Neurological consequences of congenital hypothyroidism in Fischer 344 rats

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.

Environmental exposure to perchlorate: A review of toxicology and human health

Perchlorate pharmacology and toxicology studies date back at least 65 years in the peer-reviewed literature. Repeated studies in animals and humans have demonstrated perchlorate's mechanism of action, dose-response, and adverse effects over a range of doses. The first measurable effect of perchlorate is inhibition of iodine uptake to the thyroid gland. Adequate levels of thyroid hormones are critical for the development of the fetal nervous system. With sufficient dose and exposure duration, perchlorate can reduce thyroid hormones in the pregnant or non-pregnant woman via this mechanism. The developing fetus is the most sensitive life stage for chemical agents that affect iodide uptake to the thyroid. Perchlorate has a half-life of eight hours, is not metabolized, does not bioaccumulate, is not a mutagen or carcinogen, and is not reprotoxic or immunotoxic. More recently, epidemiological and biomonitoring studies have been published in the peer-reviewed literature characterizing the thyroidal effects of perchlorate and other goitrogens. While the results from most populations report no consistent association, a few studies report thyroidal effects at environmentally relevant levels of perchlorate. We reviewed the literature on health effects of perchlorate at environmental exposure levels, with a focus on exposures during pregnancy and neurodevelopmental effects. Based on the studies we reviewed, health effects are expected to only occur at doses substantially higher than environmental levels.

A Rat Neurodevelopmental Evaluation of Offspring, Including Evaluation of Adult and Neonatal Thyroid, from Mothers Treated with Ammonium Perchlorate in Drinking Water

The purpose of this study was to evaluate the potential neurodevelopmental toxicity of perchlorate exposure during gestation and the first 10 days of lactation. Mated Sprague-Dawley rats (25/exposure group) were given continual access to 0, 0.1, 1.0, 3.0, or 10.0 mg/kg-day ammonium perchlorate (AP) in drinking water, starting gestation day 0 (mating) through lactation day 10 (DL 10). One pup/sex/litter/exposure group was assigned to (1) juvenile brain weights, morphometry, and neuropathology; (2) passive avoidance and watermaze testing; (3) motor activity and auditory startle habituation; and (4) adult regional brain weights, morphometry, and neuropathology. AP had no effect on body weights, feed consumption, clinical observations, or sexual maturation of pups at exposures as high as 10.0 mg/kg-day. There were no behavioral effects in the offspring exposed as high as 10.0 mg/kg-day as evaluated by passive avoidance, swimming watermaze, motor activity, and auditory startle. Increases in hypertrophy and hyperplasia of the thyroid follicular epithelium and a decrease in the thyroid follicle size were observed in culled male pups in the 10.0 mg/kg-day group on DL 5. The exposure level for effects on triiodothyroxine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH) levels for pups were 0.1, 1.0, and 3.0 mg/kg-day, respectively. There was an apparent increase in the thickness of the corpus callosum of the 10 mg/kg-day group pups on DL 12. The no-observed-adverse-effect level (NOAEL) for maternal toxicity was greater than 10.0 mg/kg-day. Based on the thyroid morphometric and histopathologic findings, the NOAEL for pup toxicity was 0.1 mg/kg-day.

A comparison of potency differences among thyroid peroxidase (TPO) inhibitors to induce developmental toxicity and other thyroid gland-linked toxicities in humans and rats

The potencies of resorcinol, 6-propylthiouracil (PTU) and methimazole (MMI) for inducing developmental toxicity and neurotoxicity were compared in pregnant rats, regarded as valid model for human thyroid toxicity. Profound differences on maternal thyroid hormone levels (THs), maternal toxicity as well as developmental and neurotoxicity sequelae occurred. Resorcinol affected none of those end points. PTU and MMI caused significant effects. Therapy with either PTU or MMI during the first trimester of human pregnancy can cause reductions of maternal THs, accompanied by disruptions of prenatal development. Clinical MMI studies show sporadic evidence of teratogenic effects, with equivocal relation to thyroid peroxidase (TPO) inhibition. In recent decades no MMI associated prenatal toxicity has been reported, an outcome possibly related to carefully managed therapy. Orally administered resorcinol was rapidly absorbed, metabolized and excreted and was undetectable in the thyroid. In contrast, PTU or MMI accumulated. Resorcinol's potency to inhibit TPO was profoundly lower than that of PTU or MMI. Quantum chemical calculations may explain low resorcinol reactivity with TPO. Thus, distinctions in the target organ and the TPO inhibitory potency between these chemicals are likely contributing to different reductions of maternal THs levels and affecting the potency to cause developmental toxicity and neurotoxicity.

