Subfornical organ disconnection alters Fos expression in the lamina terminalis, supraoptic nucleus, and area postrema after intragastric hypertonic NaCl

A preliminary study of the effect of a high-salt diet on transcriptome dynamics in rat hypothalamic forebrain and brainstem cardiovascular control centers

BACKGROUND

High dietary salt intake is strongly correlated with cardiovascular (CV) diseases and it is regarded as a major risk factor associated with the pathogenesis of hypertension. The CV control centres in the brainstem (the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM)) and hypothalamic forebrain (the subfornical organ, SFO; the supraoptic nucleus, SON and the paraventricular nucleus, PVN) have critical roles in regulating CV autonomic motor outflows, and thus maintaining blood pressure (BP). Growing evidence has implicated autonomic regulatory networks in salt-sensitive HPN (SSH), but the genetic basis remains to be delineated. We hypothesized that the development and/ or maintenance of SSH is reliant on the change in the expression of genes in brain regions controlling the CV system.

METHODOLOGY

We used RNA-Sequencing (RNA-Seq) to describe the differential expression of genes in SFO, SON, PVN, NTS and RVLM of rats being chronically fed with high-salt (HS) diet. Subsequently, a selection of putatively regulated genes was validated with quantitative reverse transcription polymerase chain reaction (qRT-PCR) in both Spontaneously Hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats.

RESULTS

The findings enabled us to identify number of differentially expressed genes in SFO, SON, PVN, NTS and RVLM; that are either up-regulated in both strains of rats (SON- Caprin2, Sctr), down-regulated in both strains of rats (PVN- Orc, Gkap1), up-regulated only in SHRs (SFO- Apopt1, Lin52, AVP, OXT; SON- AVP, OXT; PVN- Caprin2, Sclt; RVLM- A4galt, Slc29a4, Cmc1) or down-regulated only in SHRs (SON- Ndufaf2, Kcnv1; PVN- Pi4k2a; NTS- Snrpd2l, Ankrd29, St6galnac6, Rnf157, Iglon5, Csrnp3, Rprd1a; RVLM- Ttr, Faim).

CONCLUSIONS

These findings demonstrated the adverse effects of HS diet on BP, which may be mediated via modulating the signaling systems in CV centers in the hypothalamic forebrain and brainstem.

Estradiol and osmolality: Behavioral responses and central pathways

Regulation of appropriate osmolality of body fluid is critical for survival, yet there are sex differences in compensatory responses to osmotic challenges. Few studies have focused on the role of sex hormones such as estradiol in behavioral responses to increases or decreases in systemic osmolality, and even fewer studies have investigated whether central actions of estrogens contribute to these responses. This overview integrates findings from a series of ongoing and completed experiments conducted in my laboratory to assess estradiol effects on water and NaCl intake in response to osmotic challenges, and on activity in central pathways that mediate such responses.

Estradiol selectively reduces central neural activation induced by hypertonic NaCl infusion in ovariectomized rats

We recently reported that the latency to begin drinking water during slow, intravenous infusion of a concentrated NaCl solution was shorter in estradiol-treated ovariectomized rats compared to oil vehicle-treated rats, despite comparably elevated plasma osmolality. To test the hypothesis that the decreased latency to begin drinking is attributable to enhanced detection of increased plasma osmolality by osmoreceptors located in the CNS, the present study used immunocytochemical methods to label fos, a marker of neural activation. Increased plasma osmolality did not activate the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), or the nucleus of the solitary tract (NTS) in either oil vehicle-treated rats or estradiol-treated rats. In contrast, hyperosmolality increased fos labeling in the area postrema (AP), the paraventricular nucleus of the hypothalamus (PVN) and the rostral ventrolateral medulla (RVLM) in both groups; however, the increase was blunted in estradiol-treated rats. These results suggest that estradiol has selective effects on the sensitivity of a population of osmo-/Na(+)-receptors located in the AP, which, in turn, alters activity in other central areas associated with responses to increased osmolality. In conjunction with previous reports that hyperosmolality increases blood pressure and that elevated blood pressure inhibits drinking, the current findings of reduced activation in AP, PVN, and RVLM-areas involved in sympathetic nerve activity-raise the possibility that estradiol blunts HS-induced blood pressure changes. Thus, estradiol may eliminate or reduce the initial inhibition of water intake that occurs during increased osmolality, and facilitate a more rapid behavioral response, as we observed in our recent study.

