Anticonvulsant and proconvulsant roles of nitric oxide in experimental epilepsy models

Loss and sprouting of nitric oxide synthase neurons in the human epileptic hippocampus

PURPOSE

Nitric oxide (NO) has been implicated in a variety of functions, including the control of synaptic plasticity and sensory signaling. Current evidence suggests that this unconventional neurotransmitter mediates N-methyl-d-aspartate (NMDA) receptor-linked excitotoxicity. This study describes the expression of neuronal NO synthase (nNOS) immunoreactivity (IR) in hippocampi from patients with temporal lobe epilepsy (TLE).

METHODS

Hippocampi from patients with clinical symptoms, neuroimaging, and EEG typical of hippocampal sclerosis (HS; n = 22) were compared with those from patients with neocortical temporal lesions (NONHS; n = 4) and autopsy (AUT; n = 18) patients for total cells, and nNOS-IR neuron and puncta densities.

RESULTS

Compared with AUT, HS hippocampi had significantly less nNOS-IR neuron densities in the fascia dentata (FD); hilus, and CA4, CA3, CA2, and CA1 subfields. HS hippocampi had significantly greater nNOS-IR puncta densities in the FD, as compared with AUT and NONHS.

CONCLUSIONS

Our results show that hippocampi from TLE patients exhibit a loss of nNOS-IR neurons and an abnormal FD innervation. The release of NO can influence the dynamics of ionic channels and neurotransmitter release, thus affecting neuronal membrane potential. Because the NOergic transmission does not obey the topographic constraints imposed on conventional transmitters, target cells can be stimulated even in regions with severe deafferentation. The plastic changes described here may contribute to abnormal hippocampal excitability.

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.

Effects of L-arginine on penicillin-induced epileptiform activity in rats

It has been suggested that nitric oxide (NO) is involved in the pathophysiology of epilepsy. Data are, however controversial because it is not clear whether NO has pro- or anticonvulsant effects. The aim of this study was to investigate the effects of NO on penicillin G-induced epileptiform activity. The left cerebral cortex was exposed by craniotomy in urethane-anesthetized Wistar rats. The epileptic activity was produced by intraperitoneal injection of penicillin G (3 million U/kg, i.p.). The ECoG (electrocorticogram) activity was displayed on a four-channel recorder. At 39.7 +/- 5.4 min after penicillin administration, large amplitude sharp waves appeared in the ECoG. Mean spike frequency and mean spike amplitude were calculated as 29.5 +/- 3.2/min and 865 +/- 91 microV, respectively, at the 55th min. 7-Nitroindazole (60 mg/kg, i.p.) injection 30 min before penicillin G administration significantly reduced the latency of epileptiform activity. Intracerebroventricular administration of L-arginine (300 microg/2 microl, i.c.v.) and sodium nitroprusside (100 microg/2 microl, i.c.v.) suppressed epileptiform activity. Saline (2 microl) and D-arginine (300 microg/2 microl, i.c.v.) administration into the cerebral ventricle were completely ineffective on epileptiform activity (P<0.01). These findings suggest that NO may be an endogenous antiepileptic substance.

Anticonvulsant properties of N-salicyloyltryptamine in mice

A new tryptamine analogue, N-salicyloyltryptamine (STP), a potential central nervous system (CNS) depressant, was tested in the pentylenetetrazol (PTZ) and maximal electroshock (MES) models of epilepsy in mice. When administered concurrently, STP (100 mg/kg ip) significantly reduced the number of animals that exhibited PTZ-induced seizures and eliminated the extensor reflex of maximal electric-induced seizures test in 50% of the experimental animals. In addition, it showed protection in the PTZ test by diminishing the death rate.

The role of nitrergic system in lidocaine-induced convulsion in the mouse

The effects of N-nitro-L-arginine-methyl ester (L-NAME) a nitric oxide (NO) synthase inhibitor and L-arginine, a NO precursor, were investigated on lidocaine-induced convulsions. In the first experiment, four groups of mice received physiological saline (0.9%), L-arginine (300 mg/kg, i.p.), L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg), respectively. Thirty minutes after these injections, all mice received lidocaine (50 mg/kg, i.p.). In the second experiment, four groups of mice received similar treatment in the first experiment, and 30 min after these injections, all mice received a higher dose of lidocaine (80 mg/kg). L-NAME (100 mg/kg, i.p.) and diazepam (2 mg/kg) significantly decreased the incidence of lidocaine (50 mg/kg)-induced convulsions. In contrast, the L-arginine treatment increased the incidence of lidocaine (80 mg/kg, i.p.)-induced convulsions significantly. These results may suggest that NO is a proconvulsant mediator in lidocaine-induced convulsions.

