Arginine vasopressin as a thyrotropin-releasing hormone

Exogenous thyrotropin improves renal function in euthyroid patients, while serum creatinine levels are increased in hypothyroidism

Background

There is evidence showing that the hypothyroid state results in increased serum creatinine levels. However, whether this is only due to the peripheral thyroid hormones or if thyroid-stimulating hormone (TSH) is also involved is not known.

Methods

Serum creatinine levels and estimated glomerular filtration rate (eGFR) were assessed in thyroidectomized patients with varying thyroid hormones and TSH levels. Blood samples from Group 1 (21 patients) were obtained 1 month after complete thyroidectomy, while under a hypothyroid state (t1) and a sufficient time after thyroid hormones initiation (euthyroid state, t2). Group 2 (20 euthyroid patients) were sampled after recombinant human thyrotropin injections (rhTSH, t1) and later after rhTSH extinction (t2).

Results

In Group 1, serum creatinine levels decreased after correction of hypothyroidism (85.3 ± 4.3 versus 78.0 ± 3.9 µmol/L; P = 0.04). In Group 2, serum creatinine levels increased after rhTSH withdrawal (70.6 ± 5.7 µmol/L versus 76.5 ± 5.8 µmol/L; P = 0.007). Between t1 and t2, eGFR varied accordingly [Group 1, 71.7 ± 3.5 versus 81.2 ± 4.5 mL/min/1.73 m² (P = 0.02); Group 2, 97.7 ± 7.4 versus 87.5 ± 5.9 (P = 0.007)]. The changes in TSH and eGFR following supplementation with thyroxine were significantly correlated (r = −0.6, P = 0.0041).

Conclusions

Iatrogenic hypothyroidism significantly increases serum creatinine and reversibly impairs eGFR, while treatment with rhTSH enhances renal function in euthyroid patients, supporting the existence of an influence of TSH level on renal function. The mechanisms by which peripheral thyroid hormones and TSH influence GFR need to be identified in physiology-orientated studies.

Ontogeny and effects of hypothalamic pituitary disconnection on formation of inositol trisphosphate in fetal sheep pituitary cells

In late gestation fetal sheep, the pituitary becomes increasingly responsive to stimulation by arginine vasopressin (AVP). This change appears to be one important factor mediating the plasma cortisol surge, a critical developmental event. It is not known precisely why pituitary corticotropes become more responsive at this time. In this study we examined the possibility that changes in second messenger generation [inositol trisphosphate (IP(3))] are responsible. Two studies were undertaken. The first was an ontogeny study, where pituitaries were isolated from 100-, 120-, and 140-d gestational age (dGA) fetal sheep. Cells were cultured, stimulated with AVP, and the formation of IP(3) assessed. The amount of IP(3) generated increased with gestational age (percent increases from unstimulated controls were 4.6, 11.5, and 21.5 for 100, 120, and 140 dGA, respectively), with significant differences between the 140-dGA group and both earlier groups apparent. The second study examined the impact of 120-dGA hypothalamo-pituitary disconnection (HPD), which prevents corticotrope maturation, on responsiveness of pituitary cells isolated from 140-dGA fetuses. Cells were stimulated with AVP, and the formation of IP(3) and secretion of ACTH were assessed. Significantly less IP(3) was formed, and ACTH secreted in cells from HPD compared with control fetuses (IP(3) and ACTH levels were 50% and 35% lower, respectively). Results from the HPD study demonstrate that the ontogenic changes in IP(3) after AVP require an intact hypothalamic-pituitary-adrenal axis. These findings suggest that heightened second messenger generation may be a key reason for increased ACTH secretory responsiveness to AVP in the late gestation sheep fetus.

From Neuroendocrinology to Neuroimmunomodulation - a tribute to Prof. Dr. Samuel McCann

One of the leading experts in the field of Neuroendocrinology and Neuroimmunmodulation, Samuel Mac Donald McCann, known by all his friends as 'Don', passed away in 2007. This article pays tribute to his outstanding scientific contribution and a glimpse on his fascinating personality. A member of the National Academy of Sciences of the United States and pioneer in the field of neuroendocrine regulation, he identified numerous hormones and peptides and set the stage for basic concepts in physiology and clinical medicine.