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.

Feasibility study of the zebrafish assay as an alternative method to screen for developmental toxicity and embryotoxicity using a training set of 27 compounds

To anticipate to increased testing needs for reproductive toxicity and 3R approaches, we studied zebrafish embryo/larva as an alternative for animal testing for developmental toxicity and embryotoxicity and evaluated a training set of 27 compounds with a standardized protocol. The classification of compounds in the zebrafish embryo/larva assay, based on a prediction model using a TI (teratogenic index) cut-off value of 2, was compared to available animal and human data. When comparing the classification of compounds in the zebrafish embryo/larva assay to available animal classification, a sensitivity of 72% and specificity of 100% were obtained. The predictive values obtained in comparison to a limited set of human data were 50, 60% respectively for teratogens, non-teratogens. Overall, we demonstrated that the zebrafish embryo/larva assay, may be used as screening tool for prioritization of compounds and could contribute to reduction of animal experiments in the field of developmental toxicology.

Thyroid hormone (T3)-induced up-regulation of voltage-activated sodium current in cultured postnatal hippocampal neurons requires secretion of soluble factors from glial cells

We have previously shown that treatment with the thyroid hormone T(3) increases the voltage-gated Na(+)current density (Nav-D) in hippocampal neurons from postnatal rats, leading to accelerated action potential upstrokes and increased firing frequencies. Here we show that the Na(+) current regulation depends on the presence of glial cells, which secrete a heat-instable soluble factor upon stimulation with T(3). The effect of conditioned medium from T(3)-treated glial cells was mimicked by basic fibroblast growth factor (bFGF), known to be released from cerebellar glial cells after T(3) treatment. Neutralization assays of astrocyte-conditioned media with anti-bFGF antibody inhibited the regulation of the Nav-D by T(3). This suggests that the up-regulation of the neuronal sodium current density by T(3) is not a direct effect but involves bFGF release and satellite cells. Thus glial cells can modulate neuronal excitability via secretion of paracrinely acting factors.

Effects of prenatal exposure to diesel exhaust particles on postnatal development, behavior, genotoxicity and inflammation in mice

Background

Results from epidemiological studies indicate that particulate air pollution constitutes a hazard for human health. Recent studies suggest that diesel exhaust possesses endocrine activity and therefore may affect reproductive outcome. This study in mice aimed to investigate whether exposure to diesel exhaust particles (DEP; NIST 2975) would affect gestation, postnatal development, activity, learning and memory, and biomarkers of transplacental toxicity. Pregnant mice (C57BL/6; BomTac) were exposed to 19 mg/m³ DEP (~1·10⁶ particles/cm³; mass median diameter ≅ 240 nm) on gestational days 9–19, for 1 h/day.

Results

Gestational parameters were similar in control and diesel groups. Shortly after birth, body weights of DEP offspring were slightly lower than in controls. This difference increased during lactation, so by weaning the DEP exposed offspring weighed significantly less than the control progeny. Only slight effects of exposure were observed on cognitive function in female DEP offspring and on biomarkers of exposure to particles or genotoxic substances.

Conclusion

In utero exposure to DEP decreased weight gain during lactation. Cognitive function and levels of biomarkers of exposure to particles or to genotoxic substances were generally similar in exposed and control offspring. The particle size and chemical composition of the DEP and differences in exposure methods (fresh, whole exhaust versus aged, resuspended DEP) may play a significant role on the biological effects observed in this compared to other studies.

