Vagal paraganglia of the rat

Paraganglia are associated with every branch of the rat vagus nerve except the pharyngeal branch. Some of the paraganglia closely resemble the glomus caroticum, whereas others appear like small, intensely fluorescent (SIF) cells of autonomic ganglia. The paraganglionic cells of SIF cell-like bodies (SLB) store catecholamines (the most abundant is probably noradrenaline) and in some cases neurotensin. The innervation pattern of SLB is variable and their physiological role remains unclear. Paraganglionic cells of glomus-like bodies (GLB) predominantly store dopamine and probably also to a lesser extent noradrenaline. These putative chemoreceptor organs receive sensory innervation from nodose ganglion neurons as revealed by degeneration experiments and by anterograde neuronal tracing. Substance P- and calcitonin gene-related peptide-immunoreactive fibres seen in the region of vascular entry into the GLB may account for some of these sensory fibres, but the peptide/classical transmitter stored in sensory terminals synapsing on paraganglionic cells is unknown. Ultrastructural immunocytochemistry revealed vasoactive intestinal polypeptide (VIP)-immunoreactive fibres lying in the interstitial space between paraganglionic cells and large capillaries. These fibres may originate from VIP-immunoreactive neurons, being frequently attached to GLB. The major difference between GLB and the glomus caroticum concerns their blood supply and related innervation: Arteries and arterioles do not penetrate into GLB and, accordingly, noradrenaline- and neuropeptide Y-containing nerve fibres are lacking within GLB. This peculiar arrangement of paraganglionic parenchyma and arterial blood supply may be one of the reasons for the different physiological properties of vagal and carotid arterial chemoreceptors.