Studies on the regenerated carotid sinus nerve of the rabbit.

1. The central end of the distally cut left carotid sinus nerve was sutured to the tunica media of the external carotid artery, 1 cm cranial to the carotid bifurcation, in nineteen rabbits. The carotid body was removed in fourteen of these rabbits but left in situ in the remaining five. After 56-165 days of recovery a neuroma was identified at the site of the suture. Ventilatory reflexes mediated by both sinus nerves were tested and afferent activity recorded from the regenerated nerve. 2. Ventilatory reflex responses to hypoxia and sodium cyanide were abolished on sectioning the right sinus nerve, whilst the hypercapnic response was maintained. 3. Electrical stimulation of the regenerated sinus nerve caused hypotension and hyperventilation. These responses were attenuated compared to stimulation of the right sinus nerve. 4. A level of afferent activity equivalent to that found in non-regeneration experiments was recorded from all regenerated sinus nerves. Whole-nerve afferent activity was modulated by changes in carotid sinus blood pressure but not by changes in Pa,O2, Pa,CO2 (arterial O2 and CO2 pressures) or intracarotid injection of sodium cyanide. 5. A minimum of thirty single afferent fibres was identified in each experiment, the vast majority of which were mechanoreceptors. In only nine experiments were chemoreceptor fibres found and only twelve chemoreceptor fibres (1.7% of total) were identified in these nine experiments. In ten experiments no chemoreceptor fibres could be found. Leaving the carotid body in situ increased the incidence of chemoreceptive preparations. A small number of fibres unresponsive to mechanical stimulation and asphyxia was also identified. 6. The responses of regenerated chemoreceptor fibres to physiological and pharmacological stimuli were generally similar to those found in control carotid body preparations. Fibres unresponsive to mechanical stimulation and asphyxia did not respond to sodium cyanide, dopamine or isoprenaline; some of these fibres were excited by nicotine. 7. The receptive fields of mechanosensitive fibres were localized on or up to 2 cm away from the neuroma. Surface application of 20-40 microliters sodium cyanide (200 micrograms ml-1) was used to localize the receptive fields of seven of the twelve chemoreceptor fibres. All seven were localized to the site of the carotid body. 8. The neuroma and site of the carotid body were examined under light and electron microscopy. Glomus tissue was absent from the neuroma but was found at the site of the carotid body. 9. In conclusion, recovery of chemoreceptor function after carotid sinus nerve section appears to be associated with reinnervation of glomus tissue.