Oxytocin secretion induced by osmotic stimulation in rats during the estrous cycle and after ovariectomy and hormone replacement therapy

Hyperosmolality is a potent stimulus for the secretion of oxytocin. Oxytocinergic neurons are modulated by estrogen and oxytocin secretion in rats varies according to the phase of the estrous cycle, with higher activity during proestrus. We investigated the oxytocin secretion induced by an osmotic stimulus (0.5 M NaCl) in female rats. Plasma oxytocin and the oxytocin contents in the neurohypophysis and the paraventricular and supraoptic nuclei were determined during the morning (8-9 h) and afternoon (17-18 h) of the estrous cycle and after ovariectomy followed or not by hormone replacement. Plasma oxytocin peaked in control animals during proestrus. Oxytocin content decreased in the paraventricular and supraoptic nuclei during proestrus and estrus compared to diestrus and increased in the neurohypophysis during proestrus morning. No significant difference was observed in the oxytocin content of the neurohypophysis, nuclei or plasma between ovariectomized animals and ovariectomized animals treated with estrogen or estrogen plus progesterone. Therefore, any ovarian factor other than estrogen or progesterone seems to play a direct or indirect role in the increase in oxytocin secretion. The osmotic stimulus caused an increase in plasma oxytocin throughout the estrous cycle. A reduction in oxytocin content during diestrus and an increase during proestrus were observed in the paraventricular nuclei. In ovariectomized animals, the treatment with estrogen potentiated the response of oxytocin to the osmotic stimulus, with the response being even stronger in the case of estrogen plus progesterone. In conclusion, the ovarian steroids estrogen plus progesterone could modulate the osmoreceptor mechanisms related to oxytocin secretion.

Physiological and pathophysiological aspects of thyrotropin-releasing hormone gene expression in the human hypothalamus

Although the tripeptide thyrotropin-releasing hormone (TRH) was the first hypothalamic hormone to be isolated and characterized, only very few data were available on the central component of the hypothalamus-pituitary-thyroid (HPT) axis in the human brain until recently. We used immunocytochemistry to describe, for the first time, the distribution of TRH-containing cells and fibers in the human hypothalamus. Brain material was obtained with a short postmortem delay followed by fixation in paraformaldehyde, glutaraldehyde, and picric acid. Many TRH-containing cells were present in the paraventricular nucleus (PVN), especially in its dorsocaudal part. Some TRH cells were found in the suprachiasmatic nucleus (SCN), which is the circadian clock of the brain, and in the sexually dimorphic nucleus (SDN), which is in agreement with earlier observations in the rat hypothalamus. Dense TRH-containing fiber networks were present not only in the median eminence but also in a number of other hypothalamic areas, suggesting a physiological function of TRH as a neuromodulator or neurotransmitter in the human brain, in addition to its neuroendocrine role in pituitary secretion of thyroid-stimulating hormone (TSH). As a next step, we developed a technique for TRH mRNA in situ hybridization using a [35S] CTP-labeled TRH cRNA antisense probe in formalin-fixed paraffin-embedded sections. Numerous heavily labeled TRH mRNA-containing neurons were detected in the caudal part of the PVN, while some cells were present in the SCN and in the perifornical area. These results demonstrated the value of in situ hybridization for elucidating the chemoarchitecture of the human hypothalamus in routinely fixed autopsy tissue and enabled us to perform quantitative studies. As part of the neuroendocrine response to disease, serum concentrations of thyroid hormone decrease without giving rise to elevated concentrations of TSH, suggesting altered feedback control at the level of the hypothalamus and/or pituitary. In order to establish whether decreased activity of TRH cells in the PVN contributes to the persistence of low TSH levels in nonthyroidal illness (NTI), hypothalamic TRH gene expression was investigated in patients whose plasma concentrations of thyroid hormones had been measured just before death. Quantitative in situ hybridization showed a positive correlation of total TRH mRNA in the PVN and serum concentrations of TSH and triiodothyronine (T3) less than 24 hours before death, supporting our hypothesis. Current experiments aim at elucidating the mechanism by which hypothalamic thyroid hormone feedback control in TRH cells of patients with NTI is changed.

Altered thyroid axis function in Lewis rats with genetically defective hypothalamic CRH/VP neurosecretory cells

Lewis rats display hyporesponsive hypothalamo-pituitary-adrenocortical (HPA) axes, overproduction of cytokines, and susceptibility to inflammatory disease. The Lewis corticotropin-releasing hormone (CRH) neurosecretory system contains normal numbers of vasopressin (VP)-deficient axon varicosities, but abnormally sparse VP-containing varicosities in the external zone of the median eminence, compared to the normoresponsive Sprague Dawley (SD), Wistar and Fischer 344 strains. Since VP may act as a thyrotropin-releasing factor, we hypothesized that thyroid axis responsivity may be altered in Lewis rats. T3, T4 and TSH were measured by radioimmunoassay, and free T4 by equilibrium dialysis, in adult male Lewis and SD rats. One h cold (5 degrees C) induced significant increases in T3, T4 and TSH levels in Lewis rats but not in SD rats. Ninety min insulin-induced hypoglycemia (1 IU/kg, i.p.) induced a significant T3 increase in Lewis rats and a significant T4 increase in SD rats. Two h after ip LPS (0.25 or 0.75 mg/kg), T4 levels fell significantly in Lewis rats but not in SD rats. TSH decreases were significant in Lewis rats after 0.75 mg/kg and in SD rats after 0.25 mg/kg. Baseline hormone levels were generally higher in Lewis rats; the differences were significant for T3 and T4 in the insulin experiments and for T3, T4 and free T4 in the LPS experiments. The data suggest that reduced inhibition from the adrenocortical axis in Lewis rats leads to hyperresponsivity of the thyroid axis to cold, and greater LPS-induced decreases in T4 levels, probably due to an exaggerated inhibitory cytokine response.

