Respiration-modulated membrane potential and chemosensitivity of locus coeruleus neurones in the in vitro brainstem-spinal cord of the neonatal rat

Catecholamine neurones in rats modulate sleep, breathing, central chemoreception and breathing variability

Brainstem catecholamine (CA) neurones have wide projections and an arousal-state-dependent activity pattern. They are thought to modulate the processing of sensory information and also participate in the control of breathing. Mice with lethal genetic defects that include CA neurones have abnormal respiratory control at birth. Also the A6 region (locus coeruleus), which contains CA neurones sensitive to CO(2) in vitro, is one of many putative central chemoreceptor sites. We studied the role of CA neurones in the control of breathing during sleep and wakefulness by specifically lesioning them with antidopamine beta-hydroxylase-saporin (DBH-SAP) injected via the 4th ventricle. After 3 weeks there was a 73-84% loss of A5, A6 and A7 tyrosine hydroxylase (TH) immunoreactive (ir) neurones along with 56-60% loss of C1 and C2 phenyl ethanolamine-N-methyltransferase (PNMT)-ir neurones. Over the 3 weeks, breathing frequency decreased significantly during air and 3 or 7% CO(2) breathing in both wakefulness and non-REM (NREM) sleep. The rats spent significantly less time awake and more time in NREM sleep. REM sleep time was unaffected. The ventilatory response to 7% CO(2) was reduced significantly in wakefulness at 7, 14 and 21 days (-28%) and in NREM sleep at 14 and 21 days (-26%). Breathing variability increased in REM sleep but not in wakefulness or NREM sleep. We conclude that CA neurones (1) promote wakefulness, (2) participate in central respiratory chemoreception, (3) stimulate breathing frequency, and (4) minimize breathing variability in REM sleep.

Optical imaging of respiratory neuron activity from the dorsal view of the lower brainstem

1. We visualized respiratory-related neuron network activity in the dorsal part of the pons and medulla of an in vitro preparation from newborn rats by optical recordings using a voltage-sensitive dye. We measured optical signals from several seconds before to several seconds after the inspiratory phase using the inspiratory motor nerve discharge as the trigger signal and we averaged the optical signals of 20-50 respiratory cycles to obtain an optical image correlating specifically to inspiratory activity. 2. Four areas that were excited or inhibited corresponding to the respiratory cycles were detected. (i) The most rostral activity was in the rostral and lateral parts of the pons, with activity mainly in the inspiratory phase, corresponding to the pontine-respiratory group. (ii) In the midpontine level, inspiratory activity followed by long-lasting hyperpolarization appeared in the midlateral parts. This part was presumed to reflect activity in the locus coeruleus. The hyperpolarization became almost negligible after treatment with the alpha-adrenergic antagonist, phentolamine. (iii) In the dorsal medulla, the predominantly inspiratory activity was detected at the rostral level of the area postrema. This part was considered to reflect activity mainly of the hypoglossal nucleus. (iv) At a similar level, we also detected weak and disperse inspiratory activity extending more laterally and caudally than that of the hypoglossal nucleus activity. This might reflect activity of the dorsal respiratory group. 3. In conclusion, the present optical recording study revealed that the dorsal part of the lower brainstem in the in vitro preparation is noticeably active as well as the ventral part shown in the previous study. This method is very useful for analysis of pharmacological properties, as well as the spatio-temporal pattern of respiratory-related network activity in the brainstem.

Response of membrane potential and intracellular pH to hypercapnia in neurons and astrocytes from rat retrotrapezoid nucleus

We compared the response to hypercapnia (10%) in neurons and astrocytes among a distinct area of the retrotrapezoid nucleus (RTN), the mediocaudal RTN (mcRTN), and more intermediate and rostral RTN areas (irRTN) in medullary brain slices from neonatal rats. Hypercapnic acidosis (HA) caused pH(o) to decline from 7.45 to 7.15 and a maintained intracellular acidification of 0.15 +/- 0.02 pH unit in 90% of neurons from both areas (n = 16). HA excited 44% of mcRTN (7/16) and 38% of irRTN neurons (6/16), increasing firing rate by 167 +/- 75% (chemosensitivity index, CI, 256 +/- 72%) and 310 +/- 93% (CI 292 +/- 50%), respectively. These responses did not vary throughout neonatal development. We compared the responses of mcRTN neurons to HA (decreased pH(i) and pH(o)) and isohydric hypercapnia (IH; decreased pH(i) with constant pH(o)). Neurons excited by HA (firing rate increased 156 +/- 46%; n = 5) were similarly excited by IH (firing rate increased 167 +/- 38%; n = 5). In astrocytes from both RTN areas, HA caused a maintained intracellular acidification of 0.17 +/- 0.02 pH unit (n = 6) and a depolarization of 5 +/- 1 mV (n = 12). In summary, many neurons (42%) from the RTN are highly responsive (CI 248%) to HA; this may reflect both synaptically driven and intrinsic mechanisms of CO(2) sensitivity. Changes of pH(i) are more significant than changes of pH(o) in chemosensory signaling in RTN neurons. Finally, the lack of pH(i) regulation in response to HA suggests that astrocytes do not enhance extracellular acidification during hypercapnia in the RTN.

Role of Kir2.2 in hypercapnic ventilatory response during postnatal development of mouse

In order to determine the role of Kir2.2 in the hypercapnic ventilatory response (HCVR) during postnatal development, we measured the response of the Kir2.2-knockout (Kir2.2-/-) mouse in an unanesthetized unrestrained state by means of pressure plethysmography on postnatal days 9-10, 14-15 and 18, and compared the response with that in its wild counterpart, the FVB mouse. We also examined developmental changes in m-RNA expression of Kir2.2 in the brainstem of the FVB mouse using quantitative real-time PCR assay. Kir2.2-/- exhibited a smaller increase in tidal volume and minute ventilation volume than the FVB mouse in response to hypercapnic challenge on days 14-15. Meanwhile, the FVB mouse showed a transient increase in m-RNA expression of Kir2.2 in the brainstem on days 14-15. These findings suggest that Kir2.2 in the brainstem plays a transient role in HCVR, possibly through central ventilatory chemosensitivity, during postnatal development.

Rhythms, synchrony and electrical coupling in the Locus coeruleus

Electrical coupling of neurones is believed to promote synchronized activity. It may, however, also be a requirement for the maintenance of endogenous rhythmic activity in some systems. In en bloc isolated brainstem-spinal cord of the neonatal rat simultaneous whole cell recordings from pairs of LC neurones (n = 47 pairs) disclosed for the most part strongly synchronized activity which could take the form of tonic spiking or phasic bursts. Simultaneous whole cell recording from LC neurones and glia also revealed synchronized waves of depolarization in 7 of 17 pairs. This synchrony was partly due to respiratory-phased synaptic input and partly due to mechanisms, which were not dependent on chemical synapses. The gap junction uncoupler carbenoxolone suppressed non-synaptic rhythmic activity in LC neurones, but did not suppress either respiratory-phased synaptic input to these neurones or their excitatory response to increased CO(2). We give preliminary direct evidence for the existence of a current pathway between LC neurones, which is inhibited by carbenoxolone. Within the LC nucleus carbenoxolone-sensitive electrical coupling, which may involve neurone-glia as well as neurone-neurone interactions, may be required not just for synchronization, but also for the maintenance of rhythm.

