Cell-cell coupling between CO2-excited neurons in the dorsal medulla oblongata

The O2-sensitive brain stem, hyperoxic hyperventilation, and CNS oxygen toxicity

Central nervous system oxygen toxicity (CNS-OT) is a complex disorder that presents, initially, as a sequence of cardio-respiratory abnormalities and nonconvulsive signs and symptoms (S/Sx) of brain stem origin that culminate in generalized seizures, loss of consciousness, and postictal cardiogenic pulmonary edema. The risk of CNS-OT and its antecedent “early toxic indications” are what limits the use of hyperbaric oxygen (HBO2) in hyperbaric and undersea medicine. The purpose of this review is to illustrate, based on animal research, how the temporal pattern of abnormal brain stem responses that precedes an “oxtox hit” provides researchers a window into the early neurological events underlying seizure genesis. Specifically, we focus on the phenomenon of hyperoxic hyperventilation, and the medullary neurons presumed to contribute in large part to this paradoxical respiratory response; neurons in the caudal Solitary complex (cSC) of the dorsomedial medulla, including putative CO2 chemoreceptor neurons. The electrophysiological and redox properties of O2-/CO2-sensitive cSC neurons identified in rat brain slice experiments are summarized. Additionally, evidence is summarized that supports the working hypothesis that seizure genesis originates in subcortical areas and involves cardio-respiratory centers and cranial nerve nuclei in the hind brain (brainstem and cerebellum) based on, respectively, the complex temporal pattern of abnormal cardio-respiratory responses and various nonconvulsive S/Sx that precede seizures during exposure to HBO2.

Normobaric hyperoxia stimulates superoxide and nitric oxide production in the caudal solitary complex of rat brain slices

Central CO₂-chemosensitive neurons in the caudal solitary complex (cSC) are stimulated not only by hypercapnic acidosis, but by hyperoxia as well. While a cellular mechanism for the CO₂ response has yet to be isolated, previous data show that a redox-sensitive mechanism underlies neuronal excitability to hyperoxia. However, it remains unknown how changes in Po₂ affect the production of reactive oxygen and nitrogen species (RONS) in the cSC that can lead to increased cellular excitability and, with larger doses, to cellular dysfunction and death. To this end, we used fluorescence microscopy in real time to determine how normobaric hyperoxia increases the production of key RONS in the cSC. Because neurons in the region are CO₂ sensitive, we also examined the potential effects of CO₂ narcosis, used during euthanasia before brain slice harvesting, on RONS production. Our findings show that normobaric hyperoxia (0.4 → 0.95 atmospheres absolute O₂) increases the fluorescence rates of fluorogenic dyes specific to both superoxide and nitric oxide. Interestingly, different results were seen for superoxide fluorescence when CO₂ narcosis was used during euthanasia, suggesting long-lasting changes in superoxide production and/or antioxidant activity subsequent to CO₂ narcosis before brain slicing. Further research needs to distinguish whether the increased levels of RONS reported here are merely increases in oxidative and nitrosative signaling or, alternatively, evidence of redox and nitrosative stress.

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.

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats

This study was designed to investigate brain connections among chemosensitive areas in newborn rats. Rhodamine beads were injected unilaterally into the locus coeruleus (LC) or into the caudal part of the nucleus tractus solitarius (cNTS) in Sprague-Dawley rat pups (P7-P10). Rhodamine-labeled neurons were patched in brainstem slices to study their electrophysiological responses to hypercapnia and to determine if chemosensitive neurons are communicating between LC and cNTS regions. After 7-10 days, retrograde labeling was observed in numerous areas of the brainstem, including many chemosensitive regions, such as the contralateral LC, cNTS and medullary raphe. Whole-cell patch clamp was done in cNTS. In 4 of 5 retrogradely labeled cNTS neurons that projected to the LC, firing rate increased in response to hypercapnic acidosis (15% CO2), even in synaptic blockade medium (SNB) (high Mg(2+)/low Ca(2+)). In contrast, 2 of 3 retrogradely labeled LC neurons that projected to cNTS had reduced firing rate in response to hypercapnic acidosis, both in the presence and absence of SNB. Extensive anatomical connections among chemosensitive brainstem regions in newborn rats were found and at least for the LC and cNTS, the connections involve some CO2-sensitive neurons. Such anatomical and functional coupling suggests a complex central respiratory control network, such as seen in adult rats, is already largely present in neonatal rats by at least day P7-P10. Since the NTS and the LC play a major role in memory consolidation, our results may also contribute to the understanding of the development of memory consolidation.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Substance P differentially modulates firing rate of solitary complex (SC) neurons from control and chronic hypoxia-adapted adult rats

NK1 receptors, which bind substance P, are present in the majority of brainstem regions that contain CO2/H(+)-sensitive neurons that play a role in central chemosensitivity. However, the effect of substance P on the chemosensitive response of neurons from these regions has not been studied. Hypoxia increases substance P release from peripheral afferents that terminate in the caudal nucleus tractus solitarius (NTS). Here we studied the effect of substance P on the chemosensitive responses of solitary complex (SC: NTS and dorsal motor nucleus) neurons from control and chronic hypoxia-adapted (CHx) adult rats. We simultaneously measured intracellular pH and electrical responses to hypercapnic acidosis in SC neurons from control and CHx adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. Substance P significantly increased the basal firing rate in SC neurons from control and CHx rats, although the increase was smaller in CHx rats. However, substance P did not affect the chemosensitive response of SC neurons from either group of rats. In conclusion, we found that substance P plays a role in modulating the basal firing rate of SC neurons but the magnitude of the effect is smaller for SC neurons from CHx adult rats, implying that NK1 receptors may be down regulated in CHx adult rats. Substance P does not appear to play a role in modulating the firing rate response to hypercapnic acidosis of SC neurons from either control or CHx adult rats.

