Multiple components of calcium current in acutely dissociated dentate gyrus granule neurons

1. Specific Ca2+ channel blocker were used to isolate and characterize different components of whole cell Ba2+ current in granule neurons acutely dissociated from guinea pig hippocampal slices. 2. Granule cell Ba2+ current peaked around +5 mV, whether elicited by step or ramp commands, and was completely blocked by 100 microM Cd2+. 3. Saturating doses (> or = 5 microM) of the dihydropyridine antagonist, nimodipine, blocked 39% of the total current, and the inhibition was partially reversible. Nimodipine had a greater effect on current at the end of a depolarizing step, suggesting some voltage dependence of its action. 4. Saturating doses (> or = 3 microM) of omega-conotoxin-GVIA irreversibly blocked 21% of the total current and did not occlude subsequent nimodipine inhibition. 5. High concentrations (> or = 100 nM) of omega-agatoxin-IVA irreversibly blocked another 20% of the total current. The effect of omega-Aga-IVA was quite slow, but saturated after several minutes at 200 nM concentration. A high dose (1 microM) produced rapid effects that were used to quantify the magnitude of block. 6. When applied together, all three blockers inhibited nearly the same amount of total current (77%) as would be expected from the sum of each blocker applied individually (80%), suggesting that the three antagonists blocked different channel types. 7. Quantitatively similar results were obtained for the effect of each blocker alone and in combination on currents elicited by depolarizing ramp commands. Ramp currents blocked by each antagonist (difference ramps) all peaked near the same potential as the initial control ramp, indicating very similar activation properties for the three current components. Difference currents elicited by step commands similarly showed little difference among the three components in their kinetics of inactivation during relatively brief (30 ms) depolarizations. With longer steps, however, the omega-CgTX-GVIA-sensitive component showed some inactivation, whereas the omega-Aga-IVA-sensitive current did not inactivate. 8. The component of current resistant to all three blockers (approximately 23% of total current) was not inhibited by omega-conotoxin-MVIIC, but was half blocked by 50 microM Ni2+. This Ni(2+)-sensitive component showed relatively rapid inactivation and peaked at a somewhat lower potential (-10 mV) than the control current. The resistant current was also inactivated by approximately 50% by holding at -60 compared with -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)