Modulation of the respiratory rhythm generator by the pontine noradrenergic A5 and A6 groups in rodents

The aim of the present review is to summarise available studies dealing with the respiratory control exerted by pontine noradrenergic neurones in neonatal and adult mammals. During the perinatal period, in vitro studies on neonatal rodents have shown that A5 and A6 neurones exert opposite modulations onto the respiratory rhythm generator, inhibitory and facilitatory respectively, that the anatomical support for these modulations already exists at birth, and that genetically induced alterations in the formation of A5 and A6 neurones affect the maturation of the respiratory rhythm generator, leading to lethal respiratory deficits at birth. The A5-A6 modulation of the respiratory rhythm generator is not transient, occurring solely during the perinatal period but it persists throughout life: A5 and A6 neurones display a respiratory-related activity, receive inputs from and send information to the medullary respiratory centres and contribute to the adaptation of adult breathing to physiological needs.

CO2 central chemosensitivity: why are there so many sensing molecules?

CO2 central chemoreceptors (CCRs) play a critical role in respiratory and cardiovascular controls. Although the primary sensory cells and their neuronal networks remain elusive, recent studies have begun to shed insight into the molecular mechanisms of several pH sensitive proteins. These putative CO2/pH-sensing molecules are expressed in the brainstem, detect P(CO2) at physiological levels, and couple the P(CO2) to membrane excitability. Functional analysis suggests that multiple CO2/pH-sensing molecules are needed to achieve high sensitivity and broad bandwidth of the CCRs. In contrast to the diversity of pH sensitive molecules, molecular mechanisms for CO2 sensing are rather general. The sensing molecules detect pH changes rather than molecular CO2. One or a few titratable amino acid residues in these proteins are usually involved. Protonation of these residues may lead to a change in protein conformation that is coupled to a change in channel activity. Depending on the location of the protonation sites, a membrane protein can detect extra- and/or intracellular pH.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

P2 receptors modulate respiratory rhythm but do not contribute to central CO2 sensitivity in vitro

Multiple brainstem sites are proposed to contribute to central respiratory chemosensitivity, however, the underlying molecular mechanisms remain unknown. P2X2 subunit-containing ATP receptors, which mediate pH-sensitive currents, appear to contribute to central chemosensitivity in vivo [J. Physiol. 523 (2000) 441]. However, recent data from P2X2 knockout mice [J. Neurosci. 23 (2003) 11315] indicate that they are not essential. To further explore the role of P2 receptors in central chemosensitivity, we examined the effects of P2 receptor agonists/antagonists on respiratory-related activity and CO2-sensitivity of rhythmically-active in vitro preparations from neonatal rat. Our main findings: (i) that putative chemosensitive regions of the ventrolateral medulla are immunoreactive for the P2X2 subunit; (ii) that ATP potentiates respiratory frequency in a dose-dependent, and PPADS-sensitive (P2 receptor antagonist), manner; and (iii) that the increase in burst frequency produced by increasing CO2 is unaffected by PPADS, indicate that ATP is a potent modulator of respiratory activity, but that P2 receptors do not contribute to central chemosensitivity in vitro.

Hypercapnic exposure in congenital central hypoventilation syndrome reveals CNS respiratory control mechanisms

Congenital central hypoventilation syndrome (CCHS) patients show impaired ventilatory responses and loss of breathlessness to hypercapnia, yet arouse from sleep to high CO2, suggesting intact chemoreceptor afferents. The syndrome provides a means to differentiate brain areas controlling aspects of breathing. We used functional magnetic resonance imaging to determine brain structures responding to inspired 5% CO2-95% O2 in 14 CCHS patients and 14 controls. Global signal changes induced by the challenge were removed on a voxel-by-voxel basis. A priori-defined volume-of-interest time trends (assessed with repeated measures ANOVA) and cluster analysis based on modeling each subject to a step function (individual model parameter estimates evaluated with t-test, corrected for multiple comparisons) revealed three large response clusters to hypercapnia distinguishing the two groups, extending from the 1) posterior thalamus through the medial midbrain to the dorsolateral pons, 2) right caudate nucleus, ventrolaterally through the putamen and ventral insula to the mid-hippocampus, and 3) deep cerebellar nuclei to the dorsolateral cerebellar cortex bilaterally. Smaller clusters and defined areas of group signal differences in the midline dorsal medulla, amygdala bilaterally, right dorsal-posterior temporal cortex, and left anterior insula also emerged. In most sites, early transient or sustained responses developed in controls, with little, or inverse change in CCHS subjects. Limbic and medullary structures regulating responses to hypercapnia differed from those previously shown to mediate loaded breathing ventilatory response processing. The findings show the significant roles of cerebellar and basal ganglia sites in responding to hypercapnia and the thalamic and midbrain participation in breathing control.

Effects on breathing of focal acidosis at multiple medullary raphe sites in awake goats

To gain insight into why there are chemoreceptors at widespread sites in the brain, mircrotubules were chronically implanted at two or three sites in the medullary raphe nuclei of adult goats (n = 7). After >2 wk, microdialysis (MD) probes were inserted into the microtubules to create focal acidosis (FA) in the awake state using mock cerebral spinal fluid (mCSF) equilibrated with 6.4% (pH = 7.3), 50% (pH = 6.5), or 80% CO(2) (pH = 6.3), where MD with 50 and 80% CO(2) reduces tissue pH by 0.1 and 0.18 pH unit, respectively. There were no changes in all measured variables with MD with 6.4% at single or multiple raphe sites (P > 0.05). During FA at single raphe sites, only 80% CO(2) elicited physiological changes as inspiratory flow was 16.9% above (P < 0.05) control. However, FA with 50 and 80% CO(2) at multiple sites increased (P < 0.05) inspiratory flow by 18.4 and 30.1%, respectively, where 80% CO(2) also increased (P < 0.05) tidal volume, heart rate, CO(2) production, and O(2) consumption. FA with 80% CO(2) at multiple raphe sites also led to hyperventilation (-2 mmHg), indicating that FA had effects on breathing independent of an increased metabolic rate. We believe these findings suggest that the large ventilatory response to a global respiratory brain acidosis reflects the cumulative effect of stimulation at widespread chemoreceptor sites rather than a large stimulation at a single site. Additionally, focal acidification of raphe chemoreceptors appears to activate an established thermogenic response needed to offset the increased heat loss associated with the CO(2) hyperpnea.

Optical mapping of pontine chemosensitive regions of neonatal rat

We analyzed the neuronal response to hypercapnic acidosis, using an optical recording technique with a fluorescent voltage-sensitive dye (di-4-ANEPPS), in pontine slice preparations of neonatal rats, containing the locus coeruleus (LC), which has been electrophysiologically demonstrated to be chemosensitive. The dye-stained preparation was continuously superfused with artificial cerebrospinal fluid. Epifluorescence of the slice was detected using a high-sensitivity optical recording system. Changes in the intensity of fluorescence were serially analyzed while switching artificial cerebrospinal fluid from control to hypercapnic acidosis, or vice versa. The optical recording method revealed that the LC, as reported in previous studies, reversibly showed a depolarizing response to hypercapnic acidosis in 56% of the examined preparations. The A5 area (56%) also exhibited a reversible, depolarizing response to hypercapnic acidosis. The response was preserved under conditions in which chemical synaptic transmission was blocked by low Ca(2+)-high Mg(2+) solution. These results suggest that the optical recording method is applicable to identification of potentially chemosensitive areas, which deserve further electrophysiological analysis, and that the A5 area could be chemosensitive.

