Role of nitric oxide in 2-deoxy-D-glucose-induced hypothermia in rats.

The present study was designed to test the hypothesis that nitric oxide (NO) plays a role in 2-deoxy-D-glucose (2-DG)-induced hypothermia. The body temperature of awake, unrestrained rats was measured before and after the administration of 2-DG, or N(G)-nitro-L-arginine methyl ester (L-NAME; a non-selective NOS inhibitor) or both treatments together. We observed a significant reduction in body temperature after 2-DG injection. L-NAME alone caused no significant change in body temperature. When the two treatments were combined, a reduction in the magnitude of 2-DG-induced hypothermia was observed. The neuronal NOS inhibitor 7-nitroindazole also inhibited 2-DG-induced hypothermia. The data indicate that NO, probably produced by neuronal NOS, plays a role in 2-DG-induced hypothermia.     

Effect of central and peripheral administration of a nitric oxide synthase inhibitor on morphine hyperthermia in rats

The effect of central and peripheral administration of a nitric oxide synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME), on morphine hyperthermia was studied in male Sprague-Dawley rats. The first series of experiments examined the effect of subcutaneous (s.c.) administration of L-NAME on the hyperthermia induced by morphine given s.c. in doses of 4 and 15 mg/kg. L-NAME, at a s.c. dose of 50 mg/kg, per se, had no influence on body temperature (T(b)). Coadministration of L-NAME (50 mg/kg, s.c.) with the higher dose of morphine (15 mg/kg, s.c.) caused a significant suppression of morphine hyperthermia during the first 30 min and then produced hypothermia. In contrast, s.c. injection of L-NAME (50 mg/kg, s.c.) failed to alter the hyperthermic response induced by the lower dose of morphine (4 mg/kg). In the second series of experiments, we investigated the effect of intracerebroventricular (i.c.v.) administration of L-NAME on the hyperthermia induced by morphine given s.c. L-NAME, itself, given i.c.v. at a dose of 1 mg did not evoke any change in T(b). Intracerebroventricular administration of L-NAME (1 mg) blocked the hyperthermia induced by 15 mg/kg morphine during the first 30 min and induced a slight hypothermia but did not alter the hyperthermia induced by 4 mg/kg morphine. The results indicate that either central or peripheral NO synthesis is required for the production of hyperthermia induced by 15 mg/kg of morphine. However, NO synthesis does not seem to be involved in the hyperthermic process induced by 4 mg/kg of morphine.     

Role of nitric oxide in rat locus coeruleus in hypoxia-induced hyperventilation and hypothermia

The locus coeruleus modulates the ventilatory and thermoregulatory response to hypoxia and contains nitric oxide synthase. Therefore, we examined the effects of L-NAME unilaterally microinjected into the locus coeruleus on hypoxic hyperventilation and hypothermia. Ventilation and body temperature were measured before and after microinjection of L-NAME (100 nmol/0.5 microl) into the locus coeruleus, followed by hypoxia. Control rats received microinjection of D-NAME (an inactive enantiomer of L-NAME). Under normoxia, L-NAME treatment did not affect ventilation or body temperature. D-NAME did not affect hypoxia-induced hyperventilation and hypothermia. L-NAME treatment reduced the ventilatory response to hypoxia but did not affect hypoxia-induced hypothermia. These data suggest that nitric oxide in the locus coeruleus is involved in the ventilatory response to hypoxia, exercising an inhibitory modulation on the locus coeruleus neurons, but plays no role in hypoxia-induced hypothermia.     

Nitric oxide pathway is an important modulator of heat loss in rats during exercise

