Distinct effects of N omega-nitro-L-arginine on seizures induced by several drugs in mice

Involvement of nitrergic system in the anticonvulsant effect of the cannabinoid CB(1) agonist ACEA in the pentylenetetrazole-induced seizure in mice

Cannabinoid system plays a pivotal role in the seizure threshold modulation which is mainly mediated through activation of the cannabinoid CB(1) receptor. There is also several evidence of interaction between cannabinoid system and other neurotransmitters including nitric oxide (NO) system. Using model of clonic seizure induced by pentylenetetrazole (PTZ) in male NMRI mice, we investigated whether NO is involved in the effects of cannabinoids on the seizure threshold. Injection of the selective cannabinoid CB(1) agonist ACEA (2mg/kg, i.p.) significantly (P<0.01) increased the seizure threshold which was prevented (P<0.001) by pretreatment with the selective CB(1) antagonist AM251 (1mg/kg, i.p.). The NO precursor l-arginine (50 and 100mg/kg, i.p.) potentiated the anticonvulsant effects of the sub-effective dose of ACEA (1mg/kg, i.p.). Pretreatment with non-effective doses of the non-specific NOS inhibitor l-NAME (15 and 30mg/kg, i.p.) and the specific neuronal NOS inhibitor 7-NI (40 and 80mg/kg, i.p.) but not the inducible NOS inhibitor aminoguanidine (10, 50 and 100mg/kg, i.p.) prevented the anticonvulsant effect of ACEA (2mg/kg, i.p.). Co-administration of non-effective dose of AM251 (0.5mg/kg) with both low and per se non-effective doses of l-NAME (1mg/kg, i.p.) and 7-NI (10mg/kg, i.p.) had significant (P<0.01) effect in preventing the anticonvulsant effect of ACEA (2mg/kg, i.p.). Our findings demonstrated that central NO system could be involved in the anticonvulsant properties of the specific cannabinoid CB(1) agonist ACEA, emphasizing on the interaction between two systems in the seizure modulation.

Amino acid levels in some brain areas of inducible nitric oxide synthase knock out mouse (iNOS−/−) before and after pentylenetetrazole kindling

Inducible nitric oxide synthase knock-out (iNOS(-/-)) mice are valid models of investigation for the role of iNOS in patho-physiological conditions. There are no available data concerning neuroactive amino acid levels of iNOS(-/-) mice and their behaviour in response to pentylenetetrazole (PTZ). We found no significant differences in the convulsive dose 50 (CD(50)) between iNOS(-/-) and control (iNOS(+/+)) mice, however, iNOS(-/-) mice reach the kindled status more slowly than control, suggesting that in basal condition the GABA-benzodiazepine inhibitory inputs are unaltered by iNOS mutation. Clear differences between iNOS(+/+) and iNOS(-/-) mice amino acid concentrations were evident both in basal conditions and after kindling. Our results show that aspartate was significantly lower in all brain areas studied except the brain stem whereas glutamate and glutamine were significantly higher in the cortex, hippocampus and brain stem. GABA was slightly and not significantly higher in the cortex, hippocampus and brain stem, whereas taurine was significantly higher in all areas except diencephalon and glycine was significantly lower in the diencephalon and cerebellum. In this context, the inability of iNOS(-/-) mice to increase the NO levels following PTZ administrations indicate that NO might play a pro-epileptogenic role in the genesis and development of some types of epilepsy. Since there is no correlation between neurotransmitter levels and the development of kindling, it is possible to exclude that the difference between the two strains is due to an imbalance between the considered neurotransmitters, and it is then possible that this difference is due to the presence of iNOS, which might be involved in long term plasticity of the brain.

