Cardiorespiratory changes induced by vertebral artery injection of sodium cyanide in cats

The need for specificity in quantifying neurocirculatory vs. respiratory effects of eucapnic hypoxia and transient hyperoxia

KEY POINTS

The carotid chemoreceptor mediates the ventilatory and muscle sympathetic nerve activity (MSNA) responses to hypoxia and contributes to tonic sympathetic and respiratory drives. It is often presumed that both excitatory and inhibitory tests of chemoreflex function show congruence in the end-organ responses. Ventilatory and neurocirculatory (MSNA, blood pressure and heart rate) responses to chemoreflex inhibition elicited by transient hyperoxia and to chemoreflex excitation produced by steady-state eucapnic hypoxia were measured in a cohort of 82 middle-aged individuals. Ventilatory and MSNA responsiveness to hyperoxia and hypoxia were not significantly correlated within individuals. It was concluded that ventilatory responses to hypoxia and hyperoxia do not predict MSNA responses and it is recommended that tests using the specific outcome of interest, i.e. MSNA or ventilation, are required. Transient hyperoxia is recommended as a sensitive and reliable means of quantifying tonic chemoreceptor-driven levels of sympathetic nervous system activity and respiratory drive.

ABSTRACT

Hypersensitivity of the carotid chemoreceptor leading to sympathetic nervous system activation and ventilatory instability has been implicated in the pathogenesis and consequences of several common clinical conditions. A variety of treatment approaches aimed at lessening chemoreceptor-driven sympathetic overactivity are now under investigation; thus, the ability to quantify this outcome variable with specificity and precision is crucial. Accordingly, we measured ventilatory and neurocirculatory responses to chemoreflex inhibition elicited by transient hyperoxia and chemoreflex excitation produced by exposure to graded, steady-state eucapnic hypoxia in middle-aged men and women (n = 82) with continuous positive airway pressure-treated obstructive sleep apnoea. Progressive, eucapnic hypoxia produced robust and highly variable increases in ventilation (+83 ± 59%) and muscle sympathetic nerve activity (MSNA) burst frequency (+55 ± 31%), whereas transient hyperoxia caused marked reductions in these variables (-35 ± 14% and -42 ± 16%, respectively). Coefficients of variation for ventilatory and MSNA burst frequency responses, indicating test-retest reproducibility, were respectively 9% and 24% for hyperoxia and 35% and 28% for hypoxia. Based on statistical measures of rank correlation or even comparisons across quartiles of corresponding ventilatory and MSNA responses, we found that the magnitudes of ventilatory inhibition with hyperoxia or excitation with eucapnic hypoxia were not correlated with corresponding MSNA responses within individuals. We conclude that, in conscious, behaving humans, ventilatory sensitivities to progressive, steady-state, eucapnic hypoxia and transient hyperoxia do not predict MSNA responsiveness. Our findings also support the use of transient hyperoxia as a reliable, sensitive, measure of the carotid chemoreceptor contribution to tonic sympathetic nervous system activity and respiratory drive.

Role of baroreceptors in the zone of vertebral arteries in the reflex regulation of venous tone in the splanchnic basin

Pressure elevation in the reflexogenic zone of vertebral arteries was accompanied by a decrease in the tone of splanchnic veins, reduction of blood pressure, and suppression of external respiration. An opposite response of the cardiorespiratory functional system was observed under conditions of low baseline pressure in the vascular zone. Our findings and results of previous physiological and morphological studies indicate that the capacitance vessels play an important role in cardiorespiratory reactions.

Yawning responses induced by local hypoxia in the paraventricular nucleus of the rat

Yawing was induced by microinjections of L-glutamate, cyanide and a nitric oxide-releasing compound (NOC12) into the paraventricular nucleus of the hypothalamus (PVN) in anesthetized, spontaneously breathing rats. To evaluate physiological aspects of yawning, we monitored intercostal electromyogram (EMG) as an index of inspiratory activity, digastric EMG, blood pressure and electrocorticogram (ECoG). Microinjection of L-glutamate in the medial parvocellular subdivision (mp) elicited a stereotyped yawning response, i.e. an initial depressor response and an arousal shift in ECoG followed by a single large inspiration with mouth opening. The same sequential events were observed during spontaneous yawning, indicating that the mp is responsible for triggering yawning. Microinjection of cyanide into the mp caused the same yawning responses as the ones elicited by microinjection of L-glutamate, suggesting that the mp is sensitive to chemical hypoxia or ischemia within the PVN. Microinjection of NOC12 into the mp elicited a single large inspiration with a variable onset delay, suggesting that diffusible nitric oxide (NO) within the mp may act as a paracrine agent to cause a yawning response. We hypothesize that the mp of the PVN contains an oxygen sensor that causes a yawning response.

