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

The RVLM contains neurons that are integral to control of sympathetic nerve activity and blood pressure. These neurons are "connected" to the spinal cord and they play a part in regulating blood pressure throughout the body. Previous studies have shown that cardiovascular responses to the activation of RVLM neurons are increased in animals that are physically inactive, in comparison to physically active animals. Studies also show that when compared with physically active rats, sedentary rats show enhanced splanchnic sympathetic nerve responses. Considering these findings, the researchers hypothesized that, "Changes in the function of RVLM neurons that regulate splanchnic sympathetic outflow contribute to the development of hypertension". It has also been found that wheel running exercise affects the dendritic morphology of neurons in cardiorespiratory centers within the central nervous system. Based on this knowledge, it was hypothesized that, "Physical inactivity may increase sympathetic nerve activity by altering the structure of bulbospinal neurons in the RVLM, specifically through an increase in dendritic branching and/or an increase in the size of their cell bodies." 

To test these hypotheses the researchers observed two groups of rats, one sedentary group, and one physically active group, as well as one rat that was part of neither group . The sedentary group had no way to exercise, while the active group had a running wheel in their cage. The rats were observed for 11-12 weeks and each day running data was recorded from the active rats. After 11 or 12 weeks, depending on the rat, a retrograde tracer was injected into their spinal cord and then they were returned to their cages. One week after the injection of the tracer, all rats were perfused, brains and spinal cords were removed and postfixed for 3-4 days. Brainstems were stored briefly and then they were sectioned off and each section was distributed into a well. The sections of brainstems were distributed so that each well contained one section from a sedentary rat, and one section from a physically active rat. The brainstem of the rat that was not a part of either group was divided as well. 

The brainstem sections were then immunohistochemically stained, and embedded in resin. To detect the retrograde tracer that had previously been injected into the rat spinal cords, sections were treated with rabbit and goat antibodies. Next, the sections from the six active and five inactive rats were embedded in Durcupan resin on glass slides. The slides were then examined and photographed using a microscope equipped with a camera. Neurons were examined and counted in each brainstem section that showed the retrograde tracer, however, only neurons within a specified area of each section were counted.

After examination of these neurons, it was found that on average, neurons from sedentary versus active rates had more dendritic branch points, greater total dendrite length, greater total dendritic surface area, and more intersections in Sholl analyses. There was no difference between sedentary and active rats in cell body perimeter, cell body area, cell body shape, or number of primary dendrites. Furthermore, it was found that neurons located rostral to the caudal pole of FN had more branch points than those located caudal to the caudal pole of FN, in sedentary rats, while the morphology of neurons in physically active rats remained consistent throughout the RVLM. This finding supports the hypothesis that physical activity may alter the structure of bulbospinal neurons in RVLM. It was also found that when compared with physically active rats, sedentary rats show increased baseline renal sympathetic nerve activity. The sympathetic outflow of sedentary rats was found to be nearly twice as much as that of active rats in splanchnic sympathetic nerve activity. In addition, neurons within the RVLM show increased activation in various disorders, such as high-salt diet, hyperinsulinemia, chronic renal failure, obesity, hypertension, and chronic heart failure. These findings suggest that neurons in the RVLM are directly involved in the regulation of blood pressure within the body.