Respiratory Physiology & Neurobiology 157 (2007) 130–139 Electrical and pharmacological properties of petrosal ganglion neurons that innervate the carotid body Rodrigo Iturriaga a,∗ , Rodrigo Varas a , Julio Alcayaga b a Laboratorio de Neurobiolog´ ıa, Facultad de Ciencias Biol´ ogicas, P. Universidad Cat´ olica de Chile, Casilla 114-D, Santiago 1, Chile b Laboratorio de Fisiolog´ ıa Celular, Departamento de Biolog´ ıa, Facultad de Ciencias, Universidad de Chile, Santiago, Chile Accepted 4 December 2006 Abstract The petrosal ganglion (PG) contains the somata of primary afferent neurons that innervate the chemoreceptor (glomus) cells in the carotid body (CB). The most accepted model of CB chemoreception states that natural stimuli trigger the release of one or more transmitters from glomus cells, which in turn acting on specific post-synaptic receptors increases the rate of discharge in the nerve endings of PG neurons. However, PG neurons that project to the CB represent only small fraction (roughly 20%) of the whole PG and their identification is not simple since their electrophysiological and pharmacological properties are not strikingly different as compared with other PG neurons, which project to the carotid sinus or the tongue. In addition, differences reported on the actions of putative transmitters on PG neurons may reflect true species differences. Nevertheless, some experimental strategies have contributed to identify and characterize the properties of PG neurons that innervate the CB. In this review, we examined the electrophysiological properties and pharmacological responses of PG neurons to putative CB excitatory transmitters, focusing on the methods of study and species differences. The evidences suggest that ACh and ATP play a major role in the fast excitatory transmission between glomus cells and chemosensory nerve endings in the cat, rat and rabbit. However, the role of other putative transmitters such as dopamine, 5-HT and GABA is less clear and depends on the specie studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Carotid body; Transmitters; Petrosal ganglion 1. Introduction The petrosal ganglion (PG) is composed by a heteroge- neous population of pseudo-monopolar neurons (Stensaas and Fidone, 1977), which generate the two main branches of the glossopharyngeal nerve: the carotid sinus nerve (CSN) and the glossopharyngeal branch. The PG contains the somata of the primary sensory neurons that innervate the chemoreceptor (glo- mus) cells in the carotid body (CB). The most accepted model of CB chemoreception proposes that hypoxia, hypercapnia and acidosis increases the intracellular [Ca 2+ ] in glomus cells, releas- ing one or more transmitters (Iturriaga and Alcayaga, 2004; Nurse, 2005; Prabhakar, 2006). Accordingly, transmitters with excitatory effects, activate specific post-synaptic receptors that increase the discharge of action potentials in the nerve endings of the PG neurons projecting to the CB (Gonz´ alez et al., 1994). This ∗ Corresponding author. Fax: +56 2 222 5515. E-mail address: riturriaga@bio.puc.cl (R. Iturriaga). model of stimulus-secretion coupling is supported by the fact that PG neurons do not respond to hypoxic-hypoxia. Indeed, per- fusion of the isolated PG with hypoxic solution did not increase the frequency of discharge of the CSN, but NaCN and sodium azide applied to the PG produced dose-related increases in CSN discharges (Alcayaga et al., 1999b, 2003a). In this review, we will examine the electrophysiological properties and pharma- cological responses of PG neurons to putative CB transmitters, focusing on methods of study and species differences. We will not discuss the regulation by neurotrophic and transcriptional signals of PG neurons and their developmental properties, since these topics have been recently reviewed (see Donnelly, 2005; Katz, 2005). 2. Morphology The somata of the sensory fibers projecting through the glos- sopharyngeal nerve are mainly concentrated in the PG, but few of them are located in the superior ganglion (Eyzaguirre and Zapata, 1984). The peripheral fibers of the PG neurons 1569-9048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2006.12.006