Reelin immunoreactivity in dissociated cultures of the postnatal hippocampus

Reelin is an extracellular matrix glycoprotein expressed in different nerve cell populations in the developing, early postnatal and adult central nervous system. During histogenesis of the neocortex and hippocampus, reelin is present in Cajal-Retzius cells and other early neurons and contributes to correct layering of these regions. During early postnatal life, pioneer neurons disappear and reelin expression establishes in a subpopulation of cortical and hippocampal GABAergic interneurons, where it is maintained throughout adult life. We studied the developmental distribution pattern of reelin in dissociated cultures obtained from the early postnatal hippocampus to verify whether or not such a maturation phenomenon is maintained in vitro. Reelin is expressed both in Cajal-Retzius cells and multipolar and pyramidal neurons in younger cultures. The density of reelin-positive Cajal-Retzius cells dropped drastically by about 84% in 4-week-old cultures. Multipolar and pyramidal neurons containing reelin represented 12% of the total cell population in younger cultures and decreased by about 25% after 3 to 4 weeks of cultivation. Their density was significantly lower in cultures of the same age treated with glutamate receptor antagonists. These reelin-positive multipolar and pyramidal neurons were heterogeneous, including a larger amount of non-GABAergic, and 30-40% of GABAergic neurons. Cells double labeled for reelin and the GABA synthesizing enzyme glutamic acid decarboxylase represented about 4% of the total neuron population in culture and their density remained constant with age. It is thus possible that the decrease in the total reelin population may selectively be of importance to the larger non-GABAergic fraction of reelin cells. This study shows that reelin-expressing neurons are maintained in dissociated cultures of the neonatal hippocampus and their distribution and age-dependent changes in density resemble those of the early postnatal hippocampus in vivo.