Silvia Graciela Ruginsk

Glucocorticoid modulation of atrial natriuretic peptide, oxytocin, vasopressin and Fos expression in response to osmotic, angiotensinergic and cholinergic stimulation.

The regulation of fluid and electrolyte homeostasis involves the participation of several neuropeptides and hormones that utilize hypothalamic cholinergic, alpha-adrenergic and angiotensinergic neurotransmitters and pathways. Additionally, it has been suggested that hypothalamus-pituitary-adrenal axis activity modulates hormonal responses to blood volume expansion. In the present study, we evaluated the effect of dexamethasone on atrial natriuretic peptide (ANP), oxytocin (OT) and vasopressin (AVP) responses to i.c.v. microinjections of 0.15 M and 0.30 M NaCl, angiotensin-II (ANG-II) and carbachol. We also evaluated the Fos protein immunoreactivity in the median preoptic (MnPO), paraventricular (PVN) and supraoptic (SON) nuclei. Male Wistar rats received an i.p. injection of dexamethasone (1 mg/kg) or vehicle (0.15 M NaCl) 2 h before the i.c.v. microinjections. Blood samples for plasma ANP, OT, AVP and corticosterone determinations were collected at 5 and 20 min after stimulus. Another set of rats was perfused 120 min after stimulation. A significant increase in plasma ANP, OT, AVP and corticosterone levels was observed at 5 and 20 min after each central stimulation compared with isotonic saline-injected group. Pre-treatment with dexamethasone decreased plasma corticosterone and OT levels, with no changes in the AVP secretion. On the other hand, dexamethasone induced a significant increase in plasma ANP levels. A significant increase in the number of Fos immunoreactive neurons was observed in the MnPO, PVN and SON after i.c.v. stimulations. Pre-treatment with dexamethasone induced a significant decrease in Fos immunoreactivity in these nuclei compared with the vehicle. These results indicate that central osmotic, cholinergic, and angiotensinergic stimuli activate MnPO, PVN and SON, with a subsequent OT, AVP, and ANP release. The present data also suggest that these responses are modulated by glucocorticoids.

Glucocorticoid modulation of neuronal activity and hormone secretion induced by blood volume expansion

The present study evaluated the involvement of glucocorticoid in the activation of vasopressinergic and oxytocinergic neurons of hypothalamic nuclei and plasma levels of vasopressin (AVP), oxytocin (OT), atrial natriuretic peptide (ANP) and corticosterone (CORT) in response to both isotonic and hypertonic blood volume expansion (BVE). Rats were subjected to isotonic (0.15 M NaCl, 2 ml/100 g b.w., i.v.) or hypertonic (0.30 M NaCl, 2 ml/100 g b.w., i.v.) BVE with or without pre-treatment with dexamethasone (1 mg/kg, i.p.). Results showed that isotonic BVE increased OT, ANP and CORT, and decreased AVP plasma levels. On the other hand, hypertonic BVE enhanced AVP, ANP, OT, and CORT plasma concentrations. Both hypertonic and isotonic BVE induced an increase in the number of Fos-OT double-labeled magnocellular neurons in the PVN and SON. Pre-treatment with dexamethasone reduced OT secretion, as well as Fos-OT immunoreactive neurons in response to both isotonic and hypertonic BVE. We also observed that dexamethasone pre-treatment had no effect on AVP secretion in response to hypertonic BVE, although this effect was associated with a blockade of Fos expression in the vasopressinergic magnocellular neurons in the PVN and SON. In conclusion, these data suggest that, not only the rapid OT release from storages, but also the oxytocinergic cellular activation induced by BVE are modulated by glucocorticoids. However, this pattern of response was not observed for AVP cells, suggesting that dexamethasone is not likely to influence rapid release of AVP but seems to modulate the activation of these neurons in response to hypertonic BVE.

Hypophagia induced by glucocorticoid deficiency is associated with an increased activation of satiety-related responses