To assess the role of nitric oxide (NO) in central thermoregulatory mechanisms during exercise, 1.43 micromol (2 microL) of N(omega)-nitro-L-arginine methyl ester (L-NAME, n=6), a NO synthase inhibitor, or 2 microL of 0.15M NaCl (SAL, n=6) was injected into the lateral cerebral ventricle of male Wistar rats immediately before the animals started running (18 m min(-1), 5% inclination). Core (Tb) and skin tail (Ttail) temperatures were measured. Body heating rate (BHR), threshold Tb for tail vasodilation (TTbV), and workload (W) were calculated. During the first 11 min of exercise, there was a greater increase in Tb in the L-NAME group than in the SAL group (BRH=0.17+/-0.02 degrees C min(-1), L-NAME, versus 0.09+/-0.01 degrees C min(-1), SAL, p<0.05). Following the first 11 min until approximately 40 min of exercise, Tb levels remained stable in both groups, but levels remained higher in the L-NAME group than in the SAL group (39.16+/-0.04 degrees C, L-NAME, versus 38.33+/-0.02 degrees C, SAL, p<0.01). However, exercise went on to induce an additional rise in Tb in the SAL group prior to fatigue. These results suggest that the reduced W observed in L-NAME-treated rats (10.8+/-2.0 kg m, L-NAME, versus 25.0+/-2.1 kg m, SAL, p<0.01) was related to the increased BHR in L-NAME-treated animals observed during the first 11 min of exercise (r=0.74, p<0.01) due to the change in TTbV (39.12+/-0.24 degrees C, L-NAME, versus 38.27+/-0.10 degrees C, SAL, p<0.05). Finally, our data suggest that the central nitric oxide pathway modulates mechanisms of heat dissipation during exercise through an inhibitory mechanism.     

Reduced plasma nitric oxide metabolites before and after antipsychotic treatment in patients with schizophrenia compared to controls

BACKGROUND: Nitric oxide (NO) is believed to have a role in the pathophysiology of schizophrenia. We examined plasma levels of NO metabolites in patients with schizophrenia and normal controls. We also determined the impact of 6-week risperidone treatment on circulating NO metabolites in patients with schizophrenia. METHOD: Plasma NO metabolite (NO(x)) levels were measured in 55 schizophrenia patients before and after 6-week treatment with risperidone and in 55 normal controls. Severity of schizophrenia and response to treatment were assessed with the positive and negative syndrome scale (PANSS) for schizophrenia. NO(x) levels were estimated by the Griess method. RESULTS: Pre-treatment plasma NO(x) levels in schizophrenia patients (8.97+/-6.74 micromol/L) were lower than those of normal controls (14.51+/-6.30 micromol/L) (p<0.01). Schizophrenia patients had lower post-treatment NO(x) levels (10.99+/-8.31 micromol/L) than those of normal controls (p<0.01). There was marginal significant change between plasma NO(x) levels before and after 6-week treatment (p=0.056). Moreover, in 37 treatment responders (>/=30% improvement in PANSS score), post-treatment plasma NO(x) significantly increased in comparison to pre-treatment NO(x) (p=0.028). CONCLUSIONS: Plasma levels of NO(x) in patients with schizophrenia were significantly lower than normal controls both before and after the treatment. Our findings suggest that the improvement of psychiatric symptoms can lead to partially normalize a deficiency of NO after treatment in schizophrenia patients. Our findings support the hypothesis that the NO system is dampened in schizophrenia.     

Biopterin in the acute phase of hypoxia-ischemia in a neonatal pig model

To clarify the participation of inducible NOS (iNOS) in the hypoxia-ischemia, we examined iNOS and its tetrahydrobiopterin co-factor in the cerebral cortex and plasma in a newborn-piglet model. We also investigated the role of hypothermia in iNOS expression and biopterin production. Male newborn piglets were ventilated 6% oxygen for 45 min. Their common carotid arteries were clamped during hypoxia. Then they were resuscitated with 30% oxygen (HI group). Piglets of the hypothermia group were treated as the HI group and their body was cooled to 35.5 degrees C after hypoxic-ischemic insults. Sham-treated piglets were also reserved. In the HI group, iNOS was present in neurons and macrophages of the cerebral cortex 12h after the insult. The concentrations of nitrite and nitrate were elevated in the cerebral cortex 12h after hypoxic-ischemic insults but the biopterin level was unchanged. The plasma biopterin concentration after the insult (377.9+/-78.7 nM) was five times higher than before the insult (80.1+/-4.3 nM); this level peaked 4h after the insult (604.8+/-200.9 nM) and only slightly decreased after 12h (445.9+/-57.8 nM). In the hypothermia group, no iNOS expression was observed 12h after the insult. The plasma biopterin concentration after the insult (464.2+/-92.3 nM) was similar to that in the HI group, but was suppressed by 4h of hypothermia (229.3+/-106.8 nM). In this study, neuronal iNOS expression and increase of NO production were found in the acute phase of hypoxia-ischemia. Brain biopterin did not increase in hypoxia-ischemia although plasma biopterin was five-fold elevated. The discrepancy may also affect hypoxic-ischemic organ damage.     

