Carotid Body Afferents Research Articles

Carotid body hypersensitivity in intermittent hypoxia and obtructive sleep apnea.

The carotid bodies are the principal sensory organs for detecting changes in arterial blood oxygen concentration, and the carotid body chemoreflex is a major regulator of the sympathetic tone, blood pressure and breathing. Intermittent hypoxia is a hallmark manifestation of obstructive sleep apnea (OSA), which is a widespread respiratory disorder. The first part of this review discuss the role of carotid bodies in heightened sympathetic tone and hypertension in rodents treated with intermittent hypoxia, and the underlying cellular, molecular, and epigenetic mechanisms. We also present evidence for hitherto uncharacterized role of carotid body afferents in triggering cellular and molecular changes by intermittent hypoxia. The second part of the review presents evidence for contribution of hypersensitive carotid body to OSA and potential therapeutic intervention to mitigate OSA in a murine model. Abstract figure legend OSA = obstructive sleep apnea; IH = intermittent hypoxia; ROS = reactive oxygen species; CB = carotid body; SNA = sympathetic nerve activity; HO-2 = hemeoxygenase 2; CSE = cystathionine-ϒ-lyase. This article is protected by copyright. All rights reserved.

Time Course of Astrocyte Activation in Mouse Nucleus Tractus Solitarius During Exposure to Hypoxia

Chronic sustained hypoxia (CH) occurs in populations living at high altitude and in patients with chronic pulmonary disease. Exposure to CH produces ventilatory acclimatization to hypoxia (VAH) and increases the hypoxic ventilatory response (HVR) by mechanisms that involve areas of the brainstem that control breathing. The nucleus tractus solitarius (NTS) is a sensory integrative center in the medulla receiving carotid body afferents and known to be crucial for VAH. Our previous results demonstrated that glia cells in the rat NTS contribute to VAH but these mechanisms have not been studied in mice exposed to CH beyond 24 hours. We hypothesized that CH produces an early transient activation of astrocytes and microglia in the mouse brainstem as observed in rats. To study the activation of glial cells with CH, we exposed mice to normobaric hypoxia (10% FiO2) for 0.5, 1 and 4 hours and 1 and 7 days. Mice were perfused with 4% paraformaldehyde and brainstem sections were obtained. We quantified astrocyte activation by measuring glial fibrillary acidic protein (GFAP) intensity with immunofluorescence, and microglial activation by measuring branch morphology using Iba‐1 marker. Exposure to CH significantly increased GFAP intensity after 30 minutes and 1day while microglia morphology did not change significantly. The results differ from those in rats (J. Neurophysiol.117: 1625–1635, 2017) and suggest that time‐dependent astrocyte activation in the NTS may contribute to VAH.Support or Funding InformationSupported by NIH RO1 HL‐081823

Electrophysiological properties of laryngeal motoneurones in rats submitted to chronic intermittent hypoxia.

The respiratory control of the glottis by laryngeal motoneurones is characterized by inspiratory abduction and post-inspiratory adduction causing decreases and increases in upper airway resistance, respectively. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, exaggerated glottal abduction (before inspiration), associated with active expiration and decreased glottal adduction during post-inspiration. CIH increased the inspiratory and decreased the post-inspiratory laryngeal motoneurone activities, which is not associated to changes in their intrinsic electrophysiological properties. We conclude that the changes in the respiratory network after CIH seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia. To keep an appropriate airflow to and from the lungs under physiological conditions a precise neural co-ordination of the upper airway resistance by laryngeal motoneurones in the nucleus ambiguus is essential. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, may alter these fine mechanisms. Here, using nerve and whole cell patch clamp recordings in in situ preparations of rats we investigated the effects of CIH on the respiratory control of the upper airway resistance, on the electrophysiological properties of laryngeal motoneurones in the nucleus ambiguus, and the role of carotid body (CB) afferents to the brainstem on the underlying mechanisms of these effects. CIH rats exhibited longer pre-inspiratory and lower post-inspiratory superior laryngeal nerve activities than control rats. These changes produced exaggerated glottal abduction (before inspiration) and decreased glottal adduction during post-inspiration, indicating a reduction of upper airway resistance during these respiratory phases after CIH. CB denervation abolished these changes produced by CIH. Regarding choline acetyltransferase positive-laryngeal motoneurones, CIH increased the firing frequency of inspiratory and decreased the firing frequency of post-inspiratory laryngeal motoneurones, without changes in their intrinsic electrophysiological properties. These data show that the effects of CIH on the upper airway resistance and laryngeal motoneurones activities are driven by the integrity of CB, which afferents induce changes in the central respiratory generators in the brainstem. These neural changes in the respiratory network seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

