Respiration-related features of sympathetic discharges in the developing kitten

The objectives of the present study were to identify sympathetic discharge features, either spontaneous or evoked, which exhibited maturational changes. The spontaneous sympathetic nerve discharge of all kittens was comprised of tonic activity as well as phasic inspiratory activity; the latter occurred synchronously with the C5 phrenic discharge. Sympathetic activity during inspiration was depressed (inhibited or disfacilitated) by vagal afferent inputs as was indicated by the marked increases of amplitude following vagotomy. Hypoxia (10% O2) elicited changes in the amplitude of the sympathetic inspiration-related discharge which were identical to those of the phrenic; discharge amplitudes were either increased, or decreased, or increased and then decreased (i.e. biphasic response). Regardless of the change in sympathetic inspiration-related activity during hypoxia, the amplitude of tonic activity during expiration was often increased. Such changes in the discharge characteristics of the cervical sympathetic nerve in response to vagotomy or hypoxia were not dependent upon maturational processes as they were similar to those of adult cats. However, an age-dependent phenomenon was revealed by autopower spectral analyses which showed that the phrenic and sympathetic nerve activities in animals > or = 45 day old were modulated by inputs from a common source because their coherence spectra exhibited correlated periodicities in the 7-10 Hz range.