Role of carotid body catecholamines in chemoreceptor function

Loss of ganglioglomerular nerve input to the carotid body impacts the hypoxic ventilatory response in freely-moving rats

The carotid bodies are the primary sensors of blood pH, pO2 and pCO2. The ganglioglomerular nerve (GGN) provides post-ganglionic sympathetic nerve input to the carotid bodies, however the physiological relevance of this innervation is still unclear. The main objective of this study was to determine how the absence of the GGN influences the hypoxic ventilatory response in juvenile rats. As such, we determined the ventilatory responses that occur during and following five successive episodes of hypoxic gas challenge (HXC, 10% O2, 90% N2), each separated by 15 min of room-air, in juvenile (P25) sham-operated (SHAM) male Sprague Dawley rats and in those with bilateral transection of the ganglioglomerular nerves (GGNX). The key findings were that 1) resting ventilatory parameters were similar in SHAM and GGNX rats, 2) the initial changes in frequency of breathing, tidal volume, minute ventilation, inspiratory time, peak inspiratory and expiratory flows, and inspiratory and expiratory drives were markedly different in GGNX rats, 3) the initial changes in expiratory time, relaxation time, end inspiratory or expiratory pauses, apneic pause and non-eupneic breathing index (NEBI) were similar in SHAM and GGNX rats, 4) the plateau phases obtained during each HXC were similar in SHAM and GGNX rats, and 5) the ventilatory responses that occurred upon return to room-air were similar in SHAM and GGNX rats. Overall, these changes in ventilation during and following HXC in GGNX rats raises the possibility the loss of GGN input to the carotid bodies effects how primary glomus cells respond to hypoxia and the return to room-air.

The superior cervical ganglia modulate ventilatory responses to hypoxia independently of preganglionic drive from the cervical sympathetic chain

Superior cervical ganglia (SCG) postganglionic neurons receive preganglionic drive via the cervical sympathetic chains (CSC). The SCG projects to structures like the carotid bodies (e.g., vasculature, chemosensitive glomus cells), upper airway (e.g., tongue, nasopharynx), and to the parenchyma and cerebral arteries throughout the brain. We previously reported that a hypoxic gas challenge elicited an array of ventilatory responses in sham-operated (SHAM) freely moving adult male C57BL6 mice and that responses were altered in mice with bilateral transection of the cervical sympathetic chain (CSCX). Since the CSC provides preganglionic innervation to the SCG, we presumed that mice with superior cervical ganglionectomy (SCGX) would respond similarly to hypoxic gas challenge as CSCX mice. However, while SCGX mice had altered responses during hypoxic gas challenge that occurred in CSCX mice (e.g., more rapid occurrence of changes in frequency of breathing and minute ventilation), SCGX mice displayed numerous responses to hypoxic gas challenge that CSCX mice did not, including reduced total increases in frequency of breathing, minute ventilation, inspiratory and expiratory drives, peak inspiratory and expiratory flows, and appearance of noneupneic breaths. In conclusion, hypoxic gas challenge may directly activate subpopulations of SCG cells, including subpopulations of postganglionic neurons and small intensely fluorescent (SIF) cells, independently of CSC drive, and that SCG drive to these structures dampens the initial occurrence of the hypoxic ventilatory response, while promoting the overall magnitude of the response. The multiple effects of SCGX may be due to loss of innervation to peripheral and central structures with differential roles in breathing control.NEW & NOTEWORTHY We present data showing that the ventilatory responses elicited by a hypoxic gas challenge in male C57BL6 mice with bilateral superior cervical ganglionectomy are not equivalent to those reported for mice with bilateral transection of the cervical sympathetic chain. These data suggest that hypoxic gas challenge may directly activate subpopulations of superior cervical ganglia (SCG) cells, including small intensely fluorescent (SIF) cells and/or principal SCG neurons, independently of preganglionic cervical sympathetic chain drive.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Transient Receptor Potential Ankyrin 1 Mediates Hypoxic Responses in Mice

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel that is broadly expressed in sensory pathways, such as the trigeminal and vagus nerves. It is capable of detecting various irritants in inspired gasses and is activated during hypoxia. In this study, the role of TRPA1 in hypoxia-induced behavioral, respiratory, and cardiovascular responses was examined through four lines of experiments using TRPA1 knockout (KO) mice and wild type (WT) littermates. First, KO mice showed significantly attenuated avoidance behavior in response to a low (15%) oxygen environment. Second, the wake-up response to a hypoxic ramp (from 21 to 10% O₂ in 40 s) was measured using EEG electrodes. WT mice woke up within 30 s when oxygen was at 13–14%, but KO mice did not wake up until oxygen levels reached 10%. Histological analysis confirmed that mild (13% O₂) hypoxia resulted in an attenuation of trigeminal neuronal activation in KO mice. Third, the ventilatory response to hypoxia was measured with whole body plethysmography. KO mice showed attenuated responses to mild hypoxia (15% O₂) but not severe hypoxia (10% O₂). Similar responses were observed in WT mice treated with the TRPA1 blocker, AP-18. These data clearly show that TRPA1 is necessary for multiple mild hypoxia (13–15% O₂)-induced physiological responses. We propose that TRPA1 channels in the sensory pathways innervating the airway can detect hypoxic environments and prevent systemic and/or cellular hypoxia from occurring.

