Physiological chemoreceptor stimulation decreases enkephalin and substance P in the carotid body

Neuroactive peptides, including the enkephalins (Met- and Leu-enkephalin; ME, LE) and substance P (SP) are known to be present in the mammalian carotid body, an arterial chemoreceptor organ sensitive to the O2, CO2 and pH levels in blood. The principal parenchymal (type I) cells of the organ, which receive sensory innervation from the carotid sinus nerve (CSN), have been shown to contain both ME and SP; SP is also present in CSN afferent fibers. In the present study, rabbits were exposed in a chamber to a physiological chemoreceptor stimulus (5% O2 in N2) for one hour, then anesthetized during surgical removal of both carotid bodies for later RIA measurement of ME and SP levels in the tissue; control animals were exposed to air in the chamber, but otherwise treated as the hypoxic animals. Both ME and SP levels were significantly reduced (approximately 40%) in the carotid bodies from hypoxic rabbits, compared to their normoxic controls. The results suggest that these neuroactive peptides are released from carotid body elements during physiological stimulation, and consequently may play a role in the transduction of chemosensory information between the type I cells and their apposed afferent terminals.