Upregulation of NMDA receptor and neuronal NADPH-d/NOS expression in the nodose ganglion of acute hypoxic rats

Glutamate-dependent regulation of food intake is altered with age through changes in NMDA receptor phenotypes on vagal afferent neurons

Compared to younger individuals, older human subjects have significantly lower food intakes and an increased satiety response. N-methyl-d-aspartate (NMDA) receptors expressed by vagal afferent neurons originating from nodose ganglia (NG) are involved in modulating the satiety response. The present study investigated how NMDA receptor subunit phenotypes in NG neurons change with age and how these age-related alterations in food intake are modulated by presynaptic NMDA receptors in the NG of male Sprague Dawley rats (six week-old and sixty week-old). Food intake was measured at 30-, 60-, and 120-min following intraperitoneal administration of cholecystokinin (CCK) or the non-competitive NMDA receptor antagonist MK-801. Immunofluorescence was used to determine NMDA receptor subunit expression (NR1, NR2B, NR2C, and NR2D) in the NG. The results showed that, CCK reduced food intake at 30-, 60-, and 120-min post injection in both young and the middle-age animals, with no statistical difference between the groups at 30- and 60-min. In contrast, MK-801 produced an increase in food intake that was significantly higher in middle-age rats compared to young animals at all time points studied. NR1 subunit was expressed by almost all NG neurons in both age groups. In young rats, NR2B, NR2C, and NR2D subunits were expressed in 56.1%, 49.3%, and 13.9% of NG neurons, respectively. In contrast, only 30.3% of the neuronal population in middle-aged rats expressed NR2B subunit immunoreactivity, NR2C was present in 34.1%, and only 10.6% of total neurons expressed the NR2D subunit. In conclusion, glutamate-dependent regulation of food intake is altered with age and one of the potential mechanisms through which this age-related changes in intake occur is changes in NMDA receptor phenotypes on vagal afferent neurons located in NG.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

Changes in the Density of Nitrergic Neurons in the Hippocampus of Rats Following Kainic Acid and Melatonin Administration

Nitric oxide (NO) may play a role in the pathophysiology of excitotoxicity. It is also possible that increase in Ca2+ overload and NO-mediated events are involved in neuronal loss during excitotoxicity. Using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry, we have investigated the effects of melatonin on NADPH-d positive hippocampal neurons after kainic acid (KA) induced excitotoxicity in female rats of Wistar strain. Cytosolic Ca2+ (free calcium) in all the respective experimental groups was also studied. Kainic acid was administered, with a single dose of 10 mg/kg/bw (body weight) to the animals. KA treated rats were given melatonin at a dose of 20 mg/kg/bw (for 14 day). On the last day of treatment, animals were transcardially perfused with 4 % paraformaldehyde under deep thiopental anaesthesia. Cryostat sections (20 µm) were cut and stained for NADPH-d positive neurons. KA exposed animals showed a significantly increased number of NADPH-d positive neurons in the dorsal and ventral blade of the dentate gyrus (DG), hilus, CA1 and CA3 area of hippocampus, with a parallel increase in intracellular free Ca2+ ion concentration, as compared to the control group. KA + melatonin-treated animal groups showed reduced number of NADPH-d positive neurons in DG, hilus, CA1 and CA3 areas and a decline in cytosolic Ca2+ concentration, as compared to KA treated group. Our study suggests that the enhanced levels of cytosolic Ca2+ and nitric oxide (NO) play an important role in kainate induced excitotoxicity. Inhibition of NO production may be another means whereby melatonin can reduce oxidative damage and seems to play important role in neuroprotection.

(-)-Epigallocatechin gallate attenuates NADPH-d/nNOS expression in motor neurons of rats following peripheral nerve injury

Background

Oxidative stress and large amounts of nitric oxide (NO) have been implicated in the pathophysiology of neuronal injury and neurodegenerative disease. Recent studies have shown that (-)-epigallocatechin gallate (EGCG), one of the green tea polyphenols, has potent antioxidant effects against free radical-mediated lipid peroxidation in ischemia-induced neuronal damage. The purpose of this study was to examine whether EGCG would attenuate neuronal expression of NADPH-d/nNOS in the motor neurons of the lower brainstem following peripheral nerve crush. Thus, young adult rats were treated with EGCG (10, 25, or 50 mg/kg, i.p.) 30 min prior to crushing their hypoglossal and vagus nerves for 30 seconds (left side, at the cervical level). The treatment (pre-crush doses of EGCG) was continued from day 1 to day 6, and the animals were sacrificed on days 3, 7, 14 and 28. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to assess neuronal NADPH-d/nNOS expression in the hypoglossal nucleus and dorsal motor nucleus of the vagus.

