Precocious differentiation of mouse parotid glands and pancreas induced by hormones

Changes of alpha-amylase activity (1,4-alpha-D-glucan glucanohydrolase, EC 3.2.1.1) in mouse parotid gland and pancreas were investigated during embryonic and postnatal development. Amylase activity in the parotid gland increased from around day 12 and reached the adult level on day 30. On the other hand, the activity in the pancreas increased during the last stage of gestation, decreased after birth, and then gradually increased from around day 15, reaching the adult level on day 35. Precocious differentiation of the parotid gland was induced by injections of hydrocortisone or thyroxine after birth, but these hormones did not have additive effects on the parotid gland. Injection of insulin had little effect when given alone, but suppressed the effects of the other two hormones on the gland. Only hydrocortisone increased the amylase activity in mouse pancreas during postnatal development, the other two hormones causing slight decrease in pancreatic amylase. Adrenalectomy and injection of hydrocortisone affected the parotid gland but not the pancreas of adult mice. These results suggest that hydrocortisone is involved in cytodifferentiations of the parotid gland and pancreas, and in maintenance of the parotid gland. Thyroxine may also be important in differentiation of the parotid gland.     

Precocious differentation of mouse parotid glands and pancreas induced by hormones

Changes of α-amylase activity (1,4-α-D-glucan glucanohydrolase, EC 3.2.1.1) in mouse parotid gland pancreas were investigated during embryonic and postnatal development. Amylase activity in the parotid gland increased from around day 12 and reached the adult level on day 30. On the other hand, the activity in the pancreas increased during the last stage of gestation, decreased after birth, and then gradually increased from around day 15, reaching the adult level on day 35. Precocious differentiation of the parotid gland was induceed by injections of hydrocortisone or thyroxine after birth, but these hormones did not have additive effects on the parotid gland. Injection of insulin had little effect when given alone, but suppressed the effects of the other two hormones on the gland. Only hydrocortisone increased the amylase activity in mouse pancreas during postnatal development, the other two hormones causing slight decrease in pancreatic amylase. Adrenalectomy and injection of hydrocortisone affected the parotid gland but not the pancreas of adult mice. These results suggest that hydrocortisone involved in cytodifferentiations of the parotid gland and pancreas, and in maintenance of the parotid gland. Thyroxine may also be important in differentiation of the parotid gland.     

Ontogenesis of Monoamine-Synthesizing Enzyme Activities and Biopterin Levels in Rat Brain or Salivary Glands, and the Effect of Thyroxine Administration

Neonatal changes in the activities of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TrpH) and in the content of the co-factor, biopterin, were studied in rat midbrain for the first 20 days after birth. Changes in TH activity in the parotid and submandibular glands were also examined. Changes in TH activity per unit weight in the developing rat brain were briefly similar to those in the salivary glands; the activity increased from day 2 or 4 to day 9 after birth, and remained constant or slightly decreased at day 12, then rapidly increased on day 16. TrpH activity in the midbrain increased about twofold up to day 16. The biopterin concentration in the brain increased, reached a maximum level on day 12 after birth, and thereafter decreased. The effect of hyperthyroidism in rats given 0.2 mg/kg i.p. of thyroxine every 2 days postnatally was studied on the activity of TH in rat salivary glands at 12-day-old rats. In parotid or submandibular gland of hyperthyroid rats, TH activity increased at day 12 postnatally. In comparison with the effect on TH activity in the salivary glands, TH activity in the midbrain on day 20 postnatally was not induced by hyperthyroidism. Furthermore, increase of the TrpH activity and biopterin and catecholamine levels in the midbrain of hyperthyroid rats was not found on day 20 after birth in comparison with the corresponding controls. From these data, we suppose that postnatal hyperthyroidism may cause precocious induction of TH in rat salivary gland, but may not increase the activity of TH or TrpH, and the level of their co-factor, biopterin, in rat midbrain.     

Histochemical studies on the effect of thyroid hormone on alkaline and acid phosphatase activities in liver of fish and amphibia.

