Figure 2.
OML and IML interneurons are differentially integrated into the excitatory hippocampal circuitry as a consequence of dendritic field reorganization. A, EPSCs in OML and IML interneurons were evoked by subicular (perforant paths; filled circle) or CA3 pyramidal layer (cross) stimulation. In all experiments, an incision was made between the CA1 and CA3. All blue and red traces denote evoked responses from OML and IML, respectively. B, Subicular stimulation elicits a monosynaptic EPSC response in OML interneurons but results in a “flurry” of asynchronous EPSCs in IML interneurons. C, Addition of 10 μm GBZ has no effect on the evoked EPSC response in OML interneurons but results in a large barrage of EPSCs in IML interneurons. Traces on the right are 3–5 min after addition of GBZ. D, Similar results are attained after preincubation of slices with 50 μm picrotoxin (picro). C, D, Arrows highlight the presence of initial EPSC that is temporally very close to the stimulus. E, OML interneurons display an NMDA receptor-mediated outward EPSC, even after the application of 10 μm CNQX, indicating that the response is monosynaptic. In IML interneurons, an outward NMDA receptor-mediated response is observed that is temporally coincident with the initial inward AMPA receptor-mediated EPSC (arrowhead and arrow). This is followed by a barrage of AMPA receptor-mediated EPSCs with corresponding NMDA receptor currents. After CNQX application, only the NMDA receptor response (arrowhead) that was temporally coincident with the initial AMPA receptor response remained, indicating that this was indeed monosynaptic. F, CA3 stimulation in the presence of GBZ elicited no response in OML interneurons, but an EPSC profile similar to that seen after perforant path stimulation (in absence and presence of GBZ) was noted in IML interneurons. G, H, Biocytin filling of OML and IML interneurons shows distinct nonoverlapping dendritic fields. EC, Entorhinal cortex; hf, hippocampal fissure; hil, hilar region; sub, subiculum. Scale bar, 50 μm.