Fig. 5. LRIPs suppress SC-evoked FFI from CCK+ SR/SLM INs.

A. Confocal projection image of a CA1 PN filled with Alexa 594 (red) in a slice where CCK+ INs expressed ChR2-EGFP (green). Blue circle represents the perimeter of 470 nm light stimulus. B. Experimental scheme depicting somatic recording from a CA1 PN (red); electrical stimulation of EC inputs in SLM was paired at variable delays with photostimulation of CCK+ INs. C. IPSCs evoked by photostimulation of CCK INs (hv) recorded from soma of a voltage-clamped CA1 PN (+10 mv) during paired electrical stimulation of EC inputs (arrow) at 0, 10, 20, 30 and 40 ms delays. D. IPSCs in CA1 PNs evoked by electrical stimulation of EC inputs and photostimulation of CCK+ INs. Grey trace (ChR2 only), CA1 PN IPSC evoked by photostimulation of CCK+ IN. Black trace (EC), CA1 PN IPSC evoked by electrical stimulation of EC input. Blue trace (EC+ChR2), net IPSC evoked by pairing EC electrical stimulation with photostimulation of CCK+ IN (20 ms delay). Red trace (difference), Inferred CCK+ IN IPSC evoked when EC electrical stimulation preceded photostimulation of CCK+ IN by 20 ms. Trace obtained by subtracting EC-evoked IPSC (black trace) from IPSC evoked during paired stimulation (blue trace). E. Effect of pairing interval on EC-dependent suppression of IPSC evoked by photostimulation of CCK+ INs or PV+ INs. Mean (±SEM) amplitude of photostimulation-evoked IPSC during pairing with EC stimulation (measured as in D) normalized by photostimulated IPSC amplitude in absence of EC stimulation, plotted versus pairing interval. ChR2-EGFP expressed in either PV+ INs (magenta, 1.01 ± 0.03 fold change at −20 ms pairing interval, P = 0.3319, paired two-tailed t-test, n = 5) or CCK+ INs (green, 0.76 ± 0.03 fold decrease in IPSC at −20 ms pairing interval, P = 0.0006, paired two-tailed t-test, n = 9).