Hypothermia during kainic acid-induced seizures reduces hippocampal lesions and cerebral nitric oxide production in immature rabbits

We investigated (1) whether cerebral hypothermia during kainic acid (KA)-induced seizures was neuroprotective; and (2) whether nitric oxide (NO) production in the brain during seizures was altered by cerebral hypothermia in immature rabbits. Twelve female rabbits, aged 2 weeks, were anesthetized, paralyzed and mechanically ventilated. We continuously measured NO production in the brain by NO-selective electrode, cortical electroencephalogram (EEG), regional cerebral blood flow (rCBF) by laser Doppler flowmetry, rectal and cerebral temperatures and mean arterial blood pressure (MABP) during KA (12 mg/kg, i.v.)-induced seizures in the hypothermic group (n = 6; rectal temperature, 33 degrees C), and in the normothermic group (n = 6; rectal temperature, 37 degrees C). The normothermic group showed a gradual increase in NO generation in the brain, which was significantly inhibited in the hypothermic group. There were no significant differences in the increases in rCBF, MABP, arterial blood gases, blood glucose, or EEG abnormalities between the two groups. Neuronal damages in the hippocampus (CA3) were significantly lower in hypothermia than in normothermia. These results suggest that hypothermia attenuates NO production during drug-induced seizures and decreases hippocampal brain lesions in the immature rabbit brain. These results may help to explain the neuroprotective effects of hypothermia.

Characterization of suppression of nitric oxide production by carbon monoxide poisoning in the striatum of free-moving rats, as determined by in vivo brain microdialysis

We examined the effect of carbon monoxide (CO) poisoning on the nitric oxide (NO) system in the striatum of free-moving rats by means of in vivo brain microdialysis. The extracellular levels of the oxidative NO products, nitrite (NO(2)(-)) and nitrate (NO(3)(-)), decreased during exposure to CO at 3000 ppm for 40 min, a condition which causes CO poisoning. The extracellular levels of citrulline (Cit; a by-product of NO production) and arginine (Arg; an NO precursor) also decreased during CO exposure. Following reoxygenation by withdrawal of CO, the NO(2)(-) and NO(3)(-) levels gradually recovered to the control values, though Arg and Cit remained at lower levels, except for a rapid, but transient, recovery shortly before and after reoxygenation, respectively. Simultaneous application of exogenous L-Arg (50 and 100 mM) with CO exposure attenuated the decreases in NO(2)(-) and NO(3)(-) during the CO exposure and accelerated their recovery following reoxygenation. However, D-Arg (100 mM) had no effect on the decrease in NO(2)(-) and NO(3)(-), except for slight and transient attenuation shortly after reoxygenation. Exogenous L-Cit (10 and 100 mM) failed to attenuate the CO-induced decrease in NO(2)(-) and NO(3)(-) levels. The decrease in the NO(2)(-) and NO(3)(-) levels during 8% O(2) exposure for 40 min, which was comparable with that in response to 3000 ppm CO, was resistant to exogenous 100 mM L-Arg, but the recovery of the NO(2)(-) and NO(3)(-) levels following reoxygenation was strongly accelerated. These findings suggest that CO poisoning suppresses NO production in rat striatum in vivo though a mechanism which may not be common with that in hypoxic hypoxia.

Anticonvulsant effects of cyclosporin A on pentylenetetrazole-induced seizure and kindling: modulation by nitricoxidergic system

Cyclosporin A (CsA) is known to decrease nitric oxide (NO) release in the nervous system. The present study was aimed at investigating the effects of acute administration of CsA on pentylenetetrazole (PTZ)-induced seizure threshold and latency and probable modulation of these effects by NO synthesis substrate L-arginine, and NO synthesis inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) or aminoguanidine. Moreover, the effect of CsA per se or concomitant with L-arginine on the development of PTZ-induced kindling was assessed. CsA (0.05, 1, 5, 10 and 20 mg/kg, s.c.) dose-dependently increased PTZ-induced clonic seizure threshold and the latency for onset of myoclonic jerks, clonic seizures and clonic-tonic generalized seizures following PTZ administration. L-NAME (10 and 30 mg/kg, i.p.) but not aminoguanidine (50 and 100 mg/kg, i.p.) potentiated the anticonvulsant effects of CsA (1 and 10 mg/kg). L-arginine (60, 100 and 200 mg/kg, i.p.) inhibited the anticonvulsant effects of CsA (20 mg/kg) in a dose-related manner. The inhibitory effect of L-arginine on CsA-induced alterations of seizure threshold and latency was blocked by L-NAME but not with aminoguanidine. CsA (20 mg/kg) significantly inhibited the development of PTZ kindling and decreased the seizure intensity as tested by a challenge dose of PTZ. Pretreatment with L-arginine (60 mg/kg) reversed the inhibitory effects of CsA on kindling development. It was concluded that CsA exerts some anticonvulsant properties that may be due to its inhibition of nitric oxide synthesis.