ANG II-induced hypertension and the role of the area postrema during normal and increased dietary salt

It has been shown that the area postrema (AP) plays a role in the development of certain types of chronic angiotensin II (ANG II)-induced hypertension in the rat but is not of great importance in the salt sensitivity of arterial pressure. It has recently been proposed, however, that elevated sodium levels may exacerbate the hypertensive effects of ANG II, which by itself dramatically affects salt sensitivity, by acting at sodium-sensing neurons in certain circumventricular organs of the brain. Thus the interactions of ANG II, sodium, and the central nervous system remain to be fully understood. The purpose of this study was to examine the role of the AP in ANG II-induced hypertension during periods of normal and elevated dietary salt. We hypothesized that an intact AP was necessary for the full development of hypertension under chronic ANG II infusion and that its role would be pronounced during periods of increased dietary sodium. To test this, male Sprague-Dawley rats underwent ablation of the area postrema (APx, n = 6) or sham operation (sham, n = 6). After 3 wk of recovery, rats were instrumented with radiotelemetry transducers for constant blood pressure and heart rate monitoring and venous catheters for vehicle infusion. After a 3-day control period of 0.9% saline infusion (7 ml/day) and 0.4% dietary sodium, a 10-day period of ANG II infusion (10 ng·kg−1·min−1) was begun, immediately followed by a second 10-day period during which rats were fed a 4.0% sodium diet. By day 6 of ANG II infusion, mean arterial pressure (MAP) in APx rats had increased to 139 ± 4 mmHg, whereas MAP in sham rats had increased to 126 ± 3 mmHg. This difference was found to be significant and continued through day 1 of the high-salt period, after which MAP of the two groups had risen to similar levels. On day 9 of high salt, MAP was again observed to be significantly higher (162 ± 1 mmHg) in APx rats when compared with sham rats (147 ± 4 mmHg.) These results do not support the hypothesis that the AP is necessary for the full development of ANG II-induced hypertension at normal or elevated levels of dietary sodium.

Differential effects of intravenous hyperosmotic solutes on drinking latency and c-Fos expression in the circumventricular organs and hypothalamus of the rat

Hyperosmotic intravenous infusions of NaCl are more potent for inducing drinking and vasopressin (AVP) secretion than equally osmotic solutions of glucose or urea. The fact that all three solutes increased cerebrospinal fluid osmolality and sodium concentration led the investigators to conclude that critical sodium receptors or osmoreceptors for stimulating drinking and AVP secretion were outside the blood-brain barrier (BBB) in the circumventricular organs (CVOs). We tested an obvious prediction of this hypothesis: that all three solutes should increase c-Fos-like immunoreactivity (Fos-ir) inside the BBB, but that only NaCl should increase Fos-ir in the CVOs. We gave intravenous infusions of 3.0 Osm/l NaCl, glucose, or urea to rats for 11 or 22 min at 0.14 ml/min and perfused the rats for assay of Fos-ir at 90 min. Controls received isotonic NaCl at the same volume. Drinking latency was measured, but water was then removed. Drinking consistently occurred with short latency during hyperosmotic NaCl infusions only. Fos-ir in the forebrain CVOs, the subfornical organ, and organum vasculosum laminae terminalis was consistently elevated only by hyperosmotic NaCl. However, all three hyperosmotic solutes potently stimulated Fos-ir in the supraoptic and paraventricular nuclei of the hypothalamus inside the BBB. Hyperosmotic NaCl greatly elevated Fos-ir in the area postrema, but even glucose and urea caused moderate elevations that may be related to volume expansion rather than osmolality. The data provide strong support for the conclusion that the osmoreceptors controlling drinking are located in the CVOs.

Agonist and hypertonic saline-induced trafficking of the NK3-receptors on vasopressin neurons within the paraventricular nucleus of the hypothalamus

The neurokinin 3 receptor (NK3R) is colocalized with vasopressinergic neurons within the hypothalamic paraventricular nucleus (PVN) and intraventricular injections of NK3R agonists stimulate vasopressin (VP) release. Our objectives were to test the hypotheses that intraventricular injections of the selective NK3R agonist, succinyl-[Asp6, N-Me-Phe8] substance P (senktide), activate NK3R expressed by vasopressinergic neurons within the PVN, and see whether NK3R expressed by vasopressinergic neurons in the PVN are activated by hyperosmolarity. NK3R internalization was used as a marker of receptor activation. Immunohistochemistry revealed that NK3Rs were membrane-bound on VP immunoreactive neurons in control rats. Following senktide injection, there was a significant increase in the appearance of NK3R immunoreactivity within the cytoplasm and a morphological rearrangement of the dendrites, indicating receptor internalization, which was reversible. Furthermore, pretreatment with a selective NK3R antagonist, SB-222200, blocked the senktide-induced VP release and internalization of the NK3R in the PVN. These results show that the trafficking of the NK3R is due to ligand binding the NK3R. In a subsequent experiment, rats were administered intragastric loads of 2 or 0.15 M NaCl, and NK3R immunohistochemistry was used to track activation of the receptor. In contrast to control rats, 2 M NaCl significantly increased plasma VP levels and caused the internalization of the NK3R on VP neurons. Also, NK3R immunoreactivity was located in the nuclei of vasopressinergic neurons after senktide and 2 M NaCl treatment. These results show that hyperosmolarity stimulates the local release of an endogenous ligand in the PVN to bind to and activate NK3R on vasopressinergic neurons.