Distribution of Fos-immunoreactive cells in the ventral part of rat medulla following voluntary treadmill exercise

The ventral part of the medulla, which contains important cardiovascular regions, is reportedly activated during exercise. Nevertheless, it was uncertain which region(s) in the ventral medulla are specifically activated by exercise. The present study aimed to demonstrate a general pattern of exercise-specific distribution of excited neuronal cells in the rat ventral medulla. Via immunohistochemical experiments, we mapped tyrosine hydroxylase- and Fos-immunoreactive cells (TH-IR and Fos-IR cells, respectively) on rat medullary coronal sections following a bout of voluntary treadmill exercise, a comparative control period, or after pharmacologically induced-hypotension under anesthesia. In the ventral medulla at the rostrocaudal level adjacent, but not rostral or caudal, to the caudal edge of the facial nucleus, voluntary treadmill exercise induced significant (P < 0.05) increases in Fos expression, similar to hypotension. The rostral ventrolateral medulla (RVLM), as compared with the rostral ventromedial medulla (RVMM), displayed a greater number of Fos-IR cells due to either exercise or hypotension. In the RVLM, either exercise or hypotension induced significant expression of Fos in both TH-IR and TH non-immunoreactive cells. In the caudal ventrolateral medulla (CVLM), hypotension, but not exercise, increased the ratio of Fos-IR cells in the TH-IR population. These findings demonstrate that RVLM adrenergic and non-adrenergic neurons are specifically excited by voluntary exercise in rats, while RVMM or CVLM neurons are not. We suggest that RVLM C1/non-C1 neurons are a major part of central circuitries underlying sympathetic adjustments to exercise.