The Lata fishes (Ophicephalus punctatus) showed increased alkaline and acid phosphatase activities in liver after immersion for 15-30 days in thyroxine-containing medium (0.025 mug/ml). A single injection of thyroxine (1-2 mug/g of body weight) caused increased acid phosphatase activity in liver of Lata fish in comparison to the controls on the 5th day after experiment but the alkaline phosphatase activity remained unchanged. Both alkaline and acid phosphatases showed increased activities in liver of Lata fishes treated with a single injection of 4 mug of thyroxine per g of body weight on the 5th day. Immersion of Lata fishes in thiourea solution (1 mg/ml) for 15 days did not show any alteration in alkaline or acid phosphatase activities but these enzyme activities decreased after 30 days' immersion in thiourea solution in comparison to the controls. A seasonal variation of alkaline and acid phosphatase activities was observed in liver of Lata fishes. More alkaline phosphatase activity was found in liver of summer fishes than in winter fishes. The winter fishes showed more acid phosphatase activity than the summer fishes. Three consecutive injections of thyroxine (0.1 mug/g of body weight) to toads (Bufo melanostictus) caused increased alkaline and acid phosphatase activities in liver on the 5th day of the experiment, in comparison to the controls.     

Thyroid hormones modulate ornithine decarboxylase in the immature rat cerebellum

In this study, we measured ornithine decarboxylase (ODC) activity as a potential parameter to evaluate the response of the developing rat brain to thyroid hormones. In cerebellum, neonatal hyperthyroidism (40 micrograms thyroxine/100 g body weight daily from birth) increased ODC activity at 2 and 5 days of age and then accelerated its developmental decline. Conversely, ODC activity was decreased in 2- and 5-day-old hypothyroid rats (propylthiouracil to the mother), but it was not significantly different from normal thereafter. No significant differences were observed in the forebrain following either treatment. In hypothyroid rat cerebellum, a single injection of triiodothyronine (T3, 100 micrograms/100 g 18 h before sacrifice) increased significantly ODC activity at all ages. A dose-response study showed that 0.5 micrograms T3/100 g is sufficient to obtain maximal stimulation. Finally, administration of antiserum against rat growth hormone had no significant effect on ODC response to T3. These results show that ODC is a useful marker of thyroid state and tissue response in the neonatal rat cerebellum.     

The adaptive behaviour of isoenzyme forms of rat liver alanine amino transferases during development

1. The activities of the mitochondrial and cytosol isoenzyme forms of l-alanine–glyoxylate and l-alanine–2-oxoglutarate aminotransferases were determined in rat liver during foetal and neonatal development. 2. The mitochondrial glyoxylate aminotransferase activity begins to develop in late-foetal liver, increases rapidly at birth to a peak during suckling and then decreases at weaning to the adult value. 3. The cytosol glyoxylate aminotransferase and the mitochondrial and cytosol 2-oxoglutarate aminotransferase activities first appear prenatally, increase further after birth and then rise to the adult values during weaning. 4. In foetal liver the mitochondrial glyoxylate aminotransferase and the cytosol 2-oxoglutarate aminotransferase activities are increased after injection in utero of glucagon, dibutyryl cyclic AMP (6-N,2′-O-dibutyryladenosine 3′:5′-cyclic monophosphate) or thyroxine. The cytosol glyoxylate aminotransferase and the mitochondrial 2-oxoglutarate aminotransferase activities are increased after injection in utero of cortisol or thyroxine. 5. After birth the further normal increases in the mitochondrial and cytosol 2-oxoglutarate aminotransferase activities can be hastened by cortisol injection, whereas the increase in cytosol glyoxylate aminotransferase activity requires cortisol treatment together with the intragastric administration of casein. 6. The results are discussed with reference to the metabolic patterns and the changes in regulatory stimuli (hormonal and dietary) that occur during the period of development.     

Impairment of cold-induced increase in thyroxine 5'-deiodinase activity in mouse brown adipose tissue by the intracerebroventricular administration of bombesin.

Effects of bombesin on brown adipose tissue (BAT) thyroxine (T4) 5'-deiodinase (5'D) activity and rectal temperature were examined in male mice. Immediately following an intracerebroventricular (ICV) or intravenous (IV) injection of bombesin (0.1-100 ng/animal) or vehicle (20 mM bacitracin dissolved in 0.9% saline), the mice were placed in a room at 4 degrees C or 22 degrees C for 30, 60, 120 or 240 min. The ICV injection of bombesin dose-dependently lessened cold-induced increase in BAT 5'D activity and increased hypothermia determined at 120 min of cold exposure, whereas the IV injection of bombesin was without effect. Bombesin (ICV)-induced hypothermia preceded the inhibition of BAT 5'D activity by at least 30 min at 4 degrees C. BAT 5'D activity was not affected by ICV injection of bombesin in mice kept at 22 degrees C, although the rectal temperature was significantly decreased. Bombesin thus appears to prevent cold-induced increase in T4 5'D activity in mouse BAT by its central effect. Bombesin-induced excessive hypothermia itself and/or the decrease in sympathetic tone of BAT by bombesin might decrease cold-induced increase in BAT 5'D activity.     

