title: Characterization of functional opioid delta receptors in a luteinizing hormone-releasing hormone-producing neuronal cell line.
abstract: Endogenous opioids participate in the regulation of gonadotropin secretion through an influence on the release of the hypothalamic LHRH. However, it is not clear whether opioids exert a direct effect on LHRH-producing neurons or interfere with other systems able to influence LHRH release. A neuronal LHRH-producing cell line (GT1) developed recently provides a good model to study the mechanisms controlling LHRH release. In the present study, the presence of opioid receptors on a subclone of GT1 cells (GT1-1) has been investigated. A specific and saturable binding of the 3H-labeled nonselective opioid ligand diprenorphine ([3H]DIP) was detected by a receptor binding assay on both intact GT1-1 cells and crude membrane preparations obtained from these cells. Analysis of saturation curves revealed that [3H]DIP apparently binds to a single class of sites with a Kd of 0.2 nM and a binding capacity of 125 fmol/mg protein, corresponding to approximately 20,000 sites/cell. Selective displacement of the binding of [3H]DIP to GT1-1 cells by [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, [D-Pen2,D-Pen5]enkephalin (DPDPE), and U50488H, which are selective ligands, respectively, for mu-, delta-, and kappa-receptors, was also evaluated. Only the specific delta-ligand DPDPE produced a significant inhibition of the binding of [3H]DIP. [D-Ala2,N-Me-Phe4,Gly5-ol]Enkephalin and U50488H were totally ineffective. The inhibitory effect of the agonist DPDPE on the binding of [3H]DIP was decreased by the presence of sodium ions, a typical characteristic of the binding of agonists to opioid receptors. Finally, it has been observed that treatment with prostaglandins E1 and E2 produces a dramatic increase in cAMP accumulation in GT1-1 cells, and DPDPE is highly effective in suppressing this effect. On the basis of these results, it is possible to postulate the presence of functional delta-opioid receptors on GT1-1 cells. By extrapolation, one might suggest that endogenous opioids may affect LHRH neurons by two mechanisms: a direct one, acting via delta-receptors, and an indirect one, through the activation of neurons impinging on the LHRH system, which uses mu-receptors.
authors: Department of Endocrinology, University of Milan, Italy.
authors: R
authors: R
authors: Maggi
authors: F
authors: F
authors: Pimpinelli
authors: L
authors: L
authors: Martini
authors: F
authors: F
authors: Piva
mesh_terms: D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000037:antagonists & inhibitors; D020881:Enkephalin, D-Penicillamine (2,5)-; D004745:Enkephalins / Q000494:pharmacology; D007987:Gonadotropin-Releasing Hormone / Q000096:biosynthesis*; D051379:Mice; D009474:Neurons / Q000378:metabolism*; D017465:Receptors, Opioid, delta / Q000378:metabolism*
chemical_list: D004745:Enkephalins; D017465:Receptors, Opioid, delta; D007987:Gonadotropin-Releasing Hormone; D020881:Enkephalin, D-Penicillamine (2,5)-; D000242:Cyclic AMP