The carotid chemoreceptor input to the respiratory neurones of the nucleus of tractus solitarus

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.

Glutamatergic Receptors Modulate Normoxic but Not Hypoxic Ventilation and Metabolism in Naked Mole Rats

Naked mole rats (Heterocephalus glaber) are among the most hypoxia-tolerant mammals, but their physiological responses to acute and chronic sustained hypoxia (CSH), and the molecular underpinnings of these responses, are poorly understood. In the present study we evaluated the acute hypoxic ventilatory response and the occurrence of ventilatory acclimatization to hypoxia following CSH exposure (8–10 days in 8% O₂) of naked mole rats. We also investigated the role of excitatory glutamatergic signaling in the control of ventilation and metabolism in these conditions. Animals acclimated to normoxia (control) or CSH and then exposed to acute hypoxia (7% O₂ for 1 h) exhibited elevated tidal volume (V_T), but decreased breathing frequency (f_R). As a result, total ventilation (V._E) remained unchanged. Conversely, V_T was lower in CSH animals relative to controls, suggesting that there is ventilatory plasticity following acclimatization to chronic hypoxia. Both control and CSH-acclimated naked mole rats exhibited similar 60–65% decreases in O₂ consumption rate during acute hypoxia, and as a result their air convection requirement (ACR) increased ∼2.4 to 3-fold. Glutamatergic receptor inhibition decreased f_R, V._E, and the rate of O₂ consumption in normoxia but did not alter these ventilatory or metabolic responses to acute hypoxia in either the control or CSH groups. Taken together, these findings indicate that ventilatory acclimatization to hypoxia is atypical in naked mole rats, and glutamatergic signaling is not involved in their hypoxic ventilatory or metabolic responses to acute or chronic hypoxia.

Carotid Bodies and the Integrated Cardiorespiratory Response to Hypoxia

Advances in our understanding of brain mechanisms for the hypoxic ventilatory response, coordinated changes in blood pressure, and the long-term consequences of chronic intermittent hypoxia as in sleep apnea, such as hypertension and heart failure, are giving impetus to the search for therapies to "erase" dysfunctional memories distributed in the carotid bodies and central nervous system. We review current network models, open questions, sex differences, and implications for translational research.

Topical Application of Connexin43 Hemichannel Blocker Reduces Carotid Body-Mediated Chemoreflex Drive in Rats

The carotid body (CB) is the main arterial chemoreceptor involved in oxygen sensing. Upon hypoxic stimulation, CB chemoreceptor cells release neurotransmitters, which increase the frequency of action potentials in sensory nerve fibers of the carotid sinus nerve. The identity of the molecular entity responsible for oxygen sensing is still a matter of debate; however several ion channels have been shown to be involved in this process. Connexin-based ion channels are expressed in the CB; however a definitive role for these channels in mediating CB oxygen sensitivity has not been established. To address the role of these channels, we studied the effect of blockers of connexin-based ion channels on oxygen sensitivity of the CB. A connexin43 (Cx43) hemichannel blocking agent (CHBa) was applied topically to the CB and the CB-mediated hypoxic ventilatory response (F_iO₂ 21, 15, 10 and 5%) was measured in adult male Sprague-Dawley rats (~250 g). In normoxic conditions, CHBa had no effect on tidal volume or respiratory rate, however Cx43 hemichannels inhibition by CHBa significantly impaired the CB-mediated chemoreflex response to hypoxia. CHBa reduced both the gain of the hypoxic ventilatory response (HVR) and the maximum HVR by ~25% and ~50%, respectively. Our results suggest that connexin43 hemichannels contribute to the CB chemoreflex response to hypoxia in rats. Our results suggest that CB connexin43 hemichannels may be pharmacological targets in disease conditions characterized by CB hyperactivity.

Carotid chemoreceptors tune breathing via multipath routing: reticular chain and loop operations supported by parallel spike train correlations

We tested the hypothesis that carotid chemoreceptors tune breathing through parallel circuit paths that target distinct elements of an inspiratory neuron chain in the ventral respiratory column (VRC). Microelectrode arrays were used to monitor neuronal spike trains simultaneously in the VRC, peri-nucleus tractus solitarius (p-NTS)-medial medulla, the dorsal parafacial region of the lateral tegmental field (FTL-pF), and medullary raphe nuclei together with phrenic nerve activity during selective stimulation of carotid chemoreceptors or transient hypoxia in 19 decerebrate, neuromuscularly blocked, and artificially ventilated cats. Of 994 neurons tested, 56% had a significant change in firing rate. A total of 33,422 cell pairs were evaluated for signs of functional interaction; 63% of chemoresponsive neurons were elements of at least one pair with correlational signatures indicative of paucisynaptic relationships. We detected evidence for postinspiratory neuron inhibition of rostral VRC I-Driver (pre-Bötzinger) neurons, an interaction predicted to modulate breathing frequency, and for reciprocal excitation between chemoresponsive p-NTS neurons and more downstream VRC inspiratory neurons for control of breathing depth. Chemoresponsive pericolumnar tonic expiratory neurons, proposed to amplify inspiratory drive by disinhibition, were correlationally linked to afferent and efferent "chains" of chemoresponsive neurons extending to all monitored regions. The chains included coordinated clusters of chemoresponsive FTL-pF neurons with functional links to widespread medullary sites involved in the control of breathing. The results support long-standing concepts on brain stem network architecture and a circuit model for peripheral chemoreceptor modulation of breathing with multiple circuit loops and chains tuned by tegmental field neurons with quasi-periodic discharge patterns. NEW & NOTEWORTHY We tested the long-standing hypothesis that carotid chemoreceptors tune the frequency and depth of breathing through parallel circuit operations targeting the ventral respiratory column. Responses to stimulation of the chemoreceptors and identified functional connectivity support differential tuning of inspiratory neuron burst duration and firing rate and a model of brain stem network architecture incorporating tonic expiratory "hub" neurons regulated by convergent neuronal chains and loops through rostral lateral tegmental field neurons with quasi-periodic discharge patterns.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

No evidence of a role for neuronal nitric oxide synthase in the nucleus tractus solitarius in ventilatory responses to acute or chronic hypoxia in awake rats

