Nitric oxide synthase inhibitors L-NAME and 7-nitroindazole protect rat hippocampus against kainate-induced excitotoxicity

Wnt5a inhibits K(+) currents in hippocampal synapses through nitric oxide production

Hippocampal synapses play a key role in memory and learning processes by inducing long-term potentiation and depression. Wnt signaling is essential in the development and maintenance of synapses via several mechanisms. We have previously found that Wnt5a induces the production of nitric oxide (NO), which modulates NMDA receptor expression in the postsynaptic regions of hippocampal neurons. Here, we report that Wnt5a selectively inhibits a voltage-gated K(+) current (Kv current) and increases synaptic activity in hippocampal slices. Further supporting a specific role for Wnt5a, the soluble Frizzled receptor protein (sFRP-2; a functional Wnt antagonist) fully inhibits the effects of Wnt5a. We additionally show that these responses to Wnt5a are mediated by activation of a ROR2 receptor and increased NO production because they are suppressed by the shRNA-mediated knockdown of ROR2 and by 7-nitroindazole, a specific inhibitor of neuronal NOS. Together, our results show that Wnt5a increases NO production by acting on ROR2 receptors, which in turn inhibit Kv currents. These results reveal a novel mechanism by which Wnt5a may regulate the excitability of hippocampal neurons.

Potential pharmacological strategies for the improved treatment of organophosphate-induced neurotoxicity

Organophosphates (OP) are highly toxic compounds that cause cholinergic neuronal excitotoxicity and dysfunction by irreversible inhibition of acetylcholinesterase, resulting in delayed brain damage. This delayed secondary neuronal destruction, which arises primarily in the cholinergic areas of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, could be largely responsible for persistent profound neuropsychiatric and neurological impairments such as memory, cognitive, mental, emotional, motor, and sensory deficits in the victims of OP poisoning. The therapeutic strategies for reducing neuronal brain damage must adopt a multifunctional approach to the various steps of brain deterioration: (i) standard treatment with atropine and related anticholinergic compounds; (ii) anti-excitotoxic therapies to prevent cerebral edema, blockage of calcium influx, inhibition of apoptosis, and allow for the control of seizure; (iii) neuroprotection by aid of antioxidants and N-methyl-d-aspartate (NMDA) antagonists (multifunctional drug therapy), to inhibit/limit the secondary neuronal damage; and (iv) therapies targeting chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may prevent secondary neuronal damage in both early and late stages of OP poisoning, and thus may be a beneficial approach to treating the neuropsychological and neuronal impairments resulting from OP toxicity.

Combustion smoke-induced inflammation in the olfactory bulb of adult rats

BACKGROUND

The damaging effect of combustion smoke inhalation on the lung is widely reported but information on its effects on the olfactory bulb is lacking. This study sought to determine the effects of smoke inhalation on the olfactory bulb, whose afferent input neurons in the nasal mucosa are directly exposed to external stimuli, such as smoke.

METHODS

Adult male Sprague-Dawley rats were subjected to combustion smoke inhalation and sacrificed at different time points. Changes in olfactory bulb proteins including vascular endothelial growth factor (VEGF), inducible nitric oxide synthase (iNOS), endothelial nitric oxide synthase (eNOS), neuronal nitric oxide synthase (nNOS), Na+-K+-Cl- cotransporter 1 (NKCC1), glial fibrillary acidic protein (GFAP), and aquaporin-4 (AQP4) were evaluated by Western blot analysis. In addition, ELISA was conducted for cytokine and chemokine levels, and double immunofluorescence labeling was carried out for GFAP/VEGF, GFAP/AQP4, NeuN/nNOS, GFAP/NKCC1, NeuN/NKCC1, GFAP/Rhodamine isothiocyanate (RITC), and transferase dUTP nick end labeling (TUNEL). Aminoguanidine was administered to determine the effects of iNOS inhibition on the targets probed after smoke inhalation.

RESULTS

The results showed a significant increase in VEGF, iNOS, eNOS, nNOS, NKCC1, and GFAP expression in the bulb tissues, with corresponding increases in inflammatory cytokines and chemokines after smoke inhalation. Concurrent to this was a drastic increase in AQP4 expression and RITC permeability. Aminoguanidine administration decreased the expression of iNOS and RITC extravasation after smoke inhalation. This was coupled with a significant reduction in incidence of TUNEL + cells that was not altered with administration of L-NG-nitroarginine methyl ester (L-NAME).

CONCLUSIONS

These findings suggest that the upregulation of iNOS in response to smoke inhalation plays a major role in the olfactory bulb inflammatory pathophysiology, along with a concomitant increase in pro-inflammatory molecules, vascular permeability, and edema. Overall, these findings indicate that the olfactory bulb is vulnerable to smoke inhalation.

Combustion smoke-induced inflammation in the cerebellum and hippocampus of adult rats

AIMS

The effect of combustion smoke inhalation on the respiratory system is widely reported but its effects on the central nervous system remain unclear. Here, we aimed to determine the effects of smoke inhalation on the cerebellum and hippocampus which are areas vulnerable to hypoxia injury.

METHODS

Adult male Sprague-Dawley rats were subjected to combustion smoke inhalation and sacrificed at 0.5, 3, 24 and 72 h after exposure. The cerebellum and hippocampus were subjected to Western analysis for VEGF, iNOS, eNOS, nNOS and AQP4 expression; ELISA analysis for cytokine and chemokine levels; and immunohistochemistry for GFAP/AQP4, RECA-1/RITC and TUNEL. Aminoguanidine (AG) was administered to determine the effects of iNOS after smoke inhalation.

RESULTS

Both the cerebellum and hippocampus showed a significant increase in VEGF, iNOS, eNOS, nNOS and AQP4 expression with corresponding increases in inflammatory cytokines and chemokines and increased AQP4 expression and RITC permeability after smoke exposure. AG was able to decrease the expression of iNOS, followed by VEGF, eNOS, nNOS, RITC and AQP4 after smoke exposure. There was also a significant increase in TUNEL+ cells in the cerebellum and hippocampus which were not significantly reduced by AG. Beam walk test revealed immediate deficits after smoke inhalation which was attenuated with AG.