AMPA Receptor-Dependent Glutamatergic Signaling is Present in the Carotid Chemoreceptor

Exposure to both sustained and intermittent hypoxia for as little as a day produces sustained augmentation of carotid chemoreceptor sensitivity; however, the molecular basis for this chemoreflex plasticity remains uncertain. We previously reported that NMDA receptor-dependent glutamatergic signaling in rat carotid body played a role in altered hypoxic sensitivity after exposure to cyclic intermittent hypoxia (CIH). Here we found that mRNAs of multiple AMPA and Kainate glutamate receptors were expressed in rat carotid body. The AMPA receptor subunit GluR1 showed intense immunoreactivity in the carotid body, co-localizing with tyrosine hydroxylase in type I cells. Treatment of rat carotid body-derived primary cells with AMPA activated ERK1/2 in a time-dependent manner. Exposing Sprague-Dawley rats to CIH for 8 h/day for 3 weeks significantly enhanced the expression level of GluA1 mRNA as well as GluR1 protein in the carotid body. In addition, our results showed that multiple of vesicular glutamate transporters (VGLUTs) and excitatory amino acid transporters (EAATs) were expressed in the rat carotid body, indicating that glutamate might be as a neurotransmitter stored, released and uptake in the carotid body. Finally, we found that mRNAs of GluA1, GluA2 and GluA3 as well as PSD-95-like membrane-associated granulate kinase family members, PSD-95, PSD-93, and SAP97, were expressed in human carotid body. Our data suggest AMPA receptor-dependent glutamatergic signaling is present in the carotid body and might be involved in the carotid chemoreceptor response to hypoxia.

Regulation of phospholipase C activity by calcium ions and guanine nucleotide in the normoxic cat carotid body

The carotid bodies (CB) are a paired chemoreceptor organ located at the bifurcation of the common carotid arteries. High O2 tension suppresses while low tension activates afferent carotid chemoreceptor activity and the chemoreflex ventilatory response in the cat. The intracellular mechanism of chemotransduction is till now unknown. Previously we have shown different activities of phospholipase C (PLC) in normoxic, hypoxic and hyperoxic cat carotid body. Now we have addressed the question whether calcium ions and G-protein could be regulators of the formation of lipid derived messenger molecules in the cat carotid body. To answer this question, the PLC acting against [3H] inositol-phosphatidylinositol (PtdIns) and [3H] inositol-phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] in the cat CB were investigated using labelled phospholipids as a source of the substrate. CB homogenate was used as a source of the enzyme. The results indicate that PLC acting on PtdIns is Ca2+-dependent, in contrary to that acting on PtdIns(4,5)P2 which remains active in the presence of 10 mM EGTA. PtdIns(4,5)P2-PLC is stimulated by GTPgammaS. In the presence of Ca2+, GTPgammaS has a synergistic stimulatory effect. PLC acting on PtdIns is not activated by GTPgammaS. In the presence of calcium ions dopamine and a nonhydrozylable analogue of acetylcholine, carbachol, have a small stimulatory effect of about 30% on PLC acting on PtdIns(4,5)P2. GTPgammaS enhances this effect. These results allow us to suggest that there are two pathways of phosphoinositides degradation in the CB, one of them is regulated by calcium ions/PtdIns-PLC/, the other one by G-protein / PtdIns(4,5)P2-PLC/.

Carotid body noradrenergic sensitivity in ventilatory acclimatization to hypoxia

Norepinephrine inhibits ventilation in awake goats under normoxic, resting conditions. This inhibition is carotid body (CB) mediated and may be due to stimulation of noradrenergic receptor on the CB. Cao et al. (FASEB J. A118, 1991) recently suggested that CB noradrenergic receptors may be down regulated following 24-36 hours of hypoxic exposure in cats. Our study was aimed at determining whether a change in noradrenergic receptor sensitivity during ventilatory acclimatization to hypoxia (VAH) was responsible for the increased sensitivity of the CB to hypoxia during prolonged exposure to hypoxia in goats. We tested this hypothesis using intracarotid infusions of norepinephrine (NE) (0.5, 1.0, 5.0 micrograms.kg-1.min-1) and dopamine (DA) (1.0 micrograms.kg-1.min-1) in awake goats under control normoxic conditions, during 4 h of isocapnic hypoxia, and upon return to normoxia. NE and DA (1.0 micrograms.kg-1.min-1) both inhibited control ventilation significantly during the intracarotid infusions (56% and 62% decreases, respectively). No significant differences were found between the pre- and post-hypoxic infusions of NE and DA in normoxia. During hypoxia, inhibition of VE during NE and DA infusions was attenuated relative to control. Time-dependent change of the NE response were not apparent during the acclimatization period suggesting that a decreased carotid body chemoreceptor sensitivity to NE and DA is not responsible for the increased drive to breathe characteristic of ventilatory acclimatization to hypoxia.

