Differential subcellular localization of SK3‐containing channels in the hippocampus

Abstract

Small-conductance, Ca(2+) -activated K(+) (SK) channels are expressed in the hippocampus where they regulate synaptic responses, plasticity, and learning and memory. To investigate the expression of SK3 (KCNN3) subunits, we determined the developmental profile and subcellular distribution of SK3 in the developing mouse hippocampus using western blots, immunohistochemistry and high-resolution immunoelectron microscopy. The results showed that SK3 expression increased during postnatal development, and that the localization of SK3 changed from being mainly associated with the endoplasmic reticulum and intracellular sites during the first postnatal week to being progressively concentrated in dendritic spines during later stages. In the adult, SK3 was localized mainly in postsynaptic compartments, both at extrasynaptic sites and along the postsynaptic density of excitatory synapses. Double labelling showed that SK3 co-localized with SK2 (KCNN2) and with N-methyl-D-aspartate receptors. Finally, quantitative analysis of SK3 density revealed two subcellular distribution patterns in different hippocampal layers, with SK3 being unevenly distributed in CA1 region of the hippocampus pyramidal cells and homogeneously distributed in dentate gyrus granule cells. Our results revealed a complex cell surface distribution of SK3-containing channels and a distinct developmental program that may influence different hippocampal functions.

Ontogenic Changes and Differential Localization of T-type Ca2+ Channel Subunits Cav3.1 and Cav3.2 in Mouse Hippocampus and Cerebellum

“…The former studies, which did not perform any tests in KO mice for the specificity of their antibodies, showed a strong somata and dendritic labeling in rats, while using specific antibodies we found low labeling in somata but strong in the neuropil in mice. This particular neuropilar labeling pattern has been demonstrated for other ion channels in the brain (see for instance Fernández-Alacid et al, 2011; Ballesteros-Merino et al, 2012, 2014; Ferrándiz-Huertas et al, 2012) by many laboratories. Other than the antibody specificity, the reason for this discrepancy might reflect differences in the species used or in the immunohistochemical techniques employed.…”

“…The non-uniform distribution of Cav3.1 and Cav3.2 shown here seems to be cell type-dependent, as Cav3.1 showed a uniform distribution in relay neurons of the thalamic dorsal lateral geniculate nucleus (Parajuli et al, 2010). Non-uniform distribution patterns has been previously described for different ion channels, including Cav2.3, GIRK and SK channels (Fernández-Alacid et al, 2011; Ballesteros-Merino et al, 2012, 2014; Parajuli et al, 2012). …”

K Ca 2 channel localization and regulation in the axon initial segment

“…For instance, one study has suggested that K Ca 2 channels might be present on the AIS on the basis of increased neuronal excitability that was observed after block of axonal P/Q‐ and N‐type calcium channels (15). More recently, a study demonstrated that K Ca 2 channels are present in the synaptic terminals of CA1 pyramidal neurons (14). At the cellular level, different K Ca 2 channel subtypes occupy different subcompartments in neurons.…”

“…In most neurons, K Ca 2 channels are typically found on dendrites and at much lower levels on the soma (6, 11, 13). Although some reports have suggested that KCa2 channels might be found in the axon (14)—and the axon initial segment (AIS), in particular (15)—this has yet to be definitively established.…”

“…An earlier study has suggested that K Ca 2 channels might be present at the AIS of cortical pyramidal neurons on the basis of indirect electrophysiologic evidence (15). In contrast, an immunogold‐electron microscopy study found that K Ca 2 channels are present on synaptic terminals, which suggests an axonal, but not AIS, localization (14).…”