Melatonin has been implicated in the control of the reproductive system, and the modulatory actions of melatonin on gonadotropin-releasing hormone (GnRH) neurons have been assumed to be indirectly mediated through afferent neurons. However, our previous studies demonstrate sexually dimorphic modulation of A-type γ-aminobutyric acid (GABA) receptor (GABAAR) currents by melatonin in adult rat GnRH neurons and a preferential expression of melatonin 1a receptor (MT1) in male GnRH neurons. Using immortalized GnRH neurons (GT1–7 cells), the present study investigated the mechanism by which the expression of melatonin receptors is regulated in GnRH neurons. Like endogenous GnRH neurons, GT1–7 cells express both GnRH and GnRH receptor mRNAs, indicating that the cells have a self-stimulatory system. A 2-iodomelatonin binding assay and RT-PCR analysis demonstrated that the cells expressed neither MT1 nor MT2. However, treatment of GT1–7 cells with the GnRH antagonist cetrorelix significantly increased 2-iodomelatonin binding and induced a time- and concentration-dependent MT1 mRNA expression. The GABAAR currents were then measured using a perforated patch-clamp technique to examine whether the treatment with cetrorelix changed the responses to melatonin. Melatonin augmented the GABAAR currents in GT1–7 cells treated with 1 μM cetrorelix for 24 h, while melatonin decreased the currents in the cells not treated with cetrorelix, probably via receptor-independent processes. The present results suggest that GnRH downregulates the expression of MT1 via an autocrine-paracrine mechanism in GT1–7 cells, and modifies the melatonin-induced modulation of GABAAR currents. These findings may provide one possible mechanism for the sexually dimorphic responses to melatonin in adult rat GnRH neurons.

Keywords:
Cetrorelix, GT1–7 cells, Melatonin, Melatonin receptors, Gonadotropin-releasing hormone, Gonadotropin-releasing hormone neurons
1.
Reiter RJ: Pineal melatonin: cell biology of its synthesis and of its physiological interactions. Endocr Rev 1991;12:151–180.
2.
Aleandri V, Spina V, Morini A: The pineal gland and reproduction. Hum Reprod Update 1996;2:225–235.
3.
Reiter RJ: Melatonin and human reproduction. Ann Med 1998;30:103–108.
4.
Witt-Enderby PA, Bennett J, Jarzynka MJ, Firestine S, Melan MA: Melatonin receptors and their regulation: biochemical and structural mechanisms. Life Sci 2003;72:2183–2198.
5.
von Gall C, Stehle JH, Weaver DR: Mammalian melatonin receptors: molecular biology and signal transduction. Cell Tissue Res 2002;309:151–162.
6.
Reppert SM, Weaver DR, Ebisawa T: Cloning and characterization of a mammalian melatonin receptor that mediates reproductive and circadian responses. Neuron 1994;13:1177–1185.
7.
Roca AL, Godson C, Weaver DR, Reppert SM: Structure, characterization, and expression of the gene encoding the mouse Mel1a melatonin receptor. Endocrinology 1996;137:3469–3477.
8.
Reppert SM, Godson C, Mahle CD, Weaver DR, Slaugenhaupt SA, Gusella JF: Molecular characterization of a second melatonin receptor expressed in human retina and brain: the Mel1b melatonin receptor. Proc Natl Acad Sci USA 1995;92:8734–8738.
9.
Jin X, von Gall C, Pieschl RL, Gribkoff VK, Stehle JH, Reppert SM, Weaver DR: Targeted disruption of the mouse Mel1b melatonin receptor. Mol Cell Biol 2003;23:1054–1060.
10.
Weaver DR, Liu C, Reppert SM: Nature’s knockout: the Mel1b receptor is not necessary for reproductive and circadian responses to melatonin in Siberian hamsters. Mol Endocrinol 1996;10:1478–1487.
11.
Glass JD, Knotts LK: A brain site for the antigonadal action of melatonin in the white-footed mouse (Peromyscus leucopus): involvement of the immunoreactive GnRH neuronal system. Neuroendocrinology 1987;46:48–55.
12.
Revel FG, Saboureau M, Masson-Pevet M, Pevet P, Mikkelsen JD, Simonneaux V: Kisspeptin mediates the photoperiodic control of reproduction in hamsters. Curr Biol 2006;16:1730–1735.
13.
Lincoln GA, Maeda KI: Reproductive effects of placing micro-implants of melatonin in the mediobasal hypothalamus and preoptic area in rams. J Endocrinol 1992;132:201–215.
14.
Malpaux B, Daveau A, Maurice F, Gayrard V, Thiery JC: Short-day effects of melatonin on luteinizing hormone secretion in the ewe: evidence for central sites of action in the mediobasal hypothalamus. Biol Reprod 1993;48:752–760.
15.
Roy D, Angelini NL, Fujieda H, Brown GM, Belsham DD: Cyclical regulation of GnRH gene expression in GT1–7 GnRH-secreting neurons by melatonin. Endocrinology 2001;142:4711–4720.
16.
Roy D, Belsham DD: Melatonin receptor activation regulates GnRH gene expression and secretion in GT1–7 GnRH neurons. Signal transduction mechanisms. J Biol Chem 2002;277:251–258.
