Responses of thoracic spinothalamic neurons to intracardiac injection of bradykinin in the monkey.

Bradykinin stimulates afferent fibers arising in the heart and may be involved in the mediation of anginal pain and the pain associated with myocardial infarction. The sensation of pain requires that noxious information reach the brain. The purpose of the present study was to determine whether the spinothalamic tract is involved in transmitting noxious information from the heart to the brain. Bradykinin was injected (0.3-3.5 micrograms/kg) into the heart via a catheter in the left atrium while we recorded from single spinothalamic cells in the C8 to T5 spinal segments. Thirty-one of 41 cells responded to bradykinin. The responses of 12 cells were characterized by both an increase in discharge rate and entrainment of cell activity with the cardiac cycle. Eighteen cells responded with only an increased rate, and one cell exhibited only entrainment of cell activity with the cardiac cycle. The mean onset of increased cell activity occurred 15 seconds following drug injection, and the average duration of the response was 54 seconds. Thirty cells increased their mean discharge rate from 11 +/- 2.5 to 29 +/- 4.4 spikes/second. Thus, some spinothalamic cells probably received input from both mechanosensitive and chemosensitive afferents. Tachyphylaxis to repeated doses of bradykinin was observed in 41% of cells. Cells responding to bradykinin had a spontaneous discharge rate that was significantly greater than that of nonresponding cells. Cells did not require input from C-fiber afferents to respond to bradykinin. No statistically significant relationships were found among anatomical locations (laminae and segments) and responses to bradykinin, although cells in lamina I seemed to be less responsive than more ventrally located cells. We conclude that the spinothalamic tract may be involved in the sensation of cardiac pain.