Expression of mu- and kappa-, but not delta-, opioid receptor mRNAs is enhanced in the spinal dorsal horn of the arthritic rats

The expression of the mRNAs for mu-, delta- and kappa-opioid receptors was studied in the lumbar spinal cord of the rats with the inflammation at their unilateral hindpaw using in situ hybridization technique. On 11 days after the first adjuvant inoculation, mu- and kappa-opioid receptor mRNA levels in laminae I-II of the spinal dorsal horn ipsilateral to the inflamed hindpaw were increased to 135.3 +/- 6.4% and 130.3 +/- 5.7%, respectively, compared with the contralateral side. At this time point, no significant differences in the mu- and kappa-opioid receptor mRNA expression were observed between ipsi- and contralateral sides of other laminae. On the other hand, no significant change was observed in the delta-opioid receptor mRNA expression throughout the laminae I-IX at any time points examined. These findings suggest the increase in the synthesis of mu- and kappa-, but not delta-, opioid receptors in the spinal laminae I-II during sustained inflammatory pain.