When exposed to a hypoxic environment, the body's first response is a reflex increase in ventilation, termed the hypoxic ventilatory response (HVR). With chronic sustained hypoxia (CSH), such as during acclimatization to high altitude, an additional time-dependent increase in ventilation occurs, which increases the HVR and is termed ventilatory acclimatization to hypoxia (VAH). This secondary increase persists after exposure to CSH and involves plasticity within the circuits in the central nervous system that control breathing. The mechanisms of HVR plasticity are currently poorly understood. We hypothesized that changes in neuronal nitric oxide synthase (nNOS) activity or expression in the nucleus tractus solitarius contribute to this plasticity and underlie VAH in rats. To test this, we treated rats held in normoxia or 10% O2 (CSH, PIO2 = 70 Torr) for 7-9 days and measured ventilation in conscious, unrestrained animals before and after microinjecting the general NOS antagonist L-NG-Nitroarginine methyl ester into the nucleus tractus solitarius (NTS) or systemically injecting the nNOS-specific antagonist S-methyl-l-thiocitrulline. Localization of injection sites in the NTS was confirmed by histology following the experiment. We found that 1) neither NTS-specific nor systemic nNOS antagonism had any effect on hypoxia-mediated changes in breathing or metabolism (P > 0.05), but 2) nNOS protein expression was increased in the middle and caudal NTS by CSH. A persistent HVR after nNOS blockade in the NTS contrasts with results in awake mice, and our findings do not support the hypotheses that nNOS in the NTS contribute to the HVR or VAH in awake rats.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

The effect of combined glutamate receptor blockade in the NTS on the hypoxic ventilatory response in awake rats differs from the effect of individual glutamate receptor blockade

Ventilatory acclimatization to hypoxia (VAH) increases the hypoxic ventilatory response (HVR) and causes persistent hyperventilation when normoxia is restored, which is consistent with the occurrence of synaptic plasticity in acclimatized animals. Recently, we demonstrated that antagonism of individual glutamate receptor types (GluRs) within the nucleus tractus solitarii (NTS) modifies this plasticity and VAH (J. Physiol. 592(8):1839–1856); however, the effects of combined GluR antagonism remain unknown in awake rats. To evaluate this, we exposed rats to room air or chronic sustained hypobaric hypoxia (CSH, Pi_O2 = 70 Torr) for 7–9 days. On the experimental day, we microinjected artificial cerebrospinal fluid (ACSF: sham) and then a “cocktail” of the GluR antagonists MK‐801 and DNQX into the NTS. The location of injection sites in the NTS was confirmed by glutamate injections on a day before the experiment and with histology following the experiment. Ventilation was measured in awake, unrestrained rats breathing normoxia or acute hypoxia (10% O₂) in 15‐min intervals using barometric pressure plethysmography. In control (CON) rats, acute hypoxia increased ventilation; NTS microinjections of GluR antagonists, but not ACSF, significantly decreased ventilation and breathing frequency in acute hypoxia but not normoxia (P <0.05). CSH increased ventilation in hypoxia and acute normoxia. In CSH‐conditioned rats, GluR antagonists in the NTS significantly decreased ventilation in normoxia and breathing frequency in hypoxia. A persistent HVR after combined GluR blockade in the NTS contrasts with the effect of individual GluR blockade and also with results in anesthetized rats. Our findings support the hypotheses that GluRs in the NTS contribute to, but cannot completely explain, VAH in awake rats.

Ventilatory acclimatization to hypoxia involves plasticity in the central nervous system, as well as in arterial chemoreceptors. NMDA and AMPA glutamate receptors in the NTS contribute to different aspects of ventilatory acclimatization to hypoxia. However, total ionotropic glutamate receptor blockade in the NTS does not block acclimatization and the effects are not predictable from those of individual antagonists.

Glutamate receptors in the nucleus tractus solitarius contribute to ventilatory acclimatization to hypoxia in rat

When exposed to a hypoxic environment the body's first response is a reflex increase in ventilation, termed the hypoxic ventilatory response (HVR). With chronic sustained hypoxia (CSH), such as during acclimatization to high altitude, an additional time-dependent increase in ventilation occurs, which increases the HVR. This secondary increase persists after exposure to CSH and involves plasticity within the circuits in the central nervous system that control breathing. Currently these mechanisms of HVR plasticity are unknown and we hypothesized that they involve glutamatergic synapses in the nucleus tractus solitarius (NTS), where afferent endings from arterial chemoreceptors terminate. To test this, we treated rats held in normoxia (CON) or 10% O2 (CSH) for 7 days and measured ventilation in conscious, unrestrained animals before and after microinjecting glutamate receptor agonists and antagonists into the NTS. In normoxia, AMPA increased ventilation 25% and 50% in CON and CSH, respectively, while NMDA doubled ventilation in both groups (P < 0.05). Specific AMPA and NMDA receptor antagonists (NBQX and MK801, respectively) abolished these effects. MK801 significantly decreased the HVR in CON rats, and completely blocked the acute HVR in CSH rats but had no effect on ventilation in normoxia. NBQX decreased ventilation whenever it was increased relative to normoxic controls; i.e. acute hypoxia in CON and CSH, and normoxia in CSH. These results support our hypothesis that glutamate receptors in the NTS contribute to plasticity in the HVR with CSH. The mechanism underlying this synaptic plasticity is probably glutamate receptor modification, as in CSH rats the expression of phosphorylated NR1 and GluR1 proteins in the NTS increased 35% and 70%, respectively, relative to that in CON rats.

