Résumé
In central nervous system neurons L-type Ca2+ channels are involved in developmental processes, the integration and conduction of postsynaptic electric activity and synaptic plasticity. However, little is known about the channel isoforms underlying each of these functions or about the exact localization and targeting properties of the major L-type channel isoform alpha1C (CaV1.2) in neurons. We addressed these questions using high-resolution immunofluorescence analysis of the endogenous alpha1C and epitope-tagged recombinant channel isoforms expressed in mouse hippocampal neurons. Endogenous alpha1C and surface-expressed hemagglutinin (HA)-tagged alpha1C-HA were localized in small clusters distributed between the axon initial segment and the apical branches of the dendritic tree. The average cluster size was estimated to be eight channels per alpha1C-HA cluster. Analysis of the subcellular localization of alpha1C-HA clusters relative to known synaptic markers suggested the existence of two distinct populations of alpha1C clusters, extrasynaptic and synaptic, the latter associated with glutamatergic synapses in dendritic spines. Both glutamatergic and GABAergic neurons expressed alpha1C in the soma and dendrites. In contrast to the N-type channel GFP-alpha1B, GFP-alpha1C was excluded from distal axons and nerve terminals of mature neurons. In developing neurons, however, alpha1C and alpha1C-HA were robustly expressed in the growth cone, indicating that specific targeting properties of neuronal compartments change during differentiation. Synaptic and extrasynaptic localizations of alpha1C correspond to putative roles of L-type Ca2+ currents in synaptic modulation and in the propagation of dendritic Ca2+ spikes, respectively. The transiently expressed alpha1C in the growth cone may be involved in neurite extension and axonal pathfinding.