Suckling ability and maternal prolactin levels in hypothyroid rats

Effects of maternal hypothyroidism in the gastrointestinal system of male young offspring from Wistar rats

Alterations in the maternal environment may impact on the fetal development. The objective of this study was to investigate the gastrointestinal consequences of maternal hypothyroidism for the male offspring from Wistar rats. The pregnant rats were divided into three groups: control (C - received water), experimental 1 [E1 - received methimazole (MMI) solution] during gestation and lactation, and experimental 2 (E2 - received MMI solution) during gestation. Maternal parameters evaluated: free T3 and T4, bodyweight variation, and water/food intake. Offspring parameters evaluated: litter size, number of male/female, free T3 and T4, stomach area, gastric ulcer susceptibility, small intestine length and weight, small intestine and distal colon motility, the stomach and intestinal weight-body weight ratio (SW/BW-IW/BW), and the accumulation of intestinal fluid. Maternal T3 and T4 from E1 were decreased when compared to the other groups. There were no differences for maternal water/food intake and weight gain, litter size, and number of males and females. Regarding to offspring, free T3, SW/BW, IW/BW, and intestinal fluid accumulation were not different between the groups, but T4 was decreased in E1. However, 30-day-old pups from E1 and E2 were smaller with lower stomach and small intestine. Even more, E1 presented a lower ulcer index when compared to the C, while E2 had a higher distal colon transit. It can be concluded that maternal hypothyroidism impaired the total body development, as well as gastric and intestinal development, besides interfering with the susceptibility to the ulcer and intestinal transit of male offspring from Wistar rats.

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.

Role of sensory, social, and hormonal signals from the mother on the development of offspring

For mammals, sensory, social, and hormonal experience early in life is essential for the continuity of the infant's development. These experiences come from the mother through maternal care, and have enduring effects on the physiology and behavior of the adult organism. Disturbing the mother-offspring interaction by maternal deprivation (neglect) or exposure to adverse events as chronic stress, maltreatment, or sexual abuse has negative effects on the mental, psychological, physiological, and behavioral health. Indeed, these kinds of negative experiences can be the source of some neuropsychiatric diseases as depression, anxiety, impulsive aggression, and antisocial behavior. The purpose of this chapter is to review the most relevant evidence that supports the participation of cues from the mother and/or littermates during the postnatal preweaning period for the development of nervous system of the offspring. These findings come from the most frequently utilized experimental paradigms used in animal models, such as natural variations in maternal behavior, handling, partial maternal deprivation, and total maternal deprivation and artificial rearing. Through the use of these experimental procedures, it is possible to positively (handling paradigm), or negatively (maternal deprivation paradigms), affect the offspring's development. Finally, this chapter reviews the importance of the hormones that pups ingest through the maternal milk during early lactation on the development of several physiological systems, including the immune, endocrine systems, as well as on the adult behavior of the offspring.

Thyroid hormone influences antioxidant defense system in adult rat brain

The objective of the current study was to find out whether thyroid hormone influences antioxidant defense parameters of rat brain. Several oxidative stress and antioxidant defense parameters of mitochondrial (MF) and post-mitochondrial (PMF) fractions of cerebral cortex (CC) of adult rats were compared among euthyroid (control), hypothyroid [6-n-propylthiouracil (PTU)-challenged], and hyperthyroid (T3-treatment to PTU-challenged rats) states. Oxidative stress parameters, such as thiobarbituric acid-reactive substances (TBA-RS) and protein carbonyl content (PC), in MF declined following PTU challenge in comparison to euthyroid rats. On the other hand, when PTU-challenged rats were treated with T3, a significant increase in the level of oxidative stress parameters in MF was recorded. Hydrogen peroxide content of MF as well as PMF of CC was elevated by PTU-challenge and brought to normal level by subsequent treatment of T3. Although mitochondrial glutathione (reduced or oxidized) status did not change following PTU challenge, a significant reduction in oxidized glutathione (GSSG) level was noticed in PMF following the treatment. T3 administration to PTU-challenged rats had no effect on mitochondrial glutathione status. Total and CN-resistant superoxide dismutase (SOD) activities in MF of CC augmented following PTU challenge. CN-resistant SOD activity did not change when PTU-challenged rats were treated with T3. Although CN-sensitive SOD activity of PMF remained unaltered in response to PTU challenge, its activity increased when PTU-challenged rats were treated with T3. Catalase activity in PMF of CC of PTU-challenged rats increased, whereas the activity was decreased when hypothyroid rats were treated with T3. Similarly, total and Se-dependent glutathione peroxidase (GPx) activities of MF increased following PTU challenge and reduced following administration of T3. Se-independent GPx activity of MF and PMF and glutathione reductase activity of PMF decreased following PTU challenge and did not change further when rats were treated with T3. On the other hand, glutathione S-transferase activity of MF and PMF of CC did not change following PTU challenge but decreased below detectable level following T3 treatment. Results of the current investigation suggest that antioxidant defense parameters of adult rat brain are considerably influenced by thyroid states of the body.

