[Signal transduction of cloned opioid receptors]

Involvement of kappa opioid receptors in the inhibition of receptor desensitization and PKC activation induced by repeated morphine treatment

Analgesic tolerance to morphine can develop from long-term use of this drug for the treatment of pain. Many reports have shown that stimulation of the kappa opioid receptor (KOR) suppresses development of analgesic tolerance to morphine. Here, we studied the KOR-mediated inhibition of morphine tolerance during repeated morphine treatment, with a focus on desensitization of the receptor. The development of analgesic tolerance to morphine during repeated morphine administration (10 mg kg(-1) s.c.) was completely suppressed by U-50488H (2 mg kg(-1) i.p.), a KOR agonist. The decrease in [35S] GTPgammaS binding activity stimulated by the mu opioid receptor (MOR) agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) was also significantly inhibited by U-50488H. These results indicate that stimulation of KOR caused by repeated morphine treatment either inhibits MOR desensitization or accelerates recycling of MOR on the cell surface, thereby suppressing morphine tolerance. Furthermore, we found that activity of protein kinase C (PKC) was significantly decreased in mice treated with both U-50488H and morphine. These results suggest that the mechanisms underlying KOR-mediated inhibition of analgesic tolerance to morphine may be partly due to suppression of PKC activation and prevention of receptor desensitization.

Usefulness of niravoline, an arginine vasopressin inhibitor, on tumour-origin brain oedema

Niravoline is a selective agonist of kappa-opioid receptors having potent aquaretic activity which may peripherally be mediated in animals. This effect of niravoline is thought to be due to inhibition of arginine vasopressin secretion. Niravoline does not cause an overt excretion of electrolytes that is seen with conventional diuretics, and it is anticipated that niravoline may prove useful in the treatment of oedema. In this study, effect of niravoline on tumour-origin brain oedema was investigated in a brain tumour model created by implantation of C6 glioma cells into the brains of Wistar rats. Five weeks after the tumour cell implantation, niravoline was administered i.v. at 1 mg/kg a total of four times at one-hour intervals. In the control group (i.e., saline or vehicle-treated rats), this brain tumour model was found to result in a statistically significant increase in the water content in selected brain regions remote from the tumour. Administration of niravoline inhibited the increase in the water content in such selected brain regions. Niravoline might be useful on tumour-origin brain oedema clinically.