Blockade of Na+/H+ exchanger type 3 causes intracellular acidification and hyperexcitability via inhibition of pH-sensitive K+ channels in chemosensitive respiratory neurons of the dorsal vagal nucleus in rats

Extracellular pH (pHe) and intracellular pH (pHi) are important factors for the excitability of chemosensitive central respiratory neurons that play an important role in respiration and obstructive sleep apnea. It has been proposed that inhibition of central Na(+)/H(+) exchanger 3 (NHE-3), a key pHi regulator in the brainstem, decreases the pHi, leading to membrane depolarization for the maintenance of respiration. However, how intracellular pH affects the neuronal excitability of respiratory neurons remains largely unknown. In this study, we showed that NHE-3 mRNA is widely distributed in respiration-related neurons of the rat brainstem, including the dorsal vagal nucleus (DVN). Whole-cell patch clamp recordings from DVN neurons in brain slices revealed that the standing outward current (Iso) through pH-sensitive K(+) channels was inhibited in the presence of the specific NHE-3 inhibitor AVE0657 that decreased the pHi. Exposure of DVN neurons to an acidified pHe and AVE0657 (5 μmol/L) resulted in a stronger effect on firing rate and Iso than acidified pHe alone. Taken together, our results showed that intracellular acidification by blocking NHE-3 results in inhibition of a pH-sensitive K(+) current, leading to synergistic excitation of chemosensitive DVN neurons for the regulation of respiration.

Analyzing the effects of gap junction blockade on neural synchrony via a motoneuron network computational model

In specific regions of the central nervous system (CNS), gap junctions have been shown to participate in neuronal synchrony. Amongst the CNS regions identified, some populations of brainstem motoneurons are known to be coupled by gap junctions. The application of various gap junction blockers to these motoneuron populations, however, has led to mixed results regarding their synchronous firing behavior, with some studies reporting a decrease in synchrony while others surprisingly find an increase in synchrony. To address this discrepancy, we employ a neuronal network model of Hodgkin-Huxley-style motoneurons connected by gap junctions. Using this model, we implement a series of simulations and rigorously analyze their outcome, including the calculation of a measure of neuronal synchrony. Our simulations demonstrate that under specific conditions, uncoupling of gap junctions is capable of producing either a decrease or an increase in neuronal synchrony. Subsequently, these simulations provide mechanistic insight into these different outcomes.

Acid-sensing ion channels contribute to chemosensitivity of breathing-related neurons of the nucleus of the solitary tract

Cellular mechanisms of central pH chemosensitivity remain largely unknown. The nucleus of the solitary tract (NTS) integrates peripheral afferents with central pathways controlling breathing; NTS neurons function as central chemosensors, but only limited information exists concerning the ionic mechanisms involved. Acid-sensing ion channels (ASICs) mediate chemosensitivity in nociceptive terminals, where pH values ∼6.5 are not uncommon in inflammation, but are also abundantly expressed throughout the brain where pHi s tightly regulated and their role is less clear. Here we test the hypothesis that ASICs are expressed in NTS neurons and contribute to intrinsic chemosensitivity and control of breathing. In electrophysiological recordings from acute rat NTS slices, ∼40% of NTS neurons responded to physiological acidification (pH 7.0) with a transient depolarization. This response was also present in dissociated neurons suggesting an intrinsic mechanism. In voltage clamp recordings in slices, a pH drop from 7.4 to 7.0 induced ASIC-like inward currents (blocked by 100 μM amiloride) in ∼40% of NTS neurons, while at pH ≤ 6.5 these currents were detected in all neurons tested; RT-PCR revealed expression of ASIC1 and, less abundantly, ASIC2 in the NTS. Anatomical analysis of dye-filled neurons showed that ASIC-dependent chemosensitive cells (cells responding to pH 7.0) cluster dorsally in the NTS. Using in vivo retrograde labelling from the ventral respiratory column, 90% (9/10) of the labelled neurons showed an ASIC-like response to pH 7.0, suggesting that ASIC currents contribute to control of breathing. Accordingly, amiloride injection into the NTS reduced phrenic nerve activity of anaesthetized rats with an elevated arterial P(CO(2)) .

Cardiorespiratory and neural consequences of rats brought past their aerobic dive limit

The mammalian diving response is a dramatic autonomic adjustment to underwater submersion affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is known to be modulated by the parasympathetic nervous system, arterial blood pressure is modulated via the sympathetic system, and still other circuits modulate the respiratory changes. In the present study, we investigate the submergence of rats brought past their aerobic dive limit, defined as the diving duration beyond which blood lactate concentration increases above resting levels. Hemodynamic measurements were made during underwater submergence with biotelemetric transmitters, and blood was drawn from cannulas previously implanted in the rats' carotid arteries. Such prolonged submersion induces radical changes in blood chemistry; mean arterial PCO(2) rose to 62.4 Torr, while mean arterial PO(2) and pH reached nadirs of 21.8 Torr and 7.18, respectively. Despite these radical changes in blood chemistry, the rats neither attempted to gasp nor breathe while underwater. Immunohistochemistry for Fos protein done on their brains revealed numerous Fos-positive profiles. Especially noteworthy were the large number of immunopositive profiles in loci where presumptive chemoreceptors are found. Despite the activation of these presumptive chemoreceptors, the rats did not attempt to breathe. Injections of biotinylated dextran amine were made into ventral parts of the medullary dorsal horn, where central fibers of the anterior ethmoidal nerve terminate. Labeled fibers coursed caudal, ventral, and medial from the injection to neurons on the ventral surface of the medulla, where numerous Fos-labeled profiles were seen in the rats brought past their aerobic dive limit. We propose that this projection inhibits the homeostatic chemoreceptor reflex, despite the gross activation of chemoreceptors.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Mechanisms of the respiratory activity of leptin at the level of the solitary tract nucleus

Acute experiments on anesthetized laboratory rats were performed to study the effects microinjections of 10(-4) M leptin into the solitary tract nucleus on the extent of the Hering-Breuer inflation reflex in ventilatory reactions to hypercapnia. Local administration of leptin into this area led to inhibition of the Hering-Breuer reflex. The extent of ventilatory responses to hypercapnia, conversely, increased, which may provide evidence that leptin has a modulatory influence on central chemoreceptors. These physiological mechanisms of action probably play a leading role in mediating the stimulatory respiratory effects of leptin at the level of the solitary tract nucleus.

