Differential expression of trkB.T1 and trkB.T2, truncated trkC, and p75(NGFR) in the cochlea prior to hearing function

Prior to the onset of hearing, synchronous cellular, neuronal, and morphoge netic processes participate in the development of a functional cochlea. We have studied the expression of different splice forms of trkB and trkC as w ell as p75(NGFR) in rat and mouse cochlea within this critical developmenta l period, using in situ hybridization, PCR, Northern blotting, and immunohi stochemical analyses. An antibody to full-length trkB receptors proved to d etect full-length trkB receptors as well as truncated trkB.T2 but not trkB. T1 isoforms. Full-length trkB and trkC isoforms as well as trkB.T2 but not trkB.T1 receptors were noted in cochlear neurons. A transient expression of trkB.T1 and trkB.T2 was observed at the epithelial-mesenchymal border of t he spiral ligament during this time. A sequential appearance of trkB.T1, th e low-affinity neurotrophin receptor p75(NGFR), and trkB.T2 in epithelial c ochlear cells correlated with the formation of the inner sulcus of the orga n. A differential expression of presumptive trkB.T2 in hair and supporting cells was observed concomitant with the maturation of the distinct innervat ion pattern of these cells. A gradual shift from p75(NGFR) to truncated trk C receptors in Pillar cells occurred during the formation of the tunnel of Corti. A distinct expression of full-length trkC correlated with the time o f differentiation of the stria vascularis. Finally, an expression of trkB.T 1 and trkB.T2 in oligodendrocytes, full-length trkB and trkC in nerve fiber s, and p75(NGFR) in Schwann cells was noted at the glial interface of the V IIIth nerve during the establishment of the glial transition zone. These va rious transitory neurotrophin receptor expression patterns, which were rela ted to final maturation processes of the cochlea, may provide new insights into the as yet obscure role of neurotrophin receptors in nonneuronal tissu e. (C) 1999 Wiley-Liss, Inc.