An automated method by which effects of compounds on locomotor activity and spontaneous neuropathic pain-specific movements can be simultaneously evaluated in rats with chronic-constriction nerve injury

Abstract

Neuropathic pain patients are characterized by evoked pain (hyperalgesia and allodynia) and spontaneous pain, the latter of which is the predominant symptom. In animal models of neuropathic pain, effects of test compounds on spontaneous pain-related behaviors have been evaluated by direct visual observation. In addition, by performing another locomotor activity experiment in normal animals, it is also indispensable to examine whether test compounds cause motor impairment to avoid overestimation of their analgesic activities. In the present study, we developed spontaneous pain-specific and automated evaluation method by improving a previous method for measuring movements of the injured hind limb in unilateral chronic constriction nerve injury (CCI) rats. Rats with unilateral CCI were implanted with strong and weak magnets in each hind limb, respectively. Limb movements were automatically detected as spiked waveforms in electromagnetic field analyzing system. Movements in each limb were analyzed separately according to differences in their respective wave amplitudes, and aberrant movements of injured limb were specifically detected on basis of the asymmetry between injured and uninjured limb movements. Consequently, the incidence ratio of spontaneous pain in injured limb, which was not affected by individual locomotive activities, was able to be obtained as a new evaluation index. The incidence ratio of spontaneous pain revealed substantial difference between CCI and sham rats with only a small variation in the value. Further, according to the frequency of movement of the uninjured limb, it could be determined simultaneously whether a test compound causes sedative effect or not, resulting in eliminating the need for another experiment in normal animals.