DEVELOPMENT OF GABA-MEDIATED, CHLORIDE-DEPENDENT INHIBITION IN CA1 PYRAMIDAL NEURONS OF IMMATURE RAT HIPPOCAMPAL SLICES

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 (E(IPSP)) 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 GABA(A)-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (E(GABA)) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the E(GABA) upon the patch pipette [Cl-] ([Cl-]p), indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, E(GABA) 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 E(GABA) in older neurones was 8-18 mV more negative. 4. Examination of the relations between E(GABA), 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 E(GABA) in PN15-20 neurones. Raising the temperature moved E(GABA) to a more negative potential, with a Q10 of 5 mV. A similar change of E(GABA) in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABA(A)-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 E(IPSP) and E(GABA) are near the RMP. After the first postnatal week, E(IPSP) and E(GABA) move to potentials more negative than the RMP, partly due to the gradual development of Cl-extrusion system(s), thereby maintaining a driving force for hyperpolarizing IPSPs at the resting potential in the more mature neurones.