Release of 5-hydroxytryptamine by hypoxia from epithelioid cells of chicken thoracic aorta

Hydrogen sulfide activates TRPA1 and releases 5-HT from epithelioid cells of the chicken thoracic aorta

Epithelioid cells in the chicken thoracic aorta are chemoreceptor cells that release 5-HT in response to hypoxia. It is likely that these cells play a role in chemoreception similar to that of glomus cells in the carotid bodies of mammals. Recently, H2S was reported to be a key mediator of carotid glomus cell responses to hypoxia. The aim of the present study was to reveal the mechanism of action of H2S on 5-HT outflow from chemoreceptor cells in the chicken thoracic aorta. The 5-HT outflow induced by NaHS, an H2S donor, and Na2S3, a polysulfide, was measured by using a HPLC equipped with an electrochemical detector. NaHS (0.3-3mM) caused a concentration-dependent increase in 5-HT outflow, which was significantly inhibited by the removal of extracellular Ca(2+). 5-HT outflow induced by NaHS (0.3mM) was also significantly inhibited by voltage-dependent L- and N-type Ca(2+) channel blockers and a selective TRPA1 channel blocker. Cinnamaldehyde, a TRPA1 agonist, mimicked the secretory response to H2S. 5-HT outflow induced by Na2S3 (10μM) was also inhibited by the TRPA1 channel blocker. Furthermore, the expression of TRPA1 was localized to 5-HT-containing chemoreceptor cells in the aortic wall. These findings suggest that the activation of TRPA1 and voltage-dependent Ca(2+) channels is involved in H2S-evoked 5-HT release from chemoreceptor cells in the chicken aorta.

Endogenous 5-HT outflow from chicken aorta by 5-HT uptake inhibitors and amphetamine derivatives

Chemoreceptor cells aggregating in clusters in the chicken thoracic aorta contain 5-hydroxytryptamine (5-HT) and have voltage-dependent ion channels and nicotinic acetylcholine receptors, which are characteristics typically associated with neurons. The aim of the present study was to investigate the effects of 5-HT uptake inhibitors, fluvoxamine, fluoxetine and clomipramine (CLM), and amphetamine derivatives, p-chloroamphetamine (PCA) and methamphetamine (MET), on endogenous 5-HT outflow from the isolated chick thoracic aorta in vitro. 5-HT was measured by using a HPLC system with electrochemical detection. The amphetamine derivatives and 5-HT uptake inhibitors caused concentration-dependent increases in endogenous 5-HT outflow. PCA was about ten times more effective in eliciting 5-HT outflow than MET. The 5-HT uptake inhibitors examined had similar potency for 5-HT outflow. PCA and CLM increased 5-HT outflow in a temperature-dependent manner. The outflow of 5-HT induced by PCA or 5-HT uptake inhibitors was independent of extracellular Ca²⁺ concentration. The 5-HT outflow induced by CLM, but not that by PCA, was dependent on the extracellular NaCl concentration. These results suggest that the 5-HT uptake system of 5-HT-containing chemoreceptor cells in the chicken thoracic aorta has characteristics similar to those of 5-HT-containing neurons in the mammalian central nervous system (CNS).

Distribution and innervation of putative arterial chemoreceptors in the bullfrog (Rana catesbeiana)

Peripheral arterial chemoreceptors have been located previously in the carotid labyrinth, the aortic arch, and the pulmocutaneous artery of frogs. In the present study we used cholera toxin B neuronal tract tracing and immunohistochemical markers for cholinergic cells (vesicular acetylcholine transporter [VAChT]), tyrosine hydroxylase (TH), and serotonin (5HT) to identify putative O2-sensing cells in Rana catesbeiana. We found potential O2-sensing cells in all three vascular areas innervated by branches of the vagus nerve, whereas only cells in the carotid labyrinth were innervated by the glossopharyngeal nerve. Cells containing either 5HT or TH were found in all three sites, whereas cells containing both neurotransmitters were found only in the carotid labyrinth. Cell bodies containing VAChT were not found at any site. The morphology and innervation of putative O2-sensing cells were similar to those of glomus cells found in other vertebrates. The presence of 5HT- and TH-immunoreactive cells in the aorta, pulmocutaneous artery, and carotid labyrinth appears to reflect a phylogenetic transition between the major neurotransmitter seen in the putative O2-sensing cells of fish (5HT) and those found in the glomus cells of mammals (acetylcholine, adenosine, and catecholamines).

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.

