FOS-DETERMINED DISTRIBUTION OF NEURONS ACTIVATED DURING THE BEZOLD-JARISCH REFLEX IN THE MEDULLA-OBLONGATA IN CONSCIOUS RABBITS AND RATS

Experiments were performed in unanaesthetized rabbits and rats to inve stigate the distribution, within the medulla oblongata, of neurons act ivated during the Bezold-Jarisch reflex. Repeated intravenous injectio ns of phenylbiguanide evoked depressor and bradycardic responses in bo th rabbits and rats. Fos-positive neurons were present in the nucleus tractus solitarius and in the caudal ventrolateral medulla oblongata. Double-label tyrosine hydroxylase (TH) immunohistochemical studies in the ventrolateral medulla showed that most Fos-positive neurons in the caudal ventrolateral medulla were TH-negative neurons scattered betwe en A1 noradrenaline cells, in the rabbit and in the rat. Approximately 20% of neurons in the caudal ventrolateral medulla in rabbits, and 50 % in rats, were immunoreactive for both Fos and TH. Some Fos-positive, TH-negative neurons in the caudal ventrolateral medulla were retrogra dely labelled with cholera toxin B-Gold after injection of this tracer into the sympathoexcitatory region of the rostral ventrolateral medul la. Our data suggests that neurons in the nucleus tractus solitarius, and rostrally projecting TH-negative neurons in the caudal ventrolater al medulla, are part of the pathway by which stimulation of cardiopulm onary receptors inhibits sympathetic vasomotor tone to decrease blood pressure during the Bezold-Jarisch reflex.