Expression of Multiple P2X Receptors by Glossopharyngeal Neurons Projecting to Rat Carotid Body O₂-Chemoreceptors: Role in Nitric Oxide-Mediated Efferent Inhibition

Abstract

In mammals, ventilation is peripherally controlled by the carotid body (CB), which receives afferent innervation from the petrosal ganglion and efferent innervation from neurons located along the glossopharyngeal nerve (GPN). GPN neurons give rise to the "efferent inhibitory" pathway via a plexus of neuronal nitric oxide (NO) synthase-positive fibers, believed to be responsible for CB chemoreceptor inhibition via NO release. Although NO is elevated during natural CB stimulation by hypoxia, the underlying mechanisms are unclear. We hypothesized that ATP, released by rat CB chemoreceptors (type 1 cells) and/or red blood cells during hypoxia, may directly activate GPN neurons and contribute to NO-mediated inhibition. Using combined electrophysiological, molecular, and confocal immunofluorescence techniques, we detected the expression of multiple P2X receptors in GPN neurons. These receptors involve at least four different purinergic subunits: P2X 2 [and the splice variant P2X 2(b) ], P2X 3 , P2X 4 , and P2X 7 . Using a novel coculture preparation of CB type I cell clusters and GPN neurons, we tested the role of P2X signaling on CB function. In cocultures, fast application of ATP, or its synthetic analog 2Ј,3Ј-O-(4 benzoylbenzoyl)-ATP, caused type I cell hyperpolarization that was prevented in the presence of the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium. These data suggest that ATP released during hypoxic stress from CB chemoreceptors (and/or red blood cells) will cause GPN neuron depolarization mediated by multiple P2X receptors. Activation of this pathway will lead to calcium influx and efferent inhibition of CB chemoreceptors via NO synthesis and consequent release.

Carotid body function in aged rats: responses to hypoxia, ischemia, dopamine, and adenosine

“…These results contrast with the reduction in carotid sinus nerve discharges in response to hypoxia observed in old rats (Conde et al 2006). It is known that the glossopharyngeal nerve has, in addition to carotid sinus nerve afferent fibers, a parallel autonomic parasympathetic efferent pathway sensitive to hypoxia that is the source of CB inhibition (Campanucci et al 2006). We can speculate that the reduction in CSN discharges found in old animals (Conde et al 2006) could mainly be due to an impairment of the efferent fiber activity without changes in the chemosensory excitatory afferent pathway.…”

Signal processing at mammalian carotid body chemoreceptors

“…Mutant mice lacking neuronal nitric oxide synthase show a significantly higher basal carotid sinus nerve discharge and greater hypoxic ventilatory responses compared to wild type mice [72]. Neurons in the glossopharyngeal nerve that express neuronal nitric oxide synthase also express P2X2, P2X3, P2X4 and P2X7-containing purinoceptors [73]. Further, activation of these purinoceptors in a co-culture preparation of rat glossopharyngeal neurons and carotid body cells hyperpolarizes type I cells.…”

“…Further, activation of these purinoceptors in a co-culture preparation of rat glossopharyngeal neurons and carotid body cells hyperpolarizes type I cells. This hyperpolarization was prevented by NO scavengers [73]. Neurons in the glossopharyngeal nerve also express nicotinic ACh receptors and are themselves cholinergic, raising the possibility of an additional paracrine/autocrine pathway by which ACh receptor activation elicits NO release [69].…”

Carotid body oxygen sensing

“…1c). The CB also contains some autonomic neurons and fibres, which seem to have an efferent regulatory action on glomus cells [5].…”