Locus coeruleus is a central chemoreceptive site in toads

Chloride Modulates Central pH Sensitivity and Plasticity of Brainstem Breathing-Related Biorhythms in Zebra Finch Embryos

All terrestrial vertebrate life must transition from aquatic gas exchange in the embryonic environment to aerial or pulmonary respiration at birth. In addition to being able to breathe air, neonates must possess functional sensory feedback systems for maintaining acid-base balance. Respiratory neurons in the brainstem act as pH sensors that can adjust breathing to regulate systemic pH. The central pH sensitivity of breathing-related motor output develops over the embryonic period in the zebra finch (Taeniopygia guttata). Due to the key role of chloride ions in electrochemical stability and developmental plasticity, we tested chloride's role in the development of central pH sensitivity. We blocked gamma-aminobutyric acid-A receptors and cation-chloride cotransport that subtly modulated the low-pH effects on early breathing biorhythms. Further, chloride-free artificial cerebrospinal fluid altered the pattern and timing of breathing biorhythms and blocked the stimulating effect of acidosis in E12-14 brainstems. Early and middle stage embryos exhibited rebound plasticity in brainstem motor outputs during low-pH treatment, which was eliminated by chloride-free solution. Results show that chloride modulates low-pH sensitivity and rebound plasticity in the zebra finch embryonic brainstem, but work is needed to determine the cellular and circuit mechanisms that control functional chloride balance during acid-base disturbances.

Molecular profiling of CO2/pH-sensitive neurons in the locus coeruleus of bullfrogs reveals overlapping noradrenergic and glutamatergic cell identity

Locus coeruleus (LC) neurons regulate breathing by sensing CO2/pH. Neurons within the vertebrate LC are the main source of norepinephrine within the brain. However, they also use glutamate and GABA for fast neurotransmission. Although the amphibian LC is recognized as a site involved in central chemoreception for the control of breathing, the neurotransmitter phenotype of these neurons is unknown. To address this question, we combined electrophysiology and single-cell quantitative PCR to detect mRNA transcripts that define norepinephrinergic, glutamatergic, and GABAergic phenotypes in LC neurons activated by hypercapnic acidosis (HA) in American bullfrogs. Most LC neurons activated by HA had overlapping expression of noradrenergic and glutamatergic markers but did not show strong support for GABAergic transmission. Genes that encode the pH-sensitive K+ channel, TASK2, and acid-sensing cation channel, ASIC2, were most abundant, while Kir5.1 was present in 1/3 of LC neurons. The abundance of transcripts related to norepinephrine biosynthesis linearly correlated with those involved in pH sensing. These results suggest that noradrenergic neurons in the amphibian LC also use glutamate as a neurotransmitter and that CO2/pH sensitivity may be linkedto the noradrenergic cell identity.

Synaptic modifications transform neural networks to function without oxygen

BACKGROUND

Neural circuit function is highly sensitive to energetic limitations. Much like mammals, brain activity in American bullfrogs quickly fails in hypoxia. However, after emergence from overwintering, circuits transform to function for approximately 30-fold longer without oxygen using only anaerobic glycolysis for fuel, a unique trait among vertebrates considering the high cost of network activity. Here, we assessed neuronal functions that normally limit network output and identified components that undergo energetic plasticity to increase robustness in hypoxia.

RESULTS

In control animals, oxygen deprivation depressed excitatory synaptic drive within native circuits, which decreased postsynaptic firing to cause network failure within minutes. Assessments of evoked and spontaneous synaptic transmission showed that hypoxia impairs synaptic communication at pre- and postsynaptic loci. However, control neurons maintained membrane potentials and a capacity for firing during hypoxia, indicating that those processes do not limit network activity. After overwintering, synaptic transmission persisted in hypoxia to sustain motor function for at least 2 h.

CONCLUSIONS

Alterations that allow anaerobic metabolism to fuel synapses are critical for transforming a circuit to function without oxygen. Data from many vertebrate species indicate that anaerobic glycolysis cannot fuel active synapses due to the low ATP yield of this pathway. Thus, our results point to a unique strategy whereby synapses switch from oxidative to exclusively anaerobic glycolytic metabolism to preserve circuit function during prolonged energy limitations.

