Nitric oxide participates in excitotoxic mechanisms induced by chemical hypoxia

Distribution pattern of dorsal root ganglion neurons synthesizing nitric oxide synthase in different animal species

The main aim of the present review is to provide at first a short survey of the basic anatomical description of sensory ganglion neurons in relation to cell size, conduction velocity, thickness of myelin sheath, and functional classification of their processes. In addition, we have focused on discussing current knowledge about the distribution pattern of neuronal nitric oxide synthase containing sensory neurons especially in the dorsal root ganglia in different animal species; hence, there is a large controversy in relation to interpretation of the results dealing with this interesting field of research.

Nitric oxide as a regulatory molecule in the processing of the visual stimulus

Nitric oxide (NO) is a highly reactive gas with considerable diffusion power that is produced pre- and post synaptically in the central nervous system (CNS). In the visual system, it is involved in the processing of the visual information from the retina to superior visual centers. In this review we discuss the main mechanisms through which nitric oxide acts, in physiological levels, on the retina, lateral geniculate nucleus (LGN) and primary visual cortex. In the retina, the cGMP-dependent nitric oxide activity initially amplifies the signal, subsequently increasing the inhibitory activity, suggesting that the signal is "filtered". In the thalamus, on dLGN, neuronal activity is amplified by NO derived from brainstem cholinergic cells, in a cGMP-independent mechanism; the result is the amplification of the signal arriving from retina. Finally, on the visual cortex (V1), NO acts through changes on the cGMP levels, increasing signal detection. These observations suggest that NO works like a filter, modulating the signal along the visual pathways.

Investigating the mechanisms underlying neuronal death in ischemia using in vitro oxygen-glucose deprivation: potential involvement of protein SUMOylation

It is well established that brain ischemia can cause neuronal death via different signaling cascades. The relative importance and interrelationships between these pathways, however, remain poorly understood. Here is presented an overview of studies using oxygen-glucose deprivation of organotypic hippocampal slice cultures to investigate the molecular mechanisms involved in ischemia. The culturing techniques, setup of the oxygen-glucose deprivation model, and analytical tools are reviewed. The authors focus on SUMOylation, a posttranslational protein modification that has recently been implicated in ischemia from whole animal studies as an example of how these powerful tools can be applied and could be of interest to investigate the molecular pathways underlying ischemic cell death.

Identification of differentially expressed genes in dorsal root ganglion in early diabetic rats

OBJECTIVE

To screen and identify differentially expressed genes in the dorsal root ganglion (DRG) in early experimental diabetic rats.

METHODS

Diabetic model rats were induced by single intraperitoneal injection of streptozotocin (STZ). At the second week after STZ injection, the sensory nerve conduction velocities (SNCV) of sciatic nerve were measured as an indicator of neuropathy. The technique of silver-staining mRNA differential display polymerase chain reaction (DD-PCR) was used to detect the levels of differentially expressed genes in rat DRG. The cDNA fragments that displayed differentially were identified by reverse-hybridization, cloned and sequenced subsequently, and then confirmed by Northern blot.

RESULTS

The SNCV in the diabetic model group [n = 9, (45.25+/-10.38) m/s] reduced obviously compared with the control group [n = 8, (60.10+/-11.92) m/s] (P < 0.05). Seven distinct cDNA clones, one was up-regulated gene and the others were down-regulated ones, were isolated by silver-staining mRNA differential display method and confirmed by Northern blot. According to the results of sequence alignment with GenBank data, majority of the clones had no significant sequence similarity to previously reported genes except only one that showed high homology to 6-pyruvoyl-tetrahydropterin synthase mRNA (accession No. BC059140), which had not been reported to relate to diabetic neuropathy.

CONCLUSION

These differentially expressed genes in the diabetic DRG may contribute to the pathogenesis of diabetic peripheral neuropathy.

A pilot study of nitric oxide blood levels in patients with chronic orofacial pain

BACKGROUND

Control of pain is the major goal in the management of chronic orofacial pain (COP) patients. The pathogenesis of COP is currently not well understood. Consequently, the treatment of COP may be suboptimal or even harmful. Based on independent observations, we propose that local elevated levels of nitric oxide (NO) may have a central role in the pathogenesis of COP.

HYPOTHESIS

NO level in the orofacial region of COP patients is elevated. A regional increased level of NO causes excessive vasodilatation. This hyperperfusion is manifested by hyperthermia of the overlying skin, while NO enhances nociception, aggravating orofacial pain. An alternative mechanism involving NO as a neurotransmitter at the CNS level may contribute to orofacial pain, but seems not to account for all the known clinical observations.

METHODS

Two groups of subjects were studied: 5 patients with COP and 59 control subjects. For each subject we collected blood samples for analysis of nitrite\nitrate (or NOx).

