Acutely isolated neurons of the rat globus pallidus exhibit four types of high-voltage-activated Ca²⁺ current

1. Large, projection-like neurons from the adult (>3 wk postnatal) rat globus pallidus (GP) were acutely isolated and subjected to whole-cell voltage-clamp (n = 37). Ca²⁺ currents were isolated pharmacologically in cells with whole-cell capacitances of 15-34 pF. 2. With 5 mM Ba²⁺ as a charge carrier, whole-cell currents began to activate near -40 mV and peaked near 0 mV. Based on activation threshold and inactivation kinetics, currents appeared to be of the high-voltage-activated type. 3. Cd²⁺ blocked whole- cell currents with an IC₅₀ near 2 μM. Currents activated at negative potentials were not relatively resistant to Cd²⁺, supporting the inference that low-voltage-activated currents were not prominent in these neurons. 4. The dihydropyridine, L-channel antagonist, nifedipine (5 μM), reduced peak current by 21 ± 4% (SD) (n = 10). The dihydropyridine agonist, BayK 8644 (1- 2 μM) enhanced peak current and slowed current deactivation (n = 4). 5. The N-channel antagonist, ω-conotoxin GVIA (ω-CgTx, 2 μM) blocked 25 ± 7% of the peak whole-cell current (n = 10). The blocks produced by ω-CgTx and nifedipine were additive, blocking an average of 46 ± 8% of the current (n = 10). 6. The current resistant to the selective N- and L-channel antagonists was partially blocked by the P-channel antagonist ω-agatoxin IVA (ω-AgTx, 100 nM). ω-AgTx blocked about one-half of the current not attributable to N- and L-type channels (22 ± 5% of the total current, n = 5). 7. Single cell aRNA amplification (n = 3) revealed the presence of rbC, rbB, and rbA α1 subunit mRNA. 8. Our results suggest that GP projection neurons express four types of HVA current: L, N, P, and a current resistant to the available selective antagonists.