Neurogenesis and stereological morphometry of calretinin-immunoreactive GABAergic interneurons of the neostriatum

We determined the neurogenesis characteristics of a distinct subclass of rat striatum γ‐aminobutyric acidergic (GABAergic) interneurons expressing the calcium‐binding protein calretinin (CR). Timed‐pregnant rats were given an intraperitoneal injection of 5‐bromo‐2′‐deoxyuridine (BrdU), a marker of cell proliferation, on designated days between embryonic day 12 (E12) and E21. CR‐immunoreactive (‐IR) neurons and BrdU‐positive nuclei were labeled in the adult neostriatum by double immunohistochemistry, and the proportion of double‐labeled cells was quantified. CR‐IR interneurons of the neostriatum show maximum birth rates (>10% double labeling) between E14 and E17, with a peak at E15. CR‐IR interneurons occupying the lateral half of the neostriatum become postmitotic prior to medial neurons. In the precomissural neostriatum, the earliest‐born neurons occupy the lateral quadrants and the latest‐born neurons occupy the dorsomedial sector. No significant rostrocaudal neurogenesis gradient is observed. CR‐IR neurons make up 0.5% of the striatal population and are localized in both the patch and the matrix compartments. CR‐IR neurons of the patch compartment are born early (E13–15), with later‐born neurons (E16–18) populating mainly the matrix compartment. CR‐IR cells of the neostriatum are a distinct subclass of interneurons that are born at an intermediate time during striatal development and share common neurogenesis characteristics with other interneurons and projection neurons produced in the ventral telencephalon. J. Comp. Neurol. 469:325–339, 2004. © 2004 Wiley‐Liss, Inc.