Coexistence of kappa- and delta-opioid receptors in rat spinal cord axons.

It has been found that heterodimers of kappa- and delta-opioid receptors can occur in vitro, but it has been unclear whether they also occur in intact animals. In the present study we examined whether kappa-delta heterodimers might occur in vivo by staining for these receptors with two-color fluorescence immunocytochemistry. Sections of rat spinal cord were double-stained using rabbit anti-kappa opioid receptor combined with rat anti-delta-opioid receptor. It was found that axons in the superficial dorsal horn of the spinal cord were double-labeled. In addition, structures within axonal varicosities were sometimes double-labeled. We conclude that single axons, and single structures within axons, express both kappa- and delta-opioid receptors. These observations are consistent with heterodimers of these receptors existing in vivo.