Analysis of sympathetic discharges during cardiogenic hypertensive chemoreflex

Serotonin activates a cardiogenic hypertensive chemoreflex. During this reflex, sympathetic efferent neural discharges were examined in 10 anesthetized dogs. Thoracic nerves were identified, severed distally, and placed across electrodes. The efferent neurogram was recorded along with an electrocardiogram and blood pressure and stored on tape. Neurograms were later analyzed by a Lab 8/e computer with a Schmitt trigger. The resting tone and the elicited discharges of the multifiber preparations were plotted as poststimulus histograms. The integrals of the histograms were normalized. During the reflex, sympathetic efferent discharges averaged 4.9 (+/- 0.8) s in duration and the total discharge frequency was 199 (+/- 16)% of control tone. The evoked peak blood pressure response averaged 221/171 mmHg. Following autonomic blockade, the attenuated blood pressure response averaged 125/96 mmHg. The sympathetic efferent discharges then lasted 5.2 (+/- 0.7) s (NS). However, total discharge frequency (268 +/- 37% of control tone) was significantly greater (P less than 0.01) that before autonomic blockade. These data suggest that baroreceptors influence the rate of discharge and the total amount of neural traffic, but not the duration of the sympathetic response evoked by the cardiogenic hypertensive chemoreflex.