Co-localization of Neuroactive Agents in the Carotid Body of the Cat

Abstract

The carotid body is a chemoreceptor organ whose parenchymal cells are innervated by chemosensory axons from the sinus branch of the glosso-pharyngeal nerve. Axons penetrating the carotid body are ensheathed by type II cells and have synaptic terminals on type I parenchymal elements. Preneural type I parenchymal elements form both pre- and postsynaptic contacts, contain clear core and dense-core vesicles, and synthesize neuroactive agents thought to play an important role in chemotransduction and chemotransmission. However, the nature of their synaptic contribution to the activity of the sinus nerve afferents is uncertain. Our studies of the cat carotid body indicate an unsuspected degree of cytochemical complexity with multiple neuroactive agents in type I cells. The present immunocytochemical investigation addresses the issue of transmitter co-occurrence utilizing antibody markers for six neuroactive agents. Pairs of antigens have been localized within frozen tissue sections by a blue, granular product associated with a first primary antibody and a uniform red reaction product coupled to a second primary antibody. A high degree of variability in the incidence and distribution of these two markers has been observed, which delineates a spectrum of type I cells ranging from dark/dark to light/unstained. Step-wise comparison of the available combinations of marker pairs revealed an unexpectedly high degree of transmitter co-localization suggesting that multiple agents may exist within some cells.