Contribution of Orexin System to Hypertension

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SUMMARY Hypertension is a major risk factor for cardiovascular disease, with salt sensitive hypertension (SSH) accounting for 51% of all cases. As augmented sympathetic nerve activity (SNA) and dysregulated neuroendocrine secretion are known to play critical roles in the development of SSH, w e propose a study to investigate the mechanisms whereby hyperactivity of the brain orexin system, may critically influence SNA and neuroendocrine dysregulation, thus contributing to SSH development. Orexin A (OXA) is a multifunctional neuropeptide produced by hypothalamic neurons that plays various physiological roles, including cardiovascular function, through binding with its receptors. Recent studies show that upregulation of brain orexin signaling occurs in several animal models of hypertension suggesting implications for overactive brain orexin signaling in hypertension development. However, the impact of orexin system in the development of SSH and detailed molecular mechanisms underlying orexin system mediated hypertension development are poorly understood. Our preliminary data shows that a high salt (HS) diet results in hypertension, and significantly increases expression of OXA and orexin 1 receptor (OX1R) in the hypothalamic paraventricular nucleus (PVN) of Dahl salt sensitive (Dahl S) rats, an animal model for human SSH, but not in salt resistant rats. Interestingly, HS diet intake in Dahl S rats also increases OX1R expression in adrenal glands, a component of hypothalamic-pituitary- adrenal (HPA) axis which is a central neuroendocrine response system to combat stress and regulate multiple physiological functions. The PVN controls SNA outflow and neuroendocrine hormone, such as vasopressin (AVP) and corticotropin-releasing hormone (CRH), production. Central administration of OXA increases SNA outflow and PVN AVP expression as well as plasma corticosterone levels, a hallmark of HPA axis activation. Hyperactivity of HPA axis is involved in multiple diseases including hypertension. Lesions in the PVN prevent high salt induced hypertension in Dahl S rats. This body of evidence suggests that PVN OX1R activation is involved in SNA and HPA axis regulation, and increased PVN OXA-OX1R signaling induced by high salt intake may stimulate neuroendocrine systemic secretion, trigger long-term adaptive changes through regulating downstream gene expression, producing excessive excitatory neurochemicals, resulting in augmented PVN sympathetic tone, and leading to SSH. The objectives of this project are: (i) investigate whether chronic knockdown of PVN OX1R expression decreases plasma corticosterone levels, attenuate HS induced increase in SNA, and prevent SSH; (ii) elucidate the molecular mechanism underlying the relationship between OXA-OX1R and its augmentation of SNA and dysregulation of HPA axis. A combination of in vivo gene transduction, electrophysiological recordings, and molecular and biochemical approaches will be employed to answer our questions using salt sensitive and normotensive rat models. Our studies may identify new targets for therapeutic intervention in hypertension.