The Carotid Body “Tripartite Synapse”: Role of Gliotransmission
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In mammals, cardiorespiratory reflexes originating in the carotid body (CB) help maintain homeostasis by matching oxygen supply to oxygen demand. CB output to the brainstem is shaped by synaptic interactions at a "tripartite synapse" consisting of chemosensory (type I) cells, abutting glial-like (type II) cells, and sensory (petrosal) nerve terminals. Type I cells are stimulated by several blood-borne metabolic stimuli, including the novel chemoexcitant lactate. During chemotransduction, type I cells depolarize and release a multitude of excitatory and inhibitory neurotransmitters/neuromodulators including ATP, dopamine (DA), histamine, and angiotensin II (ANG II). However, there is a growing appreciation that the type II cells may not be silent partners. Thus, similar to astrocytes at "tripartite synapses" in the CNS, type II cells may contribute to the afferent output by releasing "gliotransmitters" such as ATP. Here, we first consider whether type II cells can also sense lactate. Next, we review and update the evidence supporting the roles of ATP, DA, histamine, and ANG II in cross talk among the three main CB cellular elements. Importantly, we consider how conventional excitatory and inhibitory pathways, together with gliotransmission, help to coordinate activity within this network and thereby modulate afferent firing frequency during chemotransduction.
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