Biological Basis and Possible Physiological Implications of Melatonin Receptor- Mediated Signaling in the Rat Epididymis

Abstract

The mammalian epididymis plays an important role in sperm maturation, an important process of male reproduction. Specific high-affinity 2-[¹²⁵I]iodomelatonin binding sites, satisfying the pharmacokinetic properties of specific receptors, have been found in the rat corpus epididymis, suggesting a direct melatonin action on epididymal physiology. Subsequent molecular and cell biology studies have identified these 2-[¹²⁵I]iodomelatonin binding sites to be mt₁ (MEL_1A) and MT₂ (MEL_1B) melatonin receptor subtypes. Changes in the binding characteristics of these receptors in the rat corpus epididymis in response to castration and steroid hormones like testosterone and hydrocortisone indicated that these membrane melatonin receptors are biologically functional receptors, whose activities are differentially regulated by testosterone and hydrocortisone. These melatonin receptors are coupled to pertussis toxin (PTX)-sensitive G_i protein and probably participate in androgenic and adrenergic regulation of rat corpus epididymal epithelial cell functions. Furthermore, rat corpus epididymal epithelial cell proliferation was stimulated by melatonin, whose action was dependent on the concentration and duration of exposure to the hormone. Interestingly, an MT₂ receptor ligand (4-phenyl-2-propionamidotetraline, 4-P-PDOT) induced a stimulatory effect on epididymal epithelial cell proliferation similar to that produced by melatonin. In contrast, a nuclear melatonin receptor agonist (1-[3-allyl-4-oxo-thiazolidine-2-ylidene]-4-methyl-thiosemi-carbazone, CGP52608) and 8-bromo-cAMP inhibited epididymal epithelial cell proliferation. Taken together, our data lead us to postulate that one of the possible physiological functions of melatonin on the rat epididymis is the stimulation of mt₁ and MT₂ melatonin receptors resulting in the inhibition of cAMP signaling and an increase in epithelial cell proliferation.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, Classification, and Pharmacology of G Protein-Coupled Melatonin Receptors

“…MT 1 and/or MT 2 melatonin receptors are expressed on LNCaP prostate tumor cells, MCF-7 breast cancer cells (Xi et al, 2001;Ram et al, 2002), colon 38 cancer cells , human choriocarcinoma JEG-3 cells (Shiu et al, 2000), human gall bladder adenocarcinoma epithelial cells (Aust et al, 2004), and malignant breast epithelium cells (Dillon et al, 2002). The oncostatic action of melatonin seems to be mediated primarily through activation of MT 1 melatonin receptors, but an action on the MT 2 receptors cannot be excluded (Shiu et al, 1999(Shiu et al, , 2000.…”

“…The oncostatic action of melatonin seems to be mediated primarily through activation of MT 1 melatonin receptors, but an action on the MT 2 receptors cannot be excluded (Shiu et al, 1999(Shiu et al, , 2000. Melatonin inhibited endometrial cancer cell growth in in vitro studies with estrogen receptorpositive Ishikawa cells (Kanishi et al, 2000).…”

Modulation of Blood Glucose by Melatonin: A Direct Action on Melatonin Receptors in Mouse Hepatocytes

“…The concentrations of the competing agents displacing 50% of the specific binding (IC 50 ) were determined from analysis of the competition curves and the commercial software (Kaleidagraph, Abelbeck Software, Reading, Pa., USA). The inhibition constant (K i ) was calculated from IC 50 values by the method of Cheng and Prusoff [42]. Values are presented as the means B SEM.…”

Inhibition of the basal and oestradiol-stimulated mitotic activity of primary spermatogonia by melatonin in the testis of the frog, Rana esculenta, in vivo and in vitro

“…In fact, consideration should be given to the fact that melatonin interferes with the activation of the oestrogen receptor and destabilizes the binding of the oestradiol-oestrogen receptor complex to the oestrogen responsive element (Rato et al, 1999). Moreover, melatonin binding sites in the gonads of several vertebrate species have been demonstrated (Vera et al, 1997;Shiu et al, 2000;Clemens et al, 2001) and melatonin receptors coupled through a pertussis toxin-sensitive G-protein are present in adult rat Leydig cells (Valenti et al, 1997). In addition, the presence of a transcript of melatonin family receptors has been described in adult rat testis (De Rienzo et al, 2001).…”