RESULTS

(1) NOx blood levels for 5 patients diagnosed with COP was 65.9 microM (SD of 10.4) verses 42.7 microM (SD of 24.2) for 59 control subjects, the difference being statistically significant, t-statistic = -2.12 (P > .05). (2) No statistical difference was found for NOx blood levels for 59 control subjects divided by gender (male vs female), with 23 female controls having NOx blood levels of 42.6 microM (SD of 25.2) and male controls having NOx blood levels of 42.8 microM (SD of 24.0), t-statistic = -0.03, P = .98.

CONCLUSION

This pilot study suggests that NO blood levels may have an association with COP. A better understanding of the mechanism of chronic orofacial pain is expected to lead to more precise diagnostic staging and management of this disorder.

Response of NADPH-Diaphorase-Exhibiting Neurons in the Medullar Reticular Formation to High Spinal Cord Injury

1. The effect of hemisection of the cervical spinal cord on NADPH-diaphorase staining in the reticular nuclei of the rabbit medulla was investigated using histochemical technique. 2. A quantitative assessment of somal and neuropil NADPH-diaphorase staining was made by an image analyzer in a selected area of each reticular nucleus of the rabbit medulla. 3. On the 7th postsurgery day, the highest up-regulation of somatic NADPH-diaphorase staining was observed in regions regulating cardiorespiratory processes; however, the highest increase of neuropil NADPH-diaphorase staining was found in the reticular nuclei modulating the tonus of postural muscles. 4. The degeneration of non-NADPH-diaphorase-stained neurons was detected throughout the reticular formation of the medulla, but the extent of neuronal death did not correlate with the up-regulation of the NADPH-diaphorase staining in the reticular nuclei of the medulla. 5. The findings provide evidence that NADPH-diaphorase-exhibiting neurons are refractory to the hemisection of the cervical spinal cord and that the neuronal up-regulation of NADPH-diaphorase at the medullar level is probably not a causative factor leading to the death of the reticulospinal neurons.

Involvement of NMDA-nitric oxide pathways in the development of tactile hypersensitivity evoked by the loose-ligation of inferior alveolar nerves in rats

To investigate whether or not NMDA/nitric oxide (NO) pathways in the trigeminal system are involved in the development and/or maintenance of such pathological pain states as the hyperalgesia and allodynia observed after dental surgery, loose-ligation on the left inferior alveolar nerves of rats were performed. The responses to mechanical stimulation were then measured using von Frey filaments. Hypersensitivity to tactile stimulation developed on the ipsilateral side in ligated animals 5 days after surgery and lasted for at least 30 days. In addition, the effects of drugs on these pain states during the period 2-3 weeks following surgery were investigated. As a result, it was observed that tactile hypersensitivity was inhibited by the intraperitoneal (i.p.) administration of both MK-801 hydrogen maleate (0.05-0.1 mg/kg) and N(G)-monomethyl-L-arginine acetate (L-NMMA: 10-100 mg/kg). Still further, NO production and the number of neuronal NO synthase (nNOS)-positive neurons in the trigeminal nucleus caudalis (SpVc) was evaluated. As a result of these experiments, it was found that the changes in NO levels evoked by the intravenous infusions of N-methyl-D-aspartate (NMDA; 10 mg/kg) and MK-801 (0.5 mg/kg) were significantly larger in the loose-ligated rats compared to the sham-operated rats. Moreover, the number of nNOS-positive neurons was found to have increased on the ipsilateral side in layers I/II of the SpVc. These results would suggest that tactile hypersensitivity develops after inferior alveolar nerve injury and that NMDA receptor/NOS/NO production pathways in the SpVc may be involved in the development of such pathophysiological states.

Changes in nitric oxide and expression of nitric oxide synthase in spinal cord after acute traumatic injury in rats

The aim of this study was to observe the time course of NO production and NOS expression in the spinal cord following acute traumatic injury. Rat spinal cord was injured by extradural static weight-compression, which resulted in an incomplete transverse spinal cord lesion with paralysis of the lower extremities. Using this model, measurement of NO by microdialysis and Griess reaction and histological and immunohistochemical examinations using polyclonal antibodies to nNOS and iNOS were performed from immediately to 14 days after injury. In injured cord, the amount of NO markedly increased immediately after injury and gradually decreased between 1 and 12 h after injury. A second wave of increase in NO level was observed at 24 h and 3 days after injury. Histologically, hematomas and necrotic changes were observed after injury and demyelination of nerve fibers increased with time in the compressed segment. Immunohistochemically, the number of cells with expression of nNOS was increased immediately to 12 h after injury. Expression of iNOS was observed from 12 h to 3 days after injury. These findings suggested that the initial maximal increase of NO production might be caused mainly by nNOS and that the second wave of increase in NO might be due mainly to iNOS.

