Expression of μ-, δ-, and κ-opioid receptor-like immunoreactivities in rat dorsal root ganglia after carrageenan-induced inflammation

Recently, antisera that recognize unique epitopes of the cloned μ-, δ-, and κ-opioid receptors (MOR, DOR, KOR, respectively) have been developed. In the present study MOR-, DOR-, and KOR-like immunoreactivities (LIs) were examined in rat dorsal root ganglia (DRGs, L4-5) after injection of carrageenan (CAR) into the hindpaw. In normal control rats, 20.9%, 13.5%, and 9% of the DRG neurons contained MOR-, DOR-, KOR-LI, respectively. A marked upregulation in MOR-LI was observed in DRG neurons 1 and 3 d after inflammation. In contrast, CAR induced a distinct downregulation in DOR- and KOR-LIs. MOR-, DOR-, and KOR-LIs were preferentially localized in small DRG neurons. MOR-LI was often located in patches in the cytoplasm, and in some cells close to the somatic plasmalemma. However, DOR- and KOR-LIs mainly showed a diffuse staining pattern within cytoplasm. Two or even all three receptors could sometimes be found to coexist in DRG neurons. In the spinal cord, these receptors were mainly confined to the superficial dorsal horn, with a somewhat diffuse staining which was strong for MOR-LI, and weak for KOR-LI. DOR-LI had a distinctly punctate, varicose distribution. CAG induced- alterations in opioid receptor staining in spinal cord were much less pronounced than those in the DRGs with a small increase in MOR-LI and a slight decrease in DOR-LI ipsilaterally. There was an accumulation of all three types of receptors in the sciatic nerve both proximal and distal to the ligation site as early as 2 hr, indicating both antero- and retrograde transport of multiple opioid receptors. However, DOR-LI accumulation was stronger than that of MOR- and KOR-LIs. Taken together, these results suggest that all three opioid receptors are involved in the response to inflammation and that they may play different roles in this pathological state. The coexistence of MOR, DOR, and KOR in at least some primary sensory neurons provides a substrate for functional interactions between these receptors.