2-AG-mediated Control of GABAergic Signaling is Impaired in a Model of Epilepsy.

Repeated seizures result in a persistent maladaptation of endocannabinoid signaling, mediated in part by anandamide signaling deficiency in the basolateral amygdala (BLA) that manifests as aberrant synaptic function and altered emotional behaviour. Here, we determined the effect of repeated seizures (kindling) on 2-AG signaling on GABA transmission by directly measuring tonic and phasic eCB-mediated retrograde signaling in an in vitro BLA slice preparation from male rats. We report that both activity-dependent and muscarinic receptor (mAChR)-mediated depression of GABA synaptic transmission was reduced following repeated seizure activity. These effects were recapitulated in sham rats by preincubating slices with the 2-AG synthesising enzyme inhibitor DO34. Conversely, preincubating slices with the 2-AG degrading enzyme inhibitor KML29 rescued activity-dependent 2-AG signaling, but not mAChR-mediated synaptic depression, over GABA transmission in kindled rats. These effects were not attributable to a change in CB1 receptor sensitivity or altered 2-AG tonic signaling since the application of the highly selective CB1 receptor agonist CP55,940 provoked a similar reduction in GABA synaptic activity in both sham and kindled rats, while no effect of either DO34 or of the CB1 inverse agonist AM251 was observed on frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) in either sham or kindled rats. Collectively, these data provide evidence that repeated amygdala seizures persistently alter phasic 2-AG mediated-retrograde signaling at BLA GABAergic synapses, probably by impairing stimulus-dependent 2-AG synthesis/release, which contributes to the enduring aberrant synaptic plasticity associated with seizure activity. SIGNIFICANT STATEMENT: The plastic reorganization of endocannabinoid (eCB) signaling after seizures and during epileptogenesis may contribute to the negative neurobiological consequences associated with seizure activity. Therefore, a deeper understanding of the molecular basis underlying the pathologic long-term eCB signaling remodelling following seizure activity will be crucial to develop novel therapies for epilepsy which not only target seizure activity, but, most importantly, the epileptogenesis and the comorbid conditions associated with epilepsy.