Effect of thyroid hormone on the development and gene expression of hormone receptors in rat testes in vivo

Thyroid hormone is known to play a pivotal role in the regulation of prepuberal rat testes development and function with specific influence on the differentiation of Sertoli cells, the only cell type that expresses thyroid hormone receptors in testes. To explore in vivo effects of thyroid hormone on testes development and the regulation of testicular gene expression, the hyper- and hypothyroid rat models were established by T3 injection to pups (ip 100 microg/kg bw) and by oral administration of 6-N-propyl-2-thiouracil (PTU) to the lactating mother from days 1 to 21 post-delivery. Half of the rats from each group were sacrificed at 21 days of age, and the other half were allowed to recover with discontinued treatments from day 22 to day 50. At 21 days of age, a significantly elevated serum T3 level was observed in hyperthyroid rats (179.5 ng/dl) vs controls (97.5 ng/dl), and in hypothyroid rats a significantly lower level of T3 was detected (26.1 ng/dl). However, serum T4 concentration was significantly lower in both hyper- (0.105 microg/dl) and hypothyroid (0.058 microg/dl) rats compared to the controls (2.48 microg/dl). In recovered rats in which the serum T3 and T4 were restored to normal, the serum T levels remained remarkably lower in both hyper- and hypothyroid rats. The significantly decreased body and testes weights observed in both hyper- and hypothyroid rats at 21 days of age were not restored by the time they were 50 days old. Histological analyses of testes of 21-day-old hypothyroid rats revealed smaller-sized seminiferous tubules, incomplete lumen formation and delayed germ cell differentiation and in hyperthyroid rats an increased number of early stage spermatocytes was found. Testicular mRNA levels of follicle-stimulating hormone receptor (FSH-R), luteinizing hormone receptor (LH-R) and androgen binding protein (ABP) were studied by Northern blot hybridization. At 21 days of age data showed that FSH-R mRNA levels were significantly higher in both hyper- and hypothyroid rat testes compared to controls, but no differences were detected in recovered 50-day-old rats. Significantly decreased ABP mRNA levels were detected only in hypothyroid rat testes compared to those in both the hyperthyroid and control groups at 21 days of age, but no significant change was observed in recovered 50-day-old rats. To further evaluate the effect of thyroid hormone on the Leydig cell function, the 2.3/2.6 kb specific LH-R hybridization bands were detected with rat LH-R cRNA probe. Significant suppression of LH-R mRNA levels was only observed in the hypothyroid rat testes at 50 days of age. The testicular thyroid hormone receptors (TRs) and the regulation of TR by thyroid hormone were investigated using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) assays. Both TRalpha and TRbeta mRNAs were identified in the testes from 21- and/or 50-day-old rats. TRalpha mRNA levels were significantly increased in hypothyroid rat testes and were suppressed in hyperthyroid rats at 21 days of age and no changes of TRalpha mRNA were found in recovered animals. Our in vivo data strongly suggest that the thyroid hormone directly affects the development of prepuberal testes and the regulation of FSH-R and ABP gene expression in Sertoli cells, as well as the LH-R mRNA levels in Leydig cells, which may lead to further modulating the effect of gonadotropins on testes function.