Nitric oxide synthase expression in the cerebral cortex of patients with epilepsy

PURPOSE

Nitric oxide (NO), a short-lived radical synthesized from L-arginine by activation of the enzyme nitric oxide synthase (NOS), has been implicated in the pathophysiology of epilepsy by some investigators. However, the current data about NO and NOS in epilepsy are controversial and are derived only from animal models of epilepsy. In this study we investigated possible changes in NOS expression in the cerebral cortex of patients with epilepsy.

METHODS

Qualitative and quantitative parameters of the immunolabeling pattern of the neuronal, endothelial, and inducible isoforms of NOS were analyzed in biopsy material obtained from patients with short and long seizure history and from patients without epilepsy.

RESULTS

The comparative study showed that in the cerebral cortex of patients with epilepsy, particularly in those with a long seizure history, the number and labeling intensity of NOS-positive neurons increased, and that a subpopulation of nonpyramidal GABAergic neurons (type II NOS neurons) was responsible for this phenomenon.

CONCLUSIONS

The fact that NOS upregulation is more evident in patients with a long seizure history suggests that this is a consequence of seizures, acting probably as an adaptative response to the sustained release of excitatory amino acids.

Inhibitory effect of carbamazepine on inflammatory mediators produced by stimulated glial cells

Exposure of rat glial cells to lipopolysaccharide (LPS) induces the production of nitric oxide (NO) and prostaglandin E(2) (PGE(2)), the inflammatory mediators implicated in the pathogenesis of seizures and epilepsy. To determine the effect of the anticonvulsant drug carbamazepine (CBZ) on the inflammatory process, LPS-stimulated rat primary glial cultures were exposed to this agent. Dose-dependent inhibition of NO and PGE(2) production was observed of up to 77 and 88%, respectively. Furthermore, a prominent (94%) decline in the secretory isoform of phospholipase A(2) (sPLA(2)) activity was found in response to CBZ and could contribute to the inhibition of PGE(2) production. Cumulatively, our findings point to the anti-inflammatory effect of CBZ on non-neuronal cells, which might, in part, contribute to its anticonvulsive effect.

Behavioral effects of intra-nigral microinjections of manganese chloride: interaction with nitric oxide

1. Microinjection of manganese chloride (MnCl2) into the rat substantia nigra pars compacta (SNc) induces a neurodegenerative process manifested by apomorphine-induced rotational behavior. Manganese intoxication produces a parkinsonism-like phenotype in humans. 2. In addition to motor control the substantia nigra has also been proposed to be related to epilepsy and emotional behavior. 3. Although nitric oxide (NO) participation in neurodegenerative processes is still questioned, neurons stained for NAPDH-diaphorase, a marker of NO-producing cells, are spared in several experimental neuronal lesions. Additionally, NO has also been suggested to participate in motor control. 4. The objective of this study was to analyze the effects of MnCl2-induced nigral degeneration in audiogenic seizure susceptibility, anxiety and motor activity. We also analyzed if NO synthesis inhibition (N(G)-nitro-L-arginine 25 mg/Kg twice a day for 4 days) modifies MnCl2-induced neurodegenerative process. 5. MnCl2 (50 microg) microinjection into the SNc caused a statistical significant higher number of apomorphine (0.75 mg/kg s.c.)-induced rotations. No sensitization to audiogenic seizure was found but the lesion induced an increase of open arm exploration in the elevated plus maze, suggesting an anxiolytic effect. 6. The MnCl2-nigral lesion was accompanied by an increased number of NADPH-d positive neurons in the ipsilateral SNc and striatum (both sides). NO synthesis inhibition potentiated the MnCl2-nigral lesion and reversed the NADPH-d cell number increase. 7. The present results show that MnCl2-nigral lesion can influence emotional behavior and suggest that NO may modify the progression of manganese-induced degenerative process.