The neural pathway involved in "efferent inhibition" of chemoreceptors in the cat carotid body

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.

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.

Immunohistochemical Characteristics of the Human Carotid Body in the Antenatal and Postnatal Periods of Development

The evolutionary and ontogenetic development of the carotid body is still understudied. Research aimed at studying the comparative morphology of the organ at different periods in the individual development of various animal species should play a crucial role in understanding the physiology of the carotid body. However, despite more than two centuries of study, the human carotid body remains poorly understood. There are many knowledge gaps in particular related to the antenatal development of this structure. The aim of our work is to study the morphological and immunohistochemical characteristics of the human carotid body in the antenatal and postnatal periods of development. We investigated the human carotid bodies from 1 embryo, 20 fetuses and 13 adults of different ages using samples obtained at autopsy. Immunohistochemistry revealed expression of βIII-tubulin and tyrosine hydroxylase in the type I cells and nerve fibers at all periods of ontogenesis; synaptophysin and PGP9.5 in the type I cells in some of the antenatal cases and all of the postnatal cases; 200 kDa neurofilaments in nerve fibers in some of the antenatal cases and all of the postnatal cases; and GFAP and S100 in the type II cells and Schwann cells in some of the antenatal cases and all of the postnatal cases. A high level of tyrosine hydroxylase in the type I cells was a distinctive feature of the antenatal carotid bodies. On the contrary, in the type I cells of adults, the expression of tyrosine hydroxylase was significantly lower. Our data suggest that the human carotid body may perform an endocrine function in the antenatal period, while in the postnatal period of development, it loses this function and becomes a chemosensory organ.

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.

Autonomic innervation of the carotid body as a determinant of its sensitivity: implications for cardiovascular physiology and pathology

The motivation for this review comes from the emerging complexity of the autonomic innervation of the carotid body (CB) and its putative role in regulating chemoreceptor sensitivity. With the carotid bodies as a potential therapeutic target for numerous cardiorespiratory and metabolic diseases, an understanding of the neural control of its circulation is most relevant. Since nerve fibres track blood vessels and receive autonomic innervation, we initiate our review by describing the origins of arterial feed to the CB and its unique vascular architecture and blood flow. Arterial feed(s) vary amongst species and, unequivocally, the arterial blood supply is relatively high to this organ. The vasculature appears to form separate circuits inside the CB with one having arterial venous anastomoses. Both sympathetic and parasympathetic nerves are present with postganglionic neurons located within the CB or close to it in the form of paraganglia. Their role in arterial vascular resistance control is described as is how CB blood flow relates to carotid sinus afferent activity. We discuss non-vascular targets of autonomic nerves, their possible role in controlling glomus cell activity, and how certain transmitters may relate to function. We propose that the autonomic nerves sub-serving the CB provide a rapid mechanism to tune the gain of peripheral chemoreflex sensitivity based on alterations in blood flow and oxygen delivery, and might provide future therapeutic targets. However, there remain a number of unknowns regarding these mechanisms that require further research that is discussed.

Autonomic innervation of the carotid body: role in efferent inhibition

The carotid body (CB) is a chemosensory organ that monitors blood chemicals and initiates compensatory reflex adjustments to maintain homeostasis. The 'afferent' sensory discharge induced by changes in blood chemicals, e.g. low PO(2) (hypoxia), is relayed by carotid sinus nerve (CSN) fibers and has been well studied. Much less is known, however, about a parallel autonomic (parasympathetic) 'efferent' pathway that is the source of CB inhibition. This pathway is the focus of this review which begins with a historical account of the early findings and links them to more recent data on the source of this innervation, and the role of endogenous neurotransmitters in efferent inhibition. We review evidence that these autonomic neurons are embedded in 'paraganglia' within the glossopharyngeal (GPN) and CSN nerves, and for the role of nitric oxide (NO) in mediating efferent inhibition. Finally, we discuss recent data linking the action of hypoxia and a key CB neurotransmitter, i.e. ATP, to potential mechanisms for activating this efferent pathway.