Serotonin and serotonin receptor expression in the aorta of carotid intact and denervated newborns

Pharmacological blocking of serotonin (5-HT) 5A receptors abolishes aortic ventilatory chemosensitivity of carotid body denervated (CBD) piglets [J. Appl. Physiol. 92 (2002) 893]. Accordingly, the purpose of the present study was to determine whether 5-HT and 5-HT receptors exist at aortic sites that are chemosensitive after CBD. Aortas from CBD and sham CBD rats and piglets and from aortic denervated (AOD) and combined AOD+CBD piglets were harvested, sectioned and then studied using immunohistochemistry and western blot techniques. 5-HT immunoreactivity in piglets and rats was concentrated in the endothelium and sub-endothelial areas in several aortic regions studied, and in some areas also in the adventitia. At the aortic chemosensitive site (descending aorta in CBD piglets and the ascending aorta in CBD rats), the immunoreactivity was greater (P < 0.05) than in other aortic regions and greater than in other groups studied. The 5-HT(5a) receptor was expressed only at the chemosensitive sites and only in aortic innervated piglets. We conclude that the data from this and a previous study [J. Appl. Physiol. 92 (2002) 893] suggest that a serotonergic mechanism contributes to the aortic ventilatory chemoreflex after CBD.

Heterogeneity of neuronal nicotinic acetylcholine receptors in 5-HT-containing chemoreceptor cells of the chicken aorta

The effects of nicotinic agonists and antagonists on whole-cell currents and 5-hydroxytryptamine (5-HT) release were studied in order to characterize nicotinic ACh receptors on the 5-HT-containing chemoreceptor cells of the chicken aorta. ACh, nicotine and dimethylphenylpiperazinium (DMPP) evoked concentration-dependent inward currents accompanied by increases in current noise at a holding potential of -70 mV. The peak amplitude of the current response to DMPP was 50% larger than that to either nicotine or ACH: Hexamethonium, alpha - bungarotoxin (alpha - BTX) and methyllycaconitine decreased nicotine-induced inward currents in a concentration-dependent manner. Although hexamethonium (0.1 mM) abolished the current response to nicotine (30 microM), a high concentration (1 microM) of alpha - BTX decreased it only by about 30% of the control response. Methyllycaconitine (0.1 microM) decreased the current response to nicotine to the same extent as did alpha - BTX whilst a high concentration (10 microM) abolished the response. ACh, nicotine and DMPP caused concentration-dependent increases in 5-HT output from the thoracic aorta which effect was blocked by hexamethonium (0.1 mM). Pre-treatment with alpha - BTX (1 microM) for 30 min reduced the output of 5-HT induced by ACh to 70% of the control response. It is suggested that neuronal nicotinic ACh receptors, sensitive and insensitive to alpha - BTX, are present on the chemoreceptor cells of the chicken aorta, the activation of which causes the release of 5-HT.

Characteristics of 5-HT-containing chemoreceptor cells of the chicken aortic body

1. Voltage-dependent and oxygen-sensitive currents in 5-HT-containing epithelioid cells isolated from chicken thoracic aorta were examined using the whole-cell patch clamp technique. 5-HT immunoreactive cells were identified with Neutral Red. The release of 5-HT from chicken thoracic aorta in the presence of excess KCl and veratridine was also examined using HPLC. 2. At a holding potential of -70 mV with CsCl pipette solution, depolarizing steps between -30 and +60 mV produced inward currents that were blocked by tetrodotoxin (0.2 microM). In the presence of tetrodotoxin and BaCl2 (5 mM), depolarizing steps evoked slow inward currents that were sensitive to CoCl2 (2 mM). Nifedipine (1 microM) decreased the currents to 79.4 +/- 1.7 %, and omega-conotoxin GVIA (1 microM) to 20.2 +/- 3.8 %. 3. When KCl pipette solution was used, depolarizing potentials positive to -40 mV caused outward currents that were inhibited by tetraethylammonium chloride. The K+ currents evoked by depolarizing steps to +20 mV were reduced to 90.3 +/- 0.8 % by hypoxia in five out of seven cells. Two cells failed to respond to hypoxia. The K+ current response was partly decreased by Neutral Red (20 microM). 4. Excess KCl (60 mM) and veratridine (30 microM) both caused the release of 5-HT from aortic strips. 5-HT outputs induced by both stimuli were partly inhibited by nifedipine (1 microM) and by omega-conotoxin GVIA (1 microM), and were abolished by these drugs in combination and by extracellular Ca2+ removal. 5. These results suggest that epithelioid cells containing 5-HT act as chemoreceptor cells in the chicken aortic body, having voltage-dependent Na+, K+, and L- and N-type Ca2+ channels, and oxygen-sensitive K+ channels.