Sensitivity to angiotensin II of neurons in the circumventricular organs of polydipsic inbred mice

Genetic Deletion of β-Arrestin 2 From the Subfornical Organ and Other Periventricular Nuclei in the Brain Alters Fluid Homeostasis and Blood Pressure

BACKGROUND

ANG (angiotensin II) elicits dipsogenic and pressor responses via activation of the canonical Gαq (G-protein component of the AT1R [angiotensin type 1 receptor])-mediated AT1R in the subfornical organ. Recently, we demonstrated that ARRB2 (β-arrestin 2) global knockout mice exhibit a higher preference for salt and exacerbated pressor response to deoxycorticosterone acetate salt. However, whether ARRB2 within selective neuroanatomical nuclei alters physiological responses to ANG is unknown. Therefore, we hypothesized that ARRB2, specifically in the subfornical organ, counterbalances maladaptive dipsogenic and pressor responses to the canonical AT1R signaling.

METHODS

Male and female Arrb2FLOX mice received intracerebroventricular injection of either adeno-associated virus (AAV)-Cre-GFP (green fluorescent protein) to induce brain-specific deletion of ARRB2 (Arrb2ICV-Cre). Arrb2FLOX mice receiving ICV-AAV-GFP were used as control (Arrb2ICV-Control). Infection with ICV-AAV-Cre primarily targeted the subfornical organ with few off targets. Fluid intake was evaluated using the 2-bottle choice paradigm with 1 bottle containing water and 1 containing 0.15 mol/L NaCl.

RESULTS

Arrb2ICV-Cre mice exhibited a greater pressor response to acute ICV-ANG infusion. At baseline conditions, Arrb2ICV-Cre mice exhibited a significant increase in saline intake compared with controls, resulting in a saline preference. Furthermore, when mice were subjected to water-deprived or sodium-depleted conditions, which would naturally increase endogenous ANG levels, Arrb2ICV-Cre mice exhibited elevated saline intake.

CONCLUSIONS

Overall, these data indicate that ARRB2 in selective cardiovascular nuclei in the brain, including the subfornical organ, counterbalances canonical AT1R responses to both exogenous and endogenous ANG. Stimulation of the AT1R/ARRB axis in the brain may represent a novel strategy to treat hypertension.

Alterations in the baroreceptor-heart rate reflex in conscious inbred polydipsic (STR/N) mice

STR/N is an inbred strain of mice which is known to exhibit extreme polydipsia and polyuria. We previously found central administration of angiotensin II enhanced cardiovascular responses in STR/N mice than normal mice, suggesting that STR/N mice might exhibit different cardiovascular responses. Therefore, in this study, we investigated daily mean arterial blood pressure and heart rate, and changes in the baroreceptor-heart rate reflex in conscious STR/N mice and control (ICR) mice. We found that variability in daily mean arterial blood pressure and heart rate was significantly larger in STR/N mice than in ICR mice (p<0.05). There was a stronger response to phenylephrine (PE) in STR/N mice than in ICR mice. For baroreceptor reflex sensitivity, in the rapid response period, the slopes of PE and sodium nitroprusside (SNP) were more negative in STR/N mice than in ICR mice. In the later period, the slopes of PE and SNP were negatively correlated between heart rate and blood pressure in ICR mice, but their slopes were positively correlated in STR/N mice. These results indicated that STR/N mice exhibited the different cardiovascular responses than ICR mice, suggesting that the dysfunction of baroreceptor reflex happened in conscious STR/N mice.

