Rhythmicity in single fiber postganglionic activity supplying the rat tail

The temporal pattern of ongoing sympathetic vasoconstrictor activity may pl ay an important role for neurovascular transmission. Here we analyzed the a ctivity of postganglionic fibers projecting into the ventral collector nerv e of anesthetized and artificially ventilated vagotomized Wistar rats with respect to the presence of rhythmic firing under normocapnic conditions. Mo st of the fibers studied were likely vasoconstrictor and involved in thermo regulation. Accumulate histograms of sympathetic activity were produced syn chronized with the,electrocardiogram to detect cardiac rhythmicity, with ph renic nerve activity to detect modulation with the central respiratory cycl e, and with tracheal pressure to uncover a reflex modulation associated wit h artificial ventilation. Sympathetic activity, phrenic activity, and trach eal pressure also were examined by spectral analysis and autocorrelation to detect rhythmicities distinct from respiration. Twenty-seven filaments con taining two to seven fibers with spontaneous activity and 51 single fibers were analyzed. Ongoing activity was 1.12 0.65 imp/s (mean +/- SD, n = 51); conduction velocity was 0.62 +/- 0.06 m/s (n = 30). Cardiac rhythmicity in sympathetic activity was weak (46.2 +/- 16.4%). The dominant rhythm in the activity of 19/27 few-fiber preparations and 37/51 single fibers correspond ed to the central respiratory cycle. The pattern consisted of an inhibition during inspiration and an activation in expiration. In 10/19 few-fiber pre parations and 21/37 single fibers of this group, there was also a concomita nt, less prominent rhythm related to artificial ventilation. By contrast, 8 /27 few-fiber preparations and 11/51 single fibers exhibited a dominant pum p-related modulation, whereas phrenic-related rhythmicity was subordinate. The dominant rhythm in the activity of two single fibers was related to nei ther central respiration nor artificial ventilation. We conclude that the o ngoing activity of most postganglionic neurons supplying the rat tail is mo dulated by the central respiratory rhythm generator, suggesting that change s in respiratory drive may alter perfusion of the tail and therefore heat d issipation. Reflex modulation in parallel with artificial ventilation, inde pendent of vagal afferents and possibly due to ventilatory changes of baror eceptor activity, is also an important source of rhythmicity in these neuro ns.