Biophysical and Pharmacological Characterization of Voltage-Dependent Ca2+ Channels in Neurons Isolated From Rat Nucleus Accumbens

Churchill, Dennis and Brian A. MacVicar. Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens. J. Neurophysiol. 79: 635–647, 1998. The nucleus accumbens (NA) has an integrative role in behavior and may mediate addictive and psychotherapeutic drug action. Whole cell recording techniques were used to characterize electrophysiologically and pharmacologically high- and low-threshold voltage-dependent Ca2+ currents in isolated NA neurons. High-threshold Ca2+ currents, which were found in all neurons studied and include both sustained and inactivating components, activated at potentials greater than −50 mV and reached maximal activation at ∼0 mV. In contrast, low-threshold Ca2+ currents activated at voltages greater than −64 mV with maximal activation occurring at −30 mV. These were observed in 42% of acutely isolated neurons. Further pharmacological characterization of high-threshold Ca2+ currents was attempted using nimodipine (Nim), ω-conotoxin-GVIA (ω-CgTx) and ω-agatoxin-IVA (ωAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5–10 μM) blocked 18%, ωCgTx (1–2 μM) blocked 25%, and ωAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, ωCgTx irreversibly blocked both sustained and inactivating components. ωAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 μM ω-conotoxin-MVIIC to eliminate a small unblocked Q-type Ca2+ current (7%), a toxin-resistant high-threshold Ca2+ current remained that was 32% of total Ca2+ current. This current inactivated much more rapidly than the other high-threshold Ca2+ currents, was depressed in 50 μM Ni2+ and reached maximal activation 5–10 mV negative to the toxin-sensitive high-threshold Ca2+ currents. Thus NA neurons have multiple types of high-threshold Ca2+ currents with a large component being the toxin-resistant “R” component.