Glutamate receptor channels in isolated patches from CA1 and CA3 pyramidal cells of rat hippocampal slices.

1. Currents activated by glutamate receptor (GluR) agonists were recorded from outside-out patches isolated from the soma of visually identified pyramidal neurones of the CA3 and CA1 region of rat hippocampal slices. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), L-glutamate (L-Glu), and kainate (KA) were delivered either by bath application through perfusion of the recording chamber or by rapid application via a piezo-driven two-barrelled fast application system. 2. Bath application of each of the three agonists activated inward currents in all patches (n = 134) at holding potentials of -50 or -60 mV. The current amplitude increased in size between 3 to 30 microM-AMPA and 100 microM to 1 mM-KA. With this slow mode of bath application, the responses showed no apparent desensitization even at saturating concentrations of AMPA (30 microM) and KA (1 mM). 3. The ratio of currents activated by 30 microM-AMPA and 300 microM-KA showed a characteristic difference between CA3 and CA1 neurones. The ratio was 0.242 +/- 0.028 (mean +/- S.E.M., n = 16) for CA3 cell patches and 0.097 +/- 0.012 (n = 8) for CA1 cell patches indicating that GluRs in the two cell populations are different. 4. The steady-state current-voltage relations (I-Vs) for AMPA- and KA-activated currents showed pronounced outward rectification for both cell types (when the main cations are Na+ in the bath and Cs+ in the pipette solution). The current reversed close to 0 mV and the ratio of chord conductances 80 mV on either side of the reversal potential was 2.66 for KA-activated currents in CA3 cell patches and 2.60 in CA1 cell patches. AMPA-activated currents showed a time-dependent increase after steps to positive membrane potentials and a decrease after steps to negative voltages, indicating that a gating process is responsible for outward rectification of the steady-state I-V. 5. The permeability (P) of GluR channels was high for Na+ as compared to Cs+ for both cell types (PNa/PCs = 0.88 and 0.84). The permeability was low for N-methyl-D-glucamine+ (PNMG/PCs < or = 0.03) and Ca2+ (PCa/PCs < or = 0.05). 6. The current noise level increased during application of AMPA or KA. Apparent single-channel conductances obtained from fluctuation analysis were higher for AMPA than for KA, but similar for both cell types. In CA3 cell patches, AMPA activated channels with an apparent chord conductance of 7.2 pS, KA of 3.0 pS conductance.(ABSTRACT TRUNCATED AT 400 WORDS)