Discharge pattern of renal sympathetic nerve activity in the conscious rat: Spectral analysis of integrated activity

We investigated the periodic characteristics of bursting discharge in renal sympathetic nerve activity (RSNA) in conscious rats. Employing a discrete fast Fourier transform algorithm, a power spectrum analysis was used to qua ntify periodicities present in rectified and integrated RSNA whose signal-t o-noise ratio in the recordings was greater than six. In conscious rats wit h intact baroreceptors, RSNA was characterized by four frequency components occurring at about 0.5, 1.5, 6, and 12 Hz, which corresponded to the low-f requency fluctuation of heart rate, respiration, and frequency of heart bea t, and its harmonics, respectively. After intravenous infusion of sodium ni troprusside (SNP) to elicit reflex increases in RSNA and heart rate, the po wer for the component at 6 Hz followed the changes in heart beat frequency and was significantly increased, while those for the three other components were attenuated or experienced no change. In sino-aortic denervated (SAD) conscious rats, all four components were abolished, and the power spectrum was well fitted by a flat or Lorentzian curve, suggesting an almost random pattern. Only a respiratory-related component, which suggested common centr al modulation, appeared sporadically for short periods but was absent for t he most part. Therefore most of this component together with the low-freque ncy component was also likely due to the baroreceptor-dependent peripheral modulation. The activity was sorted in 15 subgroups on the basis of spike a mplitudes in the RSNA. Each subgroup showed frequency characteristics simil ar to the whole nerve activity. These results suggest that all periodicity in the RSNA of conscious rats with intact baroreceptors is caused by the ba roreceptor input.