CONCLUSION

The findings suggest that iNOS plays a major role in the central nervous system inflammatory pathophysiology after smoke inhalation exposure with concomitant increase in proinflammatory molecules, vascular permeability and oedema, for which the cerebellum appears to be more vulnerable to smoke exposure than the hippocampus.

Endothelial nitric oxide synthase activity involves in the protective effect of ascorbic acid against penicillin-induced epileptiform activity

Ascorbic acid and nitric oxide are known to play important roles in epilepsy. The aim of present study was to identify the involvement of nitric oxide (NO) in the anticonvulsant effects of ascorbic acid on penicillin-induced epileptiform activity in rats. Intracortical injection of penicillin (500, International Units (IU)) into the left sensorimotor cortex induced epileptiform activity within 2-5 min. Thirty minutes after penicillin injection, nitric oxide synthase (NOS) inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME, 100mg/kg), neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI, 40 mg/kg), NO substrate, l-arginine (500 mg/kg) were administered with the most effective dose of ascorbic acid (100 mg/kg) intraperitoneally (i.p.). The administration of l-arginine significantly decreased the frequency of epileptiform activity while administration of l-NAME did not influence the mean frequency of epileptiform activity. Injection of 7-NI decreased the mean frequency of epileptiform activity but did not influence amplitude. Ascorbic acid decreased both the mean frequency and amplitude of penicillin-induced epileptiform activity in rats. The application of l-NAME partially and temporarily reversed the anticonvulsant effects of ascorbic acid. The results support the hypothesis of neuro-protective role for NO and ascorbic acid. The protective effect of ascorbic acid against epileptiform activity was partially and temporarily reversed by nonspecific nitric oxide synthase inhibitor l-NAME, but not selective neuronal nitric oxide synthase inhibitor 7-NI, indicating that ascorbic acid needs endothelial-NOS/NO route to decrease penicillin-induced epileptiform activity.

Kainic Acid-induced Neuronal Death is Attenuated by Aminoguanidine but Aggravated by L-NAME in Mouse Hippocampus

Nitric oxide (NO) has both neuroprotective and neurotoxic effects depending on its concentration and the experimental model. We tested the effects of NG-nitro-L-arginine methyl ester (L-NAME), a nonselective nitric oxide synthase (NOS) inhibitor, and aminoguanidine, a selective inducible NOS (iNOS) inhibitor, on kainic acid (KA)-induced seizures and hippocampal CA3 neuronal death. L-NAME (50 mg/kg, i.p.) and/or aminoguanidine (200 mg/kg, i.p.) were administered 1 h prior to the intracerebroventricular (i.c.v.) injection of KA. Pretreatment with L-NAME significantly increased KA-induced CA3 neuronal death, iNOS expression, and activation of microglia. However, pretreatment with aminoguanidine significantly suppressed both the KA-induced and L-NAME-aggravated hippocampal CA3 neuronal death with concomitant decreases in iNOS expression and microglial activation. The protective effect of aminoguanidine was maintained for up to 2 weeks. Furthermore, iNOS knockout mice (iNOS(-/-)) were resistant to KA-induced neuronal death. The present study demonstrates that aminoguanidine attenuates KA-induced neuronal death, whereas L-NAME aggravates neuronal death, in the CA3 region of the hippocampus, suggesting that NOS isoforms play different roles in KA-induced excitotoxicity.

GABAERGIC NEURON DEATH IN THE STRIATUM FOLLOWING KAINATE-INDUCED DAMAGE OF HIPPOCAMPAL NEURONS: EVIDENCE FOR THE ROLE OF NO IN LOCOMOTION

The authors examined the role of nitric oxide (NO) in the relationship between kainate-induced neuronal death and locomotion changes. Locomotion was significantly increased 1 h after kainate injection, suggesting that kainate induced NO and dopamine release. Cell death occurred in the CA1 (41%) and CA3 (54%) regions at 12 h. At 7 days, GABAergic neurons in striatum were lost, suggesting possible pyramidal neuron synapse with striatal GABAergic neurons, and pyramidal neuron damage leading to deafferentation and degeneration of striatal GABAergic neurons. Pre-administration of Nw-nitro-L-arginine-methyl-ester or 7-nitroindazole reduced these effects. These results indicate that NO may modulate kainate-induced neuronal death and locomotion.

Combustion smoke exposure induces up-regulated expression of vascular endothelial growth factor, aquaporin 4, nitric oxide synthases and vascular permeability in the retina of adult rats

Retinal cells respond to various experimental stimuli including hypoxia, yet it remains to be investigated whether they react to smoke inhalation. We show here that retinal cells in rats, notably the ganglion cells, Müller cells, astrocytes and blood vessels responded vigorously to a smoke challenge. The major changes included up-regulated expression of vascular endothelial growth factor (VEGF), aquaporin 4 (AQP4) and nitric oxide synthase (NOS). VEGF expression was localized in the ganglion cells, Müller cells, astrocytes and associated blood vessels. AQP4 was markedly enhanced in both astrocytes and Müller cells. Increase in vascular permeability after smoke exposure was evidenced by extravasation of serum derived rhodamine isothiocyanate which was internalized by Müller cells and ganglion cells. The tracer leakage was attenuated by aminoguanidine and N(G)-nitro-L-arginine methyl ester (L-NAME) treatment which suppressed retinal tissue NOS and nitric oxide (NO) levels concomitantly. It is suggested that VEGF, AQP4 and NO are involved in increased vascular permeability following acute smoke exposure in which hypoxia was ultimately implicated as shown by blood gases analysis. NOS inhibitors effectively reduced the vascular leakage and hence may ameliorate possible retinal edema in smoke inhalation.

