Wim J. de Greef

Effect of Starvation and Subsequent Refeeding on Thyroid Function and Release of Hypothalamic Thyrotropin-Releasing Hormone

Effects of starvation on thyroid function were studied in 5- to 6-week-old (R x U) F1 rats. Starvation lowered plasma TSH in female, but not in male rats. Plasma T4 and T3 levels decreased, whereas the dialysable T4 fraction increased during starvation. Free T4 (FT4) levels decreased rapidly in females, but only after prolonged fasting in male rats. Glucose decreased, and free fatty acid levels increased during starvation. Peripheral TRH levels did not change during food deprivation. Since effects of starvation were most apparent in young female rats, such rats were used to study hypothalamic TRH release during starvation and subsequent refeeding. Basal in vitro hypothalamic TRH secretion was less in starved rats than in control or refed animals. In vitro hypothalamic TRH release in medium with 56 mM KCl increased 3-fold compared to basal release, and in these depolarization conditions TRH release was similar between hypothalami from control, starved and refed rats. In rats starved for 2 days, TRH level in hypophysial portal blood was lower than that of controls. Thus, diminished thyroid function during starvation may at least in part be caused by a reduced hypothalamic TRH release.

LH-RH and Dopamine Levels in Hypophysial Stalk Plasma and Their Relationship to Plasma Gonadotrophins and Prolactin Levels in Male Rats Bearing a Prolactin- and Adrenocorticotrophin-Secreting Pituitary Tumor

The present study was concerned with the effects of a transplantable pituitary tumor secreting prolactin (PRL) and adrenocorticotrophin (ACTH) on the levels of LH and FSH in peripheral plasma and on the hypothalamic release of LH-RH and dopamine in the male rat. Male rats of the same age not inoculated with the tumor served as controls. Hypophysial stalk blood was collected from urethane-anesthetized rats 4-5 weeks after tumor inoculation to measure their LH-RH and dopamine content. A peripheral blood sample was withdrawn from the animals just before sectioning the hypophysial stalk to measure their content of LH, FSH and PRL. It was found that in the tumor-bearing rats the levels of PRL increased 17-fold, whereas plasma levels of LH and FSH decreased by 45 and 70% respectively, when compared with the control rats. In the tumor-bearing rats, the secretion rate of dopamine in hypophysial stalk plasma increased from 1.4 to 4.1 ng/h, whereas the secretion rate of LH-RH decreased from 122 to 61 pg/h. However, when at the time of tumor inoculation adrenalectomy was performed, the tumor did not decrease plasma levels of LH and FSH and the secretion of LH-RH into hypophysial stalk blood any longer. The effect of the tumor on hypothalamic dopamine secretion was, however, still present in the adrenalectomized rats. It is concluded that the effect of the PRL- and ACTH-secreting pituitary tumor on plasma levels of LH and FSH requires the presence of the adrenal gland and that this effect is mediated through an inhibition of the hypothalamic release of LH-RH. Furthermore, this tumor increases the hypothalamic release of dopamine independent of the presence of the adrenal gland.

Effect of Cold Exposure on the Hypothalamic Release of Thyrotropin-Releasing Hormone and Catecholamines

The effects of cold exposure on the release of thyrotropin-releasing hormone (TRH) and catecholamines as estimated by push-pull perfusion of the mediobasal hypothalamus were studied. Before cold exposure, the male rats had been kept at room temperature or at 30 degrees C for 3 weeks. Transfer to 4 degrees C increased plasma levels of thyroid-stimulating hormone (TSH), but this cold-induced TSH response was more pronounced in animals which had been acclimatized to 30 degrees C. Exposure to 4 degrees C also increased plasma thyroid hormone levels, but had no effect on plasma prolactin. The hypothalamic content of TRH and dopamine remained similar after transfer to 4 degrees C, but after 6 h of cold, the content of noradrenaline and adrenaline had increased 1.6-fold and 3-fold, respectively. In vivo hypothalamic release of TRH, adrenaline and dopamine remained similar during a 2-hour period in control rats kept at room temperature or 30 degrees C. The hypothalamic release of TRH, dopamine and adrenaline did not change in rats transferred from room temperature to 4 degrees C. The amount of dopamine and adrenaline in push-pull perfusate also remained similar in rats acclimatized to 30 degrees C after transfer to low temperatures. However, in these rats kept at 30 degrees C for 3 weeks, exposure to 4 degrees C increased TRH release in perfusate from the mediobasal hypothalamus in the first 15 min of cold exposure (2-fold increase). Thus, exposure to cold stimulates the hypothalamo-pituitary-thyroid axis and increases the hypothalamic release of TRH in rats which had been acclimatized to 30 degrees C.

In vivo Release of Dopamine, Luteinizing Hormone-Releasing Hormone and Thyrotropin-Releasing Hormone in Male Rats Bearing a Prolactin-Secreting Tumor