Nitric oxide (NO) and convulsions induced by pentylenetetrazol

Data about the role of nitric oxide (NO) in epileptogenesis are contradictory. It is found to exert both proconvulsant and anticonvulsant effects. In an attempt to elucidate the role of NO in seizures, male Wistar rats were treated intraperitoneally by pentylenetetrazol (PTZ) (60, 80, and 100 mg/kg) and by a nitric oxide synthase antagonist, N-omega-nitro-L-arginine-methyl-ester (L-NAME) (10, 40, and 70 mg/kg), applied before PTZ. The time to onset and incidence of forelimb dystonia (FLD), generalized clonic convulsions (GCC), clonic-tonic convulsions (CTC), and mortality were recorded. The most successful convulsive response and mortality prevention were found in PTZ (80 mg/kg)-treated groups, where L-NAME (70 mg/kg) decreased the incidence by 29, 50, 67 (p = 0.052), and 50%, respectively, and significantly prolonged the time to onset, except that for mortality. Unexpectedly, L-NAME (40 mg/kg) increased incidence of GCC and mortality by 16%, similar to L-NAME (10 mg/kg) in PTZ (60 mg/kg)-treated groups, where GCC, CTC, and mortality increased by 14, 14, and 28%, respectively. Convulsive latency was prolonged in some PTZ (100 mg/kg) + L-NAME (40 and 70 mg/kg)-treated groups. In the experimental model and protocol used, it is concluded that (1) the effects of NO are L-NAME- and PTZ-dose dependent; (2) clonic-tonic convulsions are more strongly influenced by NO than limbic, probably because of PTZ limbic structure overstimulation; (3) L-NAME decreases the incidence of CTC and prolongs FLD, GCC, and CTC times to onset, indicating that NO acts as a proconvulsant; and (3) increased GCC, CTC, and mortality that suggests an anticonvulsant effect of NO needs further investigation.

The additive effect of opioids and nitric oxide in increasing pentylenetetrazole-induced seizure threshold in cholestatic mice

BACKGROUND

Accumulation of endogenous opioids and overproduction of nitric oxide has been reported in cholestatic mice. It is well known that endogenous opioids and nitric oxide alter the susceptibility of experimental animals to different models of seizure.

METHODS

The alterations in clonic seizure thresholds, induced by pentylenetetrazole from 1 to 6 days after bile duct ligation, were evaluated in mice. Whether the pretreatment of cholestatic mice with different doses of opioid receptor antagonist naltrexone, or nitric oxide inhibitor N(omega)-nitro-L-arginine methyl ester would have changed the clonic seizure threshold was also examined.

RESULTS

While in sham-operated mice the clonic seizure threshold was similar to that of the thresholds in unoperated controls, a time-dependent increase in the threshold was observed in cholestatic mice, reaching a peak on day 3 after bile duct ligation and declining partially after the 4th day. Chronic pretreatment with naltrexone (2, 5 and 10 mg/kg, i.p.) reversed the increased threshold in cholestatic mice on day 3 after operation in a dose-dependent manner with the highest doses used restoring the threshold to that of the control animals. A similar reversal of the increased threshold was observed after acute (1, 3 and 10 mg/kg, i.p.) or chronic (10 mg/kg, i.p. for 4 days) pretreatment with N(omega)-nitro-L-arginine methyl ester. Moreover, concurrent administration of doses of N(omega)-nitro-L-arginine methyl ester and naltrexone that each separately induced a partial reversal of increased seizure threshold in cholestasis caused a complete restoring of the threshold to the control level.

