Microglia Activation and IL‐1β Expression in Select Ventilatory Control Regions Following Exposure to Chronic Sustained Hypoxia

Abstract

Ventilatory acclimatization to hypoxia (VAH) is defined as the time‐dependent increase in ventilation which occurs with chronic sustained hypoxia (CSH) of several hours to months. Previous research has shown that astrocytes and microglia undergo a morphology shift, indicating a possible change in activity, upon exposure to hypoxic conditions and may contribute to VAH. Understanding when and how the different cell types in respiratory control regions are activated is pertinent to understanding ventilatory control during hypoxic conditions. The first part of this study aimed to optimize the Sholl analysis method to detect morphological changes in microglia. Two different microglia antibodies, Iba‐1 and CD11b[Ox42], were compared using Sholl analysis to determine which antibody best represents the branching pattern of the individual microglia. Analysis indicated that neither antibody was statistically different from the other in terms of microglia branching (p>0.05), so CD11b[Ox42] was chosen to simplify the immunohistochemistry protocol. The second part of this study assessed the morphology shift of microglia in the nucleus tractus solitarius (NTS) and Pre‐Bötzinger Complex (PBC) following CSH exposure. Based on previous research, we hypothesized that microglia in the NTS and PBC would be activated following CSH exposure, as assessed via a morphology shift to a more amoeboid state indicated by less microglia branch crossings of the Sholl brackets. To address this hypothesis, rats were exposed to either normoxic, 60‐minutes of CSH, or 12‐hours of CSH. Microglia morphology was assessed in perfused brainstem tissue via immunohistochemistry, confocal imaging, and image analysis. In the NTS, microglia branching analysis revealed a trend towards a more amoeboid morphology at the 60‐minute CSH time point, but only one of the Sholl analysis brackets, 21–30μM, was statistically significant from normoxic conditions (p<0.05). In the PBC, microglia branching analysis also revealed a trend towards a more amoeboid morphology at the 60‐minute CSH time point with statistically significant (p<0.05) branch patterns at the 11–20μM, 21–30μM, and 31–40 μM Sholl brackets. A morphology shift of microglia to a more amoeboid state could indicate their localized response to neurotransmitters or cytokines. In a follow‐up study we investigated the expression of IL‐1β in the NTS region using immunofluorescent labeling. Rat brainstem tissue from normoxic conditions, 15‐minute CSH, and 60‐minute CSH was labeled with antibodies against IL‐1β, GFAP, and Cd11b[Ox42]. IL‐1β positive cells were counted in the NTS region. This preliminary set of cell counts (n=2) suggests that there is an increase in IL‐1β positive cells with CHS exposure (normoxic 17 ± 2 IL‐1β positive cells, 15‐minute CSH 31.5 ± 0.5 IL‐1β positive cells, 60‐minute CSH 29.5 ± 9.5 IL‐1β positive cells). Taken together these data provide a great starting point in assessing the activation profiles of glial cells and cytokine activity in select respiratory control regions in the brainstem.Support or Funding InformationCentenary College Student‐Faculty Summer Research Award 2018 and 2019 (JRF and JAS); NIH R01HL081823 (FLP)