Norepinephrine-containing glomus cells in the rabbit carotid body. I. Autoradiographic and morphometric study after tritiated norepinephrine uptake

Rabbit carotid bodies were investigated by autoradiography at both the light and electron microscope levels following tritiated norepinephrine administration either in vivo or in vitro. Two kinds of labelled structures were found: nerve fibres (absent in sympathectomized carotid bodies) and some type I glomus cells. Desipramine (a specific norepinephrine uptake inhibitor) prevented labelling. Most of the labelled cells differed from unlabelled ones by the presence of (i) large dense-cored vesicles characterized by a large halo between the membrane and an eccentric dense core; (ii) a nucleus showing a more electron dense chromatin and a more irregular shape; and (iii) relatively abundant glycogen particles. A new weakly-labelled cells were characterized by a pyknotic nucleus and very swollen dense-cored vesicles, and were presumed to be degenerating. Dense core diameters of dense-cored vesicles were distributed according to a unimodal distribution in labelled cells as in unlabelled ones but with an extension towards both large and very small diameters in labelled cells. The mean diameter was higher in labelled cells than in unlabelled ones (127 nm versus 113 nm, P less than 0.01). The labelling intensity (as estimated by the number of silver grains per unit of cytoplasmic area) was maximum in cells having dense-cored vesicles whose mean diameter was between 130 and 170 nm, but decreased for cells with mean diameter of dense cores smaller than 130 nm, or larger than 170 nm. Thus, in the rabbit carotid body, some glomus cells differ from others by their ability to take up tritiated norepinephrine and by the presence of larger dense-cored vesicles. However, this distinction is not clearcut and there are many intermediates. The observations suggest a phenomenon of evolution deriving from a unique cell type and typified by both metabolic norepinephrine uptake ability, glycogen accumulation) and morphologic changes (increase in diameter of dense-cored vesicles). It seems, therefore, more appropriate to consider these results in terms of different functional states rather than different types of glomus cells.

The co-existence of biogenic amines and neuropeptides in the type I cells of the cat carotid body

The mammalian carotid body consists of preneural type I (glomus) cells synaptically coupled to afferent axon terminals and enveloped by type II (sustentacular) cells. Recent studies indicate the presence of multiple putative neurotransmitters in this arterial chemoreceptor organ. A double-labeling immunocytochemical technique was utilized which allows simultaneous visualization of two neurochemicals in a single cell. The issue of transmitter co-occurrence in type I cells of the cat carotid body was addressed using specific antibodies for seven neurochemical agents: tyrosine hydroxylase, dopamine-beta-hydroxylase, choline acetyltransferase, serotonin, substance P, met-enkephalin and chromogranin. A high degree (greater than 70%) of co-localization was observed for most pairs of markers, indicating the co-existence of multiple neuroactive agents in type I cells of the cat carotid body. The intensity of staining varied greatly among cells but formed a pattern. Thus, for tyrosine hydroxylase and dopamine-beta-hydroxylase, the majority of double-labeled type I cells exhibited equivalently low or high levels of both, while for the neuropeptides unequal levels of the two markers predominated. Neuropeptides also co-existed in type I cells with catecholamine-synthesizing enzymes and with serotonin. The functional significance of such patterns of multiple co-existence involving biogenic amines and neuropeptides is discussed. Our results indicate a high degree of co-occurrence of reaction product for amine-synthesizing enzymes (tyrosine hydroxylase, dopamine-beta-hydroxylase and choline acetyltransferase), the indoleamine serotonin, and the neuropeptides substance P and met-enkephalin.

Carotid body amine levels in goats exposed to hypoxia or hypercapnia

The carotid body (CB) contains large amounts of several monoamines. There is considerable evidence that carotid body (CB) chemoreceptor function may be regulated by one or several of these monoamines. In order to test whether conditions stimulating the CB might change the CB content of these monoamines, we measured the norepinephrine, dopamine, 5-hydroxyindoleacetic acid, and 5-hydroxytryptamine content of carotid bodies collected from goats exposed to 4 h of either normoxic-normocapnic, hypoxic-normocapnic, or normoxic-hypercapnic gas mixtures. We found that there were no consistent changes in the levels of these amines associated with exposure to the test gases. These findings would not support the hypothesis that changes in these amine levels in the CB are responsible for the time-dependent changes in carotid chemoreceptor activity in goats exposed to 4 h of hypoxia.

Fate of the catecholamine stores in the rabbit carotid body superfused in vitro

In rabbit carotid bodies (CBs) superfused during 1-5 h, with an air-equilibrated medium containing no tyrosine (TYR), the dopamine (DA) content decreased by approximately 60% after 1 h and remained constant afterwards. TYR and 3,4-dihydroxyphenylacetic acid (DOPAC) decreased with the same time course. Noradrenaline (NA) content exhibited a biphasic decrease of lesser magnitude than that of DA. Superfusions with a TYR-containing medium did not prevent the reduction in DA and TYR. Large amounts of DA and DOPAC were recovered in the effluent during the first hour of superfusion but after 90 min the two substances had declined below the detection limits (i.e. 0.5 and 1 pmol/5 min, respectively). The DA efflux decreased exponentially during the first hour and was not altered by changing the oxygen partial pressure (PO2) of the medium. The DOPAC efflux declined after 40 min of superfusion and was modulated by PO2. The DA and the DOPAC effluxes were not suppressed by omitting calcium ions from the superfusing medium. In 4 cat CBs equal amounts of DA and NA were recovered from the effluent during the first hour of superfusion.