The role of nitric oxide in the anticonvulsant effects of pyridoxine on penicillin-induced epileptiform activity in rats

The present study was conducted to identify the role of nitric oxide (NO) in the anticonvulsant effects of pyridoxine hydrochloride on penicillin-induced epileptiform activity in rats. A single microinjection of penicillin (500 units) into the left sensorimotor cortex induced epileptiform activity within 2-4 min, progressing to full seizure activity lasting about 3-5h. Thirty minutes after penicillin injection, 20, 40, 80, and 160 mg/kg of pyridoxine hydrochloride was administered intraperitoneally (i.p.). Pyridoxine significantly reduced the frequency of penicillin-induced epileptiform activity. A low dose of pyridoxine (40 mg/kg) was the most effective in reducing both the frequency and amplitude of epileptiform activity. The effect of systemic administration of nitric oxide synthase (NOS) inhibitors, non-selective N(G)-nitro-L-arginine methyl ester (L-NAME), selective neuronal NOS inhibitor, 7-nitroindazole (7-NI) and NO substrate, L-arginine on anticonvulsive effects of pyridoxine was investigated. The administration of L-arginine (500 mg/kg, i.p.) and 7-NI (25 and 50 mg/kg, i.p.) significantly decreased the frequency of epileptiform electrocorticographical (ECoG) activity while administration of L-NAME (60 mg/kg, i.p.) and the inactive form of arginine (D-arginine) did not influence it. The administration of L-NAME (60 mg/kg, i.p.) 15 min before pyridoxine (40 mg/kg i.p.) application reversed the anticonvulsant effects of pyridoxine whereas 7-NI (25 and 50 mg/kg, i.p.) did not influence it. The same dose of its inactive enantiomer N(G)-nitro-D-arginine methyl ester (d-NAME) failed to reverse the anticonvulsant effects of pyridoxine. The administration of L-arginine (500 mg/kg, i.p.) did not affect the frequency of epileptiform ECoG activity in the pyridoxine administered group. L-arginine did not reverse the anticonvulsant effect of 7-NI in the penicillin and pyridoxine administered groups. The results of present study indicate that the inhibitory effect on the anticonvulsant activity of pyridoxine against penicillin-induced epileptiform activity was produced by L-NAME, not by 7-NI, and is probably not related to the decrease of NOS activity in the brain.

The involvement of nitric oxide in the anticonvulsant effects of α-tocopherol on penicillin-induced epileptiform activity in rats

A variety of animal seizure models exist which help to document the effects of alpha-tocopherol (Vitamin E) and specify its action. In the present study, we provide further evidence for the functional involvement of NO in the anticonvulsant effects of alpha-tocopherol on penicillin-induced epileptiform electrocorticographical (ECoG) activity in rats. The epileptiform ECoG activity was induced by microinjection of penicillin into the left sensorimotor cortex. Thirty minutes after penicillin injection, the most effective dose of alpha-tocopherol (500 mg/kg) was administrated intramuscularly (i.m.). Alpha-tocopherol decreased the frequency of penicillin-induced epileptiform ECoG activity without changing the amplitude. The effect of systemic administration of nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) and NO substrates, L-arginine and sodium nitro prusside (SNP) on anticonvulsive effects of alpha-tocopherol was investigated. The administration of L-NAME (60 mg/kg, i.p.) did not influence the frequency of epileptiform ECoG activity while administration of L-arginine (500 mg/kg, i.p.) and SNP (6 mg/kg, i.p.) significantly decreased in the penicillin-treated group. The administration of L-NAME (60 mg/kg, i.p.) 10 min after alpha-tocopherol (500 mg/kg, i.m.) application reversed the anticonvulsant effects of alpha-tocopherol. The administration of L-arginine (500 mg/kg, i.p.) and SNP (6 mg/kg, i.p.) did not affect the frequency of epileptiform ECoG activity in alpha-tocopherol supplemented group. L-arginine and SNP did not provide an additional anticonvulsant effect in alpha-tocopherol supplemented group. These results support the involvement of the nitric oxide pathway in the anticonvulsant effect of alpha-tocopherol on the penicillin-induced epileptiform ECoG activity.

Increased neuronal nuclear calcium influx in neonatal seizures

We hypothesized that neonatal seizures lead to increased Ca(2+) influx (nCa(2+)I) in neuronal nuclei of newborn rats and that such increase is nitric-oxide mediated. Neuronal nuclear (45)Ca(2+) influx (nCa(2+)I) was measured in neuronal nuclei of 25 10-day-old male rat-pups newborn brains. They were divided into five groups (n = 5/group). (I) control; (II) hypoxia without seizures; (III) hypoxia with seizures; (IV) kainate, 2 mg/kg intraperitoneal (i.p.)-induced seizures and (V) 7-nitroindazole (7-NINA), 1 mg/kg i.p. pretreated, kainate-induced seizures. nCa(2+)I was significantly (P < 0.05) increased following hypoxia or seizures (hypoxic- or kainate-induced). Post-hypoxic seizures further enhanced nCa(2+)I increase induced by hypoxia (P < 0.05). 7-NINA abated the nCa(2+)I increase induced by kainate. We conclude that (1) kainate or hypoxia-induced seizures in newborn rats modify the neuronal nuclear membrane function, resulting in increased nCa(2+)I, (2) seizures exacerbate the hypoxia-induced increased nCa(2+)I incurred after hypoxia and (3) intranuclear calcium surges during kainate-induced neonatal seizures are nitric oxide-mediated.

7-Nitroindazole potentiates the anticonvulsant action of some second-generation antiepileptic drugs in the mouse maximal electroshock-induced seizure model

The effects of 7-nitroindazole (7NI, a preferential neuronal nitric oxide synthase inhibitor) on the anticonvulsant activity of four second-generation antiepileptic drugs (AEDs: felbamate [FBM], lamotrigine [LTG], oxcarbazepine [OXC] and topiramate [TPM]) were studied in the mouse maximal electroshock-induced seizure (MES) model. Moreover, the influence of 7NI on the acute neurotoxic (adverse-effect) profiles of the studied AEDs, with regard to motor coordination, was determined in the chimney test in mice. Results indicate that 7NI (50 mg/kg; i.p.) significantly potentiated the anticonvulsant activity of OXC, but not that of FBM, LTG and TPM against MES-induced seizures and, simultaneously, it enhanced the acute neurotoxic effects of TPM, but not those of FBM, LTG and OXC in the chimney test in mice. 7NI at the lower dose of 25 mg/kg had no effect on the antiseizure activity and acute neurotoxic profiles of all investigated AEDs. Pharmacokinetic evaluation of interactions between 7NI and LTG, OXC and TPM against MES-induced seizures revealed no significant changes in free (non-protein bound) plasma AED concentrations following 7NI administration. Moreover, none of the examined combinations of 7NI with AEDs from the MES test were associated with long-term memory impairment in mice subjected to the step-through passive avoidance task. Based on our preclinical study, it can be concluded that only the combination of 7NI with OXC was beneficial, when considering its both anticonvulsant and acute neurotoxic effects. Moreover, the lack of impairment of long-term memory and no pharmacokinetic interactions in plasma of experimental animals make the combination of 7NI with OXC worthy of consideration for the treatment of patients with refractory epilepsy. The other combinations tested between 7NI and LTG, FBM and TPM were neutral, when considering their both anticonvulsant effects and acute neurotoxic profiles, therefore, no useful recommendation can be made for their clinical application.

