Ethosuximide affects paired-pulse facilitation in somatosensory cortex of WAG\Rij rats as a model of absence seizure

The interaction between somatosensory cortex and thalamus via a thalamocortical
loop is a theory behind induction of absence epilepsy. Inside peri-oral somatosensory
(S1po) and primary somatosensory forelimb (S1fl) regions, excitatory and inhibitory
systems are not balanced and GABAergic inhibitory synapses seem to play a fundamental
role in short-term plasticity alterations.
Methods: We investigated the effects of Ethosuximide on presynaptic changes by utilizing
paired-pulse stimulation that was recorded from somatosensory cortex in 18 WAG\Rij rats
during epileptic activity. A twisted tripolar electrode including two stimulating electrodes
and one recording electrode was implanted into the S1po and S1FL according to stereotaxic
landmarks. Paired-pulses (200 μs, 100-1000 μA, 0.1 Hz) were applied to somatosensory
cortex at 50, 100, 400, 500 ms inter-pulse intervals for 50 min period.
Results: The results showed that paired-pulse facilitation was significantly reduced at all
intervals in all times, but compared to the control group of epileptic WAG/Rij rats (p<0.05),
it was exceptional about the first 10 minutes after the injection. At the intervals of 50 and
100 ms, a remarkable PPD was found in second, third, fourth and fifth 10-min post
injection.
Conclusion: These experiments indicate that Ethosuximide has effects on presynaptic
facilitation in somatosensory cortex inhibitory loops by alteration in GABA levels that leads
to a markedly diminished PPF in paired-pulse stimulation