Maturational and degenerative processes in the organ of Corti after neonatal hypothyroidism

The Induction of Hypothyroidism During Gestation Decreases Outer Hair Cell Motility in Rat Offspring

Introduction Perinatal hypothyroidism has a negative repercussion on the development and maturation of auditory system function. However, its long-term effect on auditory function remains unsettled.

Objective To evaluate the effect of prenatal hypothyroidism on the auditory function of adult offspring in rats.

Methods Pregnant Wistar rats were given the antithyroid drug methimazole (0.02% -1-methylimidazole-2-thiol– MMI) in drinking water, ad libitum, from gestational day (GD) 9 to postnatal day 15 (PND15). Anesthetized offspring from MMI-treated dams (OMTD) and control rats were evaluated by tympanometry, distortion product otoacoustic emission (DPOAE), and auditory brainstem response (ABR) at PNDs 30, 60, 90, and 120.

Results Our data demonstrated no middle ear dysfunction, with the OMTD compliance lower than that of the control group. The DPOAE revealed the absence of outer hair cells function, and the ABR showed normal integrity of neural auditory pathways up to brainstem level in the central nervous system. Furthermore, in the OMTD group, hearing loss was characterized by a higher electrophysiological threshold.

Conclusion Our data suggest that perinatal hypothyroidism leads to irreversible damage to cochlear function in offspring.

Genetic variation in thyroid folliculogenesis influences susceptibility to hypothyroidism-induced hearing impairment

Maternal and fetal sources of thyroid hormone are important for the development of many organ systems. Thyroid hormone deficiency causes variable intellectual disability and hearing impairment in mouse and man, but the basis for this variation is not clear. To explore this variation, we studied two thyroid hormone-deficient mouse mutants with mutations in pituitary-specific transcription factors, POU1F1 and PROP1, that render them unable to produce thyroid stimulating hormone. DW/J-Pou1f1^dw/dw mice have profound deafness and both neurosensory and conductive hearing impairment, while DF/B-Prop1^df/df mice have modest elevations in hearing thresholds consistent with developmental delay, eventually achieving normal hearing ability. The thyroid glands of Pou1f1 mutants are more severely affected than those of Prop1^df/df mice, and they produce less thyroglobulin during the neonatal period critical for establishing hearing. We previously crossed DW/J-Pou1f1^dw/+ and Cast/Ei mice and mapped a major locus on Chromosome 2 that protects against hypothyroidism-induced hearing impairment in Pou1f1^dw/dw mice: modifier of dw hearing (Mdwh). Here we refine the location of Mdwh by genotyping 196 animals with 876 informative SNPs, and we conduct novel mapping with a DW/J-Pou1f1^dw/+ and 129/P2 cross that reveals 129/P2 mice also have a protective Mdwh locus. Using DNA sequencing of DW/J and DF/B strains, we determined that the genes important for thyroid gland function within Mdwh vary in amino acid sequence between strains that are susceptible or resistant to hypothyroidism-induced hearing impairment. These results suggest that the variable effects of congenital hypothyroidism on the development of hearing ability are attributable to genetic variation in postnatal thyroid gland folliculogenesis and function.

Moderate perinatal thyroid hormone insufficiency alters visual system function in adult rats

Thyroid hormone (TH) is critical for many aspects of neurodevelopment and can be disrupted by a variety of environmental contaminants. Sensory systems, including audition and vision are vulnerable to TH insufficiencies, but little data are available on visual system development at less than severe levels of TH deprivation. The goal of the current experiments was to explore dose-response relations between graded levels of TH insufficiency during development and the visual function of adult offspring. Pregnant Long Evans rats received 0 or 3 ppm (Experiment 1), or 0, 1, 2, or 3 ppm (Experiment 2) of propylthiouracil (PTU), an inhibitor of thyroid hormone synthesis, in drinking water from gestation day (GD) 6 to postnatal day (PN) 21. Treatment with PTU caused dose-related reductions of serum T4, with recovery on termination of exposure, and euthyroidism by the time of visual function testing. Tests of retinal (electroretinograms; ERGs) and visual cortex (visual evoked potentials; VEPs) function were assessed in adult offspring. Dark-adapted ERG a-waves, reflecting rod photoreceptors, were increased in amplitude by PTU. Light-adapted green flicker ERGs, reflecting M-cone photoreceptors, were reduced by PTU exposure. UV-flicker ERGs, reflecting S-cones, were not altered. Pattern-elicited VEPs were significantly reduced by 2 and 3 ppm PTU across a range of stimulus contrast values. The slope of VEP amplitude-log contrast functions was reduced by PTU, suggesting impaired visual contrast gain. Visual contrast gain primarily reflects function of visual cortex, and is responsible for adjusting sensitivity of perceptual mechanisms in response to changing visual scenes. The results indicate that moderate levels of pre-and post-natal TH insufficiency led to alterations in visual function of adult rats, including both retinal and visual cortex sites of dysfunction.

Thyroid hormone is required for the pruning of afferent type II spiral ganglion neurons in the mouse cochlea

Afferent connections to the sensory inner (IHCs) and outer hair cells (OHCs) in the cochlea refine and functionally mature during the thyroid hormone (TH)-critical period of inner ear development that occurs perinatally in rodents. In this study, we investigated the effects of hypothyroidism on afferent type II innervation to outer hair cells using the Snell dwarf mouse (Pit1(dw)). Using a transgenic approach to specifically label type II spiral ganglion neurons (SGNs), we found that lack of TH causes persistence of excess type II SGN connections to the OHCs, as well as continued expression of the hair cell functional marker, otoferlin (OTOF), in the OHCs beyond the maturation period. We also observed a concurrent delay in efferent attachment to the OHCs. Supplementing with TH during the early postnatal period from postnatal day (P) 3 to P4 reversed the defect in type II SGN pruning but did not alter OTOF expression. Our results show that hypothyroidism causes a defect in the large-scale pruning of afferent type II SGNs in the cochlea, and a delay in efferent attachment and the maturation of OTOF expression. Our data suggest that the state of maturation of hair cells, as determined by OTOF expression, may not regulate the pruning of their afferent innervation.

Age-Related Hearing Loss and Degeneration of Cochlear Hair Cells in Mice Lacking Thyroid Hormone Receptor β1

A key function of the thyroid hormone receptor β (Thrb) gene is in the development of auditory function. However, the roles of the 2 receptor isoforms, TRβ1 and TRβ2, expressed by the Thrb gene are unclear, and it is unknown whether these isoforms promote the maintenance as well as development of hearing. We investigated the function of TRβ1 in mice with a Thrb(b1) reporter allele that expresses β-galactosidase instead of TRβ1. In the immature cochlea, β-galactosidase was detected in the greater epithelial ridge, sensory hair cells, spiral ligament, and spiral ganglion and in adulthood, at low levels in the hair cells, support cells and root cells of the outer sulcus. Although deletion of all TRβ isoforms causes severe, early-onset deafness, deletion of TRβ1 or TRβ2 individually caused no obvious hearing loss in juvenile mice. However, over subsequent months, TRβ1 deficiency resulted in progressive loss of hearing and loss of hair cells. TRβ1-deficient mice had minimal changes in serum thyroid hormone and thyrotropin levels, indicating that hormonal imbalances were unlikely to cause hearing loss. The results suggest mutually shared roles for TRβ1 and TRβ2 in cochlear development and an unexpected requirement for TRβ1 in the maintenance of hearing in adulthood.

