Thyroxine: effects of neonatal administration on maturation, development, and behavior

Phenobarbital: effects of long-term administration on behavior and brain of artificially reared rats

Two doses of phenobarbital were given daily for 2 weeks to infant rats fed by intragastric cannulas. The larger dose (60 milligrams per kilogram of body weight) resulted in decreased spontaneous activity and increased responses to novel stimuli. The smaller dose (15 milligrams per kilogram) resulted in increased spontaneous activity and also an increase of responses to novel stimuli. The larger dose produced a 12 percent reduction in brain growth, while the smaller dose was associated with a 3 percent reduction in brain growth.

Effect of thyroxine on the development of somatosensory and visual evoked potentials in the rat

The thyroid hormone thyroxine (T4), administered post-natally to neonatal rats, has been shown to accelerate development of auditory function, as expressed by auditory nerve-brainstem evoked responses. This study investigated whether this earlier development was also reflected in other sensory modalities. Rat pups were injected with T4 from the day of birth for 10 consecutive days. Somatosensory evoked potentials, both from the cortex and from sub-cortical structures, and flash-elicited visual evoked potentials (VEP), were recorded at various ages up to 3 months. The recordings were compared with those from control rats from the same litters. Only a minimal difference was found between the experimental and control groups, the most significant being in the VEP at age 12 days, by which time the eyes of most of the experimental rats had opened, which was not the case for the majority of control rats. This difference disappeared with eye-opening in the control rats. Although T4 is known to affect myelinization and synaptic transmission in developing rat brain, this apparently only minimally affects the functioning of the brain as expressed by evoked potentials, both in the short and long term. The main effect of neonatal hyperthyroidism in these rats appeared to be accelerated development of the end organ (the eye and the ear).

Studies on L-arginase in developing rat small intestine, brain, and kidney. II. Effect of hydrocortisone and thyroxine

The influences of hydrocortisone and thyroxine on the developmental changes of arginase activity in intestine, kidney, and brain of suckling rats were studied. A single injection of hydrocortisone (50 mg/kg) into rats aged 9 days evoked premature increase of jejunal arginase activity due to precocious formation of arginase A4. Arginase A4 can be detected about 48 hr after hydrocortisone injection, whereas in intact rats the enzyme appears in the intestinal mucosa on the 19th-21st days of postnatal life. After hydrocortisone administration to rats aged 6 days, a similar pattern of arginase activity in jejunum was observed. Under the same conditions, the influence of hydrocortisone on kidney arginase was weaker. The hormone did not have any influence on the activity of brain arginase. Daily injection of thyroxine (2 mg/kg) to 6-day-old rats (for 6 consecutive days) caused a precocious increase of the arginase activity in intestine. Under the same conditions, only a slight increase of the arginase activity was observed in kidney, whereas in brain the activity was unaffected.

Age differences in water maze performance and swimming behavior in the rat

This study was designed to determine the earliest age at which a water maze task could be used to measure learning in rats. In Experiment 1, 24 litters of Sprague-Dawley rats were used. Eight litters per group were tested beginning at 18, 28, or 38 days of age for their ability to learn a positional discrimination in a Y-shaped water maze. Results indicated that the 28 day olds were better able to learn the task than were the 18 or 38 day groups. Performance by the 38 day olds was impaired by an immobilization response. This response is known to be influenced by catecholaminergically-active drugs [18, 19, 20], and may result from ongoing maturation in these systems. The difference in learning abilities between the 18 day and 28 day groups was further investigated in Experiment 2. In this study, 24 litters of Sprague-Dawley rats were tested for acquisition of the discrimination at 18, 20, or 22 days of age. At each age, 8 litters were tested. Results showed that significant improvements in the ability to learn the Y-maze task occurred between 20 and 22 days of age. Results of both experiments suggest that this measure of learning may be employed in rats as young as 22 days of age, but should be avoided in 38 day old rats.

Dietary and metabolic effects on rhesus social behaviour: neonatal thyroxine reductions

Assessment was made of the behavioural development of 3 monkeys whose levels of thyroxine were reduced by being given 131I and another 3 monkeys given a thyroxine depressant (TCAP) during the first weeks of life and maintained with low thyroxine levels for 3 months. When subsequently tested at normal thyroxine levels, these experimental subjects showed less positive social behaviour when compared with controls. When confronted with unfamiliar monkeys, the two experimental groups were less fearful than controls and also showed a lack of differential responsiveness in varying social situations, suggesting a low level of emotion. A foetal athyreotic group is also described.

Postnatal modification of hippocampal circuitry alters avoidance learning in adult rats

In rats and mice, the genetically mediated extent of the mossy fiber projection that synapses on the basal dendrites of hippocampal pyramidal cells is inversely correlated with rate of two-way avoidance (shuttle-box) learning. Postnatal hyperthyroidism, induced in 51 rat pups, resulted in marked variations of this infrapyramidal mossy fiber projection. The number of trials required for criterion performance of these rats in adulthood remained correlated with the neuroanatomical trait (r = 0.74, P less than 0.0001).

