During status epilepticus (SE), GABAergic mechanisms fail and seizures become self-sustaining and pharmacoresistant. During lithiumpilocarpine-induced SE, our studies of postsynaptic GABAAreceptors in dentate gyrus granule cells show a reduction in the amplitude of miniature IPSCs (mIPSCs). Anatomical studies show a reduction in the colocalization of the β2/β3 and γ2 subunits of GABAAreceptors with the presynaptic marker synaptophysin and an increase in the proportion of those subunits in the interior of dentate granule cells and other hippocampal neurons with SE. Unlike synaptic mIPSCs, the amplitude of extrasynaptic GABAAtonic currents is augmented during SE. Mathematical modeling suggests that the change of mIPSCs with SE reflects a decrease in the number of functional postsynaptic GABAAreceptors. It also suggests that increases in extracellular [GABA] during SE can account for the tonic current changes and can affect postsynaptic receptor kinetics with a loss of paired-pulse inhibition. GABA exposure mimics the effects of SE on mIPSC and tonic GABAAcurrent amplitudes in granule cells, consistent with the model predictions. These results provide a potential mechanism for the inhibitory loss that characterizes initiation of SE and for the pharmacoresistance to benzodiazepines, as a reduction of available functional GABAApostsynaptic receptors. Novel therapies for SE might be directed toward prevention or reversal of these losses.

The lithium-pilocarpine model of status epilepticus (SE) was used to study the type and distribution of seizure-induced neuronal injury in the rat and its consequences during development. Cell death was evaluated in hematoxylin- and eosin-stained sections and by electron microscopy. Damage to the CA1 neurons was maximal in the 2- and 3-week-old pups and decreased as a function of age. On the other hand, damage to the hilar and CA3 neurons was minimal in the 2-week-old rat pups but reached an adult-like pattern in the 3-week-old animals, and damage to amygdalar neurons increased progressively with age. The 3-week-old animals also demonstrated vulnerability of the dentate granule cells. To evaluate neuronal apoptosis, we used terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) stain, confocal fluorescence microscopy of ethidium bromide-stained sections, electron microscopy, and DNA electrophoresis. Neurons displaying all of those features of apoptotic death in response to SE were seen in the CA1 region of the 2-week-old pups and in the hilar border of the dentate granule cells of the 3-week-old animals. Some (3/11) of the animals that underwent SE at 2 weeks of age and most of the animals that underwent SE at 3 or 4 weeks of age (8/11 and 6/8, respectively) developed spontaneous seizures later in life; the latter showed SE-induced synaptic reorganization as demonstrated by Timm methodology. These results provide strong evidence for the vulnerability of the immature brain to seizure-induced damage, which bears features of both necrotic and apoptotic death and contributes to synaptic reorganization and the development of chronic epilepsy.

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Human status epilepticus (SE) is consistently associated with cognitive problems, and with widespread neuronal necrosis in hippocampus and other brain regions. In animal models, convulsive SE causes extensive neuronal necrosis. Nonconvulsive SE in adult animals also leads to widespread neuronal necrosis in vulnerable regions, although lesions develop more slowly than they would in the presence of convulsions or anoxia. In very young rats, nonconvulsive normoxic SE spares hippocampal pyramidal cells, but other types of neurons may not show the same resistance, and inhibition of brain growth, DNA and protein synthesis, and of myelin formation and of synaptogenesis may lead to altered brain development. Lesions induced by SE may be epileptogenic by leading to misdirected regeneration. In SE, glutamate, aspartate, and acetylcholine play major roles as excitatory neurotransmitters, and GABA is the dominant inhibitory neurotransmitter. GABA metabolism in substantia nigra (SN) plays a key role in seizure arrest. When seizures stop, a major increase in GABA synthesis is seen in SN postictally. GABA synthesis in SN may fail in SE. Extrasynaptic factors may also play an important role in seizure spread and in maintaining SE. Glial immaturity, increased electronic coupling, and SN immaturity facilitate SE development in the immature brain. Major increases in cerebral blood flow (CBF) protect the brain in early SE, but CBF falls in late SE as blood pressure falters. At the same time, large increases in cerebral metabolic rate for glucose and oxygen continue throughout SE. Adenosine triphosphate (ATP) depletion and lactate accumulation are associated with hypermetabolic neuronal necrosis. Excitotoxic mechanisms mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors open ionic channels permeable to calcium and play a major role in neuronal injury from SE. Hypoxia, systemic lactic acidosis, CO2 narcosis, hyperkalemia, hypoglycemia, shock, cardiac arrhythmias, pulmonary edema, acute renal tubular necrosis, high output failure, aspiration pneumonia, hyperpyrexia, blood leukocytosis and CSF pleocytosis are common and potentially serious complications of SE. Our improved understanding of the pathophysiology of brain damage in SE should lead to further improvement in treatment and outcome.

Previous studies have shown that the expression of the neuropeptide galanin in the hippocampus is altered by seizures and that exogenous administration of galanin into the hippocampus attenuates seizure severity. To address the role of endogenous galanin in modulation of hippocampal excitability and its possible role in seizure mechanisms, we studied two types of transgenic mice: mice with a targeted disruption of the galanin gene (GalKO) and mice that overexpress the galanin gene under a dopamine-beta-hydroxylase promoter (GalOE). GalKO mice showed increased propensity to develop status epilepticus after perforant path stimulation or systemic kainic acid, as well as greater severity of pentylenetetrazol-induced convulsions. By contrast, GalOE mice had increased resistance to seizure induction in all three models. Physiological tests of hippocampal excitability revealed enhanced perforant path-dentate gyrus long-term potentiation (LTP) in GalKO and reduced LTP in GalOE. GalKO showed increased duration of afterdischarge (AD) evoked from the dentate gyrus by perforant path simulation, whereas GalOE had increased threshold for AD induction. Depolarization-induced glutamate release from hippocampal slices was greater in GalKO and lower in GalOE, suggesting that alterations of physiological and seizure responses in galanin transgenic animals may be mediated through modulation of glutamate release. Our data provide further evidence that hippocampal galanin acts as an endogenous anticonvulsant and suggest that genetically induced changes in galanin expression modulate both hippocampal excitability and predisposition to epileptic seizures.