Effects of exogenous arginine vasotocin on circulating levels of thyroid hormones and melatonin in the pigeon, Columbia livia

The effects of intravenous injections of two different preparations of synthetic arginine vasotocin (AVT), deamino-dicarba-arginine vasotocin and arginine vasotocin diacetate tetrahydrate on serum levels of thyroxine (T4), triiodothyronine (T3) and melatonin were studied in the resting pigeon. Although variations depending on factors such as the type of AVT preparation and the dose used and also the duration of treatment were apparent, both preparations of AVT produced an increase in serum levels of T4 and a decrease in that of T3. The T3:T4 ratio was also significantly lower in response to AVT administration. However, no statistically significant changes in serum levels of melatonin were discernible with the administration of either of the two AVT preparations, even though melatonin levels in most AVT-treated groups showed a trend (P > 0.05) to be higher than that in the controls. The rise in T4 levels is indicative of increased release of T4 and/or the inhibition of its conversion to the active hormone, T3. The drop in T3 may account for the AVT-induced reduction in the metabolic rate of the pigeon observed in an earlier study. The absence of a significant increase in melatonin indicates that AVT could not produce such a change in the daytime in resting pigeons, in contrast to that in flown homing pigeons, suggesting that the modulating effect of AVT on melatonin levels is dependent on the physiological state of the animal.

Extrapituitary expression of the rat V1b vasopressin receptor gene

[Arg8]vasopressin (AVP) stimulates adrenocorticotropic hormone release from the anterior pituitary by acting on the V1b AVP receptor. This receptor can be distinguished from the vascular/hepatic V1a and renal V2 AVP receptors by its differential binding affinities for structural analogous of AVP. Recent studies have shown that the cloned V1a and V2 receptors are structurally related. We have isolated a clone encoding the V1b receptor from a rat pituitary cDNA library using polymerase chain reaction (PCR)-based methodology. The rat V1b receptor is a protein of 421 amino acids that has 37-50% identity with the V1a and V2 receptors. Homology is particularly high in the seven putative membrane-spanning domains of these guanine nucleotide-binding protein-coupled receptors. Expression of the recombinant receptor in mammalian cells shows the same binding specificity for AVP agonists and antagonists as the rat pituitary V1b receptor. AVP-stimulated phosphotidylinositol hydrolysis and intracellular Ca2+ mobilization in Chinese hamster ovary or COS-7 cells expressing the cloned receptor suggest second messenger signaling through phospholipase C. RNA blot analysis, reverse transcription PCR, and in situ hybridization studies reveal that V1b receptor mRNA is expressed in the majority of pituitary corticotropes as well as in multiple brain regions and a number of peripheral tissues, including kidney, thymus, heart, lung, spleen, uterus, and breast. Thus, the V1b receptor must mediate some of the diverse biological effects of AVP in the pituitary as well as other organs.

Distribution of thyrotropin-releasing hormone (TRH)-containing cells and fibers in the human hypothalamus

In the present study, we describe for the first time the distribution of thyrotropin-releasing hormone (TRH)-containing cells and fibers in the human hypothalamus using brain material obtained with a short postmortem delay. Following fixation in paraformaldehyde, glutaraldehyde and picric acid, excellent staining was obtained with two different TRH antisera. Many TRH-containing neurons were present in the paraventricular nucleus (PVN), especially in the dorsocaudal part of this nucleus. They were mostly parvicellular, but a few magnocellular TRH-positive neurons were observed as well. The PVN also contained a dense network of TRH fibers. The supraoptic nucleus (SON) did not show any TRH immunoreactivity, excluding the possibility of cross-reactivity of the antiserum with neurohypophysial hormones or their precursors. In addition, TRH cells were found in the suprachiasmatic nucleus (SCN), which is the circadian clock of the brain, in the sexually dimorphic nucleus (SDN) and dorsomedially of the SON. We observed small number of TRH cells throughout the hypothalamic gray in all subjects studied. A high density of TRH-containing fibers was seen not only in the median eminence but also in other hypothalamic areas, e.g., in the ventromedial nucleus (VM) and in the perifornical area. The results generally agree with earlier data in the rat, with the exception of the absence of TRH cells in the SON. The large number of sites of TRH-containing fiber terminations on neurons suggests important physiological functions of this neuropeptide as a neurotransmitter or neuromodulator in the human brain, in addition to its role as a neurohormone in pituitary secretion of thyroid-stimulating hormone (TSH).

