Genesis of gasping is independent of levels of serotonin in the Pet-1 knockout mouse

Eupnea is normal breathing. If eupnea fails, as in severe hypoxia or ischemia, gasping is recruited. Gasping can serve as a powerful mechanism for autoresuscitation. A failure of autoresuscitation has been proposed as a basis of the sudden infant death syndrome. In an in vitro preparation, endogenous serotonin is reported to be essential for expression of gasping. Using an in situ preparation of the Pet-1 knockout mouse, we evaluated such a critical role for serotonin. In this mouse, the number of serotonergic neurons is reduced by 85-90% compared with animals without this homozygous genetic defect. Despite this reduction in the number of serotonergic neurons, phrenic discharge in eupnea and gasping of Pet-1 knockout mice was not different from that of wild-type mice. Indeed, gasping continued unabated, even after administration of methysergide, a blocker of many types of receptors for serotonin, to Pet-1 knockout mice. We conclude that serotonin is not critical for expression of gasping. The proposal for such a critical role, on the basis of observations in the in vitro slice preparation, may reflect the minimal functional neuronal tissue and neurotransmitters in this preparation, such that the role of any remaining neurotransmitters is magnified. Also, rhythmic activity of the in vitro slice preparation has been characterized as eupnea or gasping solely on the basis of activity of the hypoglossal nerve or massed neuronal activities of the ventrolateral medulla. The accuracy of this method of classification has not been established.

Gestational cigarette smoke exposure and hyperthermic enhancement of laryngeal chemoreflex in rat pups

Laryngeal chemoreflex (LCR) apnea occurs in infant mammals of many species in response to water or other liquids in the laryngeal lumen. The apnea can last for many seconds, sometimes leading to dangerous hypoxemia, and has therefore been considered as a possible mechanism in the Sudden Infant Death Syndrome (SIDS). We have found recently that this reflex is markedly prolonged in decerebrate piglets and anesthetized rat pups that are warmed 1-3 degrees C above their normal body temperatures. We intermittently exposed pregnant rats to cigarette smoke and examined the LCR in their four- to fifteen-day-old offspring under general anesthesia, with and without whole body warming. During warming, pups of gestationally smoke-exposed dams had significantly longer LCR-induced respiratory disruption than similarly warmed control pups. The results may be significant for the pathogenesis and/or prevention of SIDS as maternal cigarette smoking during human pregnancy and heat stress in infants are known risk factors for SIDS.

Elevated concentrations of ethinylestradiol, 17beta-estradiol, and medroxyprogesterone have little effect on reproduction and survival of Ceriodaphnia dubia

Wastewater effluent contains synthetic and natural hormones, often in complex mixtures, that may be associated with reproductive abnormalities in fish and other aquatic biota. We exposed the sentinel invertebrate Ceriodaphnia dubia to the natural estrogen 17beta-estradiol (E(2)), a synthetic estrogen, ethinylestradiol (EE(2)), and a synthetic progestin, medroxyprogesterone in a 7-day test. These compounds had no significant effect on reproduction or survival even at 10(6) times the concentrations at which reproductive effects have been documented in several fish species. C. dubia is routinely used for screening the toxicity of wastewater effluent. However, in the standard chronic 7-day exposure the endpoints of survival and reproduction were insensitive to several synthetic and natural vertebrate hormones. The C. dubia 7-day chronic toxicity test is probably not a useful monitoring tool for vertebrate hormones and their pharmaceutical analogs unless other sensitive endpoints such as maturation rates, molt frequency, and offspring sex ratios are incorporated in a practical manner.

An adenosine A(2A) antagonist injected in the NTS reverses thermal prolongation of the LCR in decerebrate piglets

Hyperthermia prolongs the laryngeal chemoreflex (LCR). Under normothermic conditions, adenosine antagonists shorten and adenosine A(2A) (Ad-A(2A)) agonists prolong the LCR. Therefore, we tested the hypothesis that SCH-58261, an Ad-A(2A) receptor antagonist, would prevent thermal prolongation of the LCR when injected unilaterally within the nucleus of the solitary tract (NTS). We studied decerebrate piglets aged 4-13 days. We elicited the LCR by injecting 0.1ml of water into the larynx and recorded integrated phrenic nerve activity. The laryngeal chemoreflex was prolonged when the body temperature of each piglet was raised approximately 2.5 degrees C, and SCH-58261 reversed the thermal prolongation of the LCR when injected into the NTS (n=13), but not when injected in the nucleus ambiguus (n=9). Injections of vehicle alone into the NTS did not alter the thermal prolongation of the LCR (n=9). We conclude that activation of adenosine receptors, perhaps located on GABAergic neurons in the NTS, contributes to thermal prolongation of the LCR.

The effects of environmentally relevant mixtures of estrogens on Japanese medaka (Oryzias latipes) reproduction

Wastewater effluent contains a variety of estrogenic compounds that vary in potency, but each of which contributes to the overall estrogenicity of the effluent. We hypothesized that the effects of mixtures of estrogens on reproduction in pair breeding medaka (Oryzias latipes) could be predicted by their relative estrogenicity. Relative estrogenicity was defined by the ability of estrogenic compounds to induce vitellogenesis in various species of male fish. We exposed reproducing pairs of medaka to mixtures of the environmental estrogens nonylphenol (NP), 17alpha-ethinylestradiol (EE(2)), and natural 17beta-estradiol (E(2)), as well as treatments of equivalent estrogenicity that were composed of E(2) alone. Reproducing medaka exposed to mixtures of estrogenic compounds and equipotent treatments of estradiol alone had very similar responses in mortality and reproduction (fecundity, number of spawns). However, mixtures of NP, E(2), and EE(2) elicited lower vitellogenic induction than equipotent concentrations of E(2) alone. Therefore, relative estrogenicity was a good model for predicting some, but not all, reproductive responses, and simple additive mixture models may not predict all relevant physiological responses.

Glial modulation of CO2 chemosensory excitability in the retrotrapezoid nucleus of rodents

We investigated the possibility that astrocytes modify the extracellular milieu and thereby modify the activity of central CO2 chemosensory neurons. The ability of astrocytes to modify the extracellular milieu is heterogeneously distributed among chemosensory sites that have, at least nominally, the same function. The differences in astrocytic activity may make some central chemosensory sites better attuned to the local brain tissue environment and other chemosensory sites better suited to integrate chemosensory activity from multiple sites within and outside the central nervous system.

A computer model of mammalian central CO2 chemoreception

We developed a single compartment model of a mammalian CO2 sensitive neuron and tested the hypothesis that pH-dependent inhibition of multiple potassium channels contributes to CO2 sensitivity. pH-dependent inhibition of potassium channels by either intracellular or extracellular pH was sufficient to alter neuronal activity, but changes in neither intracellular nor extracellular pH are required to elicit a neuronal response to hypercapnic stimulation.

