Immunocytochemical localization of the alpha 1A subunit of the P/Q-type calcium channel in the rat cerebellum.

Among various types of low- and high-threshold calcium channels, the high voltage-activated P/Q-type channel is the most abundant in the cerebellum. These P/Q-type channels are involved in the regulation of neurotransmitter release and in the integration of dendritic inputs. We used an antibody specific for the alpha1A subunit of the P/Q-type channel in quantitative pre-embedding immunogold labelling combined with three-dimensional reconstruction to reveal the subcellular distribution of pre- and postsynaptic P/Q-type channels in the rat cerebellum. At the light microscopic level, immunoreactivity for the alpha1A protein was prevalent in the molecular layer, whereas immunostaining was moderate in the somata of Purkinje cells and weak in the granule cell layer. At the electron microscopic level, the most intense immunoreactivity for the alpha1A subunit was found in the presynaptic active zone of parallel fibre varicosities. The dendritic spines of Purkinje cells were also strongly labelled with the highest density of immunoparticles detected within 180 nm from the edge of the asymmetrical parallel fibre-Purkinje cell synapses. By contrast, the immunolabelling was sparse in climbing fibre varicosities and axon terminals of GABAergic cells, and weak and diffuse in dendritic shafts of Purkinje cells. The association of the alpha1A subunit with the glutamatergic parallel fibre-Purkinje cell synapses suggests that presynaptic channels have a major role in the mediation of excitatory neurotransmission, whereas postsynaptic channels are likely to be involved in depolarization-induced generation of local calcium transients in Purkinje cells.