Synaptic mechanisms during re-innervation of rat arterial chemoreceptors in co-culture

Petrosal ganglion: a more complex role than originally imagined

The petrosal ganglion (PG) is a peripheral sensory ganglion, composed of pseudomonopolar sensory neurons that innervate the posterior third of the tongue and the carotid sinus and body. According to their electrical properties PG neurons can be ascribed to one of two categories: (i) neurons with action potentials presenting an inflection (hump) on its repolarizing phase and (ii) neurons with fast and brisk action potentials. Although there is some correlation between the electrophysiological properties and the sensory modality of the neurons in some species, no general pattern can be easily recognized. On the other hand, petrosal neurons projecting to the carotid body are activated by several transmitters, with acetylcholine and ATP being the most conspicuous in most species. Petrosal neurons are completely surrounded by a multi-cellular sheet of glial (satellite) cells that prevents the formation of chemical or electrical synapses between neurons. Thus, PG neurons are regarded as mere wires that communicate the periphery (i.e., carotid body) and the central nervous system. However, it has been shown that in other sensory ganglia satellite glial cells and their neighboring neurons can interact, partly by the release of chemical neuro-glio transmitters. This intercellular communication can potentially modulate the excitatory status of sensory neurons and thus the afferent discharge. In this mini review, we will briefly summarize the general properties of PG neurons and the current knowledge about the glial-neuron communication in sensory neurons and how this phenomenon could be important in the chemical sensory processing generated in the carotid body.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Fifty years of progress in carotid body physiology--invited article

Research on arterial chemoreceptors, particularly on the carotid body, has been fruitful in the last fifty years, to which this review is addressed. The functional anatomy of the organ appears to be well established. The biophysical bases by which glomus cells transduce chemical changes in the milieu intérieur (hypoxia, hypercapnia, acidosis) into electrical and biochemical changes in glomus cells have received much attention. Physical changes (in temperature, flow and osmolarity) are also detected by the carotid body. Electrical coupling between glomus cells themselves appears as very extensive. Sustentacular cells classically considered as ensheathing glia for glomus cells and nerve endings now appear to behave as stem cells precursors for glomus cells under chronic hypoxic conditions. Many papers have been devoted to transmitters released from glomus cells (acetylcholine, dopamine, ATP) and well as to their effects upon chemosensory nerve activity. Chemosensory neurons have been explored from generation of action potentials at peripheral nerve endings, passing to properties of perikarya at petrosal ganglia and finally at characterization of synaptic transmission at solitary tract nuclei. There is abundant literature on ventilatory and cardiovascular reflexes elicited from arterial chemoreceptors. The transient effects of sudden and brief withdrawal of chemosensory discharges by hyperoxia also provide clues on the role played by carotid bodies in the homeostasis of full organisms.

Developmental profile of cholinergic and purinergic traits and receptors in peripheral chemoreflex pathway in cats

This study describes the developmental profile of specific aspects of cholinergic and purinergic neurotransmission in key organs of the peripheral chemoreflex: the carotid body (CB), petrosal ganglion (PG) and superior cervical ganglion (SCG). Using real time RT-PCR and Western blot analyses, we assessed both mRNA and protein expression levels for choline-acetyl-transferase (ChAT), nicotinic receptor (subunits alpha3, alpha4, alpha7, and beta2), ATP and purinergic receptors (P2X2 and P2X3). These analyses were performed on tissue from 1- and 15-day-old, 2-month-old, and adult cats. During development, ChAT protein expression level increased slightly in CB; however, this increase was more important in PG and SCG. In CB, mRNA level for alpha4 nicotinic receptor subunit decreased during development (90% higher in 1-day-old cats than in adults). In the PG, mRNA level for beta2 nicotinic receptor subunit increased during development (80% higher in adults than in 1-day-old cats). In SCG, mRNA for alpha7 nicotinic receptor levels increased (400% higher in adults vs. 1-day-old cats). Conversely, P2X2 receptor protein level was not altered during development in CB and decreased slightly in PG; a similar pattern was observed for the P2X3 receptor. Our findings suggest that in cats, age-related changes in cholinergic and purinergic systems (such as physiological expression of receptor function) are significant within the afferent chemoreceptor pathway and likely contribute to the temporal changes of O2-chemosensitivity during development.

Is ATP a suitable co-transmitter in carotid body arterial chemoreceptors?

A review is presented on carotid body ATP content, effects and release, receptors involved and results of their block by purinergic antagonists, and the possibility of cholinergic-purinergic co-transmission in the carotid body. Glomus cells release ACh and ATP upon physiological stimulation. Both agents and their agonists have chemo-excitatory actions and their combined effects disappear upon blocking n-ACh and P2X receptors. Both ACh and ATP also are capable of exciting the somata of chemosensory neurons of petrosal ganglia. Although a combined cholinergic-purinergic block suppresses the chemosensory activity in neurons co-cultured with glomus cells and some carotid body preparations in vitro, basal chemosensory activity and chemosensory responses to hypoxic stimuli persist in cat carotid body preparations in situ and in vitro. Therefore, ATP is an effective excitatory agent for carotid body chemosensory activity, although less potent than ACh; their joint participation may contribute to -- but does not entirely explain -- the transfer of chemoreceptor excitation from glomus cells to sensory endings in carotid body.

