PROPERTIES AND DISTRIBUTION OF SINGLE VOLTAGE-GATED CALCIUM CHANNELS IN ADULT HIPPOCAMPAL-NEURONS

1. The properties of single voltage-gated calcium channels were investigated in acutely exposed CA3 and CA1 pyramidal neurons and granule cells of area dentata in the adult guinea pig hippocampal formation. 2. Guinea pig hippocampal slices were prepared in a conventional manner, then treated with proteolytic enzymes and gently shaken to expose the somata of the three cell types studied. Standard patch-clamp techniques were used to record current flow through calcium channels in cell-attached membrane patches with isotonic barium as the charge carrier. 3. Single-channel current amplitudes were measured at different membrane potentials. Single-channel current-voltage plots were constructed and single-channel slope conductances were found to fall into three classes. These were (approximately) 8, 14, and 25 pS, and were observed in all three cell types. 4. The three groups of channels differed from each other in voltage dependence of activation: from a holding potential of -80, the small-conductance channel began to activate at about -40 to -30 mV, the medium-conductance channel at about -20 mV, and the large-conductance channel at ~0 mV. 5. Ensemble averages of single-channel currents during voltage steps revealed differences in voltage-dependent inactivation. The small-conductance channel inactivated completely within ~50 ms during steps from -80 to -10 mV or more positive. Steps to less positive potentials resulted in less inactivation. The medium-conductance channel displayed variable inactivation during steps from -80 to 0 mV. Inactivation of this channel during a 160-ms step ranged from virtually zero to ~100%. The large-conductance channel displayed no significant inactivation during steps as long as 400 ms. 6. The large-conductance channel was strikingly affected by the dihydropyridine agonist Bay K8644 (0.5-2.0 μM), resulting in a high probability of channel opening, prolonged openings, and an apparent increase in the number of channels available for activation. The medium and small-conductance channels were not noticeably affected by the drug. 7. The large-conductance channel could be induced to open at very negative membrane potentials by holding the patch for several seconds at 20 or 30 mV and stepping to -30 or -40 mV. This process was enhanced by Bay K8644, resulting in prolonged openings at potentials as negative as -100 mV. 8. All three channel types were routinely observed, often in the same patch, in recordings from CA1 and CA3 pyramidal neurons. CA3 and CA1 neurons possesed a similar distribution of channel types, with the exception of the low-conductance channel, which was observed in 72% of CA3 patches but only 32% of CA1 patches. In CA3 neurons, all three types were seen in patches taken from the basilar and apical poles of the soma, as well as the central somatic region. In granule cells, the medium-conductance channel was the predominant type observed; the small-conductance channel was seen occasionally and the large-conductance only rarely, except in the presence of Bay K8644. 9. All three principal cell types of the hippocampal formation thus contain predominantly three types of voltage-gated calcium channels, distinguishable on the basis of single-channel conductance, voltage dependence of activation and inactivation, and sensitivity to the dihydropyridine agonist Bay K8644.