Involvement of the NMDA-nitric oxide pathway in the development of hypersensitivity to tactile stimulation in dental injured rats

To investigate mechanisms in pathological pain conditions as the hyperalgesia and allodynia observed after dental surgery, we employed a rat dental-injury model involving the simultaneous pulpectomy to a lower incisor and extraction of an ipsilateral upper incisor. We found that hypersensitivity to tactile stimulation developed on both ipsilateral and contralateral sides in the dental-injured rats 5 days after the surgery and that this lasted for at least 30 days. Recovery from hypersensitivity to tactile stimulation was achieved by the intraperitoneal (i.p.) administration of MK-801 (0.05 mg/kg) or N(G)-monomethyl-L-arginine monoacetate (L-NMMA: 10 - 100 mg/kg), but not attained by N(G)-monomethyl-D-arginine monoacetate (D-NMMA: 100 mg/kg). This recovery effect of L-NMMA (50 mg/kg) was inhibited by pretreatment with L-arginine (600 mg/kg). In the trigeminal nucleus caudalis (SpVc), the changes in nitric oxide (NO) levels invoked by the intravenous (i.v.) administration of N-methyl-D-aspartate (NMDA; 10 mg/kg) were found to be significantly larger in the dental-injured rats than in sham-operated rats. The number of neuronal NO synthase (nNOS)-positive neurons increased in layers I-II and III-IV in the SpVc on both sides of the dental-injured rats. These results suggest that hypersensitivity to tactile stimulation developed following dental injury, and that NMDA receptor/NOS/NO production pathways in the SpVc may be involved in pathological conditions.

Blockade of NMDA-receptors or calcium-channels attenuates the ischaemia-evoked efflux of glutamate and phosphoethanolamine and depression of neuronal activity in rat organotypic hippocampal slice cultures

We have investigated the effects of various insults on extracellular glutamate and phosphoethanolamine levels as well as electrical activity alterations in the early period following these insults in organotypic hippocampal slice cultures. Cultures prepared from 7-day-old rats were maintained in vitro for 7-14 days and then metabolic inhibition was induced: cultures were briefly exposed to potassium cyanide to induce chemical anoxia, 2-deoxyglucose with glucose removal to produce hypoglycaemia, or a combination of both to simulate ischaemia. Chemical anoxia induced a small increase in glutamate and a reversible decrease in evoked field potentials and these were greatly potentiated following simulated ischaemia: high, biphasic glutamate efflux and irreversible field potential abolition as well as increase in phosphoethanolamine levels were observed. We have characterised the effects of treatments using NMDA-receptor antagonists and the L-type calcium channel blocker diltiazem. Anoxia-induced glutamate accumulation was prevented by MK-801 and diltiazem D-AP5. Following simulated ischaemia, diltiazem totally prevented glutamate and phosphoethanolamine accumulations, whereas MK-801 did not block the first phase of glutamate accumulation and D-AP5 prevented none. We demonstrated that glutamate and phosphoethanolamine ischaemic-evoked efflux as well as the recovery of electrical activity in organotypic hippocampal slice cultures are sensitive to both NMDA-receptor and calcium-channel blockade. This model thus represents a useful in vitro system for the study of ischaemic neurodegeneration paralleling results reported using in vivo models.

Yawning responses induced by local hypoxia in the paraventricular nucleus of the rat

Yawing was induced by microinjections of L-glutamate, cyanide and a nitric oxide-releasing compound (NOC12) into the paraventricular nucleus of the hypothalamus (PVN) in anesthetized, spontaneously breathing rats. To evaluate physiological aspects of yawning, we monitored intercostal electromyogram (EMG) as an index of inspiratory activity, digastric EMG, blood pressure and electrocorticogram (ECoG). Microinjection of L-glutamate in the medial parvocellular subdivision (mp) elicited a stereotyped yawning response, i.e. an initial depressor response and an arousal shift in ECoG followed by a single large inspiration with mouth opening. The same sequential events were observed during spontaneous yawning, indicating that the mp is responsible for triggering yawning. Microinjection of cyanide into the mp caused the same yawning responses as the ones elicited by microinjection of L-glutamate, suggesting that the mp is sensitive to chemical hypoxia or ischemia within the PVN. Microinjection of NOC12 into the mp elicited a single large inspiration with a variable onset delay, suggesting that diffusible nitric oxide (NO) within the mp may act as a paracrine agent to cause a yawning response. We hypothesize that the mp of the PVN contains an oxygen sensor that causes a yawning response.

