Identification of M1 and M2 muscarinic acetylcholine receptors in the cat carotid body chemosensory system

The ability of Ibutropin to blunt fentanyl-induced respiratory depression is independent of its activation of carotid body chemoafferents

This study examined the effects of intravenous injection of isobutyric tropine ester (Ibutropin) on ventilation in freely-moving sham-operated (SHAM) male Sprague Dawley rats and those with bilateral carotid sinus nerve transection (CSNX). This study also examined the effects of a subsequent injection of fentanyl on ventilatory parameters in both groups of rats. Ibutropin (200 μmol/kg, i.v.) elicited rapid and pronounced increases in breathing frequency, tidal volume, minute ventilation, peak inspiratory and expiratory flows, and inspiratory and expiratory drives in SHAM rats, but substantially smaller responses in CSNX rats. The subsequent injection of fentanyl (75 μg/kg, i.v.) elicited similar ventilatory responses in Ibutropin-treated SHAM and CSNX rats with markedly different changes in end-inspiratory and end-expiratory pauses, expiratory delay, and apneic pause. Moreover, the fentanyl-induced responses in Ibutropin-treated SHAM and CSNX rats were substantially smaller than in rats that were pre-injected with vehicle (saline) rather than Ibutropin. These novel findings suggest that Ibutropin acts at the carotid body-chemoafferent complex to drive ventilation by mechanisms that may involve the rapid entry of this cell-permeant tropine ester into chemoafferent nerve terminals and/or primary glomus cells. A key finding was that the ability of Ibutropin to blunt the adverse effects of fentanyl on breathing does not require functional carotid body chemoreceptor afferent input to brainstem structures controlling breathing. As such, the ability of Ibutropin to greatly diminish the adverse effects of fentanyl on breathing may involve the actions of Ibutropin within central respiratory control centers and/or peripheral structures other than the carotid bodies. SIGNIFICANCE STATEMENT: This study revealed that the ability of Ibutropin to blunt the respiratory depressant effects of fentanyl may involve mechanisms present in central respiratory control centers and/or peripheral structures other than the carotid bodies.

Neurochemical Anatomy of the Mammalian Carotid Body

Carotid body (CB) glomus cells in most mammals, including humans, contain a broad diversity of classical neurotransmitters, neuropeptides and gaseous signaling molecules as well as their cognate receptors. Among them, acetylcholine, adenosine triphosphate and dopamine have been proposed to be the main excitatory transmitters in the mammalian CB, although subsequently dopamine has been considered an inhibitory neuromodulator in almost all mammalian species except the rabbit. In addition, co-existence of biogenic amines and neuropeptides has been reported in the glomus cells, thus suggesting that they store and release more than one transmitter in response to natural stimuli. Furthermore, certain metabolic and transmitter-degrading enzymes are involved in the chemotransduction and chemotransmission in various mammals. However, the presence of the corresponding biosynthetic enzyme for some transmitter candidates has not been confirmed, and neuroactive substances like serotonin, gamma-aminobutyric acid and adenosine, neuropeptides including opioids, substance P and endothelin, and gaseous molecules such as nitric oxide have been shown to modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. It is likely that the fine balance between excitatory and inhibitory transmitters and their complex interactions might play a more important than suggested role in CB plasticity.

Intra-carotid body inter-cellular communication

The classic peripheral chemoreflex response is a critical homeostatic mechanism. In healthy individuals, appropriate chemoreflex responses are triggered by acute activation of the carotid body - the principal chemosensory organ in mammals. However, the aberrant chronic activation of the carotid body can drive the elevated sympathetic activity underlying cardio-respiratory diseases such as hypertension, diabetes and heart failure. Carotid body resection induces intolerable side effects and so understanding how to modulate carotid body output without removing it, and whilst maintaining the physiological chemoreflex response, represents the next logical next step in the development of effective clinical interventions. By definition, excessive carotid body output must result from altered intra-carotid body inter-cellular communication. Alongside the canonical synaptic transmission from glomus cells to petrosal afferents, many other modes of information exchange in the carotid body have been identified, for example bidirectional signalling between type I and type II cells via ATP-induced ATP release, as well as electrical communication via gap junctions. Thus, herein we review the carotid body as an integrated circuit, discussing a variety of different inter-cellular signalling mechanisms and highlighting those that are potentially relevant to its pathological hyperactivity in disease with the aim of identifying novel therapeutic targets.