CO2 exposure enhances Fos expression in hypothalamic neurons in rats during the light and dark phases of the diurnal cycle

Orexinergic (OX) neurons in the lateral hypothalamus (LH), perifornical area (PFA) and dorsomedial hypothalamus (DMH) play a role in the hypercapnic ventilatory response, presumably through direct inputs to central pattern generator sites and/or through interactions with other chemosensitive regions. OX neurons can produce and release orexins, excitatory neuropeptides involved in many functions, including physiological responses to changes in CO₂/pH. Thus, in the present study, we tested the hypothesis that different nuclei (LH, PFA and DMH) where the orexinergic neurons are located, show distinct activation by CO₂ during the light-dark cycle phases. For this purpose, we evaluated the Fos and OXA expression by immunohistochemistry to identify neurons that co-localize Fos + OXA in the LH, LPeF, MPeF and DMH in the light-inactive and dark-active phase in Wistar rats subjected to 3 h of normocapnia or hypercapnia (7% CO₂). Quantitative analyses of immunoreactive neurons show that hypercapnia caused an increase in the number of neurons expressing Fos in the LH, LPeF, MPeF and DMH in the light and dark phases. In addition, the number of Fos + OXA neurons increased in the LPeF and DMH independently of the phases of the diurnal cycle; whereas in the MPeF, this increase was observed exclusively in the light phase. Thus, we suggest that OX neurons are selectively activated by hypercapnia throughout the diurnal cycle, reinforcing the differential role of nuclei in the hypothalamus during central chemosensitivity.

Locus Coeruleus in Non-Mammalian Vertebrates

The locus coeruleus (LC) is a vertebrate-specific nucleus and the primary source of norepinephrine (NE) in the brain. This nucleus has conserved properties across species: highly homogeneous cell types, a small number of cells but extensive axonal projections, and potent influence on brain states. Comparative studies on LC benefit greatly from its homogeneity in cell types and modularity in projection patterns, and thoroughly understanding the LC-NE system could shed new light on the organization principles of other more complex modulatory systems. Although studies on LC are mainly focused on mammals, many of the fundamental properties and functions of LC are readily observable in other vertebrate models and could inform mammalian studies. Here, we summarize anatomical and functional studies of LC in non-mammalian vertebrate classes, fish, amphibians, reptiles, and birds, on topics including axonal projections, gene expressions, homeostatic control, and modulation of sensorimotor transformation. Thus, this review complements mammalian studies on the role of LC in the brain.

Noradrenergic circuit control of non-REM sleep substates

To understand what makes sleep vulnerable in disease, it is useful to look at how wake-promoting mechanisms affect healthy sleep. Wake-promoting neuronal activity is inhibited during non-rapid-eye-movement sleep (NREMS). However, sensory vigilance persists in NREMS in animals and humans, suggesting that wake promotion could remain functional. Here, we demonstrate that consolidated mouse NREMS is a brain state with recurrent fluctuations of the wake-promoting neurotransmitter noradrenaline on the ∼50-s timescale in the thalamus. These fluctuations occurred around mean noradrenaline levels greater than the ones of quiet wakefulness, while noradrenaline (NA) levels declined steeply in REMS. They coincided with a clustering of sleep spindle rhythms in the forebrain and with heart-rate variations, both of which are correlates of sensory arousability. We addressed the origins of these fluctuations by using closed-loop optogenetic locus coeruleus (LC) activation or inhibition timed to moments of low and high spindle activity during NREMS. We could suppress, lock, or entrain sleep-spindle clustering and heart-rate variations, suggesting that both fore- and hindbrain-projecting LC neurons show coordinated infraslow activity variations in natural NREMS. Noradrenergic modulation of thalamic, but not cortical, circuits was required for sleep-spindle clustering and involved NA release into primary sensory and reticular thalamic nuclei that activated both α1- and β-adrenergic receptors to cause slowly decaying membrane depolarizations. Noradrenergic signaling by LC constitutes a vigilance-promoting mechanism that renders mammalian NREMS vulnerable to disruption on the close-to-minute timescale through sustaining thalamocortical and autonomic sensory arousability. VIDEO ABSTRACT.

