Chronic opioid treatment of the mouse thymoma cell lines R1.G1 and R1EGO leads to down-regulation of the kappa opioid receptor without desensitization of adenylyl cyclase activity

Effect of a new opioid peptide on opioid receptor and peptide transporter gene of brain and small intestinal tissues in mice

AIM: To investigate the effect of a new opioid peptide, YPFPGPIRYG, on the opioid receptor and peptide transporter gene of brain and small intestinal tissues via gastrointestinal tracts at transcription level.

METHODS: Twenty-four mice were randomly divided into opioid group (n = 12) and control group (n = 12), which were fed with 8×10^-7 mol/L YPFPGPIRYG and double distilled water, respectively, for 2 wk. The total RNA was extracted from the brain and small intestinal tissues of mice for the detection of μ-, δ-opioid receptor and peptide transporters PepT1 gene expression by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS: In the brain and small intestinal tissues, μ- and δ-opioid receptor transcripts were detected, while peptide transporter PepT1 was only found to be expressed in the small intestine. In comparison with those in the controls, the expression of μ- and δ-opioid receptor were significantly enhanced in the small intestine (P < 0.05), but not in the brain, and PepT1 gene expression was not changed in the intestinal tissues of mice treated with YPFPGPIRYG.

CONCLUSION: Opioid receptor expression has tissue specificity. The immunomodulatory effects of opioid YPFPGPIRYG do not attribute to the interaction with the opioid receptors in central nervous system (CNS), but with d- and m-opioid receptor in small intestine.

Alteration in specific opioid-receptor labeling on peripheral blood leukocytes of bile duct-ligated rat

Cholestasis is associated with increased tonus and activity of opioidergic system. Opioid peptides have also immunomodulatory effects through stimulation of specific opioid receptors on the immune cells, or in an indirect fashion via the central nervous system. The combination of immunofluorescent technique and flow cytometry has proven to be sensitive method for the detection of leukocyte opioid receptors. This study was designed to examine the effect of cholestasis on the opioid-receptor labeling on the leukocytes from bile duct-ligated rats. Seven days after surgery, leukocytes were isolated from the peripheral blood of bile duct-ligated or sham-operated rats. The cells were incubated with naltrexone-fluorescein, in the absence or presence of unlabeled naltrexone, as a competitor and analysed by flow cytometry. Monocytes and granulocytes from bile duct-ligated rats showed an increase in the percentage of opioid-receptor labeling (29.6+/-2.08 for cholestatic versus 23+/-1.9 for sham, p<0.001; 50.6+/-3.18 for cholestatic versus 39.6+/-1.7 for sham, p<0.05; respectively). Furthermore, there was a decrease in the expression of opioid receptors on leukocytes due to cholestasis. In conclusion, changes in specific opioid-receptor labeling and percent of labeled leukocytes indicate that endogenous opioid-receptor interaction may be altered in peripheral blood leukocytes in acute cholestasis.

Signal transduction induced by opioids in immune cells: a review

New data regarding signal transduction triggered by opioid ligands in immune cells are reviewed, and the signal transduction in neuronal cells is documented. Similar signaling pathways are induced by opioids in immune as well as neuronal cells. Opioids altered second messenger cAMP, intracellular calcium, and second messenger-induced kinases in immune cells. Met-enkephalin, preferentially delta-opioid, was bimodally regulated, while kappa-opioids inhibited these second messengers. delta-, kappa- and micro-opioids altered nitric oxide secretion, inducing cGMP as the second messenger in immune cells. Coupling of opioid agonists to opioid receptors activated mitogen-activated protein/extracellular signal-regulated protein kinases and various transcription factors in immune cells. Activator protein 1 (AP-1), c-fos, and nuclear factor-kappaB (NF-kappaB) are transcription factors shared by neuronal and immune cells. Delta-opioids activated AP-1, c-fos, activating transcription factor 2, Ikaros-1 and Ikaros-2 transcription factors in immune cells. Induction of kappa-opioid receptor gene by retinoic acid resulted in increased binding of Sp1 transcription factor to the promoter of the kappa-opioid receptor. Micro-opioids inhibited synthesis of common transcription factors AP-1, c-fos, NF-kappaB, and nuclear factor of activated T cells in activated or stimulated immune cells, whereas micro-opioids activated NF-kappaB, GATA-3, and Kruppel-like factor 7 transcription factors in non-stimulated immune cells.