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

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5 ± 0.03 vs. control: 2.4 ± 0.2 Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7 ± 0.1 vs. control: 2.2 ± 0.2 Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1 ± 0.1 vs. control: 2.3 ± 0.09 Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

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.

Serotonin neurons of the caudal raphe nuclei contribute to sympathetic recovery following hypotensive hemorrhage

Serotonin is thought to contribute to the syncopal-like response that develops during severe blood loss by inhibiting presympathetic neurons of the rostroventrolateral medulla (RVLM). Here, we tested whether serotonin cells activated during hypotensive hemorrhage, i.e., express the protein product of the immediate early gene c-Fos, are critical for the normal sympathetic response to blood loss in unanesthetized rats. Serotonin-immunoreactive cells of the raphe obscurus and raphe magnus, parapyramidal cells of the B3 region, subependymal cells of the ventral parapyramidal region, and cells of the ventrolateral periaqueductal gray region were activated by hypotensive hemorrhage, but not by hypotension alone. In contrast to findings in anesthetized animals, lesion of hindbrain serotonergic cells sufficient to produce >80% loss of serotonin nerve terminal immunoreactivity in the RVLM accelerated the sympatholytic response to blood loss, attenuated recovery of sympathetic activity after termination of hemorrhage, and exaggerated metabolic acidosis. Hindbrain serotonin lesion also attenuated ventilatory and sympathetic responses to stimulation of central chemoreceptors but increased spontaneous arterial baroreflex sensitivity and decreased blood pressure variability. A more global neurotoxic lesion that also eliminated tryptophan hydroxylase-immunoreactive cells of the ventrolateral periaqueductal gray region had no further effect on the sympatholytic response to blood loss. Together, the data indicate that serotonin cells of the caudal hindbrain contribute to compensatory responses following blood loss that help maintain oxygenation of peripheral tissue in the unanesthetized rat. This effect may be related to facilitation of chemoreflex responses to acidosis.

Activation of the retrotrapezoid nucleus by posterior hypothalamic stimulation

The retrotrapezoid nucleus (RTN) contains chemically defined neurons (ccRTN neurons) that provide a pH-regulated excitatory drive to the central respiratory pattern generator. Here we test whether ccRTN neurons respond to stimulation of the perifornical hypothalamus (PeF), a region that regulates breathing during sleep, stress and exercise. PeF stimulation with gabazine increased blood pressure, phrenic nerve discharge (PND) and the firing rate of ccRTN neurons in isoflurane-anaesthetized rats. Gabazine produced an approximately parallel upward shift of the steady-state relationship between ccRTN neuron firing rate and end-tidal CO(2), and a similar shift of the relationship between PND and end-tidal CO(2). The central respiratory modulation of ccRTN neurons persisted after gabazine without a change in pattern. Morphine administration typically abolished PND and reduced the discharge rate of most ccRTN neurons (by 25% on average). After morphine administration, PeF stimulation activated the ccRTN neurons normally but PND activation and the central respiratory modulation of the ccRTN neurons were severely attenuated. In the same rat preparation, most (58%) ccRTN neurons expressed c-Fos after exposure to hypercapnic hyperoxia (6-7% end-tidal CO(2); 3.5 h; no hypothalamic stimulation) and 62% expressed c-Fos under hypocapnia (approximately 3% end-tidal CO(2)) after PeF stimulation. Under baseline conditions (approximately 3% end-tidal CO(2), hyperoxia, no PeF stimulation) few (11%) ccRTN neurons expressed c-Fos. In summary, most ccRTN neurons are excited by posterior hypothalamic stimulation while retaining their normal response to CNS acidification. ccRTN neurons probably contribute both to the chemical drive of breathing and to the feed-forward control of breathing associated with emotions and or locomotion.

