Art - Snutch (R)

407 The electrophysiological and pharmacological diversity of native calcium (Ca2+) channels (L, N, P, Q and T types) is well documented1. These subtypes have different functions; low-voltageactivated T-type channels shape action potentials and generate firing patterns2, whereas L-type channels regulate Ca2+-dependent genes and enzymes3,4, and N-type and P/Q-type channels contribute to neurotransmitter release5–8. High-threshold neuronal Ca2+ channels are heterotrimeric complexes composed of a pore-forming a1 subunit associated with b and a2d subunits. Each of the cloned a1 subunits has distinct functional characteristics that are modulated by coexpression of any of four b subunits9. Channels formed from a1C and a1D subunits have the properties of neuronal dihydropyridine-sensitive L-type channels, and a1B channels encode w-CgTx GVIA-sensitive N-type channels9. The a1E channel shares some properties with both low-threshold channels (high sensitivity to nickel block, permeation Ca2+ > Ba2+) and high-threshold Ca2+ channels (activation at more positive potentials, single-channel conductance > 10 pS)10–12, although the native counterpart of this subunit remains unclear. T-type low-threshold channels are encoded by at least three different a1 subunits (a1G, a1H, a1I) and seem not to require b and a2d subunits for functional expression13,14. P-type Ca2+ channels, originally described in cerebellar Purkinje cells15, are widely distributed16,17 and mediate neurotransmitter release in the central and peripheral nervous systems5–8. Q-type Ca2+ channels, first described in cerebellar granule cells18,19, also mediate neurotransmitter release at some synapses20. Native Pand Q-type channels differ in sensitivity to w-agatoxin IVA (w-Aga IVA; Kd ~2 nM for P-type versus > 100 nM for Q-type) and in their inactivation kinetics. (P-type currents show a non-inactivating waveform during prolonged membrane depolarization, whereas Q-type currents show a pronounced inactivation.) Some native Ca2+ channels have properties related to but distinct from either Por Q-type channels, suggesting a closely related family of channel types21,22. The a1A subunit 23,24 is highly expressed in both Purkinje and cerebellar granule cells and shares several properties with both native Pand Q-type Ca2+ currents25,26. Mutations in the a1A subunit result in several neuropathological conditions, including familial hemiplegic migraine, cerebellar ataxia and epilepsy27–29. The properties of a1A do not exactly match those of either Por Q-type Ca2+ currents, and there has also been no account for their distinct sensitivities to w-Aga IVA. Here we report that alternative splicing at distinct sites within the a1A subunit gene generate multiple types of Pand Q-type conductances that exhibit distinct gating, pharmacology and modulatory characteristics. We also suggest there is little reason that a1A channels should be selectively subdivided into distinct Pversus Qsubtypes.