Effects of combined cholinergic-purinergic block upon cat carotid body chemoreceptors in vitro

Since acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters at synapses between glomus cells and sensory nerve endings of the carotid body (CB), we tested such hypothesis by studying the effects of combined cholinergic-purinergic block on the chemosensory activity recorded from cat's carotid bodies perfused and/or superfused in vitro. The preparations were bathed with Tyrode's solution, either normoxic (PO2=98.5+/-13.5 Torr) or hypoxic (PO2=31.8+/-5.2 Torr), and the frequency of chemosensory impulses (fchi) was recorded from the carotid (sinus) nerve. Dose-response curves for fchi increases evoked by intra-stream boluses of acetylcholine, nicotine and ATP were studied. A combination of mecamylamine 2 microM and suramin 50 microM, applied through the perfusate or superfusate, suppressed nicotine- and ATP-induced increases in fchi, but the basal chemosensory activity in normoxia and the chemosensory excitation elicited by hypoxic superfusion were preserved, although variably reduced in most preparations. Thus, in spite of the excitatory effects provoked by applying ACh and ATP to the perfused/superfused CB in vitro, a co-release of these substances cannot account entirely for the chemosensory excitation induced by hypoxic stimulation of the CB.

Carotid body chemosensory activity and ventilatory chemoreflexes in cats persist after combined cholinergic-purinergic block

Acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters operating at synapses between glomus cells and sensory nerve endings of the carotid body (CB). To test such hypothesis, we performed experiments on cats under pentobarbitone anesthesia and breathing spontaneously. Cholinergic and purinergic agonists and antagonists were given into one common carotid artery. Chemoreflex ventilatory changes initiated from the ipsilateral CB or chemosensory activity from the ipsilateral carotid nerve were recorded. Agonists ACh, nicotine, epibatidine, ATP, betagamma-methylene-ATP and gammaS-ATP induced transient chemoreflex enhancements of ventilation or increased chemosensory activity. When given in combination, mecamylamine and suramin suppressed both nicotine- and ATP-induced ventilatory chemoreflexes or chemosensory responses. However, neither chemoreflex hyperventilation induced by brief hypoxic exposures or steady-state hypoxic levels, nor chemosensory excitation elicited by these maneuvers were eliminated. Asphyxia-induced chemosensory excitation was not reduced by combined blockade of ACh and ATP receptors. Furthermore, ventilatory or chemosensory depression evoked by 100% O2 tests was unmodified, thus evidencing that basal chemosensory drive in normoxia was not suppressed by combined cholinergic-purinergic blockade. Therefore, although ACh and ATP may participate in chemoexcitation of the CB, their involvement fails to explain the origin of chemosensory discharges from synaptic transmission between glomus cells and chemosensory nerve endings of the CB.

Are there interactions between acetylcholine- and ATP-induced responses at the level of a visceral sensory ganglion?

We investigate possible interactions between acetylcholine (ACh)- and adenosine 5'-triphosphate (ATP)-induced responses of petrosal ganglion, where the perikarya of most sensory neurons of the glossopharyngeal nerve are located. Experiments were performed on petrosal ganglia excised from pentobarbitone-anesthetized cats, desheathed and perfused in vitro. Separate applications of ACh and ATP to the exposed surface of the ganglion induced bursts of antidromic potentials recorded from the carotid (sinus) nerve branch of the glossopharyngeal nerve, which frequencies were dependent on the dose of the applied agonists. The simultaneous application of previously determined ED50s of ACh and ATP provoked responses corresponding closely to the simple addition of the responses elicited by the separate application of each agent. Responses usually subsided within 1 min of stimuli application but were followed by periods of refractoriness to subsequent application of the same agent. After determining the timing for recovering from desensitization to the ED50s of ACh and ATP applied separately, ACh was applied while the preparation had been desensitized to ATP and then ATP was applied during desensitization to ACh, but responses obtained were similar to control responses induced by each agent separately. In summary, ACh- and ATP-induced responses of petrosal ganglion neurons are simply additive, followed by a few minute lasting desensitization, but cross-desensitization was not observed. Thus, ACh and ATP seem to operate through independent receptors, activating separate ionic channels, whose coincident currents do not interfere each other.