Differential microglial activation between acute stress and lipopolysaccharide treatment

Acute stress was demonstrated to induce morphological microglial activation in several brain regions including the midbrain periaqueductal gray (PAG), an area that plays important roles in behavioral responses to uncontrollable stress, threat, anxiety, and pain. To determine whether neuronal activation may be involved in the stress-induced microglial activation, the present study investigated the correlation between neuronal activity measured as c-Fos expression and morphological microglial activation in the PAG. Acute stress was followed by morphological activation of microglia and increased c-Fos expression in the PAG but not in the surrounding midbrain. Double immunohistochemistry and topological analysis demonstrated that microglial activation occurred adjacent to responsive neurons. By contrast, lipopolysaccharide (LPS) treatment induced microglial activation even in the absence of neuronal responses in the PGA as well as in the rest of the midbrain. These findings suggest that the mechanism of microglial activation during stress may differ from those of infection or inflammation. It also indicates that the neuronal cells expressing c-Fos protein may play some roles to trigger microglial activation.

CO2-sensitivity of GABAergic neurons in the ventral medullary surface of GAD67-GFP knock-in neonatal mice

We investigated the CO2 responsiveness of GABAergic neurons in the ventral medullary surface (VMS), a putative chemoreceptive area using a 67-kDa isoform of GABA-synthesizing enzyme (GAD67)-green fluorescence protein (GFP) knock-in neonatal mouse, in which GFP is specifically expressed in GABAergic neurons. The slice was prepared by transversely sectioning at the level of the rostral rootlet of the XII nerve and the rostral end of the inferior olive in mock cerebrospinal fluid (CSF). Each medullary slice was continuously superfused with hypocapnic CSF. GFP-positive neurons in the VMS were selected by using fluorescent optics and their membrane potentials and firing activities were analyzed with a perforated patch recording technique. Thereafter, superfusion was changed from hypocapnic to hypercapnic CSF. In 4 out of 8 GABAergic neurons in the VMS, perfusion with hypercapnic CSF induced more than a 20% decrease in the discharge frequency and hyperpolarized the neurons. The remaining 4 GFP-positive neurons were CO2-insensitive. GABAergic neurons in the VMS have chemosensitivity. Inhibition of chemosensitive GABAergic neural activity in the VMS may induce increases in respiratory output in response to hypercapnia.

Characterization of efferent projections of chemosensitive neurons in the caudal parapyramidal area of the rat brain

The caudal parapyramidal area of the rat brain contains a population of neurons that are highly sensitive to an increase in the extracellular hydrogen ion concentration ([H+]o). Some of them fire synchronously with respiration when [H+]o is increased. These chemosensitive neurons are located in the caudal ventrolateral medulla in a medial region, closest to the pyramidal tract, and a lateral region, beneath the lateral reticular nucleus. To assess the nature of medullary connections, biotinylated dextran amine injections were performed after recordings from the neurons had been completed. The injections were located within the areas containing serotonergic neurons of the caudal parapyramidal area. The injections within the medial and lateral parts of the caudal parapyramidal region revealed bilateral terminal fields of varicosities within the nucleus of the solitary tract and the ventral respiratory column. Efferent bilateral projections to the lateral paragigantocellular, lateral reticular, and inferior olive nuclei, as well as ipsilateral projections to medial and lateral caudal parapyramidal regions were also identified. Efferent projections towards the raphe obscurus from both medial and lateral caudal parapyramidal regions were found. Medial caudal parapyramidal regions also sent efferent projections towards the raphe pallidus, B1-B3 region, and to the dorsal and ventral parts of the medullary reticular nuclei. The detection of H(+)-sensitive neurons in the caudal parapyramidal area and their projections towards the nucleus of the solitary tract and to the ventral respiratory column, associated with respiratory regulation, indicate that this region could be an excellent candidate for central chemoreception.

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.

Phosphorylation of JNK is involved in regulation of H(+)-induced c-Jun expression

Cells respond to physical and chemical stimulations mediated by pH, osmolarity, and oxidative and mechanical stresses. Various signal transduction pathways cooperate and participate in these responses. Here we describe the role of c-Jun NH2-terminal kinase (JNK) in regulation of gene transcription after an increase in extracellular H+. When cells were incubated in low pH medium, the promotion of JNK phosphorylation and c-Jun expression was clearly observed in cells in an extracellular pH- and time-dependent manner. Activation of p38 and extracellular signal-regulated kinase 1/2 (ERK1/2) was extremely weak compared with that of JNK. An increase in extracellular H+ led to enhanced nuclear translocation of phosphorylated JNK leading to augmentation of the transcriptional activity of c-Jun. Nimodipine, a blocker of voltage-gated Ca2+ ion channels, prevented the phosphorylation of JNK and expression of c-Jun in a dose-dependent manner. These results suggest a novel intracellular signalling pathway for H+-induced c-Jun expression: an increase of extracellular H+ induces JNK phosphorylation and c-Jun expression via partly extracellular Ca2+ influx through voltage-gated Ca2+ channels.

