The cardiac-related rhythm in preganglionic sympathetic activities of developing piglets.

This investigation was performed to determine whether partial spectral analysis of preganglionic sympathetic nerve discharges would reveal age-related differences in the distribution of baroreceptor afferent information to brainstem sympathetic-related neurons. Any influence of baroreceptor afferent activity on ordinary spectra of cervical sympathetic and splanchnic nerves was removed by partialization using the arterial blood pressure signal which represented baroreceptor activity. An absence of statistically significant coherence in partialized nerve spectra would indicate that sympathetic-related neurons receive peripheral baroreceptor afferent input, but are not interconnected, whereas the presence of significant coherence would mean that these neurons are interconnected. Ordinary spectral analysis did not demonstrate age-related differences in the relationship between nerve activity and baroreceptor afferent input. In many animals, large peaks, located at cardiac frequencies (range 2.75-5.6 Hz), were noted in ordinary nerve autopower spectra, and were significantly correlated in ordinary coherence spectra. Partialization of nerve spectra eliminated or reduced cardiac-related peaks in autopower spectra regardless of age, and, in 8 of 10 animals, reduced coherence estimates to non-significant values. In two animals, 19 and 36 days old, significant coherence values remained after partialization. These results demonstrated that cardiac-related peaks in coherence in spectra of preganglionic splanchnic and cervical sympathetic nerves were dependent upon peripheral afferent baroreceptor input in most animals. Further, the finding that significant residual coherence was absent in most cases suggested a paucity of intrabulbar pathways connecting brainstem sympathetic-related neurons.

Hypercapnia induces long-term changes in postganglionic renal nerve activity in the piglet.

In developing swine, time and frequency domain analyses were used to compare changes in discharge features of efferent phrenic and postganglionic renal nerve activities evoked by prolonged (1 h) exposure to severe hypercapnia (10% CO2, balance O2), before and after combined carotid sinus and aortic depressor nerve (CSN-AOD) sectioning. With intact CSN-AOD innervation, respiration-related activity in renal nerve discharge was rare (3 of 11 animals) during baseline periods with intact innervation, but was observed in most cases (10 of 11 animals) during baseline following denervation. Renal nerve respiration-related activity was recruited by hypercapnic stimulation in animals with intact CSN-AOD innervation, and was augmented in denervated animals with ongoing respiratory activity. Phrenic nerve discharge was markedly augmented during hypercapnia, whether CSN-AOD innervation was intact or not, and it did not exhibit a post-hypercapnic depression. Autopower spectra of renal nerve activity revealed the presence of two coexisting rhythms, 2-6 and 7-13 Hz, which were present whether CSN-AOD innervation was intact or not. The hypercapnic-induced increases of activity in the 2-6 and 7-13 Hz bands were not comparable, with the latter region exhibiting a much more robust response to hypercapnia, especially following CSN-AOD denervation. Thus, prolonged exposure to hypercapnia evoked changes in renal nerve discharge that involved increased coupling to neuronal ensembles shaping central inspiratory activity and those generating central sympathetic outflows, especially to networks generating 7-13 Hz rhythm. Such changes may permit more efficient modulation of innervated structures during exposure to stressors.