Chemosensory pathways in the brainstem controlling cardiorespiratory activity

Cardiorespiratory activity is controlled by a network of neurons located within the lower brainstem. The basic rhythm of breathing is generated by neuronal circuits within the medullary pre-Bötzinger complex, modulated by pontine and other inputs from cell groups within the medulla oblongata and then transmitted to bulbospinal pre-motor neurons that relay the respiratory pattern to cranial and spinal motor neurons controlling respiratory muscles. Cardiovascular sympathetic and vagal activities have characteristic discharges that are patterned by respiratory activity. This patterning ensures ventilation-perfusion matching for optimal respiratory gas exchange within the lungs. Peripheral arterial chemoreceptors and central respiratory chemoreceptors are crucial for the maintenance of cardiorespiratory homeostasis. Inputs from these receptors ensure adaptive changes in the respiratory and cardiovascular motor outputs in various environmental and physiological conditions. Many of the connections in the reflex pathway that mediates the peripheral arterial chemoreceptor input have been established. The nucleus tractus solitarii, the ventral respiratory network, pre-sympathetic circuitry and vagal pre-ganglionic neurons at the level of the medulla oblongata are integral components, although supramedullary structures also play a role in patterning autonomic outflows according to behavioural requirements. These medullary structures mediate cardiorespiratory reflexes that are initiated by the carotid and aortic bodies in response to acute changes in PO(2), PCO(2) and pH in the arterial blood. The level of arterial PCO(2) is the primary factor in determining respiratory drive and although there is a significant role of the arterial chemoreceptors, the principal sensor is located either at or in close proximity to the ventral surface of the medulla. The cellular and molecular mechanisms of central chemosensitivity as well as the neural basis for the integration of central and peripheral chemosensory inputs within the medulla remain challenging issues, but ones that have some emerging answers.

Ventilatory responses to acute hypoxia in neurokinin-1 receptor deficient mice

The regulatory effect of substance P on respiration is mediated via neurokinin (NK) receptors. While previous studies suggest that NK-1 receptors are involved, little is known about the role NK-2 receptors in ventilatory responses to hypoxia. Ventilatory responses to acute hypoxia (8% O2 in N2) were measured by indirect plethysmography in unanaesthetized, unrestrained NK-1 receptor gene deficient (NK-1-/-) and wild-type mice. In additional experiments mice were treated with an NK-2 receptor antagonist prior to hypoxic challenge. Resting ventilatory parameters were not different between groups. NK-1-/- mice displayed significantly greater shortening of expiratory time and higher increase of breathing frequency during hypoxia than wild-type mice. Treatment with the NK-2 receptor antagonist SR 48968 (1 mg/kg) resulted in a further shortening of inspiratory and expiratory time in NK-1-/- but not wild-type mice. These results demonstrate that both NK-1 and NK-2 receptors are involved in the modification of ventilation in response to acute hypoxia.

The murine neurokinin NK1 receptor gene contributes to the adult hypoxic facilitation of ventilation

Substance P and neurokinin-1 receptors (NK1) modulate the respiratory activity and are expressed early during development. We tested the hypothesis that NK1 receptors are involved in prenatal development of the respiratory network by comparing the resting respiratory activity and the respiratory response to hypoxia of control mice and mutant mice lacking the NK1 receptor (NK1-/-). In vitro and in vivo experiments were conducted on neonatal, young and adult mice from wild-type and NK1-/- strains. In the wild strain, immunohistological, pharmacological and electrophysiological studies showed that NK1 receptors were expressed within medullary respiratory areas prior to birth and that their activation at birth modulated central respiratory activity and the membrane properties of phrenic motoneurons. Both the membrane properties of phrenic motoneurons and the respiratory activity generated in vitro by brainstem-spinal cord preparation from NK1-/- neonate mice were similar to that from the wild strain. In addition, in vivo ventilation recordings by plethysmography did not reveal interstrain differences in resting breathing parameters. The facilitation of ventilation by short-lasting hypoxia was similar in wild and NK1-/- neonates but was significantly weaker in adult NK1-/- mice. Results demonstrate that NK1 receptors do appear to be necessary for a normal respiratory response to short-lasting hypoxia in the adult. However, NK1 receptors are not obligatory for the prenatal development of the respiratory network, for the production of the rhythm, or for the regulation of breathing by short-lasting hypoxia in neonates.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Medullary projections of rabbit carotid sinus nerve

In New Zealand white rabbits, cholera-toxin HRP was injected into the carotid sinus nerve just proximal to the carotid sinus. After survival periods of 3-5 days the rabbits were anesthetized and the brain fixed with aldehyde solution. Transverse sections were cut on a sledge microtome and the sections reacted with the tetramethylbenzidine procedure. HRP-positive fibers entered the ipsilateral dorsolateral medulla at the level of the acoustic tubercle, joining the tractus solitarius. Positive fibres were found principally ipsilaterally in four regions of the medulla: in the caudal two thirds of the nucleus tractus solitarius, in its dorsolateral regions and, more caudally, in its commissural subdivision; in the dorsolateral aspect of the spinal nucleus of the trigeminal nerve; in the region ventral and ventrolateral to the tractus solitarius (extending beyond the nucleus tractus solitarius); and in the ventrolateral medulla oblongata.

Evidence for the involvement of endogenous aspartate in the mediation of carotid chemoreceptor reflexes in the rostral ventrolateral medulla of the rat

2-Amino-5-phosphonovalerate (AP5; 153 pmol) injected into the rostral ventrolateral medulla (RVLM) inhibited pressor responses induced by carotid chemoreceptor stimulation. AP5 also inhibited pressor responses to aspartate (0.75 nmol) but not to glutamate (0.53 nmol) similarly injected. High K+ (50 mM) released endogenous aspartate and glutamate in a Ca2+-dependent manner from the RVLM. Chemoreceptor stimulation caused a release of aspartate but not of glutamate in the RVLM, and sinus nerve denervation abolished the release of aspartate. Increases in blood pressure induced by intravenous phenylephrine did not release aspartate. These results support the hypothesis that endogenous aspartate in the rat RVLM is involved in the mediation of chemoreceptor reflexes.

Central control mechanisms in hypertension

There is substantial evidence for an activation of the sympathetic nervous system in man as well as in genetic models of hypertension, such as the spontaneously hypertensive rat (SHR), but we are only beginning to understand the central mechanisms that generate changes in sympathetic activity and elevate blood pressure (BP). Significant recent advances have been made in defining the neural pathways involved in BP regulation and in identifying the neurotransmitters these neurones utilise. In this overview, we describe the neural pathways within the medulla oblongata and spinal cord that participate in BP control and examine the role of amino acid neurotransmitters within these pathways. We demonstrate how alterations in these pathways explain the sympathetic activation observed in the SHR and contribute to hypertension in this model. Lastly, we examine the application of modern molecular biological approaches to further our understanding of the neural regulation of the circulation. In these studies, we used the administration of antisense oligonucleotides to interrupt gene expression.