Increased thyroxine turnover after 3,3',4,4',5,5'-hexabromobiphenyl injection and lack of effect on peripheral triiodothyronine production

Juvenile male Wistar rats were injected i.p. with 0, 20, or 40 mg/kg 3,3',4,4',5,5'-hexabromobiphenyl and blood samples collected periodically up to 28 days. A dose-dependent depression of the serum thyroxine level was detected, while the circulating triiodothyronine concentration was not affected by the biphenyl congener. Thyroxine turnover in vivo 7 days after injection of the 20 mg/kg dose revealed significant increases of various clearance parameters relative to controls. The fractional clearance rate (day-1) increased by 84%, the daily metabolic clearance rate (mL.kg-1.day-1) increased by 128%, and the daily thyroxine disposal rate (ng.kg-1.day-1) increased by 41%. Also, the thyroxine distribution space (mL/kg) increased by 21%. These results indicated greater thyroxine binding in major organs as well as a marked increase in the peripheral metabolism of thyroxine. The increased thyroxine metabolism is explained by a 4.8-fold induction of uridine 5'-diphosphoglucuronyltransferase activity in liver microsomes. The type I 5'-deiodinase activity in liver homogenates and endogenous concentrations of the cofactor for this reaction, glutathione, were not affected by the biphenyl. This result means that homeostatic mechanisms involving thyroxine conversion to triiodothyronine do not explain the maintenance of serum T3 under these conditions.     

Precocious induction of pepsinogen in the stomach of suckling mice by hormones

Effects of hormones on pepsinogen activity in mouse stomach were investigated by enzyme assay and electron microscopy. Administration of hydrocortisone alone to mice on days 5–10 increased the enzyme activity in the stomach to as much as 4.5-fold that of untreated mice and the increase was dose dependent. Thyroxine also evoked precocious differentiation of the stomach. The effects of thyroxine and hydrocortisone were additive. Injections of insulin had little effect when given alone, or in combination with other hormones. Injection of hydrocortisone alone or plus thyroxine also caused morphological differentiation of the chief cells in the stomach mucosa. Administration of thyroxine to mice on days 15–20 induced as much enzyme activity as that induced by hydrocortisone, but neither of these hormones had any effect when injected after day 23.These results suggest that besides hydrocortisone, thyroxine is also involved in differentiation of the stomach in mice for the first 20 days after birth and that the normal increase of pepsinogen activity in the stomach of mice during the late suckling period is brought about by serum glucocorticoids, possibly with thyroxine.     

Acute effects of leptin on 5'-deiodinases are modulated by thyroid state of fed rats.

Leptin has been shown to modulate deiodinase type 1 (D1) and type 2 (D2) enzymes responsible for thyroxine (T4) to triiodothyronine (T3) conversion. Previously, it was demonstrated that a single injection of leptin in euthyroid fed rats rapidly increased liver, pituitary, and thyroid D1 activity, and simultaneously decreased brown adipose tissue (BAT) and hypothalamic D2 activity. We have now examined D1 and D2 activities, two hours after a single subcutaneous injection of leptin (8 microg/100 g BW) into hypo- and hyperthyroid rats. In hypothyroid rats, leptin did not modify pituitary, liver and thyroid D1, and thyroid D2 activity, while pituitary D2 was decreased by 41% (p<0.05) and hypothalamic D2 showed a 1.5-fold increase. In hyperthyroid rats, thyroid and pituitary D1, and pituitary and hypothalamic D2 were not affected by leptin injection, while liver D1 showed a 42% decrease (p<0.05). BAT D2 was decreased by leptin injection both in hypo- and hyperthyroid states (42 and 48% reduction, p<0.001). Serum TH and TSH showed the expected variations of hypo- and hyperthyroid state, and leptin had no effect. Serum insulin was lower in hypothyroid than in hyperthyroid rats and remained unchanged after leptin. Therefore, acute effects of leptin on D1 and D2 activity, expect for BAT D2, were abolished or modified by altered thyroid state, in a tissue-specific manner, showing an IN VIVO interplay of thyroid hormones and leptin in deiodinase regulation.     