Otoferlin couples to clathrin-mediated endocytosis in mature cochlear inner hair cells

The encoding of auditory information with indefatigable precision requires efficient resupply of vesicles at inner hair cell (IHC) ribbon synapses. Otoferlin, a transmembrane protein responsible for deafness in DFNB9 families, has been postulated to act as a calcium sensor for exocytosis as well as to be involved in rapid vesicle replenishment of IHCs. However, the molecular basis of vesicle recycling in IHCs is largely unknown. In the present study, we used high-resolution liquid chromatography coupled with mass spectrometry to copurify otoferlin interaction partners in the mammalian cochlea. We identified multiple subunits of the adaptor protein complex AP-2 (CLAP), an essential component of clathrin-mediated endocytosis, as binding partners of otoferlin in rats and mice. The interaction between otoferlin and AP-2 was confirmed by coimmunoprecipitation. We also found that AP-2 interacts with myosin VI, another otoferlin binding partner important for clathrin-mediated endocytosis (CME). The expression of AP-2 in IHCs was verified by reverse transcription PCR. Confocal microscopy experiments revealed that the expression of AP-2 and its colocalization with otoferlin is confined to mature IHCs. When CME was inhibited by blocking dynamin action, real-time changes in membrane capacitance showed impaired synaptic vesicle replenishment in mature but not immature IHCs. We suggest that an otoferlin-AP-2 interaction drives Ca(2+)- and stimulus-dependent compensating CME in mature IHCs.

Generation of somatic electromechanical force by outer hair cells may be influenced by prestin-CASK interaction at the basal junction with the Deiter's cell

The motor protein, prestin, situated in the basolateral plasma membrane of cochlear outer hair cells (OHCs), underlies the generation of somatic, voltage-driven mechanical force, the basis for the exquisite sensitivity, frequency selectivity and dynamic range of mammalian hearing. The molecular and structural basis of the ontogenetic development of this electromechanical force has remained elusive. The present study demonstrates that this force is significantly reduced when the immature subcellular distribution of prestin found along the entire plasma membrane persists into maturity, as has been described in previous studies under hypothyroidism. This observation suggests that cochlear amplification is critically dependent on the surface expression and distribution of prestin. Searching for proteins involved in organizing the subcellular localization of prestin to the basolateral plasma membrane, we identified cochlear expression of a novel truncated prestin splice isoform named prestin 9b (Slc26A5d) that contains a putative PDZ domain-binding motif. Using prestin 9b as the bait in a yeast two-hybrid assay, we identified a calcium/calmodulin-dependent serine protein kinase (CASK) as an interaction partner of prestin. Co-immunoprecipitation assays showed that CASK and prestin 9b can interact with full-length prestin. CASK was co-localized with prestin in a membrane domain where prestin-expressing OHC membrane abuts prestin-free OHC membrane, but was absent from this area for thyroid hormone deficiency. These findings suggest that CASK and the truncated prestin splice isoform contribute to confinement of prestin to the basolateral region of the plasma membrane. By means of such an interaction, the basal junction region between the OHC and its Deiter's cell may contribute to efficient generation of somatic electromechanical force.

Irreversible damage to auditory system functions caused by perinatal hypothyroidism in rats

We examined the effect of perinatal hypothyroidism on auditory function in rats using a prepulse inhibition paradigm. Pregnant rats were treated with the antithyroid drug methimazole (1-methyl-2-mercaptoimidazole) from gestational day 15 to postnatal day 21 via drinking water at concentrations (w/v) of 0 (control), 0.002 (low dose), or 0.02% (high dose). Rats from methimazole-treated mothers were tested at ages 1, 6, and 12months using techniques to examine prepulse inhibition and startle response. The startle stimulus consisted of 40ms of white noise at 115dB, whereas the prepulse, which preceded the startle stimulus by 30ms, consisted of 20ms of white noise at 75, 85, or 95dB. When the prepulse intensity was 75 or 85dB, the high-dose group showed decreased prepulse inhibition percentages compared with the control and low-dose groups. The reduced percentages of prepulse inhibition did not return to control levels over the 12-month study period. In contrast, no differences in prepulse inhibition were observed among the three dose groups when prepulse intensity was 95dB. Moreover, the high-dose group displayed excessive reaction to auditory startle stimuli compared with the other groups. Reductions in plasma free thyroxine and body weight gain were observed in the high-dose group. We conclude that perinatal hypothyroidism results in irreversible damage to auditory function in rats.

Deafness and permanently reduced potassium channel gene expression and function in hypothyroid Pit1dw mutants

The absence of thyroid hormone (TH) during late gestation and early infancy can cause irreparable deafness in both humans and rodents. A variety of rodent models have been used in an effort to identify the underlying molecular mechanism. Here, we characterize a mouse model of secondary hypothyroidism, pituitary transcription factor 1 (Pit1(dw)), which has profound, congenital deafness that is rescued by oral TH replacement. These mutants have tectorial membrane abnormalities, including a prominent Hensen's stripe, elevated beta-tectorin composition, and disrupted striated-sheet matrix. They lack distortion product otoacoustic emissions and cochlear microphonic responses, and exhibit reduced endocochlear potentials, suggesting defects in outer hair cell function and potassium recycling. Auditory system and hair cell physiology, histology, and anatomy studies reveal novel defects of hormone deficiency related to deafness: (1) permanently impaired expression of KCNJ10 in the stria vascularis of Pit1(dw) mice, which likely contributes to the reduced endocochlear potential, (2) significant outer hair cell loss in the mutants, which may result from cellular stress induced by the lower KCNQ4 expression and current levels in Pit1(dw) mutant outer hair cells, and (3) sensory and strial cell deterioration, which may have implications for thyroid hormone dysregulation in age-related hearing impairment. In summary, we suggest that these defects in outer hair cell and strial cell function are important contributors to the hearing impairment in Pit1(dw) mice.

