Mapping of Brain Stem Neuronal Circuitry Active during Swallowing

A poorly understood neural circuit in the brain stem controls swallowing. This experiment studied the swallowing circuit in the rat brain stem by means of fos immunocytochemistry. The fos protein is a marker of activated neurons, and under experimental conditions, repetition of a behavior causes the fos protein to be produced in the neurons involved in that behavior. The fos technique has been successfully used to delineate neural circuits involved in reflex glottic closure, cough, and vocalization; however, the technique has not been used to map the swallowing circuit. Nine rats were used in this study. Swallows were evoked in anesthetized rats for 1 hour, then, after a 4-hour delay to allow maximum fos production, the rats were painlessly sacrificed by perfusion. The brain stems were removed and sectioned in the frontal plane, and every fourth section was immunoreacted for fos protein. All sections were examined by light microscopy, and cells positive for fos were marked on drawings of brain stem structures for different levels throughout the brain stem. Control animals underwent sham experiments. After subtraction of the areas of fos labeling seen in controls, all experimental rats showed fos-labeled neurons in very discrete and localized areas, including practically all regions implicated by prior neurophysiology studies of swallowing. The distribution of labeled neurons was more dispersed through the brain stem than current theories of swallowing would suggest. Specifically, recent studies of swallowing control have focused on the nucleus of the solitary tract (NST) and the region surrounding the nucleus ambiguus (periambigual area) just rostral to the obex. These areas contained fos-labeled neurons, but unexpectedly, heavy labeling was found in the same areas caudal to the obex. Areas containing the heaviest labeling were specific subnuclei of the NST and surrounding reticular formation; the periambigual area; and the intermediate reticular zone in the pons and caudal medulla. Interestingly, none of these anatomic structures had uniform fos labeling; this finding suggests that the unlabeled areas are involved in other oromotor behaviors, or that the specific protocol did not activate the full population of swallowing-related neurons. A notable finding of this study is a candidate for the central pattern generator (CPG) of swallowing. Careful lesioning studies in cats strongly suggest that a region in the rostral-medial medulla contains the CPG for swallowing, although the exact location of the CPG was never pinpointed. In the homologous region of the rat brain stem, fos labeling was only found in a small group of neurons within the gigantocellular reticular formation that may be a candidate for the CPG. In summary, correlation with prior physiology experiments suggests that this experiment appears to have delineated many, if not all, of the components of the swallowing circuit for the first time in any mammal. In addition, other areas were found that might also be swallowing-related. One notable example is a small group of fos-labeled cells that may be the CPG for swallowing. Further studies are required to clarify the specific roles of the fos-labeled neurons seen in this study.