Results

In rats treated with high dosages of EGCG (25 or 50 mg/kg), NADPH-d/nNOS reactivity and cell death of the motor neurons were significantly decreased.

Conclusions

The present evidence indicated that EGCG can reduce NADPH-d/nNOS reactivity and thus may enhance motor neuron survival time following peripheral nerve injury.

Roles of gastro-oesophageal afferents in the mechanisms and symptoms of reflux disease

Oesophageal pain is one of the most common reasons for physician consultation and/or seeking medication. It is most often caused by acid reflux from the stomach, but can also result from contractions of the oesophageal muscle. Different forms of pain are evoked by oesophageal acid, including heartburn and non-cardiac chest pain, but the basic mechanisms and pathways by which these are generated remain to be elucidated. Both vagal and spinal afferent pathways are implicated by basic research. The sensitivity of afferent fibres within these pathways may become altered after acid-induced inflammation and damage, but the severity of symptoms in humans does not necessarily correlate with the degree of inflammation. Gastro-oesophageal reflux disease (GORD) is caused by transient relaxations of the lower oesophageal sphincter, which are triggered by activation of gastric vagal mechanoreceptors. Vagal afferents are therefore an emerging therapeutic target for GORD. Pain in the absence of excess acid reflux remains a major challenge for treatment.

Mild hypoxic preconditioning attenuates injury-induced NADPH-d/nNOS expression in brainstem motor neurons of adult rats

Excessive production of nitric oxide (NO) might have detrimental effects on the hypoxia-related neuropathology. This study aimed to test if mild hypoxic preconditioning (MHPC) would attenuate the pathological changes in the brainstem motoneurons having a different functional component after peripheral nerve crush injury (PNCI). Prior to PNCI treatment, young adult rats were caged in the mild hypoxic altitude chamber with 79Torr of the partial oxygen concentration ( pO(2)) (i.e., 0.5atm at 5500m in height) for 4 weeks to adapt the environmental changes. After that, all the animals having successfully crushed both the hypoglossal and vagus nerves (left-side) were allowed to survive for 3, 7, 14, 30 and 60 successive days in normoxic condition. Nicotinamine adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry revealed that MHPC reduces NADPH-d/nNOS expression in the hypoglossal nucleus (HN) and the dorsal motor nucleus of the vagus (DMN) at different time points after PNCI. The morphological findings were further ascertained by Western blot analysis of nNOS and nitrite assay for NO production. Both the morphological and quantitative results peaked at 7 days in HN, whereas for those in DMN were progressively increased up to 60 days following PNCI. The staining intensity of NADPH-d/nNOS(+) neurons, expression of nNOS protein, NO production levels as well as the neuronal loss in HN and DMN of MHPC rats following PNCI were attenuated, especially for those having a longer survival period over 14 days. The MHPC treatment might induce minute amounts of NO to alter the state of milieu of the experimental animals to protect against the PNCI.

Increased Membrane/Nuclear Translocation and Phosphorylation of p90 KD Ribosomal S6 Kinase in the Brain of Hypoxic Preconditioned Mice

Our previous studies have demonstrated that hypoxic precondition (HPC) increased membrane translocation of protein kinase C isoforms and decreased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the brain of mice. The goal of this study was to determine the involvement of p90 KD ribosomal S6 kinase (RSK) in cerebral HPC of mice. Using Western-blot analysis, we found that the levels of membrane/nuclear translocation, but not protein expression of RSK increased significantly in the frontal cortex and hippocampus of HPC mice. In addition, we found that the phosphorylation levels of RSK at the Ser227 site (a PDK1 phosphorylation site), but not at the Thr359/Ser363 sites (ERK1/2 phosphorylated sites) increased significantly in the brain of HPC mice. Similar results were confirmed by an immunostaining study of total RSK and phospho-Ser227 RSK. To further define the cellular populations to express phospho-Ser227 RSK, we found that the expression of phospho-Ser227 RSK co-localized with neurogranin, a neuron-specific marker, in cortex and hippocampus of HPC mice by using double-labeled immunofluorescent staining method. These results suggest that increased RSK membrane/nuclear translocation and PDK1 mediated neuron-specific phosphorylation of RSK at Ser227 might be involved in the development of cerebral HPC of mice.

