Abstracts

Rationale: In temporal lobe epilepsy, seizures initiate in or near hippocampus, which frequently displays loss of neurons. Hilar somatostatin interneurons normally are abundant. Their axons are positioned ideally to inhibit granule cells at sites of perforant path input, the major afferent of the dentate gyrus. There is substantial loss of this class of inhibitory cell in patients with temporal lobe epilepsy and rodent models. It is unclear whether surviving interneurons function normally, are impaired, or develop compensatory mechanisms. The possibility of interneuron axon sprouting is suggested by increased expression of markers of GABAergic axons and synapses in epileptic tissue. However, it is difficult to distinguish genuine axon sprouting and synaptogenesis from increased marker expression by pre-existing structures. Methods: In GIN mice (Jackson Lab), a subset of somatostatin interneurons can be visualized by green fluorescence protein (GFP)-immunoreactivity. We used GFP-immunocytochemistry in GIN mice to evaluate surviving hilar somatostatin-immunoreactive interneurons after pilocarpine-induced status epilepticus. Total numbers of GFP-positive hilar neurons per dentate gyrus were estimated using the optical fractionator method. Stereological methods were used to estimate the total length of GFP-positive axon in the granule cell layer plus molecular layer. To more directly evaluate structural changes of surviving interneurons, patch-clamp recording was used in hippocampal slices to label individual GFP-positive hilar neurons. Biocytin-labeled neurons were scanned with a confocal microscope. Somata, dendrites, and axons were reconstructed and measured. Dual whole-cell patch recording was used to test for monosynaptic connections from hilar GFP-positive neurons to granule cells. Results: GFP-immunocytochemistry and stereological analyses revealed substantial loss of hilar GFP-positive neurons but increased GFP-positive axon length per dentate gyrus in epileptic mice, despite fewer neurons. Biocytin-labeled individual GFP-positive hilar neurons in hippocampal slices from epileptic mice had larger somata, more axon in the molecular layer, and longer dendrites than controls. Unitary IPSCs recorded in control and epileptic mice were similar with respect to rise time, amplitude, charge transfer, and decay time. However, the probability of evoking uIPSCs in paired recordings was 2.6 times higher in epileptic mice compared to controls. Conclusions: Together, these findings suggest that in this mouse model of temporal lobe epilepsy, surviving hilar somatostatin interneurons enlarge, extend dendrites, sprout axon collaterals in the molecular layer, and form new synapses with granule cells. These 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 somatostatin interneurons.