Interactions between sympathetic and vagal cardiac afferents in nucleus tractus solitarii

Epicardial application of hydrogen peroxide (H2O2) reflexly elicits a sympathetically mediated pressor response. This pressor response is augmented by vagotomy and abolished by sympathectomy, suggesting an occlusive interaction between the afferents in the central nervous system (CNS). To support this observation we recorded sympathetic efferent responses from the sympathetic chain (T1-T2) before and after epicardial application of H2O2 in six cats before and after vagotomy. Cardiac sympathetic efferent responses to H2O2 were increased by vagotomy. Thus there exists an occlusive interaction between the two afferent pathways in the CNS. Because cardiopulmonary vagal afferents make their first central synapse in the nucleus tractus solitarii (NTS), we further hypothesized that cells in the NTS receive convergent inputs from sympathetic and vagal afferents and that the inputs would interact in an occlusive manner. In alpha-chloralose-anesthetized sinoaortic-denervated cats, cardiac sympathetic and vagal branches were stimulated electrically at 1 Hz, either separately or in combination. Extracellular single-unit activity was recorded in the NTS. Vagal stimuli most frequently (38%) diminished sympathetically evoked unit activity (-46.6 +/- 6.0%) versus control (1.4 +/- 1.5%). However, a few (21%) vagal and sympathetic afferent inputs were found to be additive or facilitative. We conclude that interactions occur between cardiac sympathetic and vagal afferents in the NTS. It is possible that this occlusive interaction explains the alteration in cardiac sympathetic outflow after epicardial stimulation with H2O2.