Changes in Kappa Opioid Receptor Expression During Maturation of Mouse Lymphocytes

Abstract

Dynamic interactions between the immune and nervous systems have been well documented through the study of chemical mediators, such as neurotransmitters^1,2 and neuropeptides,³ which are common to both systems. In particular, of the neuropeptides, the opioid peptides have been shown to exert profound modulating effects on various immune cell functions.^4–7 In addition, detection of messenger RNA encoding for μ,⁸ δ,^9,10 and k^10,11 opioid receptors in lymphocytes suggests the existence of these neuronal receptors on immune cell populations. However, the detection of opioid receptors on cells of the immune system has remained elusive, possibly due to a lack in abundance of opioid receptors on immune cells or due to differential receptor expression on specific subpopulations of immune cells. Recently, initiatives from our laboratory combining flow cytometric analysis with indirect immunofluorescent labeling have been successful in demonstrating k opioid receptor expression on mouse immune cells.^12–14 This sensitive method employs amplification of labeling by aκ-selective opioid, fluorescein-conjugated arylacetamide (FITC-AA), with the addition of biotinylated anti-fluorescein IgG and extravidin-R-phycoerythrin (PE). Double-labeling of immune cells using this receptor amplification procedure along with fluorophore-conjugated monoclonal antibodies (mAb) provides for the identification of specific immune cell subpopulations, which express the k opioid receptor, from within larger, heterogeneous populations. Recently, we reported that freshly isolated thymocytes from 6–8-week-old C57BL/6ByJ mice demonstrate FITC-AA/PE labeling, which is blocked by theκ-selective antagonist nor-binaltorphimine (nor-BNI).^13,15 Whereas thymocytes consist mainly of immature T-cells, the question remained as to whether mature lymphocytes express the k opioid receptor. The present study, therefore, has applied this amplification labeling procedure, along with phenotypic analysis, to both immature and mature immune cell populations to determine whether k opioid receptor expression is present throughout lymphocyte maturation.