Effects of sumatriptan on nitric oxide and superoxide balance during glyceryl trinitrate infusion in the rat. Implications for antimigraine mechanisms

Infusion of glyceryl trinitrate (GTN) into patients with migraine precipitates the onset of a migraine attack several hours after completion of the infusion. Using an infusion of GTN into anaesthetised rats, this study investigates the relationship of regional cerebral blood flux rCBF(ldf), cortical nitric oxide (NO) and cortical superoxide concentrations and the effect of sumatriptan on each variable. In saline treated animals, a 30 min infusion of GTN (2 microgram kg(-1) min(-1), i.v.) was found to markedly increase cortical rCBF(ldf) (133+/-3% of baseline) and NO concentrations (141+/-13% of baseline). Superoxide levels exhibited an inverse relationship to NO levels, decreasing below basal to 48+/-14% of baseline. It is hypothesised that high NO levels during GTN infusion may decrease the detectable superoxide due to "leeching" of the superoxide into low level peroxynitrite formation. In the presence of sumatriptan, a decrease below baseline in cortical rCBF(ldf) (82+/-5% of baseline) and NO concentration (64+/-13% of baseline) was observed throughout GTN infusion, although superoxide levels significantly increased above baseline by 105+/-14 nM (p<0.05, ANOVA post hoc LSD test). The mechanism for this action of sumatriptan is unknown but may include; modulation of cell redox state, NO scavenging or direct manipulation of superoxide release.     

Refeeding attenuates bombesin-induced hypothermia in the rat

The tetradecapeptide bombesin is a potent agent in producing hypothermia when injected centrally. Bombesin-induced hypothermia at normal ambient temperature occurs under conditions of food deprivation or insulin-induced hypoglycemia. This experiment examined the effect of refeeding on the duration of bombesin-induced hypothermia. Rats (n = 7) received microinfusions of bombesin (0.1 microgram/1.0 microliter) into the preoptic area under separate conditions of food deprivation (18 h) and insulin pretreatment (10 U/kg, IM). Core body temperature was evaluated over a period of 4 h with or without food available during testing. Hypothermia was observed under all conditions during the first 2 h. Food-deprived and insulin-pretreated rats not permitted access to food remained hypothermic until at least 4 h following bombesin. These results are discussed in terms of the possible role of glucose availability in the production and duration of bombesin-induced hypothermia.     

Release of arginine, glutamate and glutamine in the hippocampus of freely moving rats: Involvement of nitric oxide

Using in vivo microdialysis, we have monitored the release of three amino acids (arginine, glutamate and glutamine) in the hippocampus of freely moving rats in response to various drugs. In response to N-methyl-d-aspartate (NMDA) infusion, extracellular glutamate was increased, glutamine was decreased and arginine remained unchanged. By contrast, alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) elicited an increase in arginine release but had no effect on either glutamate or glutamine. When S-nitroso-N-acetylpenicillamine (SNAP), a nitric oxide (NO) donor, was infused into the hippocampus, an increase in glutamate, a decrease in glutamine and no change in arginine were recorded. The effect of SNAP on extracellular glutamine levels was reversed by prior infusion of the guanylate cyclase inhibitor oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ), however its effect on glutamate release was unchanged. Interestingly, SNAP was found to promote the release of arginine in the presence of ODQ. We also assessed the effect of two nitric oxide synthase inhibitors, N-nitro-l-arginine methylester (l-NAME) and 7-nitroindazole (7-NI), on the release of these amino acids. l-NAME was found to increase arginine and glutamate levels but decrease those of glutamine. In contrast, 7-NI reduced the release of all three amino acids. The results presented here confirm some but not all of the findings previously obtained using in vitro preparations. In addition, they suggest that complex relationships exist between the release of these amino acids, and that endogenous NO plays an important role in regulating their release.     