CONCLUSIONS

Based on these findings, both opioid receptors and nitric oxide may be involved in the dramatic increase in pentylenetetrazole-induced seizure threshold in cholestasis.

Different effects between 7-nitroindazole and L-NAME on cerebral hemodynamics and hippocampal lesions during kainic acid-induced seizures in newborn rabbits

We examined the effects of 7-nitroindazole (7-NI) and N-omega-nitro-L-arginine methyl ester (L-NAME) on the endogenous nitric oxide (NO) production in vivo, cerebral hemodynamics, and hippocampal lesions to investigate the different roles between endothelial NOS (eNOS) and neuronal NOS (nNOS) during kainic acid (KA)-induced seizures in newborn rabbits. After a pre-treatment with 7-NI (50 mg/kg, i.p.), L-NAME (20 mg/kg, i.v.) or saline (1 ml, i.v.), KA (12 mg/kg, i.v.) was administered. NO production in the brain, regional cerebral blood flow (rCBF), cerebral oxygenation (concentrations of oxyhemoglobin (HbO2), deoxyhemoglobin (HbR), and total hemoglobin (tHb) in the brain tissue), and electroencephalography (EEG) were continuously monitored throughout the experiment lasting at least 60 min after the KA administration. There was a significant increase in NO generation in the brain during KA-induced seizures, which was inhibited by a pre-treatment with 7-NI or L-NAME. KA-induced seizures also increased rCBF significantly, which was inhibited not by 7-NI but by L-NAME. L-NAME pre-treatment caused a significant decrease in HbO2 and a significant increase in HbR during KA-induced seizures, compared with 7-NI and saline pre-treatment. EEG abnormalities and Neuronal damages in the hippocampus were significantly lower in 7-NI- and L-NAME-treated animals respectively, than in saline-treated animals. The present data demonstrated that the selective nNOS inhibitor, 7-NI, attenuated neither rCBF nor cerebral oxygenation during the seizures, while the non-selective NOS (nNOS and eNOS) inhibitor, L-NAME, attenuated both. These findings suggest that NO, probably originating from eNOS, may play an important role in the cerebral circulation. Both 7-NI and L-NAME inhibited the NO production and EEG abnormalities during the seizures that led to less damage to the hippocampus.

7-Nitroindazole potentiates the antiseizure activity of some anticonvulsants in DBA/2 mice

7-Nitroindazole, a selective neuronal nitric oxide synthase inhibitor (25-200 mg kg(-1), intraperitoneally (i.p.)) antagonized audiogenic seizures in DBA/2 mice in a dose-dependent manner. We investigated the effects of 7-nitroindazole at a dose of 25 mg kg(-1) i.p., which per se did not show anticonvulsant activity against audiogenic seizures in DBA/2 mice, on the antiseizure activity of some conventional antiepileptic drugs. 7-Nitroindazole sometimes potentiated the anticonvulsant activity of carbamazepine, diazepam, lamotrigine, phenytoin, phenobarbital and valproate against audiogenic seizures in DBA/2 mice. The degree of potentiation by 7-nitroindazole was greatest for phenobarbital and diazepam, less for valproate and least for carbamazepine, lamotrigine and phenytoin. The increase in anticonvulsant activity was associated with a comparable increase in motor impairment. However, the therapeutic index of combined treatment with diazepam+7-nitroindazole, phenobarbital+7-nitroindazole or valproate+7-nitroindazole was more favourable than that of the diazepam+vehicle, phenobarbital+vehicle or valproate+vehicle treatment. The results indicate that 7-nitroindazole is able to increase the protective activity of some conventional antiepileptics and this effect appears not to result only from the impaired synthesis of nitric oxide. In fact, mice receiving 7-nitroindazole (25 mg kg(-1), i.p.) and L-arginine (30 microg/mouse, intracerebroventricularly (i.c.v.) did not show significant changes of ED(50) values in comparison to those of related groups of animals treated with 7-nitroindazole and anticonvulsants. 7-Nitroindazole was able to increase the brain levels of dopamine and noradrenaline and its anticonvulsant effects and changes in catecholamine content were antagonized by pretreatment with alpha-methyl-paratyrosine, an agent inhibiting the synthesis of catecholamines. The fact that alpha-methyl-paratyrosine reverses concomitantly both the increase in brain levels of dopamine and noradrenaline and the anticonvulsant properties of 7-nitroindazole strongly suggests an important role of catecholamines in the antiseizure activity of 7-nitroindazole. Since 7-nitroindazole did not significantly influence the total and free plasma levels of the anticonvulsant drugs studied, we suggest that pharmacokinetic interactions, in terms of total or free plasma levels, are not probable. 7-Nitroindazole did not significantly affect the hypothermic effects of the anticonvulsant compounds studied. 7-Nitroindazole showed an additive effect when administered in combination with some classical anticonvulsants, most notably diazepam, phenobarbital and valproate and its activity could be, in part, due to an increase of monoamine levels.