Effects of inactivation and stimulation of locus coeruleus on respiratory activity of neonatal rat

We examined the effects of pharmacological inactivation and electrical stimulation of the locus coeruleus (LC) on the respiratory rate (RR) in isolated brainstem-spinal cord preparations of the neonatal rat. The brainstem and spinal cord were isolated en bloc from neonatal (days 1-4) Sprague-Dawley rats and superfused with artificial cerebrospinal fluid (ACSF) equilibrated with a gas mixture containing 2% CO(2) (pH 7.8). Pharmacological inactivation of the bilateral LC by means of microinjection of tetrodotoxin or noradrenaline elicited a significant decrease in RR in preparations obtained from rats aged 3-4 days, but not in preparations of rats aged 1-2 days. Stimulation of the bilateral LC with a train of electrical pulses (25-50 microA, 30 ms, 0.5 Hz, 3-4 min) caused a marginal but significant increase in RR on days 3-4, but not on days 1-2. These results indicate that the LC exerts an excitatory effect on the medullary respiratory rhythm generator in an age-dependent manner.

Phox2a gene, A6 neurons, and noradrenaline are essential for development of normal respiratory rhythm in mice

Although respiration is vital to the survival of all mammals from the moment of birth, little is known about the genetic factors controlling the prenatal maturation of this physiological process. Here we investigated the role of the Phox2a gene that encodes for a homeodomain protein involved in the generation of noradrenergic A6 neurons in the maturation of the respiratory network. First, comparisons of the respiratory activity of fetuses delivered surgically from heterozygous Phox2a pregnant mice on gestational day 18 showed that the mutants had impaired in vivo ventilation, in vitro respiratory-like activity, and in vitro respiratory responses to central hypoxia and noradrenaline. Second, pharmacological studies on wild-type neonates showed that endogenous noradrenaline released from pontine A6 neurons potentiates rhythmic respiratory activity via alpha1 medullary adrenoceptors. Third, transynaptic tracing experiments in which rabies virus was injected into the diaphragm confirmed that A6 neurons were connected to the neonatal respiratory network. Fourth, blocking the alpha1 adrenoceptors in wild-type dams during late gestation with daily injections of the alpha1 adrenoceptor antagonist prazosin induced in vivo and in vitro neonatal respiratory deficits similar to those observed in Phox2a mutants. These results suggest that noradrenaline, A6 neurons, and the Phox2a gene, which is crucial for the generation of A6 neurons, are essential for development of normal respiratory rhythm in neonatal mice. Metabolic noradrenaline disorders occurring during gestation therefore may induce neonatal respiratory deficits, in agreement with the catecholamine anomalies reported in victims of sudden infant death syndrome.

Role of l-glutamate in the locus coeruleus of rats in hypoxia-induced hyperventilation and anapyrexia

Locus coeruleus (LC) is a noradrenergic nucleus in the pons which has been reported to play an inhibitory role in the ventilatory response to hypoxia. Since LC contains glutamatergic receptors and L-glutamate is known to participate in the ventilatory and thermoregulatory responses to hypoxia, the effects of kynurenic acid (KYN, a glutamatergic receptor antagonist) microinjected into the LC in the hypoxic hyperventilation and anapyrexia (a regulated drop in body temperature [Tb]) were examined. Ventilation (V) and Tb were measured before and after a microinjection of KYN (10 nmol/0.1 microl) into the LC, followed by hypoxia. Control rats received a saline injection. Under normoxia, KYN treatment did not affect V or Tb. Typical hypoxia-induced hyperventilation and anapyrexia were observed after saline injection. KYN injection caused an increase in the ventilatory response, acting on tidal volume (Vt), but did not affect the anapyrexic response to hypoxia. These data suggest that L-glutamate in the LC is an excitatory neurotransmitter that activates an inhibitory pathway to reduce the hypoxic ventilatory response, similarly to the data reported for rostral ventrolateral medulla (VLM). The role of L-glutamate into the LC and VLM opposes its effect on other nuclei such as the nucleus of the solitary tract and ventromedullary surface, where the neurotransmitter participates in an excitatory pathway of the ventilatory response.

Effects on breathing in awake and sleeping goats of focal acidosis in the medullary raphe

Our aim was to determine the effects of focal acidification in the raphe obscurus (RO) and raphe pallidus (RP) on ventilation and other physiological variables in both the awake and sleep states in adult goats. Through chronically implanted microtubules, 1) a focal acidosis was created by microdialysis of mock cerebrospinal fluid (mCSF), equilibrated with various levels of CO2, and 2) medullary extracellular fluid (ECF) pH was measured by using a custom-made pH electrode. Focal acidosis in the RO or RP, by dialyzing either 25 or 80% CO2 (mCSF pH approximately 6.8 or 6.3), increased (P < 0.05) inspiratory flow by 8 and 12%, respectively, while the animals were awake during the day, but not at night while they were awake or in non-rapid eye movement sleep. While the animals were awake during the day, there were also increases in heart rate and blood pressure (P < 0.05) but no significant change in metabolic rate or arterial Pco2. Dialysis with mCSF equilibrated with 25 or 80% CO2 reduced ECF pH by the same amount (25%) or three times more (80%) than when inspired CO2 was increased to 7%. During CO2 inhalation, the reduction in ECF pH was only 50% of the reduction in arterial pH. Finally, dialysis in vivo only decreased ECF pH by 19.1% of the change during dialysis in an in vitro system. We conclude that 1) the physiological responses to focal acidosis in the RO and RP are consistent with the existence of chemoreceptors in these nuclei, and 2) local pH buffering mechanisms act to minimize changes in brain pH during systemic induced acidosis and microdialysis focal acidosis and that these mechanisms could be as or more important to pH regulation than the small changes in inspiratory flow during a focal acidosis.

Respiratory responses to pH in the absence of pontine and dorsal medullary areas in the newborn mouse in vitro

The contribution of pons and dorsal medulla in establishing the pattern of fictive respiration and in mediating the respiratory response to acidification was studied using the isolated brainstem-spinal cord preparation from neonatal mouse. About 40% of ponto-medullary preparations (retaining pons) showed spontaneous, but irregular respiratory-like rhythm. In the other 60%, the elimination of the pons often was followed by the initiation of a respiratory-like rhythm. Medullary preparations, derived from either inactive or rhythmic ponto-medullary preparations, showed a regular respiratory-like rhythm, which was also of a higher frequency and a bigger amplitude than that observed in ponto-medullary preparations. In contrast, ventral medullary preparations, derived from medullary preparations by eliminating the dorsal medulla, showed an irregular rhythm with a reduced amplitude of the integrated inspiratory burst. In ponto-medullary and ventral medullary preparations, acidification of the superfusion medium increased the respiratory frequency, while in medullary preparations, it increased the frequency and reduced the amplitude of the inspiratory burst. Our results suggest that pontine structures influence negatively the rate and depth of the respiratory-like rhythm, while dorsal medullary structures influence positively the depth of the rhythm. They also suggest that the pattern of response to pH supported by the ventral medulla is modified by the input provided from pons and dorsal medulla.

