Johnson, Christopher David; (1997) Characteristics of sympathetic nerve activity recorded from blood vessels in the rat tail. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

1) In anaesthetized rats, a focal electrode was developed to record activity from single sympathetic postganglionic neurones innervating the caudal ventral artery (CVA) and the lateral vein (LV) in the tail. Experiments were performed to characterize on-going activity in terms of patterning and rhythms so that they may be related to central mechanisms controlling sympathetic outflow and to neurotransmitter release and target organ response. 2) Initial experiments tested the hypothesis that on-going activity occurs in bursts. As this was found to be the case, in subsequent experiments the hypotheses were tested that this rhythm could be different from the central respiratory rhythm and was not reliant on feedback from afferent nerves in the aortic, sinus and vagus nerves. It was also hypothesized that the rhythmic sympathetic discharge would be influenced by physiological disturbances such as hyperthermia and baroreceptor stimulation. Thus, it was possible to compare characteristics of activity to the two vessels to highlight any differences in their control. 3) Four types of preparation were used when recording activity to CVA: spontaneously breathing (group 1), artificially ventilated (group 2), artificially ventilated with vagi cut (group 3) and artificially ventilated with vagus and sino-aortic denervation (group 4). Activity to LV was recorded from group 3 only. 4) On-going sympathetic unit activity was found to have a robust, dominant rhythm in units to both vessels, the frequency of which was not significantly different, although it was often different from that of the phrenic rhythm, regardless of the preparation type. This dominant sympathetic rhythm is referred to as the tail (or T-) rhythm. 5) In activity to both vessels the mean "T-rhythm frequency was not influenced significantly by hypocapnic apnoea, but was decreased during hyperthermia whilst the phrenic rhythm frequency increased. Additionally, the T-rhythm frequency was always different from that of the artificial ventilation cycle. 8) Cardiac rhythmicity was usually absent in recordings from both vessels. However, electrical stimulation of an aortic nerve and blood volume expansion could influence T- rhythm frequency recorded from units to both vessels without influencing the phrenic rhythm frequency. 9) Characteristics of activity recorded from both vessels were similar, although activity recorded from units to LV appeared to be less rhythmical. 10) In conclusion, the frequency of the T-rhythm in activity recorded from both vessels can be different from that of central respiratory drive and is not 'driven' by afferent feedback via sinus, aortic and vagus nerves, although its frequency is influenced by stimulation of baroreceptors and by hyperthermia. Further, there is no clear evidence that control of activity to CVA is different to that of LV. 11) It is proposed that the T-rhythm is unlikely to be generated by a central respiratory oscillator.

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