β-Adrenoceptors in the hypothalamic paraventricular nucleus modulate the baroreflex in conscious rats

The role of β-adrenoceptors of the hypothalamic paraventricular nucleus (PVN) in modulation of the baroreflex was investigated in conscious rats. The baroreflex was induced by intravenous injection of phenylephrine, and then the extracellular concentration of norepinephrine in the PVN region determined using microdialysis and high-performance liquid chromatography. Next, the role of the β-adrenoceptor in modulation of the baroreflex was investigated by perfusion of its antagonist or agonist into the PVN using microdialysis. Intravenous injection of phenylephrine increased the norepinephrine concentration in the PVN by 35.83 ± 5.71%. Propranolol (an antagonist of the β-adrenoceptor) significantly decreased the gain of reflex bradycardia, but did not affect the magnitude of blood-pressure increases in the baroreflex, resulting in reduced baroreflex sensitivity. Isoprenaline (an agonist of the β-adrenoceptor) significantly increased the gain of reflex bradycardia without affecting blood-pressure increases, leading to increased baroreflex sensitivity. Our results suggest that norepinephrine in the PVN facilitates the phenylephrine-induced baroreflex via β-adrenoceptors.