Rostral ventrolateral medullary but not medullary lateral tegmental field neurons mediate sympatho-sympathetic reflexes in cats

Barman, Susan M., and Hakan S. Orer. "Rostral ventrolateral medullary but not medullary lateral tegmental field neurons mediate sympatho-sympathetic reflexes in cats." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 299.5 (2010): R1269-R1278. Previous to this study, work by Dr. Barman and her laboratory had shown that the medullary lateral tegmental field (LTF) neurons set the pattern for resting sympathetic nerve activity (SNA), not the NTS, CVLM, or RVLM. She was very open in stating that sympathetic tone to most of the vasculature was not driven by the RVLM, and that the RVLM, CVLM, and NTS were not the only neuronal pathways that contributed to SNA modulation resulting from the baroreflex, chemoreflex, or vagal afferents. However, these hypotheses were based upon various microinjecitons into directly into the LTF while recording SNA, so this study looked to further characterize the role the LTF plays in the sympatho-sympathetic reflex in response to afferent stimulation. After the separate microinjection of 1) non-NMDA receptor antagonist (NBQX) 2) NMDA receptor antagonist (D-AP5) 3) muscimol into the LTF there were no differences seen in sympathoexcitatory response recorded from the right inferior cardiac nerve, following stimulation of either the left inferior cardiac afferent nerve (CN) or sympathetic afferent nerve (SN). This same protocol was then repeated in the RVLM and injections of NBQX reduced the sympatho-sympathetic excitatory response to ~25% from baseline levels, as well as increased the onset latency following the electrical stimulation to the CN and SN. Like in the LTF, microinjections of D-AP5 produced no significant differences in the sympatho-sympathetic excitatory response. Muscimol, however, like NBQX significantly reduced the responses to CN and SN stimulation. In most cases the response seen was negligible, with little to no excitation. Conclusively, there were no changes in the excitatory responses following identical microinjections into the NTS and CVLM. Results from this study disproved the original hypothesis and instead showed that the RVLM is a critical region involved in sympatho-sympathetic cardiovascular responses. Dr. Barman also goes on to hypothesize that non-NMDA receptors play a primary role in this reflex compared to NMDA receptors. ~JI