Physical (In)Activity-Dependent Structural Plasticity in Bulbospinal Catecholaminergic Neurons of Rat Rostral Ventrolateral Medulla

Authors: Mishcel..Mueller Background: It is known that physical inactivity is a leading risk factor in cardiovascular disease, which is the number one cause of death in the United States. One potential consequence of physical inactivity is the development of hypertension that can also lead or contribute to cardiovascular disease. There are multiple forms of hypertension, however the one this paper focuses on is hypertension induced by increased sympathetic outflow (specifically splanchnic) from the brain region known as the rostral ventrolateral medulla (RVLM). Previous studies have shown that the RVLM is comprised of C1 and non-C1 spinally projecting neurons that decend down the intermediate medial column of the spinal cord and eventually target multiple areas in the body such renal, lumbar, and splanchnic nerves used to control blood pressure. Here the authors hypothesized through changes in the function of RVLM neurons that regulate splanchnic sympathetic outflow contribute to the development of hypertension. Methods: • Retrograde tracer CTB spinal cord injections • Immunohistochemistry • Light microscopy and digital reconstruction Results: • In both physically active and sedentary rats C1 neuron distribution varied depending on rostrocaudal level. It was seen rostral to the caudal pole of the facial nucleus most c1 neurons were arranged in clusters just ventral and medial to the motor neurons of the FN. However, in more caudal regions located caudal to the caudal pole of the FN C1 neurons were more evenly distributed and did not appear in tight clusters. • Morphological analysis following neuron reconstructions revealed C1 neuron dendritic structural differences in sedentary rats compared to physically active, with no differences in neuron somata. Overall dendritic surface area, branch points, and secondary dendritic length were increased in sedentary rats compared to physically active rats. • Finally, not only were structural differences observed between sedentary and active rats, but these morphological changes were seen on a rostrocaudal level, with increased branch points in more rostral regions of the RVLM. This rostrocaudal pattern in structural changes was not observed in the physically active cohort of rats. ~JI