REVERBERATION OF CHLORIDE-DEPENDENT SYNAPTIC POTENTIALS IN THE RAT ENTORHINAL CORTEX IN-VITRO

The spontaneous activity generated by rat entorhinal neurons during ap plication of 4-aminopyridine (4AP; 50 mu M) was studied with intracell ular and extracellular field-potential recordings in an vitro slice pr eparation. 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 am plitudes 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 negativ e-going field potentials, suggesting that they represented synchronous events. Membrane input resistance decreased by 79-86% during both LLD s 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- incr eased the amplitude of both types of potentials. Spontaneous LLDs cont inued to occur during application of the non-N-methyl-D-aspartate (NMD A) receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 mu M) , a procedure that abolished the subsequent rhythmic depolarizations ( n = 3). LLDs were blocked by further addition of the gamma-aminobutyri c acid (GABA)(A) receptor antagonist bicuculline methiodide (10 mu M, n = 3), Our findings demonstrate that during 4AP application entorhina l neurons generate glutamatergic-independent LLDs as well as synchrono us, Cl--dependent depolarizations that reverberate through non-NMDA-me diated excitatory circuits.