FOS IMMUNOHISTOCHEMICAL DETERMINATION OF BRAIN-STEM NEURONAL ACTIVATION IN THE MUSKRAT AFTER NASAL STIMULATION

Stimulation of the nasal passages of muskrats with either ammonia vapo urs or retrogradely-flowing water produced cardiorespiratory responses (an immediate 62% decrease in heart rate, 29% increase in mean arteri al blood pressure, and sustained expiratory apnoea). We used the immun ohistological detection of Fos, the protein product of the c-fos gene, as a marker of neuronal activation to help elucidate the brainstem ci rcuitry of this cardiorespiratory response. After repeated ammonia sti mulation of the nasal passages, increased Fos expression was detected within the spinal trigeminal nucleus (ventral laminae I and II of the medullary dorsal horn, ventral paratrigeminal nucleus, and spinal trig eminal nucleus interpolaris), an area just ventromedial to the medulla ry dorsal horn, the caudal dorsal reticular formation and the area of the A5 catecholamine group compared to control animals. Repeated water stimulation of the nasal passages produced increased Fos expression o nly in the A5 catecholamine group. There was an increase in the number of Fos-positive cells in the ammonia group in the ventral laminae I a nd II of the medullary dorsal horn and the ventral paratrigeminal nucl ei compared with the water group. We conclude that ammonia stimulation of the nasal passages produces a different pattern of neuronal activa tion within the brainstem compared with water stimulation. We also con clude that Fos immunohistochemistry is a good technique to determine f unctional afferent somatotopy, but that immunohistochemical detection of Fos is not a good technique to identify the medullary neurons respo nsible for the efferent aspects of an intermittently produced cardiore spiratory reflex. (C) 1997 IBRO.