CONTRIBUTION OF THE LOW-THRESHOLD T-TYPE CALCIUM CURRENT IN GENERATING THE POST-SPIKE DEPOLARIZING AFTERPOTENTIAL IN DENTATE GRANULE NEURONS OF IMMATURE RATS

1. The underlying ionic mechanisms of the postspike depolarizing after potential (DAP) in hippocampal dentate granule (DG) neurons of immatur e rats (postnatal 7- to 17-day-old) were examined using whole cell pat ch recordings in brain slices. 2. In current-clamp mode, the DAP follo wed each single action potential. Graded DAP-like responses were also evoked by depolarizing current pulses when the action potential was bl ocked by tetrodotoxin (TTX), demonstrating that the TTX-sensitive Naconductance is not necessary for DAP generation. The membrane resistan ce near the DAP peak was lower than at rest, suggesting activation of inward currents rather than blockade of outward currents during the DA P. The DAP peak amplitude showed a strong dependence on voltage, incre asing with membrane hyperpolarization and decreasing with depolarizati on in the range of -90 to -50 mV. Repetitive stimulation at 1-2 Hz did not change the amplitude or decay of the DAP or DAP-like response. 3. Bath application of 2 mM 4-aminopyridine (4-AP) and 5 mM tetraethylam monium chloride (TEA) prolonged the action potential and enhanced the DAP, suggesting that the DAP waveform is determined by the interaction of voltage-activated outward K+ currents and inward currents. 4. Bath application of 2 mM Co2+ depressed the DAP and the DAP-like potential . Replacement of extracellular Ca2+ with Ba2+ potentiated the DAP. Int racellular Ca2+ chelation with the fast chelator, bis-(o-aminophenoxy) -N,N,N',N'-tetraacetic acid (BAPTA), only slightly enhanced the DAP, s uggesting that the DAP is not generated by inward currents activated s econdary to Ca2+ influx. 5. In voltage-clamp mode, a low-threshold tra nsient Ca2+ current (T-type) and high-threshold voltage-activated Ca2 currents were observed. In the conventional whole cell recording mode . the high-threshold currents, but not the T-type current, showed a ti me-dependent rundown, whereas all Ca2+ currents were stable in perfora ted whole cell recordings. The activation of the T-type current, but n ot the high-threshold currents, showed a strong dependence on prepulse potentials ranging from -90 to -50 mV. Repetitive activation of the T -type current up to 2 Hz did not change its amplitude and time course, whereas repetitive activation at 0.2 Hz decreased the high-threshold currents. Bath application of 100 muM Ni2+ or 1 mM amiloride blocked b oth the T-type current and the DAP or DAP-like response, but not the h igh-threshold currents. 6. In summary, both the DAP and the T-type Ca2 + current shared the following properties: 1) dependence of amplitude on the holding potential, 2) stability in the conventional whole cell recording mode and during high-frequency stimulation, and 3) blockade by 100 muM Ni2+ and amiloride. We suggest that the activation of the T -type Ca2+ current predominantly generates the DAP in immature DG neur ons.