Comparison of the monoamine and catabolite content in the cat and rabbit carotid bodies

The monoamine and catabolite contents of a large number of rabbit (n = 95) and cat (n = 32) carotid bodies (CBs) have been measured by high performance liquid chromatography with electrochemical detection (HPLC-ED). The dopamine (DA) content as well as that of its precursors tyrosine (TYR), dihydroxyphenylalanine (DOPA) and catabolites dihydroxyphenylacetic acid (DOPAC), homovanilic acid (HVA) were approximately equal in both species. The noradrenaline (NA) content was 10 times larger in the cat than in the rabbit CBs. Twenty-nine out of the 32 cat CBs contained more NA than DA while the reverse was true in 92 out of 95 rabbit CBs. In 11 cats the right CB was sympathectomized and its DA and NA contents were compared to those of intact contralateral organs.

Mechanisms for termination of the action of dopamine in carotid body chemoreceptors

The possible presence of a high affinity uptake mechanism for dopamine and the metabolism of this amine was investigated in the rabbit carotid body. The type I cells of this sensory organ contain high levels of dopamine and share with sympathetic nerve endings (which possess a high affinity uptake mechanism for catecholamines) the properties of being presynaptic catecholaminergic elements, but also have in common with chromaffin cells from intact adrenal medulla (which lack a high affinity catecholamine uptake) a similar embryological origin and ultrastructural appearance. Our experiments revealed only a low affinity uptake process (Km = 6.76 X 10(-4) M [3H]dopamine; Vmax = 1.84 X 10(-9) mol [3H]dopamine/mg protein/min) in the rabbit carotid body. In agreement with our kinetic findings, the uptake of [3H]dopamine was found to be independent of the Na+ concentration in the incubation media. The efflux from the tissue of incorporated [3H]dopamine was very fast (88% wash-out with a half-time of 4 min). DOPAC was the principal catabolite of dopamine in the carotid body, and the organ exhibited a very low capacity for norepinephrine synthesis. Neither chronic carotid sinus nerve section nor chronic sympathectomy modified the uptake of [3H]dopamine, suggesting that the lack of expression of a high affinity uptake was not a consequence of trophic suppression by the neural innervation of the organ. We conclude that overflow, or wash-out, of released dopamine may be quantitatively the most important mechanism for the inactivation of this putative neurotransmitter in the rabbit carotid body.

The effects of hypoxia on catecholamine dynamics in the rat carotid body

The catecholamine content of the rat carotid body was assayed using high performance liquid chromatography with electrochemical detection. The concentration of dopamine (DA) was found to predominate over that of norepinephrine (NE) by a small margin (31 pmol/carotid body pair DA; 23 pmol/carotid body pair NE). The turnover rates of carotid body DA and NE were determined from the time-dependent decline in their concentrations following the blockade of synthesis with alpha-methyl-p-tyrosine. Values were obtained (DA t 1/2 = 1.9 h; NE t 1/2 = 2.3 h) which suggested a rapid turnover of carotid body catecholamines. Exposure of rats to conditions of severe hypoxia (5% O2-95% N2) failed to significantly alter either the content or turnover of carotid body catecholamines. By contrast, the concentration of carotid body DOPAC, a reflection of DA utilization, was significantly elevated following hypoxic conditions. Further, in vivo tyrosine hydroxylase activity was assessed by measuring the accumulation of carotid body DOPA after inhibiting L-aromatic amino acid decarboxylase with NSD-1015. Basal tyrosine hydroxylase activity (approximately 14-16 pmol/carotid body pair/h) also was significantly increased by acute hypoxic exposure. These results, in part, suggest that rat carotid body DA may act as a neurotransmitter whose synthesis and release are coupled to stimulus demand.

Dopamine beta-hydroxylase-like immunoreactivity in the rat and cat carotid body: a light and electron microscopic study

Immunocytochemical localization of dopamine beta-hydroxylase (DBH) was used to study the synthesis and storage sites of norepinephrine (noradrenaline) in the rat and cat carotid bodies. In the rat carotid body some parenchymal cells exhibited strong DBH-like immunoreactivity (DBH-I), while others displayed only faint DBH-I. In a typical parenchymal cell cluster, most cells with strong DBH-I were irregular in shape and appeared to partially surround those with weak DBH-I which usually were rounded in contour. In the cat carotid body most parenchymal cells showed a strong to moderate DBH-I. In both the rat and cat carotid bodies varicose nerve fibres with DBH-I were associated primarily with blood vessels. All autonomic ganglion cells examined, which were associated with the rat carotid body, showed DBH-I. Electron microscopy revealed that most DBH-I in the strongly positive cells of the rat carotid body was associated with dense granules (possibly corresponding to dense-cored vesicles of various sizes), although some was found in other sites. In oval cells with less DBH-I, reactivity resided in some of the large granules. In the cat carotid body the glomus cells contained more granules of various sizes and shapes than did those of the rat carotid body. Most of the cat glomus cell granules exhibited DBH-I activity. Our results indicate that some of glomus cells in the rat and most of the glomus cells in the cat contain DBH and therefore may be sites of norepinephrine synthesis.

