Apoptosis underlies various types of cells remodeling during advancement. foundation to apex over developmental period. T3 provided on P0 and P1 advanced the entire system of apoptosis and redesigning by ~4 times. Thyroid hormone receptor β was required for these actions suggesting a receptor-mediated process of initiation of apoptosis. Finally T3 given only at P0 or P1 resulted in deafness in adult mice therefore exposing a transient period of susceptibility to long-term damage in the neonatal auditory system. thyroid hormone receptor β gene (18-20) retard the redesigning of the K252a GER and cause deafness. In humans endemic iodine deficiency (21) early developmental hypothyroidism (22) and mutations (23 24 are associated with hearing loss. Here we statement the timecourse of apoptosis in the cochlea in mice on the 1st two postnatal weeks by analysis of triggered caspase 3 a cysteine protease effector of apoptosis and TUNEL. Furthermore since thyroid hormone is known to promote apoptosis during redesigning in tissues such as the mind and tail in amphibian metamorphosis (25 26 we investigated if ectopic triiodothyronine (T3) alters the program of apoptosis in the GER in mice. 2 Materials and Methods 2.1 Mouse strains Wild type C57BL/6J K252a pups were injected (sc at back of neck) with 1.5 μg of T3 inside a volume of 10 μl or the equivalent volume of saline at P0 and P1. Injections were performed at 2.00 pm and cochleae were collected on the indicated days at approximately 5.00 pm. For studies of TRβ-deficiency gene for T3-induced apotosis in the organ of Corti 3.3 Requirement for the gene for T3-induced cell death in the GER The gene expresses TRβ receptor isoforms in the GER in the neonatal rat and mouse (29 10 and this gene is required to facilitate morphological maturation of the organ of Corti in postnatal mice (18-20). Consequently we tested the hypothesis that is required to mediate T3-induced cell death during remodeling of the GER by comparing the response to T3 in +/+ and TRβ-deficient (gene is required to mediate T3- stimulated cell death in the GER during redesigning of the organ of Corti and support an underlying receptor-mediated mechanism. 3.4 Auditory function after short periods of T3 treatment K252a To determine if the acute T3 treatments in the neonatal period that induce premature cochlear remodeling resulted in long term defects in auditory function the auditory-evoked brainstem response (ABR) was investigated in 6 – 8 week old mice following prior injection Mouse monoclonal to CD3E with T3 at P0 or on later postnatal days (Fig. 5A). Previously we shown that T3 given on four consecutive days (P0 P1 P2 P3) induced auditory problems when analyzed at P28 (28). The present study targeted to determine the windows of sensitivity more precisely. Saline-treated organizations displayed normal ABR thresholds with normal waveforms (Fig. 5B). For any click stimulus (a broad frequency band of 1 1 – 16 kHz) reactions were evoked at a threshold of ~44.0 ± 5.4 (mean ± SEM) dB sound pressure level (dB SPL) in the normal range for mice (30)(Fig. 5C). In contrast a single injection of T3 at P0 resulted in markedly elevated average thresholds of 89.0 ± 3.0 dB SPL (p < 0.001 versus saline-treated controls). Injection of T3 at P1 offered a similarly elevated threshold whereas T3 given two days later on at P2 and P3 produced only a moderate but not significant elevation of ABR thresholds (p = 0.17 versus saline-treated organizations). K252a T3 given at P4 P5 and P6 or later on at P16 and P17 did not significantly switch thresholds compared to saline-treated organizations (Fig. 5C). Number 5 Auditory function in mice following transient short-term treatment with T3 during postnatal development The results showed that T3 treatment at P0 or P1 resulted in a severe elevation of thresholds although the residual waveforms that may be evoked suggested that auditory function in these mice was not completely abrogated. However the magnitude of the residual response that could still be evoked with high intensity stimuli was substantially diminished in mice treated with T3 as neonates. For example using a click stimulus at an intensity of 100 dB which is definitely above threshold for both saline and T3-treated mice the amplitude of the waveform was 3.5 ± 0.9 μV (mean ± SEM) in mice that had been treated with T3 at P0 compared to 12.4 ± 2.0 μV in mice treated with saline (p = 0.002). Checks using pure firmness stimuli (at 8 16 and 32 kHz) that span the range of auditory level of sensitivity in mice also exposed pronounced elevation of.