Prenatal cocaine exposure and postnatal hypoxia independently decrease carotid body dopamine in neonatal rats

The effects of prenatal cocaine exposure on the levels of carotid body dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were investigated in 5-day-old rat pups exposed to normoxic and hypoxic conditions. Timed-pregnant Sprague-Dawley rats were injected b.i.d. with either cocaine HCl (30 mg/kg) or isotonic saline (1 ml/kg) from gestational days 7-21. On the fifth postnatal day, pups were subjected to either 20 min of 0.21 or 0.08 fractional inspired oxygen (FlO2). Under a strictly timed protocol, both carotid bodies were removed from each pup, placed in an antioxidant solution to prevent DA breakdown, and subsequently analyzed via HPLC with electrochemical detection to determine carotid body DA and DOPAC content. Two-way ANOVA revealed decreases in DA in cocaine-exposed pups. No HVA was detectable in any of the samples. The 0.08 FlO2 condition decreased DA compared to 0.21 FlO2. The additive consequences of DA depletion resulting from the combination of prenatal cocaine and postnatal hypoxia decreased carotid body DA to 14% of control levels, with several animals exhibiting DA content below detection limits. Considering the role of the carotid body in the ventilatory response to hypoxia, these data suggest that prenatal cocaine exposure may adversely affect the normal chemoreceptive function of the carotid body.

Excitatory amino acid-mediated chemoreflex excitation of respiratory neurones in rostral ventrolateral medulla in rats

1. In anaesthetized rats, extracellular and intracellular recordings were made from 119 respiratory neurones in the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata. 2. Two types of active respiratory neurones were detected in RVL: expiratory (E) and pre-inspiratory (Pre-I), based on the relationship between their discharge and that of the phrenic nerve. Some Pre-I but none of the E neurones could be antidromically excited from the C(3)-C(4) level of the spinal cord. 3. E and Pre-I neurones of RVL were excited by stimulation of the arterial chemoreceptors by a close arterial injection of sodium cyanide. The reflex excitation of RVL E neurones was preceded by increased phrenic nerve activity, while the excitation of RVL Pre-I neurones preceded the increases in phrenic nerve activity. 4. The chemoreflex excitation of the two types of RVL respiratory neurones as well as their resting discharge was abolished or significantly depressed by microionophoresis of kynurenate, a wide-spectrum antagonist of excitatory amino acid receptors, while xanthurenate, an inactive analogue of kynurenate, was without effect. 5. In ventilated rats, bilateral microinjection into RVL of kynurenate, but not xanthurenate, abolished resting activity and chemoreflex excitation of phrenic nerve activity, whilst in spontaneously breathing rats, kynurenate microinjection into RVL produced apnea and silenced phrenic nerves. 6. We conclude: (a) chemoreflex excitation of the phrenic nerves is mediated by stimulating Pre-I neurones of RVL by excitatory amino acidergic inputs and (b) RVL Pre-I neurones may directly and/or indirectly excite spinal phrenic motor neurones and hence are involved in inspiratory rhythmogenesis.

Extracellular catechol and indole turnover in the nucleus of the solitary tract of spontaneously hypertensive and Wistar-Kyoto normotensive rats in response to drug-induced changes in arterial blood pressure

Drug-induced alterations in arterial blood pressure are reflected in the extracellular fluid neurotransmitter levels of the nucleus of the solitary tract (NTS). Urethane-anesthetized spontaneously hypertensive rats (SHRs) and Wistar-Kyoto normotensive (WKY) rats were used in this study. The extracellular neurochemical profile of the NTS was quantified using the in vivo microdialysis technique. In SHR, phenylephrine-induced hypertension produced no significant changes in the extracellular norepinephrine (NE) and dihydroxyphenylacetic acid concentrations, whereas a significant increase in the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) concentration was observed. Wistar normotensive rats, in response to phenylephrine-induced hypertension, showed a significant increase in extracellular NE and 5-HIAA concentrations. Hypotension produced by the intravenous infusion of nitroprusside failed to show significant changes in the extracellular neurotransmitters in both SHR and WKY rats. However, there was a significant increase in 5-HIAA concentration in SHRs during the rebound hypertension, which followed the nitroprusside-infused hypotension. No such change was observed in the case of the WKY rats. These results suggest the possible involvement of the serotonergic mechanisms of NTS in the regulation of normal arterial blood pressure in these two rat strains.

NMDA receptor-mediated transmission of carotid body chemoreceptor input to expiratory bulbospinal neurones in dogs

1. This study tested the hypothesis that excitatory amino acid receptors mediate the excitatory response of expiratory bulbospinal neurones to carotid body chemoreceptor inputs. 2. Studies were carried out in thiopental sodium anaesthetized, paralysed, ventilated, vagotomized dogs. 3. Brisk, short-duration chemoreceptor activation was produced by bilateral bolus injections of CO2-saturated saline (PCO2 > 700 mmHg) into the autoperfused carotid arteries. A pressurized-reservoir-solenoid valve system was used to deliver the CO2 bolus injections just prior to the onset of the neural expiratory phase, as determined from the phrenic neurogram, about once per minute. 4. Multibarrelled micropipettes were used to record neuronal unit activity and deliver neurotransmitter agents. Net responses of expiratory bulbospinal neurones to peripheral chemoreceptor activation were determined by subtracting the mean discharge frequencies (Fn) during three control expiratory cycles from the Fn during administration of a CO2 test bolus. The role of excitatory amino acid receptors in mediating this response was determined by comparing the baseline and bolus expiratory neuronal Fn before, during and after the pressure microejection of the NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) or the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl- benzo(f)quinoxaline (NBQX). Ejection rates of AP5 and NBQX were measured by monitoring the movement of the pipette meniscus. 5. AP5 reduced Fn during both the control and bolus cycles, as well as reducing the change in Fn between control and bolus cycles. NBQX had no effect on either baseline or bolus responses. 6. AP5 did not prevent excitation of expiratory bulbospinal neurones by AMPA. Coadministration of AMPA with AP5 prevented the AP5-mediated decrease in Fn but not the dose-dependent reduction in the CO2 bolus response. 7. Taken together, these data strongly suggest that the carotid chemoreceptor-mediated excitation of expiratory bulbospinal neurones is dependent on NMDA but not non-NMDA glutamate receptors.