Undertaking positive control studies as part of developmental neurotoxicity testing: a report from the ILSI Research Foundation/Risk Science Institute expert working group on neurodevelopmental endpoints

Developmental neurotoxicity testing involves functional and neurohistological assessments in offspring during and following maternal and/or neonatal exposure. Data from positive control studies are an integral component in developmental neurotoxicity risk assessments. Positive control data are crucial for evaluating a laboratory's capability to detect chemical-induced changes in measured endpoints. Positive control data are also valuable in a weight-of-evidence approach to help determine the biological significance of results and provide confidence in negative results from developmental neurotoxicity (DNT) studies. This review is a practical guide for the selection and use of positive control agents in developmental neurotoxicology. The advantages and disadvantages of various positive control agents are discussed for the endpoints in developmental neurotoxicity studies. Design issues specific to positive control studies in developmental neurotoxicity are considered and recommendations on how to interpret and report positive control data are made. Positive control studies should be conducted as an integral component of the incorporation and use of developmental neurotoxicity testing methods in laboratories that generate data used in risk decisions.

Thyroid hormone regulates excitability in central neurons from postnatal rats

A lack of thyroid hormone in the postnatal period causes an irreversible mental retardation, characterized by a slowing of thoughts and movements accompanied by prolonged latencies of several evoked potentials and slowed electroencephalographic rhythms. Here we show that in cultured hippocampal and cortical neurons from postnatal rats treatment with thyroid hormone not only up-regulates Na(+)-current densities but also increases rates of rise, amplitudes and firing frequencies of action potentials. Furthermore, we show that the regulation of the Na(+)-current density by thyroid hormones also occurs in vivo: recordings from acutely isolated cortical neurons obtained from hypothyroid, euthyroid and hyperthyroid postnatal rats showed that hypothyroidism decreases the ratio of Na(+) inward- to K(+) outward-currents while hyperthyroidism upregulates Na(+)-currents with respect to K(+)-currents. Our observation of a regulation of neuronal excitability by thyroid hormone offers a direct explanation for the origin of various neurological symptoms related to thyroid dysfunction.

Perinatal hypothyroidism effects on neuromotor competence, novelty-directed exploratory and anxiety-related behaviour and learning in rats

Thyroid hormone is essential for proper development of the mammalian CNS. Previous studies have documented a decrease in the ability of neonatal hypothyroid animals to learn and to habituate to maze tests and an increase in spontaneous activity. However, there is little information about the effects of perinatal (i.e. perinatal and postnatal) hypothyroidism on behaviour. The aim of the present work was to investigate the longitudinal effects of perinatal hypothyroidism on certain aspects of the behaviour in rats. Neuromotor competence was tested at 21, 40 and 60 days, novelty-directed exploratory behaviour and anxiety-related behaviour were evaluated at 40 and 60 days by means of the Boissier tests and associative learning ability was tested at 80 days by means of a step-through passive avoidance task. The persistence of the effects of perinatal hypothyroidism on psychomotor performance was highly dependent on the task examined. Perinatal hypothyroidism caused an increase of locomotor activity as revealed by the total distance travelled in the Boissier test and this increase also comprised a component of decreased anxiety-related behaviour. Methimazole-treated subjects also had higher head-dip scores than controls at 40 days while no differences were observed at 60 days. Finally, our results showed that methimazole-treated rats performed poorly in a passive avoidance learning task.

Flash-, somatosensory-, and peripheral nerve-evoked potentials in rats perinatally exposed to Aroclor 1254

Pregnant Long-Evans rats were exposed to 0, 1 or 6 mg/kg/day of Aroclor 1254 (A1254; Lot no. 124-191), a commercial mixture of polychlorinated biphenyls (PCBs), from gestation day (GD) 6 through postnatal day (PND) 21. At 128-140 days of age, male and female offspring were tested for visual-, somatosensory- and peripheral nerve-evoked potentials. The evoked responses increased in amplitude with larger stimulus intensities, and gender differences were detected for some endpoints. In contrast, developmental exposure to A1254 failed to significantly affect the electrophysiological measures. A subset of the animals were tested for low-frequency hearing dysfunction using reflex modification audiometry (RMA). An elevated threshold for a 1-kHz tone was observed, replicating previous findings of A1254-induced auditory deficits [Hear. Res. 144 (2000) 196; Toxicol. Sci. 45(1) (1998) 94; Toxicol. Appl. Pharmacol. 135(1) (1995) 77.]. These findings indicate no statistically significant changes in visual-, somatosensory- or peripheral nerve-evoked potentials following developmental exposure to doses of A1254 that produce behavioral hearing deficits. However, subtle changes in the function of the visual or somatosensory systems cannot be disproved.

