Right atrial stretch activates neurons in autonomic brain regions that project to the rostral ventrolateral medulla in the rat

Activation of the cardiac mechanoreceptors results in changes in sympathetic nerve activity and plays an important role in the responses elicited by elevated blood volume. Stimulation of the reflex influences several key autonomic regions, namely the paraventricular nucleus (PVN), the nucleus of the tractus solitarius (NTS) and the caudal ventrolateral medulla (CVLM). Neurons in these regions project directly to the rostral ventrolateral medulla (RVLM), a critical region in the generation of sympathetic vasomotor tone. The aim of the present experiments was to determine whether neurons in the PVN, NTS and CVLM that are activated by cardiac mechanoreceptor stimulation also project to the RVLM. Animals were prepared, under general anesthesia, by microinjection of a retrogradely transported tracer into the pressor region of the RVLM, and the placement of a balloon-tipped cannula at the junction of the right atrium and the superior vena cava. On the experimental day, in conscious rats, the balloon was inflated to stimulate cardiac mechanoreceptors (n=9), or left uninflated (control, n=8). Compared with controls, there was a significantly increased number of Fos-immunoreactive neurons (a marker of activation) in both the PVN (2.5-fold) and NTS (two-fold), but this was not seen in the CVLM. Compared with controls, a significant number of the neurons in the PVN (8%) and NTS (4.0%) that projected to the RVLM were activated. The data suggest that subgroups of RVLM-projecting neurons located in the PVN and NTS are involved in the central reflex pathway activated by cardiac mechanoreceptor stimulation.