Cyanide interaction with redox modulatory sites enhances NMDA receptor responses

Activation of NMDA receptors plays an important role in cyanide neurotoxicity. Cyanide indirectly activates the receptor by inducing neuronal release of glutamate and also enhances receptor-mediated responses by a direct interaction with the receptor complex. This study investigated the mechanism in cerebellar granule cells by which cyanide enhances NMDA-induced Ca2+ influx. Cyanide (50 microM) increased the influx of Ca2+ over the NMDA concentration range of 0.5-500 microM. Experiments showed that cyanide does not interact with the receptor's glycine or PKC mediated phosphorylation regulatory sites. N-ethylmaleimide, a thiol alkylating agent which inactivates the redox regulatory sites of the receptor, blocked the enhancing effect of cyanide. Pretreatment of cells with 5,5-dithio-bis-2-nitrobenzoic acid (DTNB), a compound that oxidizes the receptor redox sites, had no effect on the response to cyanide. On the other hand, the nonpermeant reducing agents, dithiothreitol or cysteine, further increased the cyanide effect. These observations can be explained by cyanide interacting with redox sensitive disulfide groups that are not accessible to the non-permeant reducing agents. It is proposed that cyanide interacts with a redox site(s) located either on the intracellular receptor domain or in the transmembrane hydrophobic domain. Furthermore the enhancement by cyanide of the excitotoxic actions of NMDA involves receptor sites that are sensitive to oxidation/reduction and this interaction contributes to the neurotoxic action of cyanide.

Responses to reversible anoxia of intracellular free and bound Ca(2+) in rat cortical slices

Severe anoxia induces destabilisation of intracellular calcium homeostasis in neurones. The mechanism of this effect, and particularly the interrelationship between changes in intracellular concentration of free Ca(2+) ions and the content of the intracellular Ca(2+) stores, during and after anoxia, is not clear. We used a superfusion system of rat olfactory cortical slices for the fluorimetric estimation of changes in the intracellular concentration of free Ca(2+) ions and in the level of bound Ca(2+), utilising the fluorescent indicators Fura-2 and chlortetracycline, respectively. It was found that 10-min normoglycaemic anoxia results in simultaneous decrease in bound and increase in free Ca(2+) levels, whereas during 60-min reoxygenation, we detected an increase in both indices. The NMDA receptor antagonists MK-801 and APV attenuated changes in free Ca(2+) level during anoxia and reoxygenation and intensified anoxia-evoked decrease in bound Ca(2+) content, whereas a late post-anoxic increase in bound Ca(2+) was abolished. These data suggest that the influx of extracellular Ca(2+) to neurones via NMDA receptors, plays a critical role in the rise of intracellular free Ca(2+) concentration during and after anoxia. Biphasic changes in bound Ca(2+) content during anoxia and reoxygenation may reflect an anoxia-induced release of Ca(2+) from intracellular stores, followed later by a neuronal calcium overload and refilling of intracellular Ca(2+) binding sites.

Microglia in organotypic hippocampal slice culture and effects of hypoxia: ultrastructure and lipocortin-1 immunoreactivity

Lipocortin-1 immunocytochemistry was used to study the various cell forms of microglia that appear during organotypic hippocampal tissue culture, as well as in the in vitro toxic hypoxia model. Antibodies against lipocortin-1 identified activated and phagocytic cells that were abundant in a slice after the plating of a culture: cells of the intermediate form at the later time-points of culturing, resting ramified microglia beginning from the seventh day of culturing, as well as activated and phagocytic cells that appeared in the slice after experimental toxic hypoxia induced by potassium cyanide treatment. Lipocortin-1-positive microglia cell forms corresponded well to the description of the microglia in vivo, and the morphology of microglia corresponded to the circumstances under which these cells were observed in slice cultures. Electron microscopic studies have demonstrated, for the first time, that microglia in organotypic slice culture preserve morphological features typical of different microglial forms in vivo, as well as specific contacts and interactions with the other neural tissue elements. After experimental toxic hypoxia, rapid changes in microglial ultrastructure and localization were observed, reminiscent of in vivo models of ischaemia. In conclusion, observations of microglial morphology and behaviour allow us to suggest that microglia in the organotypic culture preserve their essential characteristic features and properties, thus providing an important model system for studying the structure and function of these cells.

