We found that carrageenan-induced inflammation of the nonspecialized connective tissues of the low back in the rat caused altered gait, increased local mechanical sensitivity and macrophage infiltration of connective tissues. All of these effects were ameliorated by tissue stretch. Our findings suggest that the nonspecialized connective tissues of the low back could be an important therapeutic target because: 1) inflammation of these tissues can cause pain and impair function and 2) the response of these tissues to a static stretch intervention could improve these outcomes.
The majority of animal models relevant to low back pain have focused on injury associated with the spine. Only a few musculoskeletal rodent models have considered the contribution of connective tissue structures to pain 
, and no animal models so far have specifically examined inflammation of the nonspecialized connective tissues lateral of the spine. Our previous work identified and quantified calcitonin gene-related peptide (CGRP) expression in the nonspecialized connective tissues of the low back in control (no inflammation) animals, indicating a potential role for these tissues in the pathogenesis of pain. This finding was supported by the work of Tesarz et al., where CGRP and substance P innervation was quantified in the thoracolumbar fascia in the rodent 
. In the present study we aimed to gain a greater understanding of the effects of inflammation within these nonspecialized connective tissues. A strength of the model is that it permitted us to evaluate two complementary behavioral measures: local measures of nociception (Von Frey testing) and global measures of movement impairment (gait). A limitation of mechanical sensitivity testing with Von Frey filaments is that they are applied to the surface of the skin and thus do not specifically target deeper tissues, while a limitation of using walking tracks to measure gait is that the velocity of each step cannot accurately be collected. To better understand whether altered gait was due to non-specific sickness behavior 
or anxiety 
, either of which could affect gait velocity, we conducted an open field test and found no changes in overall activity, rearing, or time spent in the center of the field between experimental groups. The presence of gait abnormalities in our model suggests that the increased mechanical sensitivity detected with Von Frey testing was due to inflammation of deeper tissues (connective tissue and/or muscle) as opposed to simply the skin. Indeed, shortened stride length similar to that observed in our study was reported in a rodent model of “myofascial” inflammation produced by injection of paraformaldehyde into the multifidus muscles 
. Although it is not possible to rule out sensitization of sensory afferents within muscle in the current study, we were able to document clear evidence of carrageenan-induced inflammation within paravertebral connective tissue that improved with stretch.
With the stretching technique used in this study, the animal was encouraged to hold a position of stretch that was slightly beyond its usual range of motion. This technique is relevant to active stretch therapeutic interventions (physical therapy, yoga) that involve slow and gentle, but non-habitual, body movements. Because our animal model involves a stretch of the whole, conscious animal, anti-inflammatory effects induced by stretching could involve central as well as local mechanisms. Centrally mediated effects of stretch could include stimulation of the hypothalamic-pituitary-adrenal axis and systemic cortisol secretion with direct anti-inflammatory effect on tissues. Another possibility is that stress during tissue stretch could have activated descending pain inhibitory pathways with inhibition of neurogenic inflammation via reduced secretion of neuropeptides (Substance P, CGRP) into the tissue. Future studies will attempt to differentiate between central and/or peripheral mechanisms that may underlie the improvements observed with stretch. A potentially important difference between our stretching method and stretching methods used in humans (e.g. yoga) is stress caused by the added restraint imposed by holding the animal by the tail. Further experiments comparing stretching in conscious vs. anesthetized rats will be necessary to further explore this issue.
Alternatively, or in addition, it is possible that stretching could have had a direct anti-inflammatory effect on the peripheral connective tissues of the low back. The direct response of cells and tissues to mechanical forces varies greatly depending on the manner in which the force is applied 
. In cultured fibroblasts, repetitive or high amplitude cyclic stretch can lead to the production of pro-inflammatory cytokines 
and apoptosis 
, whereas brief static stretch has been reported to decrease pro-inflammatory cytokines IL-3 and IL-6 
. Anti-inflammatory outcomes also were found in in vitro
studies where low amplitude mechanical input was applied to chondrocytes 
and fibroblasts 
. Other potentially relevant peripheral mechanisms involve the complex relationship between TGFβfibrosis and inflammation 
. While repetitive or high amplitude mechanical input generally increases TGFβbrief static tissue stretch attenuated the increase in both soluble TGFβ-1 (ex vivo
) and type-1 procollagen (in vivo
) following tissue injury 
. It is therefore plausible that a number of potentially interrelated local and systemic mechanisms may have contributed to the reduction in tissue inflammation observed in our in vivo
animal model in response to stretch.
In conclusion, the role of the nonspecialized connective tissues in low back pain pathophysiology and treatment is not well understood but could involve localized inflammation. To address this question, we developed a rodent model of connective tissue inflammation and found that tissue stretch markedly improved both the local inflammation itself, as well as associated mechanical sensitivity and gait abnormalities. Further investigations using this model will be important to elucidate the mechanisms by which tissue stretch resulted in these therapeutic benefits.