Nitric oxide synthase in autonomic innervation of the cat carotid body

In the cat carotid body, nitric oxide synthase (NOS) immunoreactivity and NADPH diaphorase activity localize in nerve fibers mainly associated with blood vessels and occasionally lying close to glomus cells. The NOS-positive innervation originates in part from multipolar ganglion cells scattered in and around the carotid body and in the glossopharyngeal nerve. In the superior cervical ganglion, NOS and diaphorase staining localizes to many preganglionic axons and also to a small population of vasoactive intestinal peptide-positive, presumably cholinergic, ganglion cells. Positively stained ganglion cells are absent in the petrosal ganglion and very rare in the nodose ganglion, although both sensory ganglia display characteristic distributions of cells immunoreactive for calcitonin gene-related peptide, substance P and tyrosine hydroxylase. The NOS-positive innervation of the carotid body thus appears to be autonomic, originating mainly from a population of dispersed ganglion cells, and probably parasympathetic in nature. The superior cervical ganglion also may supply some pre- or postganglionic NOS-positive axons. Nitric oxide released from these nerves could affect glomus cell activity directly or indirectly by vasoregulation.

An anterograde-retrograde labeling study of the carotid sinus nerve of the Japanese monkey (Macaca fuscata)

The sites of origin and termination of efferent and afferent fibers in the carotid sinus nerve (CSN) were investigated in the Japanese monkey. After application of a mixture of horseradish peroxidase (HRP) and wheat germ aggulutinin-conjugated HRP to the central cut end of the CSN, sensory ganglion neurons were labeled in the jugular ganglion of the vagus nerve, as well as in the superior and petrosal ganglia of the glossopharyngeal nerve. Many sympathetic ganglion neurons were also labeled retrogradely in the superior cervical ganglion. In the brain, many labeled terminals were seen ipsilaterally in the lateral division of the nucleus of the solitary tract (NST). A few neuronal cell bodies were also labeled ipsilaterally in a reticular region dorsomedial to the caudal one-third of the facial nucleus. The results indicate that the CSN of the Japanese monkey is composed mainly of afferent fibers terminating in the NST, that the afferent fibers in the CSN originate not only from the superior and petrosal ganglia of the glossopharyngeal nerve but also from the jugular ganglion of the vagus nerve, and that efferent fibers contained in the CSN arise from the medullary reticular formation and superior cervical ganglion.

Nitric oxide mediates chemoreceptor inhibition in the cat carotid body

Numerous studies have demonstrated that carotid sinus nerve fibers mediate a so-called "efferent" inhibition of carotid body chemoreceptors. However, the mechanism(s) underlying this phenomenon are not understood. Recently, it has been shown that an extensive plexus of nitric oxide synthase-containing carotid sinus nerve fibers innervate the carotid body, and that many fine, beaded fibers can be seen in close proximity to small blood vessels as well as lobules of parenchymal cells. The present study examined the effects of centrifugal neural activity in the carotid sinus nerve on the accumulation of [3H]citrulline synthesized from [3H]arginine in the cat carotid body, and the possible involvement of nitric oxide in mediating "efferent" chemoreceptor inhibition. Electrical stimulation of carotid sinus nerve C-fibers evoked an increase in [3H]citrulline accumulation in the carotid body, which was Ca(2+)-dependent and blocked by L-NG-nitroarginine methylester (0.1 mM), an inhibitor of nitric oxide synthase. Using a vascularly perfused in vitro carotid body preparation, chemoreceptor activity was recorded from thin nerve filaments split-off from the main trunk of the carotid sinus nerve. Electrical stimulation of the main nerve trunk at C-fiber intensities inhibited steady-state chemoreceptor discharge, and this effect was blocked by L-NG-nitroarginine methylester. However, when the organ preparation was switched to the superfuse-only mode, carotid sinus nerve stimulation failed to alter the steady-state discharge, but under these conditions, prolonged nerve stimulation (> 5 min) did attenuate the chemoreceptor response to hypoxia, an effect which was likewise blocked by L-NG-nitroarginine methylester. The present data, together with previous anatomical findings that nitric oxide synthase immunoreactivity is present in both sensory and autonomic ganglion cells innervating the carotid body, suggest that two neural mechanisms may be involved in the inhibitory neural regulation of carotid chemoreceptors. One mechanism appears to involve nitric oxide release from intralobular sensory C-fibers, which lie in close proximity to the chemoreceptor type I cells. The other mechanism involves release of nitric oxide from perivascular terminals of autonomic microganglia neurons, which control carotid body blood flow.