Neuroprotective and neurodestructive functions of nitric oxide after spinal cord hemisection

Nitric oxide (NO) may subserve different functions in different central neurons subjected to axotomy. The difference may depend on whether the neurons basally express neuronal nitric oxide synthase (nNOS), a biosynthetic enzyme of NO. This is supported by our previous finding that suggests the differential role of NO in neurons of nucleus dorsalis (ND) and red nucleus (RN) which have different basal expression of nNOS. This study aimed to establish firmly the functions of NO, as revealed by nNOS immunoreactivity and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry, by the administration of endogenous NO donor, l-arginine (l-arg), and NOS inhibitor, l-N(G)-nitroarginine methyl ester (l-NAME). To relate the role of NO to glutamate receptors (GluR), the distributions of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-d-aspartate receptor (NMDAR) in the two nuclei were revealed by immunohistochemical techniques. nNOS immunoreactivity was void in ND neurons, but expressed weakly in the RN normally. It was induced in ipsilateral ND neurons and upregulated on both sides of RN after spinal cord hemisection. Neuronal loss in the ipsilateral ND was augmented by l-arg, but reduced by l-NAME. In the contralateral RN, l-arg attenuated neuronal loss. NMDAR1 was present in most neurons in ND. After axotomy, some NMDAR1 immunoreactive neurons of the ipsilateral ND were induced to express NOS, whereas RN neurons showed strong staining for NMDAR1 and all the AMPA subunits. Most of the NOS-positive neurons in the RN were coexistent with GluR2 in normal rats and those subjected to axotomy. The present data demonstrated that NO exerted neurodestructive function in the non-NOS-containing ND neurons characterized by NMDAR as the predominant glutamate receptor. NO might be beneficial to the NOS-containing RN neurons. This could be attributed to the presence of GluR2. Possible diverse synthesizing pathways of NO in two different central nuclei were suggested from the observation that NOS was colocalized with NADPH-d in ND neurons, but not in RN neurons.

Neuronal nitric oxide synthase mRNA upregulation in rat sensory neurons after spinal nerve ligation: lack of a role in allodynia development

Pharmacological evidence suggests a functional role for spinal nitric oxide (NO) in the modulation of thermal and/or inflammatory hyperalgesia. To assess the role of NO in nerve injury-induced tactile allodynia, we examined neuronal NO synthase (nNOS) expression in the spinal cord and dorsal root ganglia (DRG) of rats with tactile allodynia because of either tight ligation of the left fifth and sixth lumbar spinal nerves or streptozotocin-induced diabetic neuropathy. RNase protection assays indicated that nNOS mRNA (1) was upregulated in DRG, but not spinal cord, neurons on the injury side beginning 1 d after nerve ligation, (2) peaked (approximately 10-fold increase) at 2 d, and (3) remained elevated for at least 13 weeks. A corresponding increase in DRG nNOS protein was also observed and localized principally to small and occasionally medium-size sensory neurons. In rats with diabetic neuropathy, there was no significant change in DRG nNOS mRNA. However, similar increases in DRG nNOS mRNA were observed in rats that did not develop allodynia after nerve ligation and in rats fully recovered from allodynia 3 months after the nerve ligation. Systemic treatment with a specific pharmacological inhibitor of nNOS failed to prevent or reverse allodynia in nerve-injured rats. Thus, regulation of nNOS may contribute to the development of neuronal plasticity after specific types of peripheral nerve injury. However, upregulation of nNOS is not responsible for the development and/or maintenance of allodynia after nerve injury.

NADPH-diaphorase-containing neurons in cortex, subcortical white matter and neostriatum are selectively spared in Alzheimer's disease

To investigate the involvement of NADPH-diaphorase (NADPH-d)-containing neurons in Alzheimer's disease (AD), NADPH-d enzyme histochemistry in vibratome sections was applied to the superior frontal and superior temporal cortex and the neostriatum in 5 AD and 6 aged control brains. Overall there was a neuronal loss and atrophy in the cortex of AD. Despite slight morphological neuronal changes in the cortex of AD, we found no significant difference in the number of NADPH-d-positive neurons in both cortex and neostriatum between control and AD cases. These results provide further evidence for a selective preservation of NADPH-d neurons in AD. In order to check whether nNOS-immunoreactive neurons are identical to NADPH-d-positive neurons in the human brain, we examined the frontal and temporal cortex and neostriatum of normal human brains in serial cryostat sections. We found that nNOS-containing neurons paralleled NADPH-d-positive neurons in these brain regions. Copyrightz1999S.KargerAG,Basel

Nitric oxide and glutamate interaction in the control of cortical and hippocampal excitability

PURPOSE

We investigated the role of nitric oxide (NO) as a new neurotransmitter in the control of excitability of the hippocampus and the cerebral cortex, as well as the possible functional interaction between NO and the glutamate systems.

METHODS

The experiments were performed on anesthetized rats. The bioelectrical activities of the somatosensory cortex and the CA1 region of the hippocampus of these rats were recorded. Pharmacologic inhibition of NO synthase (NOS) through the nonselective and brain-selective inhibitors, N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), was performed.

