The Effects of Carotid Body Neurotransmitters on the Efferent Glossopharyngeal Neurons

Abstract

<p> The carotid body (CB) is the main peripheral chemoreceptor organ that maintains homeostatic control of the O2, CO2, glucose and pH levels in the blood. It is innervated by nerve fibers from the carotid sinus nerve (CSN) that consists of sensory afferents from the petrosal ganglion (PG) and "inhibitory" efferents from the glossopharyngeal nerve (GPN). The efferent innervation forms an elaborate network that is immuno-positive for neuronal nitric oxide synthase (nNOS), and is thought to inhibit the CB via release of nitric oxide (NO). The purpose of this study is to further understand the underlying mechanisms of this inhibition. Since the CB possesses various neurotransmitters, including the excitatory neurotransmitter, acetylcholine (ACh), I tested the hypothesis that the CB drives its own modulation during chemoexcitation by secreting ACh, which would directly act on receptors located on the GPN neurons (GPNs) and lead to nNOS activation via calcium entry and the subsequent release of NO. To address this, molecular and calcium imaging techniques were used to demonstrate the specific types of nicotinic ACh receptors (nAChRs) expressed in GPN neurons. It was shown that GPN neurons expressed the mRNA for ten subunits: α2-α9, excluding α8 and β2-β4 and they responded to ACh and nicotine, a nAChR agonist, in a dose-dependent manner via an increase in intracellular calcium. The EC50 for ACh and nicotine were ~ 9.9 and 20.5 μM respectively. The nicotine-induced calcium transients were inhibited by mecamylamine, a nAChR competitive antagonist, with an IC50 of ~ 1.2 μM. Studies using subunit-specific antagonists, dihydro-β-erythroidine (specific for α4β2 and α3β4 in particular dose ranges) and methyllycaconitine (MLA) and α-bungarotoxin (BTX; both specific for α7) revealed that the major functional nAChR expressed in GPNs were the α4β2 and α3β4 nAChRs. The results of this study show that GPN neurons respond to ACh stimulation with an increase in intracellular calcium and thus raise the possibility that ACh secreted after stimulation/activation of receptors on the CB may contribute to the synthesis of NO and negative feedback inhibition of CB function via stimulation of GPN efferent nerve fibers.</p>