Metabotropic Glutamate Receptors 1 Regulates Rat Carotid Body Response to Acute Hypoxia via Presynaptic Mechanism

Background: The carotid body (CB) plays a critical role in oxygen sensing; however, the role of glutamatergic signaling in the CB response to hypoxia remains uncertain. We previously found that functional multiple glutamate transporters and inotropic glutamate receptors (iGluRs) are expressed in the CB. The aim of this present research is to investigate the expression of group I metabotropic glutamate receptors (mGluRs) (mGluR1 and 5) in the CB and its physiological function in rat CB response to acute hypoxia.

Methods: RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of group I mGluRs in the human and rat CB. Immunofluorescence staining was performed to examine the cellular localization of mGluR1 in the rat CB. In vitro carotid sinus nerve (CSN) discharge recording was performed to detect the physiological function of mGluR1 in CB response to acute hypoxia.

Results: We found that (1) mRNAs of mGluR1 and 5 were both expressed in the human and rat CB. (2) mGluR1 protein rather than mGluR5 protein was present in rat CB. (3) mGluR1 was distributed in type I cells of rat CB. (4) Activation of mGluR1 inhibited the hypoxia-induced enhancement of CSN activity (CSNA), as well as prolonged the latency time of CB response to hypoxia. (5) The inhibitory effect of mGluR1 activation on rat CB response to hypoxia could be blocked by GABA_B receptor antagonist.

Conclusion: Our findings reveal that mGluR1 in CB plays a presynaptic feedback inhibition on rat CB response to hypoxia.

miRNA-132/212 Gene-Deletion Aggravates the Effect of Oxygen-Glucose Deprivation on Synaptic Functions in the Female Mouse Hippocampus

Cerebral ischemia and its sequelae, which include memory impairment, constitute a leading cause of disability worldwide. Micro-RNAs (miRNA) are evolutionarily conserved short-length/noncoding RNA molecules recently implicated in adaptive/maladaptive neuronal responses to ischemia. Previous research independently implicated the miRNA-132/212 cluster in cholinergic signaling and synaptic transmission, and in adaptive/protective mechanisms of neuronal responses to hypoxia. However, the putative role of miRNA-132/212 in the response of synaptic transmission to ischemia remained unexplored. Using hippocampal slices from female miRNA-132/212 double-knockout mice in an established electrophysiological model of ischemia, we here describe that miRNA-132/212 gene-deletion aggravated the deleterious effect of repeated oxygen-glucose deprivation insults on synaptic transmission in the dentate gyrus, a brain region crucial for learning and memory functions. We also examined the effect of miRNA-132/212 gene-deletion on the expression of key mediators in cholinergic signaling that are implicated in both adaptive responses to ischemia and hippocampal neural signaling. miRNA-132/212 gene-deletion significantly altered hippocampal AChE and mAChR-M1, but not α7-nAChR or MeCP2 expression. The effects of miRNA-132/212 gene-deletion on hippocampal synaptic transmission and levels of cholinergic-signaling elements suggest the existence of a miRNA-132/212-dependent adaptive mechanism safeguarding the functional integrity of synaptic functions in the acute phase of cerebral ischemia.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

G-Protein-Coupled Receptor (GPCR) Signaling in the Carotid Body: Roles in Hypoxia and Cardiovascular and Respiratory Disease

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

Receptor-Receptor Interactions of G Protein-Coupled Receptors in the Carotid Body: A Working Hypothesis

In the carotid body (CB), a wide series of neurotransmitters and neuromodulators have been identified. They are mainly produced and released by type I cells and act on many different ionotropic and metabotropic receptors located in afferent nerve fibers, type I and II cells. Most metabotropic receptors are G protein-coupled receptors (GPCRs). In other transfected or native cells, GPCRs have been demonstrated to establish physical receptor–receptor interactions (RRIs) with formation of homo/hetero-complexes (dimers or receptor mosaics) in a dynamic monomer/oligomer equilibrium. RRIs modulate ligand binding, signaling, and internalization of GPCR protomers and they are considered of relevance for physiology, pharmacology, and pathology of the nervous system. We hypothesize that RRI may also occur in the different structural elements of the CB (type I cells, type II cells, and afferent fibers), with potential implications in chemoreception, neuromodulation, and tissue plasticity. This ‘working hypothesis’ is supported by literature data reporting the contemporary expression, in type I cells, type II cells, or afferent terminals, of GPCRs which are able to physically interact with each other to form homo/hetero-complexes. Functional data about cross-talks in the CB between different neurotransmitters/neuromodulators also support the hypothesis. On the basis of the above findings, the most significant homo/hetero-complexes which could be postulated in the CB include receptors for dopamine, adenosine, ATP, opioids, histamine, serotonin, endothelin, galanin, GABA, cannabinoids, angiotensin, neurotensin, and melatonin. From a methodological point of view, future studies should demonstrate the colocalization in close proximity (less than 10 nm) of the above receptors, through biophysical (i.e., bioluminescence/fluorescence resonance energy transfer, protein-fragment complementation assay, total internal reflection fluorescence microscopy, fluorescence correlation spectroscopy and photoactivated localization microscopy, X-ray crystallography) or biochemical (co-immunoprecipitation, in situ proximity ligation assay) methods. Moreover, functional approaches will be able to show if ligand binding to one receptor produces changes in the biochemical characteristics (ligand recognition, decoding, and trafficking processes) of the other(s). Plasticity aspects would be also of interest, as development and environmental stimuli (chronic continuous or intermittent hypoxia) produce changes in the expression of certain receptors which could potentially invest the dynamic monomer/oligomer equilibrium of homo/hetero-complexes and the correlated functional implications.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

