Abstracts

Rationale: The hippocampal dentate gyrus is notable for its sparse activity and tight inhibitory regulation essential for its role in memory processing. Compromises in dentate inhibition have been implicated in epilepsy. Dentate projection neurons, the classic granule cells (GCs) and the recently characterized semilunar granule cells (SGCs), are inhibited by diverse interneuron subtypes including fast-spiking interneurons (PV-IN), neurons expressing cannabinoid receptor type 1 (CB1R) and somatostatin interneurons. Additionally, SGCs have been proposed to support sustained feedback inhibition of GCs. However, which interneuronal populations inhibit GCs and SGCs during epochs steady state, and afferent activation is not fully understood. Here we examined the contribution of PV-IN and CB1R sensitive interneurons to GC and SGC. Methods: Whole cell current and voltage clamp recordings were obtained from GCs and SGCs in acute hippocampal slices (350 µm) from Wistar rats (P20-25) and mice (C57BL/6, PV-ChR2, PV-NpHR2, 6-8 weeks). IPSCs were recorded using KCl-based internal solution in glutamate receptor antagonists or as outward currents using Cs-methanesulfonate. SGCs and GCs were identified based on post-hoc analysis of biocytin immunostaining. In-vitro full-field light activation (470nm in PV-ChR2) or suppression (589nm in PV-NpHR2) of PV-INs was used to modulate PV-INs. WIN 55,212-2 (5µM) was used to selectively suppress synaptic release for CB1R sensitive interneurons. Results: As reported in rats, SGCs in mice had wider dendritic arbors, inner molecular layer axon collaterals and lower spike frequency adaptation. SGCs received more steady-state sIPSCs than GCs (in Hz, GC: 24.10±2.08, SGC: 36.73±1.08, pWIN 55,212-2 failed to reduce baseline sIPSCs frequency in both GCs and SGCs. Optical suppression of PV-IN did not reduce sIPSC frequency in either cell type. However, light activation of PV-IN enhanced sIPSC frequency, with a greater increase in GCs than in SGCs. Although optogenetic suppression of PV-INs reduced both the number of evoked action potentials and frequency of sustained firing in YFP-expressing PV-INs, it failed to reduce the peak PP-evoked IPSC amplitude in both cell types. Optogenetic suppression of PV-INs selectively reduced the frequency of sustained evoked inhibitory response in SGCs (in % change, from 211.90±54.86 to 116.88±37.67, p<0.05) without reducing IPSC frequency in GCs. Conclusions: These data demonstrate that: 1) CB1R sensitive interneurons and PV-IN have limited contribution to steady state inhibition of GCs and SGCs. While PV-INs innervate both cell types, their activation resulted in greater enhancement of GC inhibition than of SGCs. 2) Optical suppression of PV-INs decreased PP-evoked firing in YFP labeled PV-INs; Its failure to reduce the IPSC amplitude in projection neurons suggests that circuit mechanisms may compensate for reduction in PV-IN inhibition. 3) PV-INs contribute substantively to late sustained inhibitory activity in SGCs upon paired PP-stimulation. These differences in inhibitory inputs to GCs and SGCs could support distinct functional roles for the two cell types in the dentate network. Funding: NIH/NINDS R01NS069861 and R01NS097750 (V.S)