Structure and function of neuronal Ca2+ channels and their role in neurotransmitter release.

Electrophysiological studies of neurons reveal different Ca2+ currents designated L-, N-, P-, Q-, R-, and T-type. High-voltage-activated neuronal Ca2+ channels are complexes of a pore-forming alpha 1 subunit of about 190-250 kDa, a transmembrane, disulfide-linked complex of alpha 2 and delta subunits, and an intracellular beta subunit, similar to the alpha 1, alpha 2 delta, and beta subunits previously described for skeletal muscle Ca2+ channels. The primary structures of these subunits have all been determined by homology cDNA cloning using the corresponding subunits of skeletal muscle Ca2+ channels as probes. In most neurons, L-type channels contain alpha 1C or alpha 1D subunits, N-type contain alpha 1B subunits, P- and Q-types contain alternatively spliced forms of alpha 1A subunits, R-type contain alpha 1E subunits, and T-type contain alpha 1G or alpha 1H subunits. Association with different beta subunits also influences Ca2+ channel gating substantially, yielding a remarkable diversity of functionally distinct molecular species of Ca2+ channels in neurons.