Subcellular distribution of L-type calcium channel subtypes in rat hippocampal neurons.

L-type calcium channels play an essential role in synaptic activity-dependent gene expression and are implicated in long-term alterations in synaptic efficacy underlying learning and memory in the hippocampus. The two principal pore-forming subunits of L-type Ca2+ channels expressed in neurons are the Ca(v)1.2 (alpha(1C)) or Ca(v)1.3 (alpha(1D)) subtypes. Experimental evidence suggests that calcium entry through Ca(v)1.2 and Ca(v)1.3 Ca2+ channels occurs in close proximity to key signalling molecules responsible for triggering signalling pathways leading to transcriptional responses. Determining the subcellular distribution of Ca(v)1.2 and Ca(v)1.3 L-type channels in neurons is clearly important for unravelling the molecular mechanisms underlying long-term alterations in neuronal function. In this study, we used immunogold-labelling techniques and electron-microscopy (EM) to analyse the subcellular distribution and density of both Ca(v)1.2 and Ca(v)1.3 Ca2+ channels in rat hippocampal CA1 pyramidal cells in vivo. We confirm that both Ca(v)1.2 and Ca(v)1.3 channel subtypes are predominantly but not exclusively located in postsynaptic dendritic processes and somata. Both Ca(v)1.2 and Ca(v)1.3 are distributed throughout the dendritic tree. However, the smallest (distal) dendritic processes and spines have proportionally more calcium channels inserted into their plasma membrane than located within cytoplasmic compartments indicating the potential targeting of calcium channels to microdomains within neurons. Ca(v)1.2 and Ca(v)1.3 Ca2+ channels are located at the postsynaptic density and also at extra-synaptic sites. The location of L-type Ca(v)1.2 and Ca(v)1.3 channels in distal dendrites and spines would thus place them at appropriate sites where they could initiate synapse to nucleus signalling.