Hypercapnia causes cellular oxidation and nitrosation in addition to acidosis: implications for CO2 chemoreceptor function and dysfunction

Cellular mechanisms of CO2 chemoreception are discussed and debated in terms of the stimuli produced during hypercapnic acidosis and their molecular targets: protons generated by the hydration of CO2 and dissociation of carbonic acid, which target membrane-bound proteins and lipids in brain stem neurons. The CO2 hydration reaction, however, is not the only reaction that CO2 undergoes that generates molecules capable of modifying proteins and lipids. Molecular CO2 also reacts with peroxynitrite (ONOO-), a reactive nitrogen species (RNS), which is produced from nitric oxide (*NO) and superoxide (*O2-). The CO2/ONOO- reaction, in turn, produces additional nitrosative and oxidative reactive intermediates. Furthermore, protons facilitate additional redox reactions that generate other reactive oxygen species (ROS). ROS/RNS generated by these redox reactions may act as additional stimuli of CO2 chemoreceptors since neurons in chemosensitive areas produce both *NO and *O2- and, therefore, ONOO-. Perturbing *NO, *O2-, and ONOO- activities in chemosensitive areas modulates cardiorespiration. Moreover, neurons in at least one chemosensitive area, the solitary complex, are stimulated by cellular oxidation. Together, these data raise the following two questions: 1) do pH and ROS/RNS work in tandem to stimulate CO2 chemoreceptors during hypercapnic acidosis; and 2) does nitrosative stress and oxidative stress contribute to CO2 chemoreceptor dysfunction? To begin considering these two issues and their implications for central chemoreception, this minireview has the following three goals: 1) summarize the nitrosative and oxidative reactions that occur during hypercapnic acidosis and isocapnic acidosis; 2) review the evidence that redox signaling occurs in chemosensitive areas; and 3) review the evidence that neurons in the solitary complex are stimulated by cellular oxidation.

Glia modulation of the extracellular milieu as a factor in central CO2 chemosensitivity and respiratory control

We discuss the influence of astrocytes on respiratory function, particularly central CO2 chemosensitivity. Fluorocitrate (FC) poisons astrocytes, and studies in intact animals using FC provide strong evidence that disrupting astrocytic function can influence CO2 chemosensitivity and ventilation. Gap junctions interconnect astrocytes and contribute to K+ homeostasis in the extracellular fluid (ECF). Blocking gap junctions alters respiratory control, but proof that this is truly an astrocytic effect is lacking. Intracellular pH regulation of astrocytes has reciprocal effects on extracellular pH. Electrogenic sodium-bicarbonate transport (NBCe) is present in astrocytes. The activity of NBCe alkalinizes intracellular pH and acidifies extracellular pH when activated by depolarization (and a subset of astrocytes are depolarized by hypercapnia). Thus, to the extent that astrocytic intracellular pH regulation during hypercapnia lowers extracellular pH, astrocytes will amplify the hypercapnic stimulus and may influence central chemosensitivity. However, the data so far provide only inferential support for this hypothesis. A lactate shuttle from astrocytes to neurons seems to be active in the retrotrapezoid nucleus (RTN) and important in setting the chemosensory stimulus in the RTN (and possibly other chemosensory nuclei). Thus astrocytic processes, so vital in controlling the constituents of the ECF in the central nervous system, may profoundly influence central CO2 chemosensitivity and respiratory control.

CO2 chemoreception in cardiorespiratory control

Considerable progress has been made elucidating the cellular signals and ion channel targets involved in the response to increased CO2/H+ of brain stem neurons from chemosensitive regions. Intracellular pH (pHi) does not exhibit recovery from an acid load when extracellular pH (pHo) is also acid. This lack of pHi recovery is an essential but not unique feature of all chemosensitive neurons. These neurons have pH-regulating transporters, especially Na+/H+ exchangers, but some may also contain HCO3--dependent transporters as well. Studies in locus ceruleus (LC) neurons have shown that firing rate will increase in response to decreased pHi or pHo but not in response to increased CO2 alone. A number of K+ channels, as well as other channels, have been suggested to be targets of these pH changes with a fall of pH inhibiting these channels. In neurons from some regions it appears that multiple signals and multiple channels are involved in their chemosensitive response while in neurons from other regions a single signal and/or channel may be involved. Despite the progress, a number of key issues remain to be studied. A detailed study of chemosensitive signaling needs to be done in neurons from more brain stem regions. Fully describing the chemosensitive signaling pathways in brain stem neurons will offer new targets for therapies to alter the strength of central chemosensitivity and will yield new insights into the reason why there are multiple central chemoreceptive sites.

Sensing, physiological effects and molecular response to elevated CO2 levels in eukaryotes

Carbon dioxide (CO₂) is an important gaseous molecule that maintains biosphere homeostasis and is an important cellular signalling molecule in all organisms. The transport of CO₂ through membranes has fundamental roles in most basic aspects of life in both plants and animals. There is a growing interest in understanding how CO₂ is transported into cells, how it is sensed by neurons and other cell types and in understanding the physiological and molecular consequences of elevated CO₂ levels (hypercapnia) at the cell and organism levels. Human pulmonary diseases and model organisms such as fungi, C. elegans, Drosophila and mice have been proven to be important in understanding of the mechanisms of CO₂ sensing and response.