A Bridge between the Breath and the Brain: Synchronization of Respiration, a Pupillometric Marker of the Locus Coeruleus, and an EEG Marker of Attentional Control State

Yogic and meditative traditions have long held that the fluctuations of the breath and the mind are intimately related. While respiratory modulation of cortical activity and attentional switching are established, the extent to which electrophysiological markers of attention exhibit synchronization with respiration is unknown. To this end, we examined (1) frontal midline theta-beta ratio (TBR), an indicator of attentional control state known to correlate with mind wandering episodes and functional connectivity of the executive control network; (2) pupil diameter (PD), a known proxy measure of locus coeruleus (LC) noradrenergic activity; and (3) respiration for evidence of phase synchronization and information transfer (multivariate Granger causality) during quiet restful breathing. Our results indicate that both TBR and PD are simultaneously synchronized with the breath, suggesting an underlying oscillation of an attentionally relevant electrophysiological index that is phase-locked to the respiratory cycle which could have the potential to bias the attentional system into switching states. We highlight the LC’s pivotal role as a coupling mechanism between respiration and TBR, and elaborate on its dual functions as both a chemosensitive respiratory nucleus and a pacemaker of the attentional system. We further suggest that an appreciation of the dynamics of this weakly coupled oscillatory system could help deepen our understanding of the traditional claim of a relationship between breathing and attention.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Buccal rhythmogenesis and CO2 sensitivity in Lithobates catesbeianus tadpole brainstems across metamorphosis

Bullfrog tadpoles ventilate both the buccal cavity and lung. In isolated brainstems, the midbrain/pons influences CO₂ responsiveness and timing of lung ventilatory bursting, depending on larval development. However, little is known about midbrain/pons influences on buccal burst patterns. As such, we investigated how removal of this region affects buccal burst shape and CO₂ responsiveness across development. We measured facial nerve activity in brainstems isolated from tadpoles during early and late developmental stages, under normal and elevated levels of CO₂. Brainstems were either left intact or transected by removing the midbrain/pons. In late stage preparations, buccal burst pattern differed between intact and reduced preparations, and bursts were responsive to elevated CO₂ in these reduced preparations. These results suggest the midbrain/pons affects tadpole buccal burst pattern and CO₂ responsiveness, perhaps similar to its influences on lung ventilation.

ATP in the lateral hypothalamus/perifornical area enhances the CO2 chemoreflex control of breathing

NEW FINDINGS

What is the central question of this study? ATP is known to modulate the chemosensitivity of some brain areas. However, whether the ATP contributes specifically to the mechanism of chemoreception in the lateral hypothalamus/perifornical area (LH/PFA) remains to be determined. What is the main finding and its importance? ATP, acting on the LH/PFA, enhances the hypercapnic ventilatory response in rats during wakefulness, in the dark period. Our results highlight the importance of ATP as a modulator of central chemoreception and provide new insight regarding the mechanisms involved in LH/PFA chemosensitivity and the sleep-wake differences in the CO₂ /H⁺ -dependent drive to breathe.

ABSTRACT

The lateral hypothalamus/perifornical area (LH/PFA) is a central chemoreceptor site, which acts in an arousal state-dependent manner. It has been shown that purinergic signalling through ATP influences the CO₂ /H⁺ responsiveness of other chemosensitive regions, but it is unknown whether ATP is also involved in the mechanisms that underlie LH/PFA chemoreception. Here, we studied the effects of microdialysis of a P2X-receptor agonist [α,β-methylene ATP (α,β-meATP), 10 mm] and a non-selective P2-receptor antagonist [pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 1 mm] into the LH/PFA of conscious rats on ventilation in room air and in 7% CO₂ . In the dark (active) phase, but not in the light, microdialysis of α,β-meATP caused an augmented hypercapnic ventilatory response during wakefulness, but not during non-REM sleep (P < 0.001). PPADS caused no change in CO₂ ventilatory responses in either the dark period or the light period. Our data suggest that ATP in LH/PFA contributes to the hypercapnic ventilatory response in conscious rats during wakefulness in the dark phase of the diurnal cycle.

