Development of GABA‐mediated, chloride‐dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices.

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Abstract
1. gamma‐Aminobutyric acid (GABA)‐mediated, Cl(‐)‐dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole‐cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2‐ to 5‐ (PN2‐5), 8‐ to 13‐(PN8‐13) and 15‐ to 20‐(PN15‐20)day‐old CA1 neurones. In the presence of glutamate receptor blockers 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) and D‐2‐amino‐5‐phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2‐5 neurones, but 13 and 25 mV more negative than the RMP in PN8‐13 and PN15‐20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA‐receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl‐] ([Cl‐]p). indicating that the GABA currents were mediated by a Cl‐ conductance. In PN2‐5 neurones, EGABA agreed with the value predicted by the Goldman‐Hodgkin‐Katz equation at given concentrations of internal and external anions permeable through GABA‐activated Cl‐ channels, whereas EGABA in older neurones was 8‐18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl‐]p and resting conductance indicated that the membrane of the PN2‐5 neurones was readily permeable to Cl‐ which followed a passive Donnan equilibrium. Passive distribution of Cl‐ played a decreasing role in PN8‐13 neurones and in PN15‐20 neurones. 5. To assess the contribution of outward Cl‐ co‐transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15‐20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8‐13 neurones. 6. The present data indicate the existence of GABAA‐mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl‐ ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)