Effect of early locomotor training on evoked potentials and structural organization of visual cortex dendrites of rats during ontogeny

The effect of early locomotor training (running in a squirrel cage from an age of 1 month for 3-6 months) on evoked potentials of the rat visual cortex during ontogeny was investigated. It was established that intensified proprioceptive afferentation causes a statistically significant reduction of both the latent periods and the recovery cycles of excitability of primary responses of the visual cortex to paired light flashes, whereupon the increase in functional activity is more pronounced with respect to the indices of the excitability recovery cycle than with respect to the latent periods or primary responses. The indicated functional changes correlate with the quantitative increase in the density of dendritic spines on neurons of the visual cortex of both layer V, which are primarily the integrative-triggering apparatus of the cortex, and of the complex of layers II + III, which are primarily associative with respect to their functional significance.

Olfactory granule cell development in normal and hyperthyroid rats

Dendritic development was examined in olfactory bulbs of both normal 7-, 14-, 21- and 60-day-old rats and littermates treated on postnatal days 1-4 with 1 microgram/g body weight of L-thyroxine sodium. Tissue was processed via the Golgi-Cox technique and subjected to quantitative analyses of mitral and internal layer granule cell development. These populations of granule cells were selected because their pattern of late proliferation suggested potentially greater susceptibility to postnatal hormonal alterations. Although neonatal hyperthyroidism induces widespread acceleration of maturation, including precocious chemosensitivity, granule cell development was unaffected relative to littermate controls. Both normal and hyperthyroid groups exhibited an inverted U-shaped pattern of cellular development, with rapid dendritic dendritic growth and expansion occurring during the earliest ages tested, but with loss of processes and dendritic field size occurring after day 21.

Development of serotonin-mediated behavioral inhibition in the hyperthyroid mouse

Experimental hyperthyroidism was induced in neonatal mice by thyroxine injections at 1-3 days of age. Control and thyroxine-treated mice were subsequently tested daily for locomotor activity following injections of methysergide, a serotonin antagonist, or saline from 10 through 15 days of age. Although thyroxine-treated mice were more active than controls, the ontogeny of methysergide-induced disinhibition of locomotor activity was similar for both thyroxine-treated and control mice. The results suggest that the early maturation of serotonin-mediated inhibition of behavioral arousal is not affected by thyroxine-accelerated development. Results are discussed in terms of the specificity of the pharmacological agent, methysergide.

Development of home orientation in hypothyroid and hyperthyroid rat pups

In order to assess the effects of perinatal hypothyroidism or hyperthyroidism on the development of an integrated behavioral response, we tested hypothyroid, hyperthyroid, and control pups, as well as pups receiving thyroxine replacement therapy, for the development of the home orientation response. Hypothyroidism was induced in the pups by feeding the pregnant or lactating female a diet of .2% propylthiouracil from Day 15 of gestation to Day 22 postpartum. Pups receiving replacement therapy and pups made hyperthyroid were injected daily with thyroxine, starting at birth. The ability of the pups to initiate and maintain locomotion toward the nest was assessed between Days 4 and 22. Hyperthyroid, control, and replacement therapy pups behaved very similarly on the task, showing a peak in the percentage of pups homing between Days 12 and 16. Hypothyroid pups showed a delay in the peak percentage until Day 20, although the percentage of pups was similar to that found in other treatments. An integrated behavioral response can be delayed by hypothyroidism and still emerge apparently intact at a later age.

Behavioral effects of neonatal thyroid hormones and differential postweaning rearing in rats

Male and female rats (T rats) given an injection of 75 microgram tri-iodothyronine (T3) and 25 microgram thyroxine (T4) on Day 3 after birth were raised under enriched (E) and impoverished (I) postweaning conditions. Observations of their open-field behavior and Days 63 and 112 revealed a higher rate of activity in T groups as compared to controls (C) as well as faster running times and more entries/min in Hebb-Williams maze testing on Days 74-86, indicating behavioral hyperactivity in T rats. Active-avoidance testing at age 91 days revealed a higher number of intertrial crossings and significantly faster unconditioned response and conditioned response latencies in T as compared to C groups. Enriched postweaning rearing altered the behavioral effects in T rats comparatively little, least in the open-field and more so for escape- and avoidance-latencies and for the number of errors in the Hebb-Williams maze.

Thryo-endocrine pathology, obstetric morbidity and schizophrenia: survey of a hundred families with a schizophrenic proband

This preliminary communication reports that the mothers of 104 schizophrenic patients had: (1) a significantly higher incidence of thyroid disease than a carefully matched control group; (2) significantly more abortions, still-births and greater infant mortality. The findings and possible relevance of thyroid disease to schizophrenia are discussed. Three prospective studies currently in progress are outlined.