Chronic hypoxia suppresses the CO2 response of solitary complex (SC) neurons from rats

We studied the effect of chronic hypobaric hypoxia (CHx; 10-11% O(2)) on the response to hypercapnia (15% CO(2)) of individual solitary complex (SC) neurons from adult rats. We simultaneously measured the intracellular pH and firing rate responses to hypercapnia of SC neurons in superfused medullary slices from control and CHx-adapted adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. We found that CHx caused the percentage of SC neurons inhibited by hypercapnia to significantly increase from about 10% up to about 30%, but did not significantly alter the percentage of SC neurons activated by hypercapnia (50% in control vs. 35% in CHx). Further, the magnitudes of the responses of SC neurons from control rats (chemosensitivity index for activated neurons of 166+/-11% and for inhibited neurons of 45+/-15%) were the same in SC neurons from CHx-adapted rats. This plasticity induced in chemosensitive SC neurons by CHx appears to involve intrinsic changes in neuronal properties since they were the same in synaptic blockade medium.

Development of chemosensitivity in neurons from the nucleus tractus solitarii (NTS) of neonatal rats

We studied the development of chemosensitivity during the neonatal period in rat nucleus tractus solitarii (NTS) neurons. We determined the percentage of neurons activated by hypercapnia (15% CO(2)) and assessed the magnitude of the response by calculating the chemosensitivity index (CI). There were no differences in the percentage of neurons that were inhibited (9%) or activated (44.8%) by hypercapnia or in the magnitude of the activated response (CI 164+/-4.9%) in NTS neurons from neonatal rats of all ages. To assess the degree of intrinsic chemosensitivity in these neurons we used chemical synaptic block medium and the gap junction blocker carbenoxolone. Chemical synaptic block medium slightly decreased basal firing rate but did not affect the percentage of NTS neurons that responded to hypercapnia at any neonatal age. However, in neonates aged Collapse

Key Words Collapse

MESH Headings

• Age Factors
• Analysis of Variance
• Animals
• Animals, Newborn
• Carbenoxolone/pharmacology
• Carbon Dioxide/pharmacology
• Chemoreceptor Cells/physiology
• Drug Interactions
• Electric Stimulation
• Hypercapnia/physiopathology
• In Vitro Techniques
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Patch-Clamp Techniques/methods
• Rats
• Rats, Sprague-Dawley
• Solitary Nucleus/cytology
• Solitary Nucleus/growth & development

Collapse

Grants

• R01 HL056683-08 NHLBI NIH HHS
• HL-56683 NHLBI NIH HHS
• R01 HL056683-10 NHLBI NIH HHS
• R01 HL056683-09A1 NHLBI NIH HHS
• R01 HL056683-11 NHLBI NIH HHS
• R01 HL056683 NHLBI NIH HHS

Collapse

Affiliation(s)

Susan C. Conrad Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Nicole L. Nichols Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Nick A. Ritucci Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Jay B. Dean Department of Molecular Pharmacology and Physiology, University of South Florida College of Medicine, 12901 Bruce B. Downs Boulevard, Tampa, FL 33612
Robert W. Putnam Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435

Collapse

Intrinsic and synaptic long-term depression of NTS relay of nociceptin- and capsaicin-sensitive cardiopulmonary afferents hyperactivity

The nucleus tractus solitarius (NTS) in the caudal medulla is a gateway for a variety of cardiopulmonary afferents important for homeostatic regulation and defense against airway and cardiovascular insults and is a key central target potentially mediating the response habituation to these inputs. Here, whole-cell and field population action potential recordings and infrared imaging in rat brainstem slices in vitro revealed a compartmental pain-pathway-like organization of capsaicin-facilitated vs. nocistatin-facilitated/nociceptin-suppressed neuronal clusters in an NTS region, which receives cardiopulmonary A- and C-fiber afferents with differing capsaicin sensitivities. All capsaicin-sensitive neurons and a fraction of nociceptin-sensitive neurons expressed N-methyl-D: -aspartate (NMDA) receptor-dependent synaptic long-term depression (LTD) following afferent stimulation. All neurons also expressed activity-dependent decrease of excitability (intrinsic LTD), which converted to NMDA receptor-dependent intrinsic long-term potentiation after GABA(A) receptor blockade. Thus, distinct intrinsic and synaptic LTD mechanisms in the NTS specific to the relay of A- or C-fiber afferents may underlie the response habituation to persistent afferents hyperactivity that are associated with varying physiologic challenges and cardiopulmonary derangements-including hypertension, chronic cough, asthmatic bronchoconstriction, sustained elevated lung volume in chronic obstructive pulmonary disease or in continuous positive-airway-pressure therapy for sleep apnea, metabolic acidosis, and prolonged exposure to hypoxia at high altitude.

Characterization of the chemosensitive response of individual solitary complex neurons from adult rats

We studied the CO(2)/H(+)-chemosensitive responses of individual solitary complex (SC) neurons from adult rats by simultaneously measuring the intracellular pH (pH(i)) and electrical responses to hypercapnic acidosis (HA). SC neurons were recorded using the blind whole cell patch-clamp technique and loading the soma with the pH-sensitive dye pyranine through the patch pipette. We found that SC neurons from adult rats have a lower steady-state pH(i) than SC neurons from neonatal rats. In the presence of chemical and electrical synaptic blockade, adult SC neurons have firing rate responses to HA (percentage of neurons activated or inhibited and the magnitude of response as determined by the chemosensitivity index) that are similar to SC neurons from neonatal rats. They also have a typical response to isohydric hypercapnia, including decreased DeltapH(i), followed by pH(i) recovery, and increased firing rate. Thus, the chemosensitive response of SC neurons from adults is similar to the chemosensitive response of SC neurons from neonatal rats. Because our findings for adults are similar to previously reported values for neurons from neonatal rats, we conclude that intrinsic chemosensitivity is established early in development for SC neurons and is maintained throughout adulthood.

