Brain Research 988 (2003) 154–163 www.elsevier.com / locate / brainres Research report ACh and ATP mediate excitatory transmission in cat carotid identified chemoreceptor units in vitro a b a, * Rodrigo Varas , Julio Alcayaga , Rodrigo Iturriaga a ´ ´ ´ ´ Laboratorio de Neurobiologıa, Departamento de Ciencias Fisiologicas, Facultad de Ciencias Biologicas, P . Universidad Catolica de Chile, Casilla 114-D, Santiago 1, Chile b ´ ´ Laboratorio de Fisiologıa Celular, Departamento de Biologıa, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago 1, Chile Accepted 21 July 2003 Abstract Several molecules have been proposed as excitatory transmitters between glomus (type 1) cells and nerve terminals of petrosal ganglion (PG) neurons in the carotid body (CB). We tested whether ACh and ATP have a role to play as excitatory transmitters in the cat CB by recording intracellularly from identified PG neurons functionally connected to the CB in vitro. PG neurons projecting to the CB were classified according to their intracellular responses as: (a) neurons with humped action potentials (hAP neurons) responding phasically to long-lasting depolarizing pulses (53 / 67), and (b) neurons with smooth action potentials (non-hAP neurons) that fire tonically during long-lasting depolarizations (14 / 67). CB stimulation by stop flow and / or acidosis induced activity in 28 of 39 hAP-type neurons, being classified as chemosensory, but in none of the non-hAP neurons. Hexamethonium (10 mM) and suramin (100 mM) reversibly abolished the increased discharges evoked in chemosensory neurons (8 / 9) by stop flow or acidosis. Moreover, 24 of 27 chemosensory neurons responded to ganglionar application of ACh and ATP, while two neurons responded only to ACh and one to ATP. Mechanical deformation of the carotid sinus induced firing activity in 10 of 13 non-hAP neurons, but in none of the hAP neurons tested. Interestingly, 4 / 10 non-hAP neurons, which responded to carotid sinus mechanical stimulation also responded to ganglionar application of ATP, but were insensitive to ACh. Present results favor the hypothesis that ACh and ATP are excitatory transmitters in the cat CB, acting—at least—on the PG neuron terminals in the CB.  2003 Elsevier B.V. All rights reserved. Theme: Sensory systems Topic: Somatic and visceral afferents Keywords: ACh; ATP; Carotid body; Co-transmission; Petrosal ganglion 1. Introduction CB [27]. However, the identity of the excitatory transmit- ter between glomus cells and sensory afferent neurons is The carotid body (CB) is the main chemosensory organ still controversial. Among the putative excitatory transmit- that senses the levels of pO, pCO , and pH in the arterial ters present in the CB, DA has received much attention 2 2 blood. In response to hypoxia, hypercapnia and acidosis, [27]. The proposal that DA is the excitatory transmitter the glomus (type 1) cells of the CB are expected to release was strongly supported by the observation that hypoxia one or more transmitters that increase the frequency of produces DA release from the CB. Indeed, Fidone et al. 3 action potentials in the apposed nerve endings of petrosal [19] found that after incubation with [ H]tyrosine for 2–3 3 ganglion (PG) neurons [18]. Several molecules present in h, the amount of [ H]DA released from rabbit CB super- glomus cells, such as acetylcholine (ACh), adenosine 59- fused in vitro was roughly proportional to the degree of triphosphate (ATP), dopamine (DA) and substance P have hypoxia. However, simultaneous recordings of CB been postulated to be the excitatory transmitter(s) in the chemosensory discharges and endogenous DA secretion measured by amperometry have shown a clear dissociation between chemosensory excitation and DA efflux induced *Corresponding author. Tel.: 156-2-686-2852; fax: 156-2-222-5515. E-mail address: riturria@bio.puc.cl (R. Iturriaga). by natural stimuli [10,13,30,31]. In addition, the inhibitory 0006-8993 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0006-8993(03)03366-3