Distribution of inositol-1,4,5-trisphosphate receptor isotypes and ryanodine receptor isotypes during maturation of the rat hippocampus

Abstract

Activation of inositol-1,4,5-trisphosphate receptors (InsP₃Rs) and ryanodine receptors (RyRs) can lead to the release of Ca²⁺ from intracellular stores and propagating Ca²⁺ waves. Previous studies of these proteins in neurons have focused on their distribution in adult tissue, whereas, recent functional studies have examined neural tissue extracted from prenatal and young postnatal animals. In this study we examined the distribution of InsP₃R isotypes 1–3 and RyR isotypes 1–3 in rat hippocampus during postnatal maturation, with a focus on InsP₃R1 because it is most prominent in the hippocampus. InsP₃R1 was observed in pyramidal cells and granule cells, InsP₃R2 immunoreactivity was observed in perivascular astrocytes and endothelial cells, and InsP₃R3 immunoreactivity was detected in axon terminals located in stratum pyramidale of CA1 and microvessels in stratum radiatum. RyR1 immunolabeling was enriched in CA1, RyR2 was most intense in CA3 and the dentate gyrus, and RyR3 immunolabeling was detected in all subfields of the hippocampus, but was most intense in stratum lacunosum-moleculare. During maturation from 2 to 10 weeks of age there was a shift in InsP₃R1 immunoreactivity from a high density in the proximal apical dendrites to a uniform distribution along the dendrites. Independent of age, InsP₃R1 immunoreactivity was observed to form clusters within the primary apical dendrite and at dendritic bifurcations of pyramidal neurons. As CA1 pyramidal neurons matured, InsP₃R1 was often co-localized with the Ca²⁺ binding protein calbindin D-28k. In contrast, InsP₃R1 immunolabel was never co-localized with calbindin D-28k immunopositive interneurons located outside of stratum pyramidale or with parvalbumin, typically found in hippocampal basket cells, suggesting that InsP₃R1s do not play a role in internal Ca²⁺ release in these interneurons. These findings should help to interpret past functional studies and inform future studies examining the characteristics and consequences of InsP₃R-mediated internal Ca²⁺ release and Ca²⁺ waves in hippocampal neurons.