Figure 1.

Composition of AMPARs at MF synapses on L-M interneurons. A, Schematic illustration of the hippocampal slice preparation showing the position of the concentric bipolar electrode in the SDG to stimulate MFs (MF Stim). In some experiments, a second bipolar electrode was placed in the s. pyramidale to activate the commissural/associational fibers (C/A Stim). The whole-cell recording pipette (Rec) was placed in the stratum lacunosum-moleculare (L-M) of area CA3. B, Computer reconstruction of a biocytin-filled L-M interneuron. Dendritic (red) and axonal arbors (blue) were reconstructed from serial coronal slices using the Neurolucida software. Abbreviations: SP, Stratum pyramidale; SL, stratum lucidum; SR, stratum radiatum; L-M, stratum lacunosum moleculare. C, Scatter plot from one representative experiment showing weak sensitivity of MF EPSPs (≤10%) to PhTx (5 μm). MF EPSPs were reduced by DCG-IV (1 μm) confirming their MF origin. Each dot represents a single MF EPSP evoked at 0.3 Hz. The inset shows average MF EPSP traces (10 sweeps) from the same experiment. MF EPSPs were recorded in the presence of bicuculline (10 μm) and d-AP5 (50 μm). Calibration: 2 mV, 25 ms. D, In a second group of L-M interneurons, MF EPSPs were highly sensitive to PhTx (≥50%) and were also significantly reduced by DCG-IV (1 μm), confirming their MF origin. The insets show average MF EPSP traces (10 sweeps) from the same experiment. Calibration: 2 mV, 25 ms. E, Representative example of a linear current–voltage relationship (I–V) for a CI-AMPAR MF synapse (r = 0.97; left) and a strongly inward-rectifying I–V for a CP-AMPAR MF synapse (right). Calibration: 100 pA, 10 ms. F, Summary graph showing the number CI-AMPA MF synapses on L-M interneurons (N = 36) and CP-AMPA MF synapses (N = 14) and the average block by PhTx and DCG-IV. **p < 0.001 or higher statistical significance. Error bars indicate SEM.