Brainstem interneurons necessary for vestibular influences on sympathetic outflow.

The objective of this study was to determine which general brainstem regions contain interneurons that are critical for mediating vestibulo-sympathetic responses in decerebrate cats, as a prelude for future cell recording studies. Large injections of kainic acid into the lateral reticular formation at levels caudal to the obex abolished sympathetic nerve responses elicited by electrical stimulation of the vestibular nerve. The same lesions also diminished excitatory and some inhibitory components of somato-sympathetic responses elicited by stimulation of the sciatic nerve, raising the possibility that a common pool of interneurons integrates signals from muscle, skin and the vestibular system that reflect body position in space. Large lesions of other brainstem regions containing interneurons involved in regulation of sympathetic activity, including nucleus tractus solitarius, the lateral tegmental field rostral to the obex, and the parabrachial nucleus, had no appreciable effect on the amplitude of either vestibulo-sympathetic or somato-sympathetic responses. Thus, interneurons critical for relaying vestibular and other somatic signals to descending cardiovascular-regulatory pathways appear to be located in the caudal and lateral parts of the medulla.