Arginine vasopressin (AVP) causes the reversible phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein in the ovine anterior pituitary: evidence that MARCKS phosphorylation is associated with adrenocorticotropin (ACTH) secretion

We have recently shown that AVP causes a protein kinase C (PKC)-dependent increase in ACTH release and biosynthesis in ovine anterior pituitary cells. In these cells, AVP also causes the translocation of PKC from the cytosol to the cell membrane which is maximal at 5 min, but the intracellular events distal to protein kinase C activation that underlie ACTH secretion have not been well characterized to date. Since the MARCKS protein has been implicated in neurosecretion and is phosphorylated by PKC in synaptosomes, studies were carried out to determine whether AVP might cause MARCKS phosphorylation in the ovine anterior pituitary, and to determine whether this phenomenon might be temporally correlated with PKC translocation and the release of ACTH. When cytosolic fractions of rat brain, ovine anterior pituitary, and cultured ovine anterior pituitary cells were incubated with purified PKC, several proteins were phosphorylated including those in the region of 83-85 kDa. After precipitation of the proteins with 40% acetic acid, the 83-85 kDa phosphoproteins were selectively recovered in the acid soluble phase. Phosphopeptide maps of either the 83 or 85 kDa proteins were generated with Staphylococcus aureus V8 protease and revealed 13 and 9 kDa phosphopeptides, which are characteristic of the authentic MARCKS protein. An identical phosphopeptide map was also obtained when the MARCKS protein was selectively extracted from intact 32P-labeled anterior pituitary cells. MARCKS phosphorylation was markedly increased when ovine anterior pituitary cells were exposed to 1 microM phorbol 12-myristate 13-acetate (PMA). When the cells were exposed to 1 microM AVP, MARCKS phosphorylation increased at 15 s and reached the maximal plateau value at 30 s. MARCKS phosphorylation then started to diminish at 2 min, and baseline levels were attained by 10 min. In the same cells, AVP stimulated ACTH release in a biphasic manner-during the first 30 s, there resulted a rapid burst of ACTH secretion that was followed by a slower, but sustained rate of secretion. We conclude that: (1) AVP causes a rapid, and reversible, phosphorylation of the MARCKS protein in the ovine anterior pituitary; (2) since the AVP-induced increase in MARCKS phosphorylation occurs much earlier in these cells than does PKC trans-location, MARCKS phosphorylation may provide a more sensitive index of the onset of PKC activation than the translocation assay; (3) the close temporal association between MARCKS phosphorylation and the rapid early release of ACTH suggests that MARCKS phosphorylation may be involved in the initial intracellular events that underly exocytosis of the hormone.

Arginine vasopressin (AVP) causes the reversible phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein in the ovine anterior pituitary: evidence that MARCKS phosphorylation is associated with adrenocorticotropin (ACTH) secretion

We have recently shown that AVP causes a protein kinase C (PKC)-dependent increase in ACTH release and biosynthesis in ovine anterior pituitary cells. In these cells, AVP also causes the translocation of PKC from the cytosol to the cell membrane which is maximal at 5 min, but the intracellular events distal to protein kinase C activation that underlie ACTH secretion have not been well characterized to date. Since the MARCKS protein has been implicated in neurosecretion and is phosphorylated by PKC in synaptosomes, studies were carried out to determine whether AVP might cause MARCKS phosphorylation in the ovine anterior pituitary, and to determine whether this phenomenon might be temporally correlated with PKC translocation and the release of ACTH. When cytosolic fractions of rat brain, ovine anterior pituitary, and cultured ovine anterior pituitary cells were incubated with purified PKC, several proteins were phosphorylated including those in the region of 83-85 kDa. After precipitation of the proteins with 40% acetic acid, the 83-85 kDa phosphoproteins were selectively recovered in the acid soluble phase. Phosphopeptide maps of either the 83 or 85 kDa proteins were generated with Staphylococcus aureus V8 protease and revealed 13 and 9 kDa phosphopeptides, which are characteristic of the authentic MARCKS protein. An identical phosphopeptide map was also obtained when the MARCKS protein was selectively extracted from intact 32P-labeled anterior pituitary cells. MARCKS phosphorylation was markedly increased when ovine anterior pituitary cells were exposed to 1 microM phorbol 12-myristate 13-acetate (PMA). When the cells were exposed to 1 microM AVP, MARCKS phosphorylation increased at 15 s and reached the maximal plateau value at 30 s. MARCKS phosphorylation then started to diminish at 2 min, and baseline levels were attained by 10 min. In the same cells, AVP stimulated ACTH release in a biphasic manner - during the first 30 s, there resulted a rapid burst of ACTH secretion that was followed by a slower, but sustained rate of secretion. We conclude that: (1) AVP causes a rapid, and reversible, phosphorylation of the MARCKS protein in the ovine anterior pituitary; (2) since the AVP-induced increase in MARCKS phosphorylation occurs much earlier in these cells than does PKC trans-location, MARCKS phosphorylation may provide a more sensitive index of the onset of PKC activation than the translocation assay; (3) the close temporal association between MARCKS phosphorylation and the rapid early release of ACTH suggests that MARCKS phosphorylation may be involved in the initial intracellular events that underly exocytosis of the hormone.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Regulation of vasopressin gene expression: changes in the level, but not the size, of vasopressin mRNA following endocrine manipulations