The rostral medulla of bullfrog tadpoles contains critical lung rhythmogenic and chemosensitive regions across metamorphosis

The development of amphibian breathing provides insight into vertebrate respiratory control mechanisms. Neural oscillators in the rostral and caudal medulla drive ventilation in amphibians, and previous reports describe ventilatory oscillators and CO₂ sensitive regions arise during different stages of amphibian metamorphosis. However, inconsistent findings have been enigmatic, and make comparisons to potential mammalian counterparts challenging. In the current study we assessed amphibian central CO₂ responsiveness and respiratory rhythm generation during two different developmental stages. Whole-nerve recordings of respiratory burst activity in cranial and spinal nerves were made from intact or transected brainstems isolated from tadpoles during early or late stages of metamorphosis. Brainstems were transected at the level of the trigeminal nerve, removing rostral structures including the nucleus isthmi, midbrain, and locus coeruleus, or transected at the level of the glossopharyngeal nerve, removing the putative buccal oscillator and caudal medulla. Removal of caudal structures stimulated the frequency of lung ventilatory bursts and revealed a hypercapnic response in normally unresponsive preparations derived from early stage tadpoles. In preparations derived from late stage tadpoles, removal of rostral or caudal structures reduced lung burst frequency, while CO₂ responsiveness was retained. Our results illustrate that structures within the rostral medulla are capable of sensing CO₂ throughout metamorphic development. Similarly, the region controlling lung ventilation appears to be contained in the rostral medulla throughout metamorphosis. This work offers insight into the consistency of rhythmic respiratory and chemosensitive capacities during metamorphosis.

Coupling of respiration and attention via the locus coeruleus: Effects of meditation and pranayama

The locus coeruleus (LC) has established functions in both attention and respiration. Good attentional performance requires optimal levels of tonic LC activity, and must be matched to task consistently. LC neurons are chemosensitive, causing respiratory phrenic nerve firing to increase frequency with higher CO₂ levels, and as CO₂ level varies with the phase of respiration, tonic LC activity should exhibit fluctuations at respiratory frequency. Top-down modulation of tonic LC activity from brain areas involved in attentional regulation, intended to optimize LC firing to suit task requirements, may have respiratory consequences as well, as increases in LC activity influence phrenic nerve firing. We hypothesize that, due to the physiological and functional overlaps of attentional and respiratory functions of the LC, this small neuromodulatory nucleus is ideally situated to act as a mechanism of synchronization between respiratory and attentional systems, giving rise to a low-amplitude oscillation that enables attentional flexibility, but may also contribute to unintended destabilization of attention. Meditative and pranayama practices result in attentional, emotional, and physiological enhancements that may be partially due to the LC's pivotal role as the nexus in this coupled system. We present original findings of synchronization between respiration and LC activity (via fMRI and pupil dilation) and provide evidence of a relationship between respiratory phase modulation and attentional performance. We also present a mathematical dynamical systems model of respiratory-LC-attentional coupling, review candidate neurophysiological mechanisms of changes in coupling dynamics, and discuss implications for attentional theory, meditation, and pranayama, and possible therapeutic applications.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Environmentally induced return to juvenile-like chemosensitivity in the respiratory control system of adult bullfrog, Lithobates catesbeianus

KEY POINTS

The degree to which developmental programmes or environmental signals determine physiological phenotypes remains a major question in physiology. Vertebrates change environments during development, confounding interpretation of the degree to which development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) produces phenotypes. Tadpoles mainly breathe water for gas exchange and frogs may breathe water or air depending on their environment and are, therefore, exemplary models to differentiate the degree to which life-stage vs. environmental context drives developmental phenotypes associated with neural control of lung breathing. Using isolated brainstem preparations and patch clamp electrophysiology, we demonstrate that adult bullfrogs acclimatized to water-breathing conditions do not exhibit CO₂ and O₂ chemosensitivity of lung breathing, similar to water-breathing tadpoles. Our results establish that phenotypes associated with developmental stage may arise from plasticity per se and suggest that a developmental trajectory coinciding with environmental change obscures origins of stage-dependent physiological phenotypes by masking plasticity.

