In vitro study of H+-sensitive neurons in the ventral medullary surface of neonate rats

Molecular responses to acidosis of central chemosensitive neurons in brain

Significant advances have been made in understanding how neurons sense and respond to acidosis at the cellular level. Decrease in pH of the cerebrospinal fluid followed by hypercapnia (increased arterial CO2) is monitored by the chemosensory neurons of the medulla oblongata. Then the intracellular signalling pathways are activated to regulate specific gene expression, which leads to a hyperventilatory response. However, little is known about molecular details of such cellular responses. Recent studies have identified several transcription factors such as c-Jun, Fos and small Maf proteins that may play critical roles in the brain adaptation to hypercapnia. Hypercapnic stimulation also activates c-Jun NH2-terminal kinase (JNK) cascade via influx of extracellular Ca2+ through voltage-gated Ca2+ channels. In addition, several transmembrane proteins including Rhombex-29 (rhombencephalic expression protein-29 kDa) and Past-A (proton-associated sugar transporter-A) have been implicated in regulation of H+ sensitivity and brain acidosis-mediated energy metabolism, respectively. This review discusses current knowledge on the signalling mechanisms and molecular basis of neuronal adaptation during acidosis.

Polychlorinated biphenyl (PCB) alters acid-sensitivity of cultured neurons derived from the medulla oblongata

Polychlorinated biphenyls (PCBs) are known as environmental pollutants that may cause adverse health problems. However, little is known about the effects of PCBs on acid-sensitive neurons of the medulla oblongata, which regulate respiration. Therefore, the present study was designed to examine whether PCB alters acid-sensitivity of cultured neurons derived from the rat medulla oblongata. When extracellular pH was shifted from 7.4 to 7.0, acid-sensitive neurons showed depolarization, which was measured by voltage-sensitive fluorescent dye. Exposure to PCB (Aroclor 1254) decreased the amplitude of depolarization in low pH and increased the resting membrane potential in a dose-dependent manner. Taken together, our results indicate that PCB potentially influences acid-sensitivity through alteration of the membrane potential of acid-sensitive neurons, which could affect the regulation of respiration.

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(+).

Past-A, a novel proton-associated sugar transporter, regulates glucose homeostasis in the brain

The ventral medullary surface (VMS) of the medulla oblongata is known to be the site of the central chemosensitive neurons in mammals. These neurons sense excess H+/CO2 dissolved in the CSF and induce hyperventilation. To elucidate the mechanism of neuronal cell adaptation to changes of H+/CO2, we screened for hypercapnia-induced genes in the VMS. Here, we report cloning and characterization of a novel gene called proton-associated sugar transporter-A (Past-A), which is induced in the brain after hypercapnia and mediates glucose uptake along the pH gradient. Past-A comprises 751 amino acid residues containing 12 membrane-spanning helices, several conserved sugar transport motifs, three proline-rich regions, and leucine repeats. Past-A transcript was expressed predominantly in the brain. Moreover, the Past-A-immunoreactive neural cells were found in the VMS of the medulla oblongata, and the number of immunoreactive cells was increased by hypercapnic stimulation. Transient transfection of Past-A in COS-7 cells leads to the expression of a membrane-associated 82 kDa protein that possesses a glucose transport activity. The acidification of extracellular medium facilitated glucose uptake, whereas the addition of carbonyl cyanide m-chlorophenylhydrazone, a protonophore, inhibited glucose import. Together, our results indicate that Past-A is a brain-specific glucose transporter that may represent an adaptation mechanism regulating sugar homeostasis in neuronal cells after hypercapnia.