State influences on ventral medullary surface and physiological responses to sodium cyanide challenges

Intravenous sodium cyanide (NaCN) administration lowers ventral medullary surface (VMS) activity in anesthetized cats. Sleep states modify spontaneous and blood pressure-evoked VMS activity and may alter VMS responses to chemoreceptor input. We studied VMS activation during peripheral chemoreceptor stimulation by intravenous NaCN using optical procedures in six cats instrumented for recording sleep physiology during sham saline and control site trials. Images of scattered 660-nm light were collected at 50 frames/s with an optical device after 80-100 microg total bolus intravenous NaCN delivery during waking and sleep states. Cyanide elicited an initial ventilatory decline, followed by large inspiratory efforts and an increase in respiratory rate, except in rapid eye movement sleep, in which an initial breathing increase occurred. NaCN evoked a pronounced decrease in VMS activity in all states; control sites and sham injections showed little effect. The activity decline was faster in rapid eye movement sleep, and the activity nadir occurred later in waking. Sleep states alter the time course but not the extent of decline in VMS activity.

Excitation of phrenic and sympathetic output during acute hypoxia: contribution of medullary oxygen detectors

Severe brain hypoxia results in respiratory excitation and an increase in sympathetic nerve activity. Respiratory excitation takes the form of gasping which is characterized by an abrupt onset, high amplitude, short duration burst of inspiratory activity. Recent evidence suggests that centrally-mediated hypoxic respiratory and sympathetic excitation may result from direct hypoxic stimulation of discrete hypoxia chemosensitive sites in the medulla. Thus, medullary regions involved in the generation and modulation of respiratory and sympathetic vasomotor output may contain neurons which function as central oxygen detectors, acting as medullary analogs to the peripheral (arterial) chemoreceptors. This review focuses on the medullary sites and mechanisms proposed to mediate hypoxic respiratory and sympathetic excitation in anesthetized, chemodeafferented animals, and provides the evidence suggesting a role for central oxygen detectors in the control of breathing and sympathetic vasomotor output.

Hypoxic excitation in neurons cultured from the rostral ventrolateral medulla of the neonatal rat

Neurons within cardiorespiratory regions of the rostral ventrolateral medulla (RVLM) have been shown to be excited by local hypoxia. To determine the electrophysiological properties of these excitatory responses to hypoxia, we developed a primary dissociated cell culture system to examine the intrinsic response of RVLM neurons to hypoxia. Neonatal rat neurons plated on medullary astrocyte monolayers were studied using the whole cell perforated patch-clamp technique. Sodium cyanide (NaCN, 0.5-10 mM) was used, and membrane potential (V(m)), firing frequency, and input resistance were examined. In 11 of 19 neurons, NaCN produced a V(m) depolarization, an increase in firing frequency, and a decrease in input resistance, suggesting the opening of a cation channel. The hypoxic depolarization had a linear dose response and was dependent on baseline V(m), with a greater response at more hyperpolarized V(m). In 8 of 19 neurons, NaCN produced a V(m) hyperpolarization, decrease in firing frequency, and variable changes in input resistance. The V(m) hyperpolarization exhibited an all-or-none dose response and was independent of baseline V(m). These differential responses to NaCN were retained after synaptic blockade with low Ca(2+)-high Mg(2+) or TTX. Thus hypoxic excitation 1) is maintained in cell culture, 2) is an intrinsic response, and 3) is likely due to the increase in a cation current. These hypoxia-excited neurons are likely candidates to function as central oxygen sensors.

Pre-Bötzinger complex functions as a central hypoxia chemosensor for respiration in vivo

Recently, we identified a region located in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) in which activation of ionotropic excitatory amino acid receptors using DL-homocysteic acid (DLH) elicits a variety of excitatory responses in the phrenic neurogram, ranging from tonic firing to a rapid series of high-amplitude, rapid rate of rise, short-duration inspiratory bursts that are indistinguishable from gasps produced by severe systemic hypoxia. Therefore we hypothesized that this unique region is chemosensitive to hypoxia. To test this hypothesis, we examined the response to unilateral microinjection of sodium cyanide (NaCN) into the pre-BötC in chloralose- or chloralose/urethan-anesthetized vagotomized, paralyzed, mechanically ventilated cats. In all experiments, sites in the pre-BötC were functionally identified using DLH (10 mM, 21 nl) as we have previously described. All sites were histologically confirmed to be in the pre-BötC after completion of the experiment. Unilateral microinjection of NaCN (1 mM, 21 nl) into the pre-BötC produced excitation of phrenic nerve discharge in 49 of the 81 sites examined. This augmentation of inspiratory output exhibited one of the following changes in cycle timing and/or pattern: 1) a series of high-amplitude, short-duration bursts in the phrenic neurogram (a discharge similar to a gasp), 2) a tonic excitation of phrenic neurogram output, 3) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a gasplike burst), or 4) an increase in frequency of phrenic bursts accompanied by small increases or decreases in the amplitude of integrated phrenic nerve discharge. Our findings identify a locus in the brain stem in which focal hypoxia augments respiratory output. We propose that the respiratory rhythm generator in the pre-BötC has intrinsic hypoxic chemosensitivity that may play a role in hypoxia-induced gasping.