Downregulation of AQP2 expression in the kidney of polydipsic STR/N mice

Aquaporin-2 (AQP2) is responsible for the concentration of urine in the kidney collecting tubule under the regulation of vasopressin. The mRNA level of this water channel in polydipsic STR/N mice was extremely reduced compared with that in normal ICR mice. In male mice, reduction of the AQP2 mRNA level was not evident at 3 wk of age, at which time water intake was not increased. At 10 wk of age, however, the AQP2 mRNA level was reduced to 10% of that in control mice, whereas water intake was increased by 36%. At 44 wk, the water intake became five times that of the control ICR mice, and the AQP2 mRNA level in these polydipsic mice was only approximately 5% of control. Similar changes were observed in the AQP2 protein level, suggesting that the mRNA level of AQP2 reflects the protein level of AQP2. These inverse changes in the AQP2 mRNA level and water intake were also evident in female mice. The data imply that polydipsia in STR/N mice may have affected AQP2 mRNA transcription in the kidney, resulting in reduced AQP2 expression, which would contribute to a reduction in overretention of water.

Enhanced effects of central angiotensin II on cardiovascular and drinking responses in inbred polydipsic (STR/N) mice

STR/N, an inbred strain of mice, is known to exhibit extreme polydipsia and polyuria. The objective of this study was to investigate the possible reasons for polydipsia. First, comparisons were made between STR/N mice and control mice from the Institute of Cancer Research (ICR) concerning daily drinking, urinary excretion, and basal cardiovascular function. Then, since angiotensin II (ANG II) is a potent stimulus for drinking behavior, we investigated the effects of intracerebroventricular (i.c.v.) administration of ANG II on cardiovascular and water intake responses. Daily water intake, food intake, urinary volume, and urinary electrolytes (Na and K) excretion were larger in STR/N mice than in ICR mice, and the basal blood pressure was significantly lower in STR/N mice than in ICR mice. The i.c.v. administration of ANG II (10 pmol/per mouse) resulted in increased mean arterial blood pressure (MAP) and water intake in both STR/N and ICR mice, but the changes in MAP were significantly larger in STR/N mice than in ICR mice. These results suggest that polydipsia in STR/N mice is at least partially attributable to high sensitivity of central ANG II receptors and low MAP.

Up-regulation of vasopressin and angiotensin II receptors in the thalamus and brainstem of inbred polydipsic mice

Vasopressin (AVP), angiotensin II (Ang II) and oxytocin (OT) receptors were mapped in the brain of inbred polydipsic mice of the STR/N strain by quantitative in vitro autoradiography and receptor binding levels, compared with those found in control non-polydipsic mice of the ICR strain. A remarkable difference was evidenced in the thalamic paraventricular nucleus where AVP receptor binding was 7- to 10-fold higher in polydipsic mice than in control mice. Another disparity was observed in the hypothalamic paraventricular nucleus, which contained AVP binding sites in the control mice, but was unlabelled in the polydipsic animals. Ang II receptor binding was reduced in the hypothalamic paraventricular nucleus of the polydipsic mice, whereas it was abundant in the brainstem region, encompassing area postrema and the nucleus of the solitary tract. The distribution and amount of OT receptor binding were similar in the polydipsic and control mice. Strain-related differences of AVP and Ang II receptor binding were observed both in male and female animals. A sex-related difference was seen only for OT receptor binding in the hypothalamic ventromedial nucleus, where labelling was less intense in males than in females of both strains. Altogether, our results support the view that central AVP and Ang II systems are involved in the mechanisms responsible for polydipsia in STR/N mice.

Reduced interleukin-1 responsiveness in immune system and central nervous system of inbred polydipsic (STR/N) mice

Inbred polydipsic mice (STR/N strain) have primary polydipsia. The previous studies found abnormalities in the central nervous system (CNS), especially in the hypothalamus and circumventricular organ. As a part of pursuing to find the cause of the polydipsia, we investigated immunological characteristics of STR/N mice, using the ICR strain of mice as control. Their thymic subset cells showed that CD4+CD8+ double positive cells were increased, CD4+ single positive cells were decreased and CD5 expression was deficient, compared to ICR mice. T cell proliferative response and interleukin (IL)-2 production caused by IL-1beta stimulation were reduced in STR/N mice than those in the ICR mice. In in vivo studies the degree of thymic atrophy and the increases in serum level of ACTH and corticosterone induced by intraperitoneal IL-1beta injection were much less in STR/N mice than those in controls. Furthermore, adipsic response also induced by IL-1beta injection was greatly reduced compared to their control mice. All these results suggest that the responsiveness to IL-1 is impaired both in the immune system and the CNS of STR/N mice.

