Development of home orientation in hypothyroid and hyperthyroid rat pups

Perinatal hypothyroidism increases play behaviors in juvenile rats

Thyroid hormones play an instrumental role in the development of the central nervous system. During early development, the fetus is dependent on maternal thyroid hormone production due to the dysfunction of its own thyroid gland. Thus, maternal thyroid dysfunction has been shown to elicit significant abnormalities in neural development, neurochemistry, and behavior in offspring. Previous reports have suggested that human maternal hypothyroidism may increase the chances of having children with autism spectrum disorder and attention-deficit/hyperactivity disorder. However, very few studies have evaluated social behaviors in animal models of perinatal hypothyroidism. To evaluate the possibility that hypothyroidism during development influences the expression one of the most commonly observed non-reproductive social behaviors, juvenile play, we used the validated rat model of perinatal hypothyroidism by methimazole administration (MMI; 0.025% in drinking water) from GD12-PD23. Control animals had regular drinking water. During adolescence (PD33-35), we tested subjects for juvenile play behavior by introducing them to a same-sex, unfamiliar (since weaning) littermate for 30min. Play behaviors and other behaviors (sleep, social contact, locomotion) were then scored. MMI-treated subjects played more than twice as much as control animals, and the increase in some behaviors was particularly dramatic in males. Locomotor and other affiliative social behaviors were unaffected. These data suggest that perinatal hypothyroidism may alter the organization of the neural networks regulating play behaviors, but not other social behaviors. Moreover, this implicates perinatal hypothyroidism as a potential etiological factor in the development of neurobehavioral disorders, particularly those characterized by heightened social interactions and impulsivity.

Effects of excess thyroid hormone on cell death, cell proliferation, and new neuron incorporation in the adult zebra finch telencephalon

Widespread telencephalic neuronal replacement occurs throughout life in birds. We explored the potential relationship between thyroxine (T4) and cell turnover in the adult male zebra finch. We found that many cells in the zebra finch brain, including long-projection neurons in the high vocal center (HVC), stained positively with an antibody to thyroid hormone receptors (TR). Labeling was generally weak in the ventricular zone (VZ) that gives rise to new neurons but some proliferative VZ cells and/or their progeny, identified by [3H]-thymidine labeling, co-labeled with anti-TR antibody. Acute T4 treatment dramatically increased the number of pyknotic and TUNEL-positive cells in HVC and other telencephalic regions. In contrast, degenerating cells were never observed in the archistriatum or sub-telencephalic regions, suggesting that excess T4 augments cell death selectively in regions that show naturally occurring neuronal turnover. VZ mitotic activity was not altered shortly after acute T4 treatment at a dosage that stimulated cell death, although [3H]-labeling intensity per cell was slightly reduced. Moreover, the incorporation rates for neurons formed shortly before or after acute hormone treatment were no different from control values. Chronic T4 treatment resulted in a reduction in the total number of HVC neurons. Thus, hyperthyroidism augmented neuronal death, which was not compensated for by neuronal replacement. Collectively, these results indicate that excess T4 affects adult neuronal turnover in birds, and raises the possibility that thyroxine plays an important role in the postnatal development of the avian brain and vocal behavior.

Influences of early thyroid hormone manipulations: delays in pup motor and exploratory behavior are evident in adult operant performance

The effects of thyroid hormone depletion and enhancement on litter size, survival, body mass, ambulation, quadrant crossing, home orientation, day of eye opening, and free serum T3 and T4 levels were examined in Study 1. In Study 2, the effects of the timing of prenatal insult and the level of thyroid hormone depletion on litter size, survival, body mass, and free serum T3 and T4 levels were examined. Upon the completion of Study 1, randomly selected pups were maintained on ad-libitum water and food for 2 years, and performance was evaluated on fixed and variable ratio schedules, fixed and variable interval schedules, and probability and reversal learning tasks (Study 3). In Study 4, human subjects diagnosed with and treated for either congenital hypothyroidism or congenital hyperthyroidism were tested on the operant procedures used in Study 3, as well as on a series of simple reaction time, serial timing, and conjunctive and disjunctive search tasks. Dose-dependent decreases in survival and delays in the presentation of early motor and exploratory skills were observed following thyroid hormone depletion; dose-dependent accelerations in the presentation of early motor and exploratory skills were observed following thyroid hormone enhancement. Pups that had been prenatally exposed to propylthiouracil (PTU) 1-2 years after the return of thyroid hormones to baseline levels were significantly less accurate at timing on fixed and variable interval schedules, demonstrated an inability to allocate responding on probability tasks, and committed more errors during original learning (OL) and on each reversal problem. Similar deficits were observed in follow-up tests with humans diagnosed with congenital hypothyroidism, as were deficits in serial timing and visual searching. Collectively, the present results demonstrate that the pervasive and negative effects of prenatal thyroid deficiency on early behavior are also expressed during adult operant performance.

