The effect of salt-loading on corticotropin releasing hormone and arginine vasopressin mRNA levels in the mouse hypothalamus: a quantitative in situ hybridization analysis

The physiological and molecular mechanisms to maintain water and salt homeostasis in response to high salt intake in Mongolian gerbils (Meriones unguiculatus)

Desert rodents are faced with many challenges such as high dietary salt in their natural habitats and they have evolved abilities to conserve water and tolerate salt. However, the physiological and molecular mechanisms involved in water and salt balances in desert rodents are unknown. We hypothesized that desert rodents regulated water and salt balances by altering the expression of AQP2 and α-ENaC in the kidney. Mongolian gerbils (Meriones unguiculatus), a desert species, were acclimated to drinking water with different salt contents: (0, control; 4% NaCl, moderate salt, MS; 8% NaCl, high salt, HS) for 4 weeks. The gerbils drinking salty water had lower body mass, food intake, water intake, metabolic water production and urine volume. The HS gerbils increased the expression of arginine vasopressin (AVP) in the hypothalamus, and also enhanced the expression of AQP2 and cAMP/PKA/CREB signaling pathway in the kidney. In addition, these gerbils reduced serum aldosterone levels and α-ENaC expression in the kidney. Creatinine clearance was lower in the HS group than that in the control group, but serum and urine creatinine levels did not change. These data indicate that desert rodents rely on AVP-dependent upregulation of AQP2 and aldosterone-dependent downregulation of α-ENaC in the kidney to promote water reabsorption and sodium excretion under high salt intake.

miR-7b, a microRNA up-regulated in the hypothalamus after chronic hyperosmolar stimulation, inhibits Fos translation

The transcription factor activator protein 1 (AP-1) is formed through the dimerization of immediate-early genes Fos and Jun family members. Activator protein 1 is known as a pivotal regulator of major biological events such as cell proliferation, differentiation, organogenesis, memory formation, and apoptosis. During a search for microRNAs (miRNAs; small, endogenous, noncoding RNAs that repress gene expression of target mRNAs in animals posttranscriptionally) that are differentially expressed in the mouse paraventricular and supraoptic nuclei after 10 days of drinking 2% saline, one candidate microRNA that is relatively highly expressed, mmu-miR-7b (miR-7b), was studied further because sequence analysis suggested a likely interaction with the 3' untranslated region of Fos mRNA. We show that miR-7b expression inhibits Fos translation in vitro and that it and its host gene are prominently expressed in the PVN and other brain areas, including the suprachiasmatic nucleus. No effect on Fos mRNA levels was observed. Normally, Fos is expressed at low to undetectable levels in cells, but it shows rapid induction and decay after acute stimuli. Various pathways have been identified through which Fos family proteins are degraded; our results indicate a significant additional mechanism by which Fos protein and activity may be regulated.

Quantitation of neuropeptides in Cpe(fat)/Cpe(fat) mice using differential isotopic tags and mass spectrometry

Neuroendocrine peptides play important roles as intercellular messengers. We previously developed a technique to isolate and identify a large number of neuroendocrine peptides from Cpe(fat)/Cpe(fat) mice (Che, F.; et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 9971-6); these mice lack carboxypeptidase E activity and this defect causes an accumulation of neuropeptide intermediates that contain C-terminal Lys or Arg residues (Naggert, J. K.; et al. Nat. Genet. 1995, 10, 135-42). In the present study, we have developed a differential isotopic-labeling technique that can be used to quantitate changes in neuropeptide levels in Cpe(fat)/Cpe(fat) mouse tissues. Samples are treated with either the H6 or the D6 form of acetic anhydride, peptides that contain C-terminal basic amino acids are isolated by affinity chromatography on anhydrotrypsin agarose, and the isolated peptides are analyzed by mass spectrometry. Measurement of the regulation of pituitary peptides in response to dehydration showed a decrease in vasopressin. The general method described in this report should be widely applicable to a large number of neuroendocrine peptides, known and novel, in a variety of regulatory paradigms.

Activation by serotonin and noradrenaline of vasopressin and oxytocin expression in the mouse paraventricular and supraoptic nuclei

Noradrenaline and serotonin are known to control arginine-vasopressin (AVP) and oxytocin (OT) secretion in the systemic circulation. The aim of the current study was to investigate whether these monoamines are also able to influence AVP and OT expression in the paraventricular (PVN) and supraoptic nuclei (SON). To test this hypothesis, we used the Tg8 transgenic mice KO for the monoamine oxidase-A gene, which present high levels of noradrenaline and serotonin in the brain. AVP and OT expression were evaluated at peptide and mRNA levels by immunohistochemistry, enzyme immunoassay, and in situ hybridization. Compared with wild type, the amounts of AVP, OT, AVP mRNA, and OT mRNA were increased in the PVN and SON in Tg8 mice. To distinguish the respective contributions of noradrenaline and serotonin to these modifications, we treated Tg8 mice with a synthesis inhibitor of either catecholamines [alpha-methylparatyrosine (alpha-MPT)] or serotonin [parachlorophenylalanine (pCPA)]. Administration of alpha-MPT to Tg8 mice induced a decline in the amounts of AVP, OT, and their mRNA in the PVN and SON. The pCPA treatment in Tg8 mice was also associated with a decrease in OT expression in the PVN and SON and in AVP expression in the PVN, but not in the SON. These results suggest that noradrenaline may activate AVP and OT expression in the PVN and SON. Likewise, serotonin is proposed to stimulate AVP and OT expression in the PVN and only OT expression in the SON.