Localization and actions of nitric oxide in the cat carotid body

An extensive plexus of nerve fibers capable of synthesizing nitric oxide was demonstrated in the cat carotid body by immunocytochemical and biochemical studies of nitric oxide synthase. Denervation experiments indicated that the axons originate from: (i) microganglial neurons located within the carotid body and along the glossopharyngeal and carotid sinus nerves, whose ramifications primarily innervate carotid body blood vessels; and (ii), sensory neurons in the petrosal ganglion, whose terminals end in association with lobules of type I cells. In the in vitro superfused cat carotid body, the nitric oxide synthase substrate, L-arginine, induced a dose-dependent inhibition of carotid sinus nerve discharge evoked by hypoxia. In contrast, the nitric oxide synthase inhibitor, L-NG-nitroarginine methylester, augmented the chemoreceptor response to hypoxia, and this effect was markedly enhanced when the preparation was both perfused and superfused in vitro. The nitric oxide donor, nitroglycerine, inhibited carotid sinus nerve discharge, and immunocytochemistry revealed that this drug stimulated the formation of cyclic 3',5'-guanosine monophosphate in both type I cells and blood vessels. Our data indicate that nitric oxide is an inhibitory neuronal messenger in the carotid body, which affects the process of chemoreceptor transduction/transmission via actions on both the receptor elements and their associated blood vessels.

Neurons synthesizing nitric oxide innervate the mammalian carotid body

The carotid body is an arterial chemoreceptor organ sensitive to blood levels of O2, CO2 and pH. The present immunocytochemical and neurochemical study has demonstrated the presence of an extensive plexus of nitric oxide (NO)-synthesizing nerve fibers in this organ. These nitric oxide synthase (NOS)-containing axons are closely associated with parenchymal type I cells and with blood vessels in the carotid body. Denervation and retrograde tracing experiments have revealed that these fibers arise from NOS-immunoreactive and nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase-positive neuronal cell bodies located in the petrosal ganglion and the carotid body, and dispersed along the glossopharyngeal and carotid sinus nerves (CSN). Within the petrosal ganglion, these neurons are topographically segregated from the catecholaminergic cells, and they contain the neuropeptide, substance P. NOS-positive autonomic microganglial cells in the carotid body and CSN also exhibit choline acetyltransferase (ChAT) immunoreactivity. Our results suggest that nitric oxide may be a novel neuronal messenger in the mammalian carotid body involved in the modulation of chemosensory transduction and transmission in this organ.

Transmitter diversity in carotid body afferent neurons: dopaminergic and peptidergic phenotypes

Hypoxic stimulation of carotid body chemoreceptors is conveyed to the brainstem by primary sensory neurons whose peripheral axons run in the carotid sinus nerve. While considerable attention has focused on defining chemical neuroregulators released by glomus cells in the carotid body, our understanding of the morphology, distribution and transmitter phenotype of these carotid body afferent neurons remains limited. Carotid body afferent neurons were labeled by microinjection of the retrograde tracer, Fluorogold, into the vascularly isolated rat carotid body. In addition, immunoelectron microscopy was used to correlate transmitter phenotype with ultrastructural features of afferent terminals in the carotid body. Our results indicate that 41% of all carotid body afferent neurons express tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, whereas 7% contain substance P. Tyrosine hydroxylase- and substance P-positive neurons constitute separate subpopulations of carotid body afferents, as these two phenotypes were not colocalized. Most of the tyrosine hydroxylase-containing carotid body afferent neurons were small- or medium-sized (mean cell diameter 15-20 microns) and located in the distal petrosal ganglion, whereas the majority of substance P-containing carotid body afferent neurons were medium- to large-sized (mean cell diameter 20-29 microns) and located in the proximal petrosal ganglion and jugular ganglion. These differences strengthen the notion that these catecholaminergic and peptidergic carotid body afferent neurons give rise to functionally distinct subsets of chemoafferent fibers. To further characterize the catecholaminergic phenotype expressed by tyrosine hydroxylase-positive cells in the petrosal ganglion, we examined the colocalization of tyrosine hydroxylase and DOPA decarboxylase, the dopamine-synthesizing enzyme. Eighty-six per cent of tyrosine hydroxylase-positive neurons in the distal petrosal ganglion also contained DOPA decarboxylase; as these cells do not express the norepinephrine-synthesizing enzyme, dopamine beta-hydroxylase, these data indicate that the catecholaminergic carotid body afferent neurons are dopaminergic. Finally, ultrastructural analysis of the peripheral processes of tyrosine hydroxylase-positive afferent terminals in the carotid body demonstrated endings in close opposition to Type I glomus cells, consistent with a role for dopaminergic afferent neurons in carotid body chemoreception. One possibility is that these cells, in addition to their role as afferents, constitute a morphologic substrate for dopaminergic "efferent" inhibition in the carotid body.

