Very high frequency components of renal sympathetic nerve activity in conscious rats

The aim of this study was to examine whether multifibrenal sympathetic nerve activity (RSNA) of conscious rats contains frequency components of biological interest at frequencies above 25 Hz. RSNA was recorded in 10 conscious Sprague–Dawley rats under baseline conditions and during infusion of vasoactive drugs that reflexly altered the mean RSNA level. The RSNA signal was band-pass filtered (300–3000 Hz) before being sampled at 10,000 Hz. The analytic envelope of this raw signal was then extracted using the Hilbert transform, and 132-s periods were submitted to Fourier transform analysis. Spectral power was computed from 0 to 25 Hz and from 25 to 3000 Hz (P25–3000). P25–3000 was reduced by about 80% after either ganglionic blockade or euthanasia, which indicated that it was of biological origin and derived from the activity of postganglionic sympathetic neurons. After subtraction of post-mortem spectral power, basal P25–3000 contributed 59.8 ± 2.4% of total power. P25–3000 was strongly barosensitive and thus, accounted for a major part of the reflex changes in total power. In each of the 10 rats, P25–3000 was linearly correlated with the mean RSNA level (0.984 ± 0.003) and even more so with the spectral power in the 0–25 Hz frequency range (0.994 ± 0.001). In conclusion, the RSNA of conscious rats contains very high frequency components that account for about 60% of the total spectral power and are modulated by the baroreceptor reflex. A reasonable approximation of this power can be obtained by computing spectra up to 25 Hz.