ABSTRACT

An unanswered question in developmental physiology is to what extent does the environment vs. a genetic programme produce phenotypes? Developing animals inhabit different environments and switch from one to another. Thus a developmental time course overlapping with environmental change confounds interpretations as to whether development (i.e. permanent processes) or phenotypic plasticity (i.e. reversible processes) generates phenotypes. Tadpoles of the American bullfrog, Lithobates catesbeianus, breathe water at early life-stages and minimally use lungs for gas exchange. As adults, bullfrogs rely on lungs for gas exchange, but spend months per year in ice-covered ponds without lung breathing. Aquatic submergence, therefore, removes environmental pressures requiring lung breathing and enables separation of adulthood from environmental factors associated with adulthood that necessitate control of lung ventilation. To test the hypothesis that postmetamorphic respiratory control phenotypes arise through permanent developmental changes vs. reversible environmental signals, we measured respiratory-related nerve discharge in isolated brainstem preparations and action potential firing from CO₂ -sensitive neurons in bullfrogs acclimatized to semi-terrestrial (air-breathing) and aquatic-overwintering (no air-breathing) habitats. We found that aquatic overwintering significantly reduced neuroventilatory responses to CO₂ and O₂ involved in lung breathing. Strikingly, this gas sensitivity profile reflects that of water-breathing tadpoles. We further demonstrated that aquatic overwintering reduced CO₂ -induced firing responses of chemosensitive neurons. In contrast, respiratory rhythm generating processes remained adult-like after submergence. Our results establish that phenotypes associated with life-stage can arise from phenotypic plasticity per se. This provides evidence that developmental time courses coinciding with environmental changes obscure interpretations regarding origins of stage-dependent physiological phenotypes by masking plasticity.

Effect of temperature on chemosensitive locus coeruleus neurons of savannah monitor lizards, Varanus exanthematicus

Savannah monitor lizards (Varanus exanthematicus) are unusual among ectothermic vertebrates in maintaining arterial pH nearly constant during changes in body temperature in contrast to the typical α-stat regulating strategy of most other ectotherms. Given the importance of pH in the control of ventilation, we examined the CO₂/H⁺ sensitivity of neurons from the locus coeruleus (LC) region of monitor lizard brainstems. Whole-cell patch-clamp electrophysiology was used to record membrane voltage in LC neurons in brainstem slices. Artificial cerebral spinal fluid equilibrated with 80% O₂, 0.0-10.0% CO₂, balance N₂, was superfused across brainstem slices. Changes in firing rate of LC neurons were calculated from action potential recordings to quantify the chemosensitive response to hypercapnic acidosis. Our results demonstrate that the LC brainstem region contains neurons that can be excited or inhibited by, and/or are not sensitive to CO₂ in V. exanthematicus While few LC neurons were activated by hypercapnic acidosis (15%), a higher proportion of the LC neurons responded by decreasing their firing rate during exposure to high CO₂ at 20°C (37%); this chemosensitive response was no longer exhibited when the temperature was increased to 30°C. Further, the proportion of chemosensitive LC neurons changed at 35°C with a reduction in CO₂-inhibited (11%) neurons and an increase in CO₂-activated (35%) neurons. Expressing a high proportion of inhibited neurons at low temperature may provide insights into mechanisms underlying the temperature-dependent pH-stat regulatory strategy of savannah monitor lizards.

Control of lung ventilation following overwintering conditions in bullfrogs, Lithobates catesbeianus

