Acute effects of morphine on blood lymphocyte proliferation and plasma IL-6 levels

Administration of morphine (10 mg/kg) to rats was found to decrease the proliferative potential of blood lymphocytes by 60-80% and concurrently elevate circulating levels of the cytokine, interleukin-6 (IL-6), 2- to 4-fold. Both parameters were similarly altered upon the central administration of morphine and were blocked upon pretreatment of animals with the opioid receptor antagonist, naltrexone. These results suggest that the activation of central opioid receptors is involved in morphine-induced inhibition of lymphocyte proliferation as well as increases in circulating levels of IL-6. Studies addressing the potential peripheral mechanisms demonstrated that intact ganglionic transmission was required for both effects of morphine. Although the suppression by morphine of lymphocyte proliferation appeared to be largely independent of stimulation of the hypothalamic-pituitary-adrenal axis, the elevation of IL-6 was completely abolished in adrenalectomized animals. Collectively, these results suggest that central opioid receptor activation results in changes in different immune parameters that can be mediated through distinct peripheral mechanisms.