Laryngeal apnea in rat pups: effects of age and body temperature

In neonatal mammals of many species, including human infants, apnea and other reflex responses frequently arise from stimulation of laryngeal receptors by ingested or regurgitated liquids. These reflexes, mediated by afferents in the superior laryngeal nerves (SLNs), are collectively known as the laryngeal chemoreflex (LCR) and are suspected to be responsible for some cases of the sudden infant death syndrome (SIDS). The LCR is strongly enhanced by mild increases in body temperature in decerebrate piglets, a finding that is of interest because SIDS victims are often found in overheated environments. Because of the experimental advantages of studying reflex development and mechanisms in neonatal rodents, we have developed methods for eliciting laryngeal apnea in anesthetized rat pups and have examined the influence of mild hyperthermia in animals ranging in age from 3 to 21 days. We found that apnea and respiratory disruption, elicited either by intralaryngeal water or by electrical stimulation of the SLN, occurred at all ages studied. Raising body temperature by 2–3°C prolonged the respiratory disturbance in response to either stimulus. This effect of hyperthermia was prominent in the youngest animals and diminished with age. We conclude that many studies of the LCR restricted to larger neonatal animals in the past can be performed in infant rodents using appropriate methods. Moreover, the developmental changes in the LCR and in the thermal modulation of the LCR seem to follow different temporal profiles, implying that distinct neurophysiological processes may mediate the LCR and thermal prolongation of the LCR.

Maintenance of gasping and restoration of eupnea after hypoxia is impaired following blockers of alpha1-adrenergic receptors and serotonin 5-HT2 receptors

In severe hypoxia or ischemia, normal eupneic breathing fails and is replaced by gasping. Gasping serves as part of a process of autoresuscitation by which eupnea is reestablished. Medullary neurons, having a burster, pacemaker discharge, underlie gasping. Conductance through persistent sodium channels is essential for the burster discharge. This conductance is modulated by norepinephrine, acting on alpha 1-adrenergic receptors, and serotonin, acting on 5-HT2 receptors. We hypothesized that blockers of 5-HT2 receptors and alpha 1-adrenergic receptors would alter autoresuscitation. The in situ perfused preparation of the juvenile rat was used. Integrated phrenic discharge was switched from an incrementing pattern, akin to eupnea, to the decrementing pattern comparable to gasping in hypoxic hypercapnia. With a restoration of hyperoxic normocapnia, rhythmic, incrementing phrenic discharge returned within 10 s in most preparations. Following addition of blockers of alpha 1-adrenergic receptors (WB-4101, 0.0625-0.500 microM) and/or blockers of 5-HT2 (ketanserin, 1.25-10 microM) or multiple 5-HT receptors (methysergide, 3.0-10 microM) to the perfusate, incrementing phrenic discharge continued. Fictive gasping was still induced, although it ceased after significantly fewer decrementing bursts than in preparations than received no blockers. Moreover, the time for recovery of rhythmic activity was significantly prolonged. This prolongation was in excess of 100 s in all preparations that received both WB-4101 (above 0.125 microM) and methysergide (above 2.5 microM). We conclude that activation of adrenergic and 5-HT2 receptors is important to sustain gasping and to restore rhythmic respiratory activity after hypoxia-induced depression.

Unilateral microdialysis of gabazine in the dorsal medulla reverses thermal prolongation of the laryngeal chemoreflex in decerebrate piglets

The laryngeal chemoreflex (LCR) is elicited by water in the larynx and leads to apnea and respiratory disruption in immature animals. The LCR is exaggerated by the elevation of brain temperature within or near the nucleus of the solitary tract (NTS) in decerebrate piglets. Thermal prolongation of reflex apnea elicited by superior laryngeal nerve stimulation is reduced by systemic administration of GABA(A) receptor antagonists. Therefore, we tested the hypothesis that microdialysis within or near the NTS of gabazine, a GABA(A) receptor antagonist, would reverse thermal prolongation of the LCR. We examined this hypothesis in 21 decerebrate piglets (age 3-13 days). We elicited the LCR by injecting 0.1 ml of water into the larynx before and after each piglet's body temperature was elevated by approximately 2.5 degrees C and before and after 2-5 mM gabazine was dialyzed unilaterally and focally in the medulla. Elevated body temperature failed to prolong the LCR in one piglet, which was excluded from analysis. Elevated body temperature prolonged the LCR in all the remaining animals, and dialysis of gabazine into the region near the NTS (n = 10) reversed the thermal prolongation of the LCR even though body temperature remained elevated. Dialysis of gabazine in other medullary sites (n = 10) did not reverse thermal prolongation of the LCR. Gabazine had no consistent effect on baseline respiratory activity during hyperthermia. These findings are consistent with the hypothesis that hyperthermia activates GABAergic mechanisms in or near the NTS that are necessary for the thermal prolongation of the LCR.

High-frequency stimulation of the subthalamic nucleus increases glutamate in the subthalamic nucleus of rats as demonstrated by in vivo enzyme-linked glutamate sensor

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an effective therapy for Parkinson's disease; however, the mechanism whereby DBS ameliorates the symptoms of Parkinson's disease remains an area of intense research. In the present study, we investigated the hypothesis that the neurotransmitter glutamate is released within the STN during high-frequency stimulation (HFS) of the STN. Direct measurements of extracellular glutamate concentration in the STN were made using a dual enzyme-based electrochemical sensor. The studies were carried out in ketamine/xylazine anesthetized rats placed in a Kopf stereotaxic head frame. Various electrical stimulations (100-micros cathodic pulses; 100-3000 microA; 10- to 1000-Hz frequency; 5-s to 60-min stimulus durations) using bipolar stimulating electrodes were delivered to the STN. Stimulation of the STN elevated the concentration of glutamate in the STN. The concentration of glutamate rose quickly during HFS, remained elevated for the duration of stimulation, and descended slowly towards baseline upon cessation of stimulation. Elevation of the extracellular concentration of glutamate in the STN may be an important mechanism whereby DBS in the STN improves the symptoms of Parkinson's disease. Furthermore, our data argue against the hypothesis that DBS works primarily by electrotonic inhibition of the stimulated structure.

Mechanisms of pathogenesis in the Sudden Infant Death Syndrome

The likely processes of the Sudden Infant Death Syndrome (SIDS) were identified many years ago (apnea, failed arousal, failed autoresuscitation, etc.). The neurophysiological basis of these processes and the neurophysiological reasons some infants die of SIDS and others do not are, however, only emerging now. We reviewed recent studies that have shed light on the way in which epidemiological risk factors, genetics, neurotransmitter receptor defects and neonatal cardiorespiratory reflex responses interact to lead to sudden death during sleep in a small number of normal appearing infants. As a result of this review and analysis, we hypothesize that the neurophysiological basis of SIDS resides in a persistence of fetal reflex responses into the neonatal period, amplification of inhibitory cardiorespiratory reflex responses and reduced excitatory cardiorespiratory reflex responses. The hypothesis we developed explores the ways in which multiple subtle abnormalities interact to lead to sudden death and emphasizes the difficulty of ante-mortem identification of infants at risk for SIDS, although identification of infants at risk remains an essential goal of SIDS research.