Role of nitric oxide in pentylenetetrazol-induced seizures: age-dependent effects in the immature rat

PURPOSE

Seizure susceptibility and consequences are highly age dependent. To understand the pathophysiologic mechanisms involved in seizures and their consequences during development, we investigated the role of nitric oxide (NO) in severe pentylenetetrazol (PTZ)-induced seizures in immature rats.

METHODS

Four cortical electrodes were implanted in 10-day-old (P10) and 21-day-old (P21) rats, and seizures were induced on the following day by repetitive injections of subconvulsive doses of PTZ. The effects of NG-nitro-l-arginine methyl ester (l-NAME; 10 mg/kg) and 7-nitroindazole (7NI; 40 mg/kg), two NO synthase (NOS) inhibitors, and l-arginine (l-arg; 300 mg/kg), the NOS substrate, were evaluated regarding the mean PTZ dose, seizure type and duration, and mortality rate.

RESULTS

At P10, the postseizure mortality rate increased from 18-29% for the rats receiving PTZ only to 100% and 89% for the rats receiving l-NAME and 7NI, respectively; whereas l-arg had no effect. Conversely, at P21, NOS inhibitors did not affect the 82-89% mortality rate induced by PTZ alone, whereas l-arg decreased the mortality rate to 29%. In addition, all NO-related drugs increased the duration of ictal activity at P10, whereas at P21, l-arg and l-NAME affected the first seizure type, producing clonic seizures with l-arg and tonic seizures with l-NAME.

CONCLUSIONS

The relative natural protection of very immature rats (P10) against PTZ-induced deaths could be linked to a high availability of l-arg and, hence, endogenous NO. At P21, the modulation of seizure type by NO-related compounds may be related to the maturation of the brain circuitry, in particular the forebrain, which is involved in the expression of clonic seizures.

Effects of nitric oxide synthase inhibition on the cerebral circulation and brain damage during kainic acid-induced seizures in newborn rabbits

The nitric oxide (NO) synthase inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), was used to investigate the effect of endogenous NO on the cerebral circulation and brain damage during kainic acid (KA)-induced seizures in newborn rabbits. The cerebral blood flow (CBF), by laser doppler flowmetry, cerebral oxygenation (concentrations of oxy-(HbO2), deoxy-(HbR) and total hemoglobin (tHb) in brain tissue), by near-infrared spectroscopy (NIRS), mean arterial blood pressure (MABP), electroencephalography (EEG), and hippocampal neuronal damage were evaluated. Pretreatment with L-NAME caused significant decreases in CBF, HbO2, and tHb, and a significant increase in HbR during KA-induced seizures, compared with pretreatment with saline (P < 0.05), without a significant difference in MABP. Our study also demonstrated that pretreatment with L-NAME reduced the seizure activity and neuronal cell death in the hippocampus elicited by the systemic administration of KA in the neonatal brain. These results suggest that NO is of major importance in the neurodestructive process in spite of its roles in maintaining both the CBF and cerebral oxygenation during KA-induced seizures in the neonatal brain.

