Association of kappa opioid receptor mRNA upregulation in dorsal root ganglia with mechanical allodynia in mice following nerve injury.

This study examined the contribution of nerve injury alone or nerve injury with signs of neuropathic pain to alteration of kappa opioid receptor (KOR) mRNA expression. Two groups of mice, both of them were subjected to unilateral transection of the inferior and superior caudal trunks at the S1spinal nerve, were compared with respect to KOR mRNA expression by reverse transcriptase-polymerase chain reaction. One group showed exclusive pain behavior (PB+) as mechanical allodynia, and the other group exhibited no enhanced sensitivity to innocuous mechanical stimulation to the tail (PB-). Expression of total KOR and variants B and C mRNA increased ipsilaterally in dorsal root ganglia (DRG) of PB+ mice, whereas KOR variant A mRNA was not detected in DRG. These results show that KOR mRNA expression differs between PB+ and PB- groups of mice after nerve injury, and suggest an association of KOR expression with mechanical allodynia.