Baroreceptor-mediated inhibition of respiration after peripheral and central administration of a 5-HT1Areceptor agonist in neonatal piglets

Inhibition of neurones in the ventral medulla accentuates the respiratory inhibition associated with acute blood pressure elevation in piglets. Activation of presynaptic 5-HT(1A) receptors inhibits serotonergic neurones in the ventral medulla and caudal raphé, and we tested the hypothesis that administration of 8-hydroxydipropylaminotetralin (8-OH-DPAT), a 5-HT(1A) agonist, within the rostroventral medulla and caudal raphé would enhance baroreceptor-mediated inhibition of respiratory activity in decerebrate, neonatal piglets. Baroreceptor stimulation was achieved by inflating a balloon in the distal aorta to elevate carotid blood pressure. After two to four control trials of baroreceptor stimulation, each piglet was given either a single intravenous (i.v.) dose of 10 microg kg(-1) 8-OH-DPAT or treated by adding 10 or 30 mm 8-OH-DPAT to the dialysate for approximately 10 min to inhibit serotonergic neurones, after which the baroreceptor stimulation trials were repeated. Baroreceptor stimulation reduced respiratory activity, particularly the respiratory frequency, which diminished from 35.7 +/- 3.3 to 33.8 +/- 3.1 breaths min(-1) (P < 0.02) and, following i.v. 8-OH-DPAT, baroreceptor-mediated inhibition of respiratory output was significantly accentuated (P < 0.05); the respiratory frequency declined from 34.5 +/- 3.6 to 26.5 +/- 2.9 breaths min(-1). Increasing aortic blood pressure reduced the respiratory frequency (P < 0.01), but focal dialysis of 10 or 30 mm 8-OH-DPAT had, on average, no effect on the ventilatory inhibition associated with an acute elevation of blood pressure. We conclude that activation of 5-HT(1A) receptors after systemic administration of 8-OH-DPAT enhanced baroreflex-mediated inhibition of ventilation, but this effect cannot be attributed to 5-HT(1A) receptor activation within the rostroventral medulla and caudal raphé.

Persistence of eupnea and gasping following blockade of both serotonin type 1 and 2 receptors in the in situ juvenile rat preparation

In severe hypoxia or ischemia, normal eupneic breathing is replaced by gasping, which can serve as a powerful mechanism for "autoresuscitation." We have proposed that gasping is generated by medullary neurons having intrinsic pacemaker bursting properties dependent on a persistent sodium current. A number of neuromodulators, including serotonin, influence persistent sodium currents. Thus we hypothesized that endogenous serotonin is essential for gasping to be generated. To assess such a critical role for serotonin, a preparation of the perfused, juvenile in situ rat was used. Activities of the phrenic, hypoglossal, and vagal nerves were recorded. We added blockers of type 1 and/or type 2 classes of serotonergic receptors to the perfusate delivered to the preparation. Eupnea continued following additions of any of the blockers. Changes were limited to an increase in the frequency of phrenic bursts and a decline in peak heights of all neural activities. In ischemia, gasping was induced following any of the blockers. Few statistically significant changes in parameters of gasping were found. We thus did not find a differential suppression of gasping, compared with eupnea, following blockers of serotonin receptors. Such a differential suppression had been proposed based on findings using an in vitro preparation. We hypothesize that multiple neurotransmitters/neuromodulators influence medullary mechanisms underlying the neurogenesis of gasping. In greatly reduced in vitro preparations, the importance of any individual neuromodulator, such as serotonin, may be exaggerated compared with its role in more intact preparations.

CO2chemosensitivity inHelix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains CO2-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to CO2. Cells isolated from the dorsal subesophageal ganglia retained CO2chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel ( IKAand IKDR). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that IKAwas affected by extracellular or intracellular pH, but the activity of IKDRwas modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel ( IKCa) in neurons in situ. The results of these studies support the hypothesis that IKAinitiates the chemosensory response, and IKDRand IKCaprolong the period of activation of CO2-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates CO2chemosensory responses in H. aspersa. We did not find a single “chemosensory channel,” and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein “machinery” of CO2chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory CO2chemosensor probably depends on the resting membrane potential and synaptic connectivity.

High-frequency oscillations in phrenic activity during pontile and medullary respiratory rhythms in rats

High-frequency oscillations may be signatures of the basic mechanisms underlying the neurogenesis of various patterns of automatic ventilatory activity. These high-frequency oscillations in phrenic activity differ greatly in eupnoea and gasping, implying different mechanisms of neurogenesis. In a decerebrate, in situ preparation of the rat, the peak frequency of high-frequency oscillations fell in apneusis following removal of the rostral pons. Following removal of all pons, phrenic discharge had a mixed pattern of gasps and multiple bursts; some of the latter were incrementing, as in eupnoea. Regardless of pattern, peak frequencies were significantly below those which were found during eupnoea, apneusis or gasping of the decerebrate preparation. Results do not support the concept that 'non-gasping' rhythmic patterns that can be recorded following a removal of pons are generated by the same mechanisms as those generating eupnoea. Indeed, both pons and medulla appear essential for all aspects of eupnoea to be expressed.

GABAergic processes mediate thermal prolongation of the laryngeal reflex apnea in decerebrate piglets

We tested the hypotheses that elevated body temperature would prolong reflex apnea following electrical stimulation of the superior laryngeal nerve (SLN) in decerebrate neonatal piglets and that thermal prolongation of reflex apnea after stimulation of the SLN depended on GABAergic mechanisms. These studies were conducted in 13 decerebrate piglets (age 3-15 days). The SLN was stimulated at approximately 1.5 times the threshold stimulus level for 10 s starting at the beginning of inspiration. We measured the duration of the apnea and respiratory disruption that followed SLN stimulation. Elevating body temperature prolonged the duration of the apnea and respiratory disruption that followed SLN stimulation, and treatment with antagonists of gama-aminobutyric acid A-type (GABAA) receptors reversed the thermal prolongation of reflex apnea and the period of respiratory disruption even though body temperature remained elevated. We conclude that elevated body temperature enhances or amplifies GABAergic mechanisms that prolong the respiratory inhibition following electrical stimulation of the SLN.

Focal warming in the nucleus of the solitary tract prolongs the laryngeal chemoreflex in decerebrate piglets

The laryngeal chemoreflex (LCR), elicited by a drop of water in the larynx, is exaggerated by mild hyperthermia (body temperature = 40-41 degrees C) in neonatal piglets. We tested the hypothesis that thermal prolongation of the LCR results from heating the nucleus of the solitary tract (NTS), where laryngeal afferents first form synapses in the brain stem. Three- to 13-day-old piglets were decerebrated and vagotomized and studied without anesthesia while paralyzed and ventilated. Phrenic nerve activity and rectal temperature were recorded. A thermode was placed in the medulla, and the brain tissue temperature was recorded with a thermistor approximately 1 mm from the tip of the thermode. When the thermode was inserted into the brain stem, respiratory activity was arrested or greatly distorted in eight animals. However, the thermode was inserted in nine animals without disrupting respiratory activity, and in these animals, warming the medullary thermode (thermistor temperature = 40-41 degrees C) while holding rectal temperature constant reversibly exaggerated the LCR. The caudal raphé was warmed focally by approximately 2 degrees C in four additional animals; this did not alter the duration of the LCR in these animals. Thermodes placed in the NTS did not disrupt respiratory activity, but they did prolong the LCR when warmed. Thermodes that were placed deep to the NTS in the region of the nucleus ambiguus disrupted respiratory activity, which precluded any analysis of the LCR. We conclude that prolongation of the laryngeal chemoreflex by whole body hyperthermia originates from the elevation of brain tissue temperature within in the NTS.