Disorder of salivary secretion in inbred polydipsic mouse

To find mechanisms of an extreme polydipsia in an inbred strain of mice, STR/N, this study was undertaken using Institute of Cancer Research (ICR) mice as a control. During food deprivation, daily water intake of both strains decreased. The decrement in the STR/N mice was larger than that in the ICR mice. During dehydration, daily food intake of the STR/N mice was smaller than that of the ICR mice. These data indicate that prandial drinking was more severely affected for the STR/N mice. Under anesthesia, the stimulated salivary secretion by pilocarpine of the STR/N mice was significantly smaller than that of the ICR mice. The submandibular gland of the STR/N mice was lighter and harder than that of the ICR mice. After desalivation from the major three salivary glands, the ICR mice drank as much as the STR/N mice. Young STR/N mice with undeveloped polydipsia did not show different salivary secretion stimulated by pilocarpine from the young ICR mice. These findings indicate a dysfunction with age in the salivary glands of the STR/N mice, and they suggest that the decreased saliva induces thirst and triggers extraordinary drinking in the polydipsic mice.

Analysis of the vasopressin system and water regulation in genetically polydipsic mice

Polydipsic mice, STR/N, which show extreme polydipsia and polyuria, were discovered in 1958. In the STR/N, urine outputs are much higher than in control mice. The possibility of an abnormal regulation of the arginine vasopressin (AVP) system, or an abnormality in the renal susceptibility to AVP, should be considered. In this study we investigated the AVP system and water regulation in STR/N. We sequenced the AVP and the AVP V(2)-receptor genes of the STR/N by direct sequencing. No mutation was found in either of them. AVP gene expression examined by in situ hybridization and plasma sodium in 8-wk-old STR/N was significantly lower than in control mice, whereas it was significantly higher at 20 wk. Renal sensitivity to injected AVP was attenuated in 20-wk-old STR/N. The suppression of AVP synthesis due to excessive water retention in 8-wk-old STR/N suggests that polydipsia may be the primary cause in this strain. The 20-wk-old STR/N became dehydrated with the acceleration of AVP synthesis, which might have resulted from secondary desensitization to AVP.

Increase of vasopressin mRNA in the hypothalamus of inbred polydipsic mice

The genetically inbred polydipsic mice, STR/N strain, are characterized by extreme polydipsia and polyuria without arginine vasopressin (AVP) deficiency. The expression of AVP gene in the hypothalamus of polydipsic and non-polydipsic mice was examined by Northern blot analysis and in situ hybridization histochemistry. Northern blot analysis revealed that the total amount of AVP mRNA in the hypothalamus of the STR/N mice was approximately three-fold of that in the control, ICR mice. In situ hybridization histochemistry showed that the signals of AVP mRNA in the paraventricular (PVN) and supraoptic nuclei (SON) of the STR/N were stronger than those in the ICR. Although AVP gene transcripts were detected in the anteroventral parts of the PVN (avPVN) in the STR/N, there was a few AVP transcripts in the same area (avPVN) in the ICR. There were no differences in plasma osmolality and hematocrit between STR/N and ICR mice. These results suggest that upregulation of AVP mRNA in the hypothalamus of STR/N may be involved in the central mechanism responsible for the polydipsia in genetically polydipsic mice.

