Selective blockade of Ca2+ permeable AMPA receptors in CA1 area of rat hippocampus

Using whole cell patch-clamp recording from pyramidal cells and interneurons in the CA1 area of hippocampal slices, the effect of IEM-1460, a selective channel blocker of Ca2+ permeable AMPA receptors (AMPARs), on postsynaptic currents (PSCs) was studied. Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of Schaffer collaterals (SCs) in the presence of APV and bicuculline to pharmacologically isolate the EPSCs mediated by AMPAR activation. IEM-1460 (50 microM) did not affect the amplitude of EPSCs in CA1 pyramidal cells but reversibly decreased their amplitude in interneurons of pyramidal layer (15 cells), radiatum (37 cells) and border radiatum-lacunosum-moleculare (R-LM) (55 cells) layers. The ability of IEM-1460 to decrease EPSC amplitude correlated with EPSC rectification properties in CA1 interneurons, providing evidence for synaptic localization of Ca2+ permeable AMPARs at the SC synaptic input. Independent of their localization, the majority of interneurons studied exhibited only modest sensitivity to IEM-1460 (EPSC amplitude decreased by less than 30%), while in 15% of interneurons IEM-1460 induced more than 50% reduction in EPSC amplitude. To reveal possible afferent-specific localization of Ca2+ permeable AMPARs on R-LM interneurons, the effect of IEM-1460 on EPSCs evoked by stimulation of SC was compared with that of perforant path (PP). Although average sensitivities did not differ significantly, in 61% of R-LM layer interneurons, the SC-evoked EPSCs exhibited higher sensitivity to IEM-1460 than the PP-evoked EPSCs. Moreover, in 54% of R-LM layer interneurons the EPSCs evoked by SC stimulation were complex, having an initial peak followed by one or several late components. Kinetics, latency distribution and reversal potential of late components suggest di- and polysynaptic origin of the late components. Late EPSCs were strongly and reversibly inhibited by IEM-1460 indicating that Ca2+ permeable AMPARs are involved in the indirect excitation of R-LM layer interneurons. Despite the ability to decrease the excitatory synaptic input to interneurons, IEM-1460 did not affect interneuron-mediated inhibitory postsynaptic currents (IPSCs) evoked in pyramidal neurons by SC stimulation. These data suggest that interneurons with a synaptic input highly sensitive to IEM-1460 do not contribute specifically to the feed-forward inhibition of hippocampal pyramidal neurons.