Influence of age on the carotid bodies of spontaneously hypertensive (SHR) and normotensive rats. II. Alterations of the vascular wall

The carotid bodies of spontaneously hypertensive rats (SHR) of the Okamoto-Aoki-strain and of age-matched normotensive Wistar rats (NWR) were studied with respect to their size and the histological appearance of their arterial vessels. The animals were aged 3-6 d, and 5-6, 15-20, 30-40 and 50-70 weeks. The development of hypertension in the SHR started at an age of 5-6 weeks and was fully established at 15-20 weeks (mean systemic arterial blood pressure at about 160 mm Hg). When compared with the age-matched normotensive control rats (NWR) the SHR in the established phase of hypertension showed enlarged carotid bodies, an increase of the thickness of the media of the carotid body artery and circumscript pad-like thickening of the vascular wall within the carotid bodies. Repeatedly in particular in the old SHR, also a hyalinosis of the small branches of the glomic artery was found. These pathological changes regularly narrowed the lumen of the vessels seized; sometimes to a considerable extent. Such vascular alterations were never found in the newborn (3-6 d old) SHR and were also not demonstrable in the NWR. Thus these vascular alterations in the carotid body vessels of the adult SHR are supposed to be the result of the high systemic arterial blood pressure. The data indicate that long-lasting high systemic arterial blood pressure leads to changes of the wall of the arterial vessels of the carotid and presumably also the aortic bodies thus inducing an ischemic hypoxia of the specific chemoreceptive tissue and a chronic stimulation of the arterial chemoreceptors.

beta-Adrenoceptor blockade spares chemoreceptor responsiveness to hypoxia

The effect of beta-adrenoceptor blockade on the carotid body chemoreceptor response to hypoxia was assessed in anesthetized and paralyzed cats. Propranolol, atenolol and ICI 118,551 each abolished the enhancement of chemoreceptor activity produced by i.v. infusion of exogenous isoproterenol; however, the blocking drugs did not significantly diminish the increase in chemoreceptor neural discharge induced by hypoxia. These results do not support the hypothesis that beta-adrenergic receptors play an essential role in the chemoreceptor response to oxygen deprivation.

An ultrastructural study of intrarenal catecholamine-containing elements

Histochemical visualization of catecholamines and electron microscopy in the same tissue sample were used to localize and study catecholamine-containing nerve enlargements or swellings in male Wistar and Sprague-Dawley rat kidneys. These swellings lie in the perivascular nerve plexuses of arcuate and interlobular arteries near the points of origin of arterioles, and are composed of modified axons and associated Schwann cells. Transverse sections of the enlarged nerves reveal that individual axons are also enlarged, have processes or folds and make contact with one another. The axonal enlargements contain small mitochondria with a dense matrix and clusters of small vesicles, many of which are associated with an organelle composed of parallel cisternae of smooth membranes.

Histochemically demonstrable catecholamines in sympathetic ganglia and carotid body of spontaneously hypertensive and normotensive rats

The catecholamine content and morphology of the superior cervical and the hypogastric ganglion and the carotid body were studied in Spontaneously Hypertensive Rats (SHR) before (at the age of 6 weeks) and after (at the age of 20 weeks) becoming hypertensive, with Wistar Kyoto (WKY) rats as controls. The study was performed by formaldehyde-induced fluorescence method combined with quantitative microfluorimetry of catecholamines. At the age of 6 weeks the only significant difference observed between the rat strains was a greater number of small intensely fluorescent (SIF) cells in the superior cervical ganglion of SHR. At the age of 20 weeks the fluorescence intensity was higher in the principal neurons of the superior cervical ganglion and in glomus cells of the carotid body of SHR compared to WKY. The volumes of superior cervical ganglion and carotid body were larger in 20-week-old SHR compared to WKY. In the hypogastric ganglion differences were not found between SHR and WKY rats. The present results show differences in the superior cervical ganglion and in the carotid body of adult SHR compared to controls. These differences develop during the time period when the SHR become hypertensive, and might be functionally significant in the regulation or maintenance of the increased blood pressure in SHR rats.

Direct biochemical and neuropharmacological identification of dopamine D2-receptors in the rabbit carotid body

Dopamine D2-receptors were directly identified in receptor binding assays with washed particulate preparations of rabbit carotid body using the selective ligand, [3H]domperidone. High affinity, saturable specific binding of [3H]domperidone was clearly demonstrable and chronic section of the sinus nerve resulted in a 32% decrease in the labelling of the dopamine D2-sites. Adenylate cyclase activity was also detected in rabbit carotid body homogenates and although this enzyme was stimulated 4-fold by 10 mM sodium fluoride, neither dopamine nor isoprenaline significantly altered basal activity. On the other hand, in the intact carotid body incubated in vitro, 10(-5) M isoprenaline increased the basal cyclic AMP content 6-fold, though dopamine was again ineffective. The effect of various selective dopamine receptor antagonists and agonists was also studied on chemoreceptor afferent discharge. The results confirm that depression of 'spontaneous' chemosensory discharge is the predominant effect of dopamine (0.01-100 micrograms) in rabbits. The 'selective' D2-agonist, LY 141865, proved very effective (ID50 3.3 nmol) and was equipotent with dopamine (ID50 4.2), whereas, the D1-agonist, SK & F 38393, was very ineffective (ID50 150). The D2-antagonists domperidone and (-)-sulpiride produced a dose-related decrease in the chemodepressant responses to dopamine and LY 141865. However, there was no evidence for any appreciable excitatory action of either of these agonists after blockade of their chemo-depressant effects. The D2-antagonists variably affected the spontaneous activity, there being an increase in discharge on average, whereas responses to hypoxia, cyanide and CO2 were reduced. The present results from biochemical and neuropharmacological studies, provide strong evidence for the presence of functional dopamine D2-receptors in the rabbit carotid body, and suggest that the receptor involved in dopamine-induced depression of chemosensory discharge is of D2-type.