Hypothyroidism impairs chloride homeostasis and onset of inhibitory neurotransmission in developing auditory brainstem and hippocampal neurons

Thyroid hormone (TH) deficiency during perinatal life causes a multitude of functional and morphological deficits in the brain. In rats and mice, TH dependency of neural maturation is particularly evident during the first 1-2 weeks of postnatal development. During the same period, synaptic transmission via the inhibitory transmitters glycine and GABA changes from excitatory depolarizing effects to inhibitory hyperpolarizing ones in most neurons [depolarizing-hyperpolarizing (D/H) shift]. The D/H shift is caused by the activation of the K(+)-Cl(-) co-transporter KCC2 which extrudes Cl(-) from the cytosol, thus generating an inward-directed electrochemical Cl(-) gradient. Here we analyzed whether the D/H shift and, consequently, the onset of inhibitory neurotransmission are influenced by TH. Gramicidin perforated-patch recordings from auditory brainstem neurons of experimentally hypothyroid rats revealed depolarizing glycine effects until postnatal day (P)11, i.e. almost 1 week longer than in control rats, in which the D/H shift occurred at approximately P5-6. Likewise, until P12-13 the equilibrium potential E(Gly) in hypothyroids was more positive than the membrane resting potential. Normal E(Gly) could be restored upon TH substitution in P11-12 hypothyroids. These data demonstrate a disturbed Cl(-) homeostasis following TH deficiency and point to a delayed onset of synaptic inhibition. Interestingly, immunohistochemistry demonstrated an unchanged KCC2 distribution in hypothyroids, implying that TH deficiency did not affect KCC2 gene expression but may have impaired the functional status of KCC2. Hippocampal neurons of hypothyroid P16-17 rats also demonstrated an impaired Cl(-) homeostasis, indicating that TH may have promoted the D/H shift and maturation of synaptic inhibition throughout the brain.

The influence of thyroid hormone deficiency on the development of cochlear nonlinearities

It is well known that failure to treat severe congenital hypothyroidism leads to profound auditory disability, and it has been suggested that an intracochlear defect, or defects, associated with the condition diminishes the efficacy of an active, physiologically vulnerable nonlinear transduction process commonly referred to as cochlear amplification. We address this question directly by tracking the development of threshold-frequency (tuning) curves and two-tone suppression in hypothyroid, Tshr mutant mice born to hypothyroid dams and comparing those findings with findings observed in euthyroid mice. Like sharp tuning, two-tone suppression is a product of transduction nonlinearity and is a useful indicator of the functional status of cochlear amplification. In contrast to euthyroid mice that acquire sharp tuning, normal two-tone suppression, and adultlike sensitivity by the end of the third postnatal week, as shown in earlier studies, hypothyroid mice remained grossly insensitive to sound throughout life. In addition, tuning was generally broad in hypothyroid mice, tuning curve "tips" were frequently missing, and two-tone suppression was rarely observed. However, unlike tip thresholds, tuning curve "tail" thresholds, a feature that reflects the functional status of passive elements of transduction, acquired normal values over a roughly 2-month postnatal time period. These observations collectively suggest that active transduction micromechanics, at least in the frequency region studied here, are profoundly affected by thyroid hormone and support speculation that abnormal outer hair cell function may be the cause of the primary, enduring peripheral auditory defect associated with profound, congenital hypothyroidism in the Tshr mutant mouse.

Developmental neurotoxicity of propylthiouracil (PTU) in rats: relationship between transient hypothyroxinemia during development and long-lasting behavioural and functional changes

Markedly lowered thyroid hormone levels during development may influence a child's behaviour, intellect, and auditory function. Recent studies, indicating that even small changes in the mother's thyroid hormone status early in pregnancy may cause adverse effects on her child, have lead to increased concern for thyroid hormone disrupting chemicals in the environment. The overall aim of the study was therefore to provide a detailed knowledge on the relationship between thyroid hormone levels during development and long-lasting effects on behaviour and hearing. Groups of 16-17 pregnant rats (HanTac:WH) were dosed with PTU (0, 0.8, 1.6 or 2.4 mg/kg/day) from gestation day (GD) 7 to postnatal day (PND) 17, and the physiological and behavioural development of rat offspring was assessed. Both dams and pups in the higher dose groups had markedly decreased thyroxine (T(4)) levels during the dosing period, and the weight and histology of the thyroid glands were severely affected. PTU exposure caused motor activity levels to decrease on PND 14, and to increase on PND 23 and in adulthood. In the adult offspring, learning and memory was impaired in the two highest dose groups when tested in the radial arm maze, and auditory function was impaired in the highest dose group. Generally, the results showed that PTU-induced hypothyroxinemia influenced the developing rat brain, and that all effects on behaviour and loss of hearing in the adult offspring were significantly correlated to reductions in T(4) during development. This supports the hypothesis that decreased T(4) may be a relevant predictor for long-lasting developmental neurotoxicity.

Consequences of combined maternal, fetal and persistent postnatal hypothyroidism on the development of auditory function in Tshrhyt mutant mice

Tshrhyt/hyt mutant mice express a point mutation in the gene encoding the thyrotropin receptor, and affected animals are congenitally hypothyroid and profoundly deaf as a consequence when the condition is untreated. In this investigation, a previously unrecognized developmental stage was identified in the hypothyroid, mutant progeny of hypothyroid dams by tracking developmental changes in the auditory brainstem response (ABR). ABR thresholds develop rapidly in normal, euthyroid animals, decreasing as much as 80 dB between P12 (postnatal day 12) and P15, with mature sensitivity being gradually acquired by P18. In contrast, Tshrhyt/hyt mutant mice remained profoundly deaf on P24 and although thresholds improved by approximately 30 dB by P60, residual frequency-dependent deficits of 20-70 dB were observed in animals exhibiting end-stage disease. The rate of threshold improvement in mutant mice was approximately ten times slower than in normal mice. While ABR wave latencies and interpeak intervals decreased early in postnatal life, values decreased over a delayed and protracted time period, reaching adult values well after those of controls attained maturity. As with normal mice, slopes of wave I latency-intensity curves were significantly steeper in immature animals than those observed in adults and decreased during development, but failed to achieve normal adult values and remained significantly steeper than those for controls. Findings reported here suggest that passive aspects of electromechanical transduction achieve maturity in Tshrhyt/hyt progeny of Tshrhyt/hyt mice and that development, limited as it may be, occurs most prominently in the basal half of the cochlea.

Prenatal cocaine exposure accelerates morphological changes and transient expression of tyrosine hydroxylase in the cochlea of developing rats

Prenatal cocaine exposure causes alterations in auditory brainstem response in children and experimental animals and has adverse effects on auditory information processing and language skills in children. These effects may result from lesions in the cochlea since this organ is particularly sensitive to chemical insults during the development. We have thus studied here the effect of prenatal cocaine exposure on the maturation of the rat cochlea using the transient non-catecholaminergic expression of tyrosine hydroxylase in spiral ganglion neurons as an index of cochlear maturation and morphometry to evaluate the maturation of primary auditory neurons and the organ of Corti. We showed that prenatal cocaine exposure accelerated the cochlear maturation. In the basal coil of cochleas from PND8 cocaine-treated pups, the Kölliker's organ had disappeared, the tunnel of Corti was opened, and the stria vascularis no longer contained undifferentiated marginal cells. The maximum expression of tyrosine hydroxylase in type I primary auditory neurons occurred at PND8 instead of PND12 in pair-fed controls. On the other hand, the prenatal cocaine exposure had no effect on the width and height of the organ of Corti, spiral ganglion volume and number and size of primary auditory neurons. In conclusion, our data suggest that prenatal cocaine exposure, though not lethal to primary auditory neurons, accelerates aspects of the cochlear sensorineural maturation. This accelerated cochlear maturation in cocaine-treated rat pups could cause auditory dysfunctions by desynchronizing the development of the whole auditory pathway.