Immunohistochemical evidence for species-specific coexistence of catecholamines, serotonin, acetylcholine and nitric oxide in glomus cells of rat and guinea pig aortic bodies

The aortic bodies are small paraganglia distributed along the vagus nerve and its branches in the vicinity of the aortic arch which, like the carotid bodies, act as arterial chemoreceptors. In the rat carotid body, corelease of ATP and acetylcholine (ACh) from glomus cells is considered to be the main mechanism mediating fast hypoxic chemotransmission while dopamine, serotonin, and nitric oxide (NO) exert modulating effects. The present study was aimed at determination of the endogenous sources of serotonin, ACh and NO within rat and guinea pig aortic bodies by immunohistochemical double- and triple-labeling approaches, utilizing antibodies to serotonin, the NO and ACh synthesizing enzymes neuronal NO synthase (nNOS) and choline acetyltransferase (ChAT), respectively, as well as to the vesicular acetylcholine transporter (VAChT). Additional marker antibodies were directed against the rate-limiting enzyme of catecholamine synthesis, i.e. tyrosine hydroxylase (TH), and the vesicular protein, synaptophysin (SYN). In both species, all aortic body glomus cells were immunoreactive to serotonin and cholinergic markers. In the rat, all glomus cells were additionally catecholaminergic, as indicated by TH-immunoreactivity, whereas this applied only to a subgroup of guinea pig glomus cells. On the other hand, all guinea pig glomus cells were nNOS-immunoreactive, whereas only nerve fibers but not glomus cells exhibited nNOS-immunoreactivity in the rat. These data support the concept that the chemoexcitatory transmitters ACh and serotonin are involved in hypoxic excitation of aortic chemoreceptor terminals in both species. The production of the inhibitory modulators, dopamine and NO, however, appears to be species-specifically regulated.

Presynaptic modulation of rat arterial chemoreceptor function by 5-HT: role of K+ channel inhibition via protein kinase C

The peripheral control of breathing is mediated by O2-sensitive carotid body (CB) type 1 cells, which express multiple neurotransmitters including the monoamines, dopamine and serotonin (5-HT). Whereas dopamine has been extensively studied, 5-HT has received little attention. Here, to elucidate the role of 5-HT in CB chemotransmission, we used perforated-patch recording from rat type 1 cell clusters and co-cultured petrosal (afferent) neurones. 5-HT induced action potentials and/or membrane depolarization associated with a conductance decrease in approximately 40% of recordings from type 1 cells (n = 78/192). These responses were markedly inhibited by the 5-HT2 receptor antagonist ketanserin (10-50 microM) and by the protein kinase C (PKC) inhibitor chelerythrine (50 microM). The PKC activator 1-oleoyl-2-acetylglycerol (OAG; 50 microM) mimicked the 5-HT-induced depolarization, and the combined effects of 5-HT and OAG were non-additive. The 5-HT-induced responses reversed near the potassium (K+) equilibrium potential (at approximately -82 mV; EK = -83 mV), suggesting inhibition of a resting K+ conductance. In type 1 cells (n = 7), voltage-activated outward K+ current was also inhibited by 1-50 microM 5-HT, an effect that was prevented by PKC inhibitors (chelerythrine and NPC 15437) and mimicked by OAG; the outward K+ current inhibited by 5-HT appeared to be predominantly a Ca(2+)-dependent K+ current. The 5-HT2 receptor blockers ketanserin and ritanserin reversibly inhibited spontaneous action potentials and the hypoxia-induced receptor potential recorded from clustered type 1 cells. Moreover, these blockers reversibly inhibited the hypoxic chemosensory response recorded postsynaptically in petrosal neurones that functionally innervated type 1 clusters in co-culture. RT-PCR and confocal immunofluorescence techniques revealed 5-HT2a receptor expression in rat CB type 1 cells. These results suggest that release of endogenous 5-HT regulates CB chemoreceptor function presynaptically, by a positive feedback mechanism involving autocrine-paracrine stimulation of 5-HT2a receptors and PKC modulation of resting and Ca(2+)-dependent K+ conductances.

Carotid body chemoreceptors in dissociated cell culture

Carotid body (CB) glomus or type 1 cells act as peripheral chemoreceptors which detect changes in arterial PO(2), PCO(2), and pH and help maintain homeostasis via the reflex control of ventilation. Over the last approximately 12 years significant progress has been made towards understanding chemotransduction mechanisms using freshly isolated or cultured type 1 cells. The latter preparation allows several powerful experimental manipulations (e.g., co-culture with sensory neurons) resulting in significant advances in our understanding of CB chemoreception. Here, we review several properties of type 1 cells after several days to weeks in culture. Typically, cultured type 1 cells grow in monolayer clusters enveloped by glial-like, type II, or sustentacular cells, which are immunopositive for the glial marker, glial fibrillary acid protein (GFAP). These cells can undergo DNA synthesis, evidenced by uptake of bromodeoxyuridine (BrdU), and show a limited capacity for cell division. Mitosis and survival of type 1 cells can be regulated by oxygen tension and/or growth factors (e.g., bFGF, insulin). In the rat, type 1 cells are immunopositive for several monoaminergic markers, including tyrosine hydroxylase (TH), dopamine transporter (DAT), and 5-HT. They also express cholinergic markers (e.g., vesicular acetylcholine transporter; VAChT), the highly conserved synaptic vesicle protein (SV2), and gap junctional proteins including Connexin 32 (Cx32). Moreover, in long-term culture ( approximately 2 weeks) they retain expression of O(2)-sensitive, TASK-1-like, and Ca(2+)-dependent (BK), K(+) channels as revealed by immunocytochemistry or RT-PCR analysis of mRNA extracted from type 1 clusters after removal from the culture surface.