Elevated potassium elicits recurrent surges of large GABA_A-receptormediated post-synaptic currents in hippocampal CA3 pyramidal neurons

Previously, we found that rat hippocampal CA3 interneurons become hyperactive with increasing concentrations of extracellular K⁺ up to 10 mM. However, it is unclear how this enhanced interneuronal activity affects pyramidal neurons. Here we voltage-clamped rat hippocampal CA3 pyramidal neurons in vitro at 0 mV to isolate γ-aminobutyric acid (GABA)-activated inhibitory post-synaptic currents (IPSCs) and measured these in artificial cerebrospinal fluid (aCSF) and with 10 mM K⁺ bath perfusion. In aCSF, small IPSCs were present with amplitudes of 0.053 ± 0.007 nA and a frequency of 0.27 ± 0.14 Hz. With 10 mM K⁺ perfusion, IPSCs increased greatly in frequency and amplitude, culminating in surge events with peak amplitudes of 0.56 ± 0.08 nA, that appeared and disappeared cyclically with durations lasting 2.02 ± 0.37 min repeatedly, up to 10 times over a 30-min bath perfusion of elevated K⁺. These large IPSCs were GABAA-receptor mediated and did not involve significant desensitization of this receptor. Perfusion of a GABA transporter inhibitor (NO-711), glutamate receptor inhibitors CNQX and APV, or a gap junctional blocker (carbenoxolone) prevented the resurgence of large IPSCs. Pressure ejected sucrose resulted in the abolishment of subsequent surges. No elevated K⁺-mediated surges were observed in CA3 interneurons from the stratum oriens layer. In conclusion, these cyclic large IPSC events observable in CA3 pyramidal neurons in 10 mM KCl may be due to transient GABA depletion from continuously active interneuronal afferents.