Effects of neonatal thyroid hormone stimulation and differential preweaning rearing on open-field behavior in adult rats

In 2 experiments, rats were given a mixture containing high doses of 1-tri-iodothyronine and 1-thyroxine-sodium on Day 4 after birth (Group T). Controls (C) received equivalent amounts of saline. During the preweaning period, half of the litters in the T and C groups were raised in an impoverished (I) condition, and half in an enriched (E) condition. Open-field ambulation, rearing, defecation, and, in Experiment 2, grooming behavior was recorded during a 4-day period at 210 days (Experiment 1) and at 90 days of age (Experiment 2). A consistent augmenting action of neonatal hormone stimulation on ambulation and rearing was found for both males and females. Hormone-treated animals did not habituate their rate of general motor activity with repeated testings in the open-field. An over-all pattern of a larger effect of hormone-treatment in the I condition than in the E condition was noted, particularly among the females. Thus, the experience of preweaning enrichment partly "normalized" the open-field behavior of the T groups. The results are discussed in relation to an hypothesis concerning the interaction of rate of development of the central nervous system and environmental stimulation in determining later behavior.

Excess neonatal thyroxine: effects on learning in infant and adolescent rats

Previous reports have suggested that neonatal thyroxine administration may enhance learning ability in rats as infants but the rats become deficient in learning as they grow older. This experiment compared neonatally thyroxine-injected rats with injected and nonhanded controls on passive and active avoidance and a geotactic cold-escape discrimination. Thyroxine-injected infant rats were superior to controls on active avoidance, inferior on passive avoidance, and equal on discriminated escape. Adolescent rats which had received postnatal thyroxine were inferior to controls in Lashley III maze acquisition whether or not they had received supplementary thryoxine at the time of training. These data suggest that differences in locomotor activity, rather than in learning ability, account for early behavioral effects and that changes other than endocrine disturbances account for the adult deficits. The results are compatible with previous contentions that excess neonatal thyroxine disrupts the timing of brain development.

Proteins of rat brain myelin in neonatal hypothyroidism

The amount of myelin and its protein composition was studied in hypothyroid rats during the first 30 days after birth. Although the brain weight was 92% of that in the controls, the yield of myelin in hypothyroid animals was only 60% of that in controls. The protein and glycoprotein ocmposition of the isolated myelin was similar in hypothyroid and control rats. The 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity in whole brain from hypothyroid animals was only 60% of the controls and reflected the decrease in myelin formation. In isolated myelin the specific activity of CNP was not different in hypothyroid and control animals with the exception of very immature animals (10 days). The major fucose-labeled glycoprotein in myelin of the hypothyroid rats had a slightly higher apparent molecular weight than that in myelin from age matched controls, probably reflecting a retardation of brain maturation and myelin formation.

Intestinal lactase activity in the suckling rat: influence of hypophysectomy and thyroidectomy

Intestinal lactase activity, which is high in the infant rat intestine but falls to a low level by the end of the third week, fails to decline in animals hypophysectomized at the age of 6 days. Treating these animals with thyroxine lowers lactase activity to the control level at 24 days, but cortisone is only partly effective. Thyroidectomy at 6 days also results in persistence of high lactase activity; thyroxine again is more effective than cortisone in reducing activity. The thyroid gland appears to play a previously unsuspected role in intestinal maturation.

Synaptogenesis in the rat cerebellum: effects of early hypo- and hyperthyroidism

The number of synapses in the molecular layer of the rat cerebellum is reduced by early hypo-and hyperthyroidism within 30 days. Hypothyroidism retards synaptogenesis after 10 days, while hyperthyroidism accelerates synaptogenesis initially, but by 21 days the number of synapses is reduced. The sensitivity of developing synapses to thyroid hormone may permit analysis of the events triggering synaptogenesis.

Harderian gland: development and influence of early hormonal treatment on porphyrin content

The porphyrin content of the rat Harderian gland remains low until 12 days of age at which time both porphyrin content and concentration rapidly increase. Intraperitoneal administration of tetraiodothyronine (thyroxine) into newborn animals advances the appearance of porphyrin in the gland. Conversely, a single injection of cortisol acetate into newborns retards the appearance of porphyrin. The time of porphyrin appearance in the gland parallels the time for maturation of the evoked cortical response to visual stimulation in normal and hormone-treated animals.

Hormonal effects on ontogeny of swimming ability in the rat: assessment of central nervous system development

The maturation of swimming behavior and the evoked cortical response to sciatic stimulation were studied in newborn rats receiving thyroxine or cortisol. Compared to that of controls the maturation of swimming is accelerated or delayed 2 to 3 days by thyroxine or cortisol treatment, respectively, and this corresponds to ontogenetic shifts in the characteristics of the evoked potential. Front leg movement during swimming normally diminishes at about 16 days of age and is inhibited by day 22. Thyroxine also advances and cortisol delays the age at which this inhibitory mechanism becomes evident, and compresses (thyroxine) or expands (cortisol) the time interval over which it becomes functional. During early postnatal life certain circuilating hormones can affect the rate and chronology of central nervous system maturation. Swimming behavior may be a simple model to use in studies concerned with factors affecting the functional and behavioral development of the central nervouts system.