RESULTS

The treatments caused the appearance of an interictal discharge activity in both the structures. The latency of induction and the duration of the interictal discharge activity were strictly related to the dose of NOS inhibitor used. In some cases, after L-NAME treatment at high doses, it was possible to note spike and wave afterdischarge activity in the hippocampus. All the NOS inhibitor-mediated excitatory effects were abolished by intraperitoneal (i.p.) pretreatment with the N-methyl-D-aspartic acid (NMDA) receptor antagonists (DL-2-amino-5-phosphonovaleric acid, 2-APV; dizolcipine, MK-801) and partly suppressed after the i.p. injection of the non-NMDA antagonist (6-cyano-7-nitroquinoxaline-2,3-dione; CNQX).

CONCLUSIONS

All data showed that the reduction of NO levels in the nervous system causes the functional prevalence of the excitatory neurotransmission, which is probably due to an NMDA overactivity caused by the absence of the NO-mediated modulatory action. Thus, it is possible to hypothesize a neuroprotective role for NO, probably through a selective desensitization of the NMDA receptors.

The possible role of nitric oxide in the physiopathology of pain associated with temporomandibular joint disorders

Temporomandibular joint disorders (TMD) pose a significant challenge to the practice of oral and maxillofacial surgery. When painful, TMD are generally associated with hyperthermia of the overlying skin. It is hypothesized that this skin hyperthermia, caused by regional vasodilation, is induced by nitric oxide (NO) produced in the extravascular space of the joint. Extravascular NO can be produced by osteoblasts, chondrocytes and macrophages, or by stimulated neurons. It is suggested that this kind of pain is associated with NO-enhanced sensitivity of the peripheral nociceptors. Verification and clinical implications of the proposed mechanism are discussed.

Effects of hypoxia and reoxygenation on nitric oxide production and cerebral blood flow in developing rat striatum

We investigated the role of nitric oxide (NO) in the regulation of regional cerebral blood flow (rCBF) during hypoxia and reoxygenation in developing rat striatum. The subjects were urethane-anesthetized 7- and 14-d-old rats. After 120 min of baseline measurements, the rats received an i.p. injection of either saline (as a control) or an NO synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg) 30 min before hypoxia. Then they were subjected to a 60-min hypoxia in 8% O2, followed by a 60-min recovery in 21% O2. rCBF and NO concentration in the striatum were measured by laser Doppler flowmetry and an NO electrode throughout the experimental period. In the controls, rCBF decreased to 93 +/- 3% of baseline during hypoxia and increased to 124 +/- 3% of baseline during reoxygenation in 7-d-old rats (n = 13), whereas rCBF increased during both hypoxia and reoxygenation in 14-d-old rats to 125 +/- 6% and 168 +/- 6% of baseline, respectively (n = 17). L-NAME attenuated the hyperemic response to hypoxia/reoxygenation in both ages (n = 11, in each age). Striatal NO production increased during hypoxia and reoxygenation in both ages, but the increase was significantly less in 7-d-old than in 14-d-old rats. The NO increase was associated with the increase in rCBF, and both were attenuated by L-NAME. We speculate that NO release during hypoxia/reoxygenation modulates rCBF. The immature young rat brain may have less capacity to activate NO production than the more developed brain.

Selective neuronal vulnerability and specific glial reactions in hippocampal and neocortical organotypic cultures submitted to ischemia

Neurons from cerebral neocortex and hippocampus exhibit a striking difference in vulnerability to transient global ischemia. In order to study the contribution of neuronal connections and neuron-glia interactions to this variation in neuronal vulnerability, we used hippocampal and neocortical cultures submitted to various periods of histotoxic ischemia. Organotypic cultures were exposed at 37 degrees C for 0, 7, 30 and 60 min to a glucose-free NaCN-containing medium. Histological analysis using thionin staining and MAP2 immunostaining showed differences in the temporal profile of neuronal damage in hippocampal and neocortical structures, i.e., in decreasing order, CA1 (7 min) > CA3 and neocortical layers II, III, V, VI (30 min) > DG and neocortical layer IV (60 min). In parallel to the neurodegeneration study, the time course and the regional pattern of microglial and astroglial changes were also examined using GS-B4 isolectin and GFAP as immunohistochemical markers, respectively. The GS-B4 isolectin staining revealed an early (at 7 min for the hippocampus) and a specific microglial activation located in areas undergoing neuronal damage. For both organotypic cultures, astrogliosis occurred later (after 30 min of stress) with no specific regional distribution. Both hippocampal and neocortical cultures submitted to histotoxic ischemia allowed the replication of many of the cellular events observed in response to global ischemia in vivo. These findings support the hypothesis that neuron-neuron connections as well as interactions between neurons and glial cells are essential to reproduce in vitro the selective neuronal vulnerability described in vivo.