Ranid frogs in northern latitudes survive winter at cold temperatures in aquatic habitats often completely covered by ice. Cold-submerged frogs survive aerobically for several months relying exclusively on cutaneous gas exchange while maintaining temperature-specific acid-base balance. Depending on the overwintering hibernaculum, frogs in northern latitudes could spend several months without access to air, need to breathe, or chemosensory drive to use neuromuscular processes that regulate and enable pulmonary ventilation. Therefore, we performed experiments to determine whether aspects of the respiratory control system of bullfrogs, Lithobates catesbeianus, are maintained or suppressed following minimal use of air breathing in overwintering environments. Based on the necessity for control of lung ventilation in early spring, we hypothesized that critical components of the respiratory control system of bullfrogs would be functional following simulated overwintering. We found that bullfrogs recently removed from simulated overwintering environments exhibited similar resting ventilation when assessed at 24°C compared to warm-acclimated control bullfrogs. Additionally, ventilation met resting metabolic and, presumably, acid-base regulation requirements, indicating preservation of basal respiratory function despite prolonged disuse in the cold. Recently emerged bullfrogs underwent similar increases in ventilation during acute oxygen lack (aerial hypoxia) compared to warm-acclimated frogs; however, CO2-related hyperventilation was significantly blunted following overwintering. Overcoming challenges to gas exchange during overwintering have garnered attention in ectothermic vertebrates, but this study uncovers robust and labile aspects of the respiratory control system at a time point correlating with early spring following minimal/no use of lung breathing in cold-aquatic overwintering habitats.

Activation state of the hyperpolarization-activated current modulates temperature-sensitivity of firing in locus coeruleus neurons from bullfrogs

Locus coeruleus neurons of anuran amphibians contribute to breathing control and have spontaneous firing frequencies that, paradoxically, increase with cooling. We previously showed that cooling inhibits a depolarizing membrane current, the hyperpolarization-activated current (I h) in locus coeruleus neurons from bullfrogs, Lithobates catesbeianus (Santin JM, Watters KC, Putnam RW, Hartzler LK. Am J Physiol Regul Integr Comp Physiol 305: R1451-R1464, 2013). This suggests an unlikely role for I h in generating cold activation, but led us to hypothesize that inhibition of I h by cooling functions as a physiological brake to limit the cold-activated response. Using whole cell electrophysiology in brain slices, we employed 2 mM Cs(+) (an I h antagonist) to isolate the role of I h in spontaneous firing and cold activation in neurons recorded with either control or I h agonist (cyclic AMP)-containing artificial intracellular fluid. I h did not contribute to the membrane potential (V m) and spontaneous firing at 20°C. Although voltage-clamp analysis confirmed that cooling inhibits I h, its lack of involvement in setting baseline firing and V m precluded its ability to regulate cold activation as hypothesized. In contrast, neurons dialyzed with cAMP exhibited greater baseline firing frequencies at 20°C due to I h activation. Our hypothesis was supported when the starting level of I h was enhanced by elevating cAMP because cold activation was converted to more ordinary cold inhibition. These findings indicate that situations leading to enhancement of I h facilitate firing at 20°C, yet the hyperpolarization associated with inhibiting a depolarizing cation current by cooling blunts the net V m response to cooling to oppose normal cold-depolarizing factors. This suggests that the influence of I h activation state on neuronal firing varies in the poikilothermic neuronal environment.

Neurochemical and electrical modulation of the locus coeruleus: contribution to CO2drive to breathe

The locus coeruleus (LC) is a dorsal pontine region, situated bilaterally on the floor of the fourth ventricle. It is considered to be the major source of noradrenergic innervation in the brain. These neurons are highly sensitive to CO₂/pH, and chemical lesions of LC neurons largely attenuate the hypercapnic ventilatory response in unanesthetized adult rats. Developmental dysfunctions in these neurons are linked to pathological conditions such as Rett and sudden infant death syndromes, which can impair the control of the cardio-respiratory system. LC is densely innervated by fibers that contain glutamate, serotonin, and adenosine triphosphate, and these neurotransmitters strongly affect LC activity, including central chemoreflexes. Aside from neurochemical modulation, LC neurons are also strongly electrically coupled, specifically through gap junctions, which play a role in the CO₂ ventilatory response. This article reviews the available data on the role of chemical and electrical neuromodulation of the LC in the control of ventilation.

