Calretinin-immunoreactivity during postnatal development of the rat isocortex: a qualitative and quantitative study

Postnatal development of the rat cortex is characterized by the gradual development of many calcium-dependent processes which demand a precise control of the intracellular levels of this cation; when the balance is disturbed, neuronal death ultimately ensues. Calretinin (CR), a calcium-binding protein, has been postulated to have neuroprotective capacity by buffering intracellular calcium. This putative relationship between CR and neuroprotection is still, however, a controversial issue. With a view to shedding further light on this subject, we studied the temporal and spatial distribution of CR in five different regions (the frontal- sensorimotor-, parietal-, temporal- and occipital region) of the rat cortex during postnatal development. Qualitative and quantitative immunocytochemistry of newborn, 5-, 10-, 15-, 20- and 30-day-old and adult rats revealed a profound increase in the density of the CR-positive neurons during the first two postnatal weeks in all regions examined. At the end of this period, CR-immunoreactive cells decreased sharply to adult levels. Cell classes exhibiting transient CR-immunoreactivity during the first two postnatal weeks included cells in layer I (amongst which were Cajal-Retzius cells), the subplate and pyramidal-like cells in the upper portion of layer V, most of them in the motor cortices. The above-described dynamics of CR expression were reflected also in the biochemical analysis performed (immunoblotting, ELISA). The temporal and spatial correlation with calcium-dependent events such as synaptogenesis, neurite elongation and remodelling in further support the contention that CR may play a neuroprotective role during postnatal development of the rat cortex.