Loss of cholecystokinin-containing terminals in temporal lobe epilepsy

Abstract

Altered GABA-mediated inhibition is proposed to play a role in the pathogenesis of epilepsy. Previous studies have demonstrated a loss of somatostatin-containing GABAergic interneurons innervating granule cells in epileptic animals. However, the reorganization of synapses between interneurons and granule cells has not been investigated. We studied synapse organization in an animal model of temporal lobe epilepsy (TLE) using continuous hippocampal stimulation. The distribution of axon terminals and inhibitory synapses on granule cell dendrites was studied using a combination of immunohistochemistry and pre-embedding electron microscopy techniques. A whole-cell patch-clamp technique was applied to study the functional changes in GABAergic input from different interneurons. In epileptic animals, the density of cholecystokinin (CCK)-immunoreactive (IR) fibers and α2 subunit containing GABAA receptors in the inner molecular layer of the dentate gyrus was reduced. Quantitative immuno-electron microscopy study revealed that the ratio of CCK-containing symmetric synapses to the total symmetric synapses was reduced. The frequency of GABAergic synaptic currents (sIPSC) was decreased and their amplitude was increased. The inhibitory effect of the activation of cannabinoid 1(CB1) receptors was also reduced in epileptic animals. Isolation of CCK- and parvalbumin (PV)-containing GABAergic inputs by N- and P/Q-type calcium channel blockers respectively suggested that GABA release from CCK-containing interneurons was selectively reduced in epileptic rats. This study found that there was a loss of CCK-containing GABAergic synapses to granule cells both morphologically and functionally. These studies add to our understanding of the mechanisms that contribute to altering GABAergic inhibition of granule cells in TLE.

Functional Reduction in Cannabinoid-Sensitive Heterotypic Inhibition of Dentate Basket Cells in Epilepsy: Impact on Network Rhythms

“…It is possible that sprouting of inhibitory terminals mediating distal dendritic inputs in the outer molecular layer Thind et al 2010;Peng et al 2013) contributes to the recovery of FS-BC mIPSC frequency while decreasing mIPSC amplitude at later time points after SE. In contrast, sIPSC amplitude, which was reduced after SE, increases in epileptic rats as has been reported in GCs (Sun et al 2014) suggesting that FS-BCs may show strengthening of proximal inputs with progression of disease. Yet, CB 1 R agonist consistently suppressed a smaller proportion of FS-BC sIPSCs in post-SE and epileptic rats indicating the AC-IN inhibition of FSBCs remains reduced in epilepsy.…”

“…2g: in pA, control: median (IQR) = 17.01 (11.15-29.5) , n = 10 neurons; post-SE: median (IQR) = 16.22 (10.92-26.88) , n = 9 neurons, P > 0.05, K-S test). Structural changes such as the loss of dentate interneurons, including somatostatin-expressing cells, and axon terminals after SE (Kobayashi and Buckmaster 2003;Sun et al 2014), likely contribute to the early reduction in FS-BC synaptic inhibition. However, whether functional changes in the large amplitude, presumed perisomatic, and proximal dendritic synapses contribute to deficits in FS-BCs synaptic inhibition after SE is not known.…”

“…Having confirmed that CB 1 R antagonist selectively modulates release at AC-IN and not FS-BC synapses on FS-BCs, we used the CB 1 R agonist, WIN55212-2 (5 µM) which has been adopted by earlier studies to block CB 1 R-sensitive synaptic events (Wyeth et al 2010;Sun et al 2014), to determine the contribution of CB 1 R-sensitive AC-IN inputs to FS-BC synaptic inhibition. Use of the CB 1 R agonist WIN55212-2 (5 µM) to suppress CB 1 R-sensitive events, rather than an antagonist, eliminates potential confounds that may arise from differences in baseline CB 1 R expression or activity between experimental groups.…”

Cannabinoids and Epilepsy

“…CBDV potentiated the effects of phenobarbital, ethosuximide, and valproate in 2 seizure models [181]. These studies suggest that both Δ9-THC and CBD provide significant protection from seizures [147,[154][155][156][157][158][159][160][161][162][163][164][165][166]. GABA = γ-aminobutryic acid in preclinical animal trials, presenting potential targets for human studies.…”

“…3, dark red trace). Beginning 4 days following pilocarpine-induced seizures in rats, CB 1 R expression progressively decreased in hippocampal CCK+ inhibitory nerve terminals [161], particularly in the CA1 stratum pyramidale and the dentate gyrus inner molecular layer, unaccounted for by CA1 neuronal cell loss alone [155,156,162]. By reducing CB 1 R expression on inhibitory terminals (and presumed DSI), this homoeostatic process may limit network disinhibtion and restrain elevated excitability during prolonged epileptiform activity.…”