Prevention of kainic acid-induced changes in nitric oxide level and neuronal cell damage in the rat hippocampus by manganese complexes of curcumin and diacetylcurcumin

Curcumin is a natural antioxidant isolated from the medicinal plant Curcuma longa Linn. We previously reported that manganese complexes of curcumin (Cp-Mn) and diacetylcurcumin (DiAc-Cp-Mn) exhibited potent superoxide dismutase (SOD)-like activity in an in vitro assay. Nitric oxide (NO) is a free radial playing a multifaceted role in the brain and its excessive production is known to induce neurotoxicity. Here, we examined the in vivo effect of Cp-Mn and DiAc-Cp-Mn on NO levels enhanced by kainic acid (KA) and L-arginine (L-Arg) in the hippocampi of awake rats using a microdialysis technique. Injection of KA (10 mg/kg, i.p.) and L-Arg (1000 mg/kg, i.p.) significantly increased the concentration of NO and Cp-Mn and DiAc-Cp-Mn (50 mg/kg, i.p.) significantly reversed the effects of KA and L-Arg without affecting the basal NO concentration. Following KA-induced seizures, severe neuronal cell damage was observed in the CA1 and CA3 subfields of hippocampal 3 days after KA administration. Pretreatment with Cp-Mn and DiAc-Cp-Mn (50 mg/kg, i.p.) significantly attenuated KA-induced neuronal cell death in both CA1 and CA3 regions of rat hippocampus compared with vehicle control, and Cp-Mn and DiAc-Cp-Mn showed more potent neuroprotective effect than their parent compounds, curcumin and diacetylcurcumin. These results suggest that Cp-Mn and DiAc-Cp-Mn protect against KA-induced neuronal cell death by suppression of KA-induced increase in NO levels probably by their NO scavenging activity and antioxidative activity. Cp-Mn and DiAc-Cp-Mn have an advantage to be neuroprotective agents in the treatment of acute brain pathologies associated with NO-induced neurotoxicity and oxidative stress-induced neuronal damage such as epilepsy, stroke and traumatic brain injury.

Nitric oxide synthase inhibitors in experimental ischemic stroke and their effects on infarct size and cerebral blood flow: a systematic review

Nitric oxide produced by the neuronal or inducible isoform of nitric oxide synthase (nNOS, iNOS) is detrimental in acute ischemic stroke (IS), whereas that derived from the endothelial isoform is beneficial. However, experimental studies with nitric oxide synthase inhibitors have given conflicting results. Relevant studies were found from searches of EMBASE, PubMed, and reference lists; of 456 references found, 73 studies involving 2321 animals were included. Data on the effects of NOS inhibition on lesion volume (mm3, %) and cerebral blood flow (CBF; %, ml * min(-1) * g(-1)) were analyzed using the Cochrane Review Manager software. NOS inhibitors reduced total infarct volume in models of permanent (standardized mean difference (SMD) -0.56, 95% confidence interval (95% CI) -0.86, -0.26) and transient (SMD -0.99, 95% CI -1.25, -0.72) ischemia. Cortical CBF was reduced in models of permanent but not transient ischemia. When assessed by type of inhibitor, total lesion volume was reduced in permanent models by nNOS and iNOS inhibitors, but not by nonselective inhibitors. All types of NOS inhibitors reduced infarct volume in transient models. NOS inhibition may have negative effects on CBF but further studies are required. Selective nNOS and iNOS inhibitors are candidate treatments for acute IS.

Effects of nitric oxide on dentate gyrus cell proliferation after seizures induced by pentylenetrazol in the adult rat brain

Epileptic seizures have been shown to increase the proliferation of granule cell precursors in the adult brain, but the underlying mechanisms remain largely unknown. This study examined the effect of nitric oxide (NO) on the proliferation of granule cell precursors in adult rats after pentylenetrazol (PTZ)-induced generalized clonic seizures. Using systemic bromodeoxyuridine (BrdU) to label dividing cells, we found that injection of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (50 mg/kg i.p.) 10 min before PTZ significantly reduced the number of BrdU labeled cells in the dentate gyrus 3, 7, and 14 days after seizures (P < 0.05). Administration of the inducible NOS (iNOS) inhibitor aminoguanidine (100 mg/kg i.p.) also significantly inhibited the proliferation rate of neural precursor cells in the dentate gyrus at various time points after PTZ-induced seizures. Our findings suggest that epileptic seizures lead to increased cell proliferation in the adult rat dentate gyrus through NO-dependent mechanisms. Both the NO originating from nNOS and iNOS may be involved in brain repair after seizures.

Changes of [3H]Muscimol, [3H]Flunitrazepam and [3H]MK-801 Binding in Rat Brain by Prolonged Ventricular Infusion of 7-Nitroindazole

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), to examine modulation of NMDA and GABAA receptor binding in rat brain. The duration of sleeping time was significantly increased by the pre-treatment with 7-NI (100 mg/kg) 30 min before pentobarbital (40 mg/kg) treatment in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of 7-NI into cerebroventricle for 7 days. We have investigated the effect of NOS inhibitor on NMDA and GABAA receptor binding characteristics in discrete areas of brain regions by using autoradiographic techniques. The GABAA receptors were analyzed by quantitative autoradiography using [3H]muscimol and [3H]flunitrazepam binding, and NMDA receptor binding was analyzed by using [3H]MK-801 binding in rat brain slices. Rats were infused with 7-NI (500 pmol/10 microl/h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [3H]muscimol were markedly elevated in cortex, caudate putamen, and thalamus while the levels of [3H]flunitrazepam binding were only elevated in cerebellum by NOS inhibitor. However, there was no change in the level of [3H]MK-801 binding except decreasing in the thalamus. These results show that the prolonged inhibition of NOS by 7-NI-infusion highly elevates [3H]muscimol binding in a region-specific manner and decreases the pentobarbital-induced sleep.