Carotid chemoreceptor afferent projections to leptin receptor containing neurons in nucleus of the solitary tract

Neurons expressing the leptin receptor (Ob-R) exist within the caudal nucleus of the solitary tract (NTS). Additionally, afferent neurons expressing the Ob-R have been identified within the nodose ganglion and NTS. Furthermore, systemic injections or focal injections of leptin directly into NTS potentiate the response of NTS neurons to carotid chemoreceptor activation. However, the distribution of carotid body afferents in relation to Ob-R containing neurons within NTS is not known. In this study, chemoreceptor afferent fibers were labeled following microinjection of the anterograde tract tracer biotinylated dextran amine (BDA) into the carotid body or petrosal/nodose ganglion of Wistar rats. After a survival period of 10–14 days, the NTS was processed for BDA and Ob-R immunoreactivity. Afferent axons originating in the carotid body were found to project to the lateral (Slt), gelantinosa (Sg), and medial (Sm) subnuclei of the NTS complex. A similar, but more robust distribution of BDA labeled fibers was observed in the NTS complex after injections into the petrosal/nodose ganglion. Carotid body BDA labeled fibers were observed in close apposition to Ob-R immunoreactive neurons in the region of Slt, Sg and Sm. In addition, a small number of carotid body afferents were found to contain both BDA and express Ob-R-like immunoreactivity within the regions of Slt, Sg and Sm. Taken together, these data suggest that leptin may modulate carotid chemoreceptor function not only through direct effects on NTS neurons, but also through a direct effect on carotid body primary afferent fibers that innervate NTS neurons.

Activation of mu‐opioid receptors in commissural nucleus tractus solitarius markedly depresses hypoxic, but not hypercapnic, ventilatory response in conscious rats (712.11)

Activation of cNTS MORs blocks HVR and moderately depresses HCVR (↓30%) in anesthetized rats (Zhang et al, Anesthesiology 2011) in agreement with the cNTS synaptically receiving carotid body afferents and heavily expressing MORs. Because of synergistic effect of anesthesia on opioids’ impacts and projections of bronchopulmonary C‐fibers (PCFs) to the cNTS, we asked whether stimulation of cNTS MORs would alter HVR, HCVR, and PCF‐mediated apnea in conscious state. cNTS catheter guide was instrumented in the anesthetized rats with the right site ensured by lack of HVR following local injection of DAMGO (2.5 mM, 30 nl), a MOR agonist. After recovery from surgery, three groups of conscious animals were individually placed in a whole‐body plethysmograph to: inhale 10% O2 for 2 min or 7% CO2 for 5 min, or receive right atrial bolus injection of capsaicin (CAP, 1 μg/kg) before and 5 min after intra‐cNTS microinjection of DAMGO with and without previous local blockade of MORs by CTAP (10 mM, 30 nl). Microinjection of DAMGO into the cNTS decreased baseline VE (↓16%) mainly via lowering fR and reduced HVR and PCF‐mediated apnea (63% and 20%, P < 0.01) without effect on HCVR. Microinjection of DAMGO following CTAP failed to alter HVR. Our data suggest that cNTS MORs play a critical role in depressing respiratory chemoreflexes in conscious state and this influence is largely strengthened by anesthesia.Grant Funding Source: Supported by HL‐107462

Carotid body ablation improves survival, breathing disorders and autonomic control in heart failure rats

