Effects of neonatal dysthyroidism on serotonin type 1 and type 2 receptors in rat brain

Effect of the inhibition of triiodothyronine (T3) production by thiamazole on the T3 and serotonin content of immune cells

Triiodothyronine (T3) and serotonin are present in the cells of immune system (blood, peritoneal and thymic lymphocytes, monocytes and granulocytes of the blood and peritoneal fluid, mast cells). In the present experiments the effect of thiamazole, an antithyroid drug was studied on the content of these two hormones in immune cells after one and two weeks of continuous treatment (by drinking water, containing 30 mg/100 ml thiamazole, ad libitum) in adult male rats, using flow cytometric and confocal microscopic analysis. In thymic lymphocytes both hormone contents significantly increased in both time points. A significant decrease of T3 was observed in peritoneal monocytes and granulocytes also in both time points, in peritoneal mast cells after one week and in blood lymphocytes after two weeks. Serotonin content in addition to the elevated thymic values (in both measurements) was significantly reduced in blood lymphocytes after two weeks. Confocal microscopy demonstrated heterogeneous cell populations with disparate hormone content and mostly diffuse localization The experiments call attention to the presence of T3 in the immune cells and to its variable concentration under the effect of a thyrostatic drug as well, as to the T3-serotonin relationship in the cells of the immune system.

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.

Evaluation of local cerebral glucose utilization and the permeability of the blood-brain barrier in the genetically epilepsy-prone rat

The genetically epileptic-prone rat (GEPR) is a valuable model for the study of gene-linked abnormalities involved in epilepsy. In comparison with normal Sprague-Dawley controls, we found, in GEPRs, a marked depression in local cerebral glucose utilization, widespread throughout the brain. This depression was accompanied by a significant increase of blood-brain barrier permeability and a reduction in regional blood volume. Finally GEPRs showed lower plasma levels of total triiodothyronine than normal controls. One can speculate that alterations in cerebral metabolism and microvascular regulation and thyroid hormone imbalance may be gene-linked factors involved in seizure susceptibility.

The ontogeny of [3H]5-hydroxytryptamine binding in the lamb: effects of in vivo thyroidectomy

The effects of congenital hypothyroidism in the late gestation ovine fetus include changes in serotonin concentrations in specific brain areas. To investigate possible ontogenic patterns of changes in 5-HT receptor function, we studied the binding characteristics of [3H]5-HT in the midbrain, hypothalamus and cerebral cortex in the late gestation ovine fetus and young lamb. We compared the binding characteristics of control fetuses to those of thyroidectomized fetuses, with or without thyroxine replacement therapy. In each of the areas examined, age-dependent changes in the receptor density (Bmax) for [3H]5-HT was observed. In cerebral cortex, Bmax was constant from 120 days gestation through the early neonatal period and increased significantly only at 25-30 days after birth. In hypothalamus, [3H]5-HT binding density decreased late in gestation (140-145 days) with a return after birth to values comparable to those at 120-125 and 130-135 days gestation. The midbrain also exhibited a significant age-dependent pattern of altered receptor density with a decrease in the 130-135 and 140-145 day gestational age groups compared to both younger and older lambs. In contrast, the affinity constant (Kd) for [3H]5-HT did not change over the ages evaluated in cerebral cortex or hypothalamus. In the midbrain, however, there was a significant increase in Kd at 1-5 days after birth compared to all other age groups. The ability of fetal thyroidectomy, with or without thyroxine replacement therapy, to alter patterns of [3H]5-HT binding was also tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurological consequences of congenital hypothyroidism in Fischer 344 rats

Congenital hypothyroidism was induced in rat pups by treating pregnant and lactating dams with an antithyroid drug, methimazole. Methimazole (0.00, 0.01, 0.03 or 0.10 mg/ml) was added to the drinking water of female Fischer 344 rats from gestational day 17 through lactational day 10. The same animals as pups and adults were evaluated with a developmental neurotoxicological test battery. Pups were evaluated for physical measures of maturation, thermoregulation, flash evoked potential (FEP), motor activity, and morphology of brain, thyroid and kidneys. Parameters evaluated in the same animals as adults were body weight, functional observational battery, grip strength, body temperature, and neurological tests (FEP, auditory brainstem response to 4 and 16 kHz tone pips (ABR4, ABR16) and clicks (ABRc), somatosensory evokes potentials recorded from the somatosensory cortex (SEP-S) and the cerebellum (SEP-C), and caudal nerve action potential to single and paired stimuli (CNAP). Treatment-related findings in pups included slightly decreased body weight, slightly increased kidney weights, altered thyroid morphology, delayed incisor eruption, decreased thermoregulation, and FEP changes. Although a pup no effect level was not determined, effects at 0.01 mg/ml were minimal. Adult ABR4 and ABR16 waveforms were slower than controls and had altered shapes; ABRc, SEP-S, and SEP-C waveforms exhibited reduced power, increased latency and altered shape. Effects were detected in adults at all doses and thus, the neurological characteristics of rat congenital hypothyroidism were clearly detected with this developmental neurotoxicological test battery. The effects on body weight, kidney weight and thyroid morphology, however, suggest a general developmental effect and nervous system function did not appear to be preferentially affected.

Di-iodothyronine as part of the oestradiol and catechol oestrogen receptor--the role of iodine, thyroid hormones and melatonin in the aetiology of breast cancer

Hypothyroidism and low iodine intake may be important aetiological factors in oestrogen dependent tumours of the breast, uterus and ovary. They are preventable risk factors. Iodine supplementation will hopefully lead to a decreased incidence of these cancers in future generations. The present author proposes that the tyrosyl residue in the hydrophobic oestrogen binding site of the oestrogen receptor is post translationally modified to monoiodotyrosine and hence 3,3' di-iodothyronine monoamine (T2) by peroxidase activity. He has previously proposed that various monoamine receptors are also T2 based. The densities of these receptors are increased in hypothyroidism and they exert control over release of prolactin and other hormones, including melatonin at multiple sites in the hypothalamic--pituitary axis. Melatonin is a metabolite of serotonin and hence melatonin receptors may be T2 or rT3 based as well. These factors could be significant in the aetiology of breast cancer as high prolactin and melatonin levels may be protective. Oestrogen receptor density may be increased in hypothyroidism as is certain monoamine receptor density. This would amplify the effect of high circulation oestrogen levels in hypothyroidism and may help explain why hypothyroidism and low iodine intake are risk factors for breast, uterine and ovarian cancer.

Effect of thyroid state on histamine H1 receptors in adult and developing rat brain

The effect of thyroid status on histamine H1 receptors in adult and developing rat brain was investigated using the (3H) mepyramine binding assay. Hypothyroidism induced by treatment with 6-n-propyl-2-thiouracil resulted in a 31% decrease in the density and total content of adult rat brain (3H) mepyramine binding sites and a significant retardation of the developmental increase in H1 receptor binding in neonates. At 30 days of age, when euthyroid rats reached binding levels of the adult, hypothyroid animals presented reductions of 22 and 39% in (3H) mepyramine bound per unit weight and per brain respectively. In contrast, hyperthyroidism induced by treatment with L-thyroxine did not alter H1 receptor numbers in the adult rat brain but accelerated the developmental increase in (3H) mepyramine bound per unit weight that reached normal adult levels by 21 days of age. The results suggest that thyroid dysfunction during early life and adulthood may cause derangements of the histaminergic system in the brain.