Acetylcholine contributes to hypoxic chemotransmission in co-cultures of rat type 1 cells and petrosal neurons

The neurotransmitter mechanisms that mediate chemosensory transmission in the mammalian carotid body (CB), i.e. the primary arterial P(O2) detector, are controversial. Given the inherent difficulty of recording from afferent terminals in situ, the authors have adopted an alternative approach based on co-culture of dissociated CB receptor (type 1) cell clusters and petrosal neurons (PN) from 8-14-day-old rat pups. Electrophysiological, perforated patch recordings from petrosal somas, juxtaposed to type 1 clusters, revealed the development of a high incidence of functional 'synapses' in vitro. Recent evidence has strengthened the case for acetylcholine (ACh) as a co-released transmitter: (i) cultured type 1 cells express several cholinergic markers including the vesicular ACh transporter (VAChT), intracellular acetylcholinesterase (AChE), and occasional clear cored vesicles (approximately 50 nm diameter); (ii) the frequency of spontaneous synaptic activity, as well as the hypoxia-induced depolarization recorded in 'juxtaposed' PN in co-culture, were partially suppressed by the nicotinic ACh receptor (nAChR) blocker, mecamylamine (2 microM); (iii) consistent with the presence of extracellular AChE, ACh-mediated membrane noise in type 1 cells as well as the hypoxia-evoked PN response in co-culture were potentiated in a few cases by the AChE inhibitor, eserine (100 microM). Thus, since many PN and type 1 cells express mecamylamine-sensitive nAChR, released ACh may act presynaptically on type 1 cell autoreceptors and/or postsynaptically on petrosal terminals. Other CB transmitter candidates (e.g. 5-HT and ATP) were found to excite PN, though their potential role as co-released sensory transmitters requires further investigation.