Abstract
The nucleus tractus solitarii (nTS) of the brainstem plays an important role in cardiorespiratory homeostasis, including the reflex response to hypoxia. The nTS receives, integrates, and modulates sensory afferent input from the viscera. Alterations in afferent input and nTS function occur in many cardiorespiratory conditions, including obstructive sleep apnea, hypertension, and heart failure. The vagus nerves provide sensory input to the nTS. Vagal afferents release glutamate to activate nTS neurons as well as astrocytes, modifying nTS activity and cardiorespiratory output. However, the specific role of nTS astrocytes and other glia in cardiorespiratory modulation is unknown. We hypothesized that decreased afferent input (vagotomy) modifies astrocyte morphology and is associated with impaired cardiorespiratory responses to hypoxia. Male Sprague-Dawley rats (5-7 weeks old) underwent either right cervical vagotomy caudal to the nodose ganglion (n=9), sham surgery (n=8), or no surgery/control (n=8). Prior to surgery and one week post-surgery, cardiorespiratory parameters (respiration, heart rate, blood pressure, oxygen saturation) were recorded in conscious and anesthetized animals under baseline and hypoxic (8-12% O2) conditions. Rats were then euthanized and coronal brainstem sections from three areas of the cardiorespiratory nTS were examined using immunohistochemistry. Astrocytes were identified with glial fibrillary acidic protein (GFAP) and S100 calcium binding protein B (S100B), microglia with ionized calcium-binding protein 1 (IBA-1), and glutamatergic synapses with vesicular glutamate transporter 2 (VGLUT2). Immunoreactivity (-IR) was compared among experimental groups and between the manipulated and intact sides. At the level of the area postrema (mid-AP), vagotomy increased GFAP-IR compared to other groups, and on the manipulated compared to the intact side. Similar changes were seen in the dorsal motor nucleus of the vagus (DMX), where vagal efferent cell bodies are located. Astrocyte numbers were not altered by vagotomy, but Sholl analysis revealed that vagotomy increased astrocyte branching and altered branch patterns. Vagotomy also increased both IBA-1-IR and microglial numbers in the nTS (and DMX). Microglia in vagotomized animals exhibited decreased cell area and increased density, indicative of activation (analyzed with FracLac). VGLUT2-IR particle counts were not different among groups or between sides, suggesting a lack of glutamate synapse pruning resulting from vagotomy. In general, cardiorespiratory parameters under baseline and hypoxic conditions were not different among groups. However, vagotomy decreased the number of augmented breaths (sighs) during hypoxia. These data demonstrate that decreased afferent input via vagotomy produces both astrocyte reactivity (increased GFAP and altered branching) and microglial activation (increased numbers and amoeboid morphology). This likely indicates altered glial activity and contribution to nTS function and cardiorespiratory reflex responses. Further studies are needed to determine the specific contribution of glial changes in the brainstem to homeostasis and cardiorespiratory disease.
Published Version
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