A computational analysis of central CO2 chemosensitivity in Helix aspersa

We created a single-compartment computer model of a CO(2) chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in CO(2) chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current (I(KA)), a delayed rectifier current (I(KDR)), a calcium-activated potassium current (I(KCa)), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation. pH-dependent inhibition of I(KA) initiated the response to CO(2), but hypercapnic inhibition of I(KDR) and I(KCa) affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH (pH(i)) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to CO(2) chemosensitivity, but the primary sensor is probably I(KA). pH(i) may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either pH(i) or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of pH(i) regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of pH(i) regulation during hypercapnia probably depends on intracellular activation of pH(i) regulation but extracellular inhibition of pH(i) regulation. These general principles are applicable to all CO(2) chemosensory cells in vertebrate and invertebrate neurons.

Changes in respiratory-modulated neural activities, consistent with obstructive and central apnea, during fictive seizures in an in situ anaesthetized rat preparation

Sudden unexplained death in epilepsy (SUDEP) has been proposed to result from seizure-induced changes in respiratory and cardiac function. Our purpose was to characterize changes in respiration during seizures. We used a preparation of the anaesthetized, perfused in situ rat. This preparation has the advantage over in vivo preparations in that delivery of oxygen to the brain does not depend upon the lungs or cardiovascular system. Electroencephalographic activity was recorded as were activities of the hypoglossal, vagus and phrenic nerves. The hypoglossal and vagus nerves innervate muscles of the upper airway and larynx while the phrenic nerve innervates the diaphragm. Fictive seizures were elicited by injections of penicillin into the parietal cortex or the carotid artery. Following elicitation of the fictive seizures, activities of the hypoglossal and vagal nerves declined greatly while phrenic activity was little altered. Such a differential depression of activities of nerves to the upper airway and larynx, compared to that to the diaphragm, would predispose to obstructive apnea in intact preparations. With more time, activity of the phrenic nerve also declined or ceased. These changes characterize central apnea. The major conclusion is that seizures may result in recurrent periods of obstructive and central apnea. Thus, seizures can adversely alter respiratory function in a profound manner.

Effect of prevention of lung inflation on metamorphosis and respiration in the developing bullfrog tadpole, Rana catesbeiana

We tested the hypothesis that respiratory development would be retarded in tadpoles reared in aquaria in which a barrier prevented access to the air-water interface. To test this hypothesis, we examined swimming behavior and respiration in intact tadpoles and gill and lung respiratory activity and central chemosensory responses in an in vitro brainstem preparation. The "barrier" tadpoles had significantly lower resting gill frequencies and higher lung breath attempts than control tadpoles at the same metamorphic stage. Control tadpoles swam greater distances and spent more time in the upper one third of the aquaria, while barrier tadpoles spent significantly more time at the bottom of the aquaria. There was significantly greater mortality for barrier tadpoles compared to control animals in the earliest and latest metamorphic stages. Mean body weight was significantly greater, and metamorphic rate was reduced in barrier tadpoles. Neither control nor barrier tadpole brainstem preparations demonstrated a gill ventilatory response to CO(2); however, both control and barrier preparations possessed significant lung frequency responses to central CO(2) chemoreceptor stimulation. Bath application of the GABA(A) and glycine receptor antagonists, bicuculline and strychnine, had greater effects on control tadpole gill burst activity and produced a similar large-amplitude bursting pattern in both control and barrier tadpoles, that was insensitive to CO(2) chemoreceptor stimulation. We conclude that development of the respiratory pattern was perturbed by the barrier, but the major effect was on gill ventilation rather than lung ventilation as we had expected.

Gap junction blockade does not alter eupnea or gasping in the juvenile rat

The role of gap junctions in the brainstem respiratory control system is ambiguous. In the present study, we used juvenile rats to determine whether blocking gap junctions altered eupnea or gasping in the in situ, arterially perfused rat preparation. Blockade of gap junctions with 100 microM carbenoxolone or 300 microM octanol did not produce any consistent changes in the timing or amplitude of integrated phrenic discharge or in the peak frequency in the power spectrum of phrenic nerve discharge during eupnea or ischemic gasping beyond those changes seen in time-control animals. These findings do not rule out a role for gap junctions in the expression of eupnea or gasping, but they do demonstrate that these intermembrane channels are not obligatory for either rhythm to occur.

Dopamine efflux in the rat striatum evoked by electrical stimulation of the subthalamic nucleus: potential mechanism of action in Parkinson's disease

The precise mechanism whereby continuous high-frequency electrical stimulation of the subthalamic nucleus ameliorates motor symptoms of Parkinson's disease is unknown. We examined the effects of high-frequency stimulation of regions dorsal to and within the subthalamic nucleus on dopamine efflux in the striatum of urethane-anaesthetized rats using constant potential amperometry. Complementary extracellular electrophysiological studies determined the activity of subthalamic nucleus neurons in response to similar electrical stimulation of the subthalamic nucleus. High-frequency stimulation of the subthalamic nucleus increased action potential firing in the subthalamic nucleus only during the initial stimulation period and was followed by a cessation of firing over the remainder of stimulation. Electrical stimulation of the subthalamic nucleus with 15 pulses elicited stimulus-time-locked increases in striatal dopamine efflux with maximal peak effects occurring at 50 Hz frequency and 300 microA intensity. Extended subthalamic nucleus stimulation (1000 pulses at 50 Hz; 300 microA) elicited a similar peak increase in striatal dopamine efflux that was followed by a relatively lower steady-state elevation in extracellular dopamine over the course of stimulation. In contrast, extended stimulation immediately adjacent and dorsal to the subthalamic nucleus resulted in an 11-fold greater increase in dopamine efflux that remained elevated over the course of the stimulation. Immunohistochemical staining for tyrosine hydroxylase revealed catecholaminergic fibers running immediately dorsal to and through the subthalamic nucleus. Taken together, these results suggest that enhanced dopamine release within the basal ganglia may be an important mechanism whereby high-frequency stimulation of the subthalamic nucleus improves motor symptoms of Parkinson's disease.

Laryngeal water receptors are insensitive to body temperature in neonatal piglets

Heat stress and the laryngeal chemoreflex (LCR) have both been implicated as possible contributors to the sudden infant death syndrome (SIDS). We recently reported that moderate hyperthermia, induced in decerebrate piglets by external heating, substantially prolonged the LCR elicited by injecting 0.1 ml of water into the larynx through a prepositioned transnasal catheter. To examine the question of whether hyperthermia influences the responses of laryngeal water receptors, we recorded single fiber action potentials in fine strands of the superior laryngeal nerve (SLN) in decerebrate piglets while the larynx was filled with water or isotonic saline. Water receptors, identified by their much brisker response to water than to saline, were studied with body temperature at 37.9+/-0.2 degrees C, after warming the animal to 40.6+/-0.2 degrees C and after cooling back to 37.7+/-0.3 degrees C. The results show no effect of body temperature change, in this range, on the responses of the laryngeal water receptors and thus suggest that the potentiation of the LCR by hyperthermia is mediated by a central action.