Total sleep deprivation inhibits the neuronal nitric oxide synthase and cytochrome oxidase reactivities in the nodose ganglion of adult rats

Sleep disorders are a form of stress associated with increased sympathetic activity, and they are a risk factor for the occurrence of cardiovascular disease. Given that nitric oxide (NO) may play an inhibitory role in the regulation of sympathetic tone, this study set out to determine the NO synthase (NOS) reactivity in the primary cardiovascular afferent neurons (i.e. nodose neurons) following total sleep deprivation (TSD). TSD was performed by the disc-on-water method. Following 5 days of TSD, all experimental animals were investigated for quantitative nicotinamine adenine dinucleotide phosphate-diaphorase (NADPH-d, a co-factor of NOS) histochemistry, neuronal NOS immunohistochemistry and neuronal NOS activity assay. In order to evaluate the endogenous metabolic activity of nodose neurons, cytochrome oxidase (COX) reactivity was further tested. All the above-mentioned reactivities were objectively assessed by computerized image analysis. The clinical significance of the reported changes was demonstrated by alterations of mean arterial blood pressure (MAP). The results indicated that in normal untreated rats, numerous NADPH-d/NOS- and COX-reactive neurons were found in the nodose ganglion (NG). Following TSD, however, both the labelling and staining intensity of NADPH-d/NOS as well as COX reactivity were drastically reduced in the NG compared with normal untreated ganglions. MAP was significantly higher in TSD rats (136+/-4 mmHg) than in normal untreated rats (123+/-2 mmHg). NO may serve as an important sympathoinhibition messenger released by the NG neurons, and decrease of NOS immunoexpression following TSD may account for the decrease in NOS content. In association with the reduction of NOS activity, a defect in NOS expression in the primary cardiovascular afferent neurons would enhance clinical hypertension, which might serve as a potential risk factor in the development of TSD-relevant cardiovascular disturbances.

Vagal afferent neurons projecting to the stomach and small intestine exhibit multiple N-methyl-D-aspartate receptor subunit phenotypes

Previous reports suggest that NMDA receptors participate in control of food intake via vagal afferent neurons that innervate the upper gastrointestinal (GI) tract. While messenger RNA coding for the NR1 NMDA receptor subunit is present in a majority of vagal afferent neurons of nodose ganglia (NG), immunoreactivity for other NMDA receptor subunits (NR2B, NR2C and NR2D) are expressed in more limited subpopulations of vagal afferents. To determine whether vagal afferent neurons that project to the stomach or duodenum exhibit distinct NMDA receptor subunit phenotypes, we examined immunoreactivity (IR) for NMDA receptor NR1, NR2B, NR2C and NR2D subunits in NG neurons that were labeled by injections of the retrograde tracer Fast Blue (FB) into the wall of the stomach or duodenum. FB injections into the fundus or corpus of the stomach labeled comparable numbers of neurons in both the left and right NG, while proximal duodenal injections labeled only neurons of left NG. NR1-IR expression was observed in most neurons innervating the upper GI tract (fundus, 97%; corpus, 95%; duodenum, 98%). Likewise, most neurons that innervated the upper GI tract expressed NR2B-IR (fundus, 98%; corpus, 85%; duodenum, 81%). NR2C-IR was observed in only 52%, 46% and 32% of FB-positive neurons projecting to the fundus, corpus or duodenum respectively, while NR2D-IR occurred in an even more restricted FB-labeled subpopulation (fundus, 13%; corpus, 26%; and duodenum, 18%). Our observations indicate that different subpopulations of vagal afferents express distinct NMDA receptor subunit phenotypes. However, the neuronal distribution of NMDA receptor subunits is not correlated with innervation of either the stomach or duodenum.