Rhythmic sympathetic nerve discharges in an in vitro neonatal rat brain stem-spinal cord preparation

To understand the origination of sympathetic nerve discharge (SND), I developed an in vitro brain stem-spinal cord preparation from neonatal rats. Ascorbic acid (3 mM) was added into the bath solution to increase the viability of preparations. At 24 degrees C, rhythmic SND (recorded from the splanchnic nerve) was consistently observed, but it became quiescent at <16 degrees C. Respiratory-related SND (rSND) was discernible and was well correlated with C(4) root activity. Power spectral analysis of SND revealed a dominant 2-Hz oscillation. In most preparations (86%), such oscillation was persistent, whereas it only slightly reduced its magnitude after isolation from the brain stem. The removal of neural structures rostral to the superior cerebellar artery (equivalent to the level of facial nuclei) reduced rSND, increased tonic SND, but did not affect the temporal coupling between SND and C(4) root activity. Our data suggest a prominent contribution of SND from the neural mechanisms confined within the neonatal rat spinal cord. This ascorbic acid-enhanced in vitro preparation is a very useful model to study neural mechanisms underlying sympathorespiratory integration.

Effects of oxygen deprivation on parapyramidal neurons of the ventrolateral medulla in the rat

We characterized the electrophysiological properties and responses of neurons located in the parapyramidal region of the ventral aspect of the rat medulla oblongata (parapyramidal neurons, PP neurons) to oxygen deprivation, in order to understand the mechanisms involved in hypoxia induced respiratory depression. The responses of PP neurons to oxygen deprivation were compared to those of the functionally dissimilar neurons of the dentate gyrus (DG). Neurons from the PP region were found to fire spontaneously with a frequency of 3-3.5 spikes/sec in both adults and neonates and responded to an anoxic insult with a complete loss of spontaneous firing. Discrete metabolite analysis showed a small (about 17%) decrease in tissue adenosine triphosphate (ATP) levels of the PP neurons during an anoxic insult and the decrease was significantly smaller than in the DG cell region (28%). In contrast to the DG neurons, the PP neurons recovered from an anoxic insult lasting more than 30 min, indicating a greater survival capacity of the PP neurons during oxygen deprivation. The PP neurons were also capable of withstanding successive anoxic insults better than the DG cells as demonstrated by their complete recovery following reoxygenation. It is suggested that the PP neurons may depress their electrical activity as an energy conservation mechanism, and thereby survive anoxic insults longer than the dentate neurons, whereas the loss of cellular activity in the DG neurons may be a result of energy depletion.

Response of tracheal smooth muscle tone to lower brain stem hypoxia in dogs

We examined effects of central hypoxia on tracheal smooth muscle (TSM) tone, phrenic nerve activity (PNA) and blood pressure (BP) in decerebrated, paralysed, and artificially ventilated dogs. Central hypoxia was induced by injection of N2-saturated saline (5 ml; PO, 25-32 torr) through a catheter in the vertebral artery. The effects of central hypoxia were compared with the responses to central chemoreceptors stimulation, namely central hypercapnia induced by intravertebral injection of high CO2 saline (5 ml; PCO2 90-100 torr, PO2 80-120 torr, pH 7.38-7.42) buffered by HCO3-. Central hypoxia caused relaxation of TSM accompanied by depression of PNA and elevation of BP. In contrast, central hypercapnia evoked tracheal constriction along with respiratory excitation and pressor response. The tracheal relaxation in response to central hypoxia occurred with onset and peak latencies similar to those observed in PNA depression and BP elevation. This suggests a common source for the synaptic inputs to three distinct control systems involved in cardiovascular, respiratory and airway functions. Such neuronal substrate is considered to be activated by central hypoxia.

