Jorge Arroyo

Selective activation of carotid nerve fibers by acetylcholine applied to the cat petrosal ganglion in vitro

The petrosal ganglion innervates carotid body chemoreceptors through the carotid (sinus) nerve. These primary sensory neurons are activated by transmitters released from receptor (glomus) cells, acetylcholine (ACh) having been proposed as one of the transmitters involved in this process. Since the perikarya of primary sensory neurons share several properties with peripheral sensory endings, we studied the electrical responses of the carotid nerve and glossopharyngeal branch to ACh locally applied to the cat petrosal ganglion superfused in vitro. Ganglionar applications of AChCl (1 microg-1 mg) generated bursts of action potentials conducted along the carotid nerve, while only a few spikes were exceptionally recorded from the glossopharyngeal branch in response to the largest doses. Carotid nerve responses to ACh were dose-dependent, the higher doses inducing transient desensitization. Application of nicotine to the petrosal ganglion also evoked dose-dependent excitatory responses in the carotid nerve. Responses to ACh were reversibly antagonized by adding hexamethonium to the superfusate, more intense and prolonged block of ACh responses being produced by mecamylamine. Ganglionar applications of gamma-amino butyric acid and serotonin, in doses of up to 5 mg, did not induce firing of action potentials in any of the branches of the glossopharyngeal nerve. Our results indicate that petrosal ganglion neurons projecting through the carotid nerve are selectively activated by ACh acting on nicotinic ACh receptors located in the somata of these neurons. Thus, cholinosensitivity would be shared by the membranes of peripheral endings and perikarya of primary sensory neurons involved in arterial chemoreception.

Adenosine triphosphate-induced peripheral nerve discharges generated from the cat petrosal ganglion in vitro

Since nucleotides have been postulated as transmitters between glomus cells and chemosensory nerve endings in the carotid body, we studied the effects of their application to the petrosal ganglion, where the perikarya of carotid (sinus) nerve are located. Cat petrosal ganglia were superfused in vitro, while electrical activities of their peripheral processes (carotid nerve and glossopharyngeal branch) were recorded simultaneously. Adenosine triphosphate (ATP) evoked dose-dependent bursts of impulses in carotid nerve, while those in glossopharyngeal branch were less intense and consistent. Adenosine monophosphate was less effective than ATP. ATP-induced carotid nerve responses presented no temporal desensitization and persisted after applying P(2Y) receptor blocker Reactive Blue 2 to the ganglion. The results indicate that ATP has an excitatory effect on the perikarya of the population of petrosal ganglion neurons projecting peripherally through the carotid nerve.

Dopamine modulates carotid nerve responses induced by acetylcholine on the cat petrosal ganglion in vitro.

We have recently reported that application of acetylcholine (ACh) or nicotine to the petrosal ganglion-the sensory ganglion of the glossopharyngeal nerve-elicits a burst of discharges in the carotid nerve branch, innervating the carotid body and sinus, but not in the glossopharyngeal branch, innervating the tongue and pharynx. Thus, the perikarya of sensory neurons for the carotid bifurcation exhibit selective cholinosensitivity. Since dopamine (DA) modulates carotid nerve chemosensory activity, we searched for the presence of DA sensitivity at the perikarya of these neurons in the cat petrosal ganglion superfused in vitro. Applications of DA in doses of up to 5 mg to the ganglion did not modify the rate of spontaneous discharges in the carotid nerve. However, if DA was applied 30 s before ACh injections, ACh-evoked reactions were modified: low doses of DA enhanced the subsequent responses to ACh, while high doses of DA depressed the responses to ACh. This depressant effect of DA on ACh responses was partially antagonized by adding spiroperone to the superfusate. Our results show that the response to ACh of petrosal ganglion neurons projecting through the carotid nerve is modulated by DA acting on D(2) receptors located in the somata of these neurons. Thus, dopaminergic modulation of cholinosensitivity could be shared also by the membranes of peripheral endings and perikarya of primary sensory neurons involved in arterial chemoreception.

Responses to hypoxia of petrosal ganglia in vitro.