Extracellular acidification enhances DNA binding activity of MafG-FosB heterodimer

Cells are quite sensitive to a change of the extracellular pH and respond to it through detection of the H+/HCO3- level in extracellular fluid. However, little is known about molecular details induced by acidosis, such as intracellular pathways and gene expression. Here we describe properties of gene expression, protein interaction, and DNA binding activity of basic region leucine zipper (bZIP) transcription factor Maf and FosB during extracellular acidification. When cells were incubated with low pH medium, the expressions of small Maf proteins (MafG, MafK, and MafF) and FosB were clearly increased in an extracellular pH-dependent manner and expressed transiently with a peak after 1-2 h after stimulation. Immunofluorescence and protein binding studies indicated that MafG was partially co-localized with FosB in the nucleus and MafG can form heterodimers with FosB at extracellular pH 7.40. Moreover, we found that MafG-FosB complexes are able to bind to AP-1 consensus sequence, TGACTCA. To investigate whether extracellular acidification influences to dimerization and DNA binding activity of MafG and FosB, extracellular pH of cultured cells was decreased from 7.40 to 6.80. The decrease in extracellular pH led to enhanced dimerization of MafG with FosB leading to augmentation of the DNA binding activity of the heterodimer to AP-1 consensus sequence. Moreover, extracellular acidification induces mRNA expression of matrix metalloproteinase-1, one of the genes that are regulated by AP-1. These results suggest that MafG-FosB complexes are involved in transcriptional regulation in response to extracellular acidification.

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.

Decompensated hemorrhage activates serotonergic neurons in the subependymal parapyramidal region of the rat medulla

According to prior evidence opioid and serotonin release by lower brain stem neurons may contribute to hemorrhage-induced sympathoinhibition (HISI). Here we seek direct evidence for the activation of opioidergic, GABAergic, or serotonergic neurons by severe hemorrhage in the medulla oblongata. Blood was withdrawn from awake rats (40-50% total volume) causing hypotension and profound initial bradycardia. Other rats received the vasodilator hydralazine, causing tachycardia and hypotension. Neuronal activation was gauged by the presence of Fos-immunoreactive (ir) nuclei after 2 h. Serotonergic, enkephalinergic, and GABAergic neurons were identified by the presence of a diagnostic enzyme or mRNA. Hemorrhaged rats had 30% fewer non-GABAergic Fos-ir neurons in the rostral ventrolateral medulla (RVLM) than hydralazine-treated rats, but they had six times more Fos-ir neurons within the subependymal parapyramidal nucleus (SEPPN). Fos-labeled SEPPN neurons were serotonergic (40-60%), GABAergic (31%), enkephalinergic (15%), or had mixed phenotypes. The data suggest that a reduced sympathoexcitatory drive from RVLM may contribute to HISI. SEPPN neuronal activation may also contribute to HISI or could mediate defensive thermoregulatory mechanisms triggered by hemorrhage-induced hypothermia.

The retrotrapezoid nucleus (RTN): local cytoarchitecture and afferent connections

The retrotrapezoid nucleus (RTN) provides a source of tonic drive to respiratory neurons and is one of many sites for central chemoreception. Here we evaluate in the rat the local neuronal cytoarchitecture in the RTN histologically 2-4 h after neurobiotin injection and the afferent connections to the RTN 24 h after injection. Our neurobiotin injections often overlapped the RTN and the adjacent neurons of the parapyramidal region, so we group these two regions together in this study. The RTN is made up of small and medium sized neurons and has a low neuronal density compared to other nuclei. The organization of the RTN is reticular in nature and there are prominent small neurons at the ventral medullary border. Adjacent to the pyramids there are medium sized neurons with connections to the raphé pallidus. Major afferent connections include the regions of the dorsal and ventral respiratory groups, the medullary raphé, the contralateral parapyramidal and RTN regions, portions of the nucleus paragigantocellularis lateralis, and portions of the reticular fields. Other sources of input include the Kölliker-fuse nucleus, subceruleus, A5 region, and the paralemniscal zone.

