Vagal afferent inhibition of spinothalamic cell responses to sympathetic afferents and bradykinin in the monkey.

Effects of stimulating the left thoracic vagus nerve on the responses of spinothalamic neurons to electrical stimulation of cardiopulmonary sympathetic afferent fibers and to intracardiac injections of bradykinin were determined. Experiments were performed on 39 monkeys (Macaca fascicularis) tranquilized with ketamine and anesthetized with alpha-chloralose. The 30 spinothalamic cells studied had the following characteristics. They were excited by manipulation of their somatic receptive fields, were excited by electrical stimulation of cardiopulmonary sympathetic afferent fibers, and exhibited viscerosomatic convergence. Responses of 15 of 19 cells to sympathetic afferent test stimuli were inhibited by conditioning stimuli applied to left thoracic vagus nerve. A conditioning-test interval of 40-50 msec resulted in maximal inhibition of responses to both A delta and C fiber sympathetic afferents. A long time course of inhibition was present to at least a conditioning-test interval of 200 msec. Left thoracic vagus nerve stimulation inhibited 14 of 14 cells responding to intracardiac injection of bradykinin. Entrainment of activity of five cells to the cardiac cycle occurred in response to bradykinin. In each case, left thoracic vagus nerve stimulation, in addition to reducing frequency of cell discharge, disrupted the cardiac related pattern of cell activity. Bilateral cervical vagotomy abolished all inhibitory effects of left thoracic vagus nerve stimulation. These results demonstrate that vagal afferents may participate in processing of information related to cardiac pain.