Delta opioid receptors are involved in morphine-induced inhibition of luteinizing hormone releasing hormone in SK-N-SH cells

Abstract

Opioids play an important role in the regulation of lutenizing hormone releasing hormone (LHRH). In the present study, we attempted to find out the subtype of opioid receptors involved in the inhibitory effect of morphine on LHRH. Experiments were conducted on SK-N-SH neuroblastoma cells that express both μ and δ opioid receptors, LHRH mRNA, and release the LHRH peptide. Enzyme-linked immunosorbent assay (ELISA) was employed to measure the levels of LHRH. LHRH level was decreased by 1000 μM of morphine regardless of the duration of exposure or differentiation status of the SK-N-SH cells and was not reversed by naloxone. Selective antagonism of μ opioid receptors, but not δ opioid receptors, allowed lower concentrations (1–100 μM) of morphine to inhibit LHRH. The results of this study imply that (1) δ opioid receptors may mediate the inhibitory effect of lower concentrations of morphine on LHRH levels in SK-N-SH cells, and (2) inhibition of LHRH level by high concentrations of morphine may involve systems other than opioid receptors.