Different mechanisms regulate IPSC kinetics in early postnatal and juvenile hippocampal granule cells

1. Monosynaptic inhibitory postsynaptic currents (IPSCs) were recorded from early postnatal and juvenile dentate granule cells in rat brain slices at room temperature. The focally evoked currents were mediated by gamma-aminobutyric acid-A (GABAA) receptors. 2. IPSCs were characterized by a steep rising phase and a slower, monoexponential decay time course. The decay time constant was potential dependent and average values ranged from 33 ms at a holding potential of -60 mV to 58 ms at a holding potential of +40 mV. 3. IPSCs were studied in tissue from animals between postnatal day (p) 3 and p25. All kinetic parameters as well as the mean current amplitude were unchanged during this ontogenetic period. 4. In juvenile granule cells from animals aged 13-16 days, addition of the GABA uptake blocker (R)-N-[4,4-bis (3-methyl-2-thienyl) but-3-en1-yl] nipecotic acid (tiagabine) (10 microM) prolonged the decaying phase of the IPSCs. The current decay remained monoexponential but the time constant increased to 250% of control values. Mean current amplitudes remained largely unchanged. 5. In contrast, tiagabine had no effect on IPSCs in early postnatal tissuĕ. The decay time constant remained unchanged in cells recorded from animals aged p4-p6. Other uptake blockers were also ineffective during the first postnatal week, whereas beta-alanine, NNC-711, and L-2,3-diaminoproprionic acid enhanced the decay time constant in the older tissue (p13-p16). 6. Hypoosmolaric extracellular solution was applied to restrict the extracellular space. In juvenile tissue (p13-p16), IPSCs were not affected by this treatment, whereas early postnatal granule cells (p4-p6) displayed clearly prolonged IPSC decay time constants (165% of control). 7. We conclude that the mechanism governing the kinetics of evoked IPSCs in granule cells changes during ontogenesis. Whereas in early postnatal tissue the transmitter leaves the postsynaptic site by diffusion, GABA uptake becomes time limiting after 2 wk of postnatal development.