Reverberation of chloride-dependent synaptic potentials in the rat entorhinal cortex in vitro

Abstract

The spontaneous activity generated by rat entorhinal neurons during application of 4-aminopyridine (4AP; 50μM) was studied with intracellular and extracellular field-potential recordings in an vitro slice preparation. Long-lasting depolarizations (LLDs) with amplitudes of 15 ± 7.6 mV (mean ± SD; n = 14) and durations of 1.65 ± 0.77 s (n = 14) occurred at 0.036 ± 0.01/s (n = 14). Each LLD was followed by a rhythmic sequence of depolarizing potentials (up to 22 events) with amplitudes of 4–30 mV, durations of 40–500 ms and frequency of 0.9 ± 0.2/s (n = 14). These intracellular potentials were mirrored by negative-going field potentials, suggesting that they represented synchronous events. Membrane input resistance decreased by 79–86% during both LLDs and subsequent rhythmic depolarizations. Intracellular injection of steady depolarizing or hyperpolarizing current modified the amplitude of these potentials in a similar manner; the reversal potential of the LLDs and of the rhythmic depolarizations was −66.4 ± 4 mV and −67.9 ± 3.2 mV, respectively (n = 7). Intracellular injection of Cl− increased the amplitude of both types of potentials. Spontaneous LLDs continued to occur during application of the non-N-methyl-d-aspartate (NMDA) receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10μM), a procedure that abolished the subsequent rhythmic depolarizations (n = 3). LLDs were blocked by further addition of the γ-aminobutyric acid (GABA)A receptor antagonist bicuculline methiodide (10 μM, n = 3). Our findings demonstrate that during 4AP application entorhinal neurons generate glutamatergic-independent LLDs as well as synchronous, Cl−-dependent depolarizations that reverberate through non-NMDA-mediated excitatory circuits.