Paola S. Timiras

Glutamic acid: A strong candidate as the neurotransmitter of the cerebellar granule cell

Free amino acids and cholinergic enzymes were investigated in the cerebellum of reeler and weaver mice in an attempt to identify the neurotransmitter characteristic of the granule cell population and to clarify any neurotransmitter abnormalities of their pre- and postsynaptic neurons induced by their depletion. The data indicate that glutamic acid may be the neurotransmitter of the granule cells. Pre- and postsynaptic neurotransmitter activity seemed not to be markedly altered in cerebellar granule cell dysgenesis.

Patho-physiologic findings after chronic tryptophan deficiency in rats: A model for delayed growth and aging

Long-Evans female rats three weeks, three months and 13-14 months of age were placed on tryptophan-deficient diets for periods ranging from a few months to nearly two years. Growth was interupted during the period of tryptophan-deficiency, but when the animals were returned to a complete diet, they gained weight and grew to normal size. Ability to reproduce, as indicated by litter production, was present at 17-28 months of age in rats which had been deprived of tryptophan, whereas no controls over 17 months of age produced any offspring. Other signs of delayed aging in the experimental group included, at advanced ages, greater longevity, as well as later onset in the appearance of obvious tumors, and better coat condition and hair regrowth. Many of these effects were also seen in pair-fed controls (fed a diet equal in amount to that eaten by the tryptophan-deprived rats, but with 1-tryptophan added). It is hypothesized that tryptophan deficiency delays growth, development and maturation of the central nervous system (CNS), in particular, by decreasing the levels of the neurotransmitter serotonin, for which tryptophan is the necessary precursor. In a parallel experiment, chronic treatment with d, 1-parachlorophenylalanine, an inhibitor of brain serotonin synthesis, from weaning until adulthood, also inhibited growth (body weight) and delayed sexual maturation (age of vaginal opening). These observations suggest that diets deficient in tryptophan or restricted in calories can affect maturation and aging by interfering with CNS protein synthesis, or neurotransmitter metabolism, or both.

Effect of neonatal hypothyroidism and of thyroxine onl-[14C]leucine incorporation in proteinin vivo and the relationship to ionic levels in the developing brain of the rat

Summary The rate of cerebral protein synthesis in vivo and changes in ionic levels have been investigated in infant rats following thyroid hormone deficiency induced at 1 day of age and after the administration of physiological levels of thyroxine from the 6th day of age. Following the injection of uniformly labelled l -[ 14 C]leucine in proportion to body weight, the rate of disappearance of the label from plasma and cerebral free amino acid pool was depressed in thyroid-deficient animals. The specific activity of cerebral protein-bound leucine expressed as a function of the specific activity of the label in the free amino acid pool was significantly reduced in hypothyroid rats, indicating a decrease in protein synthesis. It was inferred from the changes in cerebral Na + , K + , and Cl-ions in hypothyroid rats that the Na-K pump is altered and the transport of amino acids is adversely affected. Thyroxine therapy reversed the changes in cerebral protein synthesis and ionic concentration. It was concluded that the relationship between thyroid hormone action and brain protein synthesis is partially mediated through the effect of this hormone on ionic distribution and amino acid transport.

Effect of hypothyroidism on ionic metabolism and Na-K activated ATP phosphohydrolase activity in the developing rat brain.

The effects of neonatal hypothyroidism on electrolyte contents and the Na+ and K+ activated ATPase system was studied in the cerebral cortex and cerebellum of the developing rat. Neonatal hypothyroidism increased Na+ and CI− contents and decreased K+ and Mg2+ contents in both brain areas. Hypothyroidism also resulted in a decrease in the specific activity of the Na‐K ATPase extracted by deoxycholate treatment from brain homogenate as well as in the specific activity of this enzyme in the heavy microsomal fraction. The decrease in Mg2+ content and ATPase activity is discussed in relation to the changes occurring in Na+ and K+. Both enzymic and ionic changes may underlie the biochemical and physiological abnormalities observed when the brain is deprived of thyroxine at critical stages of its development.