Effect of hypothyroidism induced by propylthiouracil on ovarian function and structure in offspring from treated mothers (Rats)

The aim of the present study was to investigate the effects of hypothyroidism induced during the pre- and postnatal periods of life on ovarian function and structure in offspring (pups) 120 days of age. Three groups were used. In the prenatal group, treatment was given from conception to parturition. In the postnatal group, treatment was given from parturition to 25 days postpartum. Hypothyroidism was induced by administration of 0.1% 6-n-propyl-2-thiouracil (PTU) in the drinking water of mothers. Body weights of the offspring were measured weekly. In each group, ten offspring were sacrificed at 120 days of age. Postnatal PTU treated pups showed delay in eye opening, teething, fur development, and weaning (35-37 days) compared to control animals (28-30 days). Body weight of offspring in the postnatal PTU treatment group was significantly decreased (P < 0.001), while the prenatal PTU treatment group showed a significant increase (P < 0.0001) compared to control animals. There was a significant (P < 0.05) reduction in paired ovarian weight of offspring in the postnatal PTU treatment group compared to control animals. Diameter of the ovaries was not affected by any treatment. Regarding the morphometery, only offspring in the prenatal PTU treatment group showed a significant (P < 0.001) increase in the diameter of graafian follicles. No significant difference was observed in morphometery of the granulosa layer, primary, and developing follicles of control and all treated groups. Number of primary, developing, and graafian follicles of all the treated groups was similar to that of the control group. The corpora lutea of the postnatal PTU treated group contained a population of large numbers of luteal cells compared to the control group. The prenatal PTU treated group did not exhibit a profound effect on ovarian morphology, histology, and morphometery. No difference was found in the serum estradiol concentration of control and PTU treated groups. J. Exp. Zool. 293:407-413, 2002.

Essential role of thyroid hormones in maturation of olfactory receptor neurons: an immunocytochemical study of number and cytoarchitecture of OMP-positive cells in developing rats

Neurogenesis and proliferation of olfactory receptor neurons (ORNs) in the olfactory epithelium (OE) are reduced in postnatal hypothyroid rats and upregulated following restoration of thyroid function, leading to compensatory growth and restitution of these deficits [Paternostro M.A. and Meisami E. (1993). Dev. Brain Res. 76, 151-161; Paternostro M.A. and Meisami E. (1994). Dev. Brain Res. 83, 151-162]. To investigate thyroid hormonal role on maturation of ORNs, serial sections of the septal OE from normal newborn, 25- and 90-day-old rats were immunostained for olfactory marker protein (OMP), a marker for mature ORNs, and compared with the same from age-matched hypothyroid rats and those allowed to recover from thyroid deficiency at the time of weaning (day 25). The parameters studied were the localization and distribution of the OMP(+) cells within the OE and their density and total number. Hypothyroidism was induced by adding the reversible goitrogen propylthiouracil (PTU) to the rats' drinking water (1 g/l) from birth to days 25 or 90. Recovery from hypothyroidism was induced by withdrawal of PTU at day 25. The OMP(+) cells occupied a distinct, broad band in the normal rat OE, while in hypothyroid animal, this band was narrow and restricted to OE's apical zones. Recovery resulted in broadening of the OMP(+) cell band and normalized distribution of OMP(+) cells as evident in the 90-day-old recovery animals. In normal control rats, density of OMP(+) cells increased by 2.5- and 1.3-fold during the suckling and post-weaning period (days 25-90), while total numbers of these cells increased by 12- and 3-fold, respectively, during the same age periods. Hypothyroidism decreased the growth in density by 25 and 30%, while total number of OMP(+) neurons were reduced by 40 and 70% in the 25- and 90-day-old animals, respectively. Withdrawal of PTU resulted in marked restoration of these deficits so that, at 90 days, the total number of OMP(+) cells were only 20% less than 90-day-old controls. These results indicate that thyroid hormones are essential for maturation of single ORNs and accretion of new mature ORNs in the OE of suckling and post-weaning rat. Also, the process of maturation and the final number of mature ORNs show remarkable recovery from hypothyroid-induced growth retardation.