Neonatal maturation of the hypercapnic ventilatory response and central neural CO2 chemosensitivity

The ventilatory response to CO2 changes as a function of neonatal development. In rats, a ventilatory response to CO2 is present in the first 5 days of life, but this ventilatory response to CO2 wanes and reaches its lowest point around postnatal day 8. Subsequently, the ventilatory response to CO2 rises towards adult levels. Similar patterns in the ventilatory response to CO2 are seen in some other species, although some animals do not exhibit all of these phases. Different developmental patterns of the ventilatory response to CO2 may be related to the state of development of the animal at birth. The triphasic pattern of responsiveness (early decline, a nadir, and subsequent achievement of adult levels of responsiveness) may arise from the development of several processes, including central neural mechanisms, gas exchange, the neuromuscular junction, respiratory muscles and respiratory mechanics. We only discuss central neural mechanisms here, including altered CO2 sensitivity of neurons among the various sites of central CO2 chemosensitivity, changes in astrocytic function during development, the maturation of electrical and chemical synaptic mechanisms (both inhibitory and excitatory mechanisms) or changes in the integration of chemosensory information originating from peripheral and multiple central CO2 chemosensory sites. Among these central processes, the maturation of synaptic mechanisms seems most important and the relative maturation of synaptic processes may also determine how plastic the response to CO2 is at any particular age.

Physiology of breathing and respiratory control during sleep

Studies of respiratory control during sleep have revealed that multiple sites of central CO (2) chemosensitivity exist within the brainstem, and different chemosensory sites may function only during certain sleep states. In general, chemical control of respiratory function, related to both hypercapnia and hypoxia, appears to be blunted during sleep. The decline in respiratory activity during sleep is particularly marked in the muscles of the upper airway. A variety of neuromechanical factors originating in the lungs, chest wall, and upper airway also modify respiratory function during sleep. Cardiorespiratory function seems to be less stable during sleep, and arousal responses represent a final element in the control system that preserves cardiorespiratory function by terminating the sleep state and restoring more effective control mechanisms.

Abolition of spindle oscillations and 3-Hz absence seizurelike activity in the thalamus by using high-frequency stimulation: potential mechanism of action

OBJECT

The mechanism of action whereby high-frequency stimulation (HFS) in the thalamus ameliorates tremor and epilepsy is unknown. The authors studied the effects of HFS on thalamocortical relay neurons in a ferret in vitro slice preparation to test the hypothesis that HFS abolishes synchronized oscillations by neurotransmitter release.

METHODS

Intracellular and extracellular electrophysiological recordings were made in thalamic slices. The neurons in the thalamic slice spontaneously generated spindle oscillations, and treatment with picrotoxin, a gamma-aminobutyric acid A receptor antagonist, resulted in 3- to 4-Hz absence seizurelike activity. High-frequency stimulation (stimulation parameters: 10-1000-microA amplitude; l00-microsec pulse width; 100-Hz frequency; 1-60 seconds) was applied using a concentric bipolar stimulating electrode placed adjacent to the recording electrodes. High-frequency stimulation within the thalamus generated inhibitory and excitatory postsynaptic potentials, membrane depolarization, an increase in action potential firing during the stimulation period, and abolished the spindle oscillations in the thalamocortical relay neurons. High-frequency stimulation applied to 20-microM picrotoxin-treated slices eliminated the 3- to 4-Hz absence seizurelike activity.

CONCLUSIONS

High-frequency stimulation eliminates spontaneous spindle oscillations and picrotoxin-induced absence seizurelike activity in thalamic slices by synaptic neurotransmitter release; thus, HFS may abolish synchronous oscillatory activities such as those that generate tremor and seizures. Paradoxically, HFS, which is excitatory, and surgical lesions of the ventrointermedius thalamus, which are presumably inhibitory, both suppress tremors. This paradox is resolved by recognizing that HFS-mediated neurotransmitter release and thalamic surgery both disrupt the circuit generating tremor or seizure, albeit by different mechanisms.

Maintenance of eupnea and gasping following alterations in potassium ion concentration of perfusates of in situ rat preparation

Levels of extracellular potassium ion, above those in vivo, are required for the generation of rhythmic activities of in vitro preparations. Our purpose was to define whether hyperkalemia in the perfusate of an in situ preparation was likewise necessary for the expression of eupnea and gasping. Studies were performed using the in situ preparation of the juvenile rat, in which activity of the phrenic nerve was recorded as an index of the respiratory rhythm. Eupnea and gasping were impervious to modifications of potassium ion of the perfusate. Eupnea was maintained uninterrupted for more than 60 min whether the total concentration of potassium was hypokalemic (2.75 mM), normokalemic (4.0 mM), or hyperkalemic (6.25 and 7.75 mM). Gasping, with identical characteristics, was elicited at any concentration of potassium ion. We conclude that both eupnea and gasping are unaffected by modest changes in the concentration of potassium ion in the perfusate with which the in situ rat preparation is maintained. These results add further support to the conclusion that the in situ preparation represents a model that accurately reproduces mechanisms of rhythm generation of both eupnea and gasping in vivo.

Elevated body temperature enhances the laryngeal chemoreflex in decerebrate piglets

Hyperthermia and reflex apnea may both contribute to sudden infant death syndrome (SIDS). Therefore, we investigated the effect of increased body temperature on the inhibition of breathing produced by water injected into the larynx, which elicits the laryngeal chemoreflex (LCR). We studied decerebrated, vagotomized, neonatal piglets aged 3-15 days. Blood pressure, end-tidal CO(2), body temperature, and phrenic nerve activity were recorded. To elicit the LCR, we infused 0.1 ml of distilled water through a polyethylene tube passed through the nose and positioned just rostral to the larynx. Three to five LCR trials were performed with the piglet at normal body temperature. The animal's core body temperature was raised by approximately 2.5 degrees C, and three to five LCR trials were performed before the animal was cooled, and three to five LCR trials were repeated. The respiratory inhibition associated with the LCR was substantially prolonged when body temperature was elevated. Thus elevated body temperature may contribute to the pathogenesis of SIDS by increasing the inhibitory effects of the LCR.

Uncoupling of rhythmic hypoglossal from phrenic activity in the rat

During eupnoea, rhythmic motor activities of the hypoglossal, vagal and phrenic nerves are linked temporally. The inspiratory discharges of the hypoglossal and vagus motor neurones commence before the onset of the phrenic burst. The vagus nerve also discharges in expiration. Upon exposure to hypocapnia or hypothermia, the hypoglossal discharge became uncoupled from that of the phrenic nerve. This uncoupling was evidenced by variable times of onset of hypoglossal discharge before or after the onset of phrenic discharge, extra bursts of hypoglossal activity in neural expiration, or complete absence of any hypoglossal discharge during a respiratory cycle. No such changes were found for vagal discharge, which remained linked to the phrenic bursts. Intracellular recordings in the hypoglossal nucleus revealed that all changes in hypoglossal discharge were due to neuronal depolarization. These results add support to the conclusion that the brainstem control of respiratory-modulated hypoglossal activity differs from control of phrenic and vagal activity. These findings have implications for any studies in which activity of the hypoglossal nerve is used as the sole index of neural inspiration. Indeed, our results establish that hypoglossal discharge alone is an equivocal index of the pattern of overall ventilatory activity and that this is accentuated by hypercapnia and hypothermia.

