5 KCl, 1.25 NaH2PO4, 26 NaHCO3, 1 CaCl2, 5 MgCl2, 20 glucose. Slices (300 μm thick) were cut with a vibratome (Leica, Wetzlar, Germany) and incubated in ACSFsucrose at 35°C for 30 min. Subsequently slices were transferred to CHIR-99021 manufacturer a submerged holding chamber containing normal ACSF solution (in mM: 125 NaCl, 3 KCl, 1.25 NaH2PO4, 26 NaHCO3, 2.6 CaCl2, 1.3 MgCl2, 15 glucose) at room temperature. All extracellular solutions were constantly carbogenized

(95% O2, 5% CO2). Since GABAB receptors play only a minor role in the inhibition mediated by the recurrent inhibitory network in CA1 (Alger and Nicoll, 1982a, 1982b; Newberry and Nicoll, 1984), GABAB receptors were blocked with 1 μM CGP55845 (Tocris) in all experiments. Current-clamp whole-cell recordings were performed at 34 ± 1°C using a DAGAN (BVC-700A) or Multiclamp INK 128 in vitro 700B amplifier (Molecular Devices, Union City, CA) at a 100 kHz sampling rate using a Digidata (1322A, Axon Instruments) interface controlled by pClamp Software (Molecular Devices). Recording pipettes were

pulled with a vertical puller (Narishige PP-830) to 3–5 MΩ resistance resulting in series resistance ranging from 8–25 MΩ. To visualize dendrites we used a water immersion objective (Olympus 60×/NA0.9, Tokyo, Japan) on either a two-photon laser scanning microscope (TRIM Scope II; LaVision Biotec, Bielefeld, Germany) or on a Zeiss Axioskop 2 FS upright microscope with Dodt-contrast infrared illumination (TILLPhotonics, Gräfelfing, Germany). In the latter experimental setup, a monochromator with an integrated light source (TILLPhotonics) was used to excite intracellular Alexa Fluor 488 (Invitrogen). To minimize photo damage during imaging we synchronized acquisition and illumination by repetitively triggering the light source (exposure times ranged from usually 10 to a maximum ADAMTS5 of 30 ms). Most whole-cell recordings were performed using an intracellular solution resembling a physiological chloride driving force (in mM: 140 K-gluconate, 7 KCl, 5

HEPS-acid, 0.5 MgCl2, 5 phosphocreatine, 0.16 EGTA). In some recordings (Figures 2A, S4D–S4G, S6A, and S6B) a lower intracellular Cl− concentration (1 mM) was used. The cell-attached recordings were conducted with an Axopatch 200B amplifier (Molecular Devices) in voltage-clamp mode and patch pipettes (5–7 MΩ resistance) were filled with normal ACSF. To exclusively recruit the recurrent inhibitory interneuron population we electrically stimulated the CA1 pyramidal cell axons in the alveus. To achieve an isolated stimulation of CA1 axons we cut off the subiculum sparing the alveus. In addition, the CA3 subfield was separated. We placed a cluster electrode (CE2F75; FHC, Bowdoin, ME) onto the alveus on the subicular side of the cut and applied 10 (or 15 in some experiments) biphasic current pulses (0.15–0.2 ms, 0.01–0.3 mA) in 100 Hz bursts at theta frequency (5 Hz).