Potentiation of mouse vagal afferent mechanosensitivity by ionotropic and metabotropic glutamate receptors

Glutamate acts at central synapses via ionotropic (iGluR--NMDA, AMPA and kainate) and metabotropic glutamate receptors (mGluRs). Group I mGluRs are excitatory whilst group II and III are inhibitory. Inhibitory mGluRs also modulate peripherally the mechanosensitivity of gastro-oesophageal vagal afferents. Here we determined the potential of excitatory GluRs to play an opposing role in modulating vagal afferent mechanosensitivity, and investigated expression of receptor subunit mRNA within the nodose ganglion. The responses of mouse gastro-oesophageal vagal afferents to graded mechanical stimuli were investigated before and during application of selective GluR ligands to their peripheral endings. Two types of vagal afferents were tested: tension receptors, which respond to circumferential tension, and mucosal receptors, which respond only to mucosal stroking. The selective iGluR agonists NMDA and AMPA concentration-dependently potentiated afferent responses. Their corresponding antagonists AP-5 and NBQX alone attenuated mechanosensory responses as did the non-selective antagonist kynurenate. The kainate selective agonist SYM-2081 had minor effects on mechanosensitivity, and the antagonist UBP 302 was ineffective. The mGluR5 antagonist MTEP concentration-dependently inhibited mechanosensitivity. Efficacy of agonists and antagonists differed on mucosal and tension receptors. We conclude that excitatory modulation of afferent mechanosensitivity occurs mainly via NMDA, AMPA and mGlu5 receptors, and the role of each differs according to afferent subtypes. PCR data indicated that all NMDA, kainate and AMPA receptor subunits plus mGluR5 are expressed, and are therefore candidates for the neuromodulation we observed.

N-methyl-D-aspartate receptor subunit phenotypes of vagal afferent neurons in nodose ganglia of the rat

Most vagal afferent neurons in rat nodose ganglia express mRNA coding for the NR1 subunit of the heteromeric N-methyl-D-aspartate (NMDA) receptor ion channel. NMDA receptor subunit immunoreactivity has been detected on axon terminals of vagal afferents in the dorsal hindbrain, suggesting a role for presynaptic NMDA receptors in viscerosensory function. Although NMDA receptor subunits (NR1, NR2B, NR2C, and NR2D) have been linked to distinct neuronal populations in the brain, the NMDA receptor subunit phenotype of vagal afferent neurons has not been determined. Therefore, we examined NMDA receptor subunit (NR1, NR2B, NR2C, and NR2D) immunoreactivity in vagal afferent neurons. We found that, although the left nodose contained significantly more neurons (7,603), than the right (5,978), the proportions of NMDA subunits expressed in the left and right nodose ganglia were not significantly different. Immunoreactivity for NMDA NR1 subunit was present in 92.3% of all nodose neurons. NR2B immunoreactivity was present in 56.7% of neurons; NR2C-expressing nodose neurons made up 49.4% of the total population; NR2D subunit immunoreactivity was observed in just 13.5% of all nodose neurons. Double labeling revealed that 30.2% of nodose neurons expressed immunoreactivity to both NR2B and NR2C, whereas NR2B and NR2D immunoreactivities were colocalized in 11.5% of nodose neurons. NR2C immunoreactivity colocalized with NR2D in 13.1% of nodose neurons. Our results indicate that most vagal afferent neurons express NMDA receptor ion channels composed of NR1, NR2B, and NR2C subunits and that a minority phenotype that expresses NR2D also expresses NR1, NR2B, and NR2C.