17.
Kato M, Ui-Tei K, Watanabe M, Sakuma Y: Characterization of voltage-gated calcium currents in gonadotropin-releasing hormone neurons tagged with green fluorescent protein in rats. Endocrinology 2003;144:5118–5125.
18.
Kato M, Tanaka N, Usui S, Sakuma Y: The SK channel blocker apamin inhibits slow afterhyperpolarization currents in rat gonadotropin-releasing hormone neurones. J Physiol 2006;574:431–442.
19.
Hiraizumi Y, Nishimura I, Ishii H, Tanaka N, Takeshita T, Sakuma Y, Kato M: Rat GnRH neurons exhibit large conductance voltage- and Ca2+-activated K+ (BK) currents and express BK channel mRNAs. J Physiol Sci 2008;58:21–29.
20.
Sato S, Yin C, Teramoto A, Sakuma Y, Kato M: Sexually dimorphic modulation of GABAA receptor currents by melatonin in rat gonadotropin-releasing hormone neurons. J Physiol Sci 2008;58:317–322.
21.
Yin C, Ishii H, Tanaka N, Sakuma Y, Kato M: Activation of A-type γ-amino butyric acid receptors excites gonadotrophin-releasing hormone neurones isolated from adult rats. J Neuroendocrinol 2008;20:566–575.
22.
Mellon PL, Windle JJ, Goldsmith PC, Padula CA, Roberts JL, Weiner RI: Immortalization of hypothalamic GnRH neurons by genetically targeted tumorigenesis. Neuron 1990;5:1–10.
23.
Gillespie JM, Chan BP, Roy D, Cai F, Belsham DD: Expression of circadian rhythm genes in gonadotropin-releasing hormone-secreting GT1-7 neurons. Endocrinology 2003;144:5285–5292.
24.
Chan CW, Song Y, Ailenberg M, Wheeler M, Pang SF, Brown GM, Silverman M: Studies of melatonin effects on epithelia using the human embryonic kidney-293 (HEK-293) cell line. Endocrinology 1997;138:4732–4739.
25.
Conway S, Drew JE, Canning SJ, Barrett P, Jockers R, Strosberg AD, Guardiola-Lemaitre B, Delagrange P, Morgan PJ: Identification of Mel1a melatonin receptors in the human embryonic kidney cell line HEK293:evidence of G protein-coupled melatonin receptors which do not mediate the inhibition of stimulated cyclic AMP levels. FEBS Lett 1997;407:121–126.
26.
Nonno R, Lucini V, Pannacci M, Mazzucchelli C, Angeloni D, Fraschini F, Stankov BM: Pharmacological characterization of the human melatonin Mel1a receptor following stable transfection into NIH3T3 cells. Br J Pharmacol 1998;124:485–492.
27.
Nonno R, Pannacci M, Lucini V, Angeloni D, Fraschini F, Stankov BM: Ligand efficacy and potency at recombinant human MT2 melatonin receptors: evidence for agonist activity of some MT1-antagonists. Br J Pharmacol 1999;127:1288–1294.
28.
Johnston JD, Messager S, Ebling FJ, Williams LM, Barrett P, Hazlerigg DG: Gonadotropin-releasing hormone drives melatonin receptor down-regulation in the developing pituitary gland. Proc Natl Acad Sci USA 2003;100:2831–2835.
29.
Barrett P, MacLean A, Davidson G, Morgan PJ: Regulation of the Mel 1a melatonin receptor mRNA and protein levels in the ovine pars tuberalis: evidence for a cyclic adenosine 3′,5′-monophosphate-independent Mel 1a receptor coupling and an autoregulatory mechanism of expression. Mol Endocrinol 1996;10:892–902.
30.
Clemens JW, Jarzynka MJ, Witt-Enderby PA: Down-regulation of mt1 melatonin receptors in rat ovary following estrogen exposure. Life Sci 2001;69:27–35.
31.
Seltzer A, Viswanathan M, Saavedra JM: Melatonin-binding sites in brain and caudal arteries of the female rat during the estrous cycle and after estrogen administration. Endocrinology 1992;130:1896–1902.
32.
Wan Q, Man HY, Liu F, Braunton J, Niznik HB, Pang SF, Brown GM, Wang YT: Differential modulation of GABAA receptor function by Mel1a and Mel1b receptors. Nature Neuroscience 1999;2:401–403.
33.
Ishii H, Tanaka N, Kobayashi M, Kato M, Sakuma Y: Gene structures, biochemical characterization and distribution of rat melatonin receptors. J Physiol Sci 2009;59:37–47.
34.
Liu C, Weaver DR, Jin X, Shearman LP, Pieschl RL, Gribkoff VK, Reppert SM: Molecular dissection of two distinct actions of melatonin on the suprachiasmatic circadian clock. Neuron 1997;19:91–102.
35.
Wang LM, Suthana NA, Chaudhury D, Weaver DR, Colwell CS: Melatonin inhibits hippocampal long-term potentiation. Eur J Neurosci 2005;22:2231–2237.
36.
Hogan MV, El-Sherif Y, Wieraszko A: The modulation of neuronal activity by melatonin: in vitro studies on mouse hippocampal slices. J Pineal Res 2001;30:87–96.
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2009
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