Low-affinity blockade of neuronal N-type CA channels by the spider toxin ω-agatoxin-IVA

Abstract

The recognition of neuronal Ca channel diversity has led to considerable efforts to identify useful classification criteria. Here, we revisit the pharmacological definition of P- and Q-type Ca channels, which is based on their respective high and low sensitivity to the spider ω-agatoxin-IVA (ω-Aga-IVA), using whole-cell recordings of the Ca channel currents carried by 5 mM Ba2+ in isolated rat subthalamic and sympathetic neurons. In subthalamic neurons, ω-Aga-IVA (1 μM) targeted multiple Ca channels. One population was blocked with high potency. These channels carried 50.4 ± 3.4% (n = 5) of the control current and showed the same inactivation kinetics and voltage-dependent high affinity for ω-Aga-IVA as do prototypic P-type Ca channels. Other Ca channels were targeted with weaker potency. This heterogeneous population contributed to 14.0 ± 1.7% (n = 5) of the control current. It included N-type Ca channels as well as highthreshold Ca channels that displayed the pharmacological signature of Q-type Ca channels but resembled P-type Ca channels in their gating properties. N-type Ca current block by ω-Aga-IVA (1 μM) was further investigated in sympathetic neurons, which mainly express this Ca channel type. Block was incomplete (~30% of the control current). Its relief at positive potentials was consistent with ω-Aga-IVA acting as a channel-gating modifier. These effects did not reflect a complete loss of selectivity, because ω-Aga-IVA (1 μM) had no effect on subthalamic Na and K currents or their T- and L-type Ca currents. Our data confirm that ω-Aga-IVA is a selective P-type Ca channel blocker. However, its diminished selectivity in the micromolar range limits its usefulness for functional studies of Q-type Ca channels.