force (HRLF) requirements; if aging enhances this systemic inflammatory response and if this response correlates with functional motor declines; 2) To determine if performance of repetitive upper extremity tasks leads to the development of sickness behaviors (weight loss, decreased social interaction and increased aggression); 3) To determine if a relationship between repetitive upper extremity tasks lead to the development of dose- and age-dependent sickness behaviors, and, to determine if increased serum or brain inflammatory cytokines are plausible mechanisms for the induction of sickness behaviors in this model; and, lastly, 4) To determine if prophylactic or secondary systemic treatments with anti-inflammatory drugs (anti--tumor necrosis factor alpha or ibuprofen) reduces sickness behaviors in rats performing a high demand high repetition high force (HRHF) reaching and handle pulling task. This finding would be in further support of an underlying task-induced inflammatory mechanism contributing to these sickness behaviors. In the first study, the relationship between serum and grip strength was examined in aged and young adult rats performing a HRLF task. Serum levels of interleukin 1-alpha (IL-1α), interferon-gamma (IFNγ) and IL-6 were higher in aged rats in general, compared to young adult rats. Each increased more in aged trained-only and 12-week HRLF rats than in young adult trained-only and 12-week HRLF rats. Serum IL-6 showed the greatest increases, with the highest levels observed in aged 12-week HRLF rats. Grip strength declined with task performance in both age groups; however, this decline correlated negatively and only moderately with serum IL-6 levels in aged rats. Thus, aging enhanced a serum cytokine response in general, a response that was even greater with repetitive task performance. Grip strength was adversely affected by task performance in both age groups, but was apparently influenced by factors other than serum cytokine levels in young rats. In the second study, the relationship between sickness behaviors (weight loss, duration of social interaction and aggression towards novel juvenile rats), serum cytokines and brain cytokines were examined in aged and young adult rats that trained to one of two task levels before performing a HRLF task. Decreased duration of social interaction and increased aggression was greater in both young adult and aged rats that trained for 10 min/day for 4-5 weeks to learn a high force (TRHF) task, compared to young adult and aged rats that learned a low force (TRLF) task. TRHF and TRLF rats of both age groups then performed a HRLF task for 6 to 24 weeks, rats termed TRHF-HRLF and TRLF-HRLF, respectively. Declines in social interaction resolved by week 3 in young adult TRHF-HRLF rats, but were still evident in 6-week aged TRHF-HRLF rats (the final endpoint for TRHF-HRLF rats). Significant increases in aggression were observed only in TRHF-HRLF rats, in both age groups. Declines in social interaction were also observed in aged rats performing a TRLF-HRLF task through week 9, but not in young adult TRLF-HRLF rats, even those performing the HRLF task for 24 weeks. These behaviors correlated moderately with increased serum IL-6 observed in the aged task rats (both TRHF-HRLF and TRLF-HRLF) and young adult TRHF-HRLF rats, although serum TNFalpha and IL-1beta also increased with task performance. Increased IL-6 and IL-6 receptor was detected immunohistochemically in brains of aged TRHF-HRLF rats, specifically in ependymal and endothelial cells, as well as glial cells and neurons of the anterior cingulate cortex and paraventricular nucleus. Thus, training to high force, even for a short time period, induced increased sickness behaviors. Aging contributed to increased sickness behaviors in repetitive task rats, and to increased expression of IL6 and IL-6 receptor in several brain regions. In the third study, the relationships between sickness behaviors (duration of social interaction and aggression towards novel juvenile rats), serum cytokines and brain cytokines were examined in young adult rats performing a high repetition high force (HRHF) task with or without anti-inflammatory medications. Rats that trained to high force (TRHF) had decreased duration of social interaction and increased aggression; these behaviors were prevented by prophylactic anti-TNFalpha or ibuprofen treatment. Untreated TRHF rats that went on to perform a HRHF task showed decreased social interaction and increased aggression through week 12; these behaviors were attenuated by secondary anti-TNFalpha and ibuprofen treatments. Untreated HRHF rats had increased serum GroKC, IFN-gamma, IL-1beta, MIP2, MIP3a and TNFalpha. These increases were attenuated after two weeks of anti-TNFalpha treatment in HRHF weeks 5-6, and after 8 weeks of ibuprofen (in HRHF weeks 5-12). The sickness behaviors correlated moderately with increased serum Gro-KC, MIP2 and TNFalpha, and were concomitant with increased IL-1beta immunoexpression in ependymal and endothelial cells in brains of untreated TRHF and HRHF rats. Both treatments reduced the increased brain IL-1beta expression. Thus, sickness behaviors induced by overuse are attenuated by anti-inflammatory interventions that reduce task-induced increases in systemic and brain inflammatory cytokines. In conclusion, both aging and task performance increased serum inflammatory cytokine responses, the latter in an exposure-dependent manner, with a greater serum cytokine response with performance of high force tasks than low force tasks. While grip strength was adversely affected by task performance in both young and aged rats, it was influenced by factors other than serum cytokine levels. In contrast, decreased duration of social interaction and increased incidence of aggression were influenced by task-induced increases in serum and brain inflammatory cytokines, responses attenuated to baseline levels with systemic anti-inflammatory treatments. Sickness behaviors were also dose- and age-dependent, with higher incidence with performance of high force tasks than with low force tasks, and higher incidence in aged rats. We postulate that the higher incidence in aged rats is due to aged-induced brain "inflamm-aging", as they also had increased immunoexpression of IL6 and IL6 receptor in blood brain barrier cells and in glial and neurons of the hypothalamic pituitary axis. / Physical Therapy