Power spectral analysis of the baroreflex in neonatal swine

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.

Hypotension-induced expression of the c-fos gene in the medulla oblongata of piglets

Neural networks that mediate the reflex response to baroreceptor withdrawal were explored in Sus scrofa. Induction of c-fos was used as a monitor of synaptic activity in response to hypotension sustained by systemic administration of a peripheral vasodilator, sodium nitroprusside. Patterns of c-fos gene expression were compared between Saffan-anesthetized experimental animals and age-matched normotensive controls administered vehicle. Effects of other variables were controlled including 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were allowed for accumulation of transcript. The c-fos protein, Fos, was identified immunocytochemically with two rabbit antisera raised against amino acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antiserum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tractus solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matched projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the baroreceptor field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commissural field, the intermediate subnucleus and a ventral A2 noradrenergic area. In reticular formation c-fos protein was induced in circumscribed columns in the lateral tegmental field (LTF) extending from facial nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neurons expressed Fos in the porcine C1 and A1 areas of ventrolateral medulla. Fos was also induced in a dorsal intermediate reticular zone of LTF. Minor or inconsistent differences between experimental and control were observed in nucleus raphe pallidus, rostral paramedian reticular formation, upper thoracic intermediolateral cell column, and stellate ganglia. In conclusion, baroreceptor withdrawal in young animals induced patterns of neuronal response along established cardiovascular reflex pathways.