Fig. 1. Lateral entorhinal cortex provides strong long-range GABAergic inputs to local CA1 inhibitory neurons.

A. LEC and MEC viral injection sites (in green) and their hippocampal target (HC, in grey). B. TdTomato-labelled (magenta) and GFP-labeled (green) axons in SLM of CA1 from LEC and MEC Gad2-Cre⁺ LRIPs, respectively. DAPI stain in blue. C. Scheme of experiment to functionally map impact of LRIPs from LEC or MEC on CA1 INs at SR/SLM border. ChR2-EYFP was virally expressed in GABAergic neurons in the LEC or MEC using rAAV^Cre injections in Gad2-Cre mice. Patch clamp recordings obtained from a CA1 IN (red) at the border of SR/SLM that targets the CA1 PN dendrite (light blue). 470 nm laser light focused on SLM photostimulated ChR2⁺ LRIPs (green). D. 20× confocal image of ChR2-EYFP⁺ LRIP axons from LEC (green) in hippocampus from Gad2-Cre mouse. DAPI staining in blue. E. 63× confocal images showing ChR2-EYFP⁺ LRIP axons from LEC (green) in CA1 SLM region impinging upon tdTomato⁺ IN soma (magenta). F. Light-evoked inhibitory postsynaptic currents (IPSCs) recorded from CA1 SR/SLM INs in normal extracellular solution (control, blue) and in presence of glutamate receptor blockers (10 µM NBQX and 100 µM D-APV, green trace) or GABA receptor antagonists (2 µM SR95531 and 1 µM CGP55845, red trace), see fig. S1 for statistics. G. Bar (Mean ± SEM) and scatter (individual cells) plot of the light-evoked IPSCs (pA, V_m = +10 mV) from responsive CA1 SR/SLM INs with ChR2 expressed in LEC (magenta, 139 ± 24.8 pA, n = 17) or MEC (green, 37.7 ± 4.5 pA, n = 11; P < 0.005, t-test, LEC LRIP versus MEC LRIP).