Carbenoxolone induced depression of rhythmogenesis in the pre-Bötzinger Complex

Background

Carbenoxolone (CBX), a gap junction uncoupler, alters the functioning of the pre-Bötzinger Complex (preBötC), a central pattern generating neuronal network important for the production of respiratory rhythm in mammals. Even when isolated in a 1/2 mm-thick slice of medulla oblongata from neonatal mouse the preBötC continues producing periodic bursts of action potentials, termed population bursts that are thought to be important in generating various patterns of inspiration, in vivo. Whether gap junction communication contributes to preBötC rhythmogenesis remains unresolved, largely because existing gap junction uncouplers exert numerous non-specific effects (e.g., inhibition of active transport, alteration of membrane conductances). Here, we determined whether CBX alters preBötC rhythmogenesis by altering membrane properties including input resistance (R_in), voltage-gated Na⁺ current (I_Na), and/or voltage-gated K⁺ current (I_K), rather than by blocking gap junction communication. To do so we used a medullary slice preparation, network-level recordings, whole-cell voltage clamp, and glycyrrhizic acid (GZA; a substance used as a control for CBX, since it is similar in structure and does not block gap junctions).

Results

Whereas neither of the control treatments [artificial cerebrospinal fluid (aCSF) or GZA (50 μM)] noticeably affected preBötC rhythmogenesis, CBX (50 μM) decreased the frequency, area and amplitude of population bursts, eventually terminating population burst production after 45–60 min. Both CBX and GZA decreased neuronal R_in and induced an outward holding current. Although neither agent altered the steady state component of I_K evoked by depolarizing voltage steps, CBX, but not GZA, increased peak I_Na.

Conclusion

The data presented herein are consistent with the notion that gap junction communication is important for preBötC rhythmogenesis. By comparing the effects of CBX and GZA on membrane properties our data a) demonstrate that depression of preBötC rhythmogenesis by CBX results from actions on another variable or other variables; and b) show that this comparative approach can be used to evaluate the potential contribution of other non-specific actions (e.g., Ca⁺⁺ conductances or active transport) of CBX, or other uncouplers, in their alteration of preBötC rhythmogenesis, or the functioning of other networks.

Intrinsic chemosensitivity of individual nucleus tractus solitarius (NTS) and locus coeruleus (LC) neurons from neonatal rats

Chemosensitive (CS) neurons are found in discrete brainstem regions, but whether the CS response of these neurons is due to intrinsic chemosensitivity of individual neurons or is mediated by changes in chemical and/or electrical synaptic input is largely unknown. We studied the effect of synaptic blockade (11.4 mM Mg2+/0.2mM Ca2+) solution (SNB) and a gap junction uncoupling agent carbenoxolone (CAR--100 microM) on the response of neurons from two CS brainstem regions, the NTS and the LC. In NTS neurons, SNB decreased spontaneous firing rate (FR). We calculated the magnitude of the FR response to hypercapnic acidosis (HA; 15% CO2) using the Chemosensitivity Index (CI). The percentage of NTS neurons activated and CI were the same in the absence and presence of SNB. Blocking gap junctions with CAR did not significantly alter spontaneous FR. CAR did not alter the CI in NTS neurons and resulted in a small decrease in the percentage of activated neurons, which was most evident in NTS neurons from rats younger than postnatal day 10. In LC neurons, SNB resulted in an increase in spontaneous FR. As with NTS neurons, SNB did not alter the percentage of activated neurons or the CI in LC neurons. CAR resulted in a small increase in spontaneous FR in LC neurons. In contrast, CAR had a marked effect on the response of LC neurons to HA: a reduced percentage of CS LC neurons and decreased CI. In summary, both NTS and LC neurons appear to contain intrinsically CS neurons. CS neurons from the two regions receive different tonic input in slices (excitatory for NTS and inhibitory for LC); however, blocking chemical synaptic input does not affect the CS response in either region. In NTS neurons, gap junction coupling plays a small role in the CS response, but gap junctions play a major role in the chemosensitivity of many LC neurons.

Hypercapnia modulates synaptic interaction of cultured brainstem neurons

CO(2) is an important metabolic product whose concentrations are constantly monitored by CO(2) chemoreceptors. However, the high systemic CO(2) sensitivity may not be achieved by the CO(2) chemoreceptors without neuronal network processes. To show modulation of network properties during hypercapnia, we studied brainstem neurons dissociated from embryonic rats (P17-19) in multielectrode arrays (MEA) after initial period (3 weeks) of culture. Spike trains of 33,622 pairs of units were analyzed using peri-event histograms (PEH). The amplitude of peri-central peaks between two CO(2)-stimulated units increased and the peak latency decreased during hypercapnia. Similar enhancement of synaptic strength was observed in those sharing a common input. These phenomena were not seen in CO(2)-unresponsive neurons. The amplitude of peri-central peaks between two CO(2) inhibited units also increased without changing latency. Over 60% CO(2)-stimulated neurons studied received mono-/oligosynaptic inputs from other CO(2)-stimulated cells, whereas only approximately 10% CO(2)-unresponsive neurons had such synaptic inputs. A small number of brainstem neurons showed electrical couplings. The coupling efficiency of CO(2)-stimulated but not CO(2)-unresponsive units was suppressed by approximately 50% with high PCO(2). Inhibitory synaptic projections were also found, which was barely affected by hypercapnia. Consistent with the strengthening of excitatory synaptic connections, CO(2) sensitivity of post-synaptic neurons was significantly higher than presynaptic neurons. The difference was eliminated with blockade of presynaptic input. Based on these indirect assessments of synaptic interaction, our PEH analysis suggests that hypercapnia appears to modulate excitatory synaptic transmissions, especially those between CO(2)-stimulated neurons.

