Intracerebroventricular losartan infusion modulates angiotensin II type 1 receptor expression in the subfornical organ and drinking behaviour in bile-duct-ligated rats

Sex-dependent effects of chronic intermittent hypoxia: implication for obstructive sleep apnea

BACKGROUND

Obstructive sleep apnea (OSA) affects 10-26% of adults in the United States with known sex differences in prevalence and severity. OSA is characterized by elevated inflammation, oxidative stress (OS), and cognitive dysfunction. However, there is a paucity of data regarding the role of sex in the OSA phenotype. Prior findings suggest women exhibit different OSA phenotypes than men, which could result in under-reported OSA prevalence in women. To examine the relationship between OSA and sex, we used chronic intermittent hypoxia (CIH) to model OSA in rats. We hypothesized that CIH would produce sex-dependent phenotypes of inflammation, OS, and cognitive dysfunction, and these sex differences would be dependent on mitochondrial oxidative stress (mtOS).

METHODS

Adult male and female Sprague Dawley rats were exposed to CIH or normoxia for 14 days to examine the impact of sex on CIH-associated circulating inflammation (IL-1β, IL-6, IL-10, TNF-α), circulating steroid hormones, circulating OS, and behavior (recollective and spatial memory; gross and fine motor function; anxiety-like behaviors; and compulsive behaviors). Rats were implanted with osmotic minipumps containing either a mitochondria-targeting antioxidant (MitoTEMPOL) or saline vehicle 1 week prior to CIH initiation to examine how inhibiting mtOS would affect the CIH phenotype.

RESULTS

Sex-specific differences in CIH-induced inflammation, OS, motor function, and compulsive behavior were observed. In female rats, CIH increased inflammation (plasma IL-6 and IL-6/IL-10 ratio) and impaired fine motor function. Conversely, CIH elevated circulating OS and compulsivity in males. These sex-dependent effects of CIH were blocked by inhibiting mtOS. Interestingly, CIH impaired recollective memory in both sexes but these effects were not mediated by mtOS. No effects of CIH were observed on spatial memory, gross motor function, or anxiety-like behavior, regardless of sex.

CONCLUSIONS

Our results indicate that the impact of CIH is dependent on sex, such as an inflammatory response and OS response in females and males, respectively, that are mediated by mtOS. Interestingly, there was no effect of sex or mtOS in CIH-induced impairment of recollective memory. These results indicate that mtOS is involved in the sex differences observed in CIH, but a different mechanism underlies CIH-induced memory impairments.

Sex-dependent effects of chronic intermittent hypoxia: Implication for obstructive sleep apnea

Background Obstructive sleep apnea (OSA) affects 10-26% of adults in the United States with known sex differences in prevalence and severity. OSA is characterized by elevated inflammation, oxidative stress (OS), and cognitive dysfunction. However, there is a paucity of data regarding the role of sex in the OSA phenotype. Prior findings suggest women exhibit different OSA phenotypes than men, which could result in under-reported OSA prevalence in women. To examine the relationship between OSA and sex, we used chronic intermittent hypoxia (CIH) to model OSA in rats. We hypothesized that CIH would produce sex-dependent phenotypes of inflammation, OS, and cognitive dysfunction, and these sex differences would be dependent on mitochondrial oxidative stress (mtOS). Methods Adult male and female Sprague Dawley rats were exposed to CIH or normoxia for 14 days to examine the impact of sex on CIH-associated circulating inflammation (IL-1β, IL-4, IL-6, IL-10, TNF-α), circulating OS, and behavior (recollective and spatial memory; gross and fine motor function; anxiety-like behaviors; and compulsive behaviors). A subset of rats was implanted with osmotic minipumps containing either a mitochondria-targeting antioxidant (MitoTEMPOL) or saline vehicle 1 week prior to CIH initiation to examine how inhibiting mtOS would affect the CIH phenotype. Results Sex-specific differences in CIH-induced inflammation, OS, motor function, and compulsive behavior were observed. In female rats, CIH increased inflammation (plasma IL-6 and IL-6/IL-10 ratio) and impaired fine motor function. Conversely, CIH elevated circulating OS and compulsivity in males. These sex-dependent effects of CIH were blocked by inhibiting mtOS. Interestingly, CIH impaired recollective memory in both sexes but these effects were not mediated by mtOS. No effects of CIH were observed on spatial memory, gross motor function, or anxiety-like behavior, regardless of sex. Conclusions Our results indicate that the impact of CIH is dependent on sex, such as an inflammatory response and OS response in females and males, respectively, that are mediated by mtOS. Interestingly, there was no effect of sex or mtOS in CIH-induced impairment of recollective memory. These results indicate that mtOS is involved in the sex differences observed in CIH, but a different mechanism underlies CIH-induced memory impairments.

