Minocycline alters expression of inflammatory markers in autonomic brain areas and ventilatory responses induced by acute hypoxia.

NEW FINDINGS: What is the central question of this study? Microglia are presumed to be the source of inflammatory mediators that contribute to hypoxia-induced neuroinflammation. However, the relationship between microglial activity during hypoxia and inflammatory responses in specific autonomic brain regions is not well understood. Therefore, we hypothesized that acute hypoxia initiates an immune response in the central nervous system elicited by an increased expression of inflammatory mediators in specific brain areas related to autonomic control. What is the main finding and its importance? Acute hypoxia initiated neuroinflammatory mechanisms specifically in brain autonomic nuclei responsible for cardiorespiratory control, i.e. the rostral ventrolateral medulla and paraventricular nucleus of the hypothalamus. Our findings emphasize the importance of microglia for the maintenance of autonomic adjustments during physiological challenges, such as hypoxia, or during cardiorespiratory reflex activation elicited by the arterial chemoreceptors.

ABSTRACT: Prolonged and continuous exposure of mammals to a low oxygen environment (chronic hypoxia) elicits remarkable morphological and physiological adjustments. These include altered gene expression, increased peripheral chemosensitivity, enhanced respiratory drive and sympathoexcitation. The current study examines the hypothesis that acute hypoxia (AH) initiates an immune response in the central nervous system elicited by an increased expression of inflammatory mediators in specific brain areas related to autonomic control. Male Wistar rats pretreated with vehicle or minocycline (30 mg kg-1  day-1 for 5 days) were subjected to AH (8% O2 , balance N2 ) or normoxia (21% O2 ) for 3 h. AH increased interleukin (IL)-6, IL-1β and matrix metalloprotease 9 (MMP9) mRNA expression in the paraventricular nucleus of the hypothalamus (PVH) and rostral ventrolateral medulla (RVLM) and tumour necrosis factor α (TNFα) in the RVLM. Treatment with minocycline, an inhibitor of microglial activation, decreased IL-1β, TNFα and MMP9 mRNA expression in the RVLM, and increased IL-6 mRNA expression in the RVLM and PVH of rats exposed to AH. Minocycline treatment also elicited a decrease in the number of activated neurons in the RVLM/C1 neurons (expressed as Fos+ /tyrosine hydroxylase+ ), the number of Fos-activated neurons in the PVH and the increase in ventilation elicited by AH. When viewed together, these results suggest that AH modulates the expression of inflammatory mediators in autonomic brain nuclei that may be involved in the responses to chemoreceptor activation.