Ventral medullary neuronal responses to peripheral chemoreceptor stimulation

Recent findings suggest that carotid chemoreceptor input into the ventral medullary surface intermediate area during hypoxia is inhibitory (Gozal et al., (1994) Neurosci. Lett. 178, 73-76. However, systemic hypoxia is a complex stimulus, and effects of carotid chemoreceptor stimulation per se on intermediate ventral medullary surface neuronal activity are difficult to isolate. Therefore, we studied neural activation of the intermediate ventral medullary surface during peripheral chemoreceptor stimulation by intravenous sodium cyanide using optical procedures in seven pentobarbital-anesthetized cats. Control recordings were also acquired in the suprasylvian cortex of three cats. Images of reflected 660 nm light were collected at l/s with a charge-coupled device camera, triggered by the cardiac R wave, after 0.0, 0.5, 2, 5, 10, 20 and 40 micrograms/kg i.v. sodium cyanide administration before and following carotid sinus denervation. Sodium cyanide doses > 5 micrograms/kg significantly increased ventilation, an effect which was eliminated following carotid sinus denervation. A pronounced, dose-dependent activity decrease within the intermediate ventral medullary surface occurred within seconds of sodium cyanide administration, with subsequent return to baseline. Carotid sinus denervation eliminated rapid-onset neural responses to all sodium cyanide doses. However, at the 40 micrograms/kg dose, a smaller, slower onset (25 s), activity decrease occurred both pre- and postdenervation. In the neocortex, the sodium cyanide-induced fast responses were absent. Intravenous cyanide, acting via a carotid sinus nerve pathway, results in a dose-dependent decrease in neural activity within the intermediate ventral medullary surface of cats. High-dose sodium cyanide also appears to decrease intermediate ventral medullary surface neural activity directly.

In vivo and in vitro responses of neurons in the ventrolateral medulla to hypoxia

Neurons in the ventrolateral medulla (VLM) are known to be involved in several cardiorespiratory reflexes and to provide tonic drive to sympathetic preganglionic neurons. Recent studies have suggested that VLM neurons modulate the respiratory responses to hypoxia and to hypercapnia. The purpose of the present study was to determine with electrophysiological techniques if the discharge of these neurons is altered by hypoxia and/or by hypercapnia both in vivo and in vitro. Extracellular single-unit activity of VLM neurons (n = 39) was recorded during inhalation of a hypoxic gas (10% O2) and during inhalation of a hypercapnic gas (5% CO2) in anesthetized, spontaneously breathing rats (n = 16). Hypoxia elicited an increase in the discharge frequency in 64% of the VLM neurons studied; hypercapnia stimulated 42% of the neurons. Fifty-two percent of the neurons were stimulated by both hypoxia and hypercapnia. Signal averaging revealed that 76% of the hypoxia-stimulated neurons had a resting discharge related to the cardiac and/or respiratory cycle. Similar percentages of VLM neurons (35/54) were stimulated by hypoxia in a second group of animals (n = 14) that were studied after sinoaortic denervation. A rat brain slice preparation was then used to determine if hypoxia exerts a direct effect upon neurons in the VLM. Perfusing a hypoxic gas over the surface of medullary slices evoked an increase in the discharge frequency in the majority (39/49) of VLM neurons studied; responses were graded in relation to the magnitude of the hypoxic stimulus. Similar responses to hypoxia were observed in VLM neurons studied during perfusion with a synaptic blockade medium. Retrograde labeling of VLM neurons with rhodamine tagged microspheres injected into the thoracic intermediolateral cell column demonstrated that the hypoxia sensitive neurons were located in a region of the VLM that projects to the thoracic spinal cord. These results demonstrate that neurons in the ventrolateral medulla are excited by a direct effect of hypoxia; these neurons may play a critical role in the cardiorespiratory responses to hypoxia.

Intramedullary sodium cyanide injection on respiratory and vasomotor responses in cats

To examine the effect of hypoxia confined to the ventrolateral medulla we microinjected NaCN into the cat medulla (1.0 mm below the ventral surface) unilaterally and investigated cardio-respiratory changes. We studied anesthetized artificially ventilated animals and measured the electrical activity of phrenic and cervical sympathetic nerves and blood pressure. Histotoxic hypoxia depressed phrenic amplitude and elevated sympathetic tone and blood pressure. These responses were obtained predominantly from the region 5.0-8.0 mm caudal to the foramen caecum and 3.0-5.0 mm lateral to the midline (intermediate area). A study with 14C-cyanide showed that total and covalently bound cyanide was confined within a 1 mm diffusion sphere following microinjection. Isolated areas in both rostral and caudal medulla responded to cyanide with elevated sympathetic tone in the absence of phrenic nerve depression, suggesting dissociation of respiratory and vasomotor responses to hypoxia. Thus, the respiratory depression and vasomotor excitation produced by central hypoxia can be reproduced by hypoxia limited to discrete regions of the ventrolateral medulla.