Failure of kindling to alter susceptibility to kainic acid

D-Serine differently modulates NMDA receptor function in rat CA1 hippocampal pyramidal cells and interneurons

The organization of the neuronal hippocampal network depends on the tightly regulated interaction between pyramidal cells (PCs) and interneurons (Ints). NMDA receptor (NMDAR) activation requires the binding of glutamate and co-activation of the 'glycine site'. It has been reported that D-serine is a more potent endogenous agonist than glycine for that site. While many studies have focused on NMDAR function in PCs, little is known regarding the modulation of NMDARs in Ints. We studied the modulatory effect of D-serine on NMDAR EPSCs in PCs and in stratum radiatum Ints using whole-cell patch-clamp recording in rat acute hippocampal slices. We found that D-serine enhances NMDAR function and differently modulates NMDAR currents in both cell types. The augmentation of NMDAR currents by D-serine was significantly larger in PCs compared with Ints. Moreover, we found differences in the kinetics of NMDAR currents in PCs and Ints. Our findings indicate that regulation of NMDAR through the 'glycine site' depends on the cell types. We speculate that the observed differences arise from assemblies of diverse NMDAR subunits. Overall, our data suggest that D-serine may be involved in regulation of the excitation-inhibition balance in the CA1 hippocampal region.

Effects of kindling on subsequent learning, memory, behavior, and seizure susceptibility

To determine the long-term effects of seizures on the developing brain we kindled 20-, 40-, and 60-day-old rats to stage 5 seizures and then elicited an additional 15 seizures using the same kindling stimulation. At age 80 days, all animals that reached stage 5 kindling, and their respective age-matched controls, underwent behavioral testing using the Morris water maze, open field test, and handling test. Prior to euthanasia the animals had seizure threshold tested using flurothyl inhalation. No differences were noted in time to platform in the water maze or activity level in the open field test between the kindled rats and controls in any of the three age groups. Rats kindled at age 20 and 40 were more emotional than the controls in the handling test. In the flurothyl inhalation test, rats kindled at 40 and 60 days of age had a shorter latency to all seizures stages than the controls. These results demonstrate that while kindling results in no alteration of learning, memory, or activity level, it does result in altered emotionality and activity level in immature animals, as well as reduced seizure threshold in pubescent and mature rats. The animal model used appears to be an important variable in determining the long-term effects of seizures.

The long-term effects of seizures on the developing brain: clinical and laboratory issues

The long-term effects of seizures on the developing brain is a difficult clinical problem to study since cognitive impairment and behavioral abnormalities may be related to the etiological agent responsible for the seizures, age at time of onset of seizures, the type, frequency, or duration of the seizures, or the antiepileptic drugs used to treat the seizures. Many of these variables can be eliminated by using animal models of epilepsy. Work in our laboratory using the kainic acid (KA) model has demonstrated that status epilepticus in prepubescent and mature rats leads to significant deficits in memory, learning and behavior as adults when compared to control littermates without seizures. These rats also had a high incidence of spontaneous recurrent seizures (SRS) and an increased susceptibility to seizures using kindling and flurothyl. However, younger animals (less than or equal to 20 day old) with KA-induced seizures of similar severity were not associated with later neurological deficits. The immature animals also had a low rate of SRS and did not differ from controls in susceptibility to kindling or flurothyl. Studies using the continuous hippocampal stimulation model of epilepsy have also demonstrated that prolonged seizures in the developing brain are less severe than those in the mature animal. The pathophysiological mechanisms that "protect" the young brain from long-term detrimental effects of prolonged seizures are unknown.

Kindling susceptibility and genetic seizure predisposition in inbred mice

Olfactory bulb kindling rates were studied in two inbred strains of genetically seizure-prone mice (DBA/2 and El) and in three non-epileptic inbred strains [C57BL/6 (B6), ddY, and C3H/He (C3H)]. None of the DBA/2 mice had been stimulated to seizure before or during the kindling and all mice were studied at 4-6 weeks of age, before development of spontaneous or movement-induced seizure activity in the El strain. The audiogenically seizure-susceptible DBA/2 mice required the fewest number of stimulations to reach stage 5 seizure (mean +/- SE = 4.0 +/- 0.6). The nonepileptic C3H mice required the most stimulations to reach stage 5 seizures (22.6 +/- 1.4). Kindling rates for B6 (9.6 +/- 0.6), El (14.8 +/- 1.1), and ddY (18.5 +/- 1.0) strains were intermediate, and the kindling rate for each strain was significantly different from that of the other strains. These findings show that the seizure-susceptible El mouse kindles more rapidly than the genetically similar but nonepileptic ddY control and suggest that an inherited seizure susceptibility accelerates the kindling rate. Nonepileptic B6 mice kindled more rapidly than El mice, however, suggesting that genetic factors other than those that influence seizure susceptibility are of primary importance in the determination of the kindling rate.