Temperature influences neuronal activity and CO2/pH sensitivity of locus coeruleus neurons in the bullfrog, Lithobates catesbeianus

The locus coeruleus (LC) is a chemoreceptive brain stem region in anuran amphibians and contains neurons sensitive to physiological changes in CO2/pH. The ventilatory and central sensitivity to CO2/pH is proportional to the temperature in amphibians, i.e., sensitivity increases with increasing temperature. We hypothesized that LC neurons from bullfrogs, Lithobates catesbeianus, would increase CO2/pH sensitivity with increasing temperature and decrease CO2/pH sensitivity with decreasing temperature. Further, we hypothesized that cooling would decrease, while warming would increase, normocapnic firing rates of LC neurons. To test these hypotheses, we used whole cell patch-clamp electrophysiology to measure firing rate, membrane potential ( Vm), and input resistance ( Rin) in LC neurons in brain stem slices from adult bullfrogs over a physiological range of temperatures during normocapnia and hypercapnia. We found that cooling reduced chemosensitive responses of LC neurons as temperature decreased until elimination of CO2/pH sensitivity at 10°C. Chemosensitive responses increased at elevated temperatures. Surprisingly, chemosensitive LC neurons increased normocapnic firing rate and underwent membrane depolarization when cooled and decreased normocapnic firing rate and underwent membrane hyperpolarization when warmed. These responses to temperature were not observed in nonchemosensitive LC neurons or neurons in a brain stem slice 500 μm rostral to the LC. Our results indicate that modulation of cellular chemosensitivity within the LC during temperature changes may influence temperature-dependent respiratory drive during acid-base disturbances in amphibians. Additionally, cold-activated/warm-inhibited LC neurons introduce paradoxical temperature sensitivity in respiratory control neurons of amphibians.

Focal microdialysis of CO₂ in the perifornical-hypothalamic area increases ventilation during wakefulness but not NREM sleep

We investigated whether the perifornical-lateral hypothalamic area (PF-LHA), where the orexin neurons reside, is a central chemoreceptor site by microdialysis of artificial cerebrospinal fluid (aCSF) equilibrated with 25% CO(2) into PF-LHA in conscious rats. This treatment is known to produce a focal tissue acidification like that associated with a 6-7 mm Hg increase in arterial [Formula: see text] . Such focal acidification in the PF-LHA significantly increased ventilation up to 15% compared with microdialysis of normal aCSF equilibrated with 5% CO(2) only in wakefulness but not in sleep in both the dark (P=0.004) and light (P<0.001) phases of the diurnal cycle. This response was predominantly due to a significant increase in respiratory frequency (11%, P<0.001). There were no significant effects on ventilation in the group with probes misplaced outside the PF-LHA. These results suggest that PF-LHA functions as a central chemoreceptor site in the central nervous system in a vigilant state dependent manner with predominant effects in wakefulness.

Time domains of the hypoxic ventilatory response in ectothermic vertebrates

Over a decade has passed since Powell et al. (Respir Physiol 112:123-134, 1998) described and defined the time domains of the hypoxic ventilatory response (HVR) in adult mammals. These time domains, however, have yet to receive much attention in other vertebrate groups. The initial, acute HVR of fish, amphibians and reptiles serves to minimize the imbalance between oxygen supply and demand. If the hypoxia is sustained, a suite of secondary adjustments occur giving rise to a more long-term balance (acclimatization) that allows the behaviors of normal life. These secondary responses can change over time as a function of the nature of the stimulus (the pattern and intensity of the hypoxic exposure). To add to the complexity of this process, hypoxia can also lead to metabolic suppression (the hypoxic metabolic response) and the magnitude of this is also time dependent. Unlike the original review of Powell et al. (Respir Physiol 112:123-134, 1998) that only considered the HVR in adult animals, we also consider relevant developmental time points where information is available. Finally, in amphibians and reptiles with incompletely divided hearts the magnitude of the ventilatory response will be modulated by hypoxia-induced changes in intra-cardiac shunting that also improve the match between O(2) supply and demand, and these too change in a time-dependent fashion. While the current literature on this topic is reviewed here, it is noted that this area has received little attention. We attempt to redefine time domains in a more 'holistic' fashion that better accommodates research on ectotherms. If we are to distinguish between the genetic, developmental and environmental influences underlying the various ventilatory responses to hypoxia, however, we must design future experiments with time domains in mind.