1. Regulatory interactions between the hypothalamoneurohypophyseal vasopressin (VP) axis and the endocrine systems of the anterior pituitary have been investigated in the rat by observing changes in VP mRNA expression following endocrine manipulations. 2. An increase in the level, but not size, of VP mRNA was found in the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus and in the neurointermediate lobe (NIL) of the pituitary following hypothyroidism (induced by drinking 6-n-propyl-2-thiouracil; PTU) and adrenalectomy. Hypothyroidism induced by alternative procedures (surgical thyroidectomy or PTU injections) did not exert similar effects. 3. Treatment with the dopamine agonist bromocriptine to reduce prolactin secretion raised levels of VP mRNA in the NIL only. Castration did not up-regulate VP mRNA levels. 4. Since the observed effects on VP mRNA levels occur in the absence of changes in plasma osmolality, these results provide evidence of nonosmotic regulation of VP gene expression, an effect which is observed most clearly in the NIL pool of VP mRNA. Furthermore, the effects are distinct from changes in VP mRNA levels associated with raised plasma osmolality since the VP mRNA size was not increased.

Evidence that the stimulation by arginine vasopressin of the release of adrenocorticotropin from the ovine anterior pituitary involves the activation of protein kinase C

These studies were undertaken to evaluate the role of protein kinase C (PKC) in the regulation by arginine vasopressin (AVP) of adrenocorticotropin (ACTH) secretion from the ovine anterior pituitary. AVP caused the rapid translocation of PKC from the cytosol to the cell membrane in ovine anterior pituitary cells that was maximal at 5 min. This phenomenon, which is a known concomitant of C-kinase activation, was produced to a greater extent by phorbol 12-myristate 13-acetate (PMA) but not by corticotropin-releasing factor (CRF). To determine whether AVP activated corticotrope PKC, we assessed the ability of three different PKC inhibitors (H-7, sphingosine, and retinal) to modify basal, AVP-, PMA-, and CRF-stimulated ACTH release. In addition to inhibiting the in vitro activity of purified PKC, each compound also caused in vitro inhibition of the protein kinase A (PKA) catalytic subunit, indicating that none could be considered to be a specific inhibitor of PKC and the PKA catalytic subunit. As determined by the mean IC50 values required for the in vitro inhibition of PKC and the PKA catalytic subunit, sphingosine was judged to be the most selective and H-7 the least selective PKC inhibitor. A 4 h exposure to each inhibitor caused a dose-dependent increase in basal ACTH release and attenuation of both AVP- and PMA-stimulated ACTH release. H-7 and retinal, in concentrations that caused a 20-50% inhibition of PKA, also attenuated CRF-stimulated ACTH release; however, this effect was not observed with sphingosine in concentrations that caused only a 10-20% inhibition of PKA. We conclude that: (1) AVP causes the direct activation of PKC in the ovine anterior pituitary and that C kinase activation is important in mediating the effect of AVP on ACTH release; (2) the finding that inhibition of PKC elevates ACTH suggests that basal ACTH secretion is also partly regulated by PKC; (3) since CRF does not cause PKC translocation in ovine anterior pituitary cells, it is unlikely that PKC plays a physiological role in the action of CRF on the corticotrope; (4) the finding that H-7 and retinal attenuate CRF-stimulated ACTH secretion suggests that CRF activates PKA in corticotropes.