Does the brain noradrenaline network mediate the effects of the CO2 challenge?

The inhalation of carbon dioxide (CO2) is commonly used in patients and volunteers as a means of producing anxiety or panic. It is generally believed that patients with panic disorder are more vulnerable to the effects of CO2 than patients with other anxiety disorders or healthy volunteers and there is speculation and debate as to the mechanism for this apparent sensitivity. Recent work from our group has shown that a single inhalation of 35% CO2 activates the hypothalamic-pituitary-adrenocortical (HPA) axis, increases blood pressure (BP) and increases subjective fear responses in healthy volunteers. Correlation analyses reveal a relationship between the changes in BP and the cortisol increase. These findings led us to postulate that a common mechanism may mediate these and the subjective responses to inhalation of CO2. We propose that the noradrenergic system, particularly the locus coeruleus (LC), but including the A1 and A2 cell groups, may be a key mediator of these responses. This article examines the evidence and discusses the results of studies from our laboratory in relation to a neuroanatomical model centring on the LC.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Multiple targets of chemosensitive signaling in locus coeruleus neurons: role of K+ and Ca2+ channels

We studied chemosensitive signaling in locus coeruleus (LC) neurons using both perforated and whole cell patch techniques. Upon inhibition of fast Na(+) spikes by tetrodotoxin (TTX), hypercapnic acidosis [HA; 15% CO(2), extracellular pH (pH(o)) 6.8] induced small, slow spikes. These spikes were inhibited by Co(2+) or nifedipine and were attributed to activation of L-type Ca(2+) channels by HA. Upon inhibition of both Na(+) and Ca(2+) spikes, HA resulted in a membrane depolarization of 3.52 +/- 0.61 mV (n = 17) that was reduced by tetraethylammonium (TEA) (1.49 +/- 0.70 mV, n = 7; P < 0.05) and absent (-0.97 +/- 0.73 mV, n = 7; P < 0.001) upon exposure to isohydric hypercapnia (IH; 15% CO(2), 77 mM HCO(3)(-), pH(o) 7.45). Either HA or IH, but not 50 mM Na-propionate, activated Ca(2+) channels. Inhibition of L-type Ca(2+) channels by nifedipine reduced HA-induced increased firing rate and eliminated IH-induced increased firing rate. We conclude that chemosensitive signals (e.g., HA or IH) have multiple targets in LC neurons, including TEA-sensitive K(+) channels and TWIK-related acid-sensitive K(+) (TASK) channels. Furthermore, HA and IH activate L-type Ca(2+) channels, and this activation is part of chemosensitive signaling in LC neurons.

Anatomical arrangement of hypercapnia-activated cells in the superficial ventral medulla of rats

The anatomical structure of central respiratory chemoreceptors in the superficial ventral medulla of rats was studied by using hypercapnia-induced c-fos labeling to identify cells directly stimulated by extracellular pH or PCO(2). The distribution of c-fos-positive cells was found to be predominantly perivascular to surface vessels. In the superficial ventral medullary midline, parapyramidal, and ventrolateral regions where c-fos-positive cells were concentrated, we found a common, characteristic, anatomical architecture. The medullary surface showed an indentation covered by a surface vessel, and the marginal glial layer was thickened. We classified c-fos-positive cells into two types. One (type I cell) was small, was located inside the marginal glial layer and close to the medullary surface, and surrounded fine vessels. The other (type II cell) was large and located dorsal to the marginal glial layer. c-fos Expression under synaptic blockade suggested that type I cells are intrinsically chemosensitive. The chemosensitivity of surface cells (possible type I cells) surrounding vessels was confirmed electrophysiologically in slice preparations. We suggest that this characteristic anatomical structure may be the central chemoreceptor.

Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH(i)), extracellular pH (pH(o)) and molecular CO(2) were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH(i) and membrane potential (V(m)). Hypercapnic acidosis (15 % CO(2), pH(o) 6.8) resulted in a maintained fall in pH(i) of 0.31 pH units and a 93 % increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15 % CO(2), 77 mM HCO(3)(-), pH(o) 7.45) resulted in a smaller and transient fall in pH(i) of about 0.17 pH units and an increase in firing rate of 76 %. Acidified Hepes (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid)-buffered medium (pH(o) 6.8) resulted in a progressive fall in pH(i) of over 0.43 pH units and an increase in firing rate of 126 %. Isosmotic addition of 50 mM propionate to the standard HCO(3)(-)-buffered medium (5 % CO(2), 26 mM HCO(3)(-), pH(o) 7.45) resulted in a transient fall in pH(i) of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5 % CO(2), 7 mM HCO(3)(-), pH(o) 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72 % increase in firing rate. For all treatments, the changes in pH(i) preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH(o). In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH(i), indicating that a decrease in pH(i) is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.

Pharmacological characterization of vanilloid receptor located in the brain

Specific [3H]resiniferatoxin (RTX) binding detects the vanilloid receptor type I (VR1). In the present study we demonstrate specific, high-affinity, saturable [3H]RTX binding in various areas of monkey brain not known to be innervated by primary afferent neurons as well as in spinal cord and dorsal root ganglion neurons of the same origin. Detailed pharmacological characterization and comparison revealed no major difference in binding affinities between the peripheral and the central sites as measured by K(d)/K(i) values. In general, lower receptor density was measured in selected brain areas than in the periphery. Areas with higher receptor density were detected in the locus ceruleus, preoptic area, and medial basal hypothalamus of the brain. Both capsaicin and the competitive antagonist capsazepine inhibited the specific binding of [3H]RTX to membrane preparations of the dorsal horn of the spinal cord and dorsal root ganglia with K(i) values of 4.3+/-0.32 microM and 2.7+/-0.33 microM, respectively. Inhibition was observed in the central areas (hypothalamus) with K(i) values of 0.95+/-0.1 microM for capsaicin and 0.86+/-0.11 microM for capsazepine. Previous biological and pharmacological evidence suggested that vanilloid receptors were present in the brain. Our results demonstrate that the pharmacological properties of both the peripheral and central receptor sites display appropriate pharmacological similarity to represent the same receptor class. The modest differences in ligand affinities for the vanilloid receptor expressed in the brain nuclei and the dorsal root ganglion neurons may correspond to differences in sequence, modification or associated proteins.

Respiratory plasticity: differential actions of continuous and episodic hypoxia and hypercapnia

The objectives of this paper are: (1) to review advances in our understanding of the mechanisms of respiratory plasticity elicited by episodic versus continuous hypoxia in short to intermediate time domains (min to h); and (2) to present new data suggesting that different patterns of hypercapnia also elicit distinct forms of respiratory plasticity. Episodic, but not continuous hypoxia elicits long-term facilitation (LTF) of respiratory motor output. Phrenic LTF is a serotonin-dependent central neural mechanism that requires: (a) activation of spinal serotonin receptors; and (b) spinal protein synthesis. Continuous and episodic hypercapnia also elicit different mechanisms of plasticity. Continuous, severe hypercapnia (25 min of approximately 10% inspired CO(2)) elicits long-term depression (LTD) of phrenic motor output (-33+/-8% at 60 min post-hypercapnia) in anesthetized rats. In contrast, 3,5 min hypercapnic episodes do not elicit LTD (9+/-17% at 60 min). We hypothesize that the response of respiratory motoneurons to serotonergic and noradrenergic modulation may contribute to pattern sensitivity to hypoxia and hypercapnia.