Synergistic effects of thyroxine and glucocorticoid in induction of trypsin-like esteroprotease in mouse submandibular gland.

The actions of thyroxine, 5 alpha-dihydrotestosterone and hydrocortisone singly or in combination in enzyme regulation in the submandibular gland were studied in intact and adrenalectomized female mice. 1. Adrenalectomy decreased the activity of trypsin-like esteroprotease (EC 3.4.4.-), and administration of hydrocortisone to adrenalectomized mice restored the activity to normal. 2. Hydrocortisone and thyroxine had synergistic effects in induction of esteroprotease in adrenalectomized mice, but 5 alpha-dihydrotestosterone and hydrocortisone did not have synergistic effects in either intact or adrenalectomized mice. 3. The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) was not influenced by change in the glucocorticoid level, but was increased by thyroxine and 5 alpha-dihydrotestosterone in both adrenalectomized mice and intact mice.4. Isoelectric focusing in polyacrylamide gel showed that there are three distinct activities of esteroprotease in this gland with isoelectric points of 5.6, 6.2 and 7.3. Both thyroxine and 5 alpha-dihydrotestosterone similarly induced these activities and glucocorticoids did not affected the isozyme patterns induced by the other two hormones.     

Hormone action at the membrane level. VIII. Adrenergic receptors in rat heart and adipocytes and their modulation by thyroxine.

The regulation of adrenergic receptors in rat heart was measured in rats made hyperthyroid by injection with thyroxine and made hypothyroid by addition of propylthiouracil to the drinking water. Hyperthyroid rats display cardiac hypertrophy and a decrease in epididymal fat pad weight. The maximal beta-receptor level of ventricular membranes, as determined by (-)-[3H]dihydroalprenolol binding, was increased 60% by thyroxine treatment and decreased about 30% by propylthiouracil treatment. The affinity of the beta receptor was unchanged after thyroxine or propylthiouracil treatment. The maximal activity of the isoproterenol-stimulated adenylate cyclase (EC 4.6.1.1) varied with thyroid state in a manner parallel to the increase in beta-adrenergic binding sites. Thyroxine treatment also increases by 2-fold the beta receptors in isolated rat fat cells. Propylthiouracil treatment lowered the level of alpha receptors in heart by 30% as measured by [3H]dihydroergocryptine binding, but increased the affinity about 2.5-fold. The highest level of alpha receptors was seen in control hearts. These studies indicate that thyroxine may control the turnover of beta-adrenergic receptors in heart and fat cells and regulate physiological responses in these tissues via a hormone-hormone interplay system. Thyroxine treatment reduced the activity of the membrane-bound Mg2+-ATPase (EC 3.6.1.3) and 5'-mononucleotidase (EC 3.1.3.5) but appears to increase the activity of the (Na+ + K+)ATPase (EC 3.6.1.4).     

Requirement of gene transcription and protein synthesis for cold-and norepinephrine-induced stimulation of thyroxine deiodinase in rat brown adipose tissue

The increase in propylthiouracil-insensitive ‘type II’ thyroxine 5′-deiodinase activity of brown adipose tissue was investigated in rats exposed to acute cold stress or single-dose norepinephrine injection. The 20-fold cold-induced increase in enzyme activity showed a 2-h lag phase and reached a maximum after only 8 h; reacclimation occurred with a 2-h time lag and a half-life of 2.2 h. 4 h after a single norepinephrine injection, the deiodinase activity was almost identical to that after a 4-h cold stress; norepinephrine could not potentiate the effect of the cold stress. Treatment with the protein synthesis inhibitor cycloheximide before exposure to cold or before norepinephrine injection totally blocked the increase in deiodinase activity, suggesting that the increase is due to de novo protein synthesis. The half-life of the enzyme in vivo was estimated to be 0.7 h. Treatment with the RNA synthesis inhibitor actinomycin D totally abolished the cold-and norepinephrine-induced increases, indicating that the increase requires mRNA synthesis. It was concluded that the dramatic cold-induced increase in thyroxine deiodinase activity in brown adipose tissue was not due to activation of preexisting enzyme but was fully due to a norepinephrine-induced increase in expression of the gene and subsequent synthesis of the protein.     

