Jan M.M. Rondeel

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.

Suppression of thyrotropin-releasing hormone gene expression by interleukin-1-beta in the rat : implications for nonthyroidal illness

Nonthyroidal illness is characterized by low thyroid hormone levels and inappropriately normal or decreased TSH levels. To determine whether the hypothalamus contributes to these responses, TRH gene expression in hypophysiotropic neurons of the paraventricular nucleus (PVN) was investigated using semiquantitative in situ hybridization histochemistry in an animal model of nonthyroidal illness. Following the systemic administration of bacterial lipopolysaccharide (LPS; 250 micrograms/100 g BW), plasma T4, T3 and TSH were reduced but this was not associated with an increase in the content of proTRH mRNA in the PVN as occurs when plasma T4 and T3 concentrations fall during primary hypothyroidism. Constant infusion of human interleukin-1 beta (IL-1 beta) into the cerebrospinal fluid also reduced plasma T4 concentration. This persisted for the duration of the infusion but TSH was only suppressed after 7 days of infusion when body weight had declined. By 24 h, the content of proTRH mRNA in the PVN in IL-1 beta infused animals was significantly reduced from control values. These studies indicate that the peripheral administration of endotoxin or central administration of IL-1 beta in the rat is associated with a proTRH mRNA content in the PVN that may be inappropriately normal or reduced for the level of circulating thyroid hormone. We propose that the inability of hypophysiotropic neurons to induce TRH gene expression in nonthyroidal illness, when circulating thyroid hormone levels are low, is one of several factors that contributes to the inability of the anterior pituitary to increase its secretion of TSH.

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.

Effect of Suckling on the in vivo Release of Thyrotropin-Releasing Hormone, Dopamine and Adrenaline in the Lactating Rat

The present study was concerned with the effect of suckling on the hypothalamic release of thyrotropin-releasing hormone (TRH), dopamine and adrenaline in lactating rats as estimated by push-pull perfusion of the median eminence-arcuate nucleus area. The push-pull cannula was implanted on day 15 of pregnancy. This surgery did not interfere with pregnancy, time of delivery or lactation. Push-pull perfusion was performed on day 8 or 14 of lactation and 30 out of 42 perfusions were successful. On the day of perfusion mothers and young were separated. Six hours later push-pull perfusion was begun and 6 samples at 15-min intervals were collected. In control animals, not allowed to nurse pups during perfusion, the release of TRH, dopamine and adrenaline did not change during the 90-min period. In experimental animals, reunited with their litter after 30 min of perfusion, the hypothalamic release of adrenaline did not change. However, both on day 8 and 14 suckling induced a 50% decrease in the release of dopamine (p less than or equal to 0.025) which lasted for 15-30 min. Suckling on day 14 did not affect the concentration of TRH in the perfusate, but on day 8 the TRH output gradually decreased for 45 min after the onset of suckling.

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.