Sympathetic hyperactivity and breathing instability contribute to decompensation during chronic heart failure (CHF). Enhanced carotid body (CB) chemoreflex drive has been related to increased sympathetic outflow and breathing disorders in CHF. We hypothesized that ablation of CB afferents (CBA) improves cardiorespiratory function and survival in CHF. Selective CBA was performed on male Sprague‐Dawley rats after 16 weeks (wk) of coronary artery ligation (EF < 45%). Resting breathing and ventilatory responses to acute hypoxia (FiO2‐10%) were measured by plethysmography prior to and one wk after CBA/sham surgery. Heart rate (HRV) and systolic blood pressure variability (BPV) and spontaneous baroreflex gain (BG) were similarly determined at rest. Neuronal activation was assessed in the rostral ventrolateral medulla (RVLM) at the end of the study using Fra‐1 expression. CHF rats compared to sham exhibited elevated apnea incidence, hypoxic ventilatory responses, LF/HF HRV, LF‐BPV, and RVLM Fra‐1 expression and decreased BG. CBA in CHF rats decreased (pre‐ vs. post‐CBA, P < .05), LF/HF HRV (2.1±0.3 vs. 0.9±0.2), LF‐BPV (18.4±4.5 vs. 7.2±2.0 mmHg2), apnea frequency (16.8±1.8 vs. 10.6±0.6 events/h), RVLM Fra‐1 expression (1.5±0.1 vs. 0.9±0.1 a.u.), and increased BG (1.0±0.1 vs. 1.9±0.3 ms/mmHg) to levels comparable to sham rats. Mortality rate at 16 wk post infarct was elevated in CHF compared to sham (33% vs. 7%). CBA at 2 wk post infarct reduced the mortality rate in CHF rats at 16 wk to 15%. Our results show that CBA reduces hyperactivation of the RVLM and improves cardiorespiratory control and survival in CHF rats. Supported by NIH PO1‐HL62222.

Depressive cardio-respiratory effects of somatostatin in anaesthetized rats

Cardio-respiratory effects of intravenous injection of somatostatin were investigated in anaesthetized, spontaneously breathing rats that were: (i) neurally intact and subsequently bilaterally vagotomized in the neck, or (ii) midcervically vagotomized with later vagotomized at the supranodosal level, or (iii) midcervically vagotomized and subjected to section of the carotid sinus nerves (CSNs). Intravenous injection of 100mug/kg of somatostatin before and after midcervical vagotomy-induced immediate slowing down of the respiratory rate and decreased tidal volume, mean arterial blood pressure (MAP) and heart rate. Supranodose vagotomy did not abolish somatostatin-induced respiratory depression. CSNs section eliminated all respiratory effects of somatostatin challenge. In all the neural states, somatostatin caused significant falls in mean arterial blood pressure and heart rate. The results of this study suggest that hypoventilation induced by intravenous somatostatin administration occurs outside vagal afferentation to the medulla and is mediated via carotid body afferents. Sino-aortic chemoreceptors and baroreceptors do not contribute to bradycardia and a fall in blood pressure. These cardiovascular effects are presumably mediated due to excitation of somatostatin receptors within the heart and/or in the brain.

Non-vagal apnea evoked by intracommon carotid artery injection of N-methyl-D-aspartic acid (NMDA) in anesthetized rats

Respiratory effects of an intra-common carotid artery injection of N-methyl-D-aspartic acid (NMDA) were investigated in anesthetized spontaneously breathing rats, using three experimental paradigms: (1) midcervical vagotomy followed by supranodosal vagotomy, (2) midcervical vagotomy followed by section of the carotid sinus nerves (CSNs), and (3) midcervical vagotomy followed by pharmacological blockade of NMDA receptors. The intra-common carotid artery injection of NMDA (4 mg/kg) induced transient expiratory apnea followed by a brief and variably occurring period of breathing at reduced tidal volume. There were no consistent changes in respiratory rate in rats subjected to midcervical vagotomy alone. Supranodose vagotomy exerted no effect on NMDA-induced respiratory arrest, whereas CSNs' section or blockade of NMDA receptors with AP-7 abolished the apnea. These results indicate that the apnea induced by intraarterial NMDA challenge is due to activation of peripheral NMDA receptors and is mediated via carotid body afferents.

Time-dependent changes in dopamine D 2-receptor mRNA in the arterial chemoreflex pathway with chronic hypoxia

