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

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.

Mechanistic actions of oxygen and methylxanthines on respiratory neural control and for the treatment of neonatal apnea

Apnea remains one of the most concerning and prevalent respiratory disorders spanning all ages from infants (particularly those born preterm) to adults. Although the pathophysiological consequences of apnea are fairly well described, the neural mechanisms underlying the etiology of the different types of apnea (central, obstructive, and mixed) still remain incompletely understood. From a developmental perspective, however, research into the respiratory neural control system of immature animals has shed light on both central and peripheral neural pathways underlying apnea of prematurity (AOP), a highly prevalent respiratory disorder of preterm infants. Animal studies have also been fundamental in furthering our understanding of how clinical interventions (e.g. pharmacological and mechanical) exert their beneficial effects in the clinical treatment of apnea. Although current clinical interventions such as supplemental O2 and positive pressure respiratory support are critically important for the infant in respiratory distress, they are not fully effective and can also come with unfortunate, unintended (and long-term) side-effects. In this review, we have chosen AOP as one of the most common clinical scenarios involving apnea to highlight the mechanistic basis behind how some of the interventions could be both beneficial and also deleterious to the respiratory neural control system. We have included a section on infants with critical congenital heart diseases (CCHD), in whom apnea can be a clinical concern due to treatment with prostaglandin, and who may benefit from some of the treatments used for AOP.

Striatal cholinergic interneurons and Parkinson's disease

Giant, aspiny cholinergic interneurons (ChIs) have long been known to be key nodes in the striatal circuitry controlling goal-directed actions and habits. In recent years, new experimental approaches, like optogenetics and monosynaptic rabies virus mapping, have expanded our understanding of how ChIs contribute to the striatal activity underlying action selection and the interplay of dopaminergic and cholinergic signaling. These approaches also have begun to reveal how ChI function is distorted in disease states affecting the basal ganglia, like Parkinson's disease (PD). This review gives a brief overview of our current understanding of the functional role played by ChIs in striatal physiology and how this changes in PD. The translational implications of these discoveries, as well as the gaps that remain to be bridged, are discussed as well.

Age-related changes in immunoreactivity for dopamine β-hydroxylase in carotid body glomus cells in spontaneously hypertensive rats

The purpose of this study was to investigate immunoreactivity for dopamine β-hydroxylase (DBH) and tyrosine hydroxylase (TH) in carotid body (CB) glomus cells in spontaneously hypertensive rats (SHR/Izm) at 4 (prehypertensive stage), 8 (early stage of developmental hypertension), 12 (later stage of developmental hypertension), and 16weeks of age (established hypertensive stage). Age-matched Wistar Kyoto rats (WKY/Izm) were used as controls. Staining properties for TH were similar between both strains at each age. Regarding DBH immunostaining, although some glomus cells showed intense DBH immunoreactivity at 4weeks of age, these cells were rarely observed at 8, 12, and 16weeks of age in WKY/Izm. In SHR/Izm, intense DBH immunoreactivity was observed in some glomus cells at 4weeks of age, these cells were also observed at 8 and 12weeks of age, and their number increased at 16weeks of age. An image analysis showed that the percentage of DBH-immunopositive glomus cells in WKY/Izm was approximately 30% at 4weeks of age and significantly decreased to approximately 10% at 8, 12, and 16weeks of age (p<0.05). This percentage in SHR/Izm was approximately 40% at each age. The gray scale intensity for DBH immunoreactivity in DBH-immunopositive glomus cells was similar in both strains at 4weeks of age, but became significantly lower in WKY/Izm and higher in SHR/Izm with increase in age (p<0.05). These results suggest that noradrenaline in glomus cells plays an important role in the regulation of neurotransmission between CB and afferent nerves during developmental hypertension.

Presynaptic muscarinic acetylcholine autoreceptors (M1, M2 and M4 subtypes), adenosine receptors (A1 and A2A) and tropomyosin-related kinase B receptor (TrkB) modulate the developmental synapse elimination process at the neuromuscular junction

Background

The development of the nervous system involves an initially exuberant production of neurons that make an excessive number of synaptic contacts. The initial overproduction of synapses promotes connectivity. Hebbian competition between axons with different activities (the least active are punished) leads to the loss of roughly half of the overproduced elements and this refines connectivity and increases specificity. The neuromuscular junction is innervated by a single axon at the end of the synapse elimination process and, because of its relative simplicity, has long been used as a model for studying the general principles of synapse development. The involvement of the presynaptic muscarinic ACh autoreceptors may allow for the direct competitive interaction between nerve endings through differential activity-dependent acetylcholine release in the synaptic cleft. Then, the most active ending may directly punish the less active ones. Our previous results indicate the existence in the weakest axons on the polyinnervated neonatal NMJ of an ACh release inhibition mechanism based on mAChR coupled to protein kinase C and voltage-dependent calcium channels. We suggest that this mechanism plays a role in the elimination of redundant neonatal synapses.

