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Long-Range–Projecting GABAergic Neurons Modulate Inhibition in Hippocampus and Entorhinal Cortex

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Science  23 Mar 2012:
Vol. 335, Issue 6075, pp. 1506-1510
DOI: 10.1126/science.1217139

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  1. Fig. 1

    long-range–projecting hippocampal SOM+ neurons target superficial layers in the MEC. (A) Schematic drawing showing the location of retrogradely labeled SOM+ cells in CA1 and DG of the dorsal hippocampus after FG injection into the MEC. (B) Confocal images of a SOM+/FG–labeled neuron in stratum oriens of CA1 (top row) and the hilus of the DG (bottom row). Scale bar, 10 μm. (C) Coexpression of SOM and ChR2-mCherry in CA1 stratum oriens after virus injection into the dorsal hippocampus in a SOMCre mouse. ChR2-mCherry expression was restricted to SOM+ cells (yellow arrows). Scale bar, 30 μm. (D) Digitally encoded (red) mCherry+ axons of long-range–projecting hippocampal SOM+ cells detected in medial septum (MS, left) and contralateral DG (right). Scale bar, 100 μm. (E) Digitally encoded (red) mCherry+ axons of long-range–projecting hippocampal SOM+ cells in the MEC. Projections indicated by numbers are shown as higher magnification. Scale bar, 150 μm. (F) Confocal images of a mCherry and FG double-labeled cell in stratum oriens of the CA1 region after virus injection into dorsal hippocampus and FG injection into the MEC. Scale bar, 10 μm.

  2. Fig. 2

    Hippocampal SOM+-projecting neurons form inhibitory synapses onto GABAergic neurons in the MEC. (A) VGAT+ varicosities (arrows) of mCherry-labeled long-range projections in layer I/II of the MEC (left) and quantification in the indicated layers of the MEC. (Right) Bar histogram representing average number of VGAT+ varicosities (60 optical sections, 2500 μm2 each, mean ± SEM; five hemispheres of three mice). Scale bar, 5 μm. (B) Electronmicrographs showing serial sections of an immunogold-labeled ChR2-mCherry–expressing axon terminal (at) that forms a symmetric synapse onto a dendrite (d). Scale bar, 0.25 μm. (C) Fluorescence image of mCherry-labeled axons in the MEC (left). Asterisk indicates the location of a target cell shown in the DIC image (right). Presynaptic axon was stimulated with blue laser light, and PSCs were recorded in the target cell. Scale bar, 25 μm. (D) Inhibitory PSCs recorded in a target cell at indicated holding potentials. Responses could be blocked by gabazine but not CNQX and AP5. Scale bars, 20 ms and 50 pA. (E) Schematic drawing of a horizontal section showing the location of target neurons (red dots) that were biocytin-filled for subsequent reconstruction (left). Firing pattern most frequently found in MEC target cells (middle). Scale bars, 200 ms and 20 mV. (Right) A representative reconstructed target cell (dendrites, black; axon, red). Scale bar, 100 μm.

  3. Fig. 3

    MEC long-range GABAergic neurons form functional synapses onto GABAergic neurons in the hippocampus. (A) Retrogradely labeled GABAergic cell in the MEC after FG injection into the dorsal hippocampus of a GAD67EGFP mouse. Scale bar, 10 μm. (B) mCherry expression in MEC GABAergic cells subsequent to AAV DIO ChR2-mCherry injection into a GADCre mouse. Scale bar, 500 μm. (C) Schematic drawing of a sagittal hippocampal section indicating areas targeted by long-range–projecting MEC GABAergic neurons (red). (D) Fluorescent axons are located in stratum lacunosum-moleculare (lm) of the CA1 and CA3 areas and in stratum moleculare (mo) of the DG. Scale bars, 25 μm (CA1) and 12.5 μM (CA3 and DG). (E) Electronmicrographs showing serial sections of an immunogold-labeled ChR2-mCherry–expressing axon terminal (at) that forms a symmetric synapse onto a dendrite (d). Scale bar, 0.5 μm. (F) Inhibitory PSCs recorded in a target cell in stratum lacunosum-moleculare at indicated holding potentials. Response was blocked by gabazine but not CNQX or AP5. Scale bars, 20 ms and 50 pA. (G) Schematic drawing of a horizontal section of the intermediate hippocampus indicating the location of responsive target cells (red dots). Firing pattern most frequently found in hippocampal target cells (middle). Scale bars, 200 ms and 20 mV. (Right) Corresponding reconstruction in a sagittal section. Scale bar, 100 μm.

  4. Fig. 4

    Activation of GABAergic long-range projections enhances rhythmic activity in the MEC and the hippocampus. MEC target cells in hippocampal AAV DIO ChR2-mCherry–injected SOMCre mice [(A) to (C), left] and hippocampal target cells in MEC AAV DIO ChR2-mCherry–injected GADCre mice [(A) to (C), right] cells were patched and depolarized to suprathreshold potentials. Long-range projections were stimulated at 8 Hz (red ticks). (A) Overlay of 20 unfiltered traces recorded in a target cell, with indicated enlargement of action potential firing during the stimulation period. Scale bars, 40 mV and 500 ms. Histogram below indicates mean number of spikes ± SEM within the first 62.5-ms interval directly after laser stimulation (red bar) and the subsequent 62.5-ms interval (black bar). (B) Spectrogram showing that activation of long-range–projecting axons entrains target cell to fire rhythmically at theta range frequency. (C) Increase in theta power (7 to 9 Hz) in target cells during laser stimulation. (D) Representative unfiltered (top) and filtered (3 to 7 Hz, bottom) trace of DHPG/NBQX–induced CA1 theta oscillations. (Right) Averaged power of 3- to 7-Hz oscillations normalized to power before stimulation for wild-type (black) and GADCre-injected mice (red). Scale bars, 500 ms and 0.1 mV.