Phosphate energy metabolism during domoic acid-induced seizures

Effects of creatine and cyclocreatine supplementation on kainate induced injury in pre-pubescent rats

PURPOSE

To investigate if energy precursor supplementation is neuroprotective in two neuroexcitotoxicity models; the kainate and the kainate followed by chronic phenobarbital models.

METHODS

Rats in experiment 1 received 1% creatine or cyclocreatine chow from age (P) 21-65 days, underwent kainate induced status epilepticus on P35 and were compared, as adults, to kainate alone rats and to normal controls. Rats in experiment 2 received 1% creatine chow (P21-P85), underwent kainate status epilepticus on P35, received daily phenobarbital (or saline) injections (P36-P85) and were compared, as adults, to kainate, kainate-phenobarbital and to normal control rats that received regular chow.

RESULTS

In experiment 1, the cyclocreatine-kainate group had increased emotionality and visuospatial learning deficits on the handling and watermaze tests as compared to all other groups. Creatine supplementation did not have any effects. In experiment 2, creatine supplementation did not prevent spontaneous recurrent seizures, aggressivity on the handling test or hippocampal histologic injury.

CONCLUSION

Energy precursor supplementation in the doses used did not have neuroprotective effects in the kainate or kainate-phenobarbital models in pre-pubescent rats.

Electroencephalographic, behavioral, and c-fos responses to acute domoic acid exposure

Domoic acid, a potent excitotoxic analogue of glutamate and kainate, may cause seizures, amnesia, and sometimes death in humans consuming contaminated shellfish. Continuous behavioral observations and recordings of the electrocorticogram (ECoG, via bipolar, epidural electrodes) were obtained from nonanesthetized rats for 2 h after intraperitoneal injection with either saline, 2.2, or 4.4 mg/kg of domoic acid. Rats were then sacrificed for c-fos immunohistochemistry. Fast Fourier transformation (FFT) of the ECoG data to obtain the voltage as a function of frequency indicated that the lower frequency bands (theta, 4.75-6.75 Hz and delta, 1.25-4.50 Hz) were the first to respond, with a significant elevation by 30 min after the high dose of domoic acid. The lower dose of domoic acid also caused a significant elevation of ECoG voltage, but not until later in the session. Sixty minutes after dosing, the behavioral biomarkers of "ear scratching" and "rearing, praying" (RP) seizures became significantly elevated in the high-dose rats. The low-dose rats showed no significant alterations in behavior at any time during the session. In postmortem brains obtained immediately after the sessions, c-fos was activated in the anterior olfactory nucleus by both the low and high doses of domoic acid. However, only the high dose increased c-fos immunoreactivity in the hippocampus, affecting both the granule and pyramidal neurons. These data indicate that electroencephalographic and c-fos responses can be obtained at a dose of domoic acid that fails to activate the behavioral response most commonly used as a bioassay for this marine toxin: ear scratching with the ipsilateral foot.

Domoic acid-induced hippocampal CA1 hyperexcitability independent of region CA3 activity

Domoic acid (DOM) is a potent agonist of AMPA and kainic acid (KA) receptors in the CNS and is known to produce seizures acutely, and lasting excitotoxic damage in several brain regions. While the excitotoxic effects of DOM are well documented, its seizurogenic properties are less clear. In this study, we assessed the acute effects of DOM and KA in region CA1 of intact rat hippocampal slices (CA3-on) and in slices lacking region CA3 (CA3-off). Orthodromic Schaffer collateral-evoked CA1 field potentials (population spikes and somal EPSP's) were monitored during DOM and KA (10-500 nM) administration. In CA3-off slices both KA and DOM produced immediate increases in CA1 population spike amplitude. With prolonged exposure, lasting dose-dependent reductions in spike amplitude and EPSP slope were observed, possibly due to depolarising conduction block following excessive AMPA/KA receptor activation; DOM was several-fold more potent than KA in this regard. Population spike threshold did not vary with DOM, but in CA3-on slices a dose-dependent steepening of the I/O curve and increase in maximum spike amplitude was seen. CA1 hyperexcitability, as evidenced by the appearance of prominent second and third population spikes, was equivalently increased across a range of DOM concentrations in both CA3-on and CA3-off slices and, in general, DOM-induced CA1 hyperexcitability was not enhanced by the presence of CA3 for any of the other variables assessed in this study. These findings show that DOM directly promotes neuronal hyperactivity in region CA1, presumably due to tonic AMPA and/or KA-receptor mediated depolarization, and further suggests that DOM-induced hyperactivity in the recurrently networked, AMPA/KA-receptor rich region CA3 does not contribute to the onset and spread of limbic seizures during relatively mild DOM intoxication.

Altered residual ATP content in rat brain cortex subcellular fractions following status epilepticus induced by lithium and pilocarpine

Changes in residual ATP concentrations were investigated following subcellular fractionation of rat brain cortex after a prolonged period of status epilepticus induced by sequential administration of lithium and pilocarpine. After 2 h of continuous high-amplitude rapid spiking on EEG, we found significantly decreased levels of residual ATP in the homogenate and mitochondria fractions from status epilepticus rat brains compared to matched controls. No difference in residual ATP level was observed in the synaptosomal preparations of status epilepticus animals compared to controls. Inorganic phosphate concentration in the status animals was higher than controls in the cytosolic fraction only. F1-ATPase activity, an enzymatic indicator of mitochondrial ATP synthesis rate, was significantly higher in the status brains, whereas other mitochondrial enzymes were not different in the status and control rat groups. These findings, together with our earlier report of reduced synaptosomal ecto-ATPase activity, suggest that either the corresponding in vivo ATP concentrations were reduced as a result of status epilepticus or other biochemical changes had occurred that facilitated the hydrolysis of ATP following decapitation. Controls for and measurement of such other changes failed to provide an explanation for the observed changes in residual ATP.