Effects of lateral reticular nucleus lesions on the exercise pressor reflex in cats

Electrical stimulation of ventral roots gives rise to a reflex cardiovascular response similar to that observed during static exercise. Although the afferent limb of the reflex is known to be comprised of small diameter afferent fibers from the contracting muscle, little is known of the central nervous system pathway(s) involved. The lateral reticular nucleus of the brainstem is known to be an important site of integration of numerous types of visceral and somatic afferent information, many of which give rise to cardiovascular responses. However, the linkage between the small diameter muscle afferents responsible for the exercise pressor reflex and the lateral reticular nucleus has not been established. In anesthetized cats (n=7), stimulation of L₇ and S₁ ventral roots increased mean arterial pressure (18.6 ± 2.4 mm Hg) and heart rate (7.4 ≠ 1.7 beats/min). Following bilateral lesions of the lateral reticular nucleus, the increases in mean arterial pressure and in heart rate were essentially abolished (P<0.005) (mean arterial pressure increased 1.9±0.8 mm Hg and heart rate increased 0.7 ± 0.5 beats/min). Unilateral lateral reticular nucleus lesions and control lesions in pressor sites outside the lateral reticular nucleus (n=5) did not affect the exercise pressor reflex. The lateral reticular nucleus lesions also produced decreases (P<0.01) both in resting mean arterial pressure (-27 ± 5.5 mm Hg) and heart rate (-31.0 ± 8 beats/min). These data suggest that the lateral reticular nucleus is important in the central pathway of the exercise pressor reflex and mediates a tonic pressor influence at rest.