病变侧亚低温对局部脑缺血再灌流损伤有关因素的影响

目的　研究病变侧脑亚低温对脑缺血再灌流损伤梗塞体积、 Ｎ Ｏ 的影响确定病变侧亚低温的疗效, 探讨机理。方法　应用可反馈控温半导体致冷块对大鼠局灶脑缺血模型病变侧降温至３２ ～３３ ℃研究持续缺血及再灌流损伤的保护作用及有关因素的影响。结果　持续缺血１０ 分钟低温组及缺血４０ 分钟再灌流并低温组梗塞体积均小于常温对照组。亚低温组 Ｎ Ｏ 含量明显低于常温对照组。结论　病变侧亚低温对脑缺血再灌流损伤在一定时间窗内有明显保护作用, 而亚低温使 Ｎ Ｏ 产生减少可能是其脑保护作用的部分机制。     

Involvement of serotoninergic receptors in the anteroventral preoptic region on hypoxia-induced hypothermia

Hypoxia causes a regulated decrease in body temperature (Tb). There is circumstantial evidence that the neurotransmitter serotonin (5-HT) in the anteroventral preoptic region (AVPO) mediates this response. However, which 5-HT receptor(s) is (are) involved in this response has not been assessed. Thus, we investigated the participation of the 5-HT receptors (5-HT1, 5-HT2, and 5-HT7) in the AVPO in hypoxic hypothermia. To this end, Tb of conscious Wistar rats was monitored by biotelemetry before and after intra-AVPO microinjection of methysergide (a 5-HT1 and 5-HT2 receptor antagonist, 0.2 and 2 microg/100 nL), WAY-100635 (a 5-HT(1A) receptor antagonist, 0.3 and 3 microg/100 nL), and SB-269970 (a 5-HT7 receptor antagonist, 0.4 and 4 micro/100 nL), followed by 60 min of hypoxia exposure (7% O2). During the experiments, the mean chamber temperature was 24.6 +/- 0.7 degrees C (mean +/- SE) and the mean room temperature was 23.5 +/- 0.8 degrees C (mean +/- SE). Intra-AVPO microinjection of vehicle or 5-HT antagonists did not change Tb during normoxic conditions. Exposure of rats to 7% of inspired oxygen evoked typical hypoxia-induced hypothermia after vehicle microinjection, which was not affected by both doses of methysergide. However, WAY-100635 and SB-269970 treatment attenuated the drop in Tb in response to hypoxia. The effect was more pronounced with the 5-HT7 antagonist since both doses (0.4 and 4 microg/0.1 microL) were capable of attenuating the hypothermic response. As to the 5-HT(1A) antagonist, the attenuation of hypoxia-induced hypothermia was only observed at the higher dose. Therefore, the present results are consistent with the notion that 5-HT acts on both 5-HT(1A) and 5-HT7 receptors in the AVPO to induce hypothermia, during hypoxia.     

Nitric oxide in the rostral ventrolateral medulla modulates hyperpnea but not anapyrexia induced by hypoxia

Hypoxia causes hyperpnea and anapyrexia (a regulated decrease in body temperature, T(b)) but the mechanisms involved are not well understood. The nitric oxide (NO) pathway is involved in hypoxia-induced anapyrexia and hyperpnea, but the site(s) of action is not known. Nitric oxide synthase is present in the rostral ventrolateral medulla (RVLM), which is a nucleus in the medulla oblongata involved in control of breathing, and RVLM neurons have been suggested to have intrinsic hypoxic chemosensitivity. Therefore, we examined the effects of inhibition of the NO pathway in the RVLM on hypoxic hyperpnea and anapyrexia. Ventilation (VE) and body temperature (T(b)) were measured before and after bilateral microinjection of N-monomethyl-L-arginine (L-NMMA, 12.5 microg/0.1 microl, a nonselective nitric oxide synthase inhibitor) into the RVLM, followed by a 120-min period of hypoxic exposure. Control rats received microinjection of saline (vehicle). Under normoxia, L-NMMA treatment did not affect VE or T(b). Typical hypoxia-induced hyperpnea and anapyrexia were observed after saline treatment. L-NMMA treatment reduced the ventilatory response to hypoxia but did not affect hypoxia-induced anapyrexia. These data suggest that nitric oxide in the RVLM is involved in the ventilatory response to hypoxia, exercising an excitatory modulation of the RVLM neurons, but plays no role in hypoxia-induced anapyrexia.     

