Abstract
Temporal lobe epilepsy (TLE) is a neurological condition associated with neuron loss, axon sprouting, and hippocampal sclerosis, which results in modified synaptic circuitry. Cannabinoids appear to be anti-convulsive in patients and animal models of TLE, but the mechanisms of this effect are not known. A pilocarpine-induced status epilepticus mouse model of TLE was used to study the effect of cannabinoid agonists on recurrent excitatory circuits of the dentate gyrus using electrophysiological recordings in hippocampal slices isolated from control mice and mice with TLE. Cannabinoid agonists WIN 55,212-2, anandamide (AEA), or 2-arachydonoylglycerol (2-AG) reduced the frequency of spontaneous and tetrodotoxin-resistant excitatory postsynaptic currents (EPSCs) in mice with TLE, but not in controls. WIN 55,212-2 also reduced the frequency of EPSCs evoked by glutamate-photolysis activation of other granule cells in epileptic mice. Secondary population discharges evoked after antidromic electrical stimulation of mossy fibers in the hilus were also attenuated by cannabinoid agonists. Agonist effects were blocked by the cannabinoid type 1 receptor (CB1R) antagonist AM251. No change in glutamate release was observed in slices from mice that did not undergo status epilepticus. Western blot analysis suggested an up-regulation of CB1R in the dentate gyrus of animals with TLE. These findings indicate that activation of CB1R present on nerve terminals can suppress recurrent excitation in the dentate gyrus of mice with TLE. This suggests a mechanism for the anti-convulsive role of cannabinoids aimed at modulating receptors on synaptic terminals expressed de novo after epileptogenesis.
Highlights
Development of temporal lobe epilepsy (TLE) is often associated with gliosis, neuron loss, axon sprouting, and synaptic reorganization in temporal lobe brain structures
Pilocarpine-induced status epilepticus triggers a cascade of events that includes selective neuron loss, gliosis, mossy fiber sprouting with concomitant synaptic reorganization in the dentate gyrus, and, after a period of a few days, development of spontaneous recurrent behavioral seizures ensues [1,7]
Mice that survived pilocarpine-induced status epilepticus (SE) had robust mossy fiber sprouting into the inner molecular layer of the dentate gyrus
Summary
Development of temporal lobe epilepsy (TLE) is often associated with gliosis, neuron loss, axon sprouting, and synaptic reorganization in temporal lobe brain structures. These changes occur variably in concert with the development of spontaneous recurrent seizures in humans and in animal models of inducible TLE. Mossy fiber sprouting in the dentate gyrus is a widely studied correlate of adult TLE development [1,2,3,4,5,6,7]. The well-described changes in the dentate gyrus may serve as a model for altered synaptic activity in other affected brain areas
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