Expression and Function of Presynaptic Neurotransmitter Receptors in the Chemoafferent Pathway of the Rat Carotid Body

Hypoxic chemotransmission in the carotid body (CB) is mediated by the release of neurotransmitters (NTs) from type I cells onto adjacent sensory (petrosal) nerve endings (Gonzalez et al. 1994; Urena et al. 1994). In the rat, there is strong evidence of a role for the co-release of fast-acting neurotransmitters, including acetylcholine (ACh) and ATP (Zhang et al. 2000), in chemosensory signalling. In contrast, little is known about the roles of NTs in mediating autoreceptor feedback mechanisms which are thought to play a role in modulating chemoreceptor output from the CB. Here, we present evidence for the involvement of two distinct NTs, ¦Ã-aminobutyric acid (GABA) and serotonin (5-HT), in mediating autoreceptor feedback onto type I cells during hypoxia. Although their actions are directly opposite, activation of receptors for either transmitter appears to regulate the activity of O2-sensitive K+ channels, such as the background K+ channel Kcnk3 (Buckler et al. 2000). 5-HT receptor activation also regulates the activity of the O2-sensitive large-conductance Ca2+-activated K+ channel (Peers, 1990). These results demonstrate the roles of NTs in regulating secretion from type I cells, by converging signals from different sources onto the same K+ channels involved in O2 sensing and control of the receptor potential.