Congenital hypothyroidism impairs response alternation discrimination behavior

The behavior of six congenitally hypothyroid and six normal control rats was assessed under forced alternation fixed-ratio, alternating lever cyclic-ratio (ALCR) and progressive-ratio schedules of reinforcement. Hypothyroidism was produced by adding methimazole (MMI) to the drinking water of pregnant dams from embryonic day 16 to postnatal day 25. There were no differences in behavioral performance between MMI-treated and control animals under the fixed-ratio and progressive ratio schedules. There were also no differences in circulating triiodothyronine levels between groups at the end of the study. Under the ALCR schedule, when alternation of responding was forced during the first three cycles but both levers (choice) were presented during the last three cycles (correct lever active), the entire control group reached a competency criteria in nine sessions. In contrast, only two MMI-treated animals reached criteria after 17 sessions, and the remaining four MMI-treated animals did not reach criteria by 30 sessions of training. These results suggest that congenital hypothyroidism impairs learning when a discrimination between correct and incorrect operanda is made available.

The effects of thyroid hormone level and action in developing brain: are these targets for the actions of polychlorinated biphenyls and dioxins?

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

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.

Maturation of olfactory exploration in golden hamsters

Following placement into a test cage filled with pine shavings, a litter of 7-8 golden hamster pups (aged 3-18 days postnatal: P3-18) initially displays a period of locomotion which ends reliably in huddling. The latency to establish a huddle (i.e., the duration of locomotion) is significantly longer in the presence of novel odors (fresh or lemon shavings) than more familiar odors (slightly soiled fresh or lemon shavings) but only in pups aged P12 or older. Pups aged P9 or younger do not locomote differentially in the presence of novel or familiar odors. This age difference represents the emergence of olfactory exploration in hamsters between P9 and P12. Exploration of novel odors interferes with initial attempts to establish a single huddle site by a litter, but does not preclude the ultimate aggregation of all pups at a single site as guided by conspecific odors and possibly thermotactile cues as well. Such shifts in the control of behavior by non-nest and nest-related, conspecific stimuli correspond well with the first occurrence of nest exits at P11-12 (e.g., Dieterlen, 1959) coupled with the persistent return of hamster pups to the maternal nest for as long as it is maintained (Rowell, 1961).

Development of home orientation in offspring of protein-restricted cats

The development of home orientation was evaluated in 2-14-day-old kittens nursing from mothers fed a protein-restricted or control diet during late gestation and lactation. Although restricted kittens remained in the home when placed in it, their ability to return to the home was delayed when they were removed from it. Restricted kittens also exhibited aberrant locomotor development and an increased frequency of loss of balance (upsets) en route to the home. During postnatal Week 1, vocalization frequency, an index of kitten disturbance when outside the home, was increased in restricted kittens tested in the home, and in adjacent and diagonal corners. Restricted kittens persisted in vocalizing more frequently than controls in home and adjacent corner tests during Week 2. Taken together, these data suggest that maternal protein restriction during late gestation and lactation disrupts the development of home orientation behavior by impairing locomotor function and increasing emotional responsiveness.

Sensory influences on homing of stunted rat pups

The effects of 2 methods of restricting food intake--large-litter rearing and rotation between lactating and nonlactating females--on sensory factors involved in homing to the nest by rat pups were examined. Homing was observed in the unaltered home cage, when olfactory cues were altered and when visual cues were altered. Stunted animals homed less in the unaltered cage than did well-nourished controls as a result of a maturational delay. Prior to eye opening, stunted animals showed greater disruption of homing when olfactory cues were altered and after eye opening they showed greater disruption when visual cues were altered. These effects could reflect decreased sensitivity, an inability to use alternate cues, or behavioral disruption by novel stimulation. Nonnutritional factors were also found to affect homing as the 2 well-nourished groups differed in their behaviors. These differences appeared to be due to animals reared in small litters maturing more slowly than animals rotated between females.

Home-orienting behavior in rat pups surviving postnatal or intergenerational malnutrition

Growth and home-orienting behavior were studied during the 1st 12 days of life in 3 groups of rat pups: (1) those with intergenerational malnutrition; (2) those with postnatal malnutrition produced by cross-fostering pups born of well-nourished mothers to lactating females maintained on a low-protein diet; and (3) well-nourished control pups. Growth of pups was impaired in both malnourished groups, and eye-opening was similarly delayed. On tests of homing behavior, both malnourished groups were impaired to a similar extent and were less likely to find their way to the nest than were control pups. However, survival rates were significantly lower (50%) in the postnatally malnourished group but not in the intergenerationally malnourished group (73%), as compared with controls (100%). These findings suggest that adaptation occurs in long-term malnutrition with increased survival. However, physical characteristics and homing behavior in early life are not distinguishable in those surviving short- or long-term malnutrition. This is in contrast to the finding of more severe behavioral deficits in later life among rats subjected to many generations of malnutrition than among those malnourished for only 1 generation.

Development of olfactory and thermal responsiveness in hypothyroid and hyperthyroid rat pups

Hypo- and hyperthyroid rat pups were tested for their responsiveness on an olfactory and a thermal gradient in an attempt to evaluate the effects of perinatal thyroid manipulation on the ability of the pups to use these cues in orientation. Hypothyroid pups showed a delay and hyperthyroid pups an acceleration in the development of orientation along the olfactory gradient, relative to controls. None of the pups was able to utilize a thermal gradient in directed orientation but pups moved less at the warm end of the gradient than at the cool end. This differential responsiveness was strongest in the hypothyroid pups, suggesting that these pups could utilize thermal cues in an orthokinetic orientation.