NaCN is a classical stimulus used to elicit discharges from carotid body chemoreceptors. The effect is assumed to be mediated by glomus (type I) cells, which release an excitatory transmitter for the excitation of carotid nerve endings. Since the sensory perikarya of the glossopharyngeal nerve (from which the carotid nerve branches) are located in the petrosal ganglion, we tested whether application of this drug to the petrosal ganglion superfused in vitro elicits antidromic discharges in the carotid nerve. NaCN did indeed cause an intense and prolonged burst of nerve impulses in the carotid nerve, while provoking a less intense and much briefer burst of discharges in the glossopharyngeal branch. Carotid nerve responses to NaCN were reduced and shortened by prior or following application of dopamine to the ganglion. Sodium azide applied to the petrosal ganglion evoked a less intense and much briefer burst of impulses in the carotid nerve. Ganglionar application of 2,4-dinitrophenol did not induce discharges in the carotid nerve. Switching the superfusion of the ganglion from a normoxic to a hypoxic solution did not evoke discharges in the carotid nerve. Therefore, the perikarya of carotid nerve neurons are sensitive to NaCN, but are not excited by reducing the pO(2) of the superfusing solution.

Responses of Cat Petrosal Ganglion Neurons are Modified by the Presence of Carotid Body Cells in Tissue Cultures

The carotid body glomus (type I, receptor) cells are the sensory elements of the arterial chemoreceptor system. They are innervated by petrosal ganglion neurons that convey chemosensory information to the central nervous system. It is accepted that the transduction of the chemosensory stimuli in the glomus cells produces the exocytotic release of several transmitters and/or modulators that, acting on the neuronal terminal, drive the sensory activity (for review see Gonzalez et al, 1994). However, there is limited information on the membrane potential changes induced in the petrosal ganglion neurons by effect of carotid chemosensory stimulation. Intracellular recordings from the nerve terminals within the carotid body showed the existence of slow, graded potentials that could generate spikes when they reach threshold (Hayashida et al, 1980), resembling synaptic or receptor potentials. Recently, acid-induced depolarizing responses, generating trains of action potentials when threshold was reached, were recorded from vagal sensory neurons co-cultured with carotid body cells (Alcayaga & Eyzaguirre, 1990). There is though, a degree of specificity in the formation of synapses between neurons and their target tissues, so we investigated the properties and responses of petrosal ganglion neurons of adult cats, both alone and in presence of carotid body tissue.

Responses induced by acetylcholine and ATP in the rabbit petrosal ganglion

Acetylcholine and ATP appear to mediate excitatory transmission between receptor (glomus) cells and the petrosal ganglion (PG) neuron terminals in the carotid body. In most species these putative transmitters are excitatory, while inhibitory effects had been reported in the rabbit. We studied the effects of the application of acetylcholine and ATP to the PG on the carotid nerve activity in vitro. Acetylcholine and ATP applied to the PG increased the carotid nerve activity in a dose-dependent manner. Acetylcholine-induced responses were mimicked by nicotine, antagonized by hexamethonium, and enhanced by atropine. Bethanechol had no effect on basal activity, but reduced acetylcholine-induced responses. Suramin antagonized ATP-induced responses, and AMP had little effect on the carotid nerve activity. Our results suggest that rabbit PG neurons projecting through the carotid nerve are endowed with nicotinic acetylcholine and purinergic P2 receptors that increase the carotid nerve activity, while simultaneous activation of muscarinic cholinergic receptors reduce the maximal response evoked by nicotinic cholinergic receptor activation.

Petrosal ganglion responses to acetylcholine and ATP are enhanced by chronic normobaric hypoxia in the rabbit

In mammals, adaptation to chronic hypoxia requires the integrity of the arterial chemoreceptors, specially the carotid body (CB). Chronic hypoxia increases the sensibility of the CB by acting on the receptor cells, but there is limited information on the effects of chronic hypoxia on the sensory neurons that innervate the CB. Therefore, we studied the responses evoked by ACh and ATP, the main transmitters that generate the chemoafferent activity, on the petrosal ganglion (PG) of rabbits exposed to chronic normobaric hypoxia (CNH) during fourteen days. ATP and ACh increased the activity of PG neurons in a dose-dependent manner, in a similar way than in rabbits not exposed to hypoxia (naïve). However, the duration of the responses were significantly increased by CNH, with the mean maximal responses to ACh and ATP increased by a factor of two and four, respectively. Our results suggest that CNH increases duration of the responses by modifying the expression and/or content of ACh and ATP receptors.