Nutrient availability is a key determinant of reproductive function. Specialized neurons in the brain monitor cellular metabolic stasis, and signal changes in oxidizable substrate fuels to the hypothalamic‐pituitary‐gonadal (HPG) axis. The periventricular caudal hindbrain is a recognized source of glucoprivic regulatory stimuli that restrain reproductive neuroendocrine function, but the neuroanatomical and neurochemical features of the circuitry that links medullary metabolic sensing and preoptic gonadotropin‐releasing hormone (GnRH) neurons remain unclear. The ovarian steroid, estradiol, controls HPG function through negative and positive feedback; estradiol amplifies glucoprivic inhibition of luteinizing hormone (LH) secretion in ovariectomized rats, but the cellular substrates and mechanisms of such action are not known. The overarching premise of this proposal is that A2 noradrenergic neurons in the caudal dorsal vagal complex detect and signal neuroglucopenia to the HPG axis via neuroanatomical projections to the medial preoptic nucleus (MPN). It is also presumed that norepinephrine (NE)‐driven glucoprivic inhibition of LH is mediated by downstream modifications in preoptic γ‐aminobutyric acid (GABA) and kisspeptin neurotransmission.
Specific aim 1 will investigate the impact of neuroglucopenia on catecholamine biosynthetic enzyme and metabolic transducer gene profiles in MPN‐projecting A2 neurons during estrogencontrolled basal and elevated NE neurotransmission. Single‐cell quantitative real‐time RT‐PCR will be performed to assess effects of neuroglucopenia on catecholamine biosynthetic enzyme and metabolic transducer mRNA profiles in MPN‐projecting A2 neurons exposed to physiological nadir versus peak estradiol secretion. In light of our evidence that A2 neurons express estrogen receptor‐alpha (ERα) and ‐beta (ERβ) mRNA and protein, and that this gene transcript ratio is governed by estradiol concentration, pharmacological tools will be used here to discern the role of these receptor subtypes in estrogen‐controlled patterns of A2 signaling to the MPN, as well as glucoprivic enhancement versus reduction of the former and latter NEergic signals. The second project aim will characterize the role of MPN GABAergic neurons in hindbrain glucoprivic inhibition of the HPG axis, and evaluate the impact of estradiol on cellular reactivity to glucoprivic‐driven noradrenergic signaling. Proposed studies will evaluate the role of this cell population in hindbrain glucoprivic suppression of the HPG axis by examining the impact of immunolesion‐based destruction of MPN GABA neurons on glucoprivic patterns of LH secretion. Our data show that the direction of MPN GABAergic reactivity to glucoprivic NE signaling is estrogen concentration‐dependent; these results will be expanded by experiments aimed at evaluating the role of ERα and ERβ in the switch from positive to negative α1‐adrenergic receptor‐mediated control of MPN GABA neurons during nadir versus peak estradiol secretion. Finally, multi‐transcriptional profiling of individual MPN GAD‐ir neurons will be performed to determine if estrogenic and noradrenergic signals converge on these cells.
Specific aim 3 will determine if preoptic KiSS1 neurons are restrained by hindbrain glucoprivation, and examine whether these neurons function downstream of the A2 NE‐MPN GABA signaling cascade. We will investigate the impact of neuroglucopenia on AVPV KiSS1 gene expression, and examine whether inhibition of GnRH neuronal activation and LH secretion by this stimulus is reversed by exogenous kisspeptin administration into the rostral preoptic area. Since our data show that AVPV GABAA receptors mediate inhibitory effects of glucoprivic patterns of GABA neurotransmission on the HPG axis, we will examine whether pharmacological manipulation of AVPV GABAA receptors attenuates hindbrain glucoprivic regulation of AVPV KiSS1 gene expression. Lastly, we will investigate the impact of ER subtypes on KiSS1 neuronal reactivity to homeostatic and glucoprivic patterns of GABA input.
Clinical and experimental studies verify that the physiological control of fertility and energy balance is coupled. Oxidizable substrate fuel availability is a crucial determinant of reproductive function, and nutritional infertility in women, food animals, and laboratory animal models occurs when provision of these energy sources does not match the body?s overall metabolic demands. This project will improve public health by characterizing the cellular source‐of‐origin and the downstream neural circuitry that conveys signals of metabolic distress to the reproductive neuroendocrine axis. Forthcoming information will establish a foundation for investigation of the impact of distinct physiological states of estrogen secretion, including pre‐sexual maturity, pregnancy, and reproductive senescence, as well as pathophysiological alterations in CNS metabolic sensing, e.g. diabetes mellitus, on this regulatory influence on reproductive hormone secretion.