Sensory Afferent Renal Nerve Activated Gαi2 Subunit Proteins Mediate the Natriuretic, Sympathoinhibitory and Normotensive Responses to Peripheral Sodium Challenges

We have previously reported that brain Gαi2 subunit proteins are required to maintain sodium homeostasis and are endogenously upregulated in the hypothalamic paraventricular nucleus (PVN) in response to increased dietary salt intake to maintain a salt resistant phenotype in rats. However, the origin of the signal that drives the endogenous activation and up-regulation of PVN Gαi2 subunit protein signal transduction pathways is unknown. By central oligodeoxynucleotide (ODN) administration we show that the pressor responses to central acute administration and central infusion of sodium chloride occur independently of brain Gαi2 protein pathways. In response to an acute volume expansion, we demonstrate, via the use of selective afferent renal denervation (ADNX) and anteroventral third ventricle (AV3V) lesions, that the sensory afferent renal nerves, but not the sodium sensitive AV3V region, are mechanistically involved in Gαi2 protein mediated natriuresis to an acute volume expansion [peak natriuresis (μeq/min) sham AV3V: 43 ± 4 vs. AV3V 45 ± 4 vs. AV3V + Gαi2 ODN 25 ± 4, p < 0.05; sham ADNX: 43 ± 4 vs. ADNX 23 ± 6, AV3V + Gαi2 ODN 25 ± 3, p < 0.05]. Furthermore, in response to chronically elevated dietary sodium intake, endogenous up-regulation of PVN specific Gαi2 proteins does not involve the AV3V region and is mediated by the sensory afferent renal nerves to counter the development of the salt sensitivity of blood pressure (MAP [mmHg] 4% NaCl; Sham ADNX 124 ± 4 vs. ADNX 145 ± 4, p < 0.05; Sham AV3V 125 ± 4 vs. AV3V 121 ± 5). Additionally, the development of the salt sensitivity of blood pressure following central ODN-mediated Gαi2 protein down-regulation occurs independently of the actions of the brain angiotensin II type 1 receptor. Collectively, our data suggest that in response to alterations in whole body sodium the peripheral sensory afferent renal nerves, but not the central AV3V sodium sensitive region, evoke the up-regulation and activation of PVN Gαi2 protein gated pathways to maintain a salt resistant phenotype. As such, both the sensory afferent renal nerves and PVN Gαi2 protein gated pathways, represent potential targets for the treatment of the salt sensitivity of blood pressure.

Sex Differences in the Regulation of Vasopressin and Oxytocin Secretion in Bile Duct-Ligated Rats

INTRODUCTION

Hyponatremia due to elevated arginine vasopressin (AVP) secretion increases mortality in liver failure patients. No previous studies have addressed sex differences in hyponatremia in liver failure animal models.

OBJECTIVE

This study addressed this gap in our understanding of the potential sex differences in hyponatremia associated with increased AVP secretion.

METHODS

This study tested the role of sex in the development of hyponatremia using adult male, female, and ovariectomized (OVX) female bile duct-ligated (BDL) rats.