Chemoreceptor A-fibres in the human carotid body contain tyrosine hydroxylase and neurofilament immunoreactivity

Previous retrograde tracing studies on rat and guinea-pig showed a projection of sensory tyrosine hydroxylase-immunoreactive neurons to the region of the carotid bifurcation via the carotid sinus nerve. In the present study, focussing on the sensory innervation of the human carotid body, antisera to tyrosine hydroxylase and other catecholamine synthesizing enzymes were applied for an immunohistochemical investigation of carotid bodies obtained at autopsy. In addition, an array of antisera directed to non-enzyme antigens known to be present in viscero-afferent neurons were incorporated in the study. The glomic lobules consisting of glomus cells and sustentacular cells contained a variable number of enzyme-immunoreactive glomus cells. Arteries were supplied by nerve fibres displaying the full phenotype of sympathetic noradrenergic axons, i.e. immunoreactivity to tyrosine hydroxylase, aromatic-L-amino-acid-decarboxylase and dopamine-beta-hydroxylase. The glomic lobules, however, were densely innervated by tyrosine hydroxylase-immunoreactive axons lacking immunoreactivity to aromatic-L-amino-acid-decarboxylase and dopamine-beta-hydroxylase. These fibres reacted with neurofilament 160kD-antibody but were devoid of immunoreactivity to all neuropeptides tested (calcitonin gene-related peptide, somatostatin, substance P). Ultrastructurally, tyrosine hydroxylase/neurofilament 160kD-immunoreactive axons gave rise to large axonal swellings filled with mitochondria and vesicles, and established extensive contacts to glomus cells. Nerve bundles surrounded by a perineural sheath contained both myelinated (2.0-2.8 microns in diameter) and unmyelinated (0.14-3.0 microns) tyrosine hydroxylase-immunoreactive axons. Most of the unmyelinated immunoreactive axons were running singularly within a Schwann cell-sheath. Judged from the pattern of immunoreactivities as well as their preterminal and terminal ultrastructure, tyrosine hydroxylase-immunoreactive axons innervating glomus cells are of sensory origin. Although final proof by retrograde tracing cannot be presented in man, this conclusion is supported by experimental evidence in laboratory animals. The myelinated immunoreactive axons correspond to chemoreceptor A-fibres whereas the classification of the large unmyelinated immunoreactive axons has yet to be established. The lack of immunoreactivity to the dopamine-synthesizing enzyme, aromatic-L-amino-acid-decarboxylase, in this fibre type does not support the view of dopamine being the primary transmitter of chemoreceptor afferents.