Role of a pituitary-specific transcription factor (pit-1/GHF-1) or a closely related protein in cAMP regulation of human thyrotropin-beta subunit gene expression

cAMP regulation of the human thyrotropin-beta (TSH beta) gene cAMP was studied in two heterologous cell lines, a human embryonal kidney cell line (293) and a rat pituitary cell line (GH3). In 293 cells, human TSH beta gene expression was not stimulated by the adenylate cyclase activator forskolin or the cAMP analogue 8-bromo-cAMP (8-Br-cAMP). On the other hand, these agents induced human TSH beta gene expression 4-12-fold in GH3 cells. Deletion analysis demonstrated that the regions from +3 to +8 bp and from -128 to -61 bp were both necessary for cAMP stimulation. The latter region contains three DNA sequences homologous to a pituitary-specific transcription factor, Pit-1/GHF-1, DNA-binding site. Gel-mobility assays demonstrated that a radiolabeled human TSH beta probe (-128 to -61 bp) formed five specific DNA-protein complexes with mouse thyrotropic tumor (MTT) nuclear extract and two specific complexes with in vitro translated Pit-1/GHF-1. Four of the five MTT complexes and both in vitro Pit-1/GHF-1 complexes were reduced or eliminated by excess of an unlabeled Pit-1/GHF-1 DNA-binding site from the rat growth hormone gene, but not a mutated version of the same DNA fragment, suggesting that Pit-1/GHF-1 or a closely related thyrotroph protein binds to these DNA sequences. In 293 cells, co-transfection of an expression vector containing the Pit-1/GHF-1 cDNA restored cAMP-responsiveness to the human TSH beta promoter (5.2- and 6.6-fold maximal stimulation by 8-Br-cAMP and forskolin, respectively) but not the herpes virus thymidine kinase promoter (1.2-fold maximal stimulation by either agent). Thus we conclude that the human TSH beta gene is positively regulated by cAMP in GH3 but not 293 cells. Since the human TSH beta gene contains at least one high-affinity binding site for Pit-1/GHF-1 in a region necessary for cAMP stimulation and cAMP stimulation could be restored to the human TSH beta promoter in a previously nonresponsive cell line by the addition of Pit-1/GHF-1, this suggests that Pit-1/GHF-1, or a closely related protein in the thyrotroph, may be a trans-acting factor for cAMP stimulation of the TSH beta gene.

Binding Sites for Vasopressin in the Human Pituitary are Associated with Corticotrophs and may Differ from Other Known Vasopressin Receptors

Abstract In view of the fact that Vasopressin can induce pituitary adrenocorticotrophin release, we performed an autoradiographical study of [(3)H]arginine vasopressin binding in human pituitary tissue obtained post-mortem from adults and foetuses. Sites of specific, high affinity binding (IC(50) 3 to 5 nM) were detected as patches in the anterior lobe and at the junction between the anterior and neural lobes. The neural lobe was not labelled. Immunocytochemical studies performed on human pituitary tissue showed that [(3)H]arginine vasopressin only marked zones which correspond to areas rich in cells immunoreactive to adrenocorticotrophin. We conclude that in the human pituitary, corticotrophs bear vasopressin binding sites. Since non-radioactive synthetic structural analogues of vasopressin acting as V(1) and V(2) agonists or antagonists failed to compete for binding of radioligand in the human pituitary, while a V(1)-type agonist displaced [(3)H]arginine vasopressin binding in the rat pituitary, we postulate that binding sites in the human pituitary may differ from the previously known vasopressin receptors.

Multipotential pituitary cells that contain adrenocorticotropin (ACTH) and other pituitary hormones

Anterior pituitary cells that contain adrenocorticotropin (ACTH) are morphologically and functionally heterogeneous. Immunolabeling has identified stellate subtypes with scattered or peripherally arranged granules, and ovoid or irregularly shaped cells with a denser accumulation of granules, which overlap morphologically with other cell types such as small gonadotropes or thyrotropes. Dual cytochemical labeling studies on the same or serial fields have identified cells that store ACTH with LH, FSH, TSH, or prolactin. Evidence is presented to suggest that they are multipotential cells with the capacity to augment corticotropes or one of the other cell populations.

Psychiatric implications of altered limbic-hypothalamic-pituitary-adrenocortical activity

Hormones of the limbic-hypothalamic-pituitary-adrenocortical (LHPA) system are much involved in central nervous system regulation. The major LHPA neuropeptides, corticotropin-releasing hormone (CRH), vasopressin (AVP) and corticotropin (ACTH) do not only coordinate the neuroendocrine response to stress, but also induce behavioral adaptation. Transcription and post-translational processing of these neuropeptides is regulated by corticosteroids secreted from the adrenal cortex after stimulation by ACTH and other proopiomelanocortin derived peptides. These steroids play a key role as regulators of cell development, homeostatic maintenance and adaptation to environmental challenges. They execute vitally important actions through genomic effects resulting in altered gene expression and nongenomic effects leading to altered neuronal excitability. Since excessive secretory activity of this particular neuroendocrine system is part of an acute stress response or depressive symptom pattern, there is good reason to suspect that central actions of these steroids and peptides are involved in pathophysiology determining the clinical phenotype, drug response and relapse liability. This overview summarizes the clinical neuroendocrine investigations of the author and his collaborators, while they worked at the Department of Psychiatry in Mainz. The major conclusions from this work were: (1) aberrant hormonal responses to challenges with dexamethasone, ACTH or CRH are reflecting altered brain physiology in affective illness and related disorders; (2) hormones of the LHPA axis influence also nonendocrine behavioral systems such as sleep EEG; (3) physiologically significant interactions exist between LHPA hormones, the thyroid, growth hormone, gonadal and other neuroendocrine systems; (4) hormones of the LHPA axis constitute a bidirectional link between immunoregulation and brain activity; and (5) future psychiatric research topics such as molecular genetics of affective disorders, familial risk studies, drug response analysis and neurobiology of aging will benefit from extended knowledge of neural corticosteroid effects at a clinical, cellular, and molecular level.