RESULTS

All BDL rats had significantly increased liver to body weight ratios compared to sham controls. Male BDL rats had hyponatremia with significant increases in plasma copeptin and FosB expression in supraoptic AVP neurons compared to male shams (all p < 0.05; 5-7). Female BDL rats did not become hyponatremic or demonstrate increased supraoptic AVP neuron activation and copeptin secretion compared to female shams. Plasma oxytocin was significantly higher in female BDL rats compared to female sham (p < 0.05; 6-10). This increase was not observed in male BDL rats. Ovariectomy significantly decreased plasma estradiol in sham rats compared to intact female sham (p < 0.05; 6-10). However, circulating estradiol was significantly elevated in OVX BDL rats compared to the OVX and female shams (p < 0.05; 6-10). Adrenal estradiol, testosterone, and dehydroepiandrosterone (DHEA) were measured to identify a possible source of circulating estradiol in OVX BDL rats. The OVX BDL rats had significantly increased adrenal estradiol along with significantly decreased adrenal testosterone and DHEA compared to OVX shams (all p < 0.05; 6-7). Plasma osmolality, hematocrit, copeptin, and AVP neuron activation were not significantly different between OVX BDL and OVX shams. Plasma oxytocin was significantly higher in OVX BDL rats compared to OVX sham.

CONCLUSIONS

Our results show that unlike male BDL rats, female and OVX BDL rats did not develop hyponatremia, supraoptic AVP neuron activation, or increased copeptin secretion compared to female shams. Adrenal estradiol might have compensated for the lack of ovarian estrogens in OVX BDL rats.

Brain-Derived Neurotrophic Factor and Supraoptic Vasopressin Neurons in Hyponatremia

Hyponatremia due to elevated arginine vasopressin (AVP) secretion increases mortality in liver failure patients. The mechanisms causing dysregulation of AVP secretion are unknown. Our hypothesis is that inappropriate AVP release associated with liver failure is due to increased brain-derived neurotrophic factor (BDNF) in the supraoptic nucleus (SON). BDNF diminishes GABAA inhibition in SON AVP neurons by increasing intracellular chloride through tyrosine receptor kinase B (TrkB) activation and downregulation of K+/Cl- cotransporter 2 (KCC2). This loss of inhibition could increase AVP secretion. This hypothesis was tested using shRNA against BDNF (shBDNF) in the SON in bile duct ligated (BDL) male rats. All BDL rats had significantly increased liver weight (p < 0.05; 6-9) compared to shams. BDL rats with control -shRNA injections (BDL scrambled [SCR]) developed hyponatremia with increased plasma AVP and copeptin (CPP; all p < 0.05; 6-9) compared to sham groups. This is the first study to show that phosphorylation of TrkB is significantly increased along with significant decrease in phosphorylation of KCC2 in BDL SCR rats compared to the sham rats (p < 0.05;6-8). Knockdown of BDNF in the SON of BDL rats (BDL shBDNF) significantly increased plasma osmolality and hematocrit compared to BDL SCR rats (p < 0.05; 6-9). The BDL shBDNF rats had significant (p < 0.05; 6-9) decreases in plasma AVP and CPP concentration compared to BDL SCR rats. The BDNF knockdown also significantly blocked the increase in TrkB phosphorylation and decrease in KCC2 phosphorylation (p < 0.05; 6-8). The results indicate that BDNF produced in the SON contributes to increased AVP secretion and hyponatremia during liver failure.