Microinjection of acetazolamide into the fastigial nucleus augments respiratory output in the rat

The rostral fastigial nucleus (FNr) of the cerebellum facilitates the respiratory response to hypercapnia. We hypothesized that some FNr sites are chemosensitive to focal tissue acidosis and contribute, at least partially, to respiratory modulation. Minute ventilation (VE) was recorded in 21 anesthetized and spontaneously breathing rats. Acetazolamide (AZ; 50 microM) was microinjected unilaterally into the FNr while an isocapnic condition was maintained throughout the experiment. AZ (1 or 20 nl) injection into the FNr significantly elevated VE (46.0 +/- 6.7%; P < 0.05), primarily via an increase in tidal volume (31.7 +/- 3.8%; P < 0.05), with little effect on arterial blood pressure. This augmented ventilatory response was initiated at 6.3 +/- 0.8 min and reached the peak at 19.7 +/- 4.1 min after AZ administration. The same dose of AZ delivered into the interposed and lateral cerebellar nuclei, or vehicle injection into the FNr, failed to elicit detectable cardiorespiratory responses. To determine whether the ventilatory response to AZ injection into the FNr resulted from an increase in respiratory central drive, the minute phrenic nerve activity (MPN) was recorded in seven paralyzed and ventilated rats. Similar to VE, MPN was increased by 38.9 +/- 8.9% (P < 0.05) after AZ administration. Our results suggest that elevation of CO2/H+ within the FNr facilitates respiratory output, supporting the presence of ventilatory chemoreception in rat FNr.

Rhombex-29, a novel gene of the PLP/DM20-M6 family cloned from rat medulla oblongata by differential display

The ventral medullary surface (VMS) is known as the site of the central chemosensitive neurons. These neurons sense excess CO(2)/H(+) dissolved in the cerebrospinal fluid that superfuses the VMS and induce hyperventilation. We hypothesized that genes specific for hyperventilation are expressed much more highly in VMS neurons than in extra-VMS neurons in other parts of the central nervous system (CNS). Applying the differential display technique to the brain of adult rats, we differentiated the mRNAs of the VMS neurons from those of cerebral cortex neurons. Seventeen candidate clones were selected, and their sequences were analyzed. Among these 17 clones, one encodes a novel four-transmembrane protein, which we named rat Rhombex-29. Structural analysis and the phylogenic tree showed that rat Rhombex-29 is homologous to the major CNS myelin protein PLP/DM20-M6 family and belongs to the intermediate type between mouse M6b and shark DMgamma. As the embryos grew into adult rats, constant expression of rat Rhombex-29 mRNA was found in the brain. Hypercapnic stimulation increased expression of rat Rhombex-29 mRNA in the VMS neurons but not in the cerebral cortex neurons. These results indicate that the VMS neurons are endowed with a novel gene, rat Rhombex-29, that is sensitive to H(+).

Molecular cloning of Rhombex-40 a transmembrane protein from the ventral medullary surface of the rat brain by differential display

Respiration-related neurons, which detect various chemicals in cerebrospinal fluid, are localized to the ventral medullary surface (VMS). We hypothesized that expression of genes involved in respiratory function is upregulated in the VMS. By differential display, we looked for genes differentially expressed in VMS neurons and cerebral cortical neurons. Seventeen clones of interest were isolated, and sequence analysis revealed that one of these clones encoded a putative transmembrane protein, rhombencephalic expression protein-40 kDa (Rhombex-40). The rat Rhombex-40 was composed of 374 amino acid residues, and the predicted secondary structure displays a signal peptide in the N-terminus and single-pass transmembrane domain in the center of the sequence. An analysis of consensus sequences identified several phosphorylation sites in the intracellular domain. Expression of rat Rhombex-40 mRNA is high in the brain, and low in lung, liver and kidney. No homologous protein sequence was found in database searches. Whereas the biological function of this protein is presently unknown, its structural features and high expression in the brain suggest that Rhombex-40 may function as a novel transmembrane molecule in neural cells of the brain.

