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Tractographic analysis of the medulla oblongata necessitates partial re‐wiring of the contemporary neural circuit for baroreflex‐mediated sympathetic vasomotor tone
Author(s) -
Chan Samuel H.H.
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.848.15
Subject(s) - rostral ventrolateral medulla , baroreflex , medulla oblongata , vasomotor , medicine , neuroscience , baroreceptor , blood pressure , anesthesia , anatomy , heart rate , biology , central nervous system
Contemporary dogma stipulates that the caudal ventrolateral medulla (CVLM) participates in baroreflex‐mediated sympathetic vasomotor tone by acting as a synaptic relay that connects the nucleus tractus solitarii (NTS) to the rostral ventrolateral medulla (RVLM). Functionally, CVLM exerts an inhibitory action on RVLM neurons that are involved in tonic and reflex control of sympathetic vasomotor tone. Whether neuronal traffic in the above‐mentioned circuit actually takes place in the brain stem during the execution of baroreflex has never been visualized. This study was undertaken to fill this void. In male adult C57BL/6 mice instrumented for physiological measurements by radiotelemetry, intracerebroventricular (i.c.v.) infusion of angiotensin (7.5 μg/h at 0.5 μL/h) by osmotic minipump for 7 days elicited a significant elevation in mean arterial pressure (MAP) that returned to basal levels over time. Determined by the power density of the low‐frequency component of systolic blood pressure, the baroreflex‐mediated sympathetic vasomotor tone exhibited a significant increase that paralleled temporally the elevation in MAP. Tractographic analysis based on magnetic resonance imaing/diffusion tensor imaging (DTI) of the medulla oblongata using a 9.4T scanner further revealed that under basal MAP, there was robust connectivity between the NTS and RVLM. This NTS‐RVLM connectivity was substantially reduced when visualized during sustained hypertension and elevated baroreflex‐mediated sympathetic vasomotor tone. More importantly, the connectivity between the NTS and RVLM was re‐established when MAP or low‐frequency power returned to their basal levels. Quantification of the magnitude of NTS‐RVLM connectivity by two DTI indices, fractional anisotropy (FA) and number of fiber tracts, yielded comparable results. Interestingly, in an anterior‐posterior presentation of the FA map of the medulla oblongata, we rarely (P < 0.0001, Chi‐square analysis) detected the obligatory presence of connectivity between NTS and CVLM under basal conditions (3 out of 56 observations) or during neurogenic hypertension (1 out of 28 observations). Our visualization of the baroreflex at work therefore necessitates partial re‐wiring of the circuit for baroreflex‐mediated sympathetic vasomotor tone.