Effects of sympathetic stimulation on the round window compound action potential in the rat

The effect on the ear of stimulating the sympathetic nervous system was studied in rats by recording the compound action potentials (N1N2) in response to 2 kHz tonebursts presented to anesthetized rats before, during, and after electrical stimulation of the superior cervical ganglion, and evaluating the changes in N1 latency which resulted. Stimulation of the superior cervical ganglion was found to cause an increase in the N1 latency which was more pronounced at low stimulus intensities (mean value 0.09 +/- 0.04 ms (S.E.) at 5 dB above threshold) than at moderate stimulation intensities (0.08 +/- 0.04 ms at 15 dB above threshold), with little change in latency occurring at the highest intensity tested (0.02 +/- 0.01 ms at approximately 25-30 dB above threshold). In addition, individual animals varied in their responses to stimulation of the superior cervical ganglion, with some animals evidencing a great change in the latency of the response (0.4 ms increase at 10 dB above threshold) and others showing very little change in latency. This variability could not be related to the condition of the animal at the time of observation of the response. In one of the twelve animals there was a slight decrease in latency as a result of sympathetic stimulation (0.07 ms at 5 dB above threshold), and although not studied systematically, low frequencies seemed to be affected more than high frequencies. Further, the change in the amplitude of N1 was not systematically related to sympathetic stimulation. After the administration of hexamethonium (which blocks transmission in autonomic ganglia) in three rats there was no effect on the latency of the N1 potential from sympathetic stimulation as recorded before sympathetic stimulation.