Abstracts

Rationale:
The dentate gyrus is critically involved in temporal lobe epilepsy. Within the dentate gyrus, granule cells (GCs) and mossy cells (MCs) establish a recurrent excitatory circuit. Given the extensive MC projections contralaterally and along the longitudinal axis of the hippocampus and the large proportion of MCs active during exploratory behaviors, MCs have a great potential to destabilize this circuit. In addition, MCs promotes epileptic activity during early stages of epilepsy. MC repetitive activity induces BDNF/TrkB-dependent long-term potentiation at the MC-GC synapse (MC-GC LTP), which increases GC output. This plasticity promotes behavioral seizures in vivo and, left unchecked, may interfere with pattern separation, a DG-dependent form of learning that relies on sparse GC firing. Strikingly, MC axon terminals express high levels of type-1 cannabinoid receptor (CB₁R). We recently reported that CB₁R activity dampens MC-GC LTP, via multiple synaptic mechanisms, which may be a critical mechanism to suppress seizure-induced runaway excitation. We hypothesized that CB₁Rs, acting via endogenous or exogenous cannabinoids, can protect against runaway excitation and seizure generation by controlling GC output.

Methods:
Here, we used multiple complementary approaches, such as a conditional knockout strategy, in vitro electrophysiology, in vivo optogenetics, in vivo calcium imaging.

Results:
We found that selective deletion of CB₁Rs from MCs (MC CB₁R cKO) increased the severity and susceptibility to acute seizures in adult mice. Using miniature scopes to assess neuronal activity from large cell populations in the dentate gyrus, we analyzed population-level activity in MC CB₁R cKO mice. We found that adult mice injected with intraperitoneal kainic acid (KA) injection, a well-established model that leads to experimental epilepsy, displayed robust calcium waves which were exacerbated in MC CB₁R cKO mice, suggesting a strong, synchronized neural activity. Moreover, selective deletion of CB₁Rs from MCs impaired DG-dependent learning processes including spatial memory and contextual memory. Finally, we found that endocannabinoid release from GCs is sufficient to depotentiate previously strengthened synapses in a model of experimental seizures in mice. To confirm this observation in vivo, we have developed and tested a photoactivable CB₁R (opto- CB₁R) to mimic CB₁R in MCs.

Conclusions:
Our findings reveal that CB₁Rs are critical to control seizures and may be used as a novel therapeutic strategy to alleviate or prevent the development of seizures.

Funding:
National Institute of Health (NIH) R01-NS113600, R01-MH125772, R01-NS115543, R01-MH116673 to PEC. American Epilepsy Society Postdoctoral Award and Einstein Junior Investigator Neuroscience Research Award (JINRA) to C.B.