WHOLE-CELL AND SINGLE-CHANNEL CALCIUM CURRENTS IN GUINEA-PIG BASAL FOREBRAIN NEURONS

1. Whole-cell and single-channel patch-clamp recordings of calcium (Ca2+) currents were made in acutely dissociated neurons from the medial septum (MS) and nucleus of the diagonal band (nDB) of adult guinea pig. Barium (Ba2+) was used as the charge carrier across the Ca2+ channel and multiple channel types were identified in different cell types. 2. Both low-voltage-activated (LVA) and high-voltage-activated (HVA) currents were distinguished on the basis of steady-state voltage dependence, activation and inactivation properties, and pharmacological sensitivity. HVA currents had activation thresholds similar to 20 mV more positive than LVA currents. Steady-state inactivation of HVA currents was similar to 50% when the holding potential was shifted from -80 to -40 mV. 3. The dihydropyridines had consistent effects on HVA currents. The amplitude was increased and the activation threshold shifted by 10 mV in the hyperpolarizing direction in the presence of the agonist Bay K 8644 (2-5 mu M). The antagonist nifedipine ( 10 mu M) produced similar to 50% inhibition of HVA currents from a holding potential of -80 mV. 4. A second component of the HVA current was blocked by omega-conotoxin (omega-CTX) (300-700 nM). At a holding potential of -80 mV, omega-CTX inhibited 45% of the HVA current. 5. LVA currents were activated near -70 mV and displayed time-dependent inactivation during a 200- to 300-ms voltage step. Voltage-dependent inactivation of LVA currents was also observed and could be described by a single Boltzman relationship with a half-inactivation potential of -84 mV. LVA currents were not significantly changed by Bay K 8644 and were not blocked by low concentrations of nifedipine or omega-CTX. 6. Single voltage-gated Ca2+ channels were investigated using cell-attached patches. In these experiments, 100 mM Ba2+ was used in the patch pipette and the membrane potential was zeroed with isotonic potassium (K+)-aspartate. A low-conductance channel was activated at negative potentials and inactivated rapidly during a 200- to 300-ms voltage step. Unitary amplitudes were determined at different membrane potentials with single-channel conductances calculated to be 7.8+/-1.2 (SD) pS. These channels were not blocked by nifedipine (10 mu M) and appeared similar to T channels previously reported in both peripheral and central neurons. Ensemble averages from cell-attached patches of T channels resembled LVA currents recorded in the whole-cell configuration. 7. A high-conductance channel was activated at membrane potentials similar to 20-30 mV more positive than T channels and displayed very little inactivation during the 200- to 300-ms voltage step. Ensemble averages of high-conductance channels closely resembled the noninactivating nature of HVA whole-cell currents and could be antagonized by nifedipine(10 mu M). In the presence of Bay K 8644 (2 mu M) these channels displayed long openings and had single-channel conductances of 25.9+/-3.4 pS. We conclude that these channels had many properties similar to L channels described in other systems.