Hypothalamic modulation of the arterial chemoreceptor reflex in the anaesthetized cat: role of the nucleus tractus solitarii

1. There is evidence in the literature of a mutual facilitatory interaction between the arterial chemoreceptor reflex and the alerting stage of the defence reaction, particularly in relation to the patterning of cardiorespiratory activity. The present study has been designed to test the hypothesis that a portion of this interaction involves synaptic interactions within the nucleus tractus solitarii (NTS). 2. The study has involved an analysis of the effective interactions between the stimulation of the arterial chemoreceptors and the hypothalamic defence area (HDA) on the activity of NTS neurones recorded in anaesthetized, paralysed and artificially ventilated cats. 3. A group of eighteen NTS neurones was classified as chemosensitive, on the basis of displaying EPSPs on sinus nerve stimulation (SN) and their failure to show an excitatory response to baroreceptor stimulation. Thirteen of these neurones displayed pronounced excitatory responses to chemoreceptor stimulation. In sixteen of these neurones HDA stimulation elicited an EPSP; in four of these sixteen neurones this early EPSP was followed by an IPSP. In the remaining two (of 18) neurones HDA stimulation provoked no obvious synaptic response but facilitated the efficacy of both chemoreceptor inputs and SN stimulation. 4. Neurones shown to receive convergent inputs from the arterial chemoreceptors (and SN stimulation) and HDA, often displayed excitatory responses to stimulation of other peripheral inputs. Vagally evoked EPSPs were observed in nine neurones, SLN-evoked responses in seven neurones and aortic nerve-evoked EPSPs in three neurones. 5. The organization of these synaptic interactions is discussed and these data are used to explain the pattern of interaction between chemoreceptor, baroreceptor and HDA inputs within the NTS. Conclusions are drawn regarding the functional role of different classes of NTS neurone, based on the findings in this and the accompanying two papers.

Nitric oxide as a retrograde messenger in the nucleus tractus solitarii of rats during hypoxia

1. We examined the role of nitric oxide (NO) in respiratory regulation in the nucleus tractus solitarii (NTS), where L-glutamate release associated with peripheral chemoreceptor activation modulates the hypoxic ventilatory response. 2. Experiments were performed in unanaesthetized freely moving rats. First, the effects on the hypoxic ventilatory response of sodium nitroprusside (SNP, a NO donor) or NG-monomethyl-L-arginine (L-NMMA, a NO synthase inhibitor), microinjected into the NTS, were investigated. Second, using in vivo microdialysis, changes in extracellular L-glutamate during hypoxia were examined in the presence of L-NMMA. Third, the effect of L-NMMA on ventilatory augmentation by exogenous L-glutamate was examined. Furthermore, we measured extracellular L-citrulline concentration changes during hypoxia in the NTS to assess NO formation indirectly and also examined the effect of MK-801 (an NMDA receptor antagonist) on L-citrulline levels during hypoxia. 3. SNP increased ventilation during both normoxia and hypoxia. L-NMMA did not alter ventilation or L-glutamate levels during normoxia but significantly attenuated the hypoxic ventilatory response and the increase in L-glutamate during hypoxia. The inhibition by L-NMMA was blocked by L-arginine. The ventilatory augmentation by exogenous L-glutamate was attenuated by L-NMMA. L-Citrulline increased during hypoxia, and this increase was inhibited by MK-801. 4. We provide the first in vivo evidence that, in the NTS, NO works as a retrograde messenger in an L-glutamate-releasing positive feedback system contributing to the augmentation of ventilation during hypoxia.

NMDA receptors are involved at the ventrolateral nucleus tractus solitarii for termination of inspiration

The purpose of the present study was to determine whether blockade of excitatory amino acid receptors at the ventrolateral nucleus of the tractus solitarius would influence respiratory activity. This was done by microinjecting excitatory amino acid receptor antagonists into the ventrolateral nucleus of the tractus solitarius of alpha-chloralose-anesthetized animals while monitoring respiratory activity using a Fleisch pneumotachograph and arterial blood pressure and heart rate. Bilateral microinjection of the NMDA receptor antagonist, 3-[(R)-carboxypiperazin-4-yl]-propyl-1- phosphomic acid (CPP), 5.62 nmol per side, produced an increase in inspiratory duration (+4 +/- 1.6 s, n = 8) which progressed to an apneustic pattern of breathing. Similar results were obtained with CPP microinjected into the ventrolateral nucleus of the tractus solitarius of three vagotomized animals. Bilateral microinjection of a second NMDA receptor antagonist, 2-amino-7-phosphono-heptanoic acid (AP7), 562 nmol per side, produced qualitatively similar effects on respiration as seen with CPP. In contrast, blockade of non-NMDA receptors with 6-cyano-7-nitroquinoxaline-2,3-dione (CNXQ), 0.125 nmol per side, had very little effect on respiration. Activation of NMDA receptors at the ventrolateral nucleus of the tractus solitarius with bilateral microinjection of NMDA, 39 pmol, produced a large increase in expiratory duration (+11 +/- 3 s, n = 8), and apnea during the expiratory phase of the respiratory cycle in half of the animals studied. Similar results were obtained with D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazol-proprionate (AMPA). These results indicate that an endogenous excitatory amino acid released at the ventrolateral nucleus of the tractus solitarius and acting at the NMDA receptor, plays a significant role in respiratory timing.

A review of the control of breathing during exercise

During the past 100 years many experimental investigations have been carried out in an attempt to determine the control mechanisms responsible for generating the respiratory responses observed during incremental and constant-load exercise tests. As a result of these investigations a number of different and contradictory control mechanisms have been proposed to be the sole mediators of exercise hyperpnea. However, it is now becoming evident that none of the proposed mechanisms are solely responsible for eliciting the exercise respiratory response. The present-day challenge appears to be one of synthesizing the proposed mechanisms, in order to determine the role that each mechanism has in controlling ventilation during exercise. This review, which has been divided into three primary sections, has been designed to meet this challenge. The aim of the first section is to describe the changes in respiration that occur during constant-load and incremental exercise. The second section briefly introduces the reader to traditional and contemporary control mechanisms that might be responsible for eliciting at least a portion of the exercise ventilatory response during these types of exercise. The third section describes how the traditional and contemporary control mechanisms may interact in a complex fashion to produce the changes in breathing associated with constant-load exercise, and incorporates recent experimental evidence from our laboratory.