Purinergic P2 receptors modulate excitability but do not mediate pH sensitivity of RTN respiratory chemoreceptors

The cellular mechanism(s) by which the brain senses changes in pH to regulate breathing (i.e., central chemoreception) have remained incompletely understood, in large part because the central respiratory chemoreceptors have themselves eluded detection. Here, we recorded from a newly identified population of central chemoreceptors located in the retrotrapezoid nucleus (RTN) on the ventral surface of the brainstem to test a recently proposed role for purinergic P2 receptor signaling in central respiratory chemoreception (Gourine et al., 2005). Using loose-patch current-clamp recordings in brainstem slices from rat pups (postnatal day 7-12), we indeed show purinergic modulation of pH-sensitive RTN neurons: activation of P2X receptors indirectly inhibited RTN firing by increasing inhibitory input, whereas P2Y receptor stimulation caused direct excitation of RTN chemoreceptors. However, after blocking P2 receptors with the broad-spectrum antagonists PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate) or RB2 (reactive blue 2), the pH sensitivity of RTN neurons remained intact. Therefore, we conclude that purinergic signaling can modulate RTN neuron activity but does not mediate the pH sensing intrinsic to these central respiratory chemoreceptors.

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

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

Somatic vs. dendritic responses to hypercapnia in chemosensitive locus coeruleus neurons from neonatal rats

Cardiorespiratory control is mediated in part by central chemosensitive neurons that respond to increased CO(2) (hypercapnia). Activation of these neurons is thought to involve hypercapnia-induced decreases in intracellular pH (pH(i)). All previous measurements of hypercapnia-induced pH(i) changes in chemosensitive neurons have been obtained from the soma, but chemosensitive signaling could be initiated in the dendrites of these neurons. In this study, membrane potential (V(m)) and pH(i) were measured simultaneously in chemosensitive locus coeruleus (LC) neurons from neonatal rat brain stem slices using whole cell pipettes and the pH-sensitive fluorescent dye pyranine. We measured pH(i) from the soma as well as from primary dendrites to a distance 160 mum from the edge of the soma. Hypercapnia [15% CO(2), external pH (pH(o)) 7.00; control, 5% CO(2), pH(o) 7.45] resulted in an acidification of similar magnitude in dendrites and soma ( approximately 0.26 pH unit), but acidification was faster in the more distal regions of the dendrites. Neither the dendrites nor the soma exhibited pH(i) recovery during hypercapnia-induced acidification; but both regions contained pH-regulating transporters, because they exhibited pH(i) recovery from an NH(4)Cl prepulse-induced acidification (at constant pH(o) 7.45). Exposure of a portion of the dendrites to hypercapnic solution did not increase the firing rate, but exposing the soma to hypercapnic solution resulted in a near-maximal increase in firing rate. These data show that while the pH(i) response to hypercapnia is similar in the dendrites and soma, somatic exposure to hypercapnia plays a major role in the activation of chemosensitive LC neurons from neonatal rats.

TASK-like K+ channels mediate effects of 5-HT and extracellular pH in rat dorsal vagal neurones in vitro

Dorsal vagal neurones (DVN) receive serotonergic projections from the medullary raphé nuclei, suggesting that 5-HT modulates vagal activity. A previous study has shown that 5-HT excites DVN in part by inhibition of a K+ current via postsynaptic 5-HT2A receptors. As mRNA for the two-pore-domain K+ channels TASK-1 (KCNK3) and TASK-3 (KCNK9) has been found in DVN, we investigated the possibility that 5-HT exerts its effects via inhibition of these K+ channels using whole-cell patch-clamp techniques. In current clamp, 5-HT (20 microM) elicited a depolarization by 5.1+/-1.5 mV and an increase in firing rate. In voltage clamp, 5-HT reduced the standing outward current (ISO) at -20 mV by 106+/-17 pA, inhibiting a conductance (reversal, -95+/-4 mV) which displayed Goldman-Hodgkin-Katz outward rectification, supportive of a TASK-like K+ current. Since TASK channels are modulated by extracellular pH (pHo), we next investigated the pH sensitivity of ISO in Hepes-buffered ACSF. At pHo 7.3, DVN exhibited an ISO of 147+/-15 pA at -20 mV. Acidification to pHo 6.3 reduced ISO to 85+/-13 pA, whereas raising pHo to 8.5 increased ISO to 216+/-26 pA. At pHo 7.3, ISO was inhibited by BaCl2 (IC50 465 microM), but unaffected by ZnCl2 (100 microM). 5-HT (10 microM) reduced ISO by 114+/-17 pA at pHo 7.3, but at pHo 6.3 the 5-HT-induced inhibition of ISO was significantly smaller. The present data suggest that the excitatory effects of 5-HT on DVN are mediated in part by inhibition of a TASK-like, pH-sensitive K+ conductance. The pharmacological profile of this conductance excludes TASK-3 homomers, but rather implicates TASK-1-containing channels.

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(+).

Update on connexins and gap junctions in neurons and glia in the mammalian nervous system

Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.