Effect of hormones on hydrolase activities and DNA synthesis in kidney of the developing mouse

The postnatal development of brush border enzyme activities, namely maltase, trehalase, alkaline phosphatase, gamma-glutamyltranspeptidase, and leucylnaphthylamidase, as well as the ontogenic profile of DNA synthesis has been determined in the mouse kidney. In addition, these parameters were evaluated following daily administration of hormones during 3 days to 8-day-old mice. Insulin or epidermal growth factor induced a 34% increase of maltase activity over that of 11-day-old controls. Trehalase activity was precociously and significantly augmented by cortisone alone or combined with thyroxine (p less than 0.05), although thyroxine alone had no influence. Only epidermal growth factor had a significant effect on alkaline phosphatase activity. gamma-Glutamyltranspeptidase activity was significantly decreased when insulin and thyroxine were given simultaneously, but was not modified by any of the hormones injected separately. The level of leucylnaphthylamidase activity was enhanced by 70% after cortisone injection, but it was significantly reduced by thyroxine injected in combination with insulin or cortisone. The incorporation of [3H]thymidine into DNA was increased by 107% after epidermal growth factor administration, but it was decreased by 33% after the cortisone treatment. In spite of this precocious reduction, the level of incorporation was still 2 times higher than that in adult mice. These results show that hormones act separately or in cooperation to accelerate or retard the maturation of the suckling mouse kidney.     

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.     

Phenylalanine-pyruvate aminotransferase in immature and adult mammalian tissues induction in fetal rat liver

Normal human fetal liver contains little phenylalanine-pyruvate aminotransferase: between the 11th and 22nd week of gestation its activity (per g) is 8.8% of that in adult liver. In rat liver this enzyme begins to rise a few hours before birth. Precocious increases in the phenylalanine-pyruvate aminotransferase activity of fetal rat liver (but not kidney or brain) were evoked by premature delivery and also by the administration of thyroxine or glucagon in utero. These results, Discussed in relation to related observations on other enzymes, suggest that thyroxine secreted by the fetus, and also another factor relaesed at the beginning of labour, may be the natural stimuli for the developmental formation of phenylalanine-pyruvate aminotransferase.The regulation of hepatic phenylalanine-pyruvate aminotransferase and phenylalanine hydroxylase (L-phenylalanine, tetrahydropteridine:oxygen oxidoreductase (4-hydroxylating), EC 1.14.16.1) during fetal development is different: in both man and rat, phenylalanine hydroxylase begins to rise earlier and is unaffected by the treatments which enhanced the formation of phenylalanine-pyruvate aminotransferase. In suckling rats (but not in fetuses and adults), an injection of cortisol increased the levels of both enzymes. Hepatocarcinomas of the adult rat were devoid phenylalanine hydroxylase as well as phenylalanine-pyruvate aminotransferase. However, suppression in vivo by substrate analogues (α-methylphenylalanine and p-chlorophenylalanine) was unique for phenylalanine hydroxylase.     

Microsomal long-chain acyl-CoA thioesterase (carboxylesterase ES-4) is regulated by thyroxine.

Long chain acyl-CoA thioesterase activity is mainly located in microsomes after subcellular fractionation of liver from untreated rats. The physiological function and regulation of expression of this activity is not known. In the present study we have investigated the effect of thyroxine on expression of carboxylesterase ES-4, the major acyl-CoA thioesterase of liver microsomes. Thyroidectomy of rats decreased the palmitoyl-CoA thioesterase activity to about 25% of normal activity. This decrease was accompanied by similar decreases at the protein and mRNA levels (31% and 57%, respectively, of controls). Treatment with thyroxine completely reversed the effect of thyroidectomy and resulted in elevated levels in both thyroidectomized and control rats. For reasons of comparison we also studied the possibility that ES-10 and ES-2, two other members of the same gene family, are affected by thyroxine. ES-10 was not changed at the protein or mRNA level by any of the treatments, while ES-2 expression in liver was decreased by thyroxine treatment. The data shows that changes in activity and expression of ES-4 correlate to thyroxine status in the rat suggesting a physiological regulatory role by this hormone. Since thyroxine regulates the expression of lipogenic enzymes, these results are consistent with a function for this microsomal acyl-CoA thioesterase in fatty acid synthesis and/or secretion, rather than in oxidative degradation of fatty acids.     