Transection of carotid sinus nerve inhibits the turnover of dopamine and norepinephrine in long-term hypoxic carotid bodies: a biochemical and morphometric study

To assess the efferent influence of the carotid sinus nerve on the catecholamine function and the vascularity in long-term hypoxic carotid bodies, rats were exposed to hypoxia (10% O2 in nitrogen for 1 and 3 weeks) after unilateral transection of the sinus nerve. In the intact carotid bodies, long-term hypoxia elicited several biochemical and morphological alterations: increased turnover of dopamine and norepinephrine, increased volume density of blood vessels, decreased volume density of glomic tissue. Transection of the sinus nerve reduced but did not abolish the increase in turnover of dopamine and norepinephrine evoked by hypoxia without preventing the structural alterations. Immunocytochemical studies revealed the presence of nerve fibres exhibiting neuropeptide Y-like immunoreactivity which persisted after the excision of the carotid sinus nerve. Nerve fibres showing substance P-like immunoreactivity were found in the intact carotid bodies but not in the carotid bodies deprived of sinus nerve. It is concluded that the sinus nerve exerts a stimulatory efferent control on the synthesis and release of dopamine and norepinephrine in rat carotid body during long-term hypoxia, but no appreciable vasodilatory influence on the glomic vasculature. A possible role of substance P contained in the endings of the carotid sinus nerve is discussed.

Immunohistochemical distribution and colocalization of regulatory peptides in the carotid body

Current investigations on the immunohistochemical occurrence and co-occurrence of biogenic polypeptides in the mammalian carotid body were reviewed and extended by our own recent findings. The family of chromogranins and related peptides in glomus cells appears to have a widespread interspecies distribution, whereas other peptides investigated occur in a species-specific pattern. Immunoreactivity to antisera against opioids, which derive from the proenkephalin sequence, appears to be present in glomus cells of the rabbit, cat, dog, and a shrew. Conversely, glomus cells of pig and guinea pig predominantly are immunoreactive to cleavage products of prodynorphin, which co-occur in some cells with substance P and met-enkephalin-arg-phe, respectively. In the rat and Callithrix jacchus, opioid immunoreactivity is present in nerve fibres but not in glomus cells. Immunoreactivity to other peptides, such as neurotensin, cholecystokinin, neuropeptide Y, and galanin, is found only in one or two particular species. Neurotensin immunolabelling occurs in beagle dog glomus cells, which are known to lack substance P. Cholecystokinin immunoreactivity is present in glomus cells of dog and Callithrix, and co-exists with chromogranin A, neuropeptide Y, and substance P. Substance P appears to exist in both carotid body glomus cells and nerve fibres. Substance P immunoreactivity is present in glomus cells of all species investigated, except dog. Coexistence of substance P and calcitonin gene-related peptide (CGRP) is demonstrated in nerve fibres of the guinea pig carotid body, which originate in the petrosal and jugular ganglia. Other peptides visualized immunohistochemically in mammalian carotid body nerve fibres are vasoactive intestinal peptide and neuropeptide Y. The functional significance of the various peptides present in the carotid body is discussed.

Retrograde neuronal labelling and double-staining immunohistochemistry of tachykinin- and calcitonin gene-related peptide-immunoreactive pathways in the carotid sinus nerve of the guinea pig

The origin of tachykinin- and calcitonin gene-related peptide-like immunoreactive (CGRP-LI) nerve fibres in the guinea pig carotid body and carotid sinus was determined by retrograde labelling of the carotid sinus nerve with Fluoro-gold and immunohistochemical double staining with fluorescein- and rhodamine-conjugated second antisera. Fluoro-gold-labelled perikarya with characteristic features of primary sensory neurones were numerous in the glossopharyngeal (petrosal) ganglion and occurred rarely in the closely attached superior vagal (jugular) ganglion. An efferent pathway from the brainstem could not be detected. Co-existence of tachykinin- and CGRP-LI was observed in 25-47% of labelled sensory neurones; less than 1% of Fluoro-gold-containing perikarya were exclusively stained by CGRP antiserum. Co-existence of tachykinin- and CGRP-LI was also demonstrated in nerve fibres of the carotid body and carotid sinus. Somatostatin-, cholecystokinin- and dynorphin-LI did not co-exist with tachykinin-LI in these fibres. Thus, tachykinin/CGRP-LI fibres in the carotid presso- and chemoreceptive areas exhibit a peptide pattern being generally characteristic for sensory fibres supplying great vessels in the guinea pig. In view of the present findings doubt is raised as to a primary involvement of these fibres in presso- or chemoreception, although a modulatory influence on these specific functions appears to be likely.

