SUMMARY1. Intracellular recordings were made from rat CA3 hippocampal neurones in vitro during the first eighteen days of postnatal life. The cells had resting membrane potentials more negative than -51 mV, action potentials greater than 55 mV and membrane input resistances of 117 + 12 MQ. An unusual characteristic of these cells was the presence of spontaneous giant depolarizing potentials (GDPs) which were observed during the first eight postnatal (P) days in over 85 % of neurones. They were less frequent between P9 and P12 (48 %) and disappeared after P12.2. The GDPs were synchronously generated by a population of neurones; they reversed polarity at -27 mV when recorded with KCl-containing electrodes and at -51 mV with potassium acetate-or potassium methylsulphate-filled electrodes.3. The GDPs were blocked by bath application of bicuculline (10 ,UM) or picrotoxin (100-200 /bM). Exogenously applied y-aminobutyric acid (GABA; 0 2-1 mM) induced at resting membrane potential a bicuculline-sensitive membrane depolarization which reversed polarity at -25 and -51 mV when recorded with KCl-or potassium methylsulphate-filled electrodes respectively.4. The GDPs were reduced in frequency or blocked by the N-methyl-D-aspartate (NMDA) receptor antagonists DL-2-amino-7-phosphonoheptanoate (AP-7; 50 /IM), D(-)2-amino-5-phosphonovalerate ,#M) and (± )3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 10-50 pM) or NMDA channel blockers phencyclidine (2 fM) and ketamine (20 #M).5. Stimulation of the hilus during the first week of life evoked a GDP followed by a hyperpolarization. The GDPs were generated by a population of synchronized neurones and reversed polarity at -27 mV with KCl-filled electrodes and at -52 mV with potassium acetate-or potassium methylsulphate-containing electrodes. slices, interictal discharges. These reversed polarity near 0 mV with KCl-or potassium acetate-containing electrodes and were reduced in amplitude and duration by AP-5 (50 /M). 8. During the second week of life, stimulation of the hilus evoked, as in adult CA3 cells, an excitatory postsynaptic potential (EPSP) followed by a fast and a slow inhibitory postsynaptic potential (IPSP). Exogenously applied GABA induced at resting membrane potential a hyperpolarization which reversed polarity at -66 mV with potassium methylsulphate-containing electrodes.9. It is concluded that, in early postnatal life, hippocampal CA3 neurones display spontaneous and evoked GDPs which are mediated by GABA and presynaptically controlled by NMDA receptors.

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