THE WHOLE-CELL CALCIUM CURRENT IN ACUTELY DISSOCIATED MAGNOCELLULAR CHOLINERGIC BASAL FOREBRAIN NEURONS OF THE RAT

1. The electrophysiological and pharmacological characteristics of the calcium current (I(Ca)) in acutely dissociated magnocellular cholinergic basal forebrain neurones from 11- to 14-day-old post-natal rats were studied using the whole-cell patch-clamp technique. 2. All cells exhibited a small transient low-voltage-activated (LVA) current with half-activation and half-inactivation potentials of -40.2 and -49.3 mV and slope factors for activation and inactivation of 4.82 and 3-85 mV per e-fold change in membrane potential (V(m)) respectively. Activation and inactivation rates for the LVA current were highly voltage dependent. For test potential changes from -50 to -20 mV, the time-to-peak of the current decreased from 39.1 to 6.4 ms, and the time constant of current decay decreased from 81.7 to 15.5 ms. 3. A high-voltage-activated (HVA) component of I(Ca) could be elicited at threshold voltages between -46 and -30 mV from a holding potential (V(H)) of -80 mV. The HVA current peaked around 0 mV; a 10-fold increase in [Ca2+]o produced a 13 mV positive shift in the peak, whilst the amplitude of the current showed an approximately hyperbolic relationship to [Ca2+]o with half-saturation at 2.5 mm. The transient phase of the HVA current could be described by two exponential functions with time constants tau(fast) and tau(slow) of 16.2 and 301 ms. Steady-state inactivation of the transient and extrapolated true sustained (pedestal) components of HVA current were described by Boltzmann equations, with half-inactivation potentials (slope factors) of -47.3 mV, (9.04) and -29.2 mV (11.8) respectively. 4. Omega-Conotoxin (omega-CgTX; 100 nm) irreversibly inhibited a kinetically distinct component of HVA current but had no effect upon the transient LVA current. The omega-CgTX-sensitive current could not be distinguished from the control HVA current by the voltage dependence of its activation or inactivation rates. 5. Low concentrations of amiloride (less-than-or-equal-to 300 mum) or Ni2+ (less-than-or-equal-to 5 mum) selectively inhibited the transient LVA current, with IC50 values of 97 and 5 mum respectively. Cd2+ (less-than-or-equal-to 1 mum) selectively blocked a component of HVA current. At higher concentrations, Cd2+ and Ni2+ were non-selective and totally blocked all components of I(Ca). 6. The lanthanide ions Gd3+ and La3+ produced saturable incomplete block of the HVA current. Maximally effective concentrations of Gd3+ (100 mum) or La3+ (30 mum) inhibited 76-5 and 41.2% respectively of the sustained component of HVA current with corresponding IC50 values of 2-2 and 1-1 mum. Low concentrations of Gd3+ (less-than-or-equal-to 3 mum) and La3+ (less-than-or-equal-to 1 muM) blocked a substantial component of the HVA current that was pharmacologically distinct from the omega-CgTX-sensitive current. 7. The transient LVA current was inhibited by dihydropyridine (DHP) calcium channel antagonists. The order of potency was nicardipine > nimodipine = nitrendipine > nifedipine. The corresponding apparent IC50 values were 0.77, 8-2 and 97 mum. Bay K 8644 (1-3 mum) did not potentiate the LVA current. 8. DHP antagonists produced a dose-dependent block of the sustained component of HVA current. The order of potency was nicardipine > nitrendipine > nimodipine > nifedipine with corresponding IC50, values of 7, 14, 61 and 200 mum. 9. Bay K 8644 (1-3 mum) produced a small enhancement (I 1.3 % at 0 mV) of the HVA current at test potentials between - 55 and 0 mV (V(H) - 80 mV). The Bay K enhancement of the current was totally antagonized by concentrations of nifedipine (1-5 mum) that produced little block of the control HVA current. 10. These results show that the calcium current in rat magnocellular cholinergic basal forebrain neurones was composed of LVA and multiple HVA currents which display distinct kinetic and pharmacological characteristics.