Marked restoration of density and total number of mature (knob-bearing) olfactory receptor neurons in rats recovering from early hypothyroid-induced growth retardation

Our recent studies have shown that restoration of thyroid function in developing hypothyroid rats results in upregulation of olfactory neurogenesis and compensatory proliferation of olfactory receptor neurons (ORN) in the olfactory epithelium (OE) (Paternostro and Meisami, Dev. Brain Res., 76 (1993) 151-161; ibid., 83 (1994) 151-162). It was not clear, however, whether the newly forming ORNs undergo complete maturational stages. To determine the effects of restoration of thyroid function on maturation of ORNs, the density and total number of mature ORNs were estimated in the OE of euthyroid and hypothyroid rats at postnatal days 1, 12, 25 and 90 and the results were compared with those in rats allowed to recover from early thyroid deficiency at weaning (day 25). As a marker for mature ORNs, and on the basis of one olfactory dendritic knob per ORN, the density and total number of the olfactory knobs were determined in the entire extent of the OE covering the nasal septum. Hypothyroidism was induced by adding propylthiouracil (PTU) to the drinking water (1 g/l) from birth until days 12, 25 or 90 of age. Recovery from hypothyroidism was induced by withdrawal of PTU at day 25, leading to restoration of thyroid function and somatic growth recovery. The density of olfactory knobs was determined in 1 microm semi-thin sections stained with toluidine blue. In the normal rats, the number of olfactory knobs (= mature ORNs) increased 8.5- and 3-fold during postnatal days 1-25 and 25-90 respectively, reaching a mean value of 4 X 10(6)/septal OE, compared to 2.8- and 1.4-fold, respectively, for the hypothyroid rats. This led to deficits of 51% and 76% in the number of mature ORNs in the 25- and 90-day-old hypothyroid rats. In rats allowed to recover, the number of mature ORNs increased 4.5-fold during postnatal days 25-90 (3 X > hypothyroid rats and 1.5 X > controls). The results indicate marked upregulation of the maturational process of the ORNs and their compensatory accretion within the OE of the recovery group. The recovery process was not complete however, as indicated by a remaining deficit of about 25% in the total number of mature ORN, compared to normal 90-day controls. Thus thyroid hormones are essential for accretion of new mature ORNs in both the suckling and postweaning rats. Also, the ORNs show a remarkable ability to recover from severe early hypothyroid-induced growth retardation and attain normal mature state.

Quantitative [3H]thymidine autoradiography of neurogenesis in the olfactory epithelium of developing normal, hypothyroid and hypothyroid-rehabilitated rats

We recently reported that postnatal hypothyroidism results in marked reduction in surface area and cell number in the rat olfactory epithelium (OE) and recovery from this condition is accompanied by compensatory growth and restitution of these parameters. To explore the correlative changes in olfactory neurogenesis, i.e. mitotic activity of basal cells (BCs) and migration and survival of developing olfactory receptor neurons (ORNs), hypothyroid rats at postnatal (P) days of P10, P25 and P75 were injected with [3H]thymidine and OE was examined by quantitative autoradiography to determine the density of labeled nuclei at the BC and ORN zones at days 1, 5 and 15 post-injection. These data were compared with those of age-matched controls as well as young adult rats allowed to recover from hypothyroidism at the end of the suckling period (P25). Hypothyroidism was induced by administration of propylthiouracil (PTU) from birth in the drinking water (1 g/l) for 10, 25 and 90 days; recovery was induced by withdrawal of PTU at P25. The results indicated that the densities of labeled nuclei in the BC and ORN zones were not significantly altered in the suckling hypothyroid rats. In the P75 hypothyroid rats density of labeled BC nuclei was unaffected 1 day after injection but was significantly (36%) more than controls 5 days after injection; the density of neuronal nuclei in the ORN zone of P75 injected rats was markedly and significantly reduced (56% and 37% at 5- and 15-days post-injection). Data indicate that mitotic activity of BCs and their migration into the ORN zone is not affected in the hypothyroid infant rats but migration and/or survival of developing ORNs are markedly reduced in the postweaning growing rats made hypothyroid from birth. In rats allowed to recover from hypothyroidism at P25 and injected with labeled thymidine at P75, the density of labeled BC nuclei were significantly increased (48% and 43% at 1- and 5-days post-injection) compared to normal rats suggesting elevated levels of neurogenesis; density of ORN nuclei, however, were the same as controls. The results indicate critical regulatory influences of thyroid hormones on olfactory neurogenesis in the rat olfactory receptor sheet, in particular during the postweaning period.