Results

Here we used confocal microscopy and quantitative morphological analysis to count the number of brightly fluorescent axons per endplate in P7, P9 and P15 transgenic B6.Cg-Tg (Thy1-YFP)16 Jrs/J mice. We investigate the involvement of individual mAChR M₁-, M₂- and M₄-subtypes in the control of axonal elimination after the Levator auris longus muscle had been exposed to agonist and antagonist in vivo. We also analysed the role of adenosine receptor subtypes (A₁ and A_2A) and the tropomyosin-related kinase B receptor. The data show that postnatal axonal elimination is a regulated multireceptor mechanism that guaranteed the monoinnervation of the neuromuscular synapses.

Conclusion

The three receptor sets considered (mAChR, AR and TrkB receptors) intervene in modulating the conditions of the competition between nerve endings, possibly helping to determine the winner or the lossers but, thereafter, the final elimination would occur with some autonomy and independently of postsynaptic maturation.

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.

Bicarbonate-sensitive soluble and transmembrane adenylyl cyclases in peripheral chemoreceptors

Stimulation of the carotid body (CB) chemoreceptors by hypercapnia triggers a reflex ventilatory response via a cascade of cellular events, which includes generation of cAMP. However, it is not known if molecular CO2/HCO3(-) and/or H(+) mediate this effect and how these molecules contribute to cAMP production. We previously reported that the CB highly expresses HCO3(-)-sensitive soluble adenylyl cyclase (sAC). In the present study we systematically characterize the role of sAC in the CB, comparing the effect of isohydric hypercapnia (IH) in cAMP generation through activation of sAC or transmembrane-adenylyl cyclase (tmAC). Pharmacological deactivation of sAC and tmAC decreased the CB cAMP content in normocapnia and IH with no differences between these two conditions. Changes from normocapnia to IH did not effect the degree of PKA activation and the carotid sinus nerve discharge frequency. sAC and tmAC are functional in CB but intracellular elevations in CO2/HCO3(-) in IH conditions on their own are insufficient to further activate these enzymes, suggesting that the hypercapnic response is dependent on secondary acidosis.

Carotid chemoreceptor "resetting" revisited

Carotid body (CB) chemoreceptors transduce low arterial O(2) tension into increased action potential activity on the carotid sinus nerves, which contributes to resting ventilatory drive, increased ventilatory drive in response to hypoxia, arousal responses to hypoxia during sleep, upper airway muscle activity, blood pressure control and sympathetic tone. Their sensitivity to O(2) is low in the newborn and increases during the days or weeks after birth to reach adult levels. This postnatal functional maturation of the CB O(2) response has been termed "resetting" and it occurs in every mammalian species studied to date. The O(2) environment appears to play a key role; the fetus develops in a low O(2) environment throughout gestation and initiation of CB "resetting" after birth is modulated by the large increase in arterial oxygen tension occurring at birth. Although numerous studies have reported age-related changes in various components of the O(2) transduction cascade, how the O(2) environment shapes normal CB prenatal development and postnatal "resetting" remains unknown. Viewing CB "resetting" as environment-driven (developmental) phenotypic plasticity raises important mechanistic questions that have received little attention. This review examines what is known (and not known) about mechanisms of CB functional maturation, with a focus on the role of the O(2) environment.

Role of ATP and adenosine on carotid body function during development

The carotid body is the main peripheral oxygen sensor involved in cardio-respiratory control under both normoxic and hypoxic conditions. This review focuses on data from newborn animals related to the involvement of the purinergic system in carotid body function during development. We describe the potential effects mediated by ATP and adenosine receptors on ventilation, chemoreceptor activity and their influence on respiratory instability, such as apnea. The conclusions that appear from this review is that in newborn rats, activation of ATP receptors increases the carotid body function although with no age dependent manner, regulates breathing under normoxia, and enhances the initial increase in ventilation in response to hypoxia (likely reflecting carotid body responses). However, activation of adenosine receptors may play a role on carotid body function under chronic conditions, such as intermittent hypoxia or exposure to the adenosine receptor antagonist caffeine. Under the later conditions, an indirect effects involving the carotid body dopaminergic system are observed.

Impaired ventilatory acclimatization to hypoxia in female mice overexpressing erythropoietin: unexpected deleterious effect of estradiol in carotid bodies

Apart from enhancing the production of red blood cells, erythropoietin (Epo) alters the ventilatory response when oxygen supply is reduced. We recently demonstrated that Epo's beneficial effect on the ventilatory response to acute hypoxia is sex dependent, with female mice being better able to cope with reduced oxygenation. In the present work, we hypothesized that ventilatory acclimatization to chronic hypoxia (VAH) in transgenic female mice (Tg6) harboring high levels of Epo in the brain and blood will also be improved compared with wild-type (WT) animals. Surprisingly, VAH was blunted in Tg6 female mice. To define whether this phenomenon had a central (brain stem respiratory centers) and/or peripheral (carotid bodies) origin, a bilateral transection of carotid sinus nerve (chemodenervation) was performed. This procedure allowed the analysis of the central response in the absence of carotid body information. Interestingly, chemodenervation restored the VAH in Tg6 mice, suggesting that carotid bodies were responsible for the blunted response. Coherently with this observation, the sensitivity to oxygen alteration in arterial blood (Dejour test) after chronic hypoxia was lower in transgenic carotid bodies compared with the WT control. As blunted VAH occurred in female but not male transgenic mice, the involvement of sex female steroids was obvious. Indeed, measurement of sexual female hormones revealed that the estradiol serum level was 4 times higher in transgenic mice Tg6 than in WT animals. While ovariectomy decreased VAH in WT females, this treatment restored VAH in Tg6 female mice. In line with this observation, injections of estradiol in ovariectomized Tg6 females dramatically reduced the VAH. We concluded that during chronic hypoxia, estradiol in carotid bodies suppresses the Epo-mediated elevation of ventilation. Considering the increased application of recombinant Epo for a variety of disorders, our data imply the need to take the patient's hormonal status into consideration.