Effects of hypothyroidism induced by perinatal exposure to PTU on rat behavior and synaptic gene expression

Hypothyroidism in the rat induced by perinatal exposure to propylthiouracil (PTU) is a useful animal model to study molecular changes underlying neurobehavioral defects associated with this condition. Understanding the developmental alterations in gene expression related to the neurobehavioral dysfunction should help to identify molecular markers for developmental neurotoxicity at an early stage of development. In the present study, we evaluate the effects of PTU on the expression of a set of genes implicated in neural network formation or synaptic function at a minimal dose of PTU causing behavioral alteration. Various doses of PTU were administered to dams from late pregnancy to the lactation period and the expression of selected genes in the hippocampus and the cerebral cortex of offspring was examined by quantitative RT-PCR. Behavioral performance of PTU-treated rats was also assessed. PTU-treated rats showed increased motor activity and impairment of E-maze learning at weaning and after maturation. At doses causing such behavioral alteration, expression of GAP-43 and M1 mRNAs was changed during neuronal network formation, suggesting that levels of these factors during development are important for accurate postnatal development and function.

Hearing Loss in Athyroid Pax8 Knockout Mice and Effects of Thyroxine Substitution

Pax8-/- mice do not develop thyroid follicular structures and thus provide an ideal animal model to study the consequences of congenital hypothyroidism. Despite their athyroidism, Pax8-/- mice survive up to postnatal day 21 (P21). No auditory brain stem responses (ABR) to sound could be recorded in these animals at 130 dB SPL, even at P21, when hearing reaches adult sensitivity in control mice. Abnormalities in the outer and middle ear structures were found in a considerable percentage of Pax8-/- animals. Maturation of the inner ear appeared delayed by about 1 week with respect to euthyroid controls. Hearing of adult Pax8-/- mice could be nearly normalized by early postnatal substitution with thyroxine (T(4)), but structural and functional restoration of hearing was incomplete. Even when T(4) substitution was initiated at P1, ABR thresholds, measured at 6 weeks of age or more, were increased by about 20 dB, and each day of delay in the start of T(4) substitution resulted in an additional threshold loss of about 4 dB. The most prominent structural deficit in Pax8-/- animals in which T(4) substitution was started at P8 or later was an abnormally thick tectorial membrane. In these late-substituted animals, disarray of stereovilli from inner and outer hair cells was observed and also outer hair cell loss was found, predominantly in the basal part of the cochlea. The degree of structural disorder increased the later T(4) substitution was initiated. The structural and functional consequences of postnatal athyroidism observed in Pax8-/- mice are largely in agreement with and extend those data obtained from hypothyroid animal models in which hypothyroidism was induced by goitrogenic agents (methimazole, propylthiouracil) or animal models with disrupted genes for the TSH receptor or the thyroid hormone receptors. The hearing loss and also the recovery effect by T(4) substitution in Pax8-/- mice is larger than that in the other models. Although Pax8-/- mice are born by euthyroid Pax8+/- dams, the Pax8-/- phenotype could not be completely restored by immediate postnatal T(4) substitution, indicating that some deficits are the consequence of prenatal T(4) deficiency of the offspring.

Distribution of glycoconjugates during cochlea development in mice: light microscopic lectin study

During development, different epithelial cells in the mouse cochlea express different cell surface glycoconjugates, which may reflect membrane specialization. Some of the lectins tested in this study (SBA, succ-WGA, and PSA) labeled the sensory cells of the cochlea around birth. Other lectins (WGA, Con A, RCA-II, and PHA-E) labeled surfaces of the sensory cells, particularly the stereocilia, from early stages of development (gestation day (GD) 16) through 21 days after birth. These may be adhesion molecules needed to attach the newly forming tectorial membrane (TM) to the stereocilia. Lectin staining of the developing TM revealed that the substructures of the TM are biochemically distinct. Lectin staining also showed the temporal sequence of the expression of cytoplasmic glycoconjugates of the cochlear epithelium during development. Biochemical changes during development are probably the result of different cells being involved in the production of glycoconjugates, and may have functional significance, specifically with regard to the expression of adhesion and/or signaling molecules.

Delayed inner ear maturation and neuronal loss in postnatal Igf-1-deficient mice

Insulin-like growth factor-1 (IGF-1) has been shown to play a key role during embryonic and postnatal development of the CNS, but its effect on a sensory organ has not been studied in vivo. Therefore, we examined cochlear growth, differentiation, and maturation in Igf-1 gene knock-out mice at postnatal days 5 (P5), P8, and P20 by using stereological methods and immunohistochemistry. Mutant mice showed reduction in size of the cochlea and cochlear ganglion. An immature tectorial membrane and a significant decrease in the number and size of auditory neurons were also evident at P20. IGF-1-deficient cochlear neurons showed increased caspase-3-mediated apoptosis, along with aberrant expression of the early neural markers nestin and Islet 1/2. Cochlear ganglion and fibers innervating the sensory cells of the organ of Corti presented decreased levels of neurofilament and myelin P(0) in P20 mouse mutants. In addition, an abnormal synaptophysin expression in the somata of cochlear ganglion neurons and sensory hair cells suggested the persistence of an immature pattern of synapses distribution in the organ of Corti of these animals. These results demonstrate that lack of IGF-1 in mice severely affects postnatal survival, differentiation, and maturation of the cochlear ganglion cells and causes abnormal innervation of the sensory cells in the organ of Corti.

Hearing loss following exposure during development to polychlorinated biphenyls: a cochlear site of action

Maternal exposure to polyhalogenated hydrocarbons results in early postnatal hypothyroxenemia and a low-frequency hearing loss in adult offspring (Goldey et al., 1995a. Toxicol. Appl. Pharmacol. 135, 67-76; Herr et al., 1996. Fundam. Appl. Toxicol. 33, 120-128). The purpose of the present work was to determine whether the site-of-action of this auditory impairment was within the cochlea. Primiparous Long-Evans rats were given daily oral doses of corn oil (control) or 8 mg/kg of the commercial PCB mixture Aroclor 1254 (A1254) from gestation day (GD) 6 through postnatal day (PND) 21. Auditory thresholds for 1-, 4-, 16-, and 40-kHz tones were assessed using reflex modification audiometry in young adult offspring on postnatal days (PND) 92-110. Approximately 6 weeks after auditory assessments, a subset of animals (n=4 per group) were killed for histological assessment of the cochlea. Surface preparations of the organ of Corti were prepared from one cochlea per animal and modiolar sections were prepared from the opposite cochlea. Consistent with previous findings, auditory thresholds for 1-kHz tones were elevated by approximately 25 dB in the A1254-exposed animals. Thresholds for all higher frequencies were not different compared to controls. Surface preparations of the organ of Corti revealed a mild to moderate loss of outer hair cells in the upper-middle and apical turns. Inner hair cells were not affected. Modiolar sections failed to reveal alterations in any other cochlear structures. There was also no apparent loss of ganglion cells. These data clearly link the loss of low-frequency hearing caused by exposure during development to A1254 to a loss of outer hair cells in the organ of Corti. The mechanism that underlies this developmental ototoxicity remains to be determined. These data provide the first evidence of a structural deficit in the nervous system of adult animals exposed to PCBs during development.