Ability of GDNF to diminish free radical production leads to protection against kainate-induced excitotoxicity in hippocampus

The primary aim of this study is to explore the protective mechanisms of glial-derived neurotrophic factor (GDNF) during excitotoxicity by kainate in the hippocampus. After a 15-min microinjection with kainate, excitotoxicity was induced in the rat hippocampus. The protective effect of GDNF in the hippocampus was evaluated by administering GDNF 14 min after injection of kainate. The resulting hydroxyl free radicals were quantified by microdialysis of the hippocampus. The results show that GDNF can effectively suppress the production of kainate-induced hydroxyl free radical production. In addition, histological observation indicated the ability of GDNF to decrease the damage level of pyramidal neurons in the CA3 and CA4 areas of the hippocampus. Superoxide dismutase (SOD) activity in the hippocampus was elevated significantly at 30 min and 7 days after kainate induction, while glutathione peroxidase (cGPx) activity did not increase significantly until the seventh day. With GDNF treatment, SOD and cGPx activity in the hippocampus was elevated significantly 7 days after kainate induction. We suggest that mechanisms including a decrease in free radical generation and scavenging of free radicals might be involved in GDNF protection against kainate-induced excitotoxicity.

Presenilin-1-deficient neurons are nitric oxide-dependently killed by hydrogen peroxide in vitro

Presenilin-1 (PS1) is the gene responsible for the development of early-onset familial Alzheimer's disease. To probe the functions of PS1 on neuronal resistance to oxidative stress, we pharmacologically examined the death signals in PS1-deficient neurons induced by oxidative stress. Because the death of primarily cultured neurons lacking PS1 is caused by hydrogen peroxide in calcium-dependent manners in vitro [J Neurochem 78 (2001) 807], we tested the neuronal survival-promoting ability of inhibitors against calcium-dependent/cell death-related signaling molecules, such as ERKs, JNK, p38 MAP kinase, calcineurin, calpain, and nitric oxide synthase (NOS). All inhibitors tested failed to rescue the PS1-deficient neurons from the death with the exception of an inhibitor of NOS, N(G)-nitro-l-arginine methyl ester. Hemoglobin, a nitric oxide (NO) scavenger, also prevented the death of the mutant neurons. NADPH-diaphorase staining, which accounts for NOS activity, was enhanced in the mutant neurons. These results suggest that PS1 has a role for NOS activation in neurons and confers oxidative stress-resistance on neurons in calcium/NO-dependent manners.

Demonstrating the dose- and time-related effects of 7-nitroindazole on picrotoxin-induced convulsions, memory formation, brain nitric oxide synthase activity, and nitric oxide concentration in rats

In this study, the dose (50, 100, 150, and 200 mg/kg)- and time (30 and 60 min)- related effects of 7-nitroindazole (7-NI), a neuronal specific inhibitor of nitric oxide synthase (NOS) were tested on picrotoxin (5 mg/kg)-induced convulsions and memory formation in rats. The changes produced by these doses of 7-NI were determined on NOS activity and nitric oxide (NO) concentration in the brain. The effects of 7-NI were tested in animals pretreated (30 min) with L-arginine (500 and 1000 mg/kg). 7-NI, at 50 and 100 mg/kg, did not produce significant changes in NOS activity and NO concentration in the brain and memory formation. However, the convulsant action of picrotoxin was inhibited in a dose-dependent manner in these animals. A time-dependent decrease in the activity of NOS and the concentration of NO, a promotion of picrotoxin-induced convulsions, and an impairment of memory were found in animals treated with 150 and 200 mg/kg of 7-NI. The larger and not the smaller dose of L-arginine raised the concentration of NO, inhibited picrotoxin-induced convulsions and promoted memory process. Either dose of L-arginine failed to prevent 50 and 100 mg/kg of 7-NI from inhibiting convulsions. The effects of the larger doses of 7-NI (150 and 200 mg/kg) were effectively prevented by the increase of NO and not the ineffective dose of L-arginine. These results suggest that 7-NI (50 and 100 mg/kg) decreases convulsions by a nonspecific mechanism and that an inhibition of NOS by the larger doses of it (150 and 200 mg/kg) results in proconvulsant action and memory impairment. The data further show that the margin between the protective and proconvulsant doses of 7-NI is relatively narrow. These results have been taken together with the earlier reports that 7-NI produces learning impairment and fails to increase the anticonvulsant effect of traditional antiepileptic agents on experimentally induced convulsions to conclude that 7-NI can never emerge as an anticonvulsant agent for clinical use.

Prevention of kainic acid seizures-induced changes in levels of nitric oxide and high-energy phosphates by 7-nitroindazole in rat brain regions

Previous studies using the spin trapping agent N-tert-butyl-alpha-phenylnitrone (PBN) and the antioxidant vitamin E established the involvement of free radicals in kainic acid (KA)-induced neurotoxicity. In the present study, we examined the effects of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI) to establish a possible role of nitric oxide (NO) in the neurotoxicity caused by KA-induced status epilepticus (SE). A single injection of KA (15 mg/kg, s.c.) induced seizures within 40-45 min, progressing to full seizure activity lasting about 3 h. Following microwave (head-focused) irradiation, perchloric acid extracts of rat brain regions (cortex, amygdala, and hippocampus) were analyzed for citrulline (determinant of NO) and high-energy phosphates (HEP) and their metabolites using high-performance liquid chromatograph (HPLC). KA-induced seizures produced a maximum increase in NO (3- to 6-fold) and a decrease in HEP (ATP 45-51% and phosphocreatine 45-58%) 2 h after KA injection in brain regions tested. 7-NI (50 mg/kg, i.p.) when given alone, reduced citrulline/NO levels (10-24%), while repeat administration of 7-NI (60 min apart) reduced NO levels by 32-49%. Neither application of 7-NI produced changes in HEP levels or toxicity. Pretreatment with 7-NI 30 min before KA injection, delayed the onset of seizures by 15-20 min, and significantly prevented an increase in NO and a decrease in HEP. Repeat administration of 7-NI, i.e. 30 min before and 30 min after KA injection, further increased protection by the delayed onset of seizures, attenuating the increase in NO and the decrease in HEP. Neurotoxicity of seizures involves activation of nNOS and of energy consumption in affected neurons. This increased energy consumption, coupled with decreased energy production caused by NO-induced mitochondrial dysfunction, may be a contributing factor to neuronal injury in KA toxicity.