Effect of Hypo- and Hyperthyroidism on Regional Monoamine Metabolism in the Adult Rat Brain

The effects of hypo- and hyperthyroidism were investigated on brain levels and accumulation rates (after pargyline) of 5-hydroxytryptamine (5-HT), norepinephrine (NE) and dopamine (DA) in discrete brain regions of the adult rat. Whereas NE remained unchanged in all brain areas except in the cerebellum, alterations in brain 5-HT and DA suggest that the behavioral abnormalities associated with thyroid dysfunction in adulthood may be related to neurotransmission disturbances. In hypothyroidism, 5-HT content decreased in cerebral hemispheres and mesodiencephalon and DA content decreased in these regions and also in cerebellum and pons-medulla. Concomitantly, accumulation rate of 5-HT was lower in pons-medulla whereas that of DA was increased in cerebral hemispheres and mesodiencephalon. In hyperthyroidism, 5-HT levels increased in cerebral hemispheres alone. Accumulation rate of 5-HT increased in pons-medulla and that of DA increased in mesodiencephalon. These data indicate that the influence of thyroid hormones on monoamines (MAs) in the adult brain varies with the neurotransmitter and the brain area considered.

Ubiquitin-protein conjugates in Alzheimer's lesions

The ubiquitin-dependent protein degradation system plays a major role in the removal of abnormal and denatured proteins which may form insoluble aggregates in pathological conditions or during other cellular stress. Neuritic plaques and neurofibrillary tangles in sections of Alzheimers cortex contain insoluble aggregates of proteins and are shown here to specifically immunostain with an antiserum to ubiquitin-protein conjugates. Plaque core amyloid and normal neurons do not immunostain and sodium dodecyl sulphate (SDS)-insoluble tangle preparations are not ubiquitin-positive on slot blots. The possible role and consequences of ubiquitination in tangle and plaque production in Alzheimers disease are discussed.

Lipid changes with aging in cardiac mitochondrial membranes

Mitochondrial membranes were isolated from the myocardium of young (4-month-old) and aged (33-month-old) male Long-Evans rats and compared in terms of cholesterol content and phospholipid and fatty acid composition. In aged rats, as compared to young, the major observations include: markedly higher cholesterol content; increased percentage of sphingomyelin and diphosphatidylglycerol (cardiolipin); in fatty acids, variable changes, with a predominant increase in the 16:0 in most phospholipids except cardiolipin, and sporadic increase in the longer chain (20:0, 24:0) fatty acids in cardiolipin; decreased unsaturation index for most phospholipids but increased for cardiolipin. These results are tentatively interpreted as indicative of an aging-related decrease in fluidity and energy transduction of mitochondrial membrane in the heart of aged rats and may be responsible, in part, for the decrements in cardiac function with aging.

Thyroid Hormones and Nervous System Development

Regulation by hormones of nervous system development is well recognized in humans, laboratory animals and cultured nerve cells. Thyroxine (T4) and triiodothyronine (T3), the major thyroid hormones, act on brain development and maturation by binding to T3 nuclear receptors. T3 responsive genes have been identified with the T3 receptor as a superfamily of genes including cortisol and estrogens, necessary for adaptation and survival. Less defined are T3 and T4 actions on the peripheral nervous system. In chromaffin cells explanted from the adrenal of immature rats, T3 induces the enzyme tyrosine hydroxylase, involved in catecholamine synthesis. This action is similar but, so far, apparently independent from Nerve Growth Factor promotion of sympathetic and chromaffin cell growth. Mechanisms based on nuclear binding depend on multiple receptors functionally diversified; their selectivity of action over a wide range of early and late developmental patterns is an attractive hypothesis to be further explored.

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.

Histology and survival in age-delayed low-tryptophan-fed rats.

Diets containing tryptophan in concentrations 30 and 40 percent of those fed to controls from weaning to 24-30 months or more, can delay aging in Long-Evans female rats. Mortality among low-tryptophan-fed rats was greater in the juvenile period, but substantially less than controls at late ages. Histological biomarkers of aging were also delayed after tryptophan restriction in some organs (liver, heart, uterus, ovary, adrenal and spleen) but not in others (kidney, lung, aorta). Brain serotonin levels were low in tryptophan-deficient rats but showed remarkable capacity for rehabilitation. Effects on early and late mortality and brain levels of serotonin were proportional to the severity of the restriction.