Thyroid hormone deficiency before the onset of hearing causes irreversible damage to peripheral and central auditory systems

Both a genetic or acquired neonatal thyroid hormone (TH) deficiency may result in a profound mental disability that is often accompanied by deafness. The existence of various TH-sensitive periods during inner ear development and general success of delayed, corrective TH treatment was investigated by treating pregnant and lactating rats with the goitrogen methimazole (MMI). We observed that for the establishment of normal hearing ability, maternal TH, before fetal thyroid gland function on estrus days 17-18, is obviously not required. Within a crucial time between the onset of fetal thyroid gland function and the onset of hearing at postnatal day 12 (P12), any postponement in the rise of TH-plasma levels, as can be brought about by treating lactating mothers with MMI, leads to permanent hearing defects of the adult offspring. The severity of hearing defects that were measured in 3- to 9-mo-old offspring could be increased with each additional day of TH deficiency during this critical period. Unexpectedly, the active cochlear process, assayed by distortion product otoacoustic emissions (DPOAE) measurements, and speed of auditory brain stem responses, which both until now were not thought to be controlled by TH, proved to be TH-dependent processes that were damaged by a delay of TH supply within this critical time. In contrast, no significant differences in the gross morphology and innervation of the organ of Corti or myelin gene expression in the auditory system, detected as myelin basic protein (MBP) and proteolipid protein (PLP) mRNA using Northern blot approach, were observed when TH supply was delayed for few days. These classical TH-dependent processes, however, were damaged when TH supply was delayed for several weeks. These surprising results may suggest the existence of different TH-dependent processes in the auditory system: those that respond to corrective TH supply (e.g., innervation and morphogenesis of the organ of Corti) and those that do not, but require T3 activity during a very tight time window (e.g. , active cochlear process, central processes).

Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRaplha and TRbeta, indicates that TRbeta is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.

Long-term effects of sectioning the olivocochlear bundle in neonatal cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

The role of adrenocortical steroid hormones in the development of hearing

Based on the findings that adrenocortical hormones are involved in the regulation of Na+, K(+)-ATPase in several tissues and the presence of receptors for these hormones in the ear during auditory development, it has been suggested that these hormones also induce Na+, K(+)-ATPase activity and the endocochlear potential in the ear, leading to auditory function in the fetus-neonate. In order to test this hypothesis, glucocorticoid and mineralocorticoid hormones were injected into rat pups and their auditory development, compared to control litter-mates, was tested by recording auditory nerve-brainstem evoked potentials (ABR). Those who received glucocorticoid hormones had elevated ABR thresholds on post-natal day (PND) 9, others on PND 11 and still others on PND 16, compared to control litter-mates. The ABR thresholds of those injected with mineralocorticoids were not different from those in controls. These results and additional considerations related to the time sequence of the natural appearance of these hormones in the plasma, of their receptors in the ear and the onset of hearing in rat pups makes it extremely unlikely that adrenocortical hormones are involved in the initiation of Na+, K(+)-ATPase pumps and thereby of the endocochlear potential in the inner ear. It is possible that these hormones and their receptors play a role in the later regulation of the number of pumps.

Structural and ultrastructural aspects of isolated immature cochlear outer hair cells maintained in short-term culture

Immature outer hair cells (OHCs), isolated from developing rat cochlea without using proteolytic enzymes, were maintained in short-term culture in a clot of coagulated plasma. Cell viability was assessed by a laser scanning image cytometer, using double-fluorescent labeling. Light and transmission electron microscopy was used to study the morphology of isolated cells. Ten to 60 healthy OHCs were obtained from one cochlea, either as single isolated cells or clusters containing 2-10 cells from the same row. Although dead cells were observed only 1 h after dissociation, there were still viable cells after 6 h. Isolated OHCs were not perfectly cylindrical, due to the immaturity of their cortical structures. One hour after dissociation the ultrastructural organization of the isolated cells was generally well preserved, but this was followed by dilatation of the Golgi apparatus and endoplasmic reticulum. Specific changes in isolated OHCs were also observed at the subsurface cisternae and cuticular plate. Although degenerating OHCs generally showed a classic pattern of necrosis, certain morphological features reminiscent of apoptosis were also observed. This study emphasises the difficulty involved in investigating isolated immature OHCs in vitro and provides a basis for future research into the physiological requirements of isolated immature OHCs.

Hearing loss and cochlear abnormalities in the congenital hypothyroid (hyt/hyt) mouse

The congenital hypothyroid (hyt/hyt) mouse has been described as having a homozygous recessive mutation of a single locus on chromosome 12 which results in significant endocrine hypofunction and retarded growth. Although a distinct correlation between inherited hypothyroidism and hearing loss in humans exists, there has been no previous evaluation of the auditory system in these mutant mice. We determined hearing thresholds by auditory-evoked brainstem response testing and noted a 40-45 dB elevation in the hyt/hyt mouse compared to littermate heterozygote (hyt/+) animals and normal progenitor controls BALB/cByJ (+/+). Conventional light microscopy was used to examine the general anatomy of the cochlea in these animals, and the surface structure of the organ of Corti was further evaluated with scanning electron microscopy. Heterozygote and normal control mice had no significant abnormalities of the cochlea, however the hyt/hyt mice displayed consistent morphologic abnormalities of the stereocilia on both inner and outer hair cell systems. The surrounding and supporting cells were identified in the cochleas of the hypothyroid mouse and control animals and showed no significant histologic abnormalities. The auditory, histologic, and ultrastructural characterization of this model provides a foundation for evaluating the effects of true inherited hypothyroidism on auditory pathway development.

Fibroblast growth factor-2-like immunoreactivity in auditory brainstem nuclei of the developing and adult rat: correlation with onset and loss of hearing

Fibroblast growth factor-2 (FGF-2; basic FGF) is widely distributed in the developing and adult brain and has numerous effects on cultured and lesioned neural cells. The physiological role of FGF-2 in the unlesioned nervous system, however, is still not understood. We have studied the distribution of FGF-2 in the developing, adult, and functionally impaired central auditory system of the rat using specific antibodies and peroxidase-antiperoxidase immunocytochemistry. FGF-2-like immunoreactivity (FGF-2-IR) occurred in neuronal cell bodies and/or nerve fibers but was very rarely observed in glial cells. Several auditory brainstem nuclei, including the superior paraolivary nucleus, the medial superior olive, the lateral and ventral trapezoid nuclei, and the central nucleus, as well as the external cortex of the inferior colliculus, were entirely devoid of FGF-2-IR. In the dorsal cochlear nucleus, the lateral superior olive, and the nuclei of the lateral lemniscus, FGF-2-IR was not detectable in nerve cell bodies prior to adult age. Neurons in the medial geniculate body exhibited FGF-2-IR only transiently, from postnatal day (P) 5 until P16. Neurons in the medial nucleus of the trapezoid body were immunoreactive from P8 onwards. FGF-2-IR in anteroventral and posteroventral cochlear neurons disappeared at P14, i.e., at the onset of hearing, but immunoreactivity returned after P21. A transient expression of FGF-2 around the time when hearing function commences was observed in the dorsal cortex of the inferior colliculus.(ABSTRACT TRUNCATED AT 250 WORDS)