Synchronized rhythms in chemosensitive neurons of the locus coeruleus in the absence of chemical synaptic transmission

The activity of locus coeruleus (LC) neurons was examined in the en bloc isolated brainstem-spinal cord of the neonatal rat using paired whole cell or whole cell plus extracellular recording. In artificial cerebrospinal fluid (ACSF) LC neurons were synchronized by their respiratory innervation and in some neurons showing tonic or burst patterns of discharge these patterns of discharge could also be synchronized. Replacing ACSF with low Ca(2+)-high Mg(2+) generated synchronized rhythmic bursts which remained synchronized at high CO(2) (up to 20%). This rhythm was suppressed by TTX. Substitution of Ba(2+) for Ca(2+) in ACSF generated a synchronized rhythm which was TTX-insensitive. The frequency of this rhythm increased by 31+/-16% on raising CO(2) concentration from 2 to 10%. We conclude that the capacity of chemosensitive LC neurons to generate a synchronized rhythm depends on their electrical coupling, but not on chemical synaptic transmission.

Cytoarchitecture of central chemoreceptors in the mammalian ventral medulla

We reviewed the previous reports on the fine anatomy of the mammalian ventral medulla with special attention to the cytoarchitecture of the superficial chemosensitive regions to summarize what is known, what is not yet known, and what should be studied in the future. We also reviewed studies on anatomical relationship between neurons and vessels, and morphological studies on dendrites of respiratory or chemosensitive neurons. When we compared the morphological reports on the ventral and dorsal putative chemosensitive regions, similarities were found as follows. Chemosensitive cells were often found not only near the ventral surface but near the dorsal surface of the brainstem. Dendritic projection towards the surface was a common characteristic in the ventral and dorsal chemosensitive neurons. Morphological abnormality in the brainstem of sudden infant death syndrome victims was also summarized. On the basis of the previous reports we discussed the perspective on the future study on central chemoreception. Among various unanswered questions in central chemosensitivity studies, physiological significance of surface cells and surface extending dendrites is the most important topic, and must be thoroughly investigated.

Cell-cell coupling in CO(2)/H(+)-excited neurons in brainstem slices

The indirect and direct electrical and anatomical evidence for the hypothesis that central chemoreceptor neurons in the dorsal brainstem (solitary complex, SC; locus coeruleus, LC) are coupled by gap junctions, as reported primarily in rat brainstem slices, and the methods used to study gap junctions in brain slices, are critiqued and reviewed. Gap junctions allow intercellular communication that could be important in either electrical coupling (intercellular flow of ionic current), metabolic coupling (intercellular flow of signaling molecules), or both, ultimately influencing excitability within the SC and LC during respiratory acidosis. Gap junctions may also provide a mechanism for modulating neuronal activity in the network under conditions that lead to increased or decreased central respiratory chemosensitivity. Indirect measures of electrical coupling suggest that junctional conductance between chemosensitive neurons is relatively insensitive to a broad range of intracellular pH (pH(i)), ranging from pH(i) approximately 7.49 to approximately 6.71 at 35-37 degrees C. In contrast, further reductions in pH(i), down through pH(i) approximately 6.67, abolish indirect measures of electrical coupling.

Central chemosensitivity, sleep, and wakefulness

Neurons in many regions of the lower brain are chemosensitive in vitro. Focal acidification of these same and other regions in vivo can stimulate breathing indicating the presence of chemoreception. Why are there so many sites for central chemoreception? This review evaluates data obtained from unanesthetized rats at three central chemoreceptor sites, the retrotrapezoid nucleus (RTN), the medullary raphé, and the nucleus tractus solitarius (NTS) and extends ideas concerning two hypotheses, which were recently formulated (Nattie, E., 2000. Respir. Physiol. 122, 223-235). (1) The high overall sensitivity of the respiratory control system in the unanesthetized state to small increases in arterial CO(2) relies on an additive or greater effect of these multiple chemoreceptor sites. (2) Chemoreceptor sites can vary in effectiveness dependent on the state of arousal. These ideas fit into a more speculative and general hypothesis that central chemoreceptors are organized in a hierarchical manner as proposed for temperature sensing and thermoregulation (Satinoff, E., 1978. Science 201, 16-22). The presence of a number of chemosensitive sites with varying thresholds, sensitivity, and arousal dependence provides finely tuned control and stability for breathing.

Ventrolateral neurons of medullary organotypic cultures: intracellular pH regulation and bioelectric activity

The hypothesized role of the intracellular pH (pH(i)) as a proximate stimulus for central chemosensitive neurons is reviewed on the basis of data obtained from organotypic cultures of the medulla oblongata (obex level) of new born rats (OMC). Within OMC a subset of neurons responds to hypercapnia as do neurons in the same (or similar) brain areas in vivo. Maneuvers altering intra- and/or extracellular pH (pH(o)) such as hypercapnia, bicarbonate-withdrawal, or ammonium pre-pulses, evoked well defined changes of the neuronal pH(i). During hypercapnia (pH(o) 7.0) or bicarbonate-withdrawal (pH(o) 7.4) most ventrolateral neurons adopted a pH(i) which was < or = 0.2 pH units below the steady state pH(i), while signs of pH(i)-regulation occurred only in a small fraction of neurons. During all treatments leading to intracellular acidosis, bioelectric activity of chemosensitive neurons increased and was often indistinguishable from the response to hypercapnia, regardless of whether pH(o) was unchanged, decreased or increased during the treatment. These data strongly suggest that the pH(i) acts as proximate stimulus. The mode of acid extrusion of chemosensitive neurons is, therefore, of major importance for the control of central chemosensitivity. Immunocytochemical data, pH(i) measurements and neuropharmacological studies with novel drugs pointed to the Na(+)/H(+) exchanger subtype 3 (NHE3) as a main acid extruder in ventrolateral chemosensitive neurons. Possible functions and neuropharmacological strategies arising from this very local NHE3 expression are discussed.

Monoaminergic neurons, chemosensation and arousal

In recent years, immense progress has been made in understanding central chemosensitivity at the cellular and functional levels. Combining molecular biological techniques (early gene expression as an index of cell activation) with neurotransmitter immunohistochemistry, new information has been generated related to neurochemical coding in chemosensory cells. We found that CO(2) exposure leads to activation of discrete cell groups along the neuraxis, including subsets of cells belonging to monoaminergic cells, noradrenaline-, serotonin-, and histamine-containing neurons. In part, they may play a modulatory role in the respiratory response to hypercapnia that could be related to their behavioral state control function. Activation of monoaminergic neurons by an increase in CO(2)/H(+) could facilitate respiratory related motor discharge, particularly activity of upper airway dilating muscles. In addition, these neurons coordinate sympathetic and parasympathetic tone to visceral organs, and participate in adjustments of blood flow with the level of motor activity. Any deficit in CO(2) chemosensitivity of a network composed of inter-related monoaminergic nuclei might lead to disfacilitation of motor outputs and to failure of neuroendocrine and homeostatic responses to life-threatening challenges (e.g. asphyxia) during sleep.