The hypoxic ventilatory response (HVR) can be modulated by dopamine D 2-receptors (D 2-R) in both the carotid body arterial chemoreceptors and the nucleus tractus solitarius (NTS), the primary synapse site of carotid body afferents. We hypothesized that chronic hypoxia alters D 2-R gene expression to initiate changes in D 2-R modulation of the HVR and enhance ventilatory acclimatization to hypoxia. Thus, we used a competitive reverse transcription-polymerase chain reaction (RT-PCR) method to quantify changes in D 2-R mRNA levels in the rat carotid body and NTS after 0, 6, 12, 24, 48, or 168 h of hypobaric hypoxia ( P IO 2 =80 Torr). In the rostral NTS, hypoxia significantly increased D 2-R mRNA at all time points. In the caudal NTS, D 2-R mRNA levels initially increased in response to hypoxia and then significantly decreased to 71±5% and 71±6% of control after 48 and 168 h of hypoxia, respectively. In the carotid body, D 2-R mRNA levels significantly decreased to 59±2% of control after 48 h of hypoxia; however, they significantly increased to 274±22% of control after 168 h. These results suggest that changes in D 2-R mRNA in the arterial chemoreflex pathway and corresponding changes at the protein and signaling levels may contribute to the time-dependent changes in ventilation observed with chronic hypoxia. Specifically, decreased carotid body inhibition by D 2-R could increase the HVR after 2 days of hypoxia.

Galanin expression in carotid body afferent neurons.

The present study examined expression and plasticity of the neuropeptide, galanin, in carotid body afferent neurons in the petrosal ganglion of the adult rat. The pattern of galanin expression was compared with that of tyrosine hydroxylase, a selective marker of dopaminergic carotid body afferents in the petrosal ganglion. In normal animals, only 3% of tyrosine hydroxylase-containing petrosal ganglion neurons co-expressed galanin. Retrograde labeling studies, in which FluoroGold was injected into the vascularly isolated carotid body, demonstrated that all tyrosine hydroxylase-positive-galanin-positive cells in the petrosal ganglion project to this target. In addition, however, we unexpectedly found that galanin expression was markedly increased in the petrosal ganglion following FluoroGold injection into the carotid body. On the other hand, tyrosine hydroxylase expression was unchanged, indicating that monoaminergic and peptidergic traits can be differentially regulated in these cells. In summary, these data demonstrate that monoaminergic chemoafferent neurons can co-express a peptidergic trait, similar to catecholaminergic neurons within the central and autonomic nervous systems, and that these cells retain the potential for phenotypic plasticity in adulthood.

Ventral medullary surface responses to hypoxic and hyperoxic transient ventilatory challenges in the cat

Carotid body afferent contributions to activity of the intermediate area of the ventral medullary surface (IVMS) following transient hypoxia and hyperoxia were examined in 6 spontaneously breathing, pentobarbital-anesthetized cats. Two tidal breaths of 100% N 2, 100% O 2, or room air, were randomly administered before and after carotid sinus denervation (CSD). Images of scattered light from the IVMS showed that activity increased with hypoxia (10.1 ± 2.4%), and decreased with hyperoxia (4.8 ± 1.8%). CSD significantly increased the magnitude and delayed the onset of the hypoxic response, but reversed the initial component of the hyperoxic response. We conclude that carotid body afferents modulate the magnitude and timing of IVMS responses to transient respiratory challenges.

BDNF Supports Mammalian Chemoafferent Neurons in Vitro and Following Peripheral Target Removal in Vivo

Chemoreceptor neurons innervating the rat carotid body were used as a model system to define target regulation of visceral sensory development in fetal and newborn animals. In vitro, chemoafferents were selectively supported by coculture with the carotid body or by treatment with trkB ligands [brain-derived neurotrophic factor (BDNF) and neurotrophin-4], whereas nerve growth factor and neurotrophin 3 had no effect. In vivo, chemoafferent neurons died following carotid body removal at birth, indicating a predominant role of peripheral, rather than central, targets in mediating survival at this stage. However, in the absence of target tissues, a large proportion of carotid body afferents could be rescued by implants containing BDNF. Moreover, BDNF mRNA was detected in the newborn carotid body by reverse transcriptase polymerase chain reaction. These data provide the first demonstration that BDNF can substitute for peripheral target support of sensory neuron survival in vivo and indicate that trkB ligands may be particularly important for development of visceral afferents involved in cardiorespiratory control.

Afferent contributions to intermediate area of the cat ventral medullary surface during mild hypoxia

The intermediate area of the cat ventral medullary surface activates to mild hypoxia. Carotid body and vagal afferent contributions to this response were examined by recording activity levels, measured as changes in scattered 660 nm light, from the medullary surface in 7 anesthetized, spontaneously breathing cats following 12% O 2 in N 2 ventilatory challenge. A miniaturized video camera collected images synchronous with the peak of cardiac R wave at 1/s, from a 3.2 mm diameter area, before, and following bilateral carotid sinus denervation (CSD) and vagotomy. In intact animals, hypoxia increased activity; however, greater increases in activity levels followed CSD, while vagotomy elicited a marked reduction of the response. Thus, carotid body afferents exert inhibitory or disfacilitatory influences on intermediate area neurons, while the vagus appears to play an excitatory role.

