Respiratory rhythmicity in the activity of postganglionic neurones supplying the rat tail during hyperthermia.

It has been suggested that thermoregulatory stimulation changes respiration-related rhythmicity in the activity of postganglionic sympathetic neurones supplying the rat tail to a distinct modulation independent of respiration. To study this possibility, single and few fibre recordings were made from ten filaments split from the ventral collector nerves of the rat during whole body warming. Sympathetic activity was analysed by autocorrelation and phrenic-triggered summation. All neurones except one were gradually inhibited and lost their on-going activity above a core temperature of 39-39.5 degrees C while the frequency of the phrenic bursts increased significantly. During hyperthermia, all neurones tested exhibited a prominent respiratory modulation in their activity which, compared to normothermia, was significantly increased in strength, or even newly acquired. No other rhythm emerged. These results speak against the hypothesis that in the rat sympathetic pathways controlling the tail vasculature and thus involved in thermoregulation, during hyperthermia become controlled by central oscillators distinct from the respiratory rhythm generator. Rather, respiratory modulation appears to remain the dominant rhythm as is common for sympathetic neurones supplying other cardiovascular targets.