Experimental research to elucidate the pathogenesis and develop treatment for pain due to inactivity

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As experimental research with the goal of elucidating the pathogenesis and developing treatments for pain due to inactivity, we created a rat model using cast immobilization of the hindlimbs to evaluate the relationship to pain and the influence of mechanical stimulation.Pain due to inactivity was evaluated as follows. Behavioral evaluation included measurement of painthresholds using the gastrocnemius muscle and measurement of the number of escape responses of hind foot pad. Histopathological evaluation included measuring the number of necrotic fibers in the gastrocnemius muscle. Molecular biological evaluation included RNA extraction and real-time polymerase chain reaction using specimens of the dorsal root ganglion and dorsal horn of the L4 -6 spinal cord.Results: A decreased pain threshold of gastrocunemius and increased number of escape responses of hind foot pad were observed immediately after immobilization, but these parameters tended to normalize early in the intervention group with mechanical stimulation. Histopathological evaluation showed marked muscle injury in the immobilization group compared to the intervention group. In addition, molecular biological evaluation showed higher expression of brain-derived neurotrophic factor mRNA in the dorsal horn in the immobilization group.These findings show that the development of pain is due to inactivity in both muscle and skin, and that BDNF mRNA expression plays a role in the pathogenesis of this pain.