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

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

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

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

Oxidative stress decreases pHi and Na+/H+ exchange and increases excitability of solitary complex neurons from rat brain slices

Putative chemoreceptors in the solitary complex (SC) are sensitive to hypercapnia and oxidative stress. We tested the hypothesis that oxidative stress stimulates SC neurons by a mechanism independent of intracellular pH (pHi). pHi was measured by using ratiometric fluorescence imaging microscopy, utilizing either the pH-sensitive fluorescent dye BCECF or, during whole cell recordings, pyranine in SC neurons in brain stem slices from rat pups. Oxidative stress decreased pHi in 270 of 436 (62%) SC neurons tested. Chloramine-T (CT), N-chlorosuccinimide (NCS), dihydroxyfumaric acid, and H2O2 decreased pHi by 0.19 ± 0.007, 0.20 ± 0.015, 0.15 ± 0.013, and 0.08 ± 0.002 pH unit, respectively. Hypercapnia decreased pHi by 0.26 ± 0.006 pH unit ( n = 95). The combination of hypercapnia and CT or NCS had an additive effect on pHi, causing a 0.42 ± 0.03 ( n = 21) pH unit acidification. CT slowed pHi recovery mediated by Na+/H+ exchange (NHE) from NH4Cl-induced acidification by 53% ( n = 20) in [Formula: see text]-buffered medium and by 58% ( n = 10) in HEPES-buffered medium. CT increased firing rate in 14 of 16 SC neurons, and there was no difference in the firing rate response to CT with or without a corresponding change in pHi. These results indicate that oxidative stress 1) decreases pHi in some SC neurons, 2) together with hypercapnia has an additive effect on pHi, 3) partially inhibits NHE, and 4) directly affects excitability of CO2/H+-chemosensitive SC neurons independently of pHi changes. These findings suggest that oxidative stress acidifies SC neurons in part by inhibiting NHE, and this acidification may contribute ultimately to respiratory control dysfunction.

Connexin36 distribution in putative CO2-chemosensitive brainstem regions in rat

Recent work from our laboratory has demonstrated that the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) are expressed in neurons in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Whether the recently identified neuron-specific gap junction protein connexin36 (Cx36) is also present in these brainstem regions remains to be determined. Therefore, in the current experiments, immunoblot and immunohistochemical protocols were used to investigate the regional distribution and cellular localization of Cx36 in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed Cx36 expression in putative CO2-chemosensitive brainstem regions in each of the age groups examined, although both regional and developmental differences in the relative expression levels were detected. Immunohistochemical analyses confirmed Cx36 expression in neurons in each of the putative CO2-chemosensitive brainstem regions and revealed both somal and dendritic labeling patterns. These findings provide additional morphological evidence supporting the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction protein Cx36 also contributes to the neuroanatomical substrate for gap junctional communication, which is hypothesized to play a role in central CO2 chemoreception.

Gap junction blockade with carbenoxolone differentially affects fictive breathing in larval and adult bullfrogs

This study examined the role of gap junctional communication in the modulation of respiratory related motor output using in vitro brainstem preparations of larval (N=14) and adult (N=14) bullfrogs (Rana catesbeiana). Superfusion of the isolated brainstem for at least 1 h with the gap junction blocker carbenoxolone (CBX; 100 microM and 1 mM) dissolved in artificial cerebrospinal fluid (aCSF) elicited significant changes in respiratory-related burst frequency in both larval and adult preparations. In tadpole preparations, both concentrations of CBX significantly decreased gill and lung burst frequency over 20-40 min, with 1 mM CBX producing complete cessation of gill and lung burst activity by 40 min in all preparations. There was little or no change in other burst characteristics such as burst amplitude or duration. By contrast, superfusion of the adult brainstem preparation with CBX significantly increased lung burst frequency over 10-20 min, and caused cessation of lung burst activity with 100 microM CBX (five of seven preparations) and with 1 mM CBX (seven of seven preparations). Adult preparations that ceased activity with 100 microM CBX recovered in control aCSF, but those in 1 mM did not recover, despite up to 3 h superfusion with control aCSF. In two additional adult preparations, 1 h exposure to hypercapnic aCSF (7-10% CO2) following the cessation of fictive breathing with 1 mM CBX failed to evoke respiratory activity. The inhibition of fictive breathing in tadpoles suggests that gap junctional communication may be important for respiratory rhythmogenesis prior to the development of central CO2 chemosensitivity. Following metamorphosis to the terrestrial adult, however, gap junctional communication may contribute to regulation of respiratory frequency and possibly the transduction of central CO2 chemosensitivity.

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.

Neuronal sensitivity to hyperoxia, hypercapnia, and inert gases at hyperbaric pressures

As ambient pressure increases, hydrostatic compression of the central nervous system, combined with increasing levels of inspired Po2, Pco2, and N2 partial pressure, has deleterious effects on neuronal function, resulting in O2 toxicity, CO2 toxicity, N2 narcosis, and high-pressure nervous syndrome. The cellular mechanisms responsible for each disorder have been difficult to study by using classic in vitro electrophysiological methods, due to the physical barrier imposed by the sealed pressure chamber and mechanical disturbances during tissue compression. Improved chamber designs and methods have made such experiments feasible in mammalian neurons, especially at ambient pressures <5 atmospheres absolute (ATA). Here we summarize these methods, the physiologically relevant test pressures, potential research applications, and results of previous research, focusing on the significance of electrophysiological studies at <5 ATA. Intracellular recordings and tissue Po2 measurements in slices of rat brain demonstrate how to differentiate the neuronal effects of increased gas pressures from pressure per se. Examples also highlight the use of hyperoxia (<or=3 ATA O2) as a model for studying the cellular mechanisms of oxidative stress in the mammalian central nervous system.