Enhancement of seminiferous tubular growth and spermatogenesis in testes of rats recovering from early hypothyroidism: a quantitative study

Testicular weight and DNA content were markedly reduced (63 and 69%) in weanling Long-Evans rat pups rendered hypothyroid from birth by administration of propylthiouracil (PTU), a reversible goitrogen. These growth deficits worsened to > 80% by continuing hypothyroidism beyond weaning, to days 50 and 90. Recovery of thyroid function, brought about by discontinuing PTU at weaning, resulted in a paradoxical stimulation of testis growth, amounting to increased weight (40%), DNA content (60%) and size by 90 days, compared to age-matched controls. In the 25-day or older hypothyroid rats, testicular structure was immature and spermatogenesis markedly delayed, as evident by closed lumen and significantly reduced diameter of seminiferous tubules (38%), thickness of germinal layer (70%), and number of primary spermatocytes (86%), compared to control. Hypothyroidism did not alter the number of tubules per testis cross section. In the 90-day recovery rats, numbers of seminiferous tubules were unchanged but tubular diameter was significantly (20%) larger than in controls and spermatogenesis appeared very active as indicated by significantly increased germinal layer thickness (22%) and total number and density of primary spermatocytes (55% and 40%). The results show that although postnatal hypothyroidism is deleterious for testicular growth and spermatogenesis, recovery from this condition leads to enhanced seminiferous tubular growth and spermatogenesis.

Developmental plasticity of the rat olfactory receptor sheet as shown by complete recovery of surface area and cell number from extensive early hypothyroid growth retardation

To assess the effects of early thyroid deficiency, and recovery from this condition on growth and development of olfactory epithelium (OE), male Sprague-Dawley rat pups were rendered hypothyroid by addition of propylthiouracil (PTU) to their drinking water from birth. At weaning some rats continued to receive PTU while others ere allowed to recover by withdrawal of PTU. Body weights and plasma thyroxine levels were determined in all groups. At the ages of 25, 50 and 90 days, the OE of these hypothyroid and 'recovery' rats were compared with age-matched controls for surface area, epithelial thickness, density and total number of olfactory receptor neurons, basal cells and supporting cells, using morphometric and cell counting methods. Normal rats showed marked and highly significant increases in the OE surface area and olfactory neuron number (2.6- and 2.3-folds) during the post-weaning period. In the hypothyroid rats, body growth and thyroxine levels were severely suppressed. The OE in the 25-day-old hypothyroid rats showed more than 40% reduction in surface area and cell number, compared to controls, but mean epithelial thickness and surface density of cells were unchanged. In the post-weaning hypothyroid rats, the expansion of surface area was severely retarded, and increase in cell number ceased entirely. In rats allowed to recover by PTU withdrawal, by 90 days of age, body weight and size had markedly increased but had not caught up completely; however, thyroxine levels were restored to normal and the surface area and cell number in the OE had increased in a compensatory manner, completely restoring the deficiencies in OE growth, including surface area, numbers of receptor neurons, basal cells and supporting cells. The results indicate marked growth plasticity of OE in the post-weaning rats. This pronounced ability to recover from early growth retardation contrasts with that seen in central neural structures, and indicates the great potential of OE for use as a model neural system for the study of recovery from early damage and growth retardation.