Involvement of nitric oxide in cocaine-induced erections and ejaculations after paradoxical sleep deprivation

OBJECTIVES: As nitric oxide (NO) is involved in penile erectile (PE) function and also influences the sleep-wake cycle, we speculated that NO could play a role in PE and ejaculation of paradonical sleep deprivation (PSD) rats. METHODS: Animals were pretreated with N(G)-nitro-L-arginine methyl ester (L-NAME, ip) and L-arginine (ip and icv) prior to saline or cocaine injection. RESULTS: Cocaine-induced PE in 90% of PSD rats, 60% of which ejaculated. L-NAME reduced the frequency of erection, but had no effect in the proportion of PSD-cocaine-injected rats displaying this response. L-NAME had no effect in saline groups. L-Arginine in PSD-saline rats reduced the proportion of animals displaying PE at the highest dose and reduced the frequency of PE at all doses in both saline and cocaine groups. The icv administration of L-arginine reduced PE only in PSD-cocaine rats. Results indicate that common to both drugs, whether it was NO synthase (NOS) inhibitor or NO precursor, was their capacity to strongly reduce PE frequency in cocaine-treated rats. Moreover, L-arginine (ip) played a relevant inhibitory role in the erection displayed by PSD rats. CONCLUSIONS: Our findings suggest that the stimulating effects of PSD associated or not with cocaine on erection can be modified by alterations in the NO system.     

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.     

Psychological stress-induced enhancement of brain lipid peroxidation via nitric oxide systems and its modulation by anxiolytic and anxiogenic drugs in mice

We investigated the effect of psychological stress on lipid peroxidation activity in the mouse brain, the mechanism underlying the psychological stress-induced change in the activity, and the effects of anxiolytic and anxiogenic drugs on the activity in psychologically-stressed animals. Psychological stress exposure using a communication box paradigm for 2-16 h significantly increased the content of thiobarbituric acid reactive substance (TBARS), an index of lipid peroxidation activity, in the brain, and the effect was maximal after peaked by a 4-h stress exposure. In the animals stressed for over 4 h, the increased brain TBARS content lasted for 30 min after the stress exposure, while no significant increase of the TBARS content was observed in the liver or serum. Trolox (67.6 mg/kg, i.p.), an antioxidant drug, but not monoamine oxidase inhibitors, clorgyline (2.5-5 mg/kg, i.p.) or 5-(4-benzylphenyl)-3-(2-cyanoethyl)-(3H)-1,3,4-oxadiazol-2-o ne (1-5 mg/kg, i.p.), significantly suppressed the effect of psychological stress. The non-selective nitric oxide (NO) synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 10-100 mg/kg, i.p.) and the selective neuronal NOS inhibitor 7-nitroindazole (25 and 50 mg/kg, i.p.), but not the inducible NOS inhibitor aminoguanidine (1-100 mg/kg, i.p.), dose dependently suppressed the psychological stress-induced enhancement of lipid peroxidation in the brain. L-Arginine (300 mg/kg, i.p.), a substrate of NOS, antagonized the effect of L-NAME. Measurements of NO metabolites revealed a significant increase of NO production in the brains of stressed mice. The benzodiazepine (BZD) receptor agonist diazepam (0.05-0.5 mg/kg, i.p.), the 5-HT(1A) receptor agonists (+/-)-8-hydroxy-di-propylaminotetralin and buspirone (0.1-1 mg/kg, i. p.), but not the 5-HT(3) receptor agonist MDL72222, dose-dependently suppressed the psychological stress-induced enhancement of brain lipid peroxidation. In contrast, the administration of anxiogenic drugs, FG7142 (an inverse BZD agonist: 1-10 mg/kg, i.p.) and 1-(3-chlorophenyl)piperazine (a mixed 5-HT(2A/2B/2C) agonist: 0.1-1 mg/kg, i.p.), potentiated it. The effects of diazepam and FG7142 were abolished by the BZD receptor antagonist flumazenil (10 mg/kg, i.p.). These results indicate that psychological stress causes oxidative damage to the brain lipid via enhancing constitutive NOS-mediated production of NO, and that drugs with a BZD or 5-HT(1A) receptor agonist profile have a protective effect on oxidative brain membrane damage induced by psychological stress.     