Localization of angiotensin-converting enzyme, angiotensin II, angiotensin II receptor subtypes, and vasopressin in the mouse hypothalamus

The hypothalamic angiotensin II (Ang II) system plays an important role in pituitary hormone release. Little is known about this system in the mouse brain. We studied the distribution of angiotensin-converting-enzyme (ACE), Ang II, Ang II receptor subtypes, and vasopressin in the hypothalamus of adult male mice. Autoradiography of binding of the ACE inhibitor [125I]351A revealed low levels of ACE throughout the hypothalamus. Ang II- and vasopressin-immunoreactive neurons and fibers were detected in the paraventricular, accessory magnocellulary, and supraoptic nuclei, in the retrochiasmatic part of the supraoptic nucleus and in the median eminence. Autoradiography of Ang II receptors was performed using [125I]Sar1-Ang II binding. Ang II receptors were present in the paraventricular, suprachiasmatic, arcuate and dorsomedial nuclei, and in the median eminence. In all areas [125I]Sar1-Ang II binding was displaced by the AT1 receptor antagonist losartan, indicating the presence of AT1 receptors. In the paraventricular nucleus [125I]Sar1-Ang II binding was displaced by Ang II (Ki = 7.6 X 10(-9)) and losartan (Ki = 1.4 X 10(-7)) but also by the AT2 receptor ligand PD 123319 (Ki = 5.0 X 10(-7)). In addition, a low amount of AT2 receptor binding was detected in the paraventricular nucleus using [125I]CGP42112 as radioligand, and the binding was displaced by Ang II (Ki = 2.4 X 10(-9)), CGP42112 (Ki = 7.9 x 10(-10)), and PD123319 (Ki = 2.2 x 10(-7)). ACE, Ang II, and AT1 as well as AT2 receptor subtypes are present in the mouse hypothalamus. Our data are the basis for further studies on the mouse brain Ang II system.

Water deprivation induces regional expression of c-fos protein in the brain of inbred polydipsic mice

We studied the effects of water deprivation on the expression of c-fos protein (Fos) in the brain of inbred polydipsic mice, STR/N strain, that show extreme polydipsia without a lack of vasopressin in the body. Non-polydipsic mice, ICR strain, were used as controls. All male animals were deprived of water for 24 and 48 h. Fos-like immunoreactivity (Fos-LI) in the brain was studied by immunohistochemical techniques. In both groups of mice water deprivation induced a remarkable increase in Fos-LI in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, the median preoptic nucleus (MnPO), the organum vasculosum laminae terminalis (OVLT) and the subfornical organ (SFO). A far more increase, however, was seen in the MnPO, the SFO and the area postrema (AP) of the polydipsic mice compared to those of the non-polydipsic control mice. In the nucleus of the tractus solitarius (NTS) and in the anteroventral part of the PVN (avPVN), water deprivation caused a clear increase in Fos-LI in the polydipsic mice, while in the non-polydipsic mice the same treatment induced no Fos-LI in the NTS and no change in the avPVN. These results indicate that neurons in the circumventricular organs and the NTS are strongly activated by water deprivation in the polydipsic mice, suggesting that these brain structures play an important role in the polydipsia.

Differential neuronal responses to angiotensin III from the subfornical organ of normotensive and spontaneously hypertensive rats

We previously reported that chronic central administration of angiotensin III (AIII) fails to produce sustained drinking behavior in spontaneously hypertensive rats (SHR), possibly because of the development of early desensitization of the angiotensin receptors. The present study extended these findings to the cellular level, using brain-slice preparation from Wistar-Kyoto rats (WKY) and SHR, in conjunction with single-neuron recording in the subfornical organ (SFO), a target site for angiotensin II-induced drinking. We found that a majority of the SFO neurons studied (13/18 in WKY, 20/28 in SHR) responded in a dose-related manner to AIII, given in the range of 10(-6)-10(-5) M. This excitation was receptor-specific, since it was reversed by Ile7-AIII (10(-4)-10(-3) M), the selective AIII antagonist. Bestatin (10(-5)-10(-4) M), an aminopeptidase B inhibitor, did not discernibly affect basal spike frequency when delivered alone. Nevertheless, given in combination with the heptapeptide, bestatin reduced the intensity and duration of SFO neuronal response in WKY to the higher dose (10(-5) M), and in SHR to both doses (10(-6) or 10(-5) M), of AIII. These data suggest that the SFO may also be a central site of action for AIII. Moreover, prolonging the action of AIII by protecting it from being metabolized with bestatin may produce desensitization of the angiotensin receptors on SFO neurons. This was particularly so in the SHR, which are thought to be defective in the degradation of the heptapeptide in the brain.

