Synaptically activated Cl- accumulation responsible for depolarizing GABAergic responses in mature hippocampal neurons

It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABA(A) receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABA(A) receptor-permeable anions, chloride (Cl(-)) and bicarbonate (HCO(3)(-)). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl(-) accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl(-) imaging technique with a Cl(-) indicator 6-methoxy-N-ethylquinolinium iodide (MEQ). The synaptically activated Cl(-) accumulation was mediated dominantly through GABA(A) receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl(-) accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABA(A)-mediated Cl(-) accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl(-) accumulation during synaptic activation.