Central chemosensitivity of respiration: a brief overview

In this introductory article we make use of the work reviewed in detail by a number of contributors to this Special Issue (Respir. Physiol., 2001) to provide an outline of current approaches to identifying brainstem CO(2)/pH-chemosensitive neurones. The section headings which we have adopted are intended to reflect particular issues rather than experimental techniques, though some of these issues arise out of the choice of preparation and the advantages and limitations which follow from such a choice. We have also considered whether, in spite of the diversity in the kinds of neurones usually considered to be chemosensitive, there are any indications for shared or uniform features. Again, this is based on the material published together in this volume. Finally, and more speculatively, we suggest that the dendritic organization of chemosensitive neurones may play an important role in chemoreception, not simply as a means of sampling the stimulus but also as a way of compartmentalizing the effects of pH in relation to other aspects of a neurone's activity.

Intracellular pH regulation of neurons in chemosensitive and nonchemosensitive areas of brain slices

The role of changes of intracellular pH (pH(i)) as the proximal signal in central chemosensitive neurons has been studied. pH(i) recovery from acidification is mediated by Na(+)/H(+) exchange in all medullary neurons and pH(i) recovery from alkalinization is mediated by Cl(-)/HCO(3)(-) exchange in most medullary neurons. These exchangers are more sensitive to inhibition by changes in extracellular pH (pH(o)) in neurons from chemosensitive regions compared to those from nonchemosensitive regions. Thus, neurons from chemosensitive regions exhibit a maintained intracellular acidification in response to hypercapnic acidosis but they show pH(i) recovery in response to isohydric hypercapnia. A similar pattern of pH(i) response is seen in other CO(2)/H(+)-responsive cells, including glomus cells, sour taste receptor cells, and chemosensitive neurons from snails, suggesting that a maintained fall of pH(i) is a common feature of the proximal signal in all CO(2)/H(+)-sensitive cells. To further evaluate the potential role of pH(i) changes as proximal signals for chemosensitive neurons, studies must be done to: determine why a lack of pH(i) recovery from hypercapnic acidosis is seen in some nonchemosensitive neurons; establish a correlation between hypercapnia-induced changes of pH(i) and membrane potential (V(m)); compare the hypercapnia-induced pH(i) changes seen in neuronal cell bodies with those in dendritic processes; understand why the V(m) response to hypercapnia of many chemosensitive neurons is washed out when using whole cell patch pipettes; and employ knock out mice to investigate the role of certain proteins in the CO(2)/H(+) response of chemosensitive neurons.

An alternative approach to the identification of respiratory central chemoreceptors in the brainstem

Central chemoreceptors (CCRs) play a crucial role in autonomic respiration. Although a variety of brainstem neurons are CO(2) sensitive, it remains to know which of them are the CCRs. In this article, we discuss a potential alternative approach that may allow an access to the CCRs. This approach is based on identification of specific molecules that are CO(2) or pH sensitive, exist in brainstem neurons, and regulate cellular excitability. Their molecular identity may provide another measure in addition to the electrophysiologic criteria to indicate the CCRs. The inward rectifier K(+) channels (Kir) seem to be some of the CO(2) sensing molecules, as they regulate membrane potential and cell excitability and are pH sensitive. Among homomeric Kirs, we have found that even the most sensitive Kir1.1 and Kir2.3 have pK approximately 6.8, suggesting that they may not be capable of detecting hypocapnia. We have studied their biophysical properties, and identified a number of amino acid residues and molecular motifs critical for the CO(2) sensing. By comparing all Kirs using the motifs, we found the same amino acid sequence in Kir5.1, and demonstrated the pH sensitivity in heteromeric Kir4.1 and Kir5.1 channels to be pK approximately 7.4. In current clamp, we show evidence that the Kir4.1-Kir5.1 can detect P(CO(2)) changes in either hypercapnic or hypocapnic direction. Our in-situ hybridization studies have indicated that they are coexpressed in brainstem cardio-respiratory nuclei. Thus, it is likely that the heteromeric Kir4.1-Kir5.1 contributes to the CO(2)/pH sensitivity in these neurons. We believe that this line of research intended to identify CO(2) sensing molecules is an important addition to current studies on the CCRs.

Plasticity in respiratory motor control: intermittent hypoxia and hypercapnia activate opposing serotonergic and noradrenergic modulatory systems

Experimental results consistently show that the respiratory control system is plastic, such that environmental factors and experience can modify its performance. Such plasticity may represent basic neurobiological principles of learning and memory, whereby intermittent sensory stimulation produces long-term alterations (i.e. facilitation or depression) in synaptic transmission depending on the timing and intensity of the stimulation. In this review, we propose that intermittent chemosensory stimulation produces long-term changes in respiratory motor output via specific neuromodulatory systems. This concept is based on recent data suggesting that intermittent hypoxia produces a net long-term facilitation of respiratory output via the serotonergic system, whereas intermittent hypercapnia produces a net long-term depression by a mechanism associated with the noradrenergic system. There is suggestive evidence that, although both respiratory stimuli activate both modulatory systems, the balance is different. Thus, these opposing modulatory influences on respiratory motor control may provide a 'push-pull' system, preventing unchecked and inappropriate fluctuations in ventilatory drive.

Development of in vivo ventilatory and single chemosensitive neuron responses to hypercapnia in rats

We used pressure plethysmography to study breathing patterns of neonatal and adult rats acutely exposed to elevated levels of CO2. Ventilation (VE) increased progressively with increasing inspired CO2. The rise in VE was associated with an increase in tidal volume, but not respiratory rate. In all animals studied, the CO2 sensitivity (determined from the slope of the VE vs. inspired % CO2 curve) was variable on a day to day basis. Chemosensitivity was high in neonates 1 day after birth (P1) and fell throughout the first week to a minimum at about P8. Chemosensitivity rose again to somewhat higher values in P10 through adult rats. The developmental pattern of these in vivo ventilatory responses was different than individual locus coeruleus (LC) neuron responses to increased CO2. The membrane potential (V(m)) of LC neurons was measured using perforated patch (amphotericin B) techniques in brain slices. At all ages studied, LC neurons increased their firing rate by approximately 44% in response to hypercapnic acidosis (10% CO2, pH 7.0). Thus the in vivo ventilatory response to hypercapnia was not correlated with the V(m) response of individual LC neurons to hypercapnic acidosis in neonatal rats. These data suggest that CO2 sensitivity of ventilation in rats may exist in two forms, a high-sensitivity neonatal (or fetal) form and a lower-sensitivity adult form, with a critical window of very low sensitivity during the period of transition between the two (approximately P8).