Hypoxia and electrical stimulation of the carotid sinus nerve induce Fos-like immunoreactivity within catecholaminergic and serotoninergic neurons of the rat brainstem

A complete understanding of the neural mechanisms responsible for the chemoreceptor and baroreceptor reflexes requires precise knowledge of the locations and chemical phenotypes of higher-order neurons within these reflex pathways. In the present study, the protein product (Fos) of the c-fos protooncogene was used as a metabolic marker to trace central neural pathways following activation of carotid sinus nerve afferent fibers. In addition, immunohistochemical double-labeling techniques were used to define the chemical phenotypes of activated neurons. Both electrical stimulation of the carotid sinus nerve and physiological stimulation of the carotid bodies by hypoxia induced Fos-like immunoreactivity in catecholaminergic neurons containing tyrosine hydroxylase or phenylethanolamine-N-methyltransferase in the ventrolateral medulla oblongata and, to a lesser degree, in the dorsal vagal complex. Tyrosine hydroxylase/Fos colocalization was also observed in the locus coeruleus and the A5 noradrenergic cell group in pons. Many serotoninergic neurons in nucleus raphe pallidus, nucleus raphe magnus, and along the ventral medullary surface contained Fos-like immunoreactivity. In pons and midbrain, Fos-like immunoreactivity was observed in the lateral parabrachial and Kölliker-Fuse nuclei, the inferior colliculus, the cuneiform nucleus, and in the vicinity of the Edinger-Westphal nucleus, but no catecholaminergic or serotoninergic colocalization was observed in these regions. Although Fos-labeled cells were observed within and lateral to the dorsal raphe nucleus, few were catecholaminergic or serotoninergic. This study further defines a potential central neuroanatomical substrate for the chemoreceptor and/or baroreceptor reflexes.

Neural drives to breathing during exercise

This article presents the author's views about the neural drives to breathing during exercise. Two hypotheses are developed, the first being that the rapid changes in ventilation at the start and end of exercise are due to a fast neural drive whose magnitude is related to the frequency of limb movement. Experimental data are presented that this drive persists throughout exercise but declines as exercise continues. Second, the excessive increase in ventilation that occurs above the first ventilatory threshold during an incremental exercise test is due to a heavy exercise neural drive whose magnitude is related to the motor commands to the exercising muscles. Using the electromyographical activity of the working muscles as an index of the strength of the motor commands, experimental evidence is presented showing the coincidence of the first ventilatory threshold and that for the electromyographic activity of the working muscles during incremental exercise tests.

In vivo release of glutamate in nucleus tractus solitarii of the rat during hypoxia

1. An attempt has been made to test the hypothesis that, in the caudal part of nucleus tractus solitarii (NTS) where carotid sinus nerve (CSN) afferents project, L-glutamate (Glut) modulates the hypoxic ventilatory response. 2. Unanaesthetized, peripherally chemodenervated (carotid body denervated; CBD) and sham-operated, freely moving rats were used. During peripheral chemoreceptor stimulation by hypoxia (10% O2 for 30 min) or doxapram (Dox) infusion (2 mg kg-1 (30 min)-1), ventilation was recorded and successively, under the same conditions, the extracellular Glut concentration ([Glut]o) in the caudal NTS was measured by in vivo microdialysis. [Glut]o was also measured during hyperoxic hypercapnia (10% CO2-30% O2 for 30 min). 3. Furthermore, the effects on ventilation of exogenous Glut, the NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 or the ionotropic receptor antagonist kynurenate microinjected into the caudal NTS were investigated in sham-operated rats. 4. In sham-operated rats, both ventilation and [Glut]o in NTS were increased during peripheral chemoreceptor stimulation. On the other hand, no increases in either ventilation or Glut release were observed in CBD rats. In spite of ventilatory augmentation during hypercapnia, no response of [Glut]o to hypercapnia was observed in either group. 5. Local Glut application into NTS increased ventilation. Pretreatment with MK-801 or kynurenate reduced the hypoxic ventilatory response. This reduction in ventilation was mainly due to the decrease in tidal volume. 6. These results suggest that hypoxia induced the release of Glut in NTS and that this effect was mediated by arterial chemosensory input.

Excitatory amino acid receptors in the rostral ventrolateral medulla mediate hypertension induced by carotid body chemoreceptor stimulation

The rostral ventrolateral medulla (RVLM) is involved in the mediation of cardiovascular responses to peripheral chemoreceptor stimulation. To investigate whether excitatory amino acid inputs in the RVLM are related to the responses to chemoreceptor stimulation, we microinjected kynurenate, an amino acid antagonist, unilaterally into the RVLM and examined its effects on the pressor response to stimulation of carotid body chemoreceptors. Male Wistar rats were anesthetized with urethane, paralyzed and artificially ventilated. The carotid chemoreceptors were stimulated with isotonic solutions of inorganic phosphate solution. Stimulation of carotid body chemoreceptors produced increases in blood pressure. Kynurenate injected ipsilaterally but not contralaterally into the RVLM markedly inhibited the pressor response to chemoreceptor stimulation. In rats with spinal transection, stimulation of carotid body chemoreceptors also produced increases in blood pressure. The pressor response in rats with spinal transection was inhibited by intravenous injection of a vasopressin antagonist or by kynurenate injected ipsilaterally into the RVLM. Kynurenate injected into the RVLM inhibited the pressor response to NMDA, AMPA and kainate but not to acetylcholine in intact rats. These findings indicate that excitatory amino acid receptors are involved in mediating the pressor response to carotid body chemoreceptor stimulation in the rat RVLM. It appears that the chemoreceptor stimulation produces an increase in vasopressin release and the enhancement of vasopressin release is also mediated by an increase in excitatory amino acid inputs in the RVLM.

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, FluoroGold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei.

