Abstract
Introduction: A recent paradigm shift in cardiovascular pathophysiology is the impact of inflammation on hypertension. Inflammation within the paraventricular nucleus of the hypothalamus (PVN) is an important pathology of sympathetic hyperactivity, and is mainly mediated by innate immune cells, microglia. Activated microglia with alteration of their morphology produce inflammatory cytokines. Previous reports demonstrated that microglia within the PVN have activated morphology in angiotensin II-induced hypertensive rats and spontaneously hypertensive rats compared with normotensive control Sprague-Dawley rats or Wistar-Kyoto (WKY) rats. However, the role of activated microglia in the PVN in blood pressure elevation associated with sympathetic hyperactivity remains unknown. In the present study, we determined whether inhibition of microglial activation within the PVN attenuates the blood pressure elevation in genetically hypertensive rats. Methods and Results: We evaluated the activation of PVN microglia, identified by microglia specific ionized calcium-binding adaptor molecule 1 immunoreactivity, by measuring the roundness and the perimeter of microglia at 6 weeks of age, early hypertension development phase in stroke-prone spontaneously hypertensive rats (SHRSP) and compared with them in age-matched normotensive WKY rats. At 6 weeks of age, increased roundness and shortening of perimeter of microglia, indicating activated microglia, were observed in SHRSP compared with those in WKY rats. Then, we performed intracerebroventricular (ICV) administration of minocycline (5 μg/h) to deactivate microglia at 6 weeks of age for 4 weeks. ICV administration of minocycline significantly attenuated systolic blood pressure elevation in SHRSP over 4 weeks (at the end of experiments; 203.2±2.2 mm Hg vs. 215.9±2.7 mm Hg, n=8-9, P<0.05), but not in WKY rats. At 10 weeks of age, morphological analysis revealed that ICV minocycline significantly decreased the roundness and increased the perimeter of microglia, indicating deactivation of microglia, within the PVN in SHRSP. Conclusions: Hypothalamic activated microglia with morphologic changes accelerate blood pressure elevation during the hypertension development phase in SHRSP.
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