Carotid sinus nerve C-fibers release catecholamines from the cat carotid body

Previous studies have demonstrated that unmyelinated fibers in the cat carotid sinus nerve (CSN) are capable of inhibiting chemoreceptor activity from the carotid body. While the mechanism(s) underlying this phenomenon are unknown, it has been postulated that carotid body catecholamines may be involved in mediating this inhibitory effect. In this study, the in vitro cat carotid body-CSN preparation was used to investigate the effects of CSN stimulation on the release of [3H]catecholamines from the carotid body. The results showed that labeled catecholamines were released from this organ only when C-fibers were recruited by nerve stimulation.

Influence of ganglioglomerular nerve on carotid chemoreceptor activity in the cat

The dependence of the carotid chemoreceptor responses to blood-borne stimuli on the ganglioglomerular nerve (GGN) activity was investigated in cats which were anesthetized, paralyzed and artificially ventilated. The activity of a few carotid chemoreceptor afferents from a slip or from the cut left carotid sinus nerve (CSN) and the activity of a few GGN fibers were recorded. The responses of the same chemoreceptor afferents to steady-state hypoxia at a constant paCO2 and to steady-state hypercapnia during hyperoxia were compared before and after the transection of the ipsilateral ganglioglomerular nerve (IGGN). Similarly the effects of IGGN transection on the responses of the same chemoreceptor afferents to graded doses of intravenous injections of sodium cyanide (20-60 micrograms) and nicotine (20-60 micrograms) at constant blood gas levels were studied. On the average, IGGN transection during normoxia only slightly changed the carotid chemoreceptor activity. Also, it did not significantly change the hypoxic and hypercapnic responses, and those to sodium cyanide and nicotine injections. Thus, the mean carotid chemoreceptor responses to physiological and pharmacological stimuli were largely independent of the GGN. However, certain GGN fibers were strongly stimulated by hypoxia and hypercapnia. Clearly, the total GGN traffic to the carotid body was not sufficiently strong to exert a significant control over the mean carotid chemoreceptor activity.

Localization and function of cat carotid body nicotinic receptors

Acetylcholine and nicotinic agents excite cat carotid body chemoreceptors and modify their response to natural stimuli. The present experiments utilized [125I]alpha-bungarotoxin [( 125I]alpha-BGT) to localize within the chemosensory tissue the possible sites of action of exogenous and endogenous nicotinic cholinergic substances. In vitro equilibrium binding studies of intact carotid bodies determined a Kd of 5.57 nM and a Bmax of 9.21 pmol/g of tissue. Chronic section (12-15 days) of the carotid sinus nerve (CSN) did not change the amount of displaceable toxin binding. In contrast, the specific binding was reduced by 46% following removal of the superior cervical ganglion. Light microscope autoradiography of normal, CSN-denervated and sympathectomized carotid bodies revealed displaceable binding sites concentrated in lobules of type I and type II cells. Treatment of carotid bodies with 50 nM alpha-BGT in vitro reduced by 50% the release of [3H]dopamine (synthesized from [3H]tyrosine) caused by hypoxia or nicotine, and also significantly reduced the stimulus-evoked discharges recorded from the CSN. The data suggest an absence of alpha-BGT binding sites on the afferent terminals of the CSN and that nicotinic receptors located with parenchymal cell lobules may modulate the release of catecholamines from these cells.

Morphology of the rat carotid sinus nerve. I. Course, connections, dimensions and ultrastructure

