Developmental regulation of EphA4 expression in the chick auditory brainstem

The avian auditory brainstem nuclei nucleus magnocellularis (NM) and nucleu s laminaris (NL) display highly precise patterns of neuronal connectivity. NM projects tonotopically to the dorsal dendrites of ipsilateral NL neurons and to the ventral dendrites of contralateral NL neurons. The precision of this binaural segregation is evident at the earliest developmental stage a t which connections can be observed. We have begun to examine the possibili ty that Eph receptor tyrosine kinase signaling is involved in establishing these spatially segregated connections. The expression of the EphA4 tyrosin e kinase was examined at several developmental stages. EphA4 is expressed i n rhombomere 5, which contains progenitors for both NM and NL. In this rhom bomere, the labeling becomes striped during the time that precursor cells m igrate to the auditory anlage. At the precise time when NM-NL projections a re forming, EphA4 expression in NL is asymmetric, with markedly higher expr ession in the dorsal NL neuropil than in the ventral neuropil, suggesting a possible role in guiding growing axons to the appropriate region. At later embryonic ages EphA4 expression is symmetric around NL, and is absent in N M. As auditory function matures, EphA4 expression decreases so that by 4 da ys after hatch no EphA4 antibody labeling is evident in the auditory brains tem nuclei. (C) 2000 Wiley-Liss, Inc.