Normal and epilepsy-associated pathologic function of the dentate gyrus

Abstract

The dentate gyrus plays critical roles both in cognitive processing, and in regulation of the induction and propagation of pathological activity. The cellular and circuit mechanisms underlying these diverse functions overlap extensively. At the cellular level, the intrinsic properties of dentate granule cells combine to endow these neurons with a fundamental reluctance to activate, one of their hallmark traits. At the circuit level, the dentate gyrus constitutes one of the more heavily inhibited regions of the brain, with strong, fast feedforward and feedback GABAergic inhibition dominating responses to afferent activation. In pathologic states such as epilepsy, a number of alterations within the dentate gyrus combine to compromise the regulatory properties of this circuit, culminating in a collapse of its normal function. This epilepsy-associated transformation in the fundamental properties of this critical regulatory hippocampal circuit may contribute both to seizure propensity, and cognitive and emotional comorbidities characteristic of this disease state.

Keywords: Calcium imaging; Dentate granule cells; Dentate gyrus; Epilepsy; Functional imaging; GABA; Hippocampus; Patch clamp.

FIG. 1

“Gatekeeper” function of the DG is maintained by GABAergic inhibition. Simultaneous voltage-sensitive dye (A), snapshot taken at the peak of the response, (B) trace illustrating the VSD response over time, (C) patch clamp (top) and field potential recording (bottom) of DG response to perforant path activation in control ACSF. (A) Corresponding to a 10–15 mV EPSP in (B), which does not result in activation of downstream structures (note lack of response in area CA3 in (A) and (B)). This lack of CA3 activation is because DGCs do not fire APs in response to perforant path activation under these conditions. This is evident in both the patch (C, top trace, the neuron depolarized to V_m of −50 mV) and field potential recording (C, bottom trace), due to powerful feedforward inhibition activated by perforant path stimulation (C, note large IPSP in patch recording). The importance of inhibition in mediating this “gatekeeper” function is illustrated in responses in (D), (E), and (F), following perfusion with 5 μM picrotoxin, a noncompetitive GABA-A receptor antagonist. This concentration blocks 20–25% of inhibition (see inset [located above (E)] depicting an averaged spontaneous IPSC [sIPSC] before and after perfusion with 5 μM picrotoxin). During 25% GABAergic blockade, perforant path activation resulted in powerful activation of both the DG and downstream structures (CA3 and hilus; D, E). It also triggered AP firing in DGCs (see patch and field potential recordings in (F), both of which exhibit AP firing). A grayscale image of the slice, with patch and field potential recording electrode location is depicted in the inset above (A). From Coulter, D.A., Carlson, G.C., 2007. Functional regulation of the dentate gyrus by GABA-mediated inhibition. Prog. Brain Res. 163, 235–243.

FIG. 2

Postnatal development of DG gating behavior. (A) Top: A schematic illustration depicting subregions of the hippocampus. Bottom: The DG (gray box) is expanded in a VSD image with an overlay of the ROI delineating subregions used to measure DG responses elicited by perforant path stimulation. (B) VSDI time-resolved fluorescence plots for the subregions depicted in (A) for P12 (top) and P60 (bottom) animals. PP stimulation elicits comparable depolarizations in the DGC, hilus, and CA3 at P12, but little depolarization of hilus and CA3 at P60, despite robust responses in DGC. (C) DG response amplitude (ΔF/F) is comparable at all developmental ages (elicited by a 400 μA PP stimulus). (D) P12, P22, and P60 mice (n=8 slices in 3 animals, n=7 of 2 animals, and n=6 of 2 animals, respectively) show progressively less propagation of synaptic responses through DGC (green, light gray in the print version) to hilus (blue, dark gray in the print version) and CA3 (red, gray in the print version). All data points are normalized to DGC layer response at 400 μA, which is equivalent across groups (see C). (E) Plots of DG gating function, the ratio of DGC to CA3 activation intensity, depict the significant increase in the DG gating property as postnatal development progresses, at several stimulus intensities 200 μA (circle), 300 μA (square), and 400 μA (triangle). p=0.001 for the animal age factor affecting gating (two-way ANOVA). p=0.16 for stimulus intensity affecting gating (two-way ANOVA). From Yu, E.P., Dengler, C.G., Frausto, S.F., Putt, M.E., Yue, C., Takano, H., Coulter, D.A., 2013. Protracted postnatal development of sparse, specific dentate granule cell activation in the mouse hippocampus. J. Neurosci. 33, 2947–2960.

FIG. 3

Decreased DGC activation during postnatal development. (A) Top: Schematic of a hippocampal slice depicting the imaged area in the DG and a Fura2-loaded DG of a P12 mouse (370×370 μm). Bottom: Image of P12 and P60 DGCs, with ROI created on a random sample of cells (90×200 μm). Numbers denote cell identification with time-resolved fluorescence responses depicted in (B). (B) Representative traces of time-resolved calcium imaging response for the ROI in the P12 and P60 images in (A). The dotted line indicates the time when PP stimulation (400 μA) occurred. An asterisk indicates detection of a calcium transient. (C) Plot of the percentage DGC activation by PP stimulation for P12, P22, and P60 animals. Note the decrease in cell activation with postnatal development. p=<0.0001 for the animal age factor (ANOVA). *p=<0.05, significant differences between P12 and P22 (Tukey’s multiple comparison post hoc testing). **p=<0.01, significant differences between P12 and P60 groups (Tukey’s multiple comparison post hoc testing). P12: (n=322 PTX-active cells in 10 imaged regions), P22: (n=198 in 10 imaged regions), and P60: (n=239 of 12 imaged regions). From Yu, E.P., Dengler, C.G., Frausto, S.F., Putt, M.E., Yue, C., Takano, H., Coulter, D.A., 2013. Protracted postnatal development of sparse, specific dentate granule cell activation in the mouse hippocampus. J. Neurosci. 33, 2947–2960.