High salt loading increases brain derived neurotrophic factor in supraoptic vasopressin neurones

High salt loading (SL) is associated with inappropriate arginine vasopressin (AVP) release and increased mean arterial pressure. Previous work has shown that chronic high salt intake impairs baroreceptor inhibition of rat AVP neurones through brain-derived neurotrophic factor (BDNF) dependent activation of tyrosine receptor kinase B (TrkB) and down-regulation of K+/Cl- co-transporter KCC2. This mechanism diminishes the GABA_A inhibition of AVP neurones in the supraoptic nucleus (SON) by increasing intracellular chloride. However, the source of BDNF leading to this ionic plasticity is unknown. In the present study, we used adeno-associated viral vectors with short hairpin RNA against BDNF to test whether SON is the source of BDNF contributing to increased AVP release and elevated mean arterial pressure in high salt loaded rats. Virally mediated BDNF knockdown (shBDNF) in the SON of salt loaded rats significantly blocked the increases in BDNF mRNA and AVP heterogeneous RNA expression. The observed increase in the activation of TrkB receptor during salt loading is consistent with previous studies. Western blot analysis of SON punches revealed that tyrosine phosphorylation of TrkB (pTrkBY515) was significantly decreased in salt shBDNF rats compared to the salt scrambled (SCR) rats. Injections of shBDNF in the SON also significantly prevented the increase in plasma AVP concentration associated with salt loading. However, the salt loading induced increase in mean arterial pressure was not decreased with BDNF knockdown in the SON. Average daily fluid intake and urine output were significantly elevated in both salt SCR and salt shBDNF rats compared to the euhydrated controls. Daily average urine sodium concentration was significantly higher in shBDNF injected salt rats than the other groups. These findings indicate that BDNF produced in the SON contributes to the increased AVP secretion during high salt loading but not with respect to the subsequent increase in mean arterial pressure.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

ANG II receptor subtype 1a gene knockdown in the subfornical organ prevents increased drinking behavior in bile duct-ligated rats

Bile duct ligation (BDL) causes congestive liver failure that initiates hemodynamic changes, resulting in dilutional hyponatremia due to increased water intake and vasopressin release. This project tested the hypothesis that angiotensin signaling at the subfornical organ (SFO) augments drinking behavior in BDL rats. A genetically modified adeno-associated virus containing short hairpin RNA (shRNA) for ANG II receptor subtype 1a (AT1aR) gene was microinjected into the SFO of rats to knock down expression. Two weeks later, BDL or sham surgery was performed. Rats were housed in metabolic chambers for measurement of fluid and food intake and urine output. The rats were euthanized 28 days after BDL surgery for analysis. A group of rats was perfused for immunohistochemistry, and a second group was used for laser-capture microdissection for analysis of SFO AT1aR gene expression. BDL rats showed increased water intake that was attenuated in rats that received SFO microinjection of AT1aR shRNA. Among BDL rats treated with scrambled (control) and AT1aR shRNA, we observed an increased number of vasopressin-positive cells in the supraoptic nucleus that colocalized with ΔFosB staining, suggesting increased vasopressin release in both groups. These results indicate that angiotensin signaling through the SFO contributes to increased water intake, but not dilutional hyponatremia, during congestive liver failure.

Differential regulation of TRPC4 in the vasopressin magnocellular system by water deprivation and hepatic cirrhosis in the rat

Transient receptor potential canonical subtype 4 (TRPC4) is expressed in the magnocellular paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. In this study, the regulation of TRPC4 expression was investigated in water deprivation and hepatic cirrhosis. We used laser capture microdissection technique for precise dissection of pure AVP cell population in the PVN and SON followed by quantitative real-time RT-PCR, and immunodetection techniques by Western blot analysis and immunofluorescence. Bile duct ligation elevated TRPC4 transcripts in the SON but not PVN with correlated changes in the protein expression in these regions, as well as increased colocalization with AVP in the SON, with no changes in the PVN. Water deprivation resulted in increased TRPC4 mRNA expression in the PVN, while it decreased channel expression levels in the SON. In both of these regions, protein expression measured from tissue punches were unaltered following water deprivation, with no changes in the number of TRPC4-positive cells. Thus, TRPC4 expression is differentially regulated in physiological and pathophysiological models of vasopressin release.