The physiological mechanisms that control energy homeostasis are reciprocally linked to reproduction. However, the neuroendocrine circuitry that registers endocrine cues to direct homeostatic responses in energy balance and reproduction remain unknown. Neuropeptide Y (NPY) neurons have emerged as a key central target of estrogen and leptin that are capable of modulating both reproduction and energy balance. The hypothesis was generated that NPY neuronal subpopulations act as an integration centre to regulate the effects of estrogen and leptin on these important physiological processes through specific signaling pathways. Using hypothalamic cell lines that express the leptin receptor (Ob-R), estrogen receptor (ER) and NPY, this hypothesis was tested in three aims.
17β-estradiol (E2) was previously demonstrated to biphasically regulate NPY mRNA in the mHypoE-38 neuronal cell line; where 24 h E2 exposure induced NPY gene expression that our group proposed may be involved in the gonadotropin-releasing hormone (GnRH) preovulatory surge. E2 also acts as an anorexigenic hormone through unknown hypothalamic targets. E2 directly decreased NPY secretion in the mHypoE-42 and mHypoA-2/12 neuronal cell lines through ER-α. The anorexigenic action of E2 was mediated through the energy sensing 5’ AMP-activated protein kinase (AMPK) and the phosphoinositide-3-kinase (PI3K) pathway. NPY secretion was also decreased by leptin in mHypoA-59 and NPY-GFP cell models through AMPK- and PI3K-dependent mechanisms. Prolonged exposure to leptin in NPY-GFP cell lines prevented AMPK signaling and the leptin-mediated reduction in NPY secretion, indicating NPY neuronal resistance with prolonged leptin exposure. Leptin also stimulated NPY secretion in mHypoE-38 neurons, which was blocked by pharmacological inhibitors of the mitogen-activated protein kinase (MAPK) and PI3K pathways. Importantly, conditioned medium from the mHypoE-38 NPY neuronal cells induced GnRH transcripts in GT1-7 neurons, which was inhibited by Y1-receptor antagonists. Pharmacological inhibitors of the MAPK and PKA signal transduction pathways attenuated the NPY-mediated increase in GnRH transcription.
Based upon these findings, I propose NPY neurons in the hypothalamus consist of a heterogeneous population of neurons, and provide the first evidence of intrinsically different responses to function as physiological integrators for two different systems: NPY secretion can be suppressed to decrease food intake and induced to stimulate GnRH neurons.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/26143 |
| Date | 14 February 2011 |
| Creators | Dhillon, Sandeep S. |
| Contributors | Belsham, Denise |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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