Abstracts

Rationale: The dentate gyrus has been proposed to serve as a functional "gate" that regulates inputs into the hippocampus. Apart from the classical dentate projection neurons, Granule Cells (GCs), recent studies have identified a new group of excitatory neurons, Semilunar Granule Cells (SGCs). SGCs have been shown to support feedback inhibition of GCs and thereby maintain the dentate gate. We previously reported that SGCs show greater baseline frequency of spontaneous inhibitory post-synaptic currents (sIPSCs) than GCs and show opposite changes in sIPSCs frequency in the two cell types after brain injury, indicating distinctive inhibitory regulation of SGCs versus GCs. However, whether SGCs, like GCs, are subject to the strong feedback inhibitory regulation is unknown. Here we examined whether SGCs and GCs are under differential synaptic inhibition from parvalbumin-expressing presumed fast-spiking interneurons (PV-IN) and GABAergic neurons expressing the cannabinoid receptor type 1 (CB1R). Methods: Whole cell current and voltage clamp recordings were obtained from GCs and SGCs in acute hippocampal slices from rats (Wistar, P20-25) or mice (C57BL/6, PV-Chr2, PV-NpHR2, 6-8 weeks). sIPSCs were isolated by perfusion with the glutamate receptor antagonist Kyneurinic Acid. WIN 55,212-2 (WIN-2), a potent agonist for CB1R was used to examine CB1R-sensitive IPSCs. Optogenetic activation or suppression of PV-INs was used to examine PV-IN inputs. SGCs and GCs were identified based on biocytin immunostaining and post hoc morphology. Results: Consistent with earlier studies in rats, SGCs in mice can be distinguished from GCs based on somato-dendritic structure, axon collateral in the inner molecular layer (IML) axonal structure and intrinsic physiology. SGCs received a higher frequency of sIPSCs than GCs in both species (sIPSCs frequency in Hz, Rat: GC: 8.371.10, SGC: 12.101.78, p < 0.05; Mouse, GC: 9.831.95, SGC: 14.360.82, p < 0.05). WIN-2 failed to reduce baseline sIPSCs frequency in both cell types. Optogenetic inhibition of PV-IN did not have a significant effect on sIPSC frequency in both GCs and SGCs. In contrast, optogenetic activation of PV-IN resulted in a greater increase in sIPSC frequency in GCs than in SGCs (GC: 417.2526.24%, n=3, SGC: 216.4312.79%, n=3, p=0.002 by t-test). Conclusions: These data show that CB1R sensitive interneurons, which include dentate CCK-expressing and total molecular layer (TML) interneurons, have not a significant contribution to baseline inhibition of GCs or SGCs. Activation of PV-IN causes a more robust inhibition of GCs demonstrating that SGCs have reduced PV-IN inputs compared to GCs in the presence of stimulation. Delineating the distinct inhibitory regulation of GCs and SGCs is crucial to identify if SGCs could circumvent the inhibitory dentate gate under physiological conditions and compound the loss of feedback inhibition in traumatic brain injury and epilepsy. Funding: NIH/NINDS R01 NS069861 and NJCBIR CBIR14RG024 (V.S)