CO2 dialysis in the medullary raphe of the rat increases ventilation in sleep

Central chemoreceptors are widespread within the brain stem. We hypothesize that function at different sites varies with arousal state. In unanesthetized rats, we produced focal acidification at single sites by means of microdialysis using artificial cerebrospinal fluid equilibrated with 25% CO2. Tissue acidosis, measured under anesthesia, is equivalent to that observed with 63 Torr end-tidal PCO2 and is limited to 600 microm. Focal acidification of the retrotrapezoid nucleus increased ventilation by 24% only in wakefulness via an increase in tidal volume (Li A, Randall M, and Nattie E. J Appl Physiol 87: 910-919, 1999). In this study of the medullary raphe, the effect of such focal acidification was in sleep (defined by electroencephalographic and electromyographic criteria): ventilation and frequency increased by 15-20% in non-rapid eye movement sleep, and frequency increased by 15% in rapid eye movement sleep. There was no effect in wakefulness. Chemoreception in the medullary raphe appears to be responsive in sleep. Central chemoreceptors at two different locations appear to vary in effectiveness with arousal state.

Location of central respiratory chemoreceptors in the developing tadpole

The location of central respiratory chemoreceptors in amphibian larvae may change as the central chemoreceptive function shifts from driving gill to driving lung ventilation during metamorphosis. We examined this possibility in the in vitro brain stem of the pre- and postmetamorphic Rana catesbeiana tadpole by microinjecting hypercapnic artificial cerebrospinal fluid (aCSF) while recording fictive lung ventilation. The rostral and caudal brain stem were separately explored systematically using injections of 11 nl of aCSF equilibrated with 100% CO2 that transiently acidified a 500-microm region, producing a maximum reduction in pH of 0.23 +/- 0.06 at the site of injection. In postmetamorphic tadpoles, chemoreceptive sites were concentrated in the rostral compared with the caudal brain stem. No such segregation was observed in the premetamorphic tadpole. We conclude that, as in lung rhythmogenic function, respiratory chemosensitivity emerges rostrally in the amphibian brain stem during development.

Sites of respiratory rhythmogenesis during development in the tadpole

During ontogeny, amphibian larvae experience a dramatic alteration in the motor act of breathing as the premetamorphic gill breather develops into the postmetamorphic lung ventilator. We tested the hypothesis that the site of lung rhythmogenesis relocates during metamorphosis by recording fictive lung ventilation before and after transecting the in vitro brain stem of pre- and postmetamorphic Rana catesbeiana into four segments. In premetamorphic tadpoles, the two caudalmost brain stem segments combined proved to be the minimum brain stem configuration necessary and sufficient for lung burst generation. In the postmetamorphic counterpart, this function was supplied by the combination of the two rostralmost brain stem segments. In the postmetamorphic brain stem, a 500-microm segment lying just rostral to cranial nerve IX conveys rhythmogenic capability to neighboring rostral or caudal segments. We conclude that lung rhythmogenic capability translocates rostrally during development as the tadpole shifts from gill to lung ventilation.

Age-dependent chemosensitive pontine inhibition of medullary respiratory rhythm generation in the isolated brainstem of the neonatal rat

Age-dependence and chemosensitivity of the pontine inhibitory effect on medullary respiratory rhythm generation were examined in the isolated brainstem-spinal cord of the neonatal rat. In early preparations (days 1-2), the increase in RR (Delta RR) induced by the pons resection was larger in 8% CO(2) (pH 7.2) than in 2% CO(2) (pH 7.8). That difference was not found in late preparations (days 3-4). Under a given pH, the Delta RR was larger in early preparations than in late preparations.

Sensing arterial CO(2) levels: a role for medullary P2X receptors

ATP has been shown to act as an excitatory neurotransmitter in the central nervous system. In this review, evidence is presented to indicate that when ATP is micro-injected into the ventrolateral medulla (VLM) of the rat, changes in respiratory activity are elicited. These effects, and accompanying changes in heart rate and blood pressure are mediated by P2X purinoreceptors. Immunocytochemistry indicates a prevalence of P2X(2) and P2X(6) purinoreceptors in this region of the medulla. The P2 purinoceptor antagonists, suramin and PPADS blunt the respiratory responses to changes in arterial CO(2) levels when micro-injected into the VLM. This effect is shown electrophysiologically to be mediated by purinoreceptors located primarily on respiratory neurones of the VLM including the Bötzinger complex. As the effects of agonist activation of P2X(2) purinoceptors expressed in HEK293 cells and Xenopus oocytes are potentiated by lowering pH, these data imply that the central respiratory response to CO(2) depends in part on the pH sensitivity of purinoreceptors located on inspiratory neurones. The implications for respiratory activity and control are discussed.

Mammalian brainstem chemosensitive neurones: linking them to respiration in vitro

Neurones which are excited by CO2 or H+ are present in a number of brainstem structures in addition to the ventrolateral region of the medulla, the site at which the respiratory response to hypercapnia is traditionally believed to originate. In this review we examine recent work concerned with establishing the relationship between these chemosensitive neurones and respiration, the emphasis being placed on the use for this purpose of in vitro preparations of the mammalian brainstem.

Modulation of kir4.1 and kir5.1 by hypercapnia and intracellular acidosis

CO2 chemoreception may be mediated by the modulation of certain ion channels in neurons. Kir4.1 and Kir5.1, two members of the inward rectifier K+ channel family, are expressed in several brain regions including the brainstem. To test the hypothesis that Kir4.1 and Kir5. 1 are modulated by CO2 and pH, we carried out experiments by expressing Kir4.1 and coexpressing Kir4.1 with Kir5.1 (Kir4.1-Kir5. 1) in Xenopus oocytes. K+ currents were then studied using two-electrode voltage clamp and excised patches. Exposure of the oocytes to CO2 (5, 10 and 15 %) produced a concentration-dependent inhibition of the whole-cell K+ currents. This inhibition was fast and reversible. Exposure to 15 % CO2 suppressed Kir4.1 currents by approximately 20 % and Kir4.1-Kir5.1 currents by approximately 60 %. The effect of CO2 was likely to be mediated by intracellular acidification, because selective intracellular, but not extracellular, acidification to the measured hypercapnic pH levels lowered the currents as effectively as hypercapnia. In excised inside-out patches, exposure of the cytosolic side of membranes to solutions with various pH levels brought about a dose-dependent inhibition of the macroscopic K+ currents. The pK value (-log of dissociation constant) for the inhibition was 6.03 in the Kir4.1 channels, while it was 7.45 in Kir4.1-Kir5.1 channels, an increase in pH sensitivity of 1.4 pH units. Hypercapnia without changing pH did not inhibit the Kir4.1 and Kir4.1-Kir5.1 currents, suggesting that these channels are inhibited by protons rather than molecular CO2. A lysine residue in the N terminus of Kir4.1 is critical. Mutation of this lysine at position 67 to methionine (K67M) completely eliminated the CO2 sensitivity of both the homomeric Kir4. 1 and heteromeric Kir4.1-Kir5.1. These results therefore indicate that the Kir4.1 channel is inhibited during hypercapnia by a decrease in intracellular pH, and the coexpression of Kir4.1 with Kir5.1 greatly enhances channel sensitivity to CO2/pH and may enable cells to detect both increases and decreases in PCO2 and intracellular pH at physiological levels.

