Distinct thyroid hormone-dependent expression of TrkB and p75(NGFR) in nonneuronal cells during the critical TH-dependent period of the cochlea

Analyzing the thyroid hromone (TH)-dependent period of the inner ear, we ob served that the presence of triiodothyronine (T3) between postnatal day 3 ( P3) and P12 is sufficient for functional maturation of the auditory system. Within this short time period, an unusual transient TH-dependent expressio n of nonneuronal neurotrophin receptors (NT-R) trkB and p75(NGFR) was obser ved in correlation with neuronal and morphogenetic processes. The availabil ity of thyroid hormone was revealed to be invariably correlated with (a) a transient expression of full-length trkB in TR alpha 1-, TR alpha 2- and TR beta 1-expressing hair cells concomitant to the segregation of afferent fi bers and the synaptogenesis of efferent fibers; and (b) a transient express ion of p75(NGFR) in TR alpha 1- and TR beta 1-expressing great epithelia ri dge cells in direct spatiotemporal correlation with the appearance of apopt otic cells and morphogenetic maturation of the organ. For the first time, t hese data suggest a TH dependency of the expression of neurotrophin recepto rs in nonneuronal cells. A potential role of these peculiar neurotrophin re ceptor expression for the conversion of the biological function of TH on in nervation patterning and morphogenesis during the critical TH-dependent per iod of the inner ear may be considered. (C) 1999 John Wiley & Sons, Inc.