Transient co-localization of calretinin, parvalbumin, and calbindin-D28K in developing visual cortex of monkey

This paper reports a double-labelling immunocytochemical study of the three calcium-binding proteins calretinin, parvalbumin, and calbindin-D28k in developing and adult Macaca primary visual cortex. In adult visual cortex, each protein marks a subset of GABAergic neurons with a characteristic laminar distribution and virtually no co-localization was found between these three proteins, suggesting that each calcium-binding protein may serve as a marker for one or more cortical subcircuits. The immature visual cortex, immunostained using identical techniques was then analysed to determine if each calcium-binding protein could serve as a developmental marker for these circuits. The Cajal-Retzius cells of layer 1 contained all three proteins during development. Calbindin-D28k and calretinin were co-localized starting at Fd (foetal day) 45 and after Fd125, parvalbumin also was present in the same Cajal-Retzius cells. All three proteins continued to be expressed until the Cajal-Retzius disappeared postnatally. In layers 2-6 calbindin-D28k and calretinin were never co-localized. In contrast, parvalbumin and calretinin were found in neurons of deep layer 3 from Fd 155 to postnatal (P6) weeks with a few persisting even later. Before birth almost all PV+ neurons in layers 4-6 were CaB+, but by P3 weeks only a few PV+/CaB+ neurons remained in layer 4C and these completely disappeared by P6 weeks. Co-localization in layer 4 neurons overlaps the period of ocular dominance segregation, suggesting that the onset of cortical maturity coincides with segregation of calcium-binding proteins within the GABA interneurons.