Increased neuronal nitric oxide synthase (nNOS) activity triggers picrotoxin-induced seizures in rats and evidence for participation of nNOS mechanism in the action of antiepileptic drugs

Increased neuronal nitric oxide synthase (nNOS) activity was observed during the prodromal period of seizures in various rat brain regions following administration of GABA(A) receptor antagonist, picrotoxin (PCT). Pretreatment with the selective nNOS inhibitor 7-nitroindazole (7-NI), dose- and time-dependently delayed the onset of clonus with a corresponding decrease in nNOS activity. The threshold dose of antiepileptic drugs (AEDs; diazepam, phenobarbitone and gabapentin) have potentiated the anticonvulsant action by pretreatment with graded doses of 7-NI. The increase in efficacy of anticonvulsant action correlated with a corresponding decrease of PCT-evoked increase in nNOS activity. The present data support a role for abnormal nNOS activity in mechanisms that trigger seizures and suggest a possible NO-mediated interplay between GABA(A) and glutamate receptors. The results of the present study provide evidence for a trigger role of neuronally produced NO in epileptogenesis induced by PCT and the participation of nNOS inhibitory mechanisms in the action of AEDs.

Neuronal nitric oxide synthase proteolysis limits the involvement of nitric oxide in kainate-induced neurotoxicity in hippocampal neurons

In this work, we investigated the role of nitric oxide (NO) in neurotoxicity triggered by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor activation in cultured hippocampal neurons. In the presence of cyclothiazide (CTZ), short-term exposures to kainate (KA; 5 and 15 min, followed by 24-h recovery) decreased cell viability. Both NBQX and d-AP-5 decreased the neurotoxicity caused by KA plus CTZ. Long-term exposures to KA plus CTZ (24 h) resulted in increased toxicity. In short-, but not in long-term exposures, the presence of NO synthase (NOS) inhibitors (l-NAME and 7-NI) decreased the toxicity induced by KA plus CTZ. We also found that KA plus CTZ (15-min exposure) significantly increased cGMP levels. Furthermore, short-term exposures lead to decreased intracellular ATP levels, which was prevented by NBQX, d-AP-5 and NOS inhibitors. Immunoblot analysis revealed that KA induced neuronal NOS (nNOS) proteolysis, gradually lowering the levels of nNOS according to the time of exposure. Calpain, but not caspase-3 inhibitors, prevented this effect. Overall, these results show that NO is involved in the neurotoxicity caused by activation of non-desensitizing AMPA receptors, although to a limited extent, since AMPA receptor activation triggers mechanisms that lead to nNOS proteolysis by calpains, preventing a further contribution of NO to the neurotoxic process.

Involvement of nitric oxide in kainic acid-induced excitotoxicity in rat brain

The involvement of nitric oxide (NO) in kainic acid (KA)-induced excitotoxicity was studied in rat brain. With the onset of KA (15 mg kg(-1), s.c.)-induced seizures (convulsions) 30 min after injection, increases in NO, as measured by the formation of citrulline, were seen in cortex (302%), amygdala (171%) and hippocampus (203%). The highest increases were determined 90 min after onset of seizures (120 min after KA injection) with 633%, 314% and 365%, respectively. These changes in NO preceded significant decreases in ATP and phosphocreatine (PCr) ranging from 44 to 53% for ATP and from 40 to 52% for PCr in the respective brain areas. With the exception of the cortex, normal citrulline values were restored within 24 h. Pretreatment with the spin trapping agent N-tert-butyl-alpha-phenylnitrone (PBN, 200 mg kg(-1), i.p.) or the antioxidant vitamin E (Vit-E, 100 mg kg(-1) per day for 3 days) prevented the increase in citrulline and significantly attenuated the loss in ATP and PCr without affecting seizure activity. It is concluded that seizures induced by KA produced a marked increase in the free radical NO, causing oxidative stress and leading to depletion of energy stores. The prevention of the increase in NO and preservation of ATP and PCr levels by PBN and Vit-E suggests the involvement of NO and other related free radicals, such as peroxynitrite (ONOO(-)). The lack of effect of PBN and Vit-E on seizure activity, suggests that NO is not involved in mechanisms regulating KA seizure generation and propagation. PBN and Vit-E or similar compounds may be important protective agents against status epilepticus-induced neuronal degeneration.

Treatment with 7-nitroindazole enhances kainic acid induced cholinergic neurotoxicity in the rat striatum: a neuroprotective role for neuronal nitric oxide

1. In this study we investigated the effect of 7-nitroindazole (7-NI), a preferential inhibitor of neuronal nitric oxide synthase (nNOS), on kainic acid (KA) induced neurotoxicity in rats. Choline acetyltransferase activity (CAT), a cholinergic marker, and histological changes were employed to assess neurotoxicity. 2. In control rats, the local intrastriatal injection of 0.5 microg of KA reduced CAT from 22.9 +/- 2.2 to 14.7 +/- 2.0 nmol/h/mg tissue ((38 +/- 6)% reduction) (P < 0.001). Greater reductions in CAT were observed with 1 and 2 microg of KA ((70 +/- 6)% and (80 +/- 3)%, respectively). 7-NI aggravated KA-induced cholinergic and histological damage. KA reduced CAT by (68.2 +/- 4)% in 7-NI-treated rats, by (38 +/- 6)% in saline-treated controls, and by (41 +/- 4)% in peanut-oil- (7-NI-vehicle-) treated rats (P = 0.0047). 3. After KA, CAT activity averaged 14.3 +/- 2.0 in peanut-oil-treated rats and 7.9 +/- 1.0 nmol/h/mg tissue in 7-NI- (peanut-oil-) treated rats (P = 0.015). Similarly to changes in CAT, 7-NI treatment aggravated KA-induced histological changes indicative of neuronal damage (acute ischemic neuronal changes, disorganization of myelinated fibers bundle, and vacuolation changes of the neuropil). Treatment with 7-NI was not associated with increased mortality. 4. Our findings suggest that neuronal NO plays a neuroprotective action on excitotoxicity.