Localization of neurotransmitters, peptides and nNOS in the pseudobranchial neurosecretory cell system and associated carotid labyrinth of the catfish, Clarias batrachus

The carotid labyrinth is an enigmatic endocrine structure of unknown chemosensory function lying in the gill region of the catfishes. The carotid body is found at the carotid bifurcation of amphibians and all mammalian vertebrates on the evolutionary tree. It is a vascular expansion comprised of a cluster of glomus cells with associated (afferent and efferent) innervations. In the catfish species studied (Clarias batrachus) a neurosecretory cell system consisting of pseudobranchial neurosecretory cells connect the carotid labyrinth or large vessels (both the efferent branchial artery and dorsal aorta), and is likely akin to the glomus cells, but comparing these structures in widely divergent vertebrate species, the conclusion is that the structural components are more elaborate than those of terrestrial vertebrates. However, these cells reveal both an endocrine phenotype (such as the association with capillaries and large vessels) and the presence of regulatory substances such as neurotransmitters and neuropeptides producing good evidence for high levels of conservation of these substances that are present in the glomus cells of mammalian vertebrates. VIP-immunopositive neuronal cell bodies are detected in the periphery of the carotid labyrinth. They are presumptive local neurons that differ from pseudobranchial neurosecretory cells, the latter failing to express VIP in their soma.

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.

The impact of adenosine and an A2A adenosine receptor agonist on the ACh-induced increase in intracellular calcium of the glomus cells of the cat carotid body

The carotid body (CB) is a polymodal chemosensor of arterial blood located next to the internal carotid artery. The basic chemosensing unit is composed of the neurotransmitter (NT)-containing glomus cells (GCs) and the sensory afferent fibers synapsing onto the GCs. Nicotinic and muscarinic receptors have been found on both the sensory afferent fibers and on the GCs. Neural output from the CB (CBNO) increases when arterial blood perfusing it is hypoxic, hypoglycemic, hypercapnic, or acidic. The increased CBNO due to GC release of excitatory NTs must be preceded by an entrance of calcium into the GCs. With repeated release of ACh from the GCs, cholinergic receptors could become desensitized, particularly nicotinic receptors which function as calcium channels. The purpose of the present study was to see if adenosine (ADO), known to alter receptor sensitivities, could attenuate or eliminate any desensitization of the nicotinic receptors occurring during the repeated application of ACh. Cat CBs were harvested with techniques approved by the University's Animal Care/Use Committee. The GCs were cultured and prepared for detecting [Ca(++)](i) with standard techniques. Repeated application of ACh produced a progressively decreasing increase in [Ca(++)](i). With the use of ADO or an A2(A) ADO receptor agonist the decrease was avoided. Though ADO also increased GC [Ca(++)](i), the sum of ADO increase and ACh increase, when superfused separately, was less than the increase when they were both included in the same superfusion. This suggested the possible involvement of a new path in the action. Potential mechanisms to explain the phenomena are discussed.

Benzodiazepines and GABA-GABAA receptor system in the cat carotid body

Benzodiazepines (BZs) suppress ventilation possibly by augmenting the GABA(A) receptor activity in the respiratory control system, but precise sites of action are not well understood. The goals of this study were: (1) to identify GABA(A) receptor subunits in the carotid body (CB) and petrosal ganglion (PG); (2) to test if BZs exert their effects through the GABA(A) receptor in the CB chemosensory unit. Tissues were taken from euthanized adult cats. RNA was extracted from the brain, and cDNA sequences of several GABA(A) receptor subunits were determined. Subsequent RT-PCR analysis demonstrated the gene expression of alpha2, alpha3, beta3, and gamma2 subunits in the CB and the PG. Immunoreactivity for GABA and for GABA(A) receptor beta3 and gamma2 subunits was detected in chemosensory glomus cells (GCs) in the CB and neurons in the PG. The functional aspects of the GABA-GABA(A) receptor system in the CB was studied by measuring CB neural output using in vitro perfusion setup. Two BZs, midazolam and diazepam, decreased the CB neural response to hypoxia. With continuous application of bicuculline, a GABA(A) receptor antagonist, the effects of BZs were abolished. In conclusion, the GABA-GABA(A) receptor system is functioning in the CB chemosensory system. BZs inhibit CB neural response to hypoxia by enhancing GABA(A) receptor activity.