Neonatal caffeine treatment does not induce long-term consequences on TrkB receptors or BDNF expression in chemosensory organs of adult rats

Chronic treatment with caffeine during the neonatal period (neonatal caffeine treatment, NCT, 15mg/kg/day from P3 to P12, oral gavage) has long-lasting consequences on respiratory control development. In adult male (but not female) rats, prior exposure to NCT results in a greater respiratory frequency response to hypoxia. This sex-specific effect of NCT was accompanied by an augmented expression of adenosine A(2A) receptors (A(2A)R) in the carotid body (CB) but not in the nucleus tractus solitarius (NTS). Since activation of adenosine A(2A)R can directly stimulate synthesis of tyrosine kinase B receptor (TrkBR) and brain-derived neurotrophic factor (BDNF), we determined whether NCT increases TrkBR and BDNF expression levels in the CB and NTS using both RT-PCR and western blot analyses. CB, NTS, and superior cervical ganglion were collected from adult male and female rats (10-12 weeks old) previously subjected to NCT or to control (neonatal water treatment, NWT). In male rats, when NCT tended to decrease TrkBR mRNA transcript levels by about 32% in the CB and to reduce BDNF transcripts in the NTS by 22%, western blot analyses showed no parallel changes in final protein expression. NCT had no effects on TrkBR or BDNF mRNA and protein levels in the CB and NTS of female rats. Neither gene was altered by NCT in the superior cervical ganglion of male and female rats. These data suggest that NCT has no long-term effects on trophic factor BDNF and TrkBR expression at peripheral and central level of chemosensory organs involved in respiratory control.

Neonatal caffeine induces sex-specific developmental plasticity of the hypoxic respiratory chemoreflex in adult rats

Caffeine is widely used to treat apneas of prematurity during the neonatal period; however, the potential consequences of administering a neonatal caffeine treatment (NCT) during a critical period for respiratory control development are unknown. The present study therefore determined whether NCT in rats alters the hypoxic respiratory chemoreflex measured at adulthood. Newborn rats received either caffeine (15 mg/kg) or water (control) each day from postnatal day 3 to 12. The ventilatory response to a hypoxic challenge (inspired O(2) fraction = 0.12) was first evaluated in awake adult female and male rats using whole body plethysmography. Results showed that NCT increased the initial phase of the breathing frequency response to hypoxia in males only. This result was confirmed in anesthetized and artificially ventilated adult male rats where NCT also increased the phrenic burst frequency response to hypoxia. RT-PCR assessment of mRNA encoding for adenosine A(1A) and A(2A) receptors, dopamine D(2) receptors, and tyrosine hydroxylase in the rat carotid bodies showed that NCT enhanced mRNA expression levels of adenosine A(2A), dopamine D(2) receptors, and tyrosine hydroxylase of males but not females. Subsequent experiments on awake male rats showed that injection of the adenosine A(2A) receptor antagonist ZM2413855 (1 mg/kg ip) before ventilatory measurements abolished, in NCT rats, the enhanced respiratory frequency response observed during the early phase of hypoxia. We propose that NCT elicits a sex-specific increase in the hypoxic respiratory chemoreflex, which is related, at least partially, to an enhancement in adenosine A(2A) receptors in the rat carotid body.

Prenatal nicotine alters maturation of breathing and neural circuits regulating respiratory control

While perinatal nicotine effects on ventilation have been widely investigated, the prenatal impact of nicotine treatment during gestation on both breathing and neural circuits involved in respiratory control remains unknown. We examined the effects of nicotine, from embryonic day 5 (E5) to E20, on baseline ventilation, the two hypoxic ventilatory response components and in vivo tyrosine hydroxylase (TH) activity in carotid bodies and brainstem areas, assessed at postnatal day 7 (P7), P11 and P21. In pups prenatally exposed to nicotine, baseline ventilation and hypoxic ventilatory response were increased at P7 (+48%) and P11 (+46%), with increased tidal volume (p<0.05). Hypoxia blunted frequency response at P7 and revealed unstable ventilation at P11. In carotid bodies, TH activity increased by 20% at P7 and decreased by 48% at P11 (p<0.05). In most brainstem areas it was reduced by 20-33% until P11. Changes were resolved by P21. Prenatal nicotine led to postnatal ventilatory sequelae, partly resulting from impaired maturation of peripheral chemoreceptors and brainstem integrative sites.

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.

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.