Inhibition of Brainstem Microglia Modulates Neuronal Activation in Autonomic Nuclei After Acute Peripheral Pro-Inflammatory Cytokine Injection

The autonomic nervous system (ANS) coordinates the body's response to stress. Both physical and psychological stressors trigger neuroendocrine reactions from the hypothalamus, influencing peripheral systems such as cardiovascular, pulmonary, and gastrointestinal systems. Chronic stress can activate immune responses, leading to elevated levels of pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNFα), which has been associated with psychiatric disorders, including major depressive disorder. In this study, we investigated the impact of pro-inflammatory cytokines on microglia in the dorsal vagal complex (DVC) and the ventral lateral medulla (VLM), key brainstem regions involved in autonomic signal integration. The DVC includes the area postrema (AP), a sensory circumventricular organ lacking an intact blood-brain barrier. This structural feature makes the AP particularly vulnerable to circulating factors, allowing it to relay peripheral changes to brainstem neurons. We administered an acute peripheral injection of TNFα (63 µg/kg) and, two hours later, perfused the animals with paraformaldehyde. The number of cFOS immunoreactive cells, indicative of neuronal activation, was significantly elevated in the DVC (44.33 ± 7.513; p < 0.05) and the VLM (9.00 ± 1.37; p < 0.05). Microglia, being resident immune cells in the CNS, respond to pro-inflammatory cytokines, such as TNFα. To examine the role of microglia in mediating neuronal responses to peripheral TNFα, we administered minocycline (160 µg/kg) into the fourth ventricle to inhibit microglia responses caused by pro-inflammatory signaling. This treatment reduced IBA1 immunoreactivity by 71% in the AP (223.6 µm 2 ± 30.79; p < 0.002) and by 57% in the NTS (123.9 µm 2 ± 39.63; p < 0.01). Subsequently, a TNFα injection was given 24 hours after minocycline administration. With microglial polarization inhibited, the number of cFOS-positive cells was significantly reduced in both the DVC (p < 0.05) and the VLM (p < 0.002). These findings highlight the impact of acute pro-inflammatory signaling on brainstem autonomic circuits and suggest that brainstem neurons can relay peripheral signals independently of microglial activation. Future research will aim to identify the specific neuronal phenotypes affected by TNFα following microglial inhibition and their role in ANS regulation. ORWH U54-MH118919 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.