Developmental and hormonal regulation of thermosensitive neuron potential activity in rat brain

To understand the involvement of thyroid hormone on the postnatal development of hypothalamic thermosensitive neurons, we focused on the analysis of thermosensitive neuronal activity in the preoptic and anterior hypothalamic (PO/AH) regions of developing rats with and without hypothyroidism. In euthyroid rats, the distribution of thermosensitive neurons in PO/AH showed that in 3-week-old rats (46 neurons tested), 19.5% were warm-sensitive and 80.5% were nonsensitive. In 5- to 12-week-old euthyroid rats (122 neurons), 33.6% were warm-sensitive and 66.4% were nonsensitive. In 5- to 12-week-old hypothyroid rats (108 neurons), however, 18.5% were warm-sensitive and 81.5% were nonsensitive. Temperature thresholds of warm-sensitive neurons were lower in 12-week-old euthyroid rats (36.4+/-0.2 degrees C, n = 15, p<0.01,) than in 3-week-old and in 5-week-old euthyroid rats (38.5+/-0.5 degrees C, n = 9 and 38.0+/-0.3 degrees C, n = 15, respectively). The temperature thresholds of warm-sensitive neurons in 12-week-old hypothyroid rats (39.5+/-0.3 degrees C, n = 8) were similar to that of warm-sensitive neurons of 3-week-old raats (euthyroid and hypothyroid). In contrast, there was no difference in the thresholds of warm-sensitive neurons between hypothyroid and euthyroid rats at the age of 3-5 weeks. In conclusion, monitoring the thermosensitive neuronal tissue activity demonstrated the evidence that thyroid hormone regulates the maturation of warm-sensitive hypothalamic neurons in developing rat brain by electrophysiological analysis.

Neurobehavioral development of neonatal rats after in-utero hypothyroxinemia: efficacy of prenatal thyroxine treatment

The aim of this study was to assess the impact on neonatal neurobehavioral development of methimazole (MMI)-induced in-utero hypothyroxinemia and of correction by maternal-fetal thyroxine (T4) transfer in the rat. Two groups of pregnant Sprague-Dawley rats received MMI as drinking water from gestation day 10 until birth. From day 16 until parturition, one of these groups received daily intraperitoneal injections of L-T4 and the other received saline injections. A third (control) group drank tap water and received saline injections. From day of birth, offspring from all groups were raised by untreated foster dams. Their neurobehavioral development was monitored, on postnatal days 5-14 (N = 3/litter, from 30 litters) by experimenters blind to treatment group. Prenatal T4 treatment resulted in correction of MMI-induced delayed appearance of three different reflexes. Body-weight gain of treated pups was similar to that of controls and more rapid than development of rats treated with MMI-alone. T4 treatment did not prevent, however, MMI-induced delay in maturation of physiological landmarks (e.g. ear opening). At least a portion of the developmental delay resulting from prenatal (maternal) MMI administration may be reversed by maternal-fetal transfer of T4 administered to the gravid dam.

Perinatal alterations of thyroid hormones and behaviour in adult rats

Several studies have shown the relevance of the neuroendocrinological system in the development and function of the nervous system. In order to observe the influence of thyroid hormones during development on the behaviour of adult rats we induced dysthyroid states during the perinatal period. Results indicate that some behaviours are more susceptible to the action of thyroid hormones than others. We observed that the thyroid hormone deficiency causes an increase of activity in animals in spite of a large period of rehabilitation. Thyroxine-treated rats showed an anxiogenic behavioural pattern in the elevated plus-maze, while animals rehabilitated from perinatal deficit of thyroid hormones showed an anxiolitic pattern. These findings suggest that an excess of thyroid hormones has less effect on behaviour than a deficiency of these hormones.

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.

