Ionic currents and action potential induced by electrical stimulation, ACh and ATP in petrosal ganglion neurons
© BioMed Central Ltd 2001
Received: 2 August 2001
Published: 17 August 2001
The glossopharyngeal nerve contains the sensory fibers of the petrosal ganglion (PG) neurons that innervate the carotid body and sinus, through the carotid sinus nerve, and the tongue and the pharynx, through the glossopharyngeal branch. Based on the shape of the action potential, two populations of PG neurons can be recognized. However, little is known about the ionic currents underlying these action potentials. The application of ACh and ATP, transmitters putatively involved in the communication between the glomus cells and PG neuron terminals, to the isolated PG increase the frequency of discharge of the carotid sinus nerve, while only ATP increases discharges in the glossopharyngeal branch. The above suggests the presence of at least two neuronal populations.
Petrosal ganglia were excised from cats anesthetized with sodium pentobarbitone (40 mg/kg, ip), dissociated, and the neurons cultured for 3–16 days. Using whole-cell patch-clamp technique, we recorded from isolated PG neurons both action potentials and ionic currents evoked by electrical stimulation, and by application of ACh and ATP.
Neurons presented either fast action potentials (F-type) or slower spikes with an inflexion in the repolarizing phase (H-type). The F-type neurons had fast inward and a sustained outward current. The spike and inward currents were reversibly blocked when choline replaced Na+ or in the presence of 3 μM tetrodotoxin (TTX). The outward current was partly blocked by 5 mM tetraethylammonium (TEA). The H-type neurons presented an inward current with at least two components, and an outward current with a fast -partly inactivating- and a slower sustained component. When external Na+ was replaced by choline, or when 3 μM TTX was added, the amplitude of the spike and the inward current were reversibly decreased. Similarly, when intracellular K+ was replaced by Cs+, or when 5 mM TEA was added, the amplitude and duration of the spike increased and the outward current was reduced.
Under voltage-clamp, ATP induced a dose-dependent inward current that was partly desensitized during 20 s application pulses. The ATP-induced current had a threshold of 100 nM, and saturated between 20–50 μM. ACh also induced a fast, inactivating inward current, with a threshold between 10–50 μM, and saturated about 1–5 mM. From 34 neurons tested for ACh and ATP, 9% responded only to ACh, 24% to ATP only, 50% responded to both agents, and the remaining 17% were unresponsive. In current-clamp, ATP and ACh depolarized the neurons and may induce action potentials.
Our results show that H- and F-type PG neurons express different voltage-gated ionic and receptor-gated currents, which are activated either by ACh or ATP, with a half population responding to both putative transmitters.
This work was funded by grants 199–0030 and 201–0133 and approved by the Animal Ethics Committee of the P. Universidad Católica de Chile.