Three types of neurochemically defined autonomic fibres innervate the carotid baroreceptor and chemoreceptor regions in the guinea-pig.

The innervation of the carotid body, carotid sinus, and neighbouring arteries (common carotid artery; external carotid artery; occipital artery; ascending pharyngeal artery) was investigated in guinea-pigs by means of glyoxylic acid-induced catecholamine-fluorescence and immunohistochemistry using a variety of antisera against neuropeptides and tyrosine hydroxylase (TH). Fibres displaying catecholamine-fluorescence, TH- and neuropeptide Y-like immunoreactivity (NPY-LI) were less numerous in the carotid sinus than in all other arterial segments. Vasoactive intestinal polypeptide (VIP)-LI axons were almost lacking in the common carotid, external carotid and occipital arteries, consistently found in the carotid sinus, and more numerous in the ascending pharyngeal artery. Catecholaminergic, TH-, NPY- and VIP-LI fibres were observed deep in the media of the carotid sinus, where the baroreceptor terminals are located. In contrast, they did not enter the media in the adjacent arterial segments. All these fibres disappeared following excision of the superior cervical ganglion, but were unaffected by combined transection of the carotid sinus nerve and resection of the no-dose ganglion, suggesting a sympathetic origin. Double-staining immunofluorescence revealed at least three types of autonomic, presumably sympathetic fibres in the carotid sinus: 1) TH+/NPY+, 2) NPY+/VIP+, and 3) VIP+ fibres. This points to a non-noradrenergic efferent innervation of the carotid sinus in addition to the hitherto known noradrenergic sympathetic fibres. The three populations of autonomic fibres seen in the carotid sinus were also observed in the carotid body, but the paucity of NPY+/VIP+ double-labelled fibres raises doubt as to the functional significance of this particular fibre type in modulating arterial chemoreception. The multiplicity of neurochemically defined autonomic nerves to the carotid baro- and chemoreceptor regions probably reflects functionally separate pathways that are differently regulated and exert different effects.