Abstracts

Rationale: The endocannabinoid system has been shown to play a role in modulating seizure activity in several in vivo and in vitro models. Studies from our laboratory have demonstrated that cannabinoid type 1 (CB1) receptor activation is anticonvulsant in the rat pilocarpine model of acquired epilepsy, and that a unique plasticity of the CB1 receptor occurs in the hippocampus of epileptic rats (Wallace et al., JPET 2003; 307[1]:129-37; Falenski et al., Neuroscience 2007; 146[3]:1232-44). However, other regions have not yet been thoroughly evaluated for changes in the CB1 receptor. Therefore, this study was initiated to evaluate the long-term effect of pilocarpine-induced status epilepticus (SE) on CB1 receptor expression, binding, and G-protein activation throughout the epileptic rat brain. Methods: Coronal sections from control and chronically epileptic animals (1 year following pilocarpine-induced SE) were cut in a cryostat at equal intervals throughout the entire neuroaxis and processed for immunohistochemistry using an antibody to the CB1 receptor, cannabinoid receptor binding using CB1 agonist [³H]WIN55,212-2 (1 nM), or agonist-stimulated [³⁵S]GTPγS autoradiography with maximally effective concentrations of WIN55,212-2 (WIN, 10 µM). Results: In addition to areas of the hippocampus, widespread increases in CB1 receptor expression, receptor binding, and WIN-stimulated [³⁵S]GTPγS binding occurred primarily in the cortex and thalamus of epileptic animals. More specifically, increases in WIN-stimulated [³⁵S]GTPγS binding were observed in the cingulate, frontal, lateral orbital, infralimbic, insular, and temporal cortices, most notably in deep layers (layers V and VI). CB1 receptor expression, [³H]WIN55,212-2 and WIN-stimulated [³⁵S]GTPγS binding levels were also increased in the thalamus, particularly in the ventrolateral, ventromedial, ventroposterolateral, ventroposteromedial, reticular, laterodorsal, anteromedial, and lateral and medial geniculate thalamic nuclei. Increases in CB1 receptor expression and function were also observed in other regions including caudate-putamen. Decreases in CB1 receptor expression and function were primarily observed in the hippocampus, particularly the dentate gyrus polymorphic and inner molecular layers. Several regions illustrated no significant changes, including the substantia nigra, globus pallidus, and cerebellum. Conclusions: Overall, immunohistochemistry, [³H]WIN55,212-2 binding, and WIN-stimulated [³⁵S]GTPγS autoradiography revealed a unique global profile of CB1 receptor expression and function in epileptic animals, and thus revealed specific regions of the epileptic brain that may serve as targets for cannabinimimetic compounds or manipulation of the endocannabinoid system. Support: Supported by U01-NS058213-01, R01-NS052529-01, and R01-NS051505-01 to RJD, VCU AD Williams Award to LSS, F30-DA023758 to PTN