Chronic Vagotomy Blunts Cardiorespiratory Reflex Responses Through Pre- and Postsynaptic Effects in the Brainstem Nucleus Tractus Solitarii (nTS)

Abstract

Sensory information from the heart, lungs, and gastrointestinal tract travels to the brain via afferents of the paired vagus nerves, where it is first integrated in the brainstem nTS that critically contributes to cardiorespiratory function. We have shown that reducing input to the nTS by unilaterally transecting the vagus nerve (vagotomy) induces astrocyte reactivity and microglial activation, and reduces sighs in hypoxia. However, the cardiorespiratory effects of vagotomy and contributing mechanisms are not well understood. We hypothesized that chronic vagotomy blunts cardiorespiratory reflex responses to vagal afferent stimulation via reduced nTS neuronal activity. Male Sprague-Dawley rats (6-wk) underwent right cervical vagotomy (or sham surgery) caudal to the nodose ganglion. After 7 days rats were anesthetized, ventilated, and instrumented to record mean arterial pressure (MAP), heart rate (HR), splanchnic sympathetic and phrenic nerve activity (SSNA and PhrNA, respectively), which decreased in response to vagal afferent stimulation in sham animals. Vagotomy blunted these decreases in MAP, SSNA, and PhrNA compared to shams. In rats expressing the calcium indicator GCaMP8m in nTS neurons, vagotomy also reduced neuronal Ca2+ responses to afferent stimulation both in vivo (photometry) and in vitro (live slice imaging). We then hypothesized that these changes were due to altered signaling at the level of the tripartite synapse, where astrocytes enwrap the sensory afferent-nTS soma synapse. We found that there is likely a vagal afferent contribution to the altered reflex responses, as vagotomy reduced presynaptic Ca2+ responses in vagal afferent terminals to solitary tract stimulation in the brainstem slice. Glutamate agonists, which induce similar cardiorespiratory responses as vagal stimulation, were nanoinjected into the nTS to determine if there was a postsynaptic contribution to the blunted responses as well. Responses to glutamate injection were also blunted following vagotomy, indicating that astrocytic glutamate uptake or nTS soma glutamate receptor function may be altered. Blocking astrocytic excitatory amino acid transporters, which remove extracellular glutamate in the nTS induced similar baseline effects and did not restore cardiorespiratory responses to glutamate in vagotomy. However, nanoinjection of NMDA into the nTS to activate ionotropic glutamate receptors mimicked the blunted responses to glutamate following vagotomy, with no difference in responses to AMPA nanoinjection. Likewise, immunohistochemistry and Western blot showed that vagotomy reduced NMDA receptor subunit expression in the nTS. Together these results indicate that chronic vagotomy blunts cardiorespiratory responses to vagal afferent stimulation, likely through reduced presynaptic activation and decreased NMDA receptor expression and function. These studies will help improve our understanding of how afferent input induces neuroplasticity within the nTS, and the resulting impacts on cardiorespiratory function in health and disease. NIH HL132836, HL128454, HL98602, T32OD011126. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.