Blockade of nitric oxide formation enhances thermal and behavioral responses in rats during turpentine abscess

OBJECTIVE: The purpose of this study was to investigate the role of nitric oxide (NO) during the development of fever and other symptoms of sickness behavior (i.e. anorexia, cachexia) in response to localized tissue inflammation caused by injection of turpentine in freely moving biotelemetered rats. METHODS: To determine the role of NO in turpentine-induced fever, we injected the NO synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) intraperitoneally simultaneously or 5 h after turpentine injection. RESULTS: Rats responded with fever to intramuscular injection of 20 microl of turpentine that commenced 6 h after injection and reached peak values 11 h after injection. Although turpentine did not significantly alter food and water intake, it caused a drop in body weight. Rats injected with turpentine and treated with L-NAME responded with a substantial rise in fever, independently of the time of L-NAME injection. The rise in body temperature (T(b)) due to turpentine injection began slightly sooner and reached the maximal T(b) value faster in rats treated with L-NAME than in the ones treated with saline (control for L-NAME). The enhanced decrease in food and water intake in rats treated with a combination of L-NAME and turpentine was also observed. As a result, L-NAME-injected rats responded with a profound drop in body mass due to turpentine, independently of the time of L-NAME injection. L-NAME alone did not affect food and water intake, but slightly suppressed the gain of body mass. CONCLUSION: These results indirectly indicate that NO is involved in pyrogenic and behavioral responses in rats during turpentine abscess.     

Nitric Oxide Synthase Inhibition Delays Axonal Degeneration and Promotes the Survival of Axotomized Retinal Ganglion Cells

Nitric oxide (NO) synthesized by inducible nitric oxide synthase (iNOS) has been implicated in neuronal cytotoxicity following trauma to the central nervous system. The aim of the present study was to examine the role of NO in mediating axotomy-induced retinal ganglion cell (RGC) death. We observed increases in iNOS expression by microglia and Müller cells in the retina after optic nerve transection. This was paralleled by the induced expression of constitutive NOS (cNOS) in RGCs which do not normally express this enzyme. In order to determine if NO is cytotoxic to axotomized RGCs, the nonspecific NOS inhibitors Nomega-nitro-L-arginine (NOLA) or N-nitro-L-arginine methyl ester (L-NAME) were delivered to the vitreous chamber by intraocular injections. Both NOLA and L-NAME significantly enhanced RGC survival at 7, 10, and 14 days postaxotomy. The separate contributions of iNOS and cNOS to RGC degeneration were examined with intraocular injections of the specific iNOS inhibitor L-N(6)-(I-iminoethyl)lysine hydrochloride or the specific cNOS inhibitor L-thiocitrulline. Our results suggest that cNOS plays a greater role in RGC degeneration than iNOS. In addition to enhancing RGC survival, NOS inhibitors delayed the retrograde degeneration of RGC axons after axotomy. We conclude that NO synthesized by retinal iNOS and cNOS plays a major role in RGC death and retrograde axonal degeneration following axotomy.     

Effects of deep hypothermia on nitric oxide-induced cytotoxicity in primary cultures of cortical neurons

Nitric oxide (NO) is thought to play a major role during cerebral ischemia. However, the protective efficacy of hypothermia against NO-induced neurotoxicity remains to be examined. In the present study, the degree of neurotoxicity induced by NO was analyzed in two temperature groups (normothermia, 37 degrees C; deep hypothermia, 22 degrees C) of cultured E16 Wistar rat cortical neurons. Two different NO donors, 1-hydroxy-2-oxo-3-(N-ethyl-2-aminoethyl)-3-ethyl-1-triazene (NOC-12) and 1-hydroxy-2-oxo-3-(3-amynopropyl)-3-isopropyl-1-triazene (NOC-5), that have equal half-lives at 37 degrees C and 22 degrees C, respectively, were used. Cultured neurons in each temperature group were exposed to 30 and 100 micro M NOC for three different time courses, 6 hr, 12 hr, and 24 hr. The survival rates of neurons were evaluated by assessing viable neurons on photomicrographs before and after the experiments. The highest survival rate (approximately 93%) was seen in both temperature groups when neurons were exposed to 30 micro M NOC for 6 hr and 12 hr, and there was no significant difference observed between these two groups (P > 0.05). Almost equal survival rates were observed in both temperature groups following exposure to 30 micro M NOC for 24 hr (at 37 degrees C, 80.4% +/- 2.6%; at 22 degrees C, 83.2% +/- 1.6%; P > 0.05). During exposure to 100 micro M NOC, although the survival rate linearly decreased (approximately from 70% to 5%) in both temperature groups when exposed for 6-24 hr, there were no significant intergroup differences observed (P > 0.05). In conclusion, hypothermia does not provide adequate protection to the neurons by acting on the mechanisms evoked by NO, so we speculate that hypothermia may not confer neuroprotetcion once NO is released during ischemia.     