Ventral medulla pHi measured in vivo by 31P NMR is not regulated during hypercapnia in anesthetized rat

Chemoreceptors in the ventral medulla contribute to the respiratory response to hypercapnia. Do they 'sense' intracellular pH (pHi)? We measured pHi in the ventral medulla or cortex (control) using 31P-NMR obtained via a novel 3 x 5 mm2 surface coil in anesthetized rats breathing air or 7% CO2. During air breathing over 240 min, pHi decreased slightly from 7.13 +/- 0.02 to 7.05 +/- 0.02 (SEM; n = 5; 2 cortex, 3 ventral medulla). During 180 min of hypercapnia, cortical pHi (n = 4) decreased from 7.17 +/- 0.02 to 6.87 +/- 0.01 by 90 min and recovered by 150 min. Ventral medulla pHi showed no such regulation. It decreased from 7.11 +/- 0.02 to 6.88 +/- 0.02 at 90 min and recovered only after cessation of hypercapnia (n = 5), results consistent with pHi being the chemoreceptor stimulus. However, non-chemoreceptor neurons that contribute to our medullary NMR signal also do not appear to regulate pHi in vitro. Regional differences in pHi regulation between cortex and ventral medulla may be due to both chemosensitive and non-chemosensitive neurons.

Chemosensitivity of medullary inspiratory neurones: a role for GABA(A) receptors?

This study tested the hypothesis that during hypercapnia partial removal of a tonic GABA-mediated inhibition contributes to the increase in activity of the ventrolateral medulla (VLM) inspiratory neurones. Extracellular recordings were taken from 22 inspiratory neurones in the VLM of rats anaesthetised with pentobarbitone and artificially ventilated. It was found that during hypercapnia, changes in the discharge pattern (i.e. an increase in the discharge frequency during the neurone's normally active phase) and firing frequency of the VLM inspiratory neurones were similar to those evoked by GABA(A) receptor antagonist bicuculline methiodide (BMI, 10 mM, 20 nA), applied ionophoretically in conditions of normocapnia. During hypercapnia BMI (20 nA) failed to evoke a further increase in firing of these neurones. This suggests that CO2-evoked activation of VLM inspiratory neurones may involve a withdrawal in part of a tonic GABA(A) receptor-mediated inhibition. This disinhibition may play a role in the hypercapnia-induced increase in ventilatory activity.

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(+).

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

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

Brainstem and hypothalamic areas involved in respiratory chemoreflexes: a Fos study in adult rats

The adaptation to hypoxia and hypercapnia requires the activation of several anatomical structures along the neuraxis. In this study, using Fos immunoreactivity, we sought to map neuronal populations involved in chemoreflex networks activated during the responses to moderate hypoxia (O(2) 11%), and hypercapnia (CO(2) 5%) in the brainstem and the hypothalamus of the rat. In the medulla, hypoxia elicited marked and significant staining in the nucleus of the solitary tract (NTS), and in parapyramidal neurons located near the ventral surface, whereas hypercapnia evoked significantly c-fos only near the ventral surface in paraolivar neurons. In contrast, within pontine and suprapontine structures, both hypoxia and hypercapnia evoked similarly Fos immunoreactivity in the lateral parabrachialis area, the central grey, the caudal hypothalamus (dorsomedial and posterior hypothalamic nuclei), and in a ventro-lateral hypothalamic area, extending from the rostral limit of the mammillary nuclei to the retrochiasmatic area. More rostrally, labelling was observed in the paraventricular nucleus of the hypothalamus in response to hypercapnia, and in the supraoptic nucleus in response to hypoxia. These results support the hypothesis that chemoreflexes pathways are not only restricted to medulla and pons but also involved mesencephalic and hypothalamic regions. The parabrachialis area and the central grey may be key relays between caudal and ventral hypothalamic neurons, and medullary neurons involved in the response to hypoxia and hypercapnia.

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.

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.