title: Melatonin receptors are present in adult rat Leydig cells and are coupled through a pertussis toxin-sensitive G-protein.
abstract: Previous studies have suggested that melatonin (MLT) acts directly on rat Leydig cells by modulating androgen production. In the present study, the site of action of MLT was investigated. The binding of 2-[125I]iodomelatonin (125I-MLT; 7-240 pmol/l) to Leydig cell membrane fragments was tested in the presence or absence of guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 50 mumol/l). Saturation studies and Scatchard analysis revealed the existence of a high-affinity binding site with a Bmax of 46.70 +/- 3.50 fmol/mg protein and a Kd of 88.70 +/- 6.20 pmol/l; treatment with GTP-gamma-S reduced the concentration of 125I-MLT binding sites (Bmax 34.03 +/- 4.50), while increasing the Kd to 106.5 +/- 2.61 pmol/l. Pretreatment of the cells with pertussis toxin (PTX; 10 ng/ml for 16 h) resulted in a decreased binding of 125I-MLT and a lack of effect of GTP-gamma-S. Moreover, the effect of MLT on testosterone secretion induced by LH (30 mIU/ml), forskolin (1 mumol/l) and LHRH (100 nmol/l) was studied after 3-h incubation of cells which had been precultured with or without PTX. The inhibition of testosterone secretion due to MLT administration was eliminated by PTX pretreatment during forskolin and LH, but not during LHRH administration. However, 17-hydroxyprogesterone levels were higher in all groups incubated in the presence of MLT, irrespective of PTX pretreatment. Our data suggest that: (a) MLT receptors are present on the membranes of adult rat Leydig cells; (b) they couple through PTX-sensitive G-protein-coupled binding sites; (c) the mechanism by which MLT blocks 17-20 desmolase enzymatic activity (thus leading to increased 17-hydroxyprogesterone levels), and testosterone secretion during LHRH stimulation is likely to depend on one or more different mechanism(s) of action.
authors.affiliation: Department of Endocrinology and Metabolism, University School of Medicine, Genoa, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Valenti
authors.forename: M
authors.initials: M
authors.lastname: Giusti
authors.forename: R
authors.initials: R
authors.lastname: Guido
authors.forename: G
authors.initials: G
authors.lastname: Giordano
mesh_terms: D000818:Animals; D019204:GTP-Binding Proteins / Q000187:drug effects* / Q000502:physiology*; D007985:Leydig Cells / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000494:pharmacology; D037342:Pertussis Toxin; D051381:Rats; D017208:Rats, Wistar; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D044094:Receptors, Melatonin; D012016:Reference Values; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
chemical_list: D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D044094:Receptors, Melatonin; D010566:Virulence Factors, Bordetella; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins; D008550:Melatonin