Cloning of MafG homologue from the rat brain by differential display and its expression after hypercapnic stimulation

The ventral medullary surface (VMS) is a site of the medullary chemoreceptor neurons which sense excess protons (H+) derived from hypercapnia and facilitate respiration. We hypothesized that expression of genes involved in H+-sensitivity is higher in the VMS than in other central nervous system areas. By using the differential display technique, we differentiated the mRNAs of VMS neurons from those of cerebral cortical neurons. Seventeen clones of interest were isolated, and sequence analysis revealed that one of these clones had an encoding nuclear transcription factor, MafG. MafG is a member of Maf protein family, and the founding member of the family (v-Maf) was originally discovered as the transduced transforming component of avian musculoaponeurotic fibrosarcoma virus, AS42. The rat MafG was composed of 162 amino acid residues and was conserved among the primary structures of various species. Expression of rat mafG mRNA is high in the VMS, heart and skeletal muscle while the cerebral cortex, cerebellum, liver, stomach and intestine show moderate expression. To determine whether the expression of mafG mRNA is induced by hypercapnic stimulation, 7% CO2 in air was inhaled to rats for 5 min. We found that the hypercapnic stimulation induced the gene expression of mafG. These results suggest that MafG may be involved in H+-sensitivity and respiratory regulation in the VMS.

Hyperoxia increases paradoxical sleep rhythm in the pontine cat

Pontine cat is an ectothermic preparation, whose central temperature can artificially be lowered from 36 degrees C to 26 degrees C; this gradual hypothermia is accompanied by a dramatic increase in paradoxical sleep (PS). Two main hypotheses might explain this result: executive systems of PS might be switched on gradually by cold-sensitive thermodetectors, whereas inhibitory monoaminergic mechanisms appear to be warm-sensitive. On the other hand, energy saving mechanisms peculiar to hypothermia might promote PS appearance. Indeed, in normal animals, PS is selectively suppressed both by hyperthermia and hypoxia. The inhibitory effect of hypoxia might explain why hypothermia, which protects the brain against hypoxic alterations, might facilitate PS. If this last hypothesis is correct, the putative increase in cerebral oxygen supply might increase PS. For this reason, we submitted eight pontine carotid-deafferented cats, kept at the same central temperature (34 +/- 0.5 degrees C: temperature clamp) to periodic hyperoxia (PaO2 = 58 +/- 7 kPa) or room air (PaO2 = 17 +/- 2 kPa) alternatively during 4- or 12-h periods. Hyperoxia induced an 85% increase in PS, mainly due to an increase in PS rhythm (PS cycle duration was 65 +/- 4 min in normoxia and 45 +/- 4 min in hyperoxia, p<0.0001). In five animals, after hyperoxia, PS cycle returned gradually back to control values in 4 to 12 h. These findings show that PS is exquisitely sensitive to conditions that impair oxidative metabolism. The role of cholinergic executive PS systems as putative metabolic-sensitive neurons remains to be established.

H+-sensitivity of cultured neurons from the dorsomedial and ventrolateral medulla of neonate rats

The H+-sensitivity of neonate rat cultured neurons derived from the dorsomedial medulla (DMM) containing the nucleus tractus solitarii and the ventrolateral medulla (VLM) was determined by H+-sensitive fluorescent probe BCECF-AM and immunohistochemical methods. Against an extracellular pH as low as 7.2-7.3, H+-sensitivity was verified in 2.6% of the DMM neurons (46/ 1800) and 2.1% of the VLM neurons (38/1800). This H+-sensitive neurons of the DMM were immunoreactive to glutamate (52.4%) and glutamic acid decarboxylase (GAD) (28.6%), while those of the VLM were immunoreactive to glutamate (66.7%) and GAD (33.3%). There was no immunoreactivity to tyrosine hydroxylase, phenylethanolamine-N-methyltransferase or choline acetyltransferase in the H+-sensitive neurons are present in the DMM and VLM besides the ventral medullary surface, the site of the central chemoreceptors.

