Spontaneous and evoked NMDA-receptor mediated potentials in the entorhinal cortex of the neonate rat in vitro

Altered excitability and distribution of NMDA receptor subunit proteins in cortical layers of rat pups following multiple perinatal seizures

During a critical period of postnatal development the epileptogenic focus is thought to be of cortical origin. We used immunohistochemistry and Western blotting to elucidate potential mechanisms underlying an increased state of susceptibility to seizures in immature animals. Distribution patterns of N-methyl-D-aspartic acid (NMDA) (NR1 and NR2A/B) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (GluR1 and GluR2) subunits were analyzed in retrosplenial, parietal and temporal cortices during the first two postnatal weeks following three episodes of status-epilepticus. Rat pups were injected three times with kainic acid (3x KA) on P6, P9, and P13 and subsequently sacrificed 48 h after the third seizure. Cortical electroencephalography (EEG) showed increased number of spikes and bursts of longer duration after 3x KA. Immunodensity measurements after 3x KA revealed a robust increase in NR2A/B labeling specific to cortical layer V throughout the retrosplenial, parietal, and temporal cortices, with no changes noted in piriform cortex. NR1 layer V immunoreactivity was also simultaneously increased in serial sections but to a lesser degree; heightened immunodensities were specific to retrosplenial and temporal cortices. The NR1:NR2 ratio was decreased in cortical layer V of the temporal and retrosplenial cortices but not in parietal cortex despite elevated immunoreactivity. Steady levels of GluR1 and GluR2 subunits were noted in all cortical areas studied in the same animals. Thus, recurrent perinatal seizures led to selective and layer-specific increases in NMDA receptor proteins. These changes may be responsible for lowering the seizure threshold in deeper cortical areas and eventually contribute to the cortical epileptogenic focus.

Stereoselective effects of the novel anticonvulsant lacosamide against 4-AP induced epileptiform activity in rat visual cortex in vitro

We examined effects of the novel anticonvulsant lacosamide and its inactive isomer (SPM 6953) in an in vitro model of epileptiform activity. Focal field potential recordings (34+/-0.2 degrees C) were obtained from 17 to 22 day old rat brain slices. Physiological synaptic transmission (fEPSP amplitude and duration) in CA1 of rat hippocampus was not significantly altered (P > 0.05, n = 4) by lacosamide (1 microM-1 mM). Recording from visual cortex during application of 4-aminopyridine (4-AP; 100 microM) revealed both spontaneous and evoked 'ictal like' discharges. Spontaneous ictal like discharges in the visual cortex were blocked by 100 microM carbamazepine (CBZ), 100 microM pentobarbital and 200 microM phenobarbital (PHB) but were insensitive to the anti-absence drug ethosuximide (750 microM; n = 4, P > 0.05). Lacosamide reduced tonic duration and maximal firing frequency with EC(50)s of 41 and 71 microM, respectively. In contrast, the S stereoisomer (100-320 microM) produced no significant effect on spontaneous ictal activity (n = 3-4, P > 0.05). Seizures induced by high frequency (100 Hz, 1s) stimulation were selectively reduced in amplitude by PHB (200 microM) and frequency by CBZ (100 microM; n = 6) and lacosamide (100 microM; n = 4). GABAergic negative going potentials were attenuated by CBZ (irreversible with washing) and lacosamide (reversible) but not by PHB. We conclude that lacosamide blocks 4-AP induced epileptiform activity in the visual cortex. This novel anticonvulsant drug appears to inhibit epileptogenesis (seizure spread) by interacting with a stereoselective, but as yet unidentified, target site in rodent neocortex in the mid-micromolar range.

Brain-derived neurotrophic factor transgenic mice exhibit passive avoidance deficits, increased seizure severity and in vitro hyperexcitability in the hippocampus and entorhinal cortex

Transgenic mice overexpressing brain-derived neurotrophic factor from the beta-actin promoter were tested for behavioral, gross anatomical and physiological abnormalities. Brain-derived neurotrophic factor messenger RNA overexpression was widespread throughout brain. Overexpression declined with age, such that levels of overexpression decreased sharply by nine months. Brain-derived neurotrophic factor transgenic mice had no gross deformities or behavioral abnormalities. However, they showed a significant passive avoidance deficit. This deficit was dependent on continued overexpression, and resolved with age as brain-derived neurotrophic factor transcripts decreased. In addition, the brain-derived neurotrophic factor transgenic mice showed increased seizure severity in response to kainic acid. Hippocampal slices from brain-derived neurotrophic factor transgenic mice showed hyperexcitability in area CA3 and entorhinal cortex, but not in dentate gyrus. Finally, area CA1 long-term potentiation was disrupted, indicating abnormal plasticity. Our data suggest that overexpression of brain-derived neurotrophic factor in the brain can interfere with normal brain function by causing learning impairments and increased excitability. The results also support the hypothesis that excess brain-derived neurotrophic factor could be pro-convulsant in the limbic system.