title: Orexin A induces GnRH gene expression and secretion from GT1-7 hypothalamic GnRH neurons.
abstract: Orexin A, a recently discovered hypothalamic peptide, has been shown to have a stimulatory effect on release of gonadotropin-releasing hormone (GnRH) from rat hypothalamic explants in vitro. However, it is presently unclear whether in vivo this effect is mediated directly at the level of the GnRH neuron, or via multiple afferent neuronal connections. Therefore, in the present study, we investigated the direct action of orexin A on GnRH neurons using the immortalized GnRH-secreting GT1-7 hypothalamic cells. Orexin-1 receptor (OX1R) expression was detected in GT1-7 cells by RT-PCR and Western blot. Results showed that 0.1-1 nM orexin A, when administered in culture media for 4 h, can significantly stimulate GnRH mRNA expression in GT1-7 cells (p < 0.05). Administration of 1 microM OX1R antagonist, SB-334867, completely blocked the observed orexin A responses in these cells, indicating that orexin A stimulation of GnRH neurons is specifically through OX1R. Moreover, 0.1 nM orexin A stimulated GnRH release after 30-45 min. To examine possible signal transduction pathways involved in mediating these effects, a MEK inhibitor (UO-126), PKC inhibitor (calphostin C), and PKA inhibitor (H-89), were used, with each blocking orexin A-induced GnRH transcription and release from immortalized cells. Collectively, our results show that orexin A is capable of directly stimulating GnRH transcription and neuropeptide release from these immortalized hypothalamic neurons, and that the effects of orexin A appear to be mediated via the OX1R, coupled with activation of the PKC-, MAPK- and PKA-signaling pathways. It is suggested that the stimulatory effect of orexin A on GnRH transcription and release may also occur directly at the level of GnRH neurons in vivo.
authors.affiliation: Department of Reproductive Medicine, University of California at San Diego, La Jolla, CA 92093-0674, USA.
authors.forename: Ravid
authors.initials: R
authors.lastname: Sasson
authors.forename: Robert K
authors.initials: RK
authors.lastname: Dearth
authors.forename: Rachel S
authors.initials: RS
authors.lastname: White
authors.forename: Patrick E
authors.initials: PE
authors.lastname: Chappell
authors.forename: Pamela L
authors.initials: PL
authors.lastname: Mellon
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology; D005786:Gene Expression Regulation / Q000187:drug effects*; D007987:Gonadotropin-Releasing Hormone / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D007031:Hypothalamus / Q000166:cytology / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000494:pharmacology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D020929:Mitogen-Activated Protein Kinase Kinases / Q000502:physiology; D009474:Neurons / Q000378:metabolism; D009479:Neuropeptides / Q000494:pharmacology*; D018377:Neurotransmitter Agents / Q000494:pharmacology*; D064446:Orexin Receptors; D000068797:Orexins; D011493:Protein Kinase C / Q000502:physiology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism; D018013:Receptors, Neuropeptide / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
chemical_list: C000596229:HCRT protein, human; C578627:Hcrtr1 protein, rat; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D018377:Neurotransmitter Agents; D064446:Orexin Receptors; D000068797:Orexins; D012333:RNA, Messenger; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D007987:Gonadotropin-Releasing Hormone; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D020929:Mitogen-Activated Protein Kinase Kinases