Cachectin alters anterior pituitary hormone release by a direct action in vitro

Cachectin (tumor necrosis factor) is a powerful macrophage hormone released during infection, which circulates in blood to produce diverse effects in the organism. We examined the effect of cachectin on release of anterior pituitary hormones from either hemipituitaries or dispersed pituitary cells incubated in vitro. The action of cachectin on dispersed cells was demonstrable only after 2 hr of incubation. With this incubation time, the protein produced a dose-related stimulation of release of adrenocorticotropin (ACTH), growth hormone (GH), and thyrotropin (TSH), but not of prolactin (Prl), from both hemipituitaries and dispersed cells. The doses required for stimulation were low in the case of hemipituitaries, usually of the order of 10(-12) M, whereas they were higher by one or two orders of magnitude with the dispersed pituitary cells. This may be related either to loss of receptors for the protein during the dispersion procedure or to the fact that in the hemipituitary system cell interactions are facilitated because the cells are close to each other. In the dispersed cell system cachectin evoked a dose-related decrease in cyclic AMP content. Incubation with somatostatin lowered the cyclic AMP content of the cells and depressed GH output without altering output of TSH or Prl. When somatostatin and cachectin were incubated together with the cells, the suppression of cyclic AMP production was abolished; TSH and Prl release were stimulated, but the action of cachectin to stimulate GH release was blocked. The stimulation of Prl release by cachectin in the presence of somatostatin may be related to the elevation of cyclic AMP, a known stimulator of Prl release. The cyclooxygenase inhibitor indomethacin nearly completely blocked the stimulatory effect of cachectin on release of GH and TSH from dispersed pituitary cells but had only a slight and nonsignificant attenuating effect on its ACTH-releasing action. These results suggest that at least part of the stimulatory action of the peptide on pituitary hormone release is brought about by prostaglandins. The failure of indomethacin to block the release of ACTH induced by cachectin suggests that other mechanisms may be involved in the release of ACTH induced by this peptide. Since the concentrations of cachectin required to stimulate pituitary hormone release are similar to those that are encountered in plasma during infection, it is likely that this direct pituitary action has pathophysiological significance.

Physiological regulation of thyrotropin

The pattern of TSH secretion in man in pulsatile in addition to the well known circadian variation. The mechanism triggering TSH pulses remains unclear to date. Infusions of somatostatin or dopamine rapidly lowering basal TSH levels without suppressing the pulsatile pattern suggest that an episodic disinhibition exerted by a physiological inhibitor is not a likely cause. On the same basis, thyroid hormones do not appear to be candidates, since they similarly inhibit basal TSH levels after a time lag of several hours but again do not suppress pulsatile release of the hormone. In contrast, bolus injections of dexamethasone completely abolish pulsatile release of TSH for several hours despite a normal sensitivity of the pituitary to exogenous TRH, suggesting a hypothalamic action of the drug. The hypothesis that pulsatile TSH release might be governed by a pulsatile mode of a hypothalamic stimulator is supported by the observation that an infusion of nifedipine, a calcium channel blocker, which in vitro selectively inhibits the TRH effect on TSH but not prolactin secretion, exerts a comparable effect when it is infused in vivo.

Effect of lysine vasopressin on basal and TRH stimulated TSH and PRL release in normal men

In order to test the possible effects of lysine vasopressin (LVP) on basal and TRH stimulated TSH and PRL release, an iv bolus of LVP (0.06 IU/kg bw) was injected alone or just before TRH (20 or 400 micrograms iv) in 18 normal male subjects. The administration of LVP modified neither the basal secretion of TSH and PRL nor the TSH and PRL release induced by 20 or 400 micrograms TRH. These data suggest that in humans, vasopressin is not involved in the control of TSH and PRL release at the anterior pituitary level.

