Distribution of sympathetic nerve endings within the rabbit carotid body: a histochemical and ultrastructural study

The distribution of sympathetic (noradrenergic) nerves within the rabbit carotid body was investigated by fluorescence microscopy and electron microscopy. With the Falck-Hillarp method, fluorescent nerve fibres were found associated with blood vessels and type I/type II cell groups. After the injection of 6-hydroxydopamine, degenerating nerve profiles were found near blood vessels, near type I/type II cell groups and in contact with type II cells. Following the administration of [3H]noradrenaline, labelled nerve profiles were observed in close association with blood vessels and around cell groups; a few labelled nerve profiles were observed in contact with type II cells. With a modified chromaffin method, chromaffin nerve profiles were found around blood vessels, around type I/type II cell groups and their sensory nerve supply and in contact with type II cells. It was shown that for about one-third of the chromaffin nerve profiles, the nearest structure (in the plane of section) was a type I/type II cell group. Furthermore, the mean distance between chromaffin nerve profiles and blood vessels and between chromaffin nerve profiles and type I/type II cell groups was about the same (2.2 microns and 2.7 microns respectively). These results suggest that the carotid body sympathetic innervation might have, in addition to its vasomotor function, a direct action on the chemosensory structures. This hypothesis is discussed in the light of currently available physiological data and a comparison is made with the role of the sympathetic innervation in other sensory systems.