Respiration physiology最新文献

The brain is a major energy consumer and dependent on carbohydrate and oxygen supply. Electrical and synaptic activity of neurons can only be sustained given sufficient availability of ATP. Glial cells, which have long been assigned trophic functions, seem to play a pivotal role in meeting the energy requirements of active neurons. Under conditions of high neuronal activity, a number of glial functions, such as the maintenance of ion homeostasis, neurotransmitter clearance from synaptic domains, the supply of energetic compounds and calcium signalling, are challenged. In the vertebrate brain, astrocytes may increase glucose utilization and release lactate, which is taken up and consumed by neurons to generate ATP by oxidative metabolism. The CO₂ produced is processed primarily in astrocytes, which display the major activity of carboanhydrase in the brain. Protons and bicarbonate in turn may contribute to drive acid/base-coupled transporters. In the present article a scenario is discussed which couples the transfer of energy and the conversion of CO₂ with the high-affinity glutamate uptake and other transport processes at glial and neuronal cell membranes. The transporters can be linked to glial signalling and may cooperate with each other at the cellular level. This could save energy, and would render energy exchange processes between glial cells and neurons more effective. Functions implications and physiological responses, in particular in chemosensitive brain areas, are discussed.

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 ≤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.

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.

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⁻/HCO₃⁻ 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 CO₂/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 CO₂/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 CO₂/H⁺ response of chemosensitive neurons.

Central chemoreceptors (CCRs) play a crucial role in autonomic respiration. Although a variety of brainstem neurons are CO₂ 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 CO₂ 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 CO₂ 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∼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 CO₂ 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∼7.4. In current clamp, we show evidence that the Kir4.1–Kir5.1 can detect P_CO2 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 CO₂/pH sensitivity in these neurons. We believe that this line of research intended to identify CO₂ sensing molecules is an important addition to current studies on the CCRs.

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 Ca²⁺-high Mg²⁺ generated synchronized rhythmic bursts which remained synchronized at high CO₂ (up to 20%). This rhythm was suppressed by TTX. Substitution of Ba²⁺ for Ca²⁺ in ACSF generated a synchronized rhythm which was TTX-insensitive. The frequency of this rhythm increased by 31±16% on raising CO₂ 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.

Central respiratory chemoreceptors adjust respiratory drive in a homeostatic response to alterations in brain pH and/or P_CO2. Multiple brainstem sites are proposed as neural substrates for central chemoreception, but molecular substrates that underlie chemosensitivity in respiratory neurons have not been identified. In rat brainstem neurons expressing transcripts for TASK-1, a two-pore domain K⁺ channel, we characterized K⁺ currents with kinetic and voltage-dependent properties identical to cloned rat TASK-1 currents. Native currents were sensitive to acid and alkaline shifts in the same physiological pH range as TASK-1 (pK∼7.4), and native and cloned pH-sensitive currents were modulated similarly by neurotransmitters and inhalational anesthetics. This pH-sensitive TASK-1 channel is an attractive candidate to mediate chemoreception because it is functionally expressed in respiratory-related neurons, including airway motoneurons and putative chemoreceptor neurons of locus coeruleus (LC). Inhibition of TASK-1 channels by extracellular acidosis can depolarize and increase excitability in those cells, thereby contributing to chemoreceptor function in LC neurons and directly enhancing respiratory motoneuronal output.

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 ≈7.49 to ≈6.71 at 35–37 °C. In contrast, further reductions in pH_i, down through pH_i ≈6.67, abolish indirect measures of electrical coupling.

The medullary raphé contains two subtypes of chemosensitive neuron: one that is stimulated by acidosis and another that is inhibited. Both types of neuron are putative chemoreceptors, proposed to act in opposite ways to modulate respiratory output and other pH sensitive brain functions. In this review, we will discuss the cellular properties of these chemosensitive raphé neurons when studied in vitro using brain slices and primary dissociated cell culture. Quantification of chemosensitivity of raphé neurons indicates that they are highly sensitive to small changes in extracellular pH (pH_o) between 7.2 and 7.6. Stimulation by acidosis occurs only in the specific phenotypic subset of neurons within the raphé that are serotonergic. These serotonergic neurons also have other properties consistent with a specialized role in chemoreception. Homologous serotonergic neurons are present within the ventrolateral medulla (VLM), and may have contributed to localization of respiratory chemoreception to that region. Chemosensitivity of raphé neurons increases in the postnatal period in rats, in parallel with development of respiratory chemoreception in vivo. An abnormality of serotonergic neurons of the ventral medulla has been identified in victims of sudden infant death syndrome (SIDS). The cellular properties of serotonergic raphé neurons suggest that they play a role in the CNS response to hypercapnia, and that they may contribute to interactions between the sleep/wake cycle and respiratory control.

Evolution of central respiratory chemosensitivity has been linked traditionally to the need for carbon dioxide regulation that accompanied the evolution of air breathing in terresterial animals. We examined the validity of this linkage by investigating the possibility of central chemoreception in air breathing fish that diverged from the amphibian lineage long before the appearance of terrestriality. We showed that the isolated brainstem preparation of the long nose gar (Lepisosteus osseus) produces a putative motor pattern for lung ventilation, which is responsive to CO₂. These findings, together with more inferential evidence, suggest an association between air breathing and central chemosensitivity in aquatic animals that spans the major branches in vertebrate phylogeny. Furthermore, developmental observations in tadpoles suggest that the neural substrates for central chemoreception exist in proximity to that for rhythm generation. We postulate that a primitive ancestral CPG, sensitive to CO₂ is conserved and is evidenced in the intrinsic coupling of respiratory CPG and central chemoreception in modern tetrapods.