The locus coeruleus and central chemosensitivity

The locus coeruleus (LC) lies in the dorsal pons and supplies noradrenergic (NA) input to many regions of the brain, including respiratory control areas. The LC may provide tonic input for basal respiratory drive and is involved in central chemosensitivity since focal acidosis of the region stimulates ventilation and ablation reduces CO(2)-induced increased ventilation. The output of LC is modulated by both serotonergic and glutamatergic inputs. A large percentage of LC neurons are intrinsically activated by hypercapnia. This percentage and the magnitude of their response are highest in young neonates and decrease dramatically after postnatal day P10. The cellular bases for intrinsic chemosensitivity of LC neurons are comprised of multiple factors, primary among them being reduced extracellular and intracellular pH, which inhibit inwardly rectifying and voltage-gated K(+) channels, and activate L-type Ca(2+) channels. Activation of K(Ca) channels in LC neurons may limit their ultimate response to hypercapnia. Finally, the LC mediates central chemosensitivity and contains pH-sensitive neurons in amphibians, suggesting that the LC has a long-standing phylogenetic role in respiratory control.

Role of pontine neurons in central O(2) chemoreflex during development in bullfrogs (Lithobates catesbeiana)

The present study used an in vitro brainstem preparation from pre-metamorphic tadpoles and adult bullfrogs (Lithobates catesbeiana) to understand the neural mechanisms associated with central O(2) chemosensitivity and its maturation. In this species, brainstem hypoxia increases fictive lung ventilation in tadpoles but decreases in adults. Previous studies have shown that alpha(1)-adrenoceptor inactivation prevents these responses, suggesting that noradrenergic neurons are involved. We first tested the hypothesis that the pons (which includes noradrenergic neurons from the locus coeruleus; LC) plays a role in the lung burst frequency response to central hypoxia by comparing the effects of brainstem transection at the LC level between pre-metamorphic tadpoles and adults. Data show that brainstem transection prevents the lung burst frequency response in both stage groups. During development, the progressive decrease in the Na(+)/K(+)/Cl(-) co-transporter NKCC1 contributes to the maturation of neural networks. Because NKCC1 becomes activated during hypoxia, we then tested the hypothesis that NKCC1 contributes to maturation of the central O(2) chemoreflex. Double labeling experiments showed that the proportion of tyrosine hydroxylase positive neurons expressing NKCC1 in the LC decreases during development. Inactivation of NKCC1 with bumetanide bath application reversed the lung burst response to hypoxia in tadpoles. Bumetanide inhibited the response in adults. These data indicate that a structure within the pons (potentially the LC) is necessary to the central hypoxic chemoreflex and demonstrate that NKCC1 plays a role in central O(2) chemosensitivity and its maturation in this species.

Central ventilatory control in the South American lungfish, Lepidosiren paradoxa: contributions of pH and CO(2)

Lungfish represent a probable sister group to the land vertebrates. Lungfish and tetrapods share features of respiratory control, including central, peripheral and intrapulmonary CO(2) receptors. We investigated whether or not central chemoreceptors in the lungfish, L. paradoxa, are stimulated by CO(2) and/or pH. Ventilation was measured by pneumotachography for diving animals. The fourth cerebral ventricle was equipped with two catheters for superfusion. Initially, two control groups were compared: (1) catheterized animals with no superfusion and (2) animals superfused with mock CSF solutions at pH = 7.45; PCO(2) = 21 mmHg. The two groups had virtually the same ventilation of about 40 ml BTPS kg(-1) h(-1) (P > 0.05). Next, PCO(2) was increased from 21 to 42 mmHg, while pH(CSF) was kept at 7.45, which increased ventilation from 40 to 75 ml BTPS kg(-1) h(-1). Conversely, a decrease of pH(CSF) from 7.45 to 7.20 (PCO(2) = 21 mmHg) increased ventilation to 111 ml BTPS kg(-1) h(-1). Further decreases of pH(CSF) had little effect on ventilation, and the combination of pH(CSF) = 7.10 and PCO(2) = 42 mmHg reduced ventilation to 63 ml BTPS kg(-1) h(-1).