Effects of the opiates on the paraventricular nucleus in genetically polydipsic mice

The inbred mice, STR/N, are known to possess extreme polydipsia with no known abnormality in vasopressin system and the kidney function. Our previous studies indicate that the opiate antagonists given intracerebroventricularly strongly attenuated spontaneous drinking. To determine the site(s) of action, the present study was undertaken. Microinjections of naltrexone methobromide and a selective kappa-receptor antagonist, nor-binaltorphimine (nor-BNI), into the paraventricular nucleus of the hypothalamus (PVN) greatly attenuated drinking of the STR/N for 0.5 to 16 h after injections, while in the two control groups, non-polydipsic STR/1N and Swiss/Webster strains, drinking was not affected by the injections. Food intake was not much altered in all groups. Studies of PVN neurons in vitro (n = > 160 for each group) showed that basal firing rates and patterns were similar in the STR/N and the control groups. Morphine added to the medium inhibited some but excited none in all strains tested. The threshold for the inhibitory action was higher in the polydipsic STR/N mice (10(-8) M), compared to that in the control, S/W mice (10(-9) M). Further, a proportion of neurons inhibited by morphine in the PVN was significantly smaller (P < 0.01) in the STR/N (41.7%), compared to that in the control (64.9%). Dynorphin had very similar effect to that of morphine, but the proportion of cells inhibited was 25.4% in the STR/N, and 70.4% in the S/W. Prior applications of naloxone to the medium prevented the action of both morphine and dynorphin. Under the synaptic blockade (in a low Ca2+ and high Mg2+ medium) the inhibitory effect of the opiates persisted. We concluded that the PVN is at least one of the possible sites where the opiates are acting to cause the polydipsia in the STR/N mice.

Vasopressin- and oxytocin-immunoreactive hypothalamic neurones of inbred polydipsic mice

In the late 1950s the inbred polydipsic mice, STR/N, was discovered. The early studies indicated that the extreme polydipsia was not due to a lack of vasopressin but probably due to innate thirst of unknown origin. Because the recent investigation has revealed the presence of some functional abnormality in the brain of the STR/N mouse, we now investigated, using immunohistochemical techniques, distribution of vasopressin (AVP)- and oxytocin (OXT)-containing neurones in the hypothalamus of polydipsic strain of mouse and compared with that of the control. The pattern of distribution of AVP- and OXT-immunoreactive neurones in the paraventricular (PV), supraoptic (SO), and suprachiasmatic nuclei (SCN) of the STR/N polydipsic mouse was similar to that of the control, but the number of AVP-immunoreactive neurones was more numerous in the PVN and SON and less in the SCN in the polydipsic mouse than in the control. In addition, a discrete group of AVP- and OXT-containing neurones that was not clearly seen in the control was discovered in the STR/N. These results implicate that abnormal distribution in the brain AVP and OXT contribute to the mechanism responsible for the polydipsia shown by the strain STR/N.

Renal renin secretion rate and norepinephrine secretion rate in response to centrally administered angiotensin-II: role of the medial basal forebrain

The influence that centrally administered angiotensin-II (ANG-II) and saralasin (SAR) has on renal norepinephrine secretion rate (NESR) and renal renin secretion rate (RSR) were studied. Rats were given thermal lesions of the medial basal forebrain (MBF) or sham surgery. Twenty-four hours later the right kidney was vascularly isolated (but neurally intact) and perfused with an artificial plasma at either a constant pressure (100 mm Hg) or constant flow (600 microliters/min). Renal perfusate was collected before (pre-injection) and at 10 min intervals after central administration of peptides for determination of NESR and RSR. In both perfusion models, intracerebroventricular (ICV) ANG-II increased renal NESR. In MBF lesioned rats pre-injection renal NESR is reduced and the response to ICV ANG-II is blocked. In both perfusion models ICV ANG-II decreases renal RSR. Concomitant administration of SAR blocks the effect of ANG-II on both NESR and RSR. MBF lesioned rats had significantly elevated pre-injection levels of RSR and there is no change in RSR following ICV ANG-II. These experiments indicate that centrally administered ANG-II increases renal NESR concomitant with a decrease in renal RSR and that MBF lesions block those changes.

