Characterization of functional opioid delta receptors in a luteinizing hormone-releasing hormone-producing neuronal cell line

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.