Biogenic amine-stimulated cyclic adenosine-3',5'-monophosphate formation in the rat carotid body

The subcutaneous injection of isoprenaline, salbutamol, histamine, and adrenaline to rats, which were subsequently killed by microwave irradiation, resulted in a rapid increase in the cyclic AMP content of the carotid body. On the other hand, noradrenaline, dopamine, adenosine, and 5-hydroxytryptamine, at doses at least 100 times greater than that of isoprenaline, did not significantly alter the cyclic nucleotide content in vivo. The response to isoprenaline was dose related, with an ED50 of 15 micrograms X kg-1, and reached a peak level 1-1.5 min after injection. Incubation of intact carotid bodies with isoprenaline (10(-5) M) in vitro also resulted in a 10-fold increase in cyclic AMP content. The in vivo response to isoprenaline could be blocked stereo-selectively by propranolol, and ICI 118.551, a beta 2-selective antagonist, blocks the isoprenaline-elicited increase in cyclic AMP completely at a dose of 30 micrograms X kg-1; whereas betaxolol, a beta 1-selective antagonist, was ineffective, even at a dose of 300 micrograms X kg-1. Hypoxia (5% oxygen in 95% N2) did not result in a significant increase in the cyclic AMP content, nor did it significantly alter the isoprenaline-stimulated increase in the cyclic AMP content of the rat carotid body. These results suggest that some catecholamines may stimulate cyclic AMP formation by interacting with a beta 2-adrenoceptor in the rat carotid body.

Catecholamines in the carotid body of several mammalian species: effects of surgical and chemical sympathectomy

The catecholamine content of the carotid body of several mammalian species has been assayed using high performance liquid chromatography coupled to electrochemical detection and radioenzymatic assays. Although there were strain differences in the content of catecholamines in the carotid body of the rat, noradrenaline was equal to or exceeded the dopamine level in this species. No apparent differences were found in carotid bodies of animals killed by cervical dislocation or those dissected from anaesthetized animals. Noradrenaline concentrations were found to be substantially higher than those of dopamine in the cat and guinea-pig carotid body, though dopamine was the predominant amine in the rabbit and ferret. Unilateral superior cervical ganglionectomy or chemical sympathectomy with 6-hydroxydopamine substantially depleted noradrenaline without influencing dopamine in the rat carotid body. A marked selective reduction in noradrenaline was also observed in the rabbit and guinea-pig following ganglionectomy, though similar procedures in the cat failed to alter the levels of either catecholamine in the carotid body. The present data highlights the marked species variation in catecholamine content and the contribution to the latter by sympathetic innervation to this organ. This information will be useful in determining the species specificity regarding the relative roles of dopamine and noradrenaline in the modulation of chemoreceptor afferent discharge.

Adrenergic mechanisms and chemoreception in the carotid body of the cat and rabbit

1. The effect of beta-adrenergic and dopaminergic agonists and antagonists on the chemoreceptor response to graded hypoxia and hypercapnia was tested in nineteen cats and ten rabbits anaesthetized either with chloralose-urethane or pentobarbitone sodium, paralysed with pancuronium bromide and artificially ventilated.2. The inhibitory action of dopamine was confirmed. The inhibition following intra-arterial bolus injection was blocked by haloperidol; dopamine then excited and this excitation was blocked with propranolol. Adrenaline or noradrenaline caused a transient inhibition followed by a marked excitation. The inhibition was blocked with haloperidol and the excitation blocked with propranolol or metoprolol. Isoprenaline excited without inhibition and this was blocked with propranolol or metoprolol.3. A novel finding was that the chemoreceptor response to hypoxia was markedly reduced or even abolished with propranolol or metoprolol. The response was enhanced with a constant infusion of isoprenaline, adrenaline or noradrenaline in proportion to the degree of hypoxia, an effect mimicked by raising CO(2). The chemoreceptor response to hypoxia was similarly enhanced by haloperidol and depressed by a constant infusion of dopamine in proportion to the degree of hypoxia.4. The effect of these drugs on the chemoreceptor response to hypercapnia was less constant. In the majority of tests the aminergic agonists and antagonists caused a parallel shift of the CO(2) response curves in the same direction as the O(2) response curves and by amounts proportional to the degree of hypoxia. In some tests these drugs caused a change in the slope of the CO(2) response curves but only if P(a, O2) was less than 60 mmHg.5. One interpretation of these results is that hypoxia exerts a presynaptic action, causing the release of noradrenaline and dopamine from Type I cells, and that these substances act upon aminergic receptors on the sensory fibre, causing a change in potential and discharge frequency proportional to the rates of dopamine and noradrenaline release.6. An additional or alternative interpretation is that O(2) and CO(2) (the latter most probably acting on intracellular pH) alter the sensitivity of the aminergic receptors to their agonists.