Increases in plasma ACTH and cortisol after hypertonic saline infusion in patients with central diabetes insipidus

To clarify the mechanism for the potentiation of CRH-induced ACTH response by the infusion of hypertonic saline, we investigated changes in plasma ACTH concentration after infusion of 5% hypertonic saline in five patients with untreated central diabetes insipidus (DI). Basal levels of plasma ACTH and cortisol in the DI group were not significantly different from those in normal control subjects. The infusion of hypertonic saline produced an increase in plasma arginine vasopressin (AVP) in controls, but did not elevate ACTH. However, in patients with DI, the plasma AVP concentration did not change, but circulating ACTH increased 3.6-fold (7.7 +/- 1.5 to 23.0 +/- 2.7 pmol/liter; P < 0.01), and plasma cortisol also increased significantly (298 +/- 99 to 538 +/- 124 nmol/liter; P < 0.05). Moreover, a positive correlation was observed between plasma ACTH and osmolality (r = 0.72; P < 0.005). These results indicate that ACTH secretion in DI patients is regulated by a mechanism distinct from that in healthy subjects. It seems possible that the increase in plasma osmolality promotes ACTH secretion in DI patients through AVP and/or urocortin via the hypophyseal portal system, independent of the AVP secretion from magnocellular neurons.

Differential expression of vasopressin, oxytocin and corticotrophin-releasing hormone messenger RNA in the paraventricular nucleus of the prairie vole brain following stress

Forced swimming, as an effective stressor, has been found to facilitate the development of pair bonds in male but to interfere with this behaviour in female prairie voles (Microtus ochrogaster). In the present study, we found that forced swimming differentially influenced the expression of messenger RNA for vasopressin, oxytocin and corticotrophin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN) in the prairie vole brain. Forced swimming did not alter vasopressin mRNA labelling, but did induce a sustained decrease in oxytocin mRNA labelling and a progressive increase in CRH mRNA labelling in the PVN. The elevated CRH mRNA labelling appeared to be due to an increased number of cells synthesizing CRH mRNA and an enhanced ability of individual cells to produce CRH mRNA. Male and female prairie voles did not differ in the vasopressin, oxytocin or CRH mRNA expression either at the basal levels or in response to swimming stress. Together, these data indicate that the hypothalamic response of vasopressin, oxytocin and CRH messenger RNAs to swimming stress is regulated by distinct transcriptional factors. In addition, it seems unlikely that these changes are involved directly in the sex differences in pair bond formation.

Reduced corticotropin-releasing factor and enhanced vasopressin gene expression in brains of mice with autoimmunity-induced behavioral dysfunction

The spontaneous development of autoimmune disease in MRL-lpr mice induces behavioral and endocrine changes that resemble effects of chronic stressors. To further examine the correspondence between autoimmune disease and chronic stress, we asked whether the brains of autoimmune mice show a shift in the corticotropin-releasing factor (CRF) to vasopressin (AVP) ratio. Using in situ hybridization histochemistry with 35S-labelled mouse riboprobes, the levels of mRNA transcripts encoding CRF and AVP were compared between autoimmune MRL-lpr and control MRL +/+ brains. CRF transcript levels were lower in the hypothalamic paraventricular nucleus and in the central nucleus of the amygdala in MRL-lpr mice. AVP transcript levels were higher in the paraventricular and the supraoptic nuclei in MRL-lpr mice compared to controls. CRF mRNA levels were inversely related to performance in stress-sensitive tasks and to measures of autoimmunity. As found previously for behavioral performance, immunosuppressive treatment with cyclophosphamide abolished the group difference in neuropeptide gene expression. These results indicate that an autoimmune disease process is necessary for the shift in the brain CRF:AVP ratio. Furthermore, they support the parallel between chronic stress and chronic autoimmunity/inflammation, and suggest common central mechanisms relevant to endocrine function and behavior.

Changes in hypothalamic cholecystokininA and cholecystokininB receptor subtypes and associated neuropeptide expression in response to salt-stress in the rat and mouse

This study demonstrates cholecystokinin receptor plasticity in response to salt-loading in the rat and mouse hypothalamus. It identifies, for the first time, the cholecystokinin receptor subtypes involved, firstly by receptor autoradiography and secondly by in situ hybridization. Both species showed increases in hypothalamic [125I]Bolton Hunter-cholecystokinin-8 binding. Co-incubation with the specific cholecystokininA and cholecystokininB antagonists, devazepide and CI-988, indicated that in the rat cholecystokininB receptor binding markedly increased, with a small increase in cholecystokininA receptor binding. In the mouse the response was comprised solely of cholecystokininA receptors. In situ hybridization studies were carried out on a range of peptide messenger ribonucleic acids after salt-loading. In the rat large increases in hypothalamic gene expression were detected for oxytocin, vasopressin, corticotrophin-releasing factor and preprocholecystokinin. In the mouse only vasopressin messenger ribonucleic acid increased, whilst hypothalamic oxytocin, preprocholecystokinin and corticotropin-releasing factor remained unchanged. However, corticotrophin-releasing factor messenger ribonucleic acid increased in the mouse amygdala. In situ hybridization was performed using oligonucleotide probes specific for either the cholecystokininA or cholecystokininB receptor messenger ribonucleic acid, and this showed good agreement with the receptor autoradiography. CholecystokininB receptor expression was upregulated in the rat hypothalamus along with a small but significant increase in cholecystokininA receptors. In the mouse only cholecystokininA receptor expression was increased. In addition to these molecular changes rats lost about 25% of their body weight during six days of salt-challenge, whilst mice continued to grow in line with controls. This work demonstrates differential changes in cholecystokinin receptor subtype binding between the rat and the mouse. It represents the first report of differential changes in cholecystokininA and cholecystokininB receptor messenger ribonucleic acids within the brain, and shows that cholecystokinin receptors within the rodent hypothalamus are capable of plastic responses to chronic osmotic stress.