Abstract
Chronic pain animal models with nerve injuries have contributed much to understand the mechanisms underlying neuropathic pain. However, understandings of chronic pain originated in a soft tissue lesion have been remaining unclear. Certain animal models with the muscle lesion have been reported so far, but they were mostly developed on the basis of inflammatory pain.
Besides inflammation, we consider that nociceptive inputs may be also important to induce chronic pain states. In the present study, we tested the effects of 2 µg/kg of lipopolysaccharide (LPS) and 6% hypertonic saline (HS) given consecutively five times at 90-minute intervals, which were injected into the unilateral gastrocnemius muscle of the rat. Animals were received a combined treatment of LPS and HS and divided into three groups depending on intervals of injections of these agents (24, 48 and 96 hours). The number of paw elevations per five applications of three different von Frey filaments (VFFs; 980, 2940 and 3920 hPa) was examined in ascending order until 6 weeks after treatments. In addition, measurements of muscle pain thresholds and circumferences at injected sites were followed up to 1 week, when both returned to normal.
Different intervals of LPS and HS injection in combined treatment made no differences in terms of inductions of pain behaviors. Animals showed increased responses to VFFs lasting over 6 weeks after the disappearance of local changes. Long-lasting responses were induced bilaterally, but these of ipsilateral side were certainly predominant. This experimental animal model is suggested to be useful for the studies of chronic pain driven by a soft tissue injury.
