Reflex responses to baroreceptor, chemoreceptor and nociceptor inputs in single renal sympathetic neurones in the rabbit and the effects of anaesthesia on them

Reflex responses of renal postganglionic neurones to stimulation of arterial baroreceptors, arterial and central chemoreceptors and cutaneous nociceptors, and the rhythmicity of their resting activity were studied in paralyzed, artificially ventilated rabbits, anaesthetized with either alfathesin or chloralose-urethane. A 'vasoconstrictor' response pattern was seen in all units. Perivascular balloon-induced falls in blood pressure increased firing while pressure rises silenced 90% of units and reduced firing in the rest. Resting activity was linked to pressure changes within the cardiac cycle and to the artificial respiratory cycle. The largest excitation occurred during hypoxia and injections of CO2 saturated solutions into the carotid artery while hypercapnia and stimulation of cutaneous nociceptors only slightly increased firing. Parameters characterizing rhythmicities and reflex responses were unimodally distributed with no apparent subgrouping of units on quantitative grounds. Unit response patterns were similar to those recorded in the whole renal nerve. With one exception, no silent units were found which responded to the afferent inputs studied. Nor was there a small-spike fibre group which was excited by angiotensin. However, reflex responses were significantly influenced by the anaesthetic regime selected for use. Under alfathesin, baroreceptor and chemoreceptor reflexes were double those found with chloralose-urethane. Under chloralose-urethane, hypoxia increased both rhythmicities, while under alfathesin, cardiac rhythmicity was decreased and respiratory rhythmicity was variably affected. We concluded that renal sympathetic neurones are a functionally uniform population which behave like vasoconstrictors.