Neuroprotective MK801 is associated with nitric oxide synthase during hypoxia/reoxygenation in rat cortical cell cultures

The neuroprotective effect of MK801 against hypoxia and/or reoxygenation-induced neuronal cell injury and its relationship to neuronal nitric oxide synthetase (nNOS) expression were examined in cultured rat cortical cells. Treatment of cortical neuronal cells with hypoxia (95% N(2)/5% CO(2)) for 2 h followed by reoxygenation for 24 h induced a release of lactate dehydrogenase (LDH) into the medium, and reduced the protein level of MAP-2 as well. MK801 attenuated the release of LDH and the reduction of the MAP-2 protein by hypoxia, suggesting a neuroprotective role of MK801. MK801 also diminished the number of nuclear condensation by hypoxia/reoxygenation. The NOS inhibitors 7-nitroindazole (7-NI) and N (G)-nitro-L-arginine methyl ester (L-NAME), as well as the Ca(2+) channel blocker nimodipine, reduced hypoxia-induced LDH, suggesting that nitric oxide (NO) and calcium homeostasis contribute to hypoxia and/or the reoxygenation-induced cell injury. The levels of nNOS immunoactivities and mRNA by RT-PCR were enhanced by hypoxia with time and, down regulated following 24 h reoxygenation after hypoxia, and were attenuated by MK801. In addition, the reduction of nNOS mRNA levels by hypoxia/reoxygenation was also diminished by MK801. Further delineation of the mechanisms of NO production and nNOS regulation are needed and may lead to additional strategies to protect neuronal cells against hypoxic/reoxygenation insults.

Nitric oxide synthase inhibition suppresses wet dog shakes and augments convulsions in rats

Electrical stimulation of limbic structures, pharmacological interventions, and getting wet induces wet dog shakes (WDS) in rats. WDS are often associated with the occurrence of seizures. In this study, we evaluated the effects of reduced NO production on physiologically (wetting)- or pharmacologically (kainic acid; KA)-induced WDS and KA-triggered seizures. Following wetting, naive and saline-treated rats displayed more WDS than rats treated with NO synthase (NOS) inhibitor, N omega-nitro-L-arginine (L-NA). In another experiment, WDS and seizures were monitored after KA treatment alone or in combination with L-NA. Again, NOS inhibition reduced the number of KA-triggered WDS but augmented the number and severity of seizures. Our results suggest that not only do physiologically- and kainate-induced WDS share a common mechanism that includes NO, but that there is also an antagonism between WDS and convulsions.

Sustained, long-lasting inhibition of nitric oxide synthase aggravates the neural damage in some models of excitotoxic brain injury

Brain nitric oxide (NO) can be a mediator of physiological and neuroprotective actions and an effector of neural damage. The effectiveness of acute or chronic inhibition of NO production in in vivo experiments of neurotoxicity/neuroprotection is controversial. We report here on the effects of a chronic, sustained inhibition of nitric oxide synthase (NOS) on the neurodegenerative damage caused by three different excitotoxic lesions. The damage caused by intrastriatal injection of ibotenic or kainic acid was aggravated in rats subjected to chronic NOS inhibition. On the contrary, the drop of cortical cholinergic input consequent to ibotenic acid-mediated degeneration of basal forebrain neurons was not altered by chronic NOS inhibition. The worsening of the damage was not related to any overt differential sensitivity to excitotoxicity of NOS-containing striatal neurons under conditions of NOS inhibition. These results suggest that, contrary to what has been often reported for short-term, mild inhibition of NO production, chronic and sustained NOS inhibition may exacerbate neuropathology. Thus, long-lasting shortage of NO may be detrimental when neuroprotective mechanisms related to the physiological action of this free radical are severely impaired. Although we cannot exclude that inhibition of the endothelial NOS isoform could have contributed to the worsening of neuropathology, differences among the paradigms of neurotoxicity used in the present study suggest a primary involvement of the neuronal NOS isoform. In view of the potential therapeutic use of NOS inhibitors, the effects of a too drastic alteration of the balance between neuroprotective and neurodegenerative actions of NO should be carefully considered.

Changes in nitric oxide synthesis and epileptic activity in the contralateral hippocampus of rats following intrahippocampal kainate injection

PURPOSE

To investigate the effects of nitric oxide (NO) on seizure activity observed in brain areas that are remote from a primary epileptic focus.

METHODS

Following an injection of kainate (concentration 1 mg/ml, volume 1 microl) in the rat hippocampus, we measured NO synthesis in the contralateral hippocampus and epileptic activity by electroencephalogram (EEG). The NO end products, nitrite and nitrate, were measured by in vivo microdialysis combined with an automated NO end-product analyzer and then used as indices of NO synthesis. We also assessed the effect of a specific inhibitor of neuronal NO synthase (NOS) on both the epileptic activity and NO synthesis in the contralateral hippocampus. For this assessment, we administered 7-nitroindazole (7-NI) (50 mg/kg) intraperitoneally 30 min before the kainate injection.

RESULTS

Epileptic discharges in the contralateral hippocampus were frequently observed 90 min after unilateral hippocampus kainate injection. The duration of these discharges gradually increased until 240 min after the kainate injection. The NO end-product levels increased immediately after kainate injection and continued to increase gradually throughout the experiments, to a maximum of 213% of the base level. This elevation of NO end products was followed by epileptic discharges. Both the seizure activity and the elevation of contralateral hippocampus NO end-product levels were markedly attenuated in the animals that received 7-NI.

CONCLUSIONS

The results suggest that remote seizure activity caused by the transneuronal spread of kainate-induced discharges may be related to NO derived from neuronal NOS.

