Excitatory and Inhibitory Responses of the Ongoing Sympathetic Discharge in Single Renal Neurons to Liminal Stimulation of Aortic C-Fibres in the Rabbit

Excitatory and inhibitory responses of sympathetic discharge were recorded in single renal postganglionic neurons of rabbits anaesthetized with urethane and chloralose. The animals were vagotomized and had transected aortic nerves. Responses were elicited by single volleys in the aortic C-fibres. Excitatory responses consisted in short-lasting increase in the rate of ongoing sympathetic discharge and were followed by inhibitory responses. Excitatory effects together with inhibitory responses were seen in 68% of units (19/28). Only excitatory effects appeared in 2 neurons (7.1%) and only inhibitory effects in 7 neurons (25%). In renal neurons exhibiting both effects, the excitatory responses appeared after latency of 172 +/- 8 ms (x +/- S.D.) and had duration of 64 +/- 11 ms. Inhibitory effects had latency o f 257 +/- 10 ms and their duration amounted to 265 +/- 22 ms. In more than half of recordings the excitatory responses were separated from the inhibitory effects by discharge lasting 33 +/- 4 ms. Significant correlations between latencies of excitatory and inhibitory responses and between duration of excitatory and latency of inhibitory responses suggest interaction between both effects. Increase in the number of afferent volleys (1 through 5) evoked relatively small changes in duration of the excitatory effect indicating that temporal facilitation is of minor importance in generating this response. Temporal facilitation was found to play an important role in determining duration of the inhibitory response. Comparison of effects of unilateral and bilateral stimulation of the aortic C-fibres showed larger occlusion of durations of the excitatory than inhibitory responses.