Reflex carotid body chemoreceptor control of phrenic sympathetic neurons

The reflex reaction of phrenic sympathetic neurons to stimulation of carotid body chemoreceptors was tested in chloralose-anesthetized and paralyzed cats with both vago-aortic nerves cut. During systemic hypoxia (animals ventilated with 10% O2 in N2) the sympathetic phrenic nerve activity increased from 100% in the control to 269%. This increase was markedly attenuated after cutting both sinus nerves. Reflex excitatory response in phrenic sympathetic neurons with the latency of 150 msec was evoked by electrical stimulation of the right carotid sinus nerve (3 pulses of 0.2 msec, 333 Hz). The central transmission time of the reflex was about 90 msec. Injecting 0.1 ml of 1 M NaHCO3 saturated with CO2 (in order to activate carotid body chemoreceptors) into the right or left carotid sinus, evoked excitatory responses in sympathetic neurons regardless of the side. The stimulation of carotid body chemoreceptors also increased somatic phrenic nerve activity. The three methods applied to the stimulation of carotid body chemoreceptors produced increase of phrenic nerve sympathetic activity.

Absence of respiration modulation of carotid sinus nerve inputs to nucleus tractus solitarius neurons receiving arterial chemoreceptor inputs

The reflex responses to activation of the arterial chemoreceptors are dependent upon when in the respiratory cycle the chemoreceptor stimulus is given. To determine if the respiratory modulation of the chemoreflex occurs within the nucleus tractus solitarius (NTS), intracellular recordings were obtained in pentobarbital-anesthetized, paralyzed and mechanically ventilated cats, from 22 non-respiratory NTS cells which were depolarized following activation of the ipsilateral carotid body chemoreceptors (by close arterial injection of < 100 microliters CO2 saturated bicarbonate). Activation of the ipsilateral carotid body chemoreceptors evoked depolarizations with amplitudes of 2.9-4.6 mV and durations of 2.1-5.9 s. Three of these cells also received a convergent excitatory input from the carotid sinus baroreceptors. Carotid sinus nerve (CSN) stimulation evoked either an excitatory post-synaptic potential (EPSPs) (n = 14, 8 monosynaptic) or an excitatory/inhibitory sequence (EPSP/IPSPs) (n = 8, 1 monosynaptic). CSN evoked PSPs were separately averaged (25-50 sweeps) during periods of phrenic nerve activity and phrenic nerve silence and during periods when the lungs were inflated and when the lungs were deflated. No parameter of the CSN evoked PSPs (latency, peak amplitude, duration) was altered during periods of phrenic nerve activity or lung inflation (all P values > 0.12, Wilcoxon signed-rank test). The results suggest that there is no respiratory modulation of arterial chemoreceptor inputs by either central respiratory drive or lung stretch receptor afferent inputs at this early stage of the reflex arc.

Structural types of spinal cord marginal (lamina I) neurons projecting to the nucleus of the tractus solitarius in the rat

The structural types of spinal cord marginal (lamina I) neurons projecting to the nucleus of the tractus solitarius (NTS) were studied. Upon injections of cholera toxin subunit B (CTb) into the caudal part of the NTS, including its lateral and medial portions, labeled cells occurred bilaterally in laminae I, IV-VII, and X, and the lateral spinal nucleus (LSN). After injections into the lateral portion alone, only a few cells were labeled in laminae V, VII, and X, and the LSN, and none in the superficial dorsal horn. Of 1882 labeled marginal cells, 38% belonged to the flattened type, 37% to the pyramidal type, and 25% to the fusiform type. Flattened and pyramidal cells were labeled in considerably greater numbers than those reported when other supraspinal targets of these cells were injected with CTb. Since cells in the NTS are known to be under marked gamma-aminobutyric acidergic (GABA-ergic) inhibition, it is possible that only strong input conveyed by great numbers of flattened and pyramidal cells is capable of overcoming that barrier. Fusiform cells were labeled in numbers similar to those observed previously after tracer injections into the two other targets of this neuronal type, the parabrachial nuclei and the lateral reticular nucleus. Considering that these regions, as well as the NTS, control cardiovascular and respiratory functions, it is suggested that fusiform cells transmit noxious input that will influence autonomic reflexes processed in the three nuclei.

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.

The afferent and efferent connections of the nucleus submedius in the rat

The afferent and efferent connections of the nucleus submedius (Sm) in the medial thalamus of the rat were examined. Injections of wheat-germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the Sm resulted in dense terminal labeling in the middle layers of the ipsilateral ventrolateral orbital cortex (VLO). Less dense labeling was also observed in the superficial and deep layers of VLO and in the medial part of the lateral orbital cortex (LO) and in the contralateral VLO. Retrogradely labeled neurons were observed primarily in the deep layers of VLO and the dorsal peduncular cortex (DP). Labeled neurons were also observed bilaterally, in the nucleus of the horizontal limb of the diagonal band, the lateral hypothalamus, the thalamic reticular nucleus (Rt), medial parabrachial nucleus (MPB), and the laterodorsal tegmental nucleus (LDT). Many labeled neurons were also observed in the trigeminal brain-stem complex. Injections of Fluoro-Gold (FG) into Sm resulted in a very similar distribution of retrogradely labeled neurons. Injections of WGA-HRP and FG in the orbital cortex confirmed the ipsilateral Sm projection to VLO and suggested that the middle and deep layers of VLO receive a specific ipsilateral projection from the dorsal Sm and that the superficial layers receive a projection primarily from the ventral Sm. Injections of WGA-HRP into the lateral hypothalamus, LDT, and MPB confirmed the retrograde labeling findings; the lateral hypothalamus was found to send a projection to the medial Sm, the LDT region to the ventromedial Sm and the MPB to the medial and dorsal Sm. These findings confirm and extend the results of previous studies in cat and rat indicating that Sm has a major and specific reciprocal connection with VLO. This finding, in conjunction with previous studies showing direct spinal and trigeminal inputs and the existence of nociceptive neurons in Sm and VLO, provides further support for a role of Sm in nociception.