An analysis of the morphology of the carotid sinus nerve in 39 rats revealed that the nerve emerged from the glossopharyngeal nerve (nerve IX) 0.6-1.0 mm beyond the distal extent of the petrosal ganglion. In 68% of cases the nerve consisted of a single bundle of axons; two bundles were present in the other nerves. Near the rostral pole of the carotid body, the nerve divided into multiple bundles of axons. Most axon bundles entered the carotid body, but some instead joined sympathetic nerves from the superior cervical ganglion. Some of the latter group of sinus nerve axons innervated the wall of the carotid sinus. Other presumptive baroreceptor axons reached the carotid sinus from the sinus nerve by first traversing the carotid body. The sinus nerve had an average length of 2.0 +/- 0.13 mm (mean +/- S.E.M., range = 1.3-2.9 mm, N = 15) and was elliptical in cross-section (major axis = 77 microns, minor axis = 46 microns). The nerve contained an average of 625 axons, 86% of which were unmyelinated. The perineurial sheath that enveloped the circumference of the nerve was comprised of 3 or 4 layers of cells interconnected by gap junctions and tight junctions. Near the carotid body, the perineurium extended into the nerve where it compartmentalized fascicles of axons. The endoneurium consisted mainly of collagenous fibres (mean diameter 40 nm) that were only half the size of those outside the perineurium. Endoneurial connective tissue cells were sparse, being only 5% as numerous as Schwann cells. The one blood vessel in the nerve (mean luminal diameter 12.4 +/- 1.2 microns) usually arose from vessels in nerve IX and terminated in a venule at the rostral surface of the carotid body. Ganglion cells were located in nerve IX near the origin of the sinus nerve and along the length of the sinus nerve itself. All ganglion cells examined were postsynaptic to vesicle-containing nerve terminals, and therefore were presumed to be autonomic rather than sensory. Ganglion cells were more numerous in terminal branches of the sinus nerve on the ventral surface of the carotid body. Most other ganglion cells in the carotid body were located among axons of the ganglioglomerular nerve, although some were associated with a small branch of the vagus nerve. Paraganglia were located within the sinus nerve in 15% of cases, and in another 10% of cases they were found nearby in nerve IX. The paraganglia, which measured approximately 50 X 50 X 100 microns, were innervated by axons that resembled sensory nerves of the carotid body.

Morphology of the rat carotid sinus nerve. II. Number and size of axons

Ten carotid sinus nerves from five rats were examined by electron microscopy at a level of 0.5 mm from the glossopharyngeal nerve (nerve IX). The sinus nerves were found to contain from 455 to 757 (mean 625) axons per nerve, of which an average of 86.3% were unmyelinated. The unmyelinated axons had a size distribution that fitted a Gaussian distribution with a mean diameter of 0.78 micron and a variance of 0.013 micron. Such axons ranged in size from 0.17 to 1.7 microns. The myelinated axons had a unimodal size distribution skewed to the right, with a median total fibre diameter of 2.49 microns. Although total diameter of myelinated fibres ranged from 1.5 to 5.3 microns, 96% of such fibres were smaller than 4 microns. Axon diameter of myelinated fibres averaged 64% of the total diameter, but this proportion tended to increase with the size of the axon. Some 68% of myelinated fibres had axons with a diameter within the range of sizes of unmyelinated axons. The number of axons varied along the length of the sinus nerve, but no consistent pattern of change was found among different rats. The two nerves examined at 0.1 and 0.5 mm from nerve IX had 8-10 more myelinated axons at the more distal level, and the number of unmyelinated axons increased by four in one nerve but decreased by 26 in the other nerve. In three nerves examined at 0.5 and 2.0 mm from nerve IX, the number of unmyelinated axons increased from proximal to distal by 11 (2%) to 220 (43%), whereas the number of myelinated axons increased by 20 (48%) in one nerve but decreased by 7-10 (13-21%) in the others. One day after nerve IX was cut distal to the petrosal ganglion, most myelinated axons in the sinus nerve were degenerating and only 109 unmyelinated axons were still present. By four days all but two myelinated axons were gone and the normal complement of unmyelinated axons was replaced by more than 1800 rounded profiles, most of which probably were pseudopodia of reactive Schwann cells. Transection of nerve IX central to the petrosal ganglion did not produce such ultrastructural changes in Schwann cells, nor did it reduce the number of axons in the sinus nerve to a degree sufficient to be detected by the counting procedure. Although these results indicate that most axons in the sinus nerve are sensory, some nonsensory axons undoubtedly are present too. The sensory and nonsensory axons in the nerve apparently are closely associated with one another and in some cases might be enveloped by the same Schwann cells.