TRANSIENT APPEARANCE OF CALBINDIN-D28K-POSITIVE NEURONS IN THE SUPERIOR OLIVARY COMPLEX OF DEVELOPING RATS

Calbindin-D28k (CaBP) is a calcium-binding protein that is prominent i n various parts of the mammalian auditory system. In order to shed som e light on the possible role of CaBP during ontogeny, when calcium ion s play key roles in several processes, the location of CaBP was examin ed immunocytochemically in the auditory system of the developing rat. This study focuses on the principal nuclei of the superior olivary com plex, which show distinct CaBP labeling in the adult. Consistent with previous reports in the rat and other mammals, CaBP immunoreactivity i n adults was intense in somata of the medial nucleus of the trapezoid body (MNTB) and in the neuropil (presumably in axons) of the lateral s uperior olive (LSO), the superior paraolivary nucleus (SPN), and the m edial superior olive (MSO). In fetal and neonatal animals, however, th e labeling pattern was strikingly different. Around birth, MNTB neuron s are immunonegative for CaBP, whereas somata and processes in the LSO , probably neuronal, are heavily labeled at that age. This labeling pa ttern persists throughout the first week of postnatal life and begins to change at P8, when MNTB neurons become immunopositive for CaBP. Dur ing the next 10 days labeling intensity in MNTB neurons increases cons iderably, and the increase is paralleled by an increase in labeling in tensity of the neuropil in the LSO, SPN, and MSO, indicating that the labeled processes in these nuclei may be axons originating from MNTB n eurons. Immunoreactivity in LSO cells begins to decline around P8, dec ays rapidly between P10 and P18, and reaches its adult level around P2 8, when the CaBP labeling pattern in the whole superior olivary comple x is indistinguishable from that in the adult. The present results sho w that the development of CaBP immunoreactivity in the rat superior ol ivary complex is characterized by two reciprocally related processes: as immunoreactivity within MNTB somata and fibers in the SPN, the LSO, and the MSO increases between P8 and about P21, the immunoreactivity in LSO neurons declines. CaBP immunoreactivity in LSO neurons is only transiently present, suggesting a critical period in development durin g which the control of Ca2+ homeostasis in LSO neurons may be of parti cular importance. (C) 1993 Wiley-Liss, Inc.