Animal models for osmoregulatory disturbances

For the various types of di in humans, animal models are available. However, their value for explaining human di is for the major part an indirect one; by studying cellular mechanisms in these animal models, fundamental aspects of the cellular processes become available, which will help to understand similar processes in human di and subsequently lead to the molecular cause(s) of the various types of human di. Finally, it is to be expected that in the very near future transgenic animals will be raised in which very specific genetic information is overexpressed (or knocked out by homologous recombination; McMahon and Bradley, 1990). Recently hypervasopressinemia could be shown in transgenic mice, providing an animal model for the syndrome of the inappropriate VP secretion (Bartter and Schwartz, 1967), which is often observed in patients with lung cancers that ectopically express the VP gene (Habener et al., 1989). Furthermore it will be possible to study the exact cause(s) of human di by performing in vitro mutagenesis and to express the RNA constructs within a cell-free translation system and in oocytes (e.g., Schmale et al., 1989) and subsequently study the pattern of precursor synthesis, packaging and processing.

Characterization of opioid-sensitive neurons in the anteroventral third ventricle region of polydipsic inbred mice in vitro

In previous studies we found that in the extremely polydipsic special strain of mice, STR/N, spontaneous drinking was greatly attenuated by injection of the opioid antagonists given intracerebroventricularly as well as subcutaneously. Therefore, we investigated, using hypothalamic slice preparations, responses of neurons in the anteroventral third ventricle region (AV3V) of the STR/N and its control, Swiss/Webster (S/W) mice to morphine and opiate peptides. An application of morphine at 10(-6) M to the circulating medium inhibited activities of 44% of AV3V neurons (45 of 102) in the STR/N, and 59% (76/129) in the S/W, demonstrating that morphine affected a smaller proportion of neurons of the polydipsic mice than that of controls. Opioid agonists for 3 receptor types, mu, delta and kappa, at 10(-6) to 10(-5) M inhibited AV3V neurons in both the STR/N and S/W mice, but to a different degree. No cell of either strain was excited by morphine or any of the opioids. The mu-receptor agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO), was the most potent inhibitor of AV3V neurons; in the STR/N 53% (25/47), and in the S/W 77% (34/44) were inhibited. The kappa-agonist, dynorphin A-(1-13) (DYN), inhibited fewer cells in the STR/N (9%, 4/47), compared with the S/W (36%, 16/44). Only a few cells responded to the delta-agonist, [D-Pen2,5]enkephalin (DPDPE), in both strains. The inhibitory actions of the opiates were reversibly blocked by naloxone, and persisted under synaptic blockade. The threshold concentration of morphine or DAGO for inhibition of AV3V neurons was higher in the STR/N (approximately 10(-8) M for both morphine and DAGO) than in S/W mice (approximately 10(-9) M for morphine and less than 10(-9) M for DAGO). Although the AV3V also contains angiotensin II-sensitive neurons, they were not affected by morphine (10(-6) M). Similarly neurons inhibited by morphine were not excited by angiotensin II (10(-7) M); some neurons were unresponsive to both chemicals. We conclude that morphine and opiate peptides directly inhibit the AV3V neurons of both the STR/N and S/W strains of mice and the sensitivity of these neurons to the opiates is lower in the polydipsic inbred mice compared to their controls. The results, together with our behavioral studies, suggest involvement of the central opioid system in the polydipsia of the STR/N mice.