Hyperbaric oxygen and chemical oxidants stimulate CO2/H+-sensitive neurons in rat brain stem slices

Hyperoxia, a model of oxidative stress, can disrupt brain stem function, presumably by an increase in O2 free radicals. Breathing hyperbaric oxygen (HBO2) initially causes hyperoxic hyperventilation, whereas extended exposure to HBO2 disrupts cardiorespiratory control. Presently, it is unknown how hyperoxia affects brain stem neurons. We have tested the hypothesis that hyperoxia increases excitability of neurons of the solitary complex neurons, which is an important region for cardiorespiratory control and central CO2/H+ chemoreception. Intracellular recordings were made in rat medullary slices during exposure to 2-3 atm of HBO2, HBO2 plus antioxidant (Trolox C), and chemical oxidants (N-chlorosuccinimide, chloramine-T). HBO2 increased input resistance and stimulated firing rate in 38% of neurons; both effects of HBO2 were blocked by antioxidant and mimicked by chemical oxidants. Hypercapnia stimulated 32 of 60 (53%) neurons. Remarkably, these CO2/H+-chemosensitive neurons were preferentially sensitive to HBO2; 90% of neurons sensitive to HBO2 and/or chemical oxidants were also CO2/H+ chemosensitive. Conversely, only 19% of HBO2-insensitive neurons were CO2/H+ chemosensitive. We conclude that hyperoxia decreases membrane conductance and stimulates firing of putative central CO2/H+-chemoreceptor neurons by an O2 free radical mechanism. These findings may explain why hyperoxia, paradoxically, stimulates ventilation.

Blockade of brain stem gap junctions increases phrenic burst frequency and reduces phrenic burst synchronization in adult rat

Recent investigations have examined the influence of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in vitro using transverse medullary slice and en bloc brain stem-spinal cord preparations obtained from neonatal (1-5 days postnatal) mice. Gap junction proteins, however, have been identified in both neurons and glia in brain stem regions implicated in respiratory control in both neonatal and adult rodents. Here, we used an in vitro arterially perfused rat preparation to examine the role of gap junctional communication on generation and modulation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents. We recorded rhythmic inspiratory motor activity from one or both phrenic nerves before and during pharmacological blockade (i.e., uncoupling) of brain stem gap junctions using carbenoxolone (100 microM), 18alpha-glycyrrhetinic acid (25-100 microM), 18beta-glycyrrhetinic acid (25-100 microM), octanol (200-300 microM), or heptanol (200 microM). During perfusion with a gap junction uncoupling agent, we observed an increase in the frequency of phrenic bursts (~95% above baseline frequency; P < 0.001) and a decrease in peak amplitude of integrated phrenic nerve discharge (P < 0.001). The increase in frequency of phrenic bursts resulted from a decrease in both T(I) (P < 0.01) and T(E) (P < 0.01). In addition, the pattern of phrenic nerve discharge shifted from an augmenting discharge pattern to a "bell-shaped" or square-wave discharge pattern in most experiments. Spectral analyses using a fast Fourier transform (FFT) algorithm revealed a reduction in the peak power of both the 40- to 50-Hz peak (corresponding to the MFO) and 90- to 110-Hz peak (corresponding to the HFO) although spurious higher frequency activity (> or =130 Hz) was observed, suggesting an overall loss or reduction in inspiratory-phase synchronization. Although additional experiments are required to identify the specific brain stem regions and cell types (i.e., neurons, glia) mediating the observed modulations in phrenic motor output, these findings suggest that gap junction communication modulates generation of respiratory rhythm and inspiratory motoneuron synchronization in adult rodents in vitro.

Role of gap junctions in CO(2) chemoreception and respiratory control

Gap junctions are composed of connexins, which are organized into intercellular channels that form transmembrane pathways between neurons (cell-cell coupling), and in some cases, neurons and glia, for exchange of ions and small molecules (metabolic coupling) and ionic current (electrical coupling). Cell-cell coupling via gap junctions has been identified in brain stem neurons that function in CO(2)/H(+) chemoreception and respiratory rhythmogenesis; however, the exact roles of gap junctions in respiratory control are undetermined. Here we review the methods commonly used to study gap junctions in the mammalian brain stem under in vitro and in vivo conditions and briefly summarize the anatomical, pharmacological, and electrophysiological evidence to date supporting roles for cell-cell coupling in respiratory rhythmogenesis and central chemoreception. Specific research questions related to the role of gap junctions in respiratory control are suggested for future research.

Gap junctions in CO(2)-chemoreception and respiratory control

Recent evidence indicates that gap junctions play a more prominent role in normal functioning of the mammalian central nervous system (CNS) than was once believed. Accumulating evidence from both neonatal and adult rodents indicates that gap junctions participate in multiple aspects of respiratory control, including central CO(2) chemoreception, respiratory rhythmogenesis, and respiratory motoneuron output. This review provides an overview of gap junction neurobiology in the mammalian CNS and presents the anatomical and electrophysiological evidence for gap junctions in CO(2) chemoreception and respiratory control.

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.

Localization of connexin26 and connexin32 in putative CO(2)-chemosensitive brainstem regions in rat

Recent studies have suggested that cell-to-cell coupling, which occurs via gap junctions, may play a role in CO(2) chemoreception. Here, we used immunoblot and immunohistochemical analyses to investigate the presence, distribution, and cellular localization of the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) in putative CO(2)-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed that both Cx subtypes were expressed in putative CO(2)-chemosensitive brainstem regions; however, regional differences in expression were observed. Immunohistochemical experiments confirmed Cx expression in each of the putative CO(2)-chemosensitive brainstem regions, and further demonstrated that Cx26 and Cx32 were found in neurons and Cx26 was also found in astrocytes in these regions. Thus, our findings suggest the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction proteins Cx26 and Cx32, at least in part, form the neuroanatomical substrate for this gap junctional communication, which is hypothesized to play a role in central CO(2) chemoreception.

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.

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.