Activity of swallowing‐related neurons in the medulla in the perfused brainstem preparation in rats

Abstract

Objectives/Hypothesis We aimed to investigate and validate the cellular activity patterns and the potential topographical organization of neurons of the medullary swallowing pattern generator (Sw‐CPG). We used the perfused brainstem preparation as an innovative experimental model that allows for stable neuronal recording in the brainstem. Study Design Animal model. Methods Experiments were conducted in 14 juvenile Wistar rats. The activities of the phrenic, vagus, and hypoglossal nerves were recorded at baseline, and fictive swallowing was elicited by stimulation of the superior laryngeal nerve. Extracellular action potentials of 72 swallowing‐related neurons were recorded in the Sw‐CPG of the dorsal medulla oblongata. Results Neurons could be classified into three types: sensory relay, and neurons that were excited or inhibited during fictive swallowing. Approximately one‐third of the neurons likely received monosynaptic input from the laryngeal afferents. One‐third of neurons recorded showed respiratory‐related activity, most of which exhibited inspiratory modulation. The neurons were widely distributed in the nucleus tractus solitarius and reticular formation. Conclusions The perfused brainstem preparation of rat fully preserves the Sw‐CPG. The recorded cellular activities and general topographical organization of swallowing neurons are in accordance with previous in vivo studies. Thus, the perfused brainstem preparation is an ideal experimental model to advance the understanding of neuronal mechanisms underlying swallowing. Level of Evidence NA Laryngoscope, 129:E72–E79, 2019

Laryngeal afferent modulation of swallowing interneurons in the dorsal medulla in perfused rats

“…The results of previous studies have shown that the majority of the interneurons controlling the pharyngeal stage of swallowing are orthodromically activated by either the SLN or glossopharyngeal nerve, suggesting that sensory signals (produced by the bolus transit through the pharyngeal cavity) may modulate the sw‐CPG to aid an efficient food bolus transfer into the esophagus. We speculate that swallowing interneurons that were excited by concomitant SLN‐stimulation indeed could be mainly associated with the sensory feedback regulation of pharyngeal swallow rather than pre‐determined sequential output signals.…”

“…The nerve signals were integrated with a 1‐ms time constant and full‐wave rectified. The detailed surgical and recording procedures were described in a recent study to perform recordings from the swallowing‐related neurons in the dorsal medulla . All other basic experimental procedures of perfused brainstem preparation were in accordance with previous reports …”

“…The preparations were then placed in a stereotaxic frame attached to a perfusion chamber secured by ear bars and cannulated by a catheter into the descending aorta. The systemic circulation was perfused with aCSF (NaCl 125 mM, KCl 3 mM, KH2PO4 1.25 mM, CaCl 2.5 mM, MgSO4 1.25 mM, NaHCO3 25 mM, D‐glucose 10 mM, and 1.25% Ficoll) bubbled with carbogen . The perfusate was continuously circulated with a peristaltic pump set within a range of 14 to 23 mL/min depending on the individual preparation.…”

“…The laryngeal stimulation can be predominantly transmitted to the nucleus tractus solitarius (NTS) through the SLN as visceral afferent signals and then conveyed to the higher center, albeit a part of sensory inputs to the larynx could be conveyed to the somatosensory area in the cortex . Fictive swallowing was elicited by stimulation of the SLN (pulse duration, 0.2 ms; frequency, 20 Hz) unilaterally that triggered distinct bursting activity in either the VNA or HNA …”

“…The sw‐CPG can be subdivided into two main compartments, the dorsal swallowing group (DSG) that may control the initiation and the sequential output pattern of swallowing and the ventral swallowing group that may drive the motoneurons to perform the complex swallowing movements . Previous studies have proposed that the neurons of the DSG are the kernel for the mediation and generation of the sequential swallowing motor pattern in mammals in absence of sensory afferent signals . Nevertheless, several lines of evidence have supported the hypothesis that sensory inputs from the alimentary tract are involved in, not only the initiation of reflex swallowing, but also in the fine‐motor adjustments of sw‐CPG activities during an ongoing swallowing sequence .…”