Effects of 2-amino-7-phosphonohepatanoic acid, melatonin or NG-nitro-L-arginine on cyanide or N-methyl-D-aspartate-induced neurotoxicity in rat cortical cells

When cortical neuronal cells were exposed to potassium cyanide (0.01, 0.05, 0.1, 0.5, or 1.0 mM) or N-methyl-D-aspartate (NMDA: 0.005, 0.01, 0.05, 0.1, or 0.2 mM) for 24 h at 37 degrees C in a 95% air and 5% CO2 environment, lactate dehydrogenase (LDH) efflux into the extracellular fluid from the cortical cells was significantly increased in a concentration dependent manner and morphological changes were observed. The increased LDH efflux and the morphological changes in cortical cells induced by potassium cyanide or NMDA were blocked by co-exposure to 2-amino-7-phosphonoheptanoic acid (AP7: 1.0 mM), a selective antagonist of the NMDA receptor, melatonin (1.0 mM), a potent hydroxyl and peroxyl radical scavenger, or NG-nitro-L-arginine (1.0 mM), an inhibitor of nitric oxide (NO) synthase. These results suggest that activation of NMDA receptor and NO synthase and/or free radical formation may contribute to the development of neurotoxicity induced by cyanide or NMDA.

Oxygen deprivation produces delayed inhibition of long-term potentiation by activation of NMDA receptors and nitric oxide synthase

The acute and delayed effects of anoxia on synaptic transmission and long-term potentiation (LTP) were examined in the CA1 region of rat hippocampal slices. Oxygen deprivation for 20 minutes completely but reversibly depressed excitatory postsynaptic potentials mediated by both N-methyl-D-aspartate receptors (NMDAR) and non-NMDAR. Although LTP was reliably produced by a single tetanus delivered 30 minutes after reoxygenation, LTP could not be induced when a tetanus was delivered 70 to 100 minutes after reoxygenation. A tetanus delivered 100 minutes after reoxygenation produced lasting synaptic enhancement when 100 mumol/L D,L-amino-phosphonovaleric acid (APV), a competitive NMDAR antagonist, was administered during the period of oxygen deprivation. The delayed effects of oxygen deprivation were not blocked when APV was administered after oxygen deprivation. Similarly, the delayed effects on LTP induction were overcome by inhibitors of nitric oxide synthase when the nitric oxide synthase inhibitors were administered during anoxia, but not when administered after oxygen deprivation. These results suggest that untimely activation of NMDAR and nitric oxide release during anoxia produce delayed inhibition of LTP induction and may be involved in the memory defects that occur subsequent to cerebral hypoxia.

Role of nitric oxide in the physiopathology of pain

Many painful disorders, including joint dysfunctions such as rheumatoid arthritis (RA) or temporomandibular joint disorders (TMD), are associated with hyperthermia of the overlying skin. The same is true of certain intractable chronic pain conditions, such as chronic orofacial pain, which may be associated with TMD. We suggest that this skin hyperthermia, caused by regional vasodilation, is induced by extravascular nitric oxide (NO). Extravascular NO can be produced in the affected joint by osteoblasts, chondrocytes, and macrophages, by mechanical stimulation of endothelial cells, or by stimulated neurons. In view of a strong correlation between pain and skin hyperthermia in these disorders, and the evidence that NO enhances the sensitivity of peripheral nociceptors, we also suggest that at least this kind of pain is associated with excessive local level of NO. This hypothesis can be verified by dynamic area telethermometry, assessing the effect of NO on the sympathetic nervous function. This mechanism, which is in line with the general role of NO as a mediator between different organ systems, also may be relevant to any pain associated with enhanced immune response. Clinical implications of the proposed mechanism are discussed.

Nitric oxide modulates synaptic glutamate release during anoxia

The ability of nitric oxide to enhance vesicular glutamate release during anoxia was examined in the present study. Whole-cell patch clamp recordings were obtained from CA1 pyramidal neurons in rat hippocampal slices perfused in media containing tetrodotoxin. These cells exhibit spontaneous inward currents previously identified as glutamatergic miniature excitatory postsynaptic currents (mEPSCs). The frequency of these mEPSCs increases during exposure to anoxia. The anoxia-induced increase in frequency is reduced when experiments are performed in the presence of the competitive nitric oxide (NO)-synthase inhibitors N(G)-nitro-L-arginine methyl ester and N(G)-nitro-L-arginine, as well as reduced hemoglobin. Arginine reversed the suppression by the NO-synthase inhibitors. The N-methyl-D-aspartate (NMDA) receptor antagonists 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid and MK-801 also suppressed the anoxia-induced increase in mEPSC frequency. These data indicate that NMDA receptor-activated NO production may enhance vesicular synaptic glutamate release, which would in turn contribute to excitotoxicity during hypometabolic states.