Nitric Oxide Regulation of Gonadotrophin Secretion in Prepubertal Heifers1

Mechanisms responsible for the pulsatile release of gonadotrophin secretion in prepubertal heifers are not fully known. We have shown that an excitatory amino acid agonist, N-Methyl-D,L-aspartic acid (NMA), induces an immediate release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) in prepubertal heifers. Nitric oxide (NO) has also emerged as an important regulator of LH release in rats. This study was designed to test the role of NO in the regulation of gonadotrophin release as well as the possible mediation by NO of the effects of NMA and gonadotrophin releasing hormone (GnRH) on gonadotrophin secretion in heifer calves. In experiment 1, four groups of five prepubertal heifers (33 weeks old) received one of the following treatments: (1); N-G-nitro-L-arginine methyl ester (L-NAME, a NO synthase inhibitor, 35 mg/kg, i.v., once); (2) NMA (4.7 mg/kg, i.v., once); (3) L-NAME+NMA (as above); and (4) Vehicle (saline, i.v.). All heifers in all groups were also challenged with a bolus injection of GnRH (10 ng/kg, i.v., once). Blood samples were collected every 15 min for 10 h. L-NAME was injected after the first blood sample, NMA after 2 h and GnRH after 6 h of blood sampling. Administration of L-NAME alone, suppressed the spontaneous pulses of LH (P<0.04). Heifers in the NMA group responded with a significantly greater LH release than did the heifers in the L-NAME+NMA group (P<0.05). Following the GnRH challenge, heifer calves treated with L-NAME or NMA had higher LH pulse responses than the controls (P<0.05). In a second experiment, four groups of five heifer calves (34 weeks old) were given one of the following treatments: (1) L-NAME (as above); (2) L-arginine, a NO precursor (ARG, 100 mg/kg/h, i.v. drip infused for 6 h starting 2 h after first blood sample was taken); (3) L-NAME+ARG (as above); and (4) Vehicle (saline i.v. bolus and drip for 6 h). Blood samples were taken every 10 min for 8 h. Administration of L-NAME suppressed the pulsatile release of LH and FSH (P<0.05). Compared to the control group, infusion of ARG by itself did not change the pattern of LH secretion (P>0.05); however, in heifers given L-NAME, ARG restored a normal pattern of LH pulses, similar to the control values (P>0.05). It was therefore concluded that NO is involved in the regulation of LH, and possibly FSH, secretion and that NO may mediate, at least in part, the stimulatory effects of NMA on LH, and to some extent FSH, release. The responses to GnRH led us to suggest that NO may have inhibitory effects on the pituitary and NMA may have increased pituitary sensitivity to GnRH.     

Role of central adenosine in the respiratory and thermoregulatory responses to hypoxia

No reports are available about the role of central adenosine in the respiratory and thermoregulatory responses to hypoxia in conscious rats. We therefore measured ventilation (VE) and body temperature (Tb) before and after intracerebroventricular injection of saline or aminophylline (adenosine antagonist), followed by a 30-min period of hypoxia exposure. Aminophylline did not change VE or Tb during normoxia; however, during hypoxia, it caused a significant increase in VE, and significantly attenuated hypoxic hypothermia. The present data indicate that central adenosine has an inhibitory effect on hypoxic hyperventilation and partially causes hypoxic hypothermia, suggesting that the ventilatory and metabolic interaction during hypoxia does not involve opposing mechanisms.