Fig. 4. CCK IN excitation and spike firing is suppressed 15–20 ms after LRIP activation.
A. Confocal projection image (left) showing CCK+ (GFP, green), PV+ (immunostained, magenta) and SOM+ (immunostained, blue) IN soma in a hippocampal section from a CCK-Cre/DLX-Flpe/RCE dual reporter mouse. Note abundant GFP+ CCK IN soma at the SR/SLM border. B. Z-axis projection image (right) of a GFP+ CCK IN at SR/SLM border filled with neurobiotin-Alexa 555 (white). C. Zoomed in image of IN in B, showing GFP (top) and Alexa 555-neurobiotin (middle) colabeling (bottom, yellow). Scale bar,10 µm. D–F. Whole cell voltage recordings from IN in B, C. D. Spike firing and voltage sag in response to 700 ms, 200 pA depolarizing and hyperpolarizing current steps, respectively. E. Depolarizing PSP evoked by SC stimulation. F. Mixed depolarizing and hyperpolarizing PSP evoked by EC stimulation (black). Bath application of NBQX (10 µM) and D-APV (100 µM) blocked the depolarization but not the hyperpolarization (green trace). G. 63× projection image of PSAM (α–BTX-Alexa 647, blue) and ChR2-EGFP (green) showing co-expression in ~75% of EC INs in brain section from a Gad2-Cre mouse injected in LEC and MEC with rAAVCre. H. Experimental scheme showing whole cell recording from a CA1 SR/SLM IN with photostimulation of LRIPs or electrical activation of EC inputs. I. Voltage-clamped IPSCs from CCK+ IN (verified by posthoc staining) evoked by photostimulation of LRIPs in absence (blue trace) or presence (red trace) of PSEM (3 µM). J. Voltage responses in CA1 IN evoked by electrical stimulation of EC (top) or SC (bottom) inputs in absence (control, blue trace) and presence (red trace) of PSEM. K. Voltage responses of CA1 SR/SLM IN to paired electrical stimulation of EC and SC inputs (20 ms delay) in the absence (blue trace) and presence (red trace) of PSEM. L. Mean probability of SR/SLM IN spike firing (percent of stimuli eliciting a spike±SEM) in response to paired EC-SC stimulation as a function of pairing interval in absence and presence of PSEM (spike probability with −20ms EC-SC pairing: control = 18 ± 4%; PSEM = 70 ±10%; P < 0.005, n = 7).