The present study was concerned with the effects of a transplantable prolactin-secreting pituitary tumor (7315b) on the hypothalamic release of dopamine, luteinizing hormone-releasing hormone (LHRH) and thyrotropin-releasing hormone (TRH) in gonadectomized, adrenalectomized male rats bearing subcutaneously a testosterone capsule and a corticosterone pellet. Similar male rats not inoculated with tumor served as controls. The rats were studied 3-4 weeks after tumor inoculation, while they were anesthetized with urethane. Compared to the controls, prolactin levels in the tumor-bearing rats had increased 70-fold, whereas the levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) decreased to 20 and 27%, respectively. In tumor-bearing rats, the secretion of dopamine into hypophysial stalk plasma increased from 2.3 to 4.9 ng/h (p less than 0.025), whereas that of LHRH decreased from 127 to 52 ph/h (p less than 0.005). Since the use of urethane anesthesia may change quantitatively and qualitatively the effects of hyperprolactinemia, it was decided to study these effects on the in vivo release of LHRH, dopamine and TRH in conscious rats by a push-pull perfusion of the median eminence-arcuate nucleus area. Using this technique, it was found that in tumor-bearing rats the secretion of LHRH decreased from 20.0 to 9.8 pg/15 min (p less than 0.005), whereas that of dopamine increased from 118 to 246 pg/15 min (p less than 0.025). The secretion of TRH was not altered by hyperprolactinemia (4.1 vs. 4.4 pg/15 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyroid status and thyrostatic drugs on hepatic glucuronidation of lodothyronines and other substrates in rats : Induction of phenol UDP-glucuronyltransferase by methimazole.

Glucuronidation of iodothyronines in rat liver is catalyzed by at least three UDP-glucuronyltransferases (UGTs): bilirubin UGT, phenol UGT, and androsterone UGT. Bilirubin and phenol UGT activities are regulated by thyroid hormone, but the effect of thyroid status on hepatic glucuronidation of iodothyronines is unknown. We examined the effects of hypothyroidism induced by treatment of rats with propylthiouracil (PTU) or methimazole (MMI) or by thyroidectomy as well as the effects of T4-induced hyperthyroidism on the hepatic UGT activities for T4, T3, bilirubin,p-nitrophenol (PNP), and androsterone. Bilirubin UGT activity was increased in MMI- or PTU-induced hypothyroid and thyroidectomized rats, and decreased in hyperthyroid animals. T4 and, to a lesser extent, T3 UGT activities were increased in MMI- or PTU-induced hypothyroid rats, and T4 but not T3 glucuronidation also showed a significant increase in thyroidectomized rats. T4 but not T3 UGT activity was slightly decreased in hyperthyroid rats. While PNP UGT activity was decreased in thyroidectomized rats and increased in hyperthyroid animals, it was also markedly increased by MMI and slightly increased by PTU-induced hypothyroidism. In T4-substituted rats, MMI did not affect T4, T3, bilirubin and androsterone UGT activities but again strongly induced PNP UGT activity, indicating that this represented a direct induction of PNP UGT by the drug independent of its thyrostatic action. Androsterone UGT activity was hardly affected by thyroid status. Our results suggest a modest, negative control of the hepatic glucuronidation of thyroid hormone by thyroid status, which may be mediated by changes in bilirubin UGT activity. To our knowledge, this is the first report of the marked induction of a hepatic enzyme by MMI, which is not mediated by its thyroid hormone-lowering effect.

Regulation of Thyrotropin-Releasing Hormone in the Posterior Pituitary

Although the presence of thyrotropin-releasing hormone (TRH) in the posterior pituitary (PP) was reported more than one decade ago, knowledge on its origin, regulation and functional significance is lacking. In the present study we investigated the regulation of TRH in the rat PP. Analysis by specific RIA, anion and cation exchange chromatography and reverse-phase HPLC showed that all TRH immunoreactivity in the PP is accounted for by authentic TRH. Induction of hyperthyroidism with thyroxine increased levels of TRH in the PP by 20%, whereas in methimazole-treated, hypothyroid rats the content decreased by 25% versus untreated, euthyroid controls. Food deprivation for 3 days increased levels by 35% and refeeding completely normalized TRH content again. Also 14-17 days after castration, TRH in the PP was increased by 25% while testosterone substitution prevented this increase. Castration did not affect proTRH mRNA levels in the hypothalamus. One week after adrenalectomy or daily subcutaneous dexamethasone injections, TRH content in the PP was not affected. Treatment with disulfiram, an inhibitor of the peptidylglycine alpha-amidating monooxygenase (PAM), reduced levels of TRH in the PP by 20%. ProTRH and PAM mRNA levels were not affected in the hypothalamus by this treatment. Since TRH in the PP has been suggested to play a role in prolactin (PRL) release, we determined the content of TRH in the PP during a 6-hour suckling stimulus that increased PRL levels in peripheral blood 30-fold. Whereas TRH in the median eminence increased by 35%, 6 h after the initiation of suckling, TRH levels in the PP remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Neural differentiation of the human neuroblastoma cell line IMR32 induces production of a thyrotropin-releasing hormone-like peptide

The human neuroblastoma cell line IMR32 produces and secretes substantial amounts of TRH-immunoreactivity (TRH-IR) as measured with radioimmunoassay (RIA) using the nonspecific antiserum 4319. It was found that synthesis of TRH-IR is dependent on neural differentiation: under serum-free conditions these cells exhibit neural characteristics as defined by morphological and biochemical standards. After culture for 2-5 days in serum-free medium cells grew large neural processes and expressed neuron-specific markers whereas glial-specific markers were absent. TRH-IR became detectable after 4-8 days serum-free conditions. Northern blot and chromatographic analysis, however, failed to detect proTRH mRNA and authentic TRH in these cells. Moreover, TRH-IR was undetectable in the RIA using TRH-specific antiserum 8880. TRH-IR produced by differentiated cells was retained on a QAE Sephadex A-25 anion-exchange column and thus negatively charged. HPLC analysis showed coelution with the synthetic peptide pGlu-Glu-ProNH2. Study of the mechanisms regulating production of this novel peptide in these cells should further elucidate the role differentiation plays in the synthesis of neuropeptides.