Activation of TRPM8 cold receptor triggers allodynia-like behavior in spinally injured rats

Aims Pain in response to innocuous cold stimulation (cold allodynia) is a common symptom in patients with neuropathic pain. Cold allodynia is difficult to treat and its mechanisms are poorly understood. Several transient receptor potential (TRP) channels have been shown to be the molecular sensors for cold stimulation in a temperature-dependent manner, but the contribution of various TRP channels in mediating cold allodynia in neuropathic pain is unclear. We have previously shown that spinally injured rats developed neuropathic pain-like behaviors, including marked cold allodynia. We now assessed the role of TRP channels in mediating cold allodynia in rats after ischemic spinal cord injury. Methods Methods: Spinal cord injury was produced using a photochemical method. The mechanical allodynia was assessed by examining the vocalization thresholds to graded mechanical touch/pressure applied with von Frey hairs. Temperature controlled cold stimulation was produced by a Peltier thermode (active surface 25 mm × 50 mm) connected to a MSA Thermal Simulator (Somedic, Sweden) with baseline temperature of 32 °C. The rate of temperature change was 0.5 °C/s. The temperature required to elicit cold allodynia was examined. The responses of the rats to topical application of icilin or menthol, agonists of transient receptor potential melastain 8 (TRPM8), were also studied. Results Normal rats did not exhibit nociceptive responses to cooling stimulation to the trunk and back area (minimal temperature +6°C) and they also did not react aversively to topical application of icilin or menthol. After spinal cord injury, the rats developed mechanical allodynia at the trunk and back just rostral to the dermatome of the injured spinal segments. In the same area, rats exhibited significant nociceptive responses to cooling from day 1 after injury, lasting for at least 70 days which is the longest time of observation. For the first two weeks after injury, the majority of spinally injured rats had a nociceptive response to cooling above 17°C. At day 70, about 50% of rats responded to cooling above 17 °C. Topical application of 400 μM icilin or 4mM menthol also elicited pain-like responses in spinally injured rats and these two cold mimetics also significantly exacerbated existing mechanical allodynia. Conclusion Our results showed that activation of the TRPM8 channel by menthol or icilin triggers allodynia in spinally injured rats and increases, rather than decreases, mechanical allodynia. TRPM8 channels which respond to cooling above 17 ° C may be involved at least in part in mediating cold allodynia in the rat model of neuropathic spinal cord injury pain. Implications The work introduced a method of quantitative testings of responses of rats to cold stimulation and may contribute to the understanding of mechanisms of cold allodynia after injury to the nervous system.