Cellular and molecular characterization of Ca²⁺ currents in acutely isolated, adult rat neostriatal neurons

Ca²⁺ currents in acutely isolated, adult rat neostriatal neurons were studied with whole-cell voltage-clamp techniques. In the vast majority of neurons (~90%, n > 250), currents were exclusively of the high-voltage- activated (HVA) type. HVA currents activated near -40 mV and reached their maximum amplitude near 0 mV. Quasi-steady-state inactivation curves in many neurons were well fitted only with a sum of Boltzmann functions, suggesting that the HVA currents were heterogeneous. Although the block of whole-cell current by Cd²⁺ was well fitted with a single isotherm having an IC₅₀ of near 1 μM, experiments with organic channel antagonists suggested that at least four types of HVA channels were expressed by most cells. On average, the L-channel antagonist nifedipine (5-10 μM) blocked 31 ± 10% of the whole-cell current (n = 20), the N-channel antagonist ω-conotoxin GVIA (ω- CgTx) (2-5 μM) blocked 27 ± 11% (n = 20), and the P-channel antagonist ω- agatoxin IVA (100-500 nM) blocked 21 ± 10% (n = 18). In many neurons, the block by ω-CgTx was partially or completely reversible. In cells tested with a combination of these antagonists, 34 ± 17% of the peak Ca²⁺ current remained unblocked (n = 13). Single-cell expression profiling of medium- sized neurons revealed the presence of rbA and rbB Ca²⁺ channel α1 subunit mRNAs but low or undetectable levels of rbC mRNA (n = 12). These findings suggest that although adult neostriatal projection neurons do not express significant levels of LVA Ca²⁺ current, they do express a pharmacologically and structurally heterogeneous population of HVA currents.