Automated measurement of spontaneous pain-associated limb movement and drug efficacy evaluation in a rat model of neuropathic pain.

BACKGROUND: The withdrawal response elicited by a nociceptive stimulus, i.e., evoked pain measure, is commonly used as an efficacy endpoint in neuropathic pain animal models. It, however, has several limitations, which highlight the importance of examining spontaneous pain. The present study describes an automated method for measuring spontaneous pain behaviour in a rat model of neuropathic pain caused by chronic constriction injury (CCI) of sciatic nerve. METHODS: After CCI surgery, a small magnet was implanted into the operated limb. The rat was placed in a test chamber that was surrounded by wire coil. Limb movements, including lifting/guarding, flinching/shaking, licking and walking in the operated limb, caused changes in the electromagnetic field, including a change in voltage and transformed into a signal via an amplifier. RESULTS: CCI rats consistently showed more frequent limb movement than sham rats. There was no significant correlation between the frequency of spontaneous pain behaviour and the evoked pain symptoms. Treatment with duloxetine (30 mg/kg p.o.) and amitriptyline (30 and 100 mg/kg p.o.) significantly reduced this frequency. Pregabalin at 30 mg/kg p.o. tended to reduce the frequency, and diclofenac up to 10 mg/kg p.o. had no effect. CONCLUSION: A non-subjective automated method for measuring spontaneous pain behaviour in an animal model of neuropathic pain was established. It is expected that the current system will greatly enhance the analysis of spontaneous pain-related behaviour, which is a predominant symptom in patients with neuropathic pain. The current system may also be valuable in the screening of potential analgesic treatments.