Role of nitric oxide in the ethylcholine aziridinium model of delayed apoptotic neurodegeneration in vivo and in vitro

The involvement of nitric oxide in neurodegenerative processes still remains incompletely characterized. Although nitric oxide has been reported to be an important mediator in neuronal degeneration in different models of cell death involving NMDA-receptor activation, increasing evidence for protective mechanisms has been obtained. In this study the role of nitric oxide was investigated in a model of NMDA-independent, delayed apoptotic cell death, induced by the neurotoxin ethylcholine aziridinium ethylcholine aziridinium both in vivo and in vitro. For the in vivo evaluation rats received bilateral intracerebroventricular injections of ethylcholine aziridinium (2nmol/ventricle) or vehicle. In the hippocampus a transient decrease in nitric oxide synthase activity occurred, reaching its lowest levels three days after ethylcholine aziridinium treatment (51.7+/-9.8% of controls). The decrease coincided with the maximal reduction in choline acetyltransferase activity as marker for the extent of cholinergic lesion. The effect of pharmacological inhibition of nitric oxide synthase was tested by application of various nitric oxide synthase inhibitors with different selectivity for the nitric oxide synthase-isoforms. Unspecific nitric oxide synthase inhibition resulted in a significant potentiation of the loss of choline acetyltransferase activity in the hippocampus measured seven days after ethylcholine aziridinium application, whereas the specific inhibition of neuronal or inducible nitric oxide synthase was ineffective. These pharmacological data are suggestive for a neuroprotective role of nitric oxide generated by endothelial nitric oxide synthase. In vitro experiments were performed using serum-free primary neuronal cell cultures from hippocampus, cortex and septum of E15-17 Wistar rat embryos. Ethylcholine aziridinium-application in a range of 5-80microM resulted in delayed apoptotic neurodegeneration with a maximum after three days as confirmed by morphological criteria, life-death assays and DNA laddering. Nitric oxide synthase activity in harvested cells decreased in a dose- and time-dependent manner. Nitric oxide production as determined by measurement of the accumulated metabolite nitrite in the medium was equally low in controls and in ethylcholine aziridinium treated cells (range 0.77-1.86microM nitrite). An expression of inducible nitric oxide synthase messenger RNA could not be detected by semiquantitative RT-PCR 13h after ethylcholine aziridinium application. The present data indicate that in a model of delayed apoptotic neurodegeneration as induced by ethylcholine aziridinium neuronal cell death in vitro and in vivo is independent of the cytotoxic potential of nitric oxide. This is confirmed by a decrease in nitric oxide synthase activity, absence of nitric oxide production and absence of inducible nitric oxide synthase expression. In contrast, evidence for a neuroprotective role of nitric oxide was obtained in vivo as indicated by the exaggeration of the cholinergic lesion after unspecific nitric oxide synthase inhibition by N-nitro-L-arginine methylester.

Tacrine, a reversible acetylcholinesterase inhibitor, induces myopathy

Tacrine, an acetylcholinesterase (AChE) inhibitor that has been used in the treatment of Alzheimer's disease, increases available acetylcholine (Ach) levels in the synaptic cleft thereby enhancing the activity of cholinergic pathways. However, excessive stimulation of nicotinic receptors at the neuromuscular junction results in muscle deterioration. We tested whether reversible AChE inhibitors such as tacrine may induce similar effects. In the present study, tacrine administration (7.5 mg/kg twice daily) to rats produces a 20 and 30-fold increase in the number of degenerating cells in leg and diaphragm muscle, respectively, as compared to control. This myopathy is significantly decreased by co-administration of tacrine with the nitric oxide (NO) synthase inhibitor L-NAME. These results show that tacrine can induce myopathy which may be mediated by increased NO production.

Inhibitors of NO-synthase and donors of NO modulate kainic acid-induced damage in the rat hippocampus

The effects of nitric oxide synthase (NOS) inhibitors, N(omega)-nitro-L-arginine and 7-nitroindazole, and the NOS substrate L-arginine on kainic acid (KA)-induced microglial reactivity and stress response were studied in the hippocampus 7 and 1 days after KA, respectively. Density of peripheral-type benzodiazepine receptors was measured as an index of microglial reactivity. Histological damage in hippocampus was evaluated at 7 days by neuronal counting. KA increased the maximal number of binding sites (B(max)) versus controls. Administration of either 7-nitroindazole (25 mg/kg) or N(omega)-nitro-L-arginine (20 and 50 mg/kg) 24 hr before KA, further increased B(max). This later effect was abolished by L-arginine (1 g/kg), which given 24 hr before KA decreased B(max) to control values. Also, KA-induced HSP72 stress response was attenuated by pre-treatment with L-arginine. Histological evaluation showed reduced cell numbers in the pyramidal cell layer of the hippocampus in groups receiving KA, either alone or in combination with 7-nitroindazole. Administration of L-arginine before KA attenuated neuronal loss in CA3 but not CA1. A clear protective effect was observed, however, in CA1 and CA3, in rats receiving both L-arginine plus 7-nitroindazole before KA. The results show that the combination of a NO substrate with a NOS inhibitor reduces the neurotoxic effects of KA in the rat hippocampus. This study suggests that extremely fine regulation of NO levels in the different neural cell types can modulate excitotoxicity.

Effect of redox-active drugs on superoxide generation from nitric oxide synthases: biological and toxicological implications

In this article, we address the mechanism of superoxide formation from constitutive nitric oxide synthases (NOS). Merits and drawbacks of the various superoxide detection assays are reviewed. One of the most viable techniques for measuring superoxide from NOS is electron spin resonance (ESR) spin-trapping using a novel phosphorylated spin trap. Implications of superoxide and peroxynitrite formation from NOS enzymes in cardiovascular and cerebrovascular disorders are discussed.

EUK-134, a synthetic superoxide dismutase and catalase mimetic, prevents oxidative stress and attenuates kainate-induced neuropathology

The present study tested the effects of EUK-134, a synthetic superoxide dismutase/catalase mimetic, on several indices of oxidative stress and neuropathology produced in the rat limbic system as a result of seizure activity elicited by systemic kainic acid (KA) administration. Pretreatment of rats with EUK-134 did not modify the latency for or duration of KA-induced seizure activity. It did produce a highly significant reduction in increased protein nitration, activator protein-1- and NF-kappaB-binding activity, and spectrin proteolysis as well as in neuronal damage resulting from seizure activity in limbic structures. These results support the hypothesis that kainate-induced excitotoxicity is caused, at least in part, by the action of reactive oxygen species. Furthermore, they suggest that synthetic superoxide dismutase/catalase mimetics such as EUK-134 might be used to prevent excitotoxic neuronal injury.