Effects of exposure to a simulated altitude of 5500 m on energy metabolic pathways in rats

We examined the effect of exposure to 5500 m on three closely related metabolic pathways: anaerobic glycolysis, the pentose phosphate shunt (PPS), and fatty acid metabolism. Rats were exposed to simulated altitude of 5500 m for up to 3 months. The maximal rate of lactate production in tissue homogenates, tissue lactic acid dehydrogenase and blood lactate levels were measured to evaluate the capacity for anaerobic glycolysis. The uptake of 14C-1-palmitate, oxidation of 14C-1-palmitate to 14CO2, incorporation of 14C-1-palmitate into tissue lipids, plasma and tissue free fatty acids (FFA) levels and total lipid contents were measured to assess the magnitude of lipid metabolism. Activities of glucose-6-phosphate dehydrogenase (G-6-PD) and 6-phophogluconate dehydrogenase (6-PGD) in the PPS pathway were measured to assess the capacity to generate reducing power. Acute and chronic hypoxia did not affect most of the measurements of anaerobic glycolysis, but depressed lactate production in liver and kidney. Chronic hypoxia enhanced all aspects of lipid metabolism in liver and enhanced the uptake and oxidation to CO2 of palmitate in skeletal muscle. Chronic hypoxia did not alter the activity of the G-6-PD in any tissue studied, but the activity of 6-PGD was depressed in heart, kidney, thymus and adrenal gland. The lack of major changes in the capacities of anaerobic glycolytic pathways and the activities of the PPS dehydrogenases is consistent with the maintenance of normal aerobic metabolism in rats at 5500 m. We found no evidence that anaerobic metabolic processes were upregulated to sustain energy consumption during chronic hypoxia. On the other hand, enhanced fatty acid metabolism may spare carbohydrate for metabolic fuel under conditions of extreme hypoxic limitation.

Phrenic, vagal and hypoglossal activities in rat: pre-inspiratory, inspiratory, expiratory components

During eupnea in an in situ perfused preparation of the rat, inspiratory activities of the hypoglossal and vagal nerves commence before the phrenic; the vagus also discharges in expiration. The hypoglossal discharge has a prominent "pre-inspiratory" component. Power spectral analysis indicated that peak frequencies of oscillations in phrenic, hypoglossal and vagal inspiratory and expiratory activities were the same during eupnea. "Pre-inspiratory" hypoglossal activity had significantly lower peak frequencies. In gasping, "pre-inspiratory" hypoglossal activity ceased and all neural activities became purely inspiratory. High frequency oscillations of phrenic and vagal activities during gasping were shifted upward, compared to those in eupnea, whereas that of the hypoglossal was unaltered. In gasping, the temporal patterns of activities of the phrenic, hypoglossal and vagal nerves, and the level of coherence between these activities implies a restricted and shared set of pre-motor neurons. During eupnea, the activity patterns in the phrenic, hypoglossal and vagal nerves seem to originate from different sets of pre-motor neurons.

Ventilatory effects of gap junction blockade in the NTS in awake rats

We tested the hypothesis that focally perfusing carbenoxolone, which blocks gap junctions, into the nucleus tractus solitarius (NTS) would reduce the ventilatory response to CO(2). We measured minute ventilation (V(E)), tidal volume (V(T)) and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (F(I)(CO(2) = 0-8%) in rats during wakefulness. Focal perfusion of acetazolamide (10 microM) into the NTS increased V(E) and V(T) during exposure to room air. Carbenoxolone (300 microM) decreased the V(E) and V(T) response to CO(2) when perfused within, but not adjacent to the NTS in animals less than 10 weeks of age. F(R) was decreased at F(I)(CO(2) = 4% in these animals. Carbenoxolone did not decrease V(E), V(T) or F(R) in animals 10 weeks of age and older. Carbenoxolone did not decrease V(E), V(T) or F(R) when focally perfused outside the NTS at any age tested. The NTS is an important CO(2) chemosensory site at all ages, and gap junctions amplify the ventilatory response to CO(2) in animals less than 10 weeks of age.

Ventilatory effects of gap junction blockade in the RTN in awake rats

We tested the hypothesis that carbenoxolone, a pharmacological inhibitor of gap junctions, would reduce the ventilatory response to CO(2) when focally perfused within the retrotrapezoid nucleus (RTN). We tested this hypothesis by measuring minute ventilation (V(E)), tidal volume (V(T)), and respiratory frequency (F(R)) responses to increasing concentrations of inspired CO(2) (Fi(CO(2)) = 0-8%) in rats during wakefulness. We confirmed that the RTN was chemosensitive by perfusing the RTN unilaterally with either acetazolamide (AZ; 10 microM) or hypercapnic artificial cerebrospinal fluid equilibrated with 50% CO(2) (pH approximately 6.5). Focal perfusion of AZ or hypercapnic aCSF increased V(E), V(T), and F(R) during exposure to room air. Carbenoxolone (300 microM) focally perfused into the RTN decreased V(E) and V(T) in animals <11 wk of age, but V(E) and V(T) were increased in animals >12 wk of age. Glyzyrrhizic acid, a congener of carbenoxolone, did not change V(E), V(T), or F(R) when focally perfused into the RTN. Carbenoxolone binds to the mineralocorticoid receptor, but spironolactone (10 microM) did not block the disinhibition of V(E) or V(T) in older animals when combined with carbenoxolone. Thus the RTN is a CO(2) chemosensory site in all ages tested, but the function of gap junctions in the chemosensory process varies substantially among animals of different ages: gap junctions amplify the ventilatory response to CO(2) in younger animals, but appear to inhibit the ventilatory response to CO(2) in older animals.

Ventilation of patients with acute lung injury and acute respiratory distress syndrome: has new evidence changed clinical practice?

OBJECTIVES

A recent randomized trial of mechanical ventilation in acute lung injury (ALI)/adult respiratory distress syndrome (ARDS) demonstrated a 22% relative reduction in mortality rate using 6 mL/kg predicted body weight tidal volume vs. 12 mL/kg predicted body weight tidal volume. We determined whether publication of these findings changed clinical practice.

DESIGN

Retrospective cohort, 12 months before (Pre) and 12 months after publication (Post) of a randomized trial supporting the use of a 6 mL/kg predicted body weight tidal volume strategy.

SETTING

Three tertiary care hospitals in northern New England.

PATIENTS

From a sample of 943 patients receiving prolonged mechanical ventilation between 1998 and 1999 (Pre) and between 2000 and 2001 (Post), 300 patients meeting the American-European Consensus Conference definition of ALI or ARDS were selected for analysis.

INTERVENTIONS

The tidal volume, tidal volume/kg predicted body weight, and proportion receiving tidal volume/kg > or =6 mL/kg and < or =12 mL/kg predicted body weight were recorded at noon the first day after the diagnosis of ALI or ARDS was established.

MEASUREMENTS AND MAIN RESULTS

Pre and Post mean tidal volume (+/- sd) size and tidal volume size/kg predicted body weight were 759 +/- 158 mL (median 750 mL) vs. 639 +/- 138 mL (median 600 mL, p <.001) and 12.3 +/- 2.7 mL/kg (median 11.7 mL/kg) vs. 10.6 +/- 2.4 mL/kg (median 10.7 mL/kg, p <.001) respectively. Pre and Post plateau pressures and peak airway pressures were similar.

CONCLUSION

Publication of a trial demonstrating large mortality reductions using small tidal volume was associated with significant reductions in tidal volume delivered to patients with ALI/ARDS. However, wide variation in practice persists, and the proportion of patients receiving tidal volumes within recommended limits (< or =8 mL/kg) remains modest.

Heart rate variability during sleep and the early development of posttraumatic stress disorder

BACKGROUND

Noradrenergic function has been linked to posttraumatic stress disorder (PTSD) and might have a role in mediating sleep disturbances of the disorder. Our objective was to relate a peripheral manifestation of noradrenergic function, sympathetic nervous system activity as indexed by heart rate variability during sleep, to the development of PTSD in subjects with recent traumatic injuries.

