Endogenous Nitric Oxide Is a Key Promoting Factor for Initiation of Seizure-Like Events in Hippocampal and Entorhinal Cortex Slices

Abstract

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal-entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg 2ϩ concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2Ј,7Ј-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal-entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-L-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS Ϫ/Ϫ mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

Ischemic-LTP in Striatal Spiny Neurons of both Direct and Indirect Pathway Requires the Activation of D1-Like Receptors and NO/Soluble Guanylate Cyclase/cGMP Transmission

“…In line with this observation, it has been reported that in the hippocampus and entorhinal cortex NO is involved in the induction, but not the maintenance of seizure-like activity. 45 NO is a diffusible neuronal messenger that activates sGC and increases intracellular cGMP levels during an ischemic episode in the brain. 46 Accordingly, we found that inhibition of sGC by ODQ, and of PKG by Rp-8Br-PET-cGMP prevented i-LTP induction.…”

Guanosine prevents nitroxidative stress and recovers mitochondrial membrane potential disruption in hippocampal slices subjected to oxygen/glucose deprivation

“…Here, we used NOS activity inhibitors to understand the involvement of the NOS isoforms in the mechanism of OGD-induced cell damage and in the neuroprotective effect of GUO. The non-selective NOS inhibitor L-NAME (1 mM), the selective iNOS inhibitor 1400 W (50 μM), and the selective nNOS inhibitor 7-NI (100 μM) were used [10,35]. GUO prevented cell viability decrease in hippocampal slices subjected to OGD, similarly to L-NAME (Fig.…”

“…However, the hyperactivation of NMDA receptors in postsynaptic neurons produces excessive nNOS activity and subsequently inflammatory reactions inducing iNOS expression at glial cells, allowing massive and uncontrolled NO release that may be damaging for the neighboring neurons [8]. Furthermore, the negative effects of excessive NO generation are caused by the endogenous formation of the peroxynitrite anion (ONOO − ) rather than NO itself [10]. ONOO − is formed by the spontaneous reaction of NO with superoxide anion (O 2 − ) [9].…”

Is Encephalopathy a Mechanism to Renew Sulfate in Autism?

“…In vitro experiments have demonstrated that nitric oxide produced by nNOS initiates seizure-like events in the hippocampus and entorhinal cortex [128]. Furthermore, an increase in superoxide synthesis follows immediately in response to the seizure.…”