Rhythmicities in sympathetic discharge: a signal of cardiorespiratory integration in developing animals

We have been pursuing various avenues of investigation to elucidate the postnatal maturation of neural regulation of cardiovascular and respiratory integration. In this paper we present our results from a systematic analysis of age-related modulations of sympathetic (SYMP) activity with respect to experimental alterations in baroreceptor afferent inputs. The three age groups of piglets were chosen based on different responses to a complex stimulus, i.e., the Valsalva maneuver. Postnatal maturation of SYMP activity was examined by spectral analysis of SYMP discharge using cross-power, full and partial coherence. Three general oscillations were observed in spontaneous SYMP discharges in the 0-30 Hz range. We divided that range into five frequency bands (0-2, 2-6, 6-12, 12-20, 20-30 Hz), which included periodicities in phase with both central respiratory activity and the cardiac cycle. Spectral analyses of SYMP activity after either baroreceptor activation (phenylephrine) or deactivation (nitroprusside) revealed that respiratory modulation was age-related across all frequencies while baroreceptor modulation was usually age-related within three of the five frequency bands. These results lead to questions concerning the possible role of the autonomic nervous system and/or central interactions between the respiratory and SYMP rhythm generators in the etiology of sudden infant death syndrome (SIDS).