Endogenous nitric oxide is a key promoting factor for initiation of seizure-like events in hippocampal and entorhinal cortex slices.
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(2+) concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2',7'-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(-/-) 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.
Kovács
Richard
R
Institute for Neurophysiology, Charité-Universitätsmedizin Berlin, 10117 Berlin, Germany.
Rabanus
Alexander
A
Otáhal
Jakub
J
Patzak
Andreas
A
Kardos
Julianna
J
Albus
Klaus
K
Heinemann
Uwe
U
Kann
Oliver
O
D000704:Analysis of Variance; D000818:Animals; D000831:Animals, Newborn; D000880:Anthraquinones / Q000378:metabolism; D004558:Electric Stimulation / Q000379:methods; D018728:Entorhinal Cortex / Q000503:physiopathology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D006624:Hippocampus / Q000503:physiopathology*; D008274:Magnesium / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D009569:Nitric Oxide / Q000378:metabolism*; D052248:Nitric Oxide Synthase Type I / Q000172:deficiency / Q000378:metabolism; D009924:Organ Culture Techniques; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D012640:Seizures / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D015398:Signal Transduction / Q000187:drug effects; D055366:Synaptic Potentials / Q000187:drug effects / Q000235:genetics
D000880:Anthraquinones; D004791:Enzyme Inhibitors; D009569:Nitric Oxide; D052248:Nitric Oxide Synthase Type I; C496321:Nos1 protein, mouse; C118201:1,2-diaminoanthraquinone; D008274:Magnesium; D011188:Potassium