MICROGLIA RESPONSES IN A RAT MODEL OF CHRONIC SLEEP RESTRICTION

Abstract

Recent research suggests that the brain and body adapt to chronic sleep restriction (CSR) resulting in only modest attempts to recover lost sleep and negatively impacting health over time. Microglia, the immune cells of the brain, are possible mediators of this adaptation to CSR, yet their responses to CSR in key keep/wake regulatory brain regions remained to be characterized. Therefore, I investigated microglia responses to CSR using a rat model featuring slowly rotating wheels (3 h on/1 h off). Adult male rats were sleep restricted for 3, 27 or 99 h, or 99 h followed by 6 days of undisturbed recovery sleep along with several time-matched housing and activity control groups. First, using the microglia marker ionized calcium-binding adaptor molecule-1 (Iba1), I found that the number of Iba1-immuoreactive cells and the density of Iba1 immunoreactivity were increased in 4/10 brain regions involved in sleep/wake regulation and cognition after 27 and/or 99 h of CSR. Neither microglia proliferation nor impaired blood–brain barrier permeability appeared to contribute to the increased Iba1 immunoreactivity. In the frontal cortex and lateral hypothalamus, two brain regions that showed increased Iba1 in response to CSR, Iba1+ microglia appeared ramified in all treatment groups with no effect of CSR. Next, I demonstrated using qRT-PCR that after 27 h, but not 99 h, of CSR, mRNA levels of the anti-inflammatory cytokine interleukin-10 were increased in the frontal cortex. Pro- inflammatory cytokine mRNA levels (tumor necrosis factor-α, interleukin-1β, and interleukin-6) were unchanged. Furthermore, cortical microglia were not immunoreactive for several inflammatory markers but were immunoreactive for the P2Y12 receptor. Collectively, these results suggest a homeostatic microglia phenotype. Finally, I investigated the functional role of CSR-responsive microglia by examining the structural relationship of microglia with two major populations of sleep/wake-regulatory neurons, namely melanin-concentrating hormone (MCH) and orexin (ORX) neurons in the lateral hypothalamus. Microglia were found in close contact with both MCH and ORX neurons and this microglia–neuron contact remained unaltered following CSR. These results suggest that microglia respond to CSR while remaining in a physiological state and may contribute to the previously reported homeostatic and adaptive responses to CSR.