CO(2) inhibits specific inward rectifier K(+) channels by decreases in intra- and extracellular pH

Hypercapnia has been shown to affect cellular excitability by modulating K(+) channels. To understand the mechanisms for this modulation, four cloned K(+) channels were studied by expressing them in Xenopus oocytes. Exposures of the oocytes to CO(2) for 4-6 min produced reversible and concentration-dependent inhibitions of Kir1.1 and Kir2.3 currents, but had no effect on Kir2.1 and Kir6.1 currents. Intra- and extracellular pH (pH(i), pH(o)) dropped during CO(2) exposures. The inhibition of Kir2.3 currents was mediated by reductions in both intra- and extracellular pH, whereas the suppression of Kir1.1 resulted from intracellular acidification. In cell-free excised inside-out patches with cytosolic-soluble factors washed out, a decrease in pH(i) produced a fast and reversible inhibition of macroscopic Kir2.3 currents. The degree of this inhibition was similar to that produced by hypercapnia when compared at the same pH(i) level. Exposure of cytosolic surface of patch membranes to a perfusate bubbled with 15% CO(2) without changing pH failed to inhibit the Kir2.3 currents. These results therefore indicate that (1) hypercapnia inhibits specific K(+) channels, (2) these inhibitions are caused by intra- and extracellular protons rather than molecular CO(2), and (3) these effects are independent of cytosol-soluble factors.

Chemosensitivity of non-respiratory rat CNS neurons in tissue culture

Neurons from many brainstem nuclei involved in respiratory control increase their firing rate in response to acidosis in vitro, suggesting that they are central chemoreceptors. This property has been considered to be either unique to neurons involved in respiratory control, or at least very unusual for non-respiratory neurons. However, recordings of intrinsic pH responses of neurons have not been made from enough non-respiratory regions of the CNS to be certain this assumption is true. Here, we have quantified changes in firing rate of neurons cultured from the hippocampus (n=43), neocortex (n=33), and cerebellum (n=29) in response to changes in CO(2) between 3% and 9% (pH approximately 7.6-7.2) after blockade of glutamatergic and GABAergic transmission. The responses of neurons from these three regions were similar, with a subset of neurons (12% of the total 105) inhibited by acidosis, decreasing their firing rate to a mean of 70% of control in response to a decrease in pH of 0.2. Some neurons (5% of total) were stimulated by acidosis, with an increase in firing rate to a mean of 175% of control in response to a decrease in pH of 0.2. We previously quantified chemosensitivity of neurons from the medullary raphe using the same methods [W. Wang, J.H. Pizzonia, G.B. Richerson, Chemosensitivity of rat medullary raphe neurones in primary tissue culture, J. Physiol., 511 (1998) 433-450]. Compared to these non-respiratory neurons, more raphe neurons were stimulated by acidosis (22%), and the average response was greater (to 300% of control) in response to the same stimulus. Thus, over a physiologically relevant pH range, stimulation by acidosis occurs in a significant percentage of neurons not involved in respiratory chemoreception. However, the degree of chemosensitivity of these neurons was less than medullary raphe neurons under the same conditions. Chemosensitivity is not an all-or-none neuronal property, and the degree of chemosensitivity may be relevant to the role neurons play in sensing pH in vivo.

ATP as a mediator of mammalian central CO2 chemoreception

1. A role for P2 purinoceptors in the chemosensory response of respiratory neurones localised in the ventrolateral medulla to changes in arterial CO2 levels was investigated in the anaesthetised rat. Extracellular recordings were made from different classes of respiratory neurone and the effects of P2 receptor blockade on CO2-evoked changes in activity investigated. 2. Increasing inspired CO2 excited 85 % of inspiratory neurones in the pre-Botzinger complex. In all cases, CO2-evoked excitation was blocked by ionophoretic application of the P2 receptor antagonists suramin (0.02 M) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 100 microM), but not the adenosine receptor antagonist 8-phenyltheophylline (8-PT; 100 microM). Suramin and PPADS often reduced ongoing activity, and blocked the excitatory effects of ATP. Inspiratory neurones were also excited by the P2X receptor agonist alphabeta-methyleneATP, suggesting a specific role for P2X receptors. 3. Sixty-six per cent of pre-inspiratory neurones were also excited by CO2. This effect was reduced or abolished by prior application of P2 receptor antagonists. Although post-inspiratory and expiratory neurones were excited by increasing levels of CO2, and also by ionophoretically applied ATP, the CO2-evoked effects were unaffected by P2 receptor blockade. 4. We suggest that ATP, possibly acting via P2X purinoceptors localised within the ventral respiratory group, is involved in central chemoreception. Specifically, these distinctive CO2-P2X-mediated actions were observed only in inspiratory neurones (incrementing inspiratory neurones and pre-inspiratory neurones), which appear to have purinoceptors with pH sensitivity that can account for the actions of CO2 in modifying ventilatory activity.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Locus coeruleus neurones in vitro: pH-sensitive oscillations of membrane potential in an electrically coupled network

The response to hypercapnic acidosis (2-8% CO2, bath pH 7.8-7.2) was examined in whole cell recordings from neonatal (P1 to P5) rat Locus coeruleus (LC) neurones in the in vitro brainstem-spinal cord preparation exposed to low Ca2+ (0.2 mM)-high Mg2+ (5 mM). This medium suppressed chemical synaptic transmission and resulted in a pattern of subthreshold oscillations of membrane potential and rhythmic burst discharge which was synchronized throughout the network. The oscillation was suppressed, and the discharge of individual neurones desynchronized, by the gap junction uncoupler, carbenoxolone, indicating that in low Ca2+-high Mg2+ LC neurones form an electrically coupled network. Switching from 2 to 8% CO2 decreased the oscillation amplitude and increased its frequency. The oscillation was suppressed by external Cd2+ and by TTX. but persisted during injection into the cell soma of QX-314. We conclude that in LC neurones acidosis increases the frequency of a Ca2+- and Na+-dependent dendritic oscillator which is synchronized by gap junction coupling throughout the network. This coupling is retained during acidosis.

Central CO2 chemoreception: a mechanism involving P2 purinoceptors localized in the ventrolateral medulla of the anaesthetized rat

1. The involvement of P2 purinoceptors in chemosensory function in the ventrolateral regions of the medulla oblongata was investigated in the anaesthetized rat. We have investigated the effect of antagonizing, or desensitizing, P2 receptors in the retrofacial area of the ventrolateral medulla on factors modifying respiratory activity. 2. Bilateral microinjection of suramin (50 nl, 0.02 M), a P2 purinoceptor antagonist, into the retrofacial area in the artificially ventilated rat reduced resting phrenic nerve discharge. It also markedly affected the response of the phrenic nerve to increases in arterial CO2. Under conditions of hyperoxic, hypocapnic apnoea, the mean threshold for inducing phrenic nerve activity was raised significantly (from an end-tidal CO2 of 2.5 % to 4.5 %, n = 9). 3. In addition, the slope of the respiratory response curve to increases in CO2 was reduced after suramin. A similar effect was observed after desensitization of certain P2X receptors with alphabeta-methyleneATP. As arterial levels of O2 were greater than 100 mmHg, and an equivalent pattern of response was observed in sino-aortically denervated and vagotomized animals, we believe any contribution of the peripheral chemoreceptors to be minimal. 4. Our data suggest that respiratory neurones within the retrofacial area (Botzinger complex) represent part of the central site of action of CO2 on respiration. Moreover, our observations lead us to suggest that CO2-evoked changes in respiration are mediated at least in part by P2X purinoceptors.