Fos-like protein is induced in neurons of the medulla oblongata after stimulation of the carotid sinus nerve in awake and anesthetized rats

The protooncogene c-fos is expressed rapidly, transiently and polysynaptically within neurons in response to synaptic activation and voltage-gated calcium entry into the cell. The nuclear protein product of this gene (Fos) is detectable immunohistochemically 20-90 min after cell activation and remains within the nucleus for hours after expression. The present study was undertaken to identify cells within the rat medulla oblongata that express Fos-like protein in response to stimulation of afferent fibers of the carotid sinus nerve (CSN). Direct electrical stimulation of the CSN in anesthetized animals or hypoxic stimulation in either anesthetized or awake animals resulted in a consistent and discrete distribution of Fos-like immunoreactivity (Fos-LI). Fos-LI was observed bilaterally within nucleus tractus solitarius (NTS) and the ventrolateral medulla (VLM), within area postrema and nucleus raphe pallidus, and bilaterally along the ventral medullary surface. Unstimulated animals were devoid of Fos-LI within the medulla oblongata. Furthermore, neither the surgical preparations alone nor the effects of anesthesia could account for the extent of Fos-LI observed. We believe these cells represent second- and higher-order neurons within the baroreceptor and chemoreceptor reflex pathways.

Synaptic response of bulbar respiratory neurons to hypercapnic stimulation in peripherally chemodenervated cats

Effects of hypercapnia on the membrane potential and synaptic activity of bulbar respiratory neurons were studied in decerebrate, vagotomized, glomectomized and artificially ventilated cats. Coaxial multibarrelled electrodes were used for intracellular recording and extracellular iontophoresis of drugs. Hypoventilation with oxygen-enriched air (hyperoxic hypercapnia) produced an increase of depolarization together with an increase of spiking during the active phase and an increase of hyperpolarization during the inactive phase of each respiratory cycle in the inspiratory, postinspiratory and expiratory neurons of the ventral respiratory group. Both depolarizing and hyperpolarizing effects were associated with a decrease in input resistance. Intracellular injection of Cl- reversed the polarity of the hyperpolarizing synaptic wave to depolarization during the inactive phase, and hypercapnia increased the depolarization at that phase. Iontophoresis of tetrodotoxin eliminated the CO2-induced changes in membrane potential and input resistance. In 20 out of 58 neurons examined, iontophoretically applied atropine partly or totally suppressed the depolarizing response to hypercapnia. For these neurons, iontophoresed acetylcholine produced a sustained depolarization that was antagonized by atropine, but not by hexamethonium. The present study shows that both depolarizing and hyperpolarizing responses of medullary respiratory neurons to hyperoxic hypercapnia are synaptically mediated. A muscarinic mechanism is involved in part of the respiratory neuronal excitation evoked by hypercapnia.

Carotid chemosensory timing effects on cervical sympathetic discharges in the cat

The hypothesis that respiratory phase-related sympathetic nerve activity would manifest timing effect of carotid chemosensory input with respect to the central respiratory drive was tested in the anesthetized, paralyzed and artificially ventilated cats which were also vagotomized and tracheostomized. Preganglionic cervical sympathetic nerve fibers (PSNF) which were clearly responsive to carotid chemosensory stimulation were selected for the test. Simultaneously with the PSNF, phrenic nerve (PN) and internal intercostal expiratory nerve (IICEN) activities were also recorded. In most instances, carotid chemosensory discharges were monitored in order to register the precise timing of its input to the brain-stem. Timed stimulation of carotid chemoreceptors was elicited by bolus injections of cyanide (10-20 micrograms), nicotine (10-20 micrograms) and CO2-saturated saline (0.2-0.5 ml) into the base of the common carotid artery. Two types of PSNFs were identified: type I discharged only in synchrony with the PN activity (13/47) and type II fired independently of, but entrained to, PN activity (34/47). Type I displayed characteristic timing effects of carotid chemoreflex on PN discharges. However, the sympathetic activity did not share the after-discharge of PN which persisted beyond the carotid chemoreceptor stimulation. Type II did not manifest any timing effect of carotid chemoreflex with respect to PN and IICEN activities. These results suggest the following model of carotid chemoreflex effects on sympathetic neuron activities: a group is exclusively gated by the properties of central respiratory drive whereas the other is not gated but entrained by it in the spontaneously breathing anesthetized cat.

Efferent phrenic nerve and respiratory neuron activities in the developing kitten: spontaneous discharges and hypoxic responses

Efferent phrenic nerve and medullary respiratory neuron discharges were examined for age-dependent changes of activities during normocapnic hyperoxia and hypoxia in anesthetized and decerebrate kittens (22-150 days old). In animals less than 39 days of age, phrenic power spectra during hyperoxia were dominated by components in the medium-frequency band (20-50 Hz), whereas spectra of animals of at least 39 days of age were dominated by components in the high-frequency band (50-100 Hz). Such high-frequency oscillations were also observed in the power spectra of some inspiratory neurons in animals of at least 43 days old. In hypoxia, the amplitude of phrenic discharge exhibited an initial facilitation followed by a diminution (i.e. biphasic response) in animals 39 days old or younger. In animals older than 39 days, however, hypoxia elicited a sustained facilitation of phrenic discharge amplitude. In contrast, no such age-dependent change in response pattern to hypoxia was observed for neuronal discharges; rather, responses of most neurons consisted of either decreases of discharge frequency, or complete abolishment of discharges.

The role of chemical (CO2) drive in the apnea induced by high frequency ventilation in the cat

The purpose of the present study was to compare the relative roles of PaCO2 and pulmonary mechanoreceptors in the generation of apnea during high frequency ventilation (HFV) and conventional mechanical ventilation (CMV) in the anesthetized cat. Our hypothesis was that PaCO2 primarily determines the appearance of apnea, while mechanoreceptor input plays an important but secondary role. We calculated the tidal volume which would have a 50% chance of inducing apnea (VT50) for each combination of ventilator settings, and used it as a standard for comparison of mechanical dynamic inputs. When either 2% or 5% CO2 was added to the bias flow during HFV, the VT50 was significantly increased over that seen with a room air bias flow. Apnea was observed during either mode of ventilation only when PaCO2 was reduced below normocapnic levels. The mean tidal volume associated with each apnea group was larger than that for the corresponding nonapnea group. There was no difference in the PaCO2 for apnea between HFV and CMV, but the EMG of the diaphragm did differ. With HFV-apnea tonic activity was observed, while CMV-apnea showed abolition of all activity. We conclude that both HFV- and CMV-induced apnea depend primarily on lowering the chemical drive (PaCO2), and secondarily on inhibitory mechanical input (VT). The excitation of rapidly adapting receptors during HFV could explain the tonic EMG activity.