Reduced cannabinoid 2 receptor activity increases susceptibility to induced seizures in mice

Objective: The endocannabinoid system (ECS) is comprised of cannabinoid receptors 1 and 2 (CB1R and CB2R), endogenous ligands, and regulatory enzymes, and serves to regulate several important physiological functions throughout the brain and body. Recent evidence suggests that the ECS may be a promising target for the treatment of epilepsy, including epilepsy subtypes that arise from mutations in the voltage-gated sodium channel SCN1A. The objective of this study was to explore the effects of modulating CB2R activity on seizure susceptibility. Methods: We examined susceptibility to induced seizures using a number of paradigms in CB2R knockout mice (Cnr2(-/-)), and determined the effects of the CB2R agonist, JWH-133, and the CB2R antagonist, SR144528, on seizure susceptibility in wild-type mice. We also examined seizure susceptibility in Cnr2 mutants harboring the human SCN1A R1648H (RH) epilepsy mutation and performed Electroencephalography (EEG) analysis to determine whether the loss of CB2Rs would increase spontaneous seizure frequency in Scn1a RH mutant mice. Results: Both heterozygous (Cnr2(+/-)) and homozygous (Cnr2(-/-)) knockout mice exhibited increased susceptibility to pentylenetetrazole (PTZ)-induced seizures. The CB2R agonist JWH-133 did not significantly alter seizure susceptibility in wildtype mice; however, administration of the CB2R antagonist SR144528 resulted in increased susceptibility to PTZ-induced seizures. In offspring from a cross between the Cnr2 x RH lines, both Cnr2 and RH mutants were susceptible to PTZ-induced seizures; however, seizure susceptibility was not significantly increased in mutants expressing both mutations. No spontaneous seizures were observed in either RH or Cnr2/RH mutants during 336-504 hours of continuous EEG recordings. Significance: Our results demonstrate that reduced CB2R activity is associated with increased seizure susceptibility. CB2Rs might therefore provide a therapeutic target for the treatment of some forms of epilepsy.