Induction of NADPH-diaphorase activity in the forebrain in a model of intracerebral hemorrhage and its inhibition by the traditional Chinese medicine complex Nao Yi An

Induction of NADPH-diaphorase (NDP) activity in the rat cerebral cortex was studied after autologous blood injection into the internal capsule as experimental model of intracerebral hemorrhage. The potential inhibitory effect on NDP induction by Nao Yi An (NYA), a complex derived from materials of animal and plant origin used in the treatment of intracerebral hemorrhage in traditional Chinese medicine, was also investigated. In animals without therapeutic treatment 2 and 4 days after injection of autologous blood, NDP activity was highly induced in pyramidal neurons in the neocortex, piriform, and entorhinal cortices, in astrocytes and in phagocytes in the hematoma and the area surrounding it, as well as in the subcortical white matter, and in endothelial cells in both the cortex and subcortical white matter bilaterally. Oral administration of NYA failed to inhibit NDP induction in endothelial cells but demonstrated a strong inhibitory effect on NDP activity induced in pyramidal neurons and astrocytes. NDP induction in phagocytes was also inhibited by the administration of NYA. Altogether the present results suggest that intracerebral hemorrhage in the internal capsule may induce nitric oxide synthase activity in different cell populations in the cortex and that administration of NYA can selectively inhibit such induction and, thus, potentially play a neuroprotective role.

Involvement of nitric oxide in acute spinal cord injury: an immunocytochemical study using light and electron microscopy in the rat

The possibility that nitric oxide participates in the pathophysiology of spinal cord injury was examined using a constitutive isoform of neuronal nitric oxide synthase immunoreactivity in a rat model. Spinal cord trauma was produced by making an incision into the right dorsal horn of the T10-11 segments. Five h after trauma, a marked upregulation of NOS-immunostained neurons was seen in the perifocal T9 and T12 segments of the cord. The immunolabelling was most pronounced in the dorsal horn of the ipsilateral side. Topical application of an antiserum to nitric oxide synthase (NOS) 2 min after injury prevented the trauma-induced upregulation of NOS-immunoreactivity. In contrast, application of preabsorbed serum or L-NAME, an inhibitor to NOS, was ineffective in reducing the induction of NOS-immunoreactivity. Trauma caused a marked expansion of the cord and resulted in marked cell changes. This expansion and cell reaction was significantly reduced following application of NOS antiserum but it was not seen after application of preabsorbed antiserum or L-NAME. Our results for the first time show that a focal trauma to the spinal cord has the capacity to upregulate neuronal NOS immunoreactivity and that application of NOS antiserum has a neuro protective effect. This indicates that nitric oxide is somehow involved in the pathogenesis of secondary injuries after spinal cord trauma.

Association of dopaminergic terminals and neurons releasing nitric oxide in the rat striatum: an electron microscopic study using NADPH-diaphorase histochemistry and tyrosine hydroxylase immunohistochemistry

To examine synaptic input and association of terminals containing dopamine and other transmitters to rat striatal nitric oxide synthase-expressing neurons, an electron microscopic study using tyrosine hydroxylase (TH) immunohistochemistry combined with histochemistry for NADPH-diaphorase (NADPHd) was performed. NADPHd-positive neurons had medium-sized cell bodies containing a highly invaginated nucleus and received relatively sparse synaptic input; 3.6% of boutons apposed to the NADPHd-positive neurons were TH-immunoreactive. Of these TH-immunoreactive boutons, two synaptic contacts showing symmetrical synaptic specializations were found on a cell body and a proximal dendrite of a NADPHd-positive neuron. Other nonsynaptic TH-immunoreactive boutons were occasionally associated with unlabeled terminals adjacent to the NADPHd-positive dendrites and also forming asymmetric synaptic contacts with unlabeled spinous or dendritic profiles. These results suggest that activity of the striatal neurons that release nitric oxide may be regulated by direct synaptic input from dopaminergic neurons and also suggest that the TH-immunoreactive terminals associated with the dendrites of nitric oxide synthase-expressing neurons provide the sites where nitric oxide influences dopamine release from neighboring terminals.

Nitric oxide mediates the sustained opening of NMDA receptor-gated ionic channels which follows transient excitotoxic exposure in hippocampal slices

In the rat hippocampal slice, a brief exposure to glutamate and glycine increased MK-801 binding to 246% of controls. Increased binding persisted 90 min after removal of those amino acids from the incubation medium. Posttreatment with the competitive substrate inhibitor of nitric oxide synthase, N omega-nitro-L-arginine or with hemoglobin, which binds NO extracellularly, inhibited this postexcitotoxic increase in MK-801 binding. L-Arginine reversed this inhibitory effect but D-arginine did not. The combination of tetrodotoxin and low Ca2+, which blocks transmitter release prevented the poststimulation increase in MK-801 binding, suggesting a presynaptic component. These findings suggest that the sustained opening of NMDA receptor-gated ionic channels seen after transient glutamate/glycine stimulation is mediated by NO.