Microsomal long-chain acyl-CoA thioesterase (carboxylesterase ES-4) is regulated by thyroxine

Long chain acyl-CoA thioesterase activity is mainly located in microsomes after subcellular fractionation of liver from untreated rats. The physiological function and regulation of expression of this activity is not known. In the present study we have investigated the effect of thyroxine on expression of carboxylesterase ES-4, the major acyl-CoA thioesterase of liver microsomes. Thyroidectomy of rats decreased the palmitoyl-CoA thioesterase activity to about 25% of normal activity. This decrease was accompanied by similar decreases at the protein and mRNA levels (31% and 57%, respectively, of controls). Treatment with thyroxine completely reversed the effect of thyroidectomy and resulted in elevated levels in both thyroidectomized and control rats. For reasons of comparison we also studied the possibility that ES-10 and ES-2, two other members of the same gene family, are affected by thyroxine. ES-10 was not changed at the protein or mRNA level by any of the treatments, while ES-2 expression in liver was decreased by thyroxine treatment. The data shows that changes in activity and expression of ES-4 correlate to thyroxine status in the rat suggesting a physiological regulatory role by this hormone. Since thyroxine regulates the expression of lipogenic enzymes, these results are consistent with a function for this microsomal acyl-CoA thioesterase in fatty acid synthesis and/or secretion, rather than in oxidative degradation of fatty acids.     

Role of Thyroid Hormone and Nerve Growth Factor in the Development of Choline Acetyltransferase and Other Cell-Specific Marker Enzymes in the Basal Forebrain of the Rat

The effects of treatment with L-thyroxine (subcutaneously 0.3 microgram/g body weight daily from birth, i.e., day 1) and 2.5S nerve growth factor (NGF; intraventricularly 2 micrograms on 1, 3, 5, 7, and 9 postnatal days), separately and together, were studied on the biochemical development of different cell types in the basal forebrain of 10-day-old rats. The development of cholinergic, gamma-aminobutyric acid-ergic (GABAergic), and glutamatergic neurons was monitored respectively in terms of choline acetyltransferase (ChAT), glutamate decarboxylase (GAD), and glutaminase activities, whereas glutamine synthetase (GS) and 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNPase) activities were used to judge the maturation of astroglial and oligodendroglial cells. Treatment with either thyroid hormone or NGF from birth significantly increased the expression of ChAT activity in the basal forebrain of neonatal rats. When both agents were administered to the same animal, in agreement with our earlier in vitro findings, the stimulation in ChAT activity was much greater than the sum of the individual effects. In hypothyroid rats, significant effects of NGF at the low doses used were not detectable, although the increase of ChAT activity induced by thyroxine was potentiated by NGF in these animals. Under the present experimental conditions neither thyroxine nor NGF treatment had an appreciable effect on the activities of glutaminase, GS, and lactate dehydrogenase. However, the administration of thyroxine markedly increased CNPase activity in normal rats, whereas in hypothyroid rats the effect on both CNPase and GAD was also significant. Similar elevations in CNPase and GAD activities were not observed after NGF treatment, suggesting that the effect of NGF was specific to the cholinergic cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyroxine transport in choroid plexus

The role of the choroid plexus in thyroid hormone transport between body and brain, suggested by strong synthesis and secretion of transthyretin in this tissue, was investigated in in vitro and in vivo systems. Rat choroid plexus pieces incubated in vitro were found to accumulate thyroid hormones from surrounding medium in a non-saturable process. At equilibrium, the ratio of thyroid hormone concentration in choroid plexus pieces to that in medium decreased upon increasing the concentration of transthyretin in the medium. Fluorescence quenching of fluorophores located at different depths in liposome membranes showed maximal hormone accumulation in the middle of the phospholipid bilayer. Partition coefficients of thyroxine and triiodothyronine between lipid and aqueous phase were about 20,000. After intravenous injection of 125I-labeled thyroid hormones, choroid plexus and parts of the brain steadily accumulated 125I-thyroxine, but not [125I]triiodothyronine, for many hours. The accumulation of 125I-thyroxine in choroid plexus preceded that in brain. The amount of 125I-thyroxine in non-brain tissues and the [125I]triiodothyronine content of all tissues decreased steadily beginning immediately after injection. A model is proposed for thyroxine transport from the bloodstream into cerebrospinal fluid based on partitioning of thyroxine between choroid plexus and surrounding fluids and binding of thyroxine to transthyretin newly synthesized and secreted by choroid plexus.