Neural and endocrine development after chronic tryptophan deficiency in rats: II. Pituitary-thyroid axis

Food for thoughts: feeding time and hormonal secretion

Many daily cycles are imposed on us by our environment, such as alternating days and nights, temperature fluctuations or rhythms in food availability. When food is accessible every day at the same time, animals will adapt their physiology and behaviour to match the daily meal. They will anticipate the access to food by waking up and being active in the hours prior to feeding, foraging for food. Adaptation of physiology to changing conditions of food availability is not only evident at the behavioural level, but also for hormonal systems. Thus, corticosteroids, melatonin, leptin/ghrelin, insulin/glucagon, orexins and thyroid hormones, which show rhythmic profiles of secretion in ad libitum feeding conditions, are sensitive to increase and/or depletion in energy supplies and will be influenced when food sources are limited or available at unusual times. The present review reports the influence of restricted feeding schedules on secretion profiles of diverse hormones compared to normal ad libitum feeding conditions in rodents. In the end, the interplay between these systems and their response to environmental challenges will allow the animal to maintain their fitness for survival.

Tissue uptake of thyroid hormone by amino acid transporters

Thyroid hormones (THs) -- thyroxine (T4) and tri-iodothyronine (T3) -- are iodinated derivatives of the amino acid tyrosine, which regulates growth, development and critical metabolic functions. THs are taken up by target cells and act at the genomic level via nuclear thyroid receptors. Saturable transport mechanisms mediate the greater part of TH movement across the plasma membrane. System L1 permease is a transporter of THs and amino acids in mammalian adipose tissue, placenta and brain. T(3) is also a substrate of a putative System T transporter, which is selective for aromatic amino acids. The activity and functional mechanisms of these transporters can be crucial to cells in determining both their hormone sensitivity and their responses to change in circulating hormone concentrations or availability of competing substrates (e.g. amino acids). TH transporters are potentially important pharmacological targets in the design of novel or improved therapies for thyroid-related disorders.

Lifetime brain serotonin: regional effects of age and precursor availability

In the rat, regional brain serotonin levels which do not change from 2-30 months of age are increased at 36 months. Corresponding catecholamine levels progressively decrease. Feeding a diet restricted in the amino acid tryptophan (the precursor of serotonin) from weaning to two years of age markedly reduces serotonin levels in all brain regions and lowers norepinephrine levels in the cerebral hemispheres. Regional activity of synthesizing (tyrosine and tryptophan hydroxylases) and catabolizing enzymes (MAO-A) does not change markedly with age or dietary manipulation except for sporadic increases in tryosine hydroxylase activity in pair-fed animals. Returning the tryptophan-deficient animals to a normal diet produces a certain degree of rehabilitation the effectiveness of which varies with the function considered: Impaired brain serotonin levels recover moderately but remain lower than controls as late as 36 months, growth is never completely compensated, and norepinephrine levels show a rebound increase.

Low tryptophan diets delay reproductive aging

Newly weaned female rats fed diets severely deficient in the essential amino acid tryptophan show marked delays in reproductive aging, with conception and delivery occurring as late as 36 months. The rate of aging in these rats seems inversely related to both their early growth rates and the accessibility of brain tryptophan. The subsequent age retardation may depend on a reduction in both early cell loss and rate of brain maturation.

Pituitary-thyroid activity and longevity in neonatally thyroxine-treated rats

Wistar rats were made hypothyroidic by intraperitoneal thyroxine (T4) injection during the first 10 days of neonatal life. Levels of T4, 3,3',5-triiodothyronine (T3), thyroid-stimulating hormone (TSH) and prolactin in the blood of these rats were measured by radioimmunoassay. The T4 levels are about two-thirds of control values up to 20 months of age. T3 level is low only at a young age. TSH level shows no significant difference from control, but is about half that of control after the stimulation of secretion by 6-propyl-2-thiouracil. The level of prolactin is much higher in the T4-treated group than in controls. In male rats, life duration of hypothyroid rats was longer than control by about 4 months. The life extension effect of hypothyroidism was observed also in females, although the difference was smaller than that in males. The concentration of T4 in the blood of male rats is higher than females, and the decrease in T4 level by neonatal T4 treatment is also more marked in males.