Ototoxicity in developing mammals

Developing mammals are more sensitive to noise, chemical and drug-induced ototoxicity than adults, with maximum sensitivity occurring during periods of anatomical and functional maturation of the cochlea. Normal physiological development of resting potentials (the endocochlear potential) and sound-evoked potentials including cochlear microphonics, summating potentials, compound action potentials, auditory brainstem responses and more recently distortion-product otoacoustic emissions have been characterized in several species including rats, mice, kittens, gerbils and guinea pigs. All of these responses are significantly impaired following acoustic trauma and/or exposure to a variety of ototoxic agents including aminoglycoside antibiotics, loop diuretics, antithyroid and antitumor drugs (alpha-difluoromethylornithine) and excitatory amino acids. Coupled with physiological and anatomical development is the maturation of specific biochemical pathways, which may be vulnerable targets of environmental noise and chemicals, excitatory amino acids and therapeutic drugs with ototoxic potentials.

Functional development of auditory sensitivity in the fetus and neonate

The human fetus responds to sound stimuli while still in utero. The rat and cat begin to hear only after birth. Therefore neonatal rat and cat are used as models of the development of auditory sensitivity in the human fetus. The inner ear of rat responds to stimuli delivered directly to it (bone conduction) before the middle ear can conduct sounds to the inner ear. During this period, middle ear development involves mesenchyme resorption, ossicular hardening and opening of the external canal. The latter stages of inner ear development involve increased magnitude of the endocochlear potential which augments cochlear transduction and the active cochlear amplifier. These developmental stages are probably controlled by thyroid hormone which activates several genes leading to the synthesis of proteins and enzymes required for the structural and functional maturation of the ear. This likely includes the Na+,K(+)-ATPase of the stria vascularis which generates the endocochlear potential. The magnitude of the endocochlear potential is dependent on oxygen supply so that the human fetus in utero whose blood carries less oxygen than the newborn has a hypoxia-induced sensorineural hearing loss. Upon birth and transition from placental to pulmonary oxygenation, the oxygen content of blood is increased, the magnitude of the endocochlear potential is elevated and auditory sensitivity is enhanced.

H and B human blood-group antigen expression in cochlear hair cells is modulated by thyroxine

The presence of human blood-group antigens in developing and adult hypothyroid rat cochleas was analyzed using antibodies directed against antigens H and B. During postnatal development, hypothyroid rat cochleas exhibited a highly selective expression of both B and H antigens, mainly at the hair cell level. Labeling for antigen B was found throughout the hair cells, whereas the antibody directed against antigen H selectively labeled the apical part of these cells. These immunostaining patterns were similar to those found in normal (euthyroid) rat cochleas, but antigenic expression periods were clearly prolonged. Thus, whereas in normal rat cochleas, the B and H antigenic expression disappears from postnatal day (PD) 9 on, in cochleas of hypothyroid rats the reactivity was intense until PD15; it decreased from this developmental stage, and was negative or only faintly positive at PD30. Therefore, in congenital hypothyroidism, hair cell immunoreactivity is present at developmental stages that are negative in normal rat cochleas. These results suggest that human blood-group antigen expression on the developing cochlear hair cells of rats is modulated by thyroxine and that thyroxine is necessary for the temporal expression pattern and secretion of normal glycoproteins.

Organization of auditory callosal connections in hypothyroid adult rats

Callosal connections were studied with tracers (horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP)) in normal rats and rats deprived of thyroid hormones with methimazole (Sigma) since embryonic day 14 and thyroidectomized at postnatal day 6. In hypothyroid rats, the auditory areas, in particular the primary auditory area, showed cytoarchitectonic changes including blurred lamination and decrease in the size of layer V pyramidal neurons. In control rats, callosally-projecting neurons were found between layers II and VI with a peak in layer III and upper layer IV. In hypothyroid rats, labelled neurons were found between layers IV and VI with two peaks corresponding to layer IV and upper layer V, and in upper layer VI. Quantitative analysis of radial distribution of callosally-projecting neurons confirmed their shift to infragranular layers in hypothyroid rats. Three-dimensional reconstructions showed a more continuous tangential distribution of callosally-projecting neurons in hypothyroid rats which may be due to the maintenance of a juvenile 'exuberant' pattern of projections. These changes in cortical connectivity may be relevant for understanding epilepsy and mental retardation associated with early hypothyroidism in humans and to clarify basic mechanisms of cortical development.

Thyroid hormone induces earlier onset of auditory function in neonatal rats

The effect of thyroid hormone injection on the development of auditory function in neonatal rats was evaluated using auditory nerve-brainstem evoked responses (ABR). The hormone induced earlier onset of auditory function. In order to differentiate between conductive and sensorineural factors, both air-conducted (AC) and bone-conducted (BC) ABR responses were recorded. Neonatal rats were injected with thyroxine (T4), or with saline (control animals), from day of birth (post-natal day-PND-0), daily, until PND 9. AC- and BC-ABRs were recorded from PND 6 up to PND 20. It was found that both AC- and BC-ABR thresholds were lower in the T4-injected rats up to PND 15, after which no difference was found between the two groups. This indicated earlier maturity of both conductive (external and middle ears) and sensorineural (inner ear) factors and is probably due to the earlier appearance in the blood of higher T4 levels, following injection, than that occurring naturally during the neonatal period in these animals.

Incorporation of D3H glucosamine to the adult and developing cochlear tectorial membrane of normal and hypothyroid rats

The uptake of D-3H-glucosamine by the developing cochlea of normal and hypothyroid rats was examined using light microscopic radioautography. During postnatal development, normal and hypothyroid rat cochleas exhibited a layer of radiolabelling in the tectorial membrane (TM). This layer first appeared in the TM region which covers the spiral limbus and the Kölliker's organ (KO), then progressively reached the apical part of the TM covering the organ of Corti. Radiolabelling was significantly greater in hypothyroid than in normal cochleas. These findings suggests that the enormous size reached by the TM in the congenital hypothyroidism could be related to an increase of epithelial secretion, at least for carbohydrates. It also suggests that TM, in normal and hypothyroid cochleas, could be formed during development by the addition of successive layers. Older layers could be displaced upwards by the new ones. Cochleas of normal young adult rats, treated with D-3H-glucosamine, showed a very scarce and diffuse radiolabelling. Cochleas of hypothyroid young adult rats exhibited a thickened and distorted TM, which incorporated a significant amount of carbohydrates. These results suggest that TM secretion is highly reduced in young adult normal animals, while in young adult hypothyroid ones it is still active. During cochlear maturation, thyroxine seems to be necessary, not only for the synthesis of normal glycoproteins (as suggested by previous reports), but also for the control of glycoprotein secretion.