METHODS

Subjects who had recall of life-threatening experiences were recruited from one of two regional trauma centers. Select subjects received a polysomnographic recording within 1 month of the trauma. Digitized electrocardiogram recordings were extracted from early and late rapid-eye-movement (REM) and preceding non-REM sleep periods. Autoregression was applied to R-R interval time series to calculate the ratios of low-frequency to high-frequency spectral densities (LF/HF ratios), which index sympathetic activation. Posttraumatic stress disorder status was determined at 2 months.

RESULTS

There was a significant state x group interaction: LF/HF ratios were higher during the REM sleep of the nine subjects who were positive for PTSD symptoms, compared with the 10 subjects who were PTSD negative.

CONCLUSIONS

Our findings are consistent with the possibility that increased noradrenergic activity during REM sleep contributes to the development of PTSD.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

Heterogeneous patterns of pH regulation in glial cells in the dorsal and ventral medulla

We examined pH regulation in two chemosensitive areas of the brain, the retrotrapezoid nucleus (RTN) and the nucleus tractus solitarius (NTS), to identify the proton transporters involved in regulation of intracellular pH (pHi) in medullary glia. Transverse brain slices from young rats [postnatal day 8 (P8) to P20] were loaded with the pH-sensitive probe 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein after kainic acid treatment removed neurons. Cells were alkalinized when they were depolarized (extracellular K+ increased from 6.24 to 21.24 mM) in the RTN but not in the NTS. This alkaline shift was inhibited by 0.5 mM DIDS. Removal of CO2/HCO3- or Na+ from the perfusate acidified the glial cells, but the acidification after Na+ removal was greater in the RTN than in the NTS. Treatment of the slice with 5-(N-ethyl-N-isopropyl)amiloride (100 microM) in saline containing CO2/HCO3- acidified the cells in both nuclei, but the acidification was greater in the NTS. Restoration of extracellular Cl- after Cl- depletion during the control condition acidified the cells. Immunohistochemical studies of glial fibrillary acid protein demonstrated much denser staining in the RTN compared with the NTS. We conclude that there is evidence of Na+-HCO3- cotransport and Na+/H+ exchange in glia in the RTN and NTS, but the distribution of glia and the distribution of these pH-regulatory functions are not identical in the NTS and RTN. The differential strength of glial pH regulatory function in the RTN and NTS may also alter CO2 chemosensory neuronal function at these two chemosensitive sites in the brain stem.

High-frequency oscillations of phrenic activity in eupnea and gasping of in situ rat: influence of temperature

We hypothesized that the in situ perfused preparation of the juvenile rat exhibits patterns of ventilatory activity comparable to eupnea and gasping in vivo. To evaluate this hypothesis, we examined high-frequency oscillations of activity of the phrenic nerve at 27-34 degrees C. The peak frequency of these high-frequency oscillations was defined from power spectral analysis. In situ, recordings were obtained in hyperoxic normocapnia, during ventilatory cycles in which the peak of integrated phrenic activity was achieved late in the burst, as in eupnea in vivo. Recordings were also obtained in hypoxic hypercapnia, when the peak of integrated phrenic activity occurred in the first half of the burst, as in gasping in vivo. In situ, peak frequencies in the power spectra were significantly higher in gasping than during eupnea. Frequencies during eupnea and gasping were progressively elevated as the temperature of the in situ preparation was increased. The shift in peak frequencies between eupnea and gasping and the temperature sensitivity of frequencies in situ were the same as in vivo. Results provide additional support for the conclusion that the in situ preparation demonstrates distinctly different patterns of automatic ventilatory activity, comparable to eupnea and gasping in vivo.

Prolongation of the laryngeal chemoreflex after inhibition of the rostral ventral medulla in piglets: a role in SIDS?

We tested the hypothesis that inhibition of neurons within the rostral ventral medulla (RVM) would prolong the laryngeal chemoreflex (LCR), a putative stimulus in the sudden infant death syndrome (SIDS). We studied the LCR in 19 piglets, age 3-16 days, by injecting 0.05 ml of saline or water into the larynx during wakefulness, non-rapid eye movement (NREM) sleep, and REM sleep, before and after 1 or 10 mM muscimol dialysis in the RVM. Muscimol prolonged the LCR (P < 0.05), and the prolongation was greater when the LCR was stimulated with water compared with saline (P < 0.02). The LCR was longer during NREM sleep than during wakefulness and longest during REM sleep (REM compared with wakefulness). Muscimol had no effect on the likelihood of arousal from sleep after LCR stimulation. We conclude that the RVM provides a tonic facilitatory drive to ventilation that limits the duration of the LCR, and loss of this drive may contribute to the SIDS when combined with stimuli that inhibit respiration.

Central CO2 chemoreception in developing bullfrogs: anomalous response to acetazolamide

Central CO(2) chemoreception and the role of carbonic anhydrase were assessed in brain stems from Rana catesbeiana tadpoles and frogs. Buccal and lung rhythms were recorded from cranial nerve VII and spinal nerve II during normocapnia and hypercapnia before and after treatment with 25 microM acetazolamide. The lung response to acetazolamide mimicked the hypercapnic response in early-stage and midstage metamorphic tadpoles and frogs. In late-stage tadpoles, acetazolamide actually inhibited hypercapnic responses. Acetazolamide and hypercapnia decreased the buccal frequency but had no effect on the buccal duty cycle. Carbonic anhydrase activity was present in the brain stem in every developmental stage. Thus more frequent lung ventilation and concomitantly less frequent buccal ventilation comprised the hypercapnic response, but the response to acetazolamide was not consistent during metamorphosis. Therefore, acetazolamide is not a useful tool for central CO(2) chemoreceptor studies in this species. The reversal of the effect of acetazolamide in late-stage metamorphosis may reflect reorganization of central chemosensory processes during the final transition from aquatic to aerial respiration.

Reduced respiratory-related evoked activity in subjects with obstructive sleep apnea syndrome

Midlatency respiratory-related evoked potentials were measured during wakefulness by using a 60-electrode array placed over the cortical region of the scalp. We studied the responses evoked by 200-ms pressure pulses at -5 and -10 cmH(2)O applied at inspiratory onset and during control tests (no pressure applied) in 14 subjects with obstructive sleep apnea syndrome (OSAS) and 18 normal subjects. Wavelet decomposition was used to smooth and dissect the respiratory-related evoked potentials in frequency and time in 8 frequency bands. After denoising, selected wavelet scales were used to reconstruct the respiratory-related evoked potentials, which were quantified by using global field power estimates. The time course of the global field power activity in OSAS subjects compared with normal subjects was significantly depressed in the period 55-70 ms poststimulus onset, a time when afferent traffic from upper airway receptors arrives in normal subjects. The reduced evoked response in subjects with OSAS suggests that these subjects receive less afferent input from upper airway mechanoreceptors. This may reflect reduced sensitivity of mechanoreceptors or reduced mechanoreceptor stimulation due to decreased upper airway compliance during wakefulness in OSAS.

