Ca(2+)-permeable AMPA and NMDA receptor channels in basket cells of rat hippocampal dentate gyrus

1. Glutamate receptor (GluR) channels were studied in basket cells in the dentate gyrus of rat hippocampal slices. Basket cells were identified by their location, dendritic morphology and high frequency of action potentials generated during sustained current injection. 2. Dual-component currents were activated by fast application of glutamate to outside-out membrane patches isolated from basket cell somata (10 μM glycine, no external Mg2+). The fast component was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the slow component by D-2-amino-5-phosphonopentanoic acid (D-AP5). This suggests that the two components were mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR)/kainate receptor and N-methyl-D-aspartate receptor (NMDAR) channels, respectively. The mean ratio of the peak current of the NMDAR component to that of the AMPAR/kainate receptor component was 0.22 (1 ms pulses of 10 mM glutamate). 3. The AMPAR/kainate receptor component, which was studied in isolation in the presence of D-AP5, was identified as AMPAR mediated on the basis of the preferential activation by AMPA as compared with kainate, the weak desensitization of kainate-activated currents, the cross-desensitization between AMPA and kainate, and the reduction of desensitization by cyclothiazide. 4. Deactivation of basket cell AMPARs following 1 ms pulses of glutamate occurred with a time constant (τ) of 1.2 ± 0.1 ms (mean ± S.E.M.). During 100 ms glutamate pulses, AMPARs desensitized with a τ of 3.7 ± 0.2 ms. 5. The peak current-voltage (I-V) relation of AMPAR-mediated currents in Na+-rich extracellular solution showed a reversal potential of -4.0 ± 2.6 mV and was characterized by a doubly rectifying shape. The conductance of single AMPAR channels was estimated as 22.6 ± 1.6 pS using non-stationary fluctuation analysis. AMPARs expressed in hippocampal basket cells mere highly Ca2+ permeable (P(Ca)/P(K) = 1.79). 6. NMDARs in hippocampal basket cells were studied in isolation in the presence of CNQX. Deactivation of NMDARs activated by glutamate pulses occurred bi-exponentially with mean τ values of 266 ± 23 ms (76%) and 2620 ± 383 ms (24%). 7. The peak I-V relation of the NMDAR-mediated component in Na+-rich extracellular solution showed a reversal potential of 1.5 ± 0.6 mV and a region of negative slope at negative membrane potentials in the presence of external Mg2+, due to voltage-dependent block by these ions. The conductance of single NMDAR channels in the main open state was 50.2 ± 1.8 pS. NMDARs in hippocampal basket cells were highly permeable to Ca2+ (P(Ca)/P(K) = 6.68). 8. AMPARs in hippocampal basket cells are characterized by about threefold faster kinetics and twentyfold higher Ca2+ permeability than AMPARs in hippocampal granule or pyramidal cells. Simulations show that the Ca2+ influx through basket cell AMPARs is comparable to that through NMDARs at negative membrane potentials with physiological concentrations of Ca2+ and Mg2+. This suggests a dual pathway of synaptically mediated Ca2+ entry into interneurones.