Biophysical and pharmacological characterization of voltage-dependent Ca2+ channels in neurons isolated from rat nucleus accumbens.

Abstract

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 approximately 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), omega-conotoxin-GVIA (omega-CgTx) and omega-agatoxin-IVA (omegaAga), which are thought to identify the L, N, and P/Q subtypes of Ca2+ currents, respectively. Nim (5-10 muM) blocked 18%, omegaCgTx (1-2 muM) blocked 25%, and omegaAga (200 nM) blocked 17% of total Ca2+ current. Nim primarily blocked a sustained high-threshold Ca2+ current in a partially reversible manner. In contrast, omegaCgTx irreversibly blocked both sustained and inactivating components. omegaAga irreversibly blocked only a sustained component. In all three of these Ca2+ channel blockers, plus 5 muM omega-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 muM 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.