Distribution of voltage-gated potassium and hyperpolarization-activated channels in sensory afferent fibers in the rat carotid body

The chemosensory glomus cells of the carotid body (CB) detect changes in O2 tension. Carotid sinus nerve fibers, which originate from peripheral sensory neurons located within the petrosal ganglion, innervate the CB. Release of transmitter from glomus cells activates the sensory afferent fibers to transmit information to the nucleus of the solitary tract in the brainstem. The ion channels expressed within the sensory nerve terminals play an essential role in the ability of the terminal to initiate action potentials in response to transmitter-evoked depolarization. However, with a few exceptions, the identity of ion channels expressed in these peripheral nerve fibers is unknown. This study addresses the expression of voltage-gated channels in the sensory fibers with a focus on channels that set the resting membrane potential and regulate discharge patterns. By using immunohistochemistry and fluorescence confocal microscopy, potassium channel subunits and HCN (hyperpolarization-activated) family members were localized both in petrosal neurons that expressed tyrosine hydroxylase and in the CSN axons within the carotid body. Channels contributing to resting membrane potential, including HCN2 responsible in part for I(h) current and the KCNQ2 and KCNQ5 subunits thought to underlie the neuronal "M current," were identified in the sensory neurons and their axons innervating the carotid body. In addition, the results presented here demonstrate expression of several potassium channels that shape the action potential and the frequency of discharge, including Kv1.4, Kv1.5, Kv4.3, and K(Ca) (BK). The role of these channels should be considered in interpretation of the fiber discharge in response to perturbation of the carotid body environment.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.

Developmental pattern of M1 and M2 muscarinic gene expression and receptor levels in cat carotid body, petrosal and superior cervical ganglion

Using real-time reverse transcriptase polymerase chain reaction, Northern blot, and Western blot analyses, we evaluated the developmental pattern of mRNA and protein expression level of muscarinic M1 and M2 receptors in the carotid body, petrosal ganglion and superior cervical ganglion of 1-day, 15-day, 2-month-old and adult cats. mRNA expression and protein levels of tyrosine hydroxylase, the rate limiting enzyme for dopamine synthesis, were also assessed. Carotid body M1 receptor mRNA, increased significantly by approximately 100% and 300% in 2-month and adult vs. 1- and 15-day-old cats, but protein level decreased gradually being approximately 50% lower compared with 1-day-old cats. In the petrosal ganglion, muscarinic M1 receptor mRNA level was higher in 15-day-old cats vs. 1-day-old, 2-month-old and adult cats and protein levels were about 30% lower than in 1- and 15-day-old cats. In the superior cervical ganglion, muscarinic M1 receptor mRNA was approximately 50% and 80% higher in 2-month-old and adult cats than 1- and 15-day-old, but no changes in the protein level except in 15-day-old cats which was approximately 40% higher than 1-day-old. There was no change of muscarinic M2 receptor mRNA or protein level in the carotid body or petrosal ganglion. However, in the superior cervical ganglion, the significant increase of mRNA of 30% and 50% in 2-month-olds and adults, respectively was not associated with an increase in receptor protein. Tyrosine hydroxylase mRNA and protein level decreased significantly with age in the carotid body and petrosal ganglion. In the superior cervical ganglion, the age dependent increase in tyrosine hydroxylase mRNA was not associated with any changes in the protein level. These results show that the expression of muscarinic M1 and M2 receptors are age and organ-dependent in cats. Consequently, these changes may modulate chemosensory activity during development since muscarinic M1 receptor is predominantly involved in postsynaptic chemosensory activity, while muscarinic M2 receptor modulates acetylcholine and dopamine release from chemosensitive cells.

Neurotransmitters in carotid body development

This review examines the possible role of neurotransmitters present in the carotid body on the functional expression of chemosensory activity during postnatal development. In particular, dopamine, acetylcholine, adenosine and neuropeptides are reviewed. Evidence to date shows involvement of these transmitters in signal transmission from the chemoreceptor cells to chemosensory afferent fibers of the sinus nerve, with clear age- or maturation-dependence of some aspects. However, it remains unresolved whether these neurotransmitters, some of which are expressed in the carotid body before birth, are directly involved in the maturation of the functional properties of the carotid chemoreceptors in sensing oxygen or other stimuli during postnatal development.