Trophic regulation of carotid body afferent development.

The ventilatory reflex response to hypoxia matures postnatally, from the biphasic pattern seen in the newborn to the more sustained hyperventilation typical of the adult (Eden & Hanson, 1987). Several mechanisms have been proposed to underlie this maturation, including an increase in chemoreceptor sensitivity and a decreased suprapontine inhibitory influence. However, the role of chemosensory afferent development in shaping the ventilatory response has been relatively unexplored. Primary afferent neurons in the glossopharyngeal petrosal ganglion (PG) convey chemosensory information from the carotid body to the brainstem. A large subset of carotid body afferents express dopaminergic traits including tyrosine hydroxylase (TH) and dopa decarboxylase, and 84% of these cells project to the carotid body via the sinus nerve (Katz et al., 1987). Moreover, TH immunoreactive (TH+) cells in the PG make synaptic contact with carotid body type I cells, further supporting a role in chemoreception (Finley et al., 1992; Katz, unpublished). Immunoreactivity for TH can therefore be used as a highly selective marker for carotid body afferents in the PG. The observation that the number of TH+ neurons in the PG increases during the last trimester of gestation, following the onset of carotid body innervation, suggests that the carotid body may regulate maturation of these neurons (Katz & Erb 1990). This is consistent with the concept that development of innervating neurons requires interaction with target tissues. The present study demonstrates that glomectomy in newborn rats causes a marked loss of TH+ neurons in the PG and that this loss can be attenuated by administration of brain-derived neurotrophic factor (BDNF).

Ventilatory and metabolic responses to cold and CO2 in intact and carotid body-denervated awake rats.

We investigated in conscious rats the characteristics and modes of action of CO2 on thermoregulation and ventilatory control during cold stress. In a group of 10 rats studied intact and after carotid body denervation, measurements of metabolic rate (VO2), ventilation (V), shivering, and colonic temperature (Tc) were made at controlled ambient temperatures (Ta) of 25, 20, 15, 10, and 5 degrees C. Animals were exposed on different days to 1) normoxia, 2) normoxia and 4% CO2, 3) 12% hypoxia, or 4) 10.8% hypoxia and 4% CO2. The following results were obtained. 1) During CO2 exposure in normoxia or hypoxia, VO2 is increased at Ta of 25 degrees C and decreased for lower Ta. These effects are partly mediated by carotid body afferents. 2) Shivering and nonshivering thermogenesis and therefore Tc regulation are affected by CO2 exposure as shown by relationships between VO2-Tc and VO2-shivering intensity. 3) V is controlled by PO2 and PCO2 directly through their peripheral and central actions but also indirectly through their effects on VO2. Our conclusions are as follows. 1) Control of Tc is markedly dependent on PCO2 level. Carotid body afferents play a role, but direct central effects acting on the different sources of thermogenesis and possibly on thermolysis are most prominent. 2) As far as control of V is concerned, during hypercapnia in normoxia or hypoxia, several analogies may be formed between exposure to cold and muscular exercise, both of which increase VO2 and V, suggesting common integrative mechanisms at the central nervous system level.

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

Hypoxie 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 μm) 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 μm) 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.

Ventilatory and metabolic responses to cold and hypoxia in intact and carotid body-denervated rats.

The effects of hypoxia on thermoregulation and ventilatory control were studied in conscious rats before and after carotid denervation (CD). Measurements of metabolic rate (VO2), ventilation (V), shivering intensity (SI), and colonic temperature (Tc) were made in groups of eight rats subjected to three protocols. In protocols 1 and 2, at ambient temperature (Ta) of 25 and 5 degrees C, respectively, rats were exposed to normoxia and hypoxia [inspired O2 fraction (FIO2) 0.13-0.11]. In protocol 3, Ta was decreased from 25 to 5 degrees C in 30-min steps of 5 degrees C. Recordings were made in normoxia and hypoxia (FIO2 0.12). The results show that in both intact and CD rats 1) in normoxia, cold exposure increased VO2, V, and SI, and these increases were proportional to the decrease in Ta; 2) hypoxia induced only a transient decrease in SI, and, for a given Ta, VO2 was reduced whereas V and SI were increased; and 3) in CD rats, V increased less during cold exposure in both normoxia and hypoxia; VO2 and Tc were more depressed during hypoxia. It is concluded that 1) the interaction between Ta and FIO2 in the control of V is partly dependent on the carotid body afferents, 2) shivering thermogenesis may be transiently affected by hypoxia independently of the carotid body afferents, and 3) nonshivering thermogenesis may be directly inhibited by hypoxia, especially during cold exposure.

