Abstracts

Rationale: Cannabinoids have been recently shown to have significant therapeutic potentials for temporal lobe epilepsy (TLE). Anandamide, an endocannabinoid (eCB), has the ability to act on both cannabinoid (CB1R) and vanilloid (TRPV1) receptors. Depending on the receptor activated by anandamide, the effect could be either suppression or enhancement of synaptic activity, respectively. It is imperative to understand this interaction so that eCB-based treatments are developed for TLE with regard for potentially adverse side effects of activating multiple receptor types.Methods: CD1 mice were injected with scopolamine (1mg/kg, ip). After 15 minutes, mice were injected with pilocarpine (283-292mg/kg, ip) to induce status epilepticus (SE). The animals were continuously monitored for 2 hours. Mice developed spontaneous seizures beginning about 2 weeks post-SE. Whole-cell patch clamp recordings were made from dentate gyrus granule cells in transverse hippocampal slices. Slices were perfused with ACSF containing low [Mg2+] and bicuculline methiodide to enhance synaptic excitation. Synaptic responses to selective agonists and antagonists of CB1R and TRPV1 receptors were analyzed. Timm staining was done on the slices after the experiment.Results: Anandamide (10μM) increased spontaneous EPSC frequency when CB1R receptors were blocked with AM251 (10μM), but reduced synaptic activity when TRPV1 receptors were blocked with capsazepine (10μM). Selective CB1R agonists, 2-AG (10μM) and WIN 55,212-2 (1-10μM) suppressed, and the selective TRPV1 agonist capsaicin (1μM) enhanced, synaptic activity in the reorganized dentate gyrus. None of the drugs had any effect on EPSC amplitude in the presence of TTX, suggesting a presynaptic location for both CB1R and TRPV1 receptors on recurrent excitatory terminals.Conclusions: In addition to a CB1R-mediated suppression of EPSCs, anandamide can activate TRPV1 in the dentate gyrus of mice with mossy fiber sprouting. Anandamide can therefore increase or decrease glutamate release in the reorganized dentate gyrus, depending upon receptor availability. Understanding this interaction between cannabinoid and vanilloid systems is essential for the development of new therapeutic strategies for treating TLE based on the cannabinoid system. Inclusion of TRPV1 antagonists may enhance any antiepileptic effects of eCBs like anandamide. (Sources of funding: NIH/NINDS R21 NS052302)