The effects of seizures on the hippocampus of the immature brain

Long-Term Effects of Early Life Seizures on Endogenous Local Network Activity of the Mouse Neocortex

Understanding the long term impact of early life seizures (ELS) is of vital importance both for researchers and clinicians. Most experimental studies of how seizures affect the developing brain have drawn their conclusions based on changes detected at the cellular or behavioral level, rather than on intermediate levels of analysis, such as the physiology of neuronal networks. Neurons work as part of networks and network dynamics integrate the function of molecules, cells and synapses in the emergent properties of brain circuits that reflect the balance of excitation and inhibition in the brain. Therefore, studying network dynamics could help bridge the cell-to-behavior gap in our understanding of the neurobiological effects of seizures. To this end we investigated the long-term effects of ELS on local network dynamics in mouse neocortex. By using the pentylenetetrazole (PTZ)-induced animal model of generalized seizures, single or multiple seizures were induced at two different developmental stages (P9-15 or P19-23) in order to examine how seizure severity and brain maturational status interact to affect the brain's vulnerability to ELS. Cortical physiology was assessed by comparing spontaneous network activity (in the form of recurring Up states) in brain slices of adult (>5 mo) mice. In these experiments we examined two distinct cortical regions, the primary motor (M1) and somatosensory (S1) cortex in order to investigate regional differences in vulnerability to ELS. We find that the effects of ELSs vary depending on (i) the severity of the seizures (e.g., single intermittent ELS at P19-23 had no effect on Up state activity, but multiple seizures induced during the same period caused a significant change in the spectral content of spontaneous Up states), (ii) the cortical area examined, and (iii) the developmental stage at which the seizures are administered. These results reveal that even moderate experiences of ELS can have long lasting age- and region-specific effects in local cortical network dynamics.

Pediatric posttraumatic seizures: epidemiology, putative mechanisms of epileptogenesis and promising investigational progress

Posttraumatic seizures and epilepsy are common in children experiencing traumatic brain injury and portend worse functional outcome. Unfortunately, the pathogenesis of pediatric posttraumatic seizures and epilepsy remains poorly understood, and no efficacious preventive therapy for post-traumatic epilepsy has been identified. This article reviews the epidemiology of pediatric posttraumatic seizures, discusses prominent putative mechanisms of posttraumatic epileptogenesis and highlights recent promising progress in experimental investigations of posttraumatic seizures and epilepsy.

Disruption of cortical development as a consequence of repetitive pilocarpine-induced status epilepticus in rats

PURPOSE

The aim of the present study was to observe possible cortical abnormalities after repetitive pilocarpine-induced status epilepticus (SE) in rats during development.

METHODS

Wistar rats received intraperitoneal injection of pilocarpine hydrochloride 2% (380 mg/kg) at P7, P8, and P9. All experimental rats displayed SE after pilocarpine injections. Rats were killed at P10 and P35, and immunocytochemistry procedures were performed on 50-microm vibratome sections, by using antibodies against nonphosphorylated neurofilament (SMI-311), parvalbumin (PV), calbindin (CB), calretinin (CR), and glutamate decarboxylase (GAD-65). Selected sections were used for the TUNEL method and double-labeling experiments, with different mixtures of the same markers.

RESULTS

The major findings of the present work were (a) altered intracortical circuitry development; (b) anticipation of PV immunoreactivity in neocortical interneurons; (c) increased GAD-65 immunoreactivity; and (d) reduced neocortical apoptotic process.

CONCLUSIONS

From these results, we suggest that previously healthy brain, without genetic abnormalities, might develop an "acquired" disruption of cortical development whose evolution reproduces some characteristics of the childhood epilepsies associated with cognitive impairment.

Neocortical and hippocampal changes after multiple pilocarpine-induced status epilepticus in rats

PURPOSE

Multiple episodes of pilocarpine-induced status epilepticus (SE) in developing rats (P7-P9) lead to progressive epileptiform activity and severe cognitive impairment in adulthood. The present work studied possible underlying abnormalities in the neocortex and hippocampus of pilocarpine-treated animals.