Modification of mitochondrial respiration by aging and dietary restriction

Effects of aging and of dietary restriction on mitochondrial recovery and respiratory capacities have been assessed in mice. Old mice (23-26 months) did not differ from adult mice (9-12 months) in amounts of protein recovered in mitochondrial fractions of liver, brain and spleen, but did show a decline in specific activity of cytochrome c oxidase (cyt. c ox.) in liver and spleen. Age effects on in vitro respiration by mitochondria occurred in liver and spleen. In liver, only one substrate (beta-hydroxybutyrate) of four tested was respired at a different rate by old than by young mitochondria. Depression of state 3 respiration and 2,4-dinitrophenol (DNP)-uncoupled rates was observed for this substrate; however, this effect depended on expressing respiration on the basis of mitochondrial protein and was less overt if data were expressed per unit of cyt. c ox. activity. Old spleen mitochondria exhibited a grosser defect, showing a 40% decrease in the respiratory control index (RCI) for (succinate + rotenone)- supported respiration (the only substrate tested) due to a possible increase in state 4 rates. Effects of dietary restriction were assessed in liver and brain of 3-7-month-old mice underfed since weaning. Dietary restriction reduced recovery of total liver mitochondrial protein and liver cyt. c ox. specific activity. Liver mitochondria from restricted mice generally showed increased state 3 rates with no differences from controls in state 4 rates for respiration supported by glutamate or pyruvate + malate, resulting in an increased RCI for these substrates. DNP-uncoupled rates were also raised by dietary restriction. Unlike effects observed in old versus young mice, these differences obtained whether the data were expressed on the basis of mitochondrial protein or on cyt. c ox. activity. Electron microscopy of liver mitochondrial preparations revealed more non-mitochondrial contaminants in old mice and larger mitochondria in dietarily restricted mice. These findings are compatible with reports of age-dependent losses of liver mitochondria and suggest that dietary restriction may retard this loss.

Changes in extrathyroidal conversion of thyroxine (T4) to 3,3',5-triiodothyronine (T3) in vitro during development and aging of the rat

Changes with age in the rate of in vitro conversion of thyroxine (T4) to triiodothyronine (T3) by organ homogenates were determined in female Wistar rats from 6 days to 18 months old. The conversion rates in liver and kidney homogenates of pups before day 13 are very low, and then rise to reach the maximum value at around day 23. During the period of sexual maturation the conversion activity suddenly decreases, and shows afterwards a gradual decline with age. The activity in both liver and kidney of male rats is higher than that of females except in the immature liver. A sudden rise was observed in the conversion activity of organ homogenates when the rats were released from a period of restricted food intake which started at an early age before maturity was achieved.

Functional implications of changes in the senescent brain: a review

The morphological, chemical, and physiological changes in the brain accompanying old age are reviewed. The deterioration of the striatal and hypothalamic dopaminergic systems were implicated in the onset of age related Parkinsonian-like slowing of performance and altered affect. Cholinergic hippocampal and neocortical systems were chemically and physiologically abnormal in the aged. The implications for slowed cognitive processing and persistance of the memory trace are presented.

Neural and endocrine development after chronic tryptophan deficiency in rats: I. Brain monoamine and pituitary responses

Caloric restriction and tryptophan deficient diets have been shown to delay aging in the immature laboratory rat. Studies of monoamine levels in the brain of developing female rats fed on these diets show changes in serotonin but not norepinephrine or dopamine levels: in tryptophan-restricted rats serotonin levels were reduced in all brain areas studied, whereas in caloric-restricted animals serotonin levels were increased in the cerebral hemispheres only. Another group of animals, in which growth and maturation was delayed by feeding d,1-parachlorophenylalanine (PCPA) showed decreases in serotonin, norepinephrine and dopamine concentrations in all brain regions investigated. All treatments employed to arrest growth and maturation resulted in pituitary alterations manifested by gross, histological and ultrastructural changes. It is postulated that there maturation- and age-retarding treatments delay the development of the central nervous system resulting in postponed maturation of the neuroendocrine axis, with consequent hypoactivity of certain pituitary functions and a resultant delay in the onset of maturation and senescence.