Further studies on anti- and proconvulsant effects of inhibitors of nitric oxide synthase in rodents

We confirmed that the effects of inhibitors of nitric oxide (NO) synthase, such as Nomega-nitro-L-arginine methyl ester and Nomega-nitro-L-arginine, differ depending on several experimental factors. Both compounds but not their less active enantiomers delayed picrotoxin-induced clonus in mice yet increased the incidence of clonus following low-dose picrotoxin. Nomega-nitro-L-arginine methyl ester significantly reduced the latencies of both myoclonus and clonus in older but not younger Sprague-Dawley rats receiving pentylenetetrazol s.c. By contrast, there was no significant change in the latencies for myoclonus and clonus in Wistar rats (older and younger). However, when pentylenetetrazol was administered i.p. rather than s.c., Nomega-nitro-L-arginine methyl ester dramatically increased latencies of convulsive indicators, including tonus, in both Sprague-Dawley and Wistar rats. Nomega-nitro-L-arginine methyl ester also delayed tonus but not myoclonus or clonus in mice, regardless of the systemic route of administration of pentylenetetrazol. Both Nomega-nitro-L-arginine methyl ester and NG-nitro-L-arginine increased the tonic CD50 of pentylenetetrazol in mice and Nomega-nitro-L-arginine methyl ester delayed 4-aminopyridine-induced tonus. However, Nomega-nitro-L-arginine methyl ester reduced the tonic CD50 of both picrotoxin and 4-aminopyridine in mice and failed to suppress tonus following maximal electroshock. Evidently, inhibitors of NO synthase are not universally effective antitonic drugs.

Anticonvulsant effects of 7-nitroindazole in rodents with reflex epilepsy may result from L-arginine accumulation or a reduction in nitric oxide or L-citrulline formation

1. To investigate the role of nitric oxide in epilepsy we have studied the effects of agents which affect nitric oxide synthesis in sound-induced seizures in DBA/2 mice and in genetically epilepsy-prone (GEP) rats. 2. The neuronal selective nitric oxide synthase inhibitor, 7-nitroindazole (7-NI) is anticonvulsant in these models with ED50 values against clonic seizures in mg kg-1 i.p. (times following injection) of: 74 (+0.25 h), 120 (+1 h) in DAB/2 mice, and 56 (+0.25 h), 42 (+0.5 h), 36 (+1 h), 28 (+2 h), 38 (+4 h), 93 (+8 h) in GEP rats. 3. Therapeutic indices (locomotor deficit ED50/anticonvulsant ED50) for 7-NI are low, ranging from 0.6 to 1.1 at +0.25 h to +1 h after administration in GEP rats, but are more favourable at later times (1.6 at +2 h and 2.9 at +4 h). 4. The substrate for nitric oxide synthase, L-arginine (500-5000 mg kg-1, i.p. or 100-300 micrograms, i.c.v.) but not D-arginine (300 micrograms i.c.v.) is anticonvulsant in DBA/2 mice. L-Arginine (500-5000 mg kg-1, i.p. or 1800-6000 micrograms, i.c.v.) is a more potent anticonvulsant than D-arginine (1500-2500 mg kg-1, i.p. or 6000 micrograms, i.c.v.) in GEP rats. 5. In DBA/2 mice, L-arginine (30 micrograms i.c.v.) reverses the anticonvulsant effect of 7-NI (50 mg kg-1, i.p.). 6. In GEP rats, low dose L-arginine (25-50 mg kg-1, i.p.) but not D-arginine (50 mg kg-1, i.p.) reverses the anticonvulsant effect of low dose 7-NI (25 mg kg-1, i.p.). A higher dose of L-arginine (500 mg kg-1, i.p.) or 7-NI (50 mg kg-1, i.p.) produces summation of anticonvulsant effect. 7. The product for nitric oxide synthase, L-citrulline (250-831 micrograms i.c.v.), is convulsant in DBA/2 mice. 8. The anticonvulsant effect of the neuronal selective nitric oxide synthase inhibitor, 7-nitroindazole, may therefore be mediated by L-arginine accumulation, as well as by a reduction in nitric oxide and L-citrulline formation in rodent models of reflex epilepsy.