Baroreceptor and chemoreceptor reflexes are actively involved in the control of fetal blood pressure and in the maintenance of blood flow to the placenta and to the metabolizing tissues. The present project is an investigation into the mechanism of the baro- and chemoreflexes. In the first funding period of this project, we discovered an important interaction between endogenously generated prostanoids and reflex activity. During the initial funding period, we made the following conclusions: 1) Endogenous generation of prostanoids augment the reflex mechanisms controlling cardiovascular and endocrine responses to hypotension. 2) Neurons, as well as vasculature, contain the enzymes (PGHS-1, PGHS-2, and thromboxane synthase) needed for synthesis of prostanoids. These enzymes are found in neurons important for reflex control of the cardiovascular system (in nucleus of the tractus solitarius, rostral and ventrolateral medulla, intermediolateral column of the spinal cord, paraventricular nucleus). There are ontogenetic changes in the concentrations of these enzymes. 3) PGE2 is released into the interstitial fluid of the brain and thromboxane A2 in released into the bloodstream during hypotension. 4) The so-call 'inducible' form of cyclooxygenase, PGHS-2, is upregulated at both the protein and mRNA level in fetal brain after hypotension. The upregulation of the protein for PGHS-2 is both in neurons and vasculature in the cardiovascular reflex-controlling regions of the fetal brain. 5) Injections of PGE2 or U46619 (thromboxane mimetic) into the cerebrospinal fluid of fetal sheep stimulates responses which are similar to the efferent responses to hypotension (vasoconstriction, secretion of adrenocorticotropin and vasopressin). These discoveries suggest that the production of prostanoids within afferent neural pathways stimulate or modulate reflex responses to cerebral hypoperfusion in the fetus. These findings have led us to propose the following specific aims, to propose to answer the following questions: 1) what are the relative contributions of PGHS-1 and PGHS-2 to the control of fetal reflex responses to hypotension? We will answer this question using both in vivo (chronically-catheterized fetal sheep), and in vitro (enzymology) techniques. 2) Does sinoaortic denervation alter the expression and distribution of PGHS-1 and PGHS-2 in fetal brain and does sinoaortic denervation block the PGHS-2 response to hypotension? We will answer this question using immunohistochemistry and RT-PCR. 3) Does hypotension cause the phosphorylation of PGHS-2, rapidly increasing its activity? We will answer this question using immunoprecipitation and immunoblot techniques. 4) Do prostanoids inhibit chloride uptake by GABAA receptors? We will answer this question using synaptoneurosomes (chloride uptake), in situ quantitative autoradiography, and immunohistochemical co-localization studies. In the proposed 5-year continuation of this project, the answers to these questions will significantly enhance our understanding of the mechanism by which prostanoids act within the brain to alter cardiovascular function.
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