Thyroid hormones and testis development: a model system for increasing testis growth and sperm production

In conclusion, the unique transient hypothyroidism system described here provides a noninvasive means by which testicular size and sperm production can be increased to unprecedented degrees. This system may be useful as a model for understanding the factors which regulate testicular growth and the eventual size that the testes attain, as well as the factors which normally establish and then maintain the amount of sperm produced by an animal.

Lack of thyroid hormones but not their excess affects the maturation of olfactory receptor neurons: a quantitative morphologic study in the postnatal rat

To study quantitatively actions of thyroid hormones on maturation of olfactory receptor neurons (ORN), surface density and total number of receptor knobs (1 knob/ORN) were measured in 1 mu sections from septal olfactory epithelium of newborn, 12- and 25 day normal, hypo- and hyperthyroid rats. Hypothyroidism was induced by adding to drinking water n-propylthiouracil (0.1% w/v) from birth. Hyperthyroidism was induced by daily injection of pups with T4 (1-thyroxine, 0.3 microgram/g b.w., s.c.). Experimental pups showed all the signs of hypo- and hyperthyroidism. Between days 1-25, normal pups showed marked increase in surface area of septal olfactory epithelium (6x), total number (12x) and surface density (#/mm2, 2x) of mature ORNs. Thyroid deficient rats showed, by day 12, marked reductions in epithelial surface area and total number of mature ORNs; these and the surface density deficits became very pronounced by 25 day (30% area, 27% density, 47% # mature ORNs). Hyperthyroid rats, however, did not show an increase in any of these parameters over controls. Although total number of ORNs (mature and immature), as measured by number of nuclei, was also reduced in hypothyroid pups, surface density was not altered, indicating that maturation of ORNs, but not their local accretion is altered in thyroid deficiency. The results indicate that thyroid hormones are essential for normal proliferative expansion of olfactory epithelium and for maturation of ORNs postnatally. These actions of thyroid hormones are not increased or accelerated by excess T4 suggesting saturation of the hormone receptor system at the normal plasma level.

Selective effects of thyroid hormonal deprivation on growth and development of olfactory receptor sheet during the early postnatal period: a morphometric and cell count study in the rat

Light microscopic numerical and morphometric studies were conducted on the olfactory epithelium of postnatal normal and hypothyroid rats. The normal rat olfactory epithelium undergoes marked growth and development during the suckling period (days 1-25): thickness, 50%; area, x 8, total number of olfactory neurons, basal and supporting cells, x 10, x 11 and x 8, respectively. The effects of thyroid hormonal deprivation on these proliferative postnatal growth changes were studied by adding PTU (n-propylthiouracil, a reversible antithyroid goitrogen) to the litter's drinking water from birth to weaning (day 25). The general architecture of naso-olfactory cavities as well as the histology and thickness of the olfactory epithelium were unaffected in the hypothyroid pups. However, the surface area of the olfactory receptor sheet was reduced by 40%, the reduction occurring throughout the cavity, though not uniformly. The total number of olfactory neurons, supporting and basal cells were reduced by 33, 45 and 47%, respectively. These results indicate that the postnatal vertical accretion of olfactory neurons occurring across the epithelial thickness is unaffected in the hypothyroid pups, while the horizontal proliferation of neurons accompanying the expansion of the sheet's surface area is markedly reduced. The results suggest differential effects of thyroid hormones on these modes of proliferative growth and imply further that in addition to possible direct effects, the influence of thyroid hormones on developmental growth of the olfactory epithelial sheets may be secondary to effects on the underlying submucosal connective tissue.

Acquisition of acoustic startle response in relation to growth and thyroid function in rats

Thiourea was administered by gavage to pregnant and lactating rats from gestational day 18 until postnatal day 10, at doses of 100 or 250 mg per day (approximately 350 or 900 mg/kg/day). From the day after birth until postnatal day 14 the neonates were weighed, and beginning on postnatal day 10 acquisition of acoustic startle reflex was assessed. Serum thyroxine and thyrotropin were measured on postnatal day 14. High dose thiourea severely inhibited thyroid function in pups, significantly depressed body growth, and retarded development of the acoustic startle response. At the low dose there was a significant reduction in growth, but no effect on acquisition of acoustic startle reflex. Low dose pups appeared to be functionally euthyroid (free T4 and thyrotropin were not significantly different from control). In conjunction with other studies, these results strongly suggest that, while development of acoustic startle is dependent upon thyroid hormone, it is not readily affected by retarded body growth.