Melatonin restores the cytochrome oxidase reactivity in the nodose ganglia of acute hypoxic rats

This study aimed to elucidate whether melatonin would exert beneficial effects on the neuronal functions of the nodose ganglion (NG) following acute hypoxic insult. The cytochrome oxidase (COX) and the nicotinamine adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry along with the nitric oxide synthase (NOS) immunofluorescence were used to examine the metabolic stage and nitric oxide production in nodose neurons respectively. Adult rats were injected intraperitoneally with melatonin at 5 or 100 mg/kg. Hypoxia was achieved by placing the rats into an altitude chamber (PO2 = 43 torr) for 4 hr. The results show that in normal untreated rats, nearly all and about 43% of the NG neurons displayed COX and NOS/NADPH-d reactivities with various staining intensities respectively. However, COX reactivity was drastically decreased while NOS/NADPH-d reactivity was significantly upregulated following hypoxia treatment. In melatonin pretreated rats, the hypoxia-induced reduction of COX reactivity was obviously prevented and the augmentation of NOS/NADPH-d reactivity was successfully suppressed. The deficit in the metabolic stage and the over-activation of NOS would contribute to the generation of oxidative stress. By effectively preventing the metabolic disruption, melatonin may have potential utility in therapeutic treatment of neuronal dysfunctions where oxidative stress is a participant.

Neuronal NADPH-d/NOS expression in the nodose ganglion of severe hypoxic rats with or without mild hypoxic preconditioning

This study aimed to test the hypothesis that mild hypoxic preconditioning (MHPC)-induced NOS expression would attenuate the neuropathological changes in the nodose ganglion (NG) of severe hypoxic exposure (SHE) rats. Thus, the young adult rats were caged in the altitude chamber for 4 weeks prior to SHE for 4 h to gain hypoxic preconditioning. The altitude chamber was used to set the height at the level from 5500 m (0.50 atm; pO2=79 Torr) to 10,000 m (0.27 atm; pO2=43 Torr) for MHPC and SHE, respectively. The experimental animals were allowed to survive for 0, 7, 14, 30 and 60 successive days, respectively. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to detect NADPH-d/nNOS reactivity in the NG at various time points following hypoxic exposure. The present results showed that about 38% of the neurons in the NG displayed NADPH-d/nNOS positive [NADPH-d/nNOS(+)] in normoxic rats. In SHE rats, a peak in the percentage (71%) and staining intensity (230%) of NADPH-d/nNOS(+) nodose neurons at 0 day, which then gradually decreased at 7-60 days. About 25% of the nodose neurons died 60 days after SHE. However, in MHPC rats subjected to SHE, NADPH-d/nNOS(+) neurons peaked in the percentage (51%) and staining intensity (171%) at 0 day, which then decreased at 7-60 days. In addition, neuronal survival was markedly increased by MHPC. These results suggested that MHPC might have a neuroprotective effect that reduces the susceptibility of the nodose neurons to NOS mediated neuropathy subsequent to SHE.

Differential expression of calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT) in the axotomized motoneurons of normoxic and hypoxic rats

We employed a double injury model (axotomy along with hypoxia) to determine how nerve injury and hypoxic insult would affect the expression of calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT) in the hypoglossal nucleus (HN) and nucleus ambiguus (NA). Adult rats were subjected to unilateral vagus and hypoglossal nerve transection, following which half of the animals were kept in an altitude chamber (PO2=380 Torr). The immunoexpression of CGRP and ChAT (CGRP-IR/ChAT-IR) were examined by quantitative immunohistochemistry at 3, 7, 14, 30 and 60 days post-axotomy. The results revealed that CGRP-IR in the HN was increased at 3 days but decreased to basal levels at 7 days following nerve injury. The decline was followed by a second rise in CGRP-IR at 30 days post-axotomy, followed again by a return to basal levels at 60 days. In the NA, CGRP-IR was up-regulated at 3 days and remained increased for up to 60 days after nerve injury. Animals treated with a double injury showed a greater CGRP-IR than normoxic group in both nuclei at all post-axtomized periods. In contrast to CGRP, ChAT-IR was markedly reduced in the HN and NA at 3 days reaching its nadir at 14 days following nerve injury. Hypoxic animals showed a stronger reduction of ChAT-IR in both nuclei at all post-axtomized periods. Results of cell counting showed that neuronal loss was somewhat obvious in hypoxic HN than that of normoxic ones. The present results suggest that up-regulation of CGRP-IR may exert its trophic effects while down-regulation of ChAT-IR may correlate with the poor neurotransmission within the injured neurons. It is speculated that the enhanced expression of CGRP-IR and the pronounced reduction of ChAT-IR in hypoxic rats may result from a drastic shift of intracellular metabolic pathways, which in turn could lead to more metabolic loading to the severely damaged neurons following the double insult.