Morphine upregulates kappa-opioid receptors of human lymphocytes

Opioids such as morphine are potent analgesic and addictive compounds. Chronic morphine use also induces immunomodulatory and immunosuppressive effects, as especially evident in HIV-infected patients. Morphine acts on the immune cells primarily through its binding to mu-opioid receptors on the plasma membrane. However, morphine modulation of immune functions still exists in mu-opioid receptor knockout mice, suggesting that in addition to the mu opioid receptors, morphine may also act by mechanisms mediated by either delta or kappa opioid receptors. To determine whether morphine activates kappa opioid receptors (KOR), a quantitative competitive RT-PCR procedure was utilized to quantify the KOR gene expression of morphine-treated cells. A segment of KOR transcript spanning the second extracellular loop, which has the reported dynorphin specificity, and the seventh transmembrane domain of the receptor was amplified from the total RNA of morphine-treated CEM x174 lymphocytes, along with a competitor molecule. The competitor was constructed by deleting a 33-nucleotide fragment from KOR. The results of the competitive RT/PCR indicated that CEM x174 cells expressed KOR mRNA constitutively, in the order of femto-grams. Treatment of 10 microM of morphine resulted in the up-regulation of KOR gene expression 24 hr post-treatment. The observed morphine effect could be reversed by treating the cells with either naloxone (a KOR-partially selective antagonist) or nor-Binaltorphimine (a KOR-selective antagonist).