Abstracts

Rationale: In adult rats, the synthetic cannabinoid agonist, WIN55212, suppresses spontaneous seizure activity and IPSCs of DHPG-sensitive hippocampal neurons. Anticonvulsant effects were attributed to reducing pre-synaptic release of glutamate via G-protein coupled CB1 receptors. In postnatal (P) day 20 rats, we found that kainate (KA)-induced epileptic activity is inversely related to WIN55212 dosage after peripheral injection. Higher doses were proconvulsant and injurious whereas lower doses were anticonvulsant and neuroprotective. Methods: To determine whether the hippocampus was responsible for this inverse relationship on the seizure threshold at P20, stereotaxic microinfusions of WIN55212 (0.5 µg) were delivered to the CA1 followed by NMDA (0.1 µg) or KA (0.3 µg) in the presence and absence of mGluR1α, mGluR5, and CB1 receptor antagonists, LY367385, MPEP or AM251, respectively. Electroencephalography (EEG) recordings from the CA1 were simultaneously determined with a bipolar/cannula assembly. CB1 and NMDA receptor proteins were examined. Results: Intrahippocampal WIN 55,212 alone caused rapid sedation and low amplitude synchronous oscillations in the EEG. NMDA (50 nM) produced early spikes within seconds and late spike activity and seizure behavior after 20 min. In the presence of WIN55212, NMDA-induced spikes and epileptic currents were delayed and attenuated. Blockade of mGluR1α receptors with LY367385 or CB1 receptors with AM251 (but not MPEP) prevented the attenuation. In contrast, microinfusion of a saturating concentration of KA led to increased periodic oscillations containing high frequency spikes in the presence and absence of WIN55212, LY367385 or AM251. Immunohistochemistry revealed a dose-dependent reduction in CB1 receptor expression with increasing concentrations of WIN55212 in presence or absence of KA seizures. There were steady levels of mGluR1α receptors, and variable distribution of NR1 and the NMDA C2 terminal splice variant patterns. Conclusions: Differential responses of NMDA and KA on seizure threshold subsequent to WIN55212 application suggests selective cooperation between NMDA and mGluR receptors such that NMDA is expected to co-activate mGluR group I receptors to synergistically activate release of endocannabinoids to further attenuate EEG spiking. Unexpectedly, mGluR1α subunits appear required for the inhibitory actions of WIN5521 on NMDA currents and that KA receptor activation is not coupled to the cannabinoid/mGluR pathway. CB1 receptor re-distributions caused by seizures may also impart different action on regulatory mechanisms of transmitter release to control the seizure threshold and associated brain injury.