The peptide angiotensin II (Ang II) is known to be a major participant in the central mechanisms that are responsible for neurogenic hypertension. Ang II acts within the brain to increase blood pressure, and the physiological mechanisms involved in this effect include angiotensin type 1 receptor (AT1-R)-mediated activation of sympathetic vasomotor neurons at the paraventricular nucleus of the hypothalamus (PVN). The PVN is a key relay point for integrating information on cardiovascular status and elicits increased sympathetic outflow and blood pressure, via increases in the activity of efferents projecting to areas such as the rostral ventrolateral medulla (RVLM). There is much evidence that this central action of Ang II is amplified in and contributes to neurogenic hypertension. Thus, understanding how Ang II regulates cardiovascular function via the brain in normal animals, including mechanisms of action and regulation, is critical and highly significant to uncovering the reasons for persistent over activity of central Ang II/AT1R mechanisms in high blood pressure and ultimately to the development of new therapeutic interventions. We established previously that in normal rats MIF is an Ang II-inducible intracellular negative regulator of the AT1R mediated excitation of PVN to RVLM sympathetic neurons and Ang II-induced pressor effect. We also determined that this MIF regulatory mechanism is dysfunctional in animal models of neurogenic hypertension that exhibit chronic central AT1R activation, including spontaneously hypertensive rats (SHR). The broad goal of the current proposal is to investigate the role of angiotensin type 2 receptors (AT2R), in concert with MIF, in regulating the cardiovascular actions of Ang II via AT1R at the PVN in normal and hypertensive rats. This strategy is based on our preliminary observations that both Ang II and MIF elicit profound increases in the expression of AT2R in PVN neurons, and that increased levels of AT2R in normotensive rat PVN blunts the AT1R-mediated pressor response to CNS-injected Ang II. Based on this, our published work on Ang II and MIF in the PVN control of blood pressure, and documented neuronal inhibitory actions of AT2R we developed the following overall hypotheses: (a) MIF induced expression of functional endogenous AT2R in PVN to RVLM neurons is a critical element of the longer term negative regulation by this protein over Ang II/AT1R cardiovascular actions; (b) This MIF/AT2R regulatory mechanism is absent from the PVN of SHR, but can be restored by the increased neuronal expression of MIF at this site. These hypotheses will be tested via the following aims: (1) Investigate the effects of MIF on AT2R expression in PVN to RVLM neurons of normotensive rats and SHR;(2) Determine whether AT2R, induced in PVN to RVLM neurons in response to MIF, produce effects that are antagonistic to AT1R-mediated excitation of the same cells;(3) Investigate the role of AT2R in the PVN in the inhibitory effects of MIF over Ang II/AT1R-induced cardiovascular responses in normotensive rats and SHR.
These aims will be studied through a multidisciplinary (cellular, molecular, physiological) approach.
In the broadest sense our research is aimed at understanding how the central nervous system (CNS) controls blood pressure in healthy individuals, and how faults in these control mechanisms contribute to hypertension. We have identified macrophage migration inhibitory factor in the brain as a regulator that helps to keep blood pressure in check and stop it from exceeding normal limits. The immediate goal of the proposed studies is to determine how macrophage migration inhibitory factor works in concert with angiotensin type 2 receptors in the CNS to help regulate blood pressure under normal conditions, and whether this regulation fails in hypertension. Information gained from these studies may lead to the development of novel therapeutic targets for hypertension.
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