The development of the anterior commissure in normal and hypothyroid rats

The development of axon number in the anterior commissure (AC) was analyzed in 39 normal and 37 hypothyroid rats using conventional electron microscopy. Hypothyroid rats underwent antithyroid treatment with methimazole from embryonic day (E) 14 onwards, followed in a fraction of the animals by thyroidectomy at postnatal day (P) 6. In normal rats, the midsagittal cross-sectional anterior commissure area (ACA) increased throughout their life; in hypothyroid rats, ACA was stationary from P4 onwards and at P174-180 it was reduced by 39% relative to normal rats. In normal rats, the number of AC axons increased rapidly from 168,500 at E18 to, on average, 1,049,000 from P4 onwards. Similarly, in hypothyroid rats, the number of axons increased from 135,000 at E18 to, on average, 1,052,000 from P4 onwards. At all ages, the number of axons was similar in normal and hypothyroid rats. During development of the AC, the evolution of axon number observed in normal and hypothyroid rats is different from what was reported for other telencephalic commissures, including the AC of the monkey, where an important fraction of the axons are eliminated postnatally. Antithyroid treatment dissociated ACA from total number of AC axons.

Neurotoxicologic examination of rats exposed to 1,1,1-trichloroethane vapor for 13 weeks

Large evoked potential and EEG changes occurred in a pilot study in Fisher 344 rats during exposure to 2000 ppm of 1,1,1-trichloroethane (1,1,1-T; a cleaning solvent with anesthetic properties). In the main study, rats were evaluated for persistent nervous system effects the week following exposure to 0, 200, 630, or 2000 ppm 1,1,1-T for 6 h/day, 5 days/week, for 13 weeks. Rats were clinically examined regularly and were given a functional observational battery monthly (FOB, including forelimb and hindlimb grip performance testing). After 13 weeks of exposure, the rats were evaluated by FOB and by visual, auditory, somatosensory, and caudal nerve-evoked potentials. After functional testing, a subgroup of rats had histopathologic examination of brain, spinal cord, peripheral nerves, and limb muscles. There were no post-exposure treatment-related findings in any parameter (FOB observations plus 39 dependent variables) except for a slightly smaller forelimb grip performance in the 2000-ppm exposure group. There was no recognized toxicologic significance for the difference in forelimb grip performance; a lack of findings in any other clinical, evoked potential or morphologic parameter did not support a diagnosis of neurotoxicity.

Neurotoxicologic evaluation of rats after 13 weeks of inhalation exposure to dichloromethane or carbon monoxide

Male and female Fischer 344 rats were exposed to dichloromethane (methylene chloride, DCM) or carbon monoxide (CO) for 6 hr/day, 5 days/week, for 13 weeks. Since oxidative metabolism of DCM to CO and CO2 is a saturable process, DCM exposure concentrations were selected clearly below saturation (50 ppm), just below saturation (200 ppm), and well above saturation (2000 ppm). At saturation of metabolism, metabolic CO causes about 10% carboxyhemoglobinemia (COHb). Therefore, as a control for CO effects, a separate group of rats was exposed to 135 ppm CO to induce approximately 10% COHb. Postexposure functional tests included an observational battery, hindlimb grip strength, and a battery of evoked potentials (flash, auditory brainstem, somatosensory, caudal nerve). After functional tests were completed, rats from all groups were perfused with fixative and a comprehensive set of nervous tissues from the high DCM exposure group and from controls were examined by light microscopy. Although some miscellaneous functional and morphologic variations were recorded, none were related to treatment. Thus, subchronic exposures as high as 2000 ppm DCM or 135 ppm CO had no deleterious effects on any of the measures of this study.

Screening for neurotoxicity: complementarity of functional and morphologic techniques

Our philosophy is that screening tests should be applicable across species and emphasize complementarity to neuropathology. Within this context, electrophysiological tests comparable to those in human clinical neurology are powerful screening tools. For example, while histopathologic evaluation of the cochlea for ototoxicity is difficult, evoked potential audiometry is fast and easy. In this instance, one might routinely screen for deficits in auditory function, and reserve morphologic techniques for a characterization role rather than one of discovery. Lesions of neurons, axons and myelin are, however, readily assessed by light microscopy. A suitable combination of functional and morphologic screening tests, therefore, enhances the ability to discover neurotoxicity, and these data often are ideal for generation of refined hypotheses for subsequent characterization studies.