PURPOSE: Genetically epileptic WAG/Rij rats develop spontaneous absence-like seizures after 3 months of age. We used WAG/Rij rats to examine whether absence seizures are associated with changes in the expression of type-1 cannabinoid (CB₁) receptors. METHODS: Receptor expression was examined by in situ hybridization and western blot analysis in various brain regions of "presymptomatic" 2-month old and "symptomatic" 8-month-old WAG/Rij rats relative to age-matched nonepileptic control rats. Furthermore, we examined whether pharmacologic activation of CB₁ receptor affects absence seizures. We recorded spontaneous spike-wave discharges (SWDs) in 8-month old WAG/Rij rats systemically injected with the potent CB₁ receptor agonist, R(+)WIN55,212-2 (3-12 mg/kg, s.c.), given alone or combined with the CB₁ receptor antagonist/inverse agonist, AM251 (12 mg/kg, s.c.). RESULTS: Data showed a reduction of CB₁ receptor mRNA and protein levels in the reticular thalamic nucleus, and a reduction in CB₁ receptor protein levels in ventral basal thalamic nuclei of 8-month-old WAG/Rij rats, as compared with age-matched ACI control rats. In vivo, R(+)WIN55,212-2 caused a dose-dependent reduction in the frequency of SWDs in the first 3 h after the injection. This was followed by a late increase in the mean SWD duration, which suggests a biphasic modulation of SWDs by CB₁ receptor agonists. Both effects were reversed or attenuated when R(+)WIN55,212-2 was combined with AM251. DISCUSSION:These data indicate that the development of absence seizures is associated with plastic modifications of CB₁ receptors within the thalamic-cortical-thalamic network, and raise the interesting possibility that CB₁ receptors are targeted by novel antiabsence drugs.
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