Abstracts

Rationale: The neurotransmitter glutamate is widely implicated in the cause of certain types of epilepsy, including childhood absence epilepsy (CAE), characterized by EEG recordings of bilaterally synchronous 2.5 to 3.5 Hz spike and wave discharges (SWD) concurrent with behavioral arrest. Previously, we have shown that the metabotropic glutamate receptor subtype 2 (mGluR2) acts a presynaptic autoreceptor to modulate excessive amounts of corticothalamic input. Here, we tested the antiepileptic effects of the mGluR2 agonist, LY379268 in vivo in the lethargic (lh/lh) mouse, a well established rodent model of CAE. Methods: For in vivo recordings, male lh/lh mice (8 weeks, n=4) were chronically implanted with EEG electrodes over the frontal and occipital cortices and EMG wires inserted in the dorsal nuchal muscles. Testing began 1 week after surgery. During EEG recording, the testing session was organized into epochs ranging from 1 to 10 second intervals of continuous recording (onset of dark phase (5 PM)) beginning 24 hr prior to injection of vehicle or LY379268 (0.1, 1 and 4.0 mg/kg, i.p.) and continued 24 hr following injection. We measured seizure frequency and duration in 30 min bins every 2 hours. For in vitro recordings, we used whole cell patch clamp electrophysiology to test the effect of LY379268, along with the positive allosteric modulator BINA (kindly provided by PJ Conn, Vanderbilt), on the frequency and amplitude of spontaneously occurring excitatory post-synaptic currents (sEPSCs) in the deep layers of somatosensory cortex, a putative initiation site of absence seizures in rodents. Results: Using continuous EEG recording, we found that in the 30 min pre-injection epoch, lh/lh mice experienced approximately 97.22 ± 42.21 SWD events, with a total average duration of 4.14 ± 0.44 min of the 30 min in absence seizures. In addition, we found a significant diurnal waxing and waning of SWD frequency. I.P. administration of the vehicle increased both frequency and duration of SWD at 30 min and 2 hr compared to pre-injection levels, suggesting a procedural or handling effect not yet reported in the literature. At the 1mg/kg dose, frequency and duration of SWD were reduced 20 and 27.78% respectively 2 hr post-injection and 31.3 and 47.87 % respectively 4 hr post-injection. Using whole cell patch clamp electrophysiology, we found that the application of LY379268 (100 nM) combined with BINA (1 μM) significantly increased the interevent interval (IEI) of cortical sEPSCs from a normalized baseline of 1 to an IEI of 2.4±0.171 (F(3, 12) = 8.375, p< 0.05, one-way ANOVA with Dunnett’s post-hoc, n = 6). Application of LY379268 alone did not significantly alter IEI from baseline. The amplitude of sEPSCs was unaffected by either LY379268 administration alone or in combination with BINA. Conclusions: We conclude that activation of mGluR2 by LY379268 or allosteric modulators has an anti-seizure effect in the lh/lh mouse model of CAE. Part of this effect may be due to reduced frequency of sEPSCs.