DEVELOPMENTAL-CHANGES IN CALRETININ EXPRESSION IN GABAERGIC AND NONGABAERGIC NEURONS IN MONKEY STRIATE CORTEX

The development of the calcium-binding protein calretinin (CaR) and it s co-localization with GABA was studied in the striate cortex of Macac a monkeys from fetal day (Fd) 45 to adult. At Fd45, early neurons rese mbling Cajal-Retzius cells are stained in the marginal zone (MZ). At F d55 the MZ is filled with CaR+ Cajal-Retzius cells and their processes , and scattered CaR+ cells are also found in deep cortical plate (CP), intermediate zone (IZ), and subventricular zone (SVZ). At Fd66, a ban d of CaR+ fibers appears in the IZ, corresponding to the location of t he geniculocortical axons. This fiber band labels heavily until Fd130 but then ceases to be immunoreactive by postnatal (P) 16 weeks. At Fd8 5-101, the number of CaR+ cells in the CP, SVZ, and ventricular zone ( VZ) reaches its highest cell density. After Fd130, CaR+ cells are conc entrated in layer II and upper layer III, and this distribution change s little into adulthood. After mid-gestation, there is a progressive l oss of CaR+ cell bodies and processes in the MZ, and these are rare in the adult cortex. Just before birth, a weakly stained CaR+ cell band appears in layer NA at the border between layer NA and IVB, but this b and disappears immediately after birth. Another CaR+ cell band appears transiently in upper layer V just below the border with layers IV at P6 months. These results suggest that CaR is expressed early in fetal development in the cell populations that are immunoreactive for CaR in the adult. However, developmental events related to cortical maturati on during late prenatal and early postnatal stages result in transient expression of CaR in neurons that are not immunoreactive for CaR in t he adult. CaR-immunoreactivity is colocalized with GABA in almost all CaR+ cells with the exception of Cajal-Retzius cells in the MZ and som e large cells observed at Fd70-101 in the VZ. The band of CaR+ fibers in the IZ is GABA-. At Fd90, almost all (>968) CaR+ cells are GABA+ in the CP and the first developed layers V/VI. This percentage declines later, so that on average 80% of CaR+ cells are GABA+ in adult cortex. At Fd135, 53% of GABA+ neurons located in layers II/III are CaR+; thi s percentage declines to 37% in the adult. These double-label patterns suggest that early in fetal development the majority of GABA+ cells s tain for CaR and that expression of CaR may be related to the migratio n of these neurons into the cortical plate. Once they attain their fin al position in the cortex many GABA+ cells loose CaR-immunoreactivity, so that in postnatal life only a minority of GABA+ neurons contain th is calcium-binding protein. (C) 1995 Wile) -Liss, Inc.