Study On The Effect And Mechanism Of NE In The Hypothalamic Paraventricular Nucleus On Regulation Of Baroreflex

The hypothalamic paraventricular nucleus (PVN) is a central site under cortex for the integration of the neuroendocrine system and the autonomic nervous system. A great deal of studies have suggested that the PVN appears to be an important region in the regulation of cardiovascular function and homeostasis. It is well known that baroreflex plays important roles in the maintenance of normal blood pressure and in the regulation of cardiovascular functions. Although medullary structures including nucleus tractus solitarius (NTS), Rostral ventrolateral medulla (RVLM) and Caudal ventrolateral medulla (CVLM) appear to be the primary pathway involved in baroreflex, the baroreflex is also modulated by supra-medullary structures, such as the insular cortex, bed nucleus of the stria terminalis, and hypothalamus. Despite accumulating evidences show that PVN play important roles in the regulation of baroreflex, the chemical mediators in the PVN responsible for mediating the baroreflex are not well understood.Norepinephrine(NE) has been recognized as an important neurotransmitter in the central nervous system, and has physiological effects on regulating cardiovascular functions, temperature, pain, food and so on. The PVN receives projections from the noradrenergic neurons in the CVLM, NTS, and locus ceruleus, and both a and β adrempceptors are expressed in the PVN. Although some studies indicate that NE in the PVN is involved in the modulation of blood pressure, the effects of NE in the PVN on regulation of baroreflex remain to be elucidated. Therefore, in the present study, we have used microdialysis, high performance liquid chromatography, immunohisto-chemistry, microinjection and neuropharmacology methods to study the effect and mechanism of NE in the PVN on regulation of baroreflex. Four parts of experiment were carried out:1. The baroreflex was induced by intravenous injection of phenylephrine (60μg/kg) in conscious rats, and the extracellular concentration of NE in the PVN was measured by microdialysis and high performance liquid chromatography (HPLC) techniques.2. For determining whether the measured NE release was involved in the baroreflex, antagonist and agonist of adrenoceptors were perfused into the PVN region during baroreflex, and the blood pressure, heart rate and sensitivity of baroreflex were measured.3. Kainic acid (2nmol/L,0.3μl) was used to create a chemical lesion to destroy neuronal perikarya in the bilateral NTS but spare axons of passage and terminals in the vicinity of the injection site. And then we measured the extracellular concentrations of NE in the PVN region along with the blood pressure, heart rate and sensitivity of baroreflex.4. The c-Fos protein expression in the RVLM was observed by using immunohistochemistry when antagonists of adrenoceptors were administrated into PVN during baroreflex.Results:1. Intravenous injection of phenylephrine increased the mean arterial pressure (MAP) from (85.95±1.79) mmHg to (115.95±3.05) mmHg (P<0.05), and decreased the heart rate (HR) from (409.08+12.01) beats/min to (375.02+12.99) beats/min (P<0.05); the extracellular levels of NE in the PVN region showed an immediate increase during baroreflex, which reached to135.83±5.71%of the basal levels (98.92±5.25%)(P<0.05, n=8);2. Direct perfusion of prazosin (an antagonist of α1adrenoceptor)(1mmol/L) in the PVN region significantly increased the baroreflex sensitivity(n=8, P<0.05); direct perfusion of phenylephrine(lmmol/L)(an agonist of α1adrenoceptor) in the PVN region significantly decreased the baroreflex sensitivity (n=8, P<0.05); 3. Direct perfusion of yohimbine(lmmol/L)(an antagonist of α2adrenoceptor) in the PVN region significantly decreased the baroreflex sensitivity(n=8, P<0.05); direct perfusion clonidine (200nmol/L)(an agonist of α2adrenoceptor) in the PVN region significantly increased the baroreflex sensitivity (n=6, P<0.05);4. Direct perfusion of propranolol (10μmol/L)(an antagonist of P adrenoceptor) in the PVN region significantly decreased the baroreflex sensitivity (n=8, P<0.05); direct perfusion of isoprenaline (100nmol/L)(an agonist of β adrenoceptor) in the PVN region significantly increased the baroreflex sensitivity (n=6, P<0.05);5. Direct perfusion of NE in different concentration (10nM and50nM),50nM NE in the PVN region significantly decreased the baroreflex sensitivity (n=6, P<0.05);6. Selective lesion of the neuronal perikarya in the NTS with kainic acid induced significant increases in both MAP and HR and attenuation in baroreflex sensitivity and the extracellular levels of NE in the PVN region (n=6, P<0.05);7. Perfusion of prazosin into the PVN did not effect the expression of c-Fos protein in the RVLM region, but significantly enhanced the increases of the expression of c-Fos protein in the RVLM during baroreflex (n=6, F=31.11, P<0.05);8. Perfusion of yohimbine into the PVN induced significant increases in the expression of c-Fos protein in the RVLM region (n=6, P<0.05), but did not effect the increases of the expression of c-Fos protein in the RVLM during baroreflex (n=6, F=0.37, P>0.05);9. Perfusion of propranolol into the PVN induced significant increases in the expression of c-Fos protein in the RVLM region (n=6, P<0.05), but did not effect the increases of the expression of c-Fos protein in the RVLM during baroreflex (n=6, F=0.37, P>0.05);Conclusions1. NE in the PVN adjusts the phenylephrine-induced baroreflex in two-way by adrenoceptor;2. Activation of α1adrenoceptor can inhibit the baroreflex, whereas the baroreflex is facilitated by activation of α2or β adrenoceptor, and the dominant regulation is α1 adrenoceptor mediated inhibitory regulation;3. The increase of NE in the PVN during braroreflex partly comes from noradrenergic neurons of NTS which produce an inhibitory effect on brarofiex;4. NE and α1adrenoceptor in the PVN modulates the baroreflex partly via affecting the signal efference in the RVLM.