Exploratory behavior, learning ability, and thyroid hormonal responses to stress in female rats rehabilitating from postnatal hypothyroidism

Newborn female Long-Evans rats were divided into groups of normal, hypothyroid [0.1% propylthiouracil (PTU) a reversible antithyroid goitrogen in the litter's drinking water], and hypothyroid rehabilitated (PTU water from birth to day 25, normal water thereafter). The rats were tested for several adaptive behavioral tasks between 40 and 90 days of age. At day 50, serum concentration of TSH and thyroid hormones revealed no detectable amounts of T4 and a 10-fold increase in TSH in the hypothyroid rats. At the same age in the rehabilitated animals, TSH levels were still below normal, a deficit fully normalized by day 90. Normal 50-day-old rats responded to pain stress (electric footshocks) by a significant depression of serum T4 and elevation of T3 levels within 10 min of treatment, whereas the rehabilitated animals exhibited an opposite pattern of response, i.e., an increase in the circulating T4 and a decrease in T3. At 50 days of age, both hypothyroid and rehabilitated rats showed decreased exploratory activity and no habituation in the hole-board test, whereas the locomotor activity of the rehabilitated females was significantly higher than that of the normals. No differences were found in the scores of passive avoidance learning (one trial step-through) among the three groups. Similarly, the rate of acquisition of the active one-way conditioned avoidance response (CAR) of the hypothyroid and rehabilitated rats did not differ significantly from that of the controls. However, the hypothyroid rats required significantly more unconditioned stimuli (footshocks) to acquire CAR and showed longer response latency and less intertrial responses. Although the hypothyroid rats showed no extinction of CAR, the rehabilitated rats were capable of extinction to an extent indistinguishable from normal rats. But compared with the normal animals, the rehabilitated rats showed significantly higher intertrial activity during both the acquisition and extinction phases of CAR.

Rehabilitation from neonatal hypothyroidism: spontaneous motor activity, exploratory behavior, avoidance learning and responses of pituitary--thyroid axis to stress in male rats

Long-Evans male rats were made hypothyroid from birth by the addition of 6-N-propylthiouracil (PTU) to their drinking water (0.1%). A group of animals was rehabilitated beginning at postnatal day 25 by withdrawal of the PTU from the drinking water. Subsequently, the rats were tested for a variety of behavioral tasks. Serum concentrations of thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) were determined by radioimmunoassay. At 50 days of age, PTU-treated rats had non-detectable levels of T4 but an eight-fold increase of TSH. In 50-day-old, neonatally hypothyroid but rehabilitated rats, serum TSH and T3 were normal, although T4 was still significantly lower. At 90 days of age, basal levels of TSH and thyroid hormones were normal in the rehabilitated rats, but thyroid hormone secretion in response to various types of neural stress was markedly altered. Comparison of passive avoidance learning revealed no significant alteration in the memory retention of either PTU-treated or rehabilitated animals. The 50-day-old, rehabilitated rats showed increased locomotor activity both in running-wheel and in hole-board tests; this hyperactivity, though markedly reduced, still persisted at day 90. In the early phase of rehabilitation (50 days of age), decreases in exploratory activity and lack of habituation occurred with the hole-board test; by the late phase of rehabilitation (90 days of age) these behavioral parameters had become normal. These results suggest generally longer periods of plasticity of the brain and better prospects for rehabilitation from neonatal cretinoid retardation than commonly believed. Specifically, the pituitary-thyroid system and neural mechanisms integrating adaptive behavior possess considerable capacity for spontaneous recovery from hypothyroidism; certain types of altered neuroendocrine and behavioral responses appear to be less amenable to rehabilitation or require longer periods for complete rehabilitation.