Abstracts

The use of cannabinoid (CB) agonists for treating seizure disorders has been proposed, but results of case study analyses have been mixed. Most known effects of CBs in the brain are mediated by CB type 1 receptors, usually located on presynaptic GABA terminals. In the pilocarpine-treated mouse, development of spontaneous seizures is associated with mossy fiber sprouting, recurrent excitatory synapse formation between granule cells of the dentate gyrus, and increased synaptic excitability. These experiments were designed to determine how cannabinoids modulate neural activity in the dentate gyrus in a murine model of temporal lobe epilepsy (TLE) in order to understand effects of the substance on a system that has undergone synaptic reorganization. Pilocarpine-induced seizures in CD1 mice led to spontaneous seizures, mossy fiber sprouting, and recurrent excitatory circuit formation in the dentate gyrus. Extracellular field potential and whole-cell patch-clamp (voltage-clamp) recordings were made from dentate gyrus granule cells in transverse hippocampal slices from epileptic and control mice in the presence of bicuculline and low extracellular Mg2+. Antidromic activity was evoked after stimulation of mossy fibers in the hilus, and spontaneous excitatory postsynaptic currents (EPSCs) were also recorded. Effects of CB receptor agonists on synaptically-driven population responses following antidromic stimulation and spontaneous EPSCs were examined. Timm staining was performed on recorded slices. Electrical stimulation of mossy fibers produced an antidromic population spike followed by prolonged bursts of activity in the dentate gyrus of epileptic mice with mossy fiber sprouting. Bursts were sensitive to glutamate receptor antagonists, indicating they were synaptic in nature. Application of the CB receptor agonists, anandamide (1-10 uM; n=5) or WIN55,212-2 (10 uM; n=2) reversibly inhibited the synaptic responses. Spontaneous bursts of EPSCs and synaptically-driven activity were also observed in epileptic mice, which were attenuated by anandamide (n=4). Timm staining showed mossy fiber sprouting in epileptic mice but not controls. Little or no effects of the agonists were observed in slices from control mice. Enhanced excitatory synaptic activity in the dentate gyrus in mice with pilocarpine-induced TLE is attributable to activation of newly-formed recurrent excitatory circuits between granule cells. In the absence of GABAA receptor-mediated inhibition, cannabinoids can inhibit synaptically-induced epileptiform activity in the dentate gyrus. Whereas CB receptor agonists often suppress GABA release in the normal hippocampus, the inhibition of epileptiform activity and glutamatergic EPSCs in the pilocarpine-treated mouse dentate gyrus suggests treatment strategies specific for TLE patients, whose brains may have undergone synaptic reorganization. (Supported by Louisiana Board of Regents and the Epilepsy Foundation of America)