Localization of anionic sulfate groups in the tectorial membrane

Colloidal iron hydroxide (CIH) staining demonstrates the existence of anionic sulfate groups of glycoconjugates associated with several constituents of the tectorial membrane (TM). In the adult animal, labelling in the main body of the TM appears as long, electron-dense patches surrounding type A fibrils which show alternating stained and unstained zones. On the other hand, labelling of the fibrils of the matrix of the TM appears as single, CIH particles with no special arrangement. Some of the structurally distinct regions of the TM are also labelled (limbal zone, Hensen's stripe and inner portions of the cover net), while others are not (marginal band and outer portions of the cover net). Staining of type A fibrils in the major TM is already present in newborn animals; while, both the outermost region of the TM closest to the cells of the organ of Kölliker and the minor TM are not labelled. The implications of these distributions of sulfated glycoconjugates for the electrochemical properties of the TM are discussed.

Lectin staining of saccharides in the normal and hypothyroid developing organ of Corti

Lectin staining has been used to detect mono- and oligosaccharides in normal and hypothyroid developing organs of Corti in the rat. Eight developmental stages were studied (1, 5, 8, 10, 15, 20, 50 and 60 days after birth). Congenital hypothyroidism was induced by oral administration of propylthyouracil to pregnant rats. Labelling of the tectorial membrane with 3 lectins, Ulex europaeus agglutinin-I (UEA-I), Lens culinaris agglutinin (LCA) and Ricinus communis agglutinin-I (RCA-I) showed no significant differences between normal and hypothyroid animals. Staining with peanut agglutinin (PNA) showed that the hypothyroid adult tectorial membrane (but not the normal one) possesses the disaccharide galactose + N-acetyl galactosamine. Phaseolus vulgaris agglutinin-L (PHA-L) labels the whole tectorial membrane in both groups of animals, but the staining is more intense in the hypothyroid one for a narrow band of oligosaccharide located just between the tectorial membrane and the underlying organ of Kölliker. Both soybean agglutinin (SBA) and succinylated wheat germ agglutinin (WGA) stain the tectorial membrane as well as the cytoplasm of the cells constituting the inner portion of the organ of Kölliker; this latter feature disappears in the normal animals about the 8th postnatal day, but it is abnormally preserved until the 60th postnatal day in the hypothyroid ones. In the adult hypothyroid animals, 3 of the lectins (LCA, PHA-L and WGA) stain extracellular conglomerates located under the synaptic pole of the outer hair cells.

Developmental delays associated with prenatal alcohol exposure are reversed by thyroid hormone treatment

Exposure to alcohol in utero has been associated with hypothyroidism and a variety of developmental defects characteristic of thyroid dysfunction. The present work examined whether these abnormalities could be reversed in infant rats treated with thyroid hormones. Subjects were offspring of dams which were on the following diet regimen during gestation: (1) free access to liquid diet containing ethanol (alcohol pups); (2) an equal volume of isocaloric liquid diet (pair-fed pups); or (3) ad libitum control diet (control pups). Neonates from each group were foster-nursed by control dams, and received triiodothyronine (T3; 0.1 mg/kg/day; s.c.) or saline treatments on postnatal days 1 to 10. The alcohol neonates displayed reduced serum thyroxine which was restored to normal by postnatal day 14. In addition, these pups showed a delayed appearance of developmental landmarks, including righting reflex, dental eruption, auditory startle response and eye opening. The retarded incisor eruption and eye opening were reversed in alcohol pups by T3 treatments. The present data suggest that at least some of the developmental abnormalities associated with prenatal alcohol exposure are attributable to perinatal hypothyroidism and can be restored by early hormone replacement therapy.

The effect of hypothyroidism on the development of the glycogen content of organ of Corti's hair cells

The density of glycogen particles in organ of Corti's sensory cells was measured to determine the effect of congenital hypothyroidism upon the normal development of this energy source. This density in both normal and hypothyroid inner hair cells remains in low values from birth to adulthood. On the other hand, that of normal outer hair cells undergoes a great increase between the 10th and the 20th postnatal days, coinciding with the maturation of both the efferent innervation of these cells and the tuning properties of the auditory receptor. The glycogen stores of the hypothyroid outer hair cells do not show any significant increase from birth to adulthood. This latter fact suggests that the congenital hypothyroidism restrains the development of an important energy source of outer hair cells, most surely disturbing the physiological processes relying on glycogen metabolism.

Neurological consequences of congenital hypothyroidism in Fischer 344 rats

Congenital hypothyroidism was induced in rat pups by treating pregnant and lactating dams with an antithyroid drug, methimazole. Methimazole (0.00, 0.01, 0.03 or 0.10 mg/ml) was added to the drinking water of female Fischer 344 rats from gestational day 17 through lactational day 10. The same animals as pups and adults were evaluated with a developmental neurotoxicological test battery. Pups were evaluated for physical measures of maturation, thermoregulation, flash evoked potential (FEP), motor activity, and morphology of brain, thyroid and kidneys. Parameters evaluated in the same animals as adults were body weight, functional observational battery, grip strength, body temperature, and neurological tests (FEP, auditory brainstem response to 4 and 16 kHz tone pips (ABR4, ABR16) and clicks (ABRc), somatosensory evokes potentials recorded from the somatosensory cortex (SEP-S) and the cerebellum (SEP-C), and caudal nerve action potential to single and paired stimuli (CNAP). Treatment-related findings in pups included slightly decreased body weight, slightly increased kidney weights, altered thyroid morphology, delayed incisor eruption, decreased thermoregulation, and FEP changes. Although a pup no effect level was not determined, effects at 0.01 mg/ml were minimal. Adult ABR4 and ABR16 waveforms were slower than controls and had altered shapes; ABRc, SEP-S, and SEP-C waveforms exhibited reduced power, increased latency and altered shape. Effects were detected in adults at all doses and thus, the neurological characteristics of rat congenital hypothyroidism were clearly detected with this developmental neurotoxicological test battery. The effects on body weight, kidney weight and thyroid morphology, however, suggest a general developmental effect and nervous system function did not appear to be preferentially affected.

Cholecalcin (28-kDa CaBP) in the rat cochlea. Development in normal and hypothyroid animals. An immunocytochemical study

The distribution of cholecalcin (28-kDa calcium-binding protein) in the cochlea of developing rats was followed by immunocytochemistry. In normal animals, the protein first appeared in utero in the cells of Kölliker's organ, a structure involved in the secretion of the tectorial membrane. The inner hair cells, followed by the outer hair cells, then became immunoreactive from the base of the cochlea to the apex. Their cuticular plate, the anchoring structure for stereocilia, was particularly labeled. The cells of Kölliker's organ lost their immunoreactivity after the first postnatal week, the time when they lose their secretory activity. During the same period, when the tunnel of Corti and the space of Nuel open, labeling appeared in the supporting cells. The neurons of the spiral ganglion were stained from the second postnatal week and the fibers of the cochlear nerve after the end of the first month. No difference was induced by thyroid deficiency until the second postnatal week. Thereafter, Kölliker's organ did not transform and its cholecalcin immunoreactivity persisted, together with the secretory activity. As a result, the tectorial membrane was greatly distorted and the mechanical properties of the organ of Corti were dramatically impaired. The supporting cells were labeled although the tunnel of Corti and the space of Nuel did not open. Staining of the spiral ganglion neurons was delayed. All the nervous structures of the cochlea were, however, stained at the end of the first postnatal month, as in normal animals, despite the irreversible complete deafness. Cholecalcin is thus present during development of the cochlea in both non-neuronal and neuronal structures, and is probably involved in very different processes in various types of cells.