Anomalous effects of external TEA on permeation and gating of the A-type potassium current in H. aspersa neuronal somata

The model proposed for external TEA block of Shaker K+ channels predicts a proportional relationship between TEA sensitivity and calculated electrical distance derived from measurements of voltage dependence of TEA block. In the present study, we examined this relationship for the A-type K+ current (IA) of Helix aspersa in neuronal somata using the whole-cell patch-clamp technique. External TEA inhibited IA with strong voltage dependence, such that the TEA dissociation constant was increased at depolarized test potentials. The half-inhibition constant (V0.5) for TEA block was approximately 21 mM at 0 mV, and V0.5 increased to approximately 67 mM at 50 mV. The calculated electrical distance for TEA block suggested that TEA traversed 65% of the way into the membrane electrical field. TEA also caused significant shifts in the voltage-dependence of A-type K+ channel gating. For example, at TEA concentrations below that required to fully suppress delayed outward currents, TEA caused depolarizing shifts in the voltage-dependence of A-type channel activation, steady-state inactivation, time for removal of inactivation, and slowed channel activation kinetics. Taken together, these observations suggest that TEA biased the local field potential near voltage-sensing domains of A-type K+ channels, causing the transmembrane electrical field to be relatively hyperpolarized in the presence of TEA. In summary, the calculated electrical distance of TEA block of A-type K+ channels in H. aspersa neurons is unprecedented among other K+ channels. This raises concerns about the conventional interpretation of this value. Furthermore, the voltage-dependent properties of IA are modified by TEA at concentrations previously used to isolate delayed rectifier potassium channels (IKDR) selectively. This lack of specificity has important implications for recent, as well as future studies of IA in H. aspersa and possibly other snail neurons.

Gasping is elicited by briefer hypoxia or ischemia following blockade of glycinergic transmission

The 'switching model' for generation of respiratory rhythms holds that gasping represents the release of a rostral medullary pacemaker mechanism from the pontomedullary neuronal circuit that generates eupnea. In a perfused preparation of the decerebrate juvenile rat, exposure to ischemia or hypoxic-hypercapnia caused an alteration in integrated phrenic activity from the incrementing pattern of eupnea to the decrementing pattern of gasping. The time required to elicit gasping was not altered by multiple exposures to ischemia or hypoxic-hypercapnia. Furthermore, this time to gasping was not altered following addition to the perfusate of increasing concentrations of bicuculline or picrotoxin; both block GABA(A) receptors. Addition to the perfusate of strychnine, a glycine antagonist, significantly shortened the duration of ischemia or hypoxic-hypercapnia required to elicit gasping. These results support the concept that a loss of inhibitory glycinergic transmission is a critical factor in release of pacemaker mechanisms for gasping from the pontomedullary neuronal circuit for eupnea.

Termination of inspiration by phase-dependent respiratory vagal feedback in awake normal humans

Imperceptible levels of proportional assist ventilation applied throughout inspiration reduced inspiratory time (TI) in awake humans. More recently, the reduction in TI was associated with flow assist, but flow assist also reaches a maximum value early during inspiration. To test the separate effects of flow assist and timing of assist, we applied a pseudorandom binary sequence of flow-assisted breaths during early, late, or throughout inspiration in eight normal subjects. We hypothesized that imperceptible flow assist would shorten TI most effectively when applied during early inspiration. Tidal volume, integrated respiratory muscle pressure per breath, TI, and TE were recorded. All stimuli (early, late, or flow assist applied throughout inspiration) resulted in a significant increase in inspiratory flow; however, only when the flow assist was applied during early inspiration was there a significant reduction in TI and the integrated respiratory muscle pressure per breath. These results provide further evidence that vagal feedback modulates breathing on a breath-by-breath basis in conscious humans within a physiological range of breath sizes.

Modulation of breathing using imperceptible unloading

We investigated the role of V(T) and V(T)/T(I) modulation of breathing in awake human subjects. We applied a PRBS of volume (incrementing ramp) or flow (decrementing wave) assist at levels below the perceptual threshold in order to stimulate respiratory feedback. We modeled the PRBS data with linear difference equations to obtain impulse-response profiles of V(T), V(T)/T(I), T(I) and factorial(P(MUS)). We limited cortical responses to our stimuli by applying sub-threshold levels of assist, and we limited humoral effects (O2 and CO2) by augmenting mechanical respiratory output intermittently and by small amounts. We found that flow or volume assist elicited similar significant increases in V(T) and V(T)/T(I). During flow assist there was a significant decrease in factorial(P(MUS)) and T(I) was reduced, albeit not significantly; however, volume assist did not modify T(I) or factorial(P(MUS)). The earlier onset of flow assist, relative to volume assist, may explain the difference between the responses. We conclude that vagally mediated inspiratory flow receptors in the chest wall or lungs may modulate breathing on a breath by breath basis when small, imperceptible increases in airflow occur early during inspiration. Furthermore, lung volume feedback during imperceptible unloading (occurring at the end of inspiration) was less effective. Finally, pseudorandom unloading with imperceptible stimuli provides a useful tool to study reflex regulation of ventilation in awake subjects without confounding cortical influences.

Enhanced baroreflex-mediated inhibition of respiration after muscimol dialysis in the rostroventral medulla

The rostral ventral medulla (RVM) may be important in the control of cardiorespiratory interactions. We hypothesized that inhibition of the RVM would enhance inhibition of breathing associated with transient blood pressure elevations. In 25 piglets 3-16 days of age, we studied the effect of acutely increasing blood pressure, by systemic infusion of phenylephrine, on respiratory activity before and after inhibition of neural activity in the RVM by dialysis of 10 mM muscimol, a GABA(A)-receptor agonist. Muscimol dialysis through probes that were placed along the ventral medullary surface from approximately 1 mm rostral to the facial nucleus to approximately 0.5 mm caudal to the facial nucleus augmented the respiratory inhibition associated with acute increases in blood pressure. No similar enhancement of respiratory inhibition after phenylephrine treatment was seen in six control animals that did not receive muscimol dialysis. We conclude that the piglet RVM participates in cardiorespiratory interactions and that dysfunction of homologous regions in the human infant could lead to cardiorespiratory instability and may be involved in the pathogenesis of sudden infant death syndrome.

Developmental changes in intracellular pH regulation in medullary neurons of the rat

We examined intracellular pH (pH(i)) regulation in the retrotrapezoid nucleus (RTN), a CO(2)-sensitive site, and the hypoglossal nucleus, a nonchemosensitive site, during development (postnatal days 2-18) in rats. Respiratory acidosis [10% CO(2), extracellular pH (pH(o)) 7.18] caused acidification without pH(i) recovery in the RTN at all ages. In the hypoglossal nucleus, pH(i) recovered in young animals, but as animal age increased, the slope of pH(i) recovery diminished. In animals older than postnatal day 11, the pH(i) responses to hypercapnia were identical in the hypoglossal nucleus and the RTN, but hypoglossal nucleus and RTN neurons could regulate pH(i) during intracellular acidification at constant pH(o) at all ages. Recovery of pH(i) from acidification in the RTN depended on extracellular Na+ and was inhibited by amiloride but was unaffected by DIDS, suggesting a role for Na+/H+ exchange. Hence, pH(i) regulation during acidosis is more effective in the hypoglossal nucleus in younger animals, possibly as a requirement of development, but in older juvenile animals (older than postnatal day 11), pH(i) regulation is relatively poor in chemosensitive (RTN) and nonchemosensitive nuclei (hypoglossal nucleus).