Dysfunction of the dentate basket cell circuit in a rat model of temporal lobe epilepsy

High ratio of synaptic excitation to synaptic inhibition in hilar ectopic granule cells of pilocarpine-treated rats

After experimental status epilepticus, many dentate granule cells born into the postseizure environment migrate aberrantly into the dentate hilus. Hilar ectopic granule cells (HEGCs) have also been found in persons with epilepsy. These cells exhibit a high rate of spontaneous activity, which may enhance seizure propagation. Electron microscopic studies indicated that HEGCs receive more recurrent mossy fiber innervation than normotopic granule cells in the same animals but receive much less inhibitory innervation. This study used hippocampal slices prepared from rats that had experienced pilocarpine-induced status epilepticus to test the hypothesis that an imbalance of synaptic excitation and inhibition contributes to the hyperexcitability of HEGCs. Mossy fiber stimulation evoked a much smaller GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSC) in HEGCs than in normotopic granule cells from either control rats or rats that had experienced status epilepticus. However, recurrent mossy fiber-evoked excitatory postsynaptic currents (EPSCs) of similar size were recorded from HEGCs and normotopic granule cells in status epilepticus-experienced rats. HEGCs exhibited the highest frequency of miniature excitatory postsynaptic currents (mEPSCs) and the lowest frequency of miniature inhibitory postsynaptic currents (mIPSCs) of any granule cell group. On average, both mEPSCs and mIPSCs were of higher amplitude, transferred more charge per event, and exhibited slower kinetics in HEGCs than in granule cells from control rats. Charge transfer per unit time in HEGCs was greater for mEPSCs and much less for mIPSCs than in the normotopic granule cell groups. A high ratio of excitatory to inhibitory synaptic function probably accounts, in part, for the hyperexcitability of HEGCs.

Receptors with low affinity for neurosteroids and GABA contribute to tonic inhibition of granule cells in epileptic animals

Neurosteroid sensitivity of GABA(A) receptor mediated inhibition of the hippocampal dentate granule cells (DGCs) is reduced in animal models of temporal lobe epilepsy. However, the properties and subunit composition of GABA(A) receptors mediating tonic inhibition in DGCs of epileptic animals have not been described. In the DGCs of epileptic animals, allopregnanolone and L-655708 sensitivity of holding current was diminished and δ subunit was retained in the endoplasmic reticulum and its surface expression was decreased the in the hippocampus. Ro15-4513 and lanthanum had distinct effects on holding current recorded from DGCs of control and epileptic animals suggesting that the pharmacological properties of GABA(A) receptors maintaining tonic inhibition in DGCs of epileptic animals were similar to those containing the α4βxγ2 subunits. Furthermore, surface expression of the α4 subunit increased and a larger fraction of the subunit co-immunoprecipitated with theγ2 subunit in hippocampi of epileptic animals. Together, these studies revealed that functional α4βxδ and α5βxγ2 receptors were reduced in the hippocampi of epileptic animals and that novel α4bxγ2 receptors contributed to the maintenance of tonic inhibition. The presence of α4βxγ2 receptors resulted in low GABA affinity and neurosteroid sensitivity of tonic currents in the DGCs of epileptic animals that could potentially increase seizure vulnerability. These receptors may represent a novel therapeutic target for anticonvulsant drugs without sedative actions.

Initial loss but later excess of GABAergic synapses with dentate granule cells in a rat model of temporal lobe epilepsy

Many patients with temporal lobe epilepsy display neuron loss in the dentate gyrus. One potential epileptogenic mechanism is loss of GABAergic interneurons and inhibitory synapses with granule cells. Stereological techniques were used to estimate numbers of gephyrin-positive punctae in the dentate gyrus, which were reduced short-term (5 days after pilocarpine-induced status epilepticus) but later rebounded beyond controls in epileptic rats. Stereological techniques were used to estimate numbers of synapses in electron micrographs of serial sections processed for postembedding GABA-immunoreactivity. Adjacent sections were used to estimate numbers of granule cells and glutamic acid decarboxylase-positive neurons per dentate gyrus. GABAergic neurons were reduced to 70% of control levels short-term, where they remained in epileptic rats. Integrating synapse and cell counts yielded average numbers of GABAergic synapses per granule cell, which decreased short-term and rebounded in epileptic animals beyond control levels. Axo-shaft and axo-spinous GABAergic synapse numbers in the outer molecular layer changed most. These findings suggest interneuron loss initially reduces numbers of GABAergic synapses with granule cells, but later, synaptogenesis by surviving interneurons overshoots control levels. In contrast, the average number of excitatory synapses per granule cell decreased short-term but recovered only toward control levels, although in epileptic rats excitatory synapses in the inner molecular layer were larger than in controls. These findings reveal a relative excess of GABAergic synapses and suggest that reports of reduced functional inhibitory synaptic input to granule cells in epilepsy might be attributable not to fewer but instead to abundant but dysfunctional GABAergic synapses.

Surviving hilar somatostatin interneurons enlarge, sprout axons, and form new synapses with granule cells in a mouse model of temporal lobe epilepsy

In temporal lobe epilepsy, seizures initiate in or near the hippocampus, which frequently displays loss of neurons, including inhibitory interneurons. It is unclear whether surviving interneurons function normally, are impaired, or develop compensatory mechanisms. We evaluated GABAergic interneurons in the hilus of the dentate gyrus of epileptic pilocarpine-treated GIN mice, specifically a subpopulation of somatostatin interneurons that expresses enhanced green fluorescence protein (GFP). GFP-immunocytochemistry and stereological analyses revealed substantial loss of GFP-positive hilar neurons (GPHNs) but increased GFP-positive axon length per dentate gyrus in epileptic mice. Individual biocytin-labeled GPHNs in hippocampal slices from epileptic mice also had larger somata, more axon in the molecular layer, and longer dendrites than controls. Dual whole-cell patch recording was used to test for monosynaptic connections from hilar GPHNs to granule cells. Unitary IPSCs (uIPSCs) recorded in control and epileptic mice had similar average rise times, amplitudes, charge transfers, and decay times. However, the probability of finding monosynaptically connected pairs and evoking uIPSCs was 2.6 times higher in epileptic mice compared to controls. Together, these findings suggest that surviving hilar somatostatin interneurons enlarge, extend dendrites, sprout axon collaterals in the molecular layer, and form new synapses with granule cells. These epilepsy-related changes in cellular morphology and connectivity may be mechanisms for surviving hilar interneurons to inhibit more granule cells and compensate for the loss of vulnerable interneurons.