Two distinct pools of large-conductance calcium-activated potassium channels in the somatic plasma membrane of central principal neurons

Although nerve cell membranes are often assumed to be uniform with respect to electrical properties, there is increasing evidence for compartmentalization into subdomains with heterogeneous impacts on the overall cell function. Such microdomains are characterized by specific sets of proteins determining their functional properties. Recently, clustering of large-conductance calcium-activated potassium (BKCa) channels was shown at sites of subsurface membrane cisterns in cerebellar Purkinje cells (PC), where they likely participate in building a subcellular signaling unit, the 'PLasmERosome'. By applying SDS-digested freeze-fracture replica labeling (SDS-FRL) and postembedding immunogold electron microscopy, we have now studied the spatial organization of somatic BKCa channels in neocortical layer 5 pyramidal neurons, principal neurons of the central and basolateral amygdaloid nuclei, hippocampal pyramidal neurons and dentate gyrus (DG) granule cells to establish whether there is a common organizational principle in the distribution of BKCa channels in central principal neurons. In all cell types analyzed, somatic BKCa channels were found to be non-homogenously distributed in the plasma membrane, forming two pools of channels with one pool consisting of clustered channels and the other of scattered channels in the extrasynaptic membrane. Quantitative analysis by means of SDS-FRL revealed that about two-thirds of BKCa channels belong to the scattered pool and about one-third to the clustered pool in principal cell somata. Overall densities of channels in both pools differed in the different cell types analyzed, although being considerably lower compared to cerebellar PC. Postembedding immunogold labeling revealed association of clustered channels with subsurface membrane cisterns and confirmed extrasynaptic localization of scattered channels. This study indicates a common organizational principle for somatic BKCa channels in central principal neurons with the formation of a clustered and a scattered pool of channels, and a cell-type specific density of this channel type.