Pattern of corticostriatal innervation in organotypic cocultures is dependent on the age of the cortical tissue

Abstract

The patch–matrix organization of the striatum is defined by the selective expression of neuronal markers and a semisegregated pattern of afferents and efferents that develops before birth in all mammals. Differential projections from ‘limbic’ and ‘somatomotor’ cortices contribute to the selective circuitry of patch (“striosome”) and matrix compartments. Organotypic cultures were used to determine the pattern of early corticostriatal innervation as a first step toward understanding the role of cortical innervation in the development of striatal patch–matrix organization. Perinatal striatum (E19–P4) was cocultured with the cortex obtained from same-age or different-age rats in the presence or absence of substantia nigra obtained from E14–15 fetuses. After 4–21 days in vitro, crystals of biocytin were placed directly onto the cortical piece to trace cortical projections into the striatal piece. Cortex obtained from fetuses (E19–22) or neonatal (P0–1) rats gave rise to a dense innervation of both prenatal and postnatal striatal slices; however, the pattern of biocytin-labeled fibers was found to be highly dependent on the age of the cortical tissue used. Cortex derived from rats between E20 and P1 gave rise to a heterogeneous distribution of fibers indicative of striatal patches when combined with striatal slices from same-age or younger (E18–19) fetuses. Cortex from E18–19 fetuses produced a homogeneous innervation even when cocultured with older striatal tissue in which the striatal patches were already present. The postnatal cortex (P2–P5) gave rise to little to no innervation of striatum of all ages. Similar findings were obtained with the use of either prelimbic or somatosensory cortex. In double- and triple-labeled cultures, the distribution of corticostriatal fibers overlapped substantially with patches of developing striatal neurons, as revealed by DARPP-32 immunocytochemistry. Dopaminergic innervation present when the substantia nigra was included in the cocultures also distributed preferentially to the developing patch compartment, but it did not substantially alter the pattern of corticostriatal innervation. These findings suggest that the cortex provides directive signals to the developing striatum rather than simply responding to the presence of patches that have already formed.