Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels

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Abstract
Hardebo, J. E. 1992. Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels. Acta Physiol Scand144, 333–339. Received 18 June, accepted 7 November 1991. ISSN 0001–6772. Departments of Medical Cell research and Neurology, University of Lund, Lund, Sweden. Earlier findings suggested that the physiological firing rate in sympathetic nerves did not exceed 10 Hz. Later recordings have revealed that this is a net value: impulses are usually not continuous but occur in bursts of high frequencies separated by quiet periods. The effects of continuous and burst-like neurogenic activation in various isolated blood vessels were compared. In the first part of the study changes in vascular tone were registered. Electrical field stimulatory parameters were chosen to give tetrodotoxin-blockable, neurogenic responses. From dose-response experiments in rat caudal artery a net frequency of 6 Hz was chosen, for bursts usually designed as 30 Hz during 0.2 s followed by a quiet period of 0.8 s. In the rabbit ear artery the neurogenic contraction was enhanced by a mean of near 50% during stimulation with bursts. Now appeared a minor phentolamine-resistant portion, which was nor due to release of NPY, 5–HT, histamine or ATP. Also in the rat caudal artery and monkey pial artery a significant enhancement of contraction was seen during burst stimulation, whereas in the rabbit facial vein no significant difference in dilatation through β-receptor activation was obtained. In vessels that do not normally respond with purely neurogenic contractions/dilatations during continuous stimulation, like pial arteries from the rat and rabbit, not even bursts revealed a neurogenic response. In a second series of experiments the influence of continuous and burst-like nerve activation on the release of [14C]noradrenaline was studied in monkey, rabbit and rat pial arteries, rat caudal artery and rabbit central ear artery and facial vein. In all vessels except rabbit pial artery a significantly higher release was obtained with burst-like compared to continuous stimulation.

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