Acoustic overstimulation alters the morphology of the tectorial membrane

After a permanent threshold shift was induced by exposing guinea pigs to a 1 kHz pure tone at 105 dB(A) for 72 h, light microscopic observations of freshly dissected and stained tectorial membranes showed an increased waviness and clumping of the fibers of the middle zone. Hensen's stripe was not seen as a continuous dense structure running through the middle zone but was at times discontinuous and curved. As measured from cross-sections of the cochlea, the thickness of the tectorial membrane was decreased after acoustic overstimulation. The stereocilia of the inner and outer hair cells lie directly under the middle zone. Visual detection levels of threshold of tectorial membrane movement was determined by stimulating the marginal zone of the tectorial membrane of isolated cochlear coils by an oscillating water jet. After acoustic overstimulation the tectorial membrane became more complaint. The tectorial membrane abnormalities were restricted to the regions of the cochlea that demonstrated a 40-50 dB hearing loss.

Tonotopic evolution during development

The evolution of cochlear tonotopy can be observed by the study of the modification of tuning curves as obtained from auditory nerve fibers in the kitten. The differential development of the two components of the tuning curves, i.e. the tail, which is restricted to lower frequencies that appear at first during ontogenesis, and the tip which is restricted to higher frequencies that appear later, can explain the ontogenetic variations of tonotopy observed in the past. An hypothesis is presented where the tail of the tuning curve is related to the first functioning of the basal inner hair cells during development, whereas the delayed appearance of the tip is related to the basal outer hair cells. The base-apex gradient of maturation of receptors, along with their lateral gradient of development can explain the tonotopic shift observed during the cochlear ontogenesis.

Somatosensory evoked potentials and auditory brain-stem responses in congenital hypothyroidism. I. A longitudinal study before and after treatment in six infants detected in the neonatal period

We report the results of a longitudinal study of auditory brain-stem responses (ABRs) and somatosensory evoked potentials (SEPs) performed in 6 children with congenital hypothyroidism. These infants were detected by the Quebec Network for Genetic Medicine and treated early. ABRs and SEPs were recorded both before and 2 weeks after the initiation of therapy and at 6 months of age. Before treatment, for SEP, we found increased wave N19, P22 latencies and N13-N19, N19-P22 interpeak latencies (IPLs) in congenital hypothyroid (CH) children. For ABR, there were increased wave I latencies with normal I-V IPLs. Substitutive therapy improved these abnormalities although this improvement was more evident after a shorter period of time for ABRs than for SEPs. Even at 6 months, 2 CH children still showed increased N13-N19 IPLs. Both had very low serum T4 levels at the time of diagnosis and one had also a very small knee surface area, both criteria indicating a severe hypothyroidism. It will be interesting to verify if initial and persisting increase of N13-N19 IPL is associated with later neuropsychological problems.

Audiological and temporal bone findings in myxedema

Fifteen patients with confirmed myxedema at a median age of 48 years (range 32 to 60 years) were referred for audiological evaluation before and after treatment with levothyroxine. The median interval between the pretreatment and posttreatment investigations was 18 months (range 9 to 27 months). In addition, 13 patients at a median age of 78 years (range 64 to 95 years) were audiologically reexamined after long-standing levothyroxine treatment. The observation period upon treatment with levothyroxine was 40 months (range 32 to 46 months). No improvement in hearing sensitivity could be demonstrated either in the younger patients or in the elderly. When compared to an age- and sex-matched unscreened population, the myxedematous patients did not demonstrate any different degree of hearing loss. Histological investigation of the temporal bones from an 83-year-old woman with myxedema, however, showed no morphological changes or deposition of glycosaminoglycans, changes which are compatible with true age-related hearing loss. It is concluded that no association exists between myxedema and hearing impairment and that no morphological or structural changes due to myxedema can be demonstrated in the temporal bones.

Periods of sensitivity to thyroid hormone during the development of the organ of Corti

Cochlear structures are sensitive to the morphogenetic effect of thyroid hormone during the whole duration of maturation. For each structure, there exists a period of maximal sensitivity to thyroid hormone which corresponds to the period of development during which the structure of interest undergoes its main morphological changes (6 to 13 days for the inner sulcus epithelium, 6 to 10 days for the pillars, the 2nd and a part of the 3rd postnatal week for OHCs and their efferent innervation in rats). These periods of sensitivity can be considered as critical periods because cochlear structures are maximally vulnerable to thyroid deficiency during these periods.

Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat. I. Morphological study

In order to study the corrective effects of thyroxine on the cochlear abnormalities induced by congenital hypothyroidism, small doses of thyroxine were injected in propylthiouracil-treated rat pups for 2 consecutive days during selected periods of development (days 3 and 4, 6 and 7, 9 and 10, 12 and 13, 18 and 19). Some animals also received thyroid replacement therapy from days 12 to 17. Corrective effects of thyroxine on cochlear structures were observed using light microscopy and transmission electron microscopy. The corrective effects not only depended on the period of administration of the hormone, but also on the structure investigated within the organ of Corti. For a given structure, a period of maximal sensitivity to thyroxine exists which corresponds to the period of development during which that structure undergoes its main morphological changes (i.e. from 6 to 13 days for the inner sulcus epithelium, the first postnatal week for the tectorial membrane, from 6 to 10 days for the pillars and the tunnel of Corti, the second and probably a part of the third postnatal week for outer hair cell synaptogenesis).

Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat. II. Electrophysiological study

In order to study the corrective effects of thyroxine (T4) on functional abnormalities induced by congenital hypothyroidism, small doses of T4 were injected to propylthiouracil-treated (PTU-treated) rat pups for 2 consecutive days on selected periods of development (days 3 and 4, 6 and 7, 9 and 10, 12 and 13, 18 and 19). Some animals also received thyroid replacement therapy from days 12 to 17. The animals were tested electrophysiologically on day 30, by recording the compound action potential and the cochlear microphonic from the round window after click and tone burst stimulation. PTU-treated animals given T4 for 2 consecutive days demonstrated both AP and CM threshold shifts. On the contrary, PTU-treated animals given T4 from days 12 to 17 demonstrated a normal CM output of the cochlea, but still showed elevated AP thresholds. These results are discussed with previous data concerning the corrective effects of T4 on cochlear structures in PTU-treated rats previously described.