The central organization of carotid body afferent projections to the brainstem of the rat

Despite increasing focus on brainstem respiratory control mechanisms in the rat, relatively little is known about the central organization of chemoreceptor pathways in this species. To approach this issue, the present study sought to selectively define the central projections of primary sensory neurons that innervate the carotid body. Afferent projections were visualized by horseradish peroxidase histochemistry following microinjection of wheat germ agglutinin-horseradish peroxidase into the vascularly isolated carotid body in situ. Labeled afferent fibers were found in several discrete regions of the dorsomedial and ventrolateral medulla. Heaviest labeling was seen bilaterally in the commissural and medial subnuclei of the caudal nucleus tractus solitarius (nTS); more moderate labeling was found bilaterally in the intermediate, interstitial, and dorsolateral subnuclei and ipsilaterally in the ventrolateral subnuclei. In addition, we observed a prominent projection to the caudal ventrolateral medulla in the region of the nucleus retroambigualis. Sparse labeling was also seen in the dorsal motor nucleus of the vagus nerve and the area postrema. These findings support the existence of multiple pathways by which peripheral chemoreceptor inputs may influence central respiratory neurons. In addition to the classically defined relay in nTS, carotid body afferents may also interact more directly with respiratory- or cardiovascular-related neurons in other regions such as the ventrolateral medulla.

Thermal effects on ventilation in cats: participation of carotid body chemoreceptors

In pentobarbitone anesthetized cats, raising body temperature from 37 to 40 °C by external heat increased respiratory frequency, tidal volume, frequency of spontaneous gasps and mean inspiratory flow. It reduced end-tidal CO 2 pressure, together with inspiratory and expiratory durations. After bilateral section of the carotid nerves, raising body temperature still induced hyperventilation, but the increase in gasp frequency was less pronounced and no significant change in tidal volume was observed. In comparison to steady ventilatory values in the intact condition, significant reductions in tidal volume at 38 °C and in gasp frequency at 37, 39 and 40 °C were observed after bilateral caurotomy. Brief hyperoxic tests induced transient decreases in tidal volume and increases in end-tidal CO 2 pressure which were significantly larger at 40 °C than at 37 °C. These changes disappeared after bilateral carotid neurotomy. Anesthetic block of both carotid nerves produced transient reduction in tidal volume at any given temperature. We conclude that carotid body afferents contribute to the hyperventilation evoked by hyperthermia. After their interruption, such contribution is replaceable from other thermal afferents.

Internal intercostal nerve discharges in the cat: influence of chemical stimuli.

We studied the influence of central and peripheral chemoreceptor stimulation on the activities of the phrenic and internal intercostal (iic) nerves in decerebrate, vagotomized, and paralyzed cats with bilateral pneumothoraces. Whole iic nerves of the rostral thorax (T2-T5) usually discharged during neural inspiration, whereas those of the caudal thorax (T7-T11) were primarily active during neural expiration. Filaments of rostral iic nerves that terminated in iic muscles generally discharged during expiration, suggesting that inspiratory activity recorded in whole iic nerves may have innervated other structures, possibly parasternal muscles. All nerves were phasically active at hyperoxic normocapnia and increased their activities systematically with hypercapnia. Isocapnic hypoxia or intra-arterial NaCN injection consistently increased phrenic and inspiratory iic nerve activities. In contrast, expiratory iic nerve discharges were either decreased (10 cats) or increased (7 cats) by hypoxia. Furthermore, expiratory responses to NaCN were highly variable and could not be predicted from the corresponding response to hypoxia. The results show that central and peripheral chemoreceptor stimulation can affect inspiratory and expiratory motoneuron activities differentially. The variable effects of hypoxia on expiratory iic nerve activity may reflect a relatively weak influence of carotid body afferents on expiratory bulbospinal neurons. However, the possibility that the magnitude of expiratory motoneuron activity is influenced by the intensity of the preceding centrally generated inspiratory discharge is also discussed.