Pharmacological properties of the CO2/H+-sensitive area in the ventral medullary surface assessed by the effects of chemical stimulation on respiration

We recently discovered that CO2/H+-sensitive neurons in the ventral medullary surface (VMS) are immunoreactive to glutamate, glutamic acid decarboxylase (GAD), calcineurin and cAMP. We then tested the hypothesis that glutamate, GABA, calcineurin and cAMP affect the activity of CO2/H+-sensitive neurons in the VMS. Using male Wistar rats anesthetized with urethane and pentobarbital, we checked for changes in relative tidal volume (VT) and respiratory frequency (f) in response to injecting the VMS with a variety of test agents dissolved in mock CSF. Respiratory changes occurred immediately and were dose-dependent. (1) 200-1600 pmol Glutamate increased VT but decreased f. The glutamate effect was never abolished by concomitant injection of AP5, a NMDA receptor antagonist, but was abolished by CNQX, an AMPA receptor antagonist, indicating predominance of AMPA receptors in the CO2/H+-sensitive neurons in the VMS. (2) 200-1600 pmol GABA decreased both VT and f. The GABA effect was never abolished by concomitant injection of saclofen, a GABA(B) receptor antagonist, but was abolished by bicuculline, a GABA(A) receptor antagonist, indicating predominance of GABA(A) receptors in the CO2/H+-sensitive neurons in the VMS. (3) 4-32 microg Calcineurin, a Ca2+/calmodulin-dependent protein phosphatase 2B, and 200-1600 pmol FK506, selective inhibitor of calcineurin, had no effect on respiration when they were applied extracellularly, but 400-3200 pmol BAPTA-AM, an intracellular Ca2+-chelating agent, decreased both VT and f, indicating involvement of intracellular Ca2+ in the excitatory mechanisms of respiration. (4) 100-800 pmol IBMX, an enhancer of intracellular cAMP, decreased both VT and f, indicating involvement of cAMP in the inhibitory mechanisms of respiration. These results indicate that the CO2/H+-sensitive neurons in the VMS contain glutamate and/or GABA in cytoplasma, possess AMPA and/or GABA(A) receptors on surface of plasma membrane, and compose the internal circuit, and that their activities are regulated by Ca2+ and cAMP.

Topology and immunohistochemistry of proton-sensitive neurons in the ventral medullary surface of rats

We aimed to clarify the topology and immunohistochemistry of CO2/H+-sensitive neurons in the ventral medullary surface (VMS), the central chemoreceptor area in rats. Inhalation of 3 and 7% CO2 in air significantly decreased pH in arterial blood and increased paCO2, which caused hyperpneic and tachypneic responses. Following inhalation of 3 and 7% CO2 in air for 5 min, the density of c-Fos-immunoreactive (IR) neurons increased stepwise not only in the 3rd-5th divisions of the VMS (between the caudal end of the nucleus corporis trapezoidei and the caudal end of the area postrema), but also in the rostroventromedial medulla (RVMM). Following inhalation of 7% CO2 in air for 5 min, glutamate-, glutamic acid decarboxylase (GAD)-, calcineurin- and cAMP-IR neurons were found not only in the VMS, but also in the RVMM. The topology of these neurons was similar to that of the c-Fos-IR neurons. No immunoreactivity was found for serotonin, substance P, somatostatin, cholecystokinin-octapeptide, methionine-enkephalin, choline acetyltransferase, tyrosine hydroxylase, phenylethanolamine N-methyltransferase, NO-synthase, S-100, calbindin-D, calmodulin, or parvalbumin. The densities of c-Fos-, glutamate-, GAD-, calcineurin- and cAMP-IR neurons were almost zero in the 1st division of the VMS, but became higher along the 2nd-4th divisions of the VMS. Regression lines of the density against the 1st-4th divisions of the VMS were significantly linear. These results indicate that H+-sensitive neurons are common in the 4th-5th divisions of the VMS, and that they are glutamatergic, GABAergic, and containing calcineurin and cAMP.