Localization of enkephalin-like immunoreactivity in the cat carotid and aortic body chemoreceptors

The purpose of this study was to determine if enkephalin-like immunoreactivity was present in the glomus cells of the carotid and aortic body peripheral arterial chemoreceptors. Cat carotid and aortic bodies were reacted with antisera to met- and leu-enkephalin using the indirect peroxidase-antiperoxidase immunocytochemical method of Sternberger (1979). Both the carotid and aortic bodies demonstrated clusters of immunoreactive cells for both met- and leu-enkephalin. Additionally, met-enkephalin-like immunoreactivity was observed in many of the dense-core vesicles of the glomus cells of the carotid body. The glomus cells of these chemoreceptors are known to contain catecholamines which may modulate chemoreceptor activity. The presence of opioid peptide-like substances co-existing with the glomus cell catecholamines, perhaps in the same vesicles, may have important implications for a trophic influence of these peptides on glomus cell chemoreceptor modulation.

Effects of vagotomy and glossopharyngectomy on respiratory response to dopamine-agonists

In normal rats lightly anesthetized with halothane apomorphine increased both resting and CO2-dependent minute ventilation (VM) by stimulating respiratory frequency (RF) whereas tidal volume (VT) was slightly decreased. Acute bilateral glossopharyngectomy, which impaired carotid body function, did not change the apomorphine effects in contrast to bilateral vagotomy, which abolished the RF response of the drug, but now increased VT. Intravenous infusion of dopamine increased VM by elevating RF, and this effect was only slightly blunted by bilateral glossopharyngectomy but nearly abolished by vagotomy and totally eliminated by the combined procedures. The respiratory response to dopamine was depressed in rats with chronically destroyed central catecholaminergic neurons. These findings indicate that there may be two different dopaminergic stimulatory mechanisms that modulate RF-one peripheral and one central-and both depend upon afferent vagal activity. With impaired vagal function, however, two other dopaminergic stimulatory mechanisms effecting VT are evident-one central, and one peripheral which involves the carotid body.

Adrenergic mechanisms and chemoreception in the carotid body of the cat and rabbit

1. The effect of beta-adrenergic and dopaminergic agonists and antagonists on the chemoreceptor response to graded hypoxia and hypercapnia was tested in nineteen cats and ten rabbits anaesthetized either with chloralose-urethane or pentobarbitone sodium, paralysed with pancuronium bromide and artificially ventilated.2. The inhibitory action of dopamine was confirmed. The inhibition following intra-arterial bolus injection was blocked by haloperidol; dopamine then excited and this excitation was blocked with propranolol. Adrenaline or noradrenaline caused a transient inhibition followed by a marked excitation. The inhibition was blocked with haloperidol and the excitation blocked with propranolol or metoprolol. Isoprenaline excited without inhibition and this was blocked with propranolol or metoprolol.3. A novel finding was that the chemoreceptor response to hypoxia was markedly reduced or even abolished with propranolol or metoprolol. The response was enhanced with a constant infusion of isoprenaline, adrenaline or noradrenaline in proportion to the degree of hypoxia, an effect mimicked by raising CO(2). The chemoreceptor response to hypoxia was similarly enhanced by haloperidol and depressed by a constant infusion of dopamine in proportion to the degree of hypoxia.4. The effect of these drugs on the chemoreceptor response to hypercapnia was less constant. In the majority of tests the aminergic agonists and antagonists caused a parallel shift of the CO(2) response curves in the same direction as the O(2) response curves and by amounts proportional to the degree of hypoxia. In some tests these drugs caused a change in the slope of the CO(2) response curves but only if P(a, O2) was less than 60 mmHg.5. One interpretation of these results is that hypoxia exerts a presynaptic action, causing the release of noradrenaline and dopamine from Type I cells, and that these substances act upon aminergic receptors on the sensory fibre, causing a change in potential and discharge frequency proportional to the rates of dopamine and noradrenaline release.6. An additional or alternative interpretation is that O(2) and CO(2) (the latter most probably acting on intracellular pH) alter the sensitivity of the aminergic receptors to their agonists.

Responses of carotid body chemosensory activity and blood flow to stimulation of sympathetic nerves in the cat

1. The effects of electrical stimulation of sympathetic nerves on sinus nerve chemosensory activity and carotid body blood flow were investigated in anaesthetized cats. 2. Two categories, designated as types I and II, of excitatory responses of chemosensory discharges to sympathetic stimulation were distinguished. Type I responses displayed elevations in impulse frequencies which were usually maximal in the initial 10-20 sec of stimulation, resisted alpha-adrenoceptor antagonism induced by phentolamine or phenoxybenzamine and were enhanced after administration of the dopamine antagonist, haloperidol. Type II responses showed increases in impulse frequencies which became more pronounced as stimulation progressed. These responses were susceptible to alpha-adrenoceptor blockade, were unaffected by haloperidol administration and were usually recorded during systemic hypotension. 3. Inhibitory changes due to activation of sympathetic fibres were recorded in 10% of chemosensory preparations. These effects were usually either abolished or replaced by type I excitatory responses after haloperidol administration. 4. Sympathetic stimulation caused reductions of carotid body blood flow during both natural and artificial perfusion of the organ. This effect was abolished or considerably attenuated by alpha-adrenoceptor antagonism and was unaffected by haloperidol administration. 5. Possible mechanisms which could account for the influences of sympathetic stimulation on chemoreceptor activity and carotid body blood flow are discussed. It is concluded that inhibitory and type I excitatory responses probably arise from activation of sympathetic fibres with non-vascular terminations within the carotid body. Type II excitatory responses are most likely due to blood flow changes.