METHODS

Wistar rats were submitted to pilocarpine-induced SE at P7, P8, and P9, and were killed at P35. Immunocytochemistry was performed on 50-microm vibratome sections, by using antibodies against nonphosphorylated neurofilament (SMI-311), parvalbumin (PV), calbindin (CB), calretinin (CR), and glutamate decarboxylase (GAD-65). Ten-micron cryostat sections were processed for immunohistoblot by using antibodies against GluR1, GluR2/3, and GluR4 alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits and NR2ab N-methyl-D-aspartate (NMDA) receptor subunit.

RESULTS

Adult rats submitted to SE at P7-9 showed: (a) altered distribution of neocortical interneurons; (b) increased cortical and reduced hippocampal GAD-65 expression; and (c) altered expression of hippocampal AMPA and NMDA receptors.

CONCLUSIONS

We conclude that multiple SE episodes during P7-9 generate long-lasting disturbances that underlie behavioral and electrographic abnormalities later in life.

Seizures decrease postnatal neurogenesis and granule cell development in the human fascia dentata

PURPOSE

There is considerable controversy whether childhood seizures damage existing neurons and/or adversely affect neurogenesis and synaptogenesis. This study addressed this question by examining fascia dentata neurogenesis, cell death, and aberrant axon connections in hippocampi from children with extratemporal seizure foci.

METHODS

Surgically resected (n = 53) and age-comparable autopsy (n = 22) hippocampi were studied for neuronal densities, polysialic acid (PSA) neural cell adhesion molecule (NCAM) immunoreactivity (IR), TUNEL, and neo-Timm's histochemistry.

RESULTS

Compared with autopsy cases, hippocampi from children with frequent seizures showed (a) decreased fascia dentata granule cell densities; (b) decreased PSA NCAM IR cell densities in the stratum granulosum, infragranular, and hilar regions; (c) no positive TUNEL-stained cells; and (d) aberrant supragranular mossy fiber axon connections.

CONCLUSIONS

These results indicate that severe seizures during early childhood are associated with anatomic signs of decreased postnatal granule cell neurogenesis (PSA NCAM IR) and aberrant mossy fiber axon connections (neo-Timm's) without evidence of seizure-induced cell death (TUNEL). In humans, these results support the concept that seizures do not damage existing neurons, but adversely affect processes involved with normal postnatal neuronal development such as neurogenesis and axon formation. Such alterations probably negatively affect normal brain development, and/or promote epileptogenesis.

Local cerebral blood flow during lithium-pilocarpine seizures in the developing and adult rat: role of coupling between blood flow and metabolism in the genesis of neuronal damage

Coupling between local cerebral blood flow and local cerebral metabolic rate for glucose is involved in the pathogenesis of epilepsy-related neuronal damage in the adult brain; however, its role in the immature brain is unknown. Lithium-pilocarpine-induced status epilepticus is associated with extended damage in adult rats, mostly in the forebrain limbic areas and thalamus, whereas damage was moderate in 21-day-old rats (P21) or absent in P10 rats. The quantitative autoradiographic [14C]iodoantipyrine technique was applied to measure the consequences of lithium-pilocarpine status epilepticus on local cerebral blood flow. In adult and P21 rats, local cerebral blood flow rates increased by 50% to 400%; the highest increases were recorded in regions showing damage in adults. At P10, local cerebral blood flow rates decreased by 40% to 60% in most areas, except in some forebrain regions showing no change during status epilepticus. In areas injured when status epilepticus was induced in adults, a strong hypermetabolism (Fernandes et al., 1999) not matched by comparable local cerebral blood flow increases was present in rats of all ages, whereas in damage-resistant areas, local cerebral metabolic rate for glucose and local cerebral blood flow remained coupled in the three age groups. Thus, the level of coupling between blood flow supply and metabolism is not involved in seizure-related brain damage in the developing brain, which appears to be resistant to the consequences of such a mismatch.