Effects of TRH on hormone release from pituitaries of the lizard, Anolis carolinensis

The ability of thyrotropin-releasing hormone (TRH) to stimulate thyrotropin (TSH) from pituitaries of adult male lizards, Anolis carolinensis, was tested in vivo and in vitro. TSH output by pituitaries in vitro was determined by bioassay of the incubation medium using in vitro T4 output by thyroids from the same lizards. Pituitaries incubated without TRH had no detectable TSH secretion during two consecutive (2 or 3 hr) periods of incubation. Incubation in 10 or 100 ng/ml TRH for 2 or 3 hr significantly augmented release of TSH bioactivity in a dose-dependent manner. Pituitaries taken from goitrogen (100 micrograms methimazole/day for 10 days)-treated lizards had elevated basal TSH secretion but did not respond to TRH. TRH injection in vivo (5 micrograms/hr for 10 hr) appeared to stimulate acute release of MSH activity as judged by darkening of skin color after each injection, and plasma T4 was significantly elevated at the end of treatment. These results provide additional evidence that the reptilian thyrotrope has functional TRH receptors and the TSH-stimulating activity of the tripeptide along with its effects on other pituitary cells was present at an early stage of reptilian evolution.

Hypophysiotrophic function of vasopressin and oxytocin

We have investigated the effects of lesioning the hypothalamic paraventricular nucleus (PVN) on the secretion of two corticotropin-releasing neurohormones, vasopressin (VP) and oxytocin (OT), at the median eminence. The experimental model was the median eminence incubated in vitro, the secretion of neurohormones was stimulated by adding 48 mM KCl to the incubation medium. In addition, immunohistochemical staining was performed to correlate the changes in neuropeptide secretion with the distribution of VP and OT immunoreactive elements in the median eminence. Lesioning of the PVN abolished the KCl-induced release of VP 1 week after hypothalamic surgery. After a longer period of postoperative survival (6 weeks), VP release was restored towards normal. The secretion of OT was reduced by 50% at 1 week after lesioning and rose to 400% of control at six weeks. The changes in VP and OT release at the median eminence largely correlated with the immunohistochemical distribution of VP and OT immunopositive nerve fibers in the external zone of the median eminence. Most importantly, 6 weeks after the PVN lesion a dense network of OT immunoreactive varicosities was observed around primary portal capillaries, where normally OT fiber density is very low. These results demonstrate the functional and structural plasticity of VP- and OT-ergic neuronal systems that project to the median eminence. Furthermore, when taken together with earlier studies on the regulation of corticotropin secretion in long-term PVN-lesioned rats, the data indicate an important role for OT in the regulation of pituitary-adrenocortical function in PVN-lesioned rats.

Arginine vasotocin from the pituitary gland of the lamprey (Petromyzon marinus): isolation and amino acid sequence

Arginine vasotocin (AVT) was isolated from extracts of sea lamprey pituitary glands (Petromyzon marinus). The amino acid sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2 is identical to the molecule isolated from teleosts and tetrapods. The total yield was estimated to be 9.6 pmol per gland. No evidence could be found for the existence of a second neurohypophyseal nonapeptide in the lamprey pituitary.

Multifactorial control of pituitary hormone secretion: the "wheels" of the brain

An attempt is made to deal with the complexity of the nerve fibers in the median eminence. Visual aids are presented in the shape of "wheels" that depict a dynamic interplay of neurochemicals which result in the release of hormones from the anterior pituitary gland. The multiplicity of neurochemicals in the median eminence is perceived to be responsible for the integrated control of pituitary hormone releasing factors.

Receptors mediating the CRH effects of vasopressin and oxytocin

The data highlighted here suggest that the CRH effects of vasopressin and oxytocin are mediated by one and the same hypophysial receptor which has unique pharmacological specificity. The nomenclature for this receptor type is not established; both V3(53) and V1 beta 39 have been proposed. The former proposal is logical if the pharmacology of ligand recognition is emphasized, whereas the latter designation takes into account that transmembrane signalling from V1 receptors occurs via coupling proteins Go and Gi but not Gs. Such issues are best resolved after cDNA cloning of the genes for the receptors: in the meanwhile the working definition V3 seems more convenient. Several studies show that pituitary V3 receptors are regulated by the concentration of vasopressin in hypophysial portal blood and the amount of glucocorticoid hormones in the circulation (see Ref. 9 for review). Work in this area should clarify further the intracellular mechanism of the CRH action of vasopressin, as well as the factors that determine the responsiveness of corticotrophs to various secretagogues. Most recently, it has been shown that vasopressin is a potent thyrotropin-releasing hormone. This finding extends further the growing concept that there is considerable "cross-talk" between the classical neuroendocrine axes. These were previously thought to be separated by the hypothalamic organization of "final common pathways" of neuroendocrine motoneurons in the hypothalamus, each producing a unique neurohormone to regulate a single type of adenohypophysial cell. It seems that the days of the validity of this hypothesis are numbered, and an important task will be to determine the possible physiological significance of the "cross-talk" within the hypothalamo-pituitary unit in the regulation adrenocortical function.