Motor unit firing and its relation to tremor in the tonic vibration reflex of the decerebrate cat

1. The discharge of single motor units has been recorded from the soleus muscle of the decerebrate cat during the tonic vibration reflex elicited isometrically, to further understanding of the tremor that is seen in the reflex contraction. The reflex was elicited by pulses of vibration of 50 micrometers amplitude at 150 Hz, and up to four units were studied concurrently. 2. Individual units fired rather regularly and at a low frequency (range 4-14 Hz). The rate of firing of any unit normally fell within the frequency band of the tremor recorded at the same time. On comparing different preparations a higher frequency of tremor was associated with a higher frequency of motor firing. 3. The responses of pairs of motor units recorded concurrently during repeated production of the reflex were compared by cross-correlation analysis; over 1000 spikes from each train were normally used for this. The major of the cross-correlograms were flat with no overt sign of any synchronization between the units other than that due to the vibration. 4. Clear indications of correlated motor unit firing could be produced deliberately by modulating the amplitude of vibration at a frequency comparable to that of the normal tremor and thereby introducing a rhythmic component into the tonic vibration reflex. 5. About 20% of the cross-correlograms obtained during normal tremor showed varying amounts of an irregular 'waviness' suggesting a possible correlation between the times of firing of a pair of units. But such waves never developed steadily throughout the period of analysis, in contrast to the comparable waves produced on modulating the vibration. Similar waves were seen on cross-correlating a motor unit with an electronic oscillator, confirming that their occurrence does not necessarily demonstrate the existence of active neural interactions. 6. It is concluded that there is no strong and widespread neural synchronizing mechanism active during the tonic vibration reflex, although the possibility of some weak neural interactions has not been excluded. The findings favour the idea that the tremor in this preparation is simply the inevitable result of motor units discharging asynchronously, but at closely similar subtetanic frequencies.

Inhibitory and excitatory effects of dopamine on carotid chemoreceptors

The effect of intravenous dopamine on carotid body chemoreceptor activity was investigated in 6 anesthetized cats which were paralyzed and artificially ventilated. Studies were performed at 4 steady-state PaO2 levels at a constant PaCO2 and at 4 levels of PaCO2 during hyperoxia. Dopamine inhibited carotid chemoreceptors before and excited them after haloperidol. Moderate stimulation of the receptors by hypoxia and hypercapnia augmented dopamine's effects. These results indicate that both inhibitory and excitatory dopamine receptors are present in the carotid body.

Transduction mechanisms in carotid body: glomus cells, putative neurotransmitters, and nerve endings

Carotid body chemoreceptors are activated by low PO2, high PCO2, acidity, increased temperature, and tonicity. These receptors are important in homeostasis and mediate their reflex effects on the CNS through sensory discharges of the carotid (sinus) nerve. The receptor complex is formed by glomus (type I) cells and carotid nerve endings, which, morphologically, appear to form a sensory synapse. The junction between glomus cells and nerve endings is enveloped by processes of sustentacular (type II) cells. The mechanisms of chemoreceptor transduction are complex; there is no agreement about the identity of the primary receptor element (glomus cell or nerve terminal) or what mechanisms are responsible for the onset of the sensory discharge in the carotid nerve. There is increasing evidence that integrity of the glomus cell is essential for normal transduction and that the receptor synapse described by morphologists may be functionally active. There is no conclusive evidence, however, that the glomus cell is the primary site of sensory transduction. Stimuli act on the glomus cell to release "transmitter" and/or "modulator" substances; but it is unknown if the released chemicals are directly responsible for the accompanying change in sensory impulse frequency or merely modify an already ongoing discharge. Interactions between glomus cells and nerves may be complicated enough to make it very difficult to resolve this question.

Enkephalin-, VIP- and substance P-like immunoreactivity in the carotid body

The carotid body type I cell contains amines and has features, both morphological and cytochemical, which indicate that it may also produce a peptide. Many regulatory peptides are now known to be present in both central and peripheral tissues. In the periphery these neuropeptides occur in both classical endocrine (APUD) cells and the neurones of the autonomic nervous system. We have now investigated the possible presence of neuropeptides in the cat carotid body using both immunocytochemistry and radioimmunoassay. Met- and Leu-enkephalin-like material occurred in considerable quantities in carotid body extracts and enkephalin-like immunoreactivity was localised in type I cells. Both vasoactive intestinal polypeptide (VIP)- and substance P-like immunoreactivity was also present but was localised in nerve fibres distributed throughout the organ. These active neuropeptides are widely distributed in mammalian tissues, forming a diffuse regulatory system which now seems to include the carotid body.