Medullary pathways involved in cardiac sympathoexcitatory reflexes in the cat

Stimulation of cardiac sympathetic afferents evokes excitatory cardiovascular reflexes. However, the exact regions in the brain that integrate these reflexes have not been identified. Expression of c-Fos in the neurons provides a useful marker of the activated neurons. In the present study, we examined the response of c-Fos within the medulla of the cat to chemical stimulation of cardiac sympathetic afferents. After bilateral sinoaortic denervation and cervical vagotomy, we applied bradykinin (BK, 1-10 microg, n=7) six times to the anterior ventricular surface every 20 min. We observed consistent increases in blood pressure and heart rate while the vehicle for BK (0.9% saline, n=6) produced no responses. Ninety minutes after the end of the sixth treatment, transcardial perfusion was performed with 4% paraformaldehyde and the brainstem was harvested for immunohistochemical staining. Compared to the control animals, we noted Fos immunoreactive neurons in the nucleus of the solitary tract, lateral tegmental field, caudal and rostral ventrolateral medulla (VLM), and vestibular nucleus in the BK-treated cats (all P<0.05). Fos immunoreactivity was found in catecholaminergic neurons of the VLM. These findings indicate that the activated neurons in the medulla, especially in the VLM, are involved in integration of cardiac-cardiovascular sympathoexcitatory reflexes.