Glucocorticoids have major effects on food intake, demonstrated by the decrease of food intake following adrenalectomy. Satiety signals are relayed to the nucleus of the solitary tract (NTS), which has reciprocal projections with the arcuate nucleus (ARC) and paraventricular nucleus (PVN) of the hypothalamus. We evaluated the effects of glucocorticoids on the activation of hypothalamic and NTS neurons induced by food intake in rats subjected to adrenalectomy (ADX) or sham surgery 7 days before the experiments. One-half of ADX animals received corticosterone (ADX+B) in the drinking water (B: 25 mg/l). Fos/tyrosine hydroxylase (TH), Fos/corticotrophin-releasing factor (CRF) and Fos immunoreactivity were assessed in the NTS, PVN, and ARC, respectively. Food intake and body weight were reduced in the ADX group compared with sham and ADX+B groups. Fos and Fos/TH in the NTS, Fos, and Fos/CRF immunoreactive neurons in the PVN and Fos in the ARC were increased after refeeding, with higher number in the ADX group, compared with sham and ADX+B groups. CCK administration showed no hypophagic effect on ADX group despite a similar increase of Fos/TH immunoreactive neurons in the NTS compared with sham and ADX+B groups, suggesting that CCK alone cannot further increase the anorexigenic effect induced by glucocorticoid deficiency. The present data indicate that glucocorticoid withdrawal reduced food intake, which was associated with higher activation of ARC, CRF neurons of the PVN, and catecholaminergic neurons of the NTS. In the absence of glucocorticoids, satiety signals elicited during a meal lead to an augmented activation of brain stem and hypothalamic pathways.

CB1 modulation of hormone secretion, neuronal activation and mRNA expression following extracellular volume expansion

The endocannabinoid system includes important signaling molecules that are involved in several homeostatic and neuroendocrine functions. In the present study, we evaluated the effects of the type 1 cannabinoid (CB(1)) receptor antagonist, rimonabant (10 mg/kg, p.o.), on hormone secretion, neuronal activation and mRNA expression in the hypothalamus following isotonic (I-) or hypertonic (H-) extracellular volume expansion (EVE). The total nitrate content in the PVN and SON was also assessed under the same experimental conditions. Our results showed that OT and AVP plasma concentrations were increased in response to H-EVE, while decreased AVP levels were found following I-EVE. Accordingly, both I- and H-EVE stimulated oxytocinergic neuronal activation, as evidenced by the increased number of c-Fos/OT double labeled neurons in the hypothalamus. The vasopressinergic cells of the PVN and SON, however, were only activated in response to H-EVE. Furthermore, increased amounts of both AVP and OT mRNAs were found in the hypothalamus following EVE. Pretreatment with rimonabant significantly potentiated hormone secretion and also vasopressinergic and oxytocinergic neuronal activation induced by EVE, although decreased AVP and OT mRNA expression was found in the hypothalami of rimonabant pretreated groups. In addition, the nitrate content in the PVN and SON was not altered in response to EVE or rimonabant pretreatment. Taken together, these results suggest that the CB(1) receptor may modulate several events that contribute to the development of appropriate responses to increased fluid volume and osmolality.

Role of nitric oxide in lipopolysaccharide-induced release of vasopressin in rats.

This study evaluated the role of nitric oxide (NO) in vasopressin (AVP) release induced by intravenous lipopolysaccharide (LPS) in rats previously treated with intracerebroventricular (i.c.v.) saline, L-NAME, L-arginine or sodium nitroprusside (SNP). In control rats given i.c.v. saline, L-NAME, L-arginine or SNP, AVP levels did not change from baseline. After LPS, plasma AVP increased, reaching a peak at 60 min, and returning to basal levels 4 h later in all i.c.v. pre-treated groups (P<0.05). The LPS administration in rats previously treated with L-NAME induced higher AVP levels (P<0.05) that remained elevated throughout the period of the experiment (P<0.05). These findings confirm the inhibitory role of NO in AVP secretion induced by LPS.

Neuroendocrine Regulation of Hydromineral Homeostasis

Since the crucial evolutionary change from an aqueous to a terrestrial environment, all living organisms address the primordial task of equilibrating the ingestion/production of water and electrolytes (primarily sodium) with their excretion. In mammals, the final route for the excretion of these elements is mainly through the kidneys, which can eliminate concentrated or diluted urine according to the requirements. Despite their primary role in homeostasis, the kidneys are not able to recover water and solutes lost through other systems. Therefore, the selective stimulation or inhibition of motivational and locomotor behavior becomes essential to initiate the search and acquisition of water and/or sodium from the environment. Indeed, imbalances affecting the osmolality and volume of body fluids are dramatic challenges to the maintenance of hydromineral homeostasis. In addition to behavioral changes, which are integrated in the central nervous system, most of the systemic responses recruited to restore hydroelectrolytic balance are accomplished by coordinated actions of the cardiovascular, autonomic and endocrine systems, which determine the appropriate renal responses. The activation of sequential and redundant mechanisms (involving local and systemic factors) produces accurate and self-limited effector responses. From a physiological point of view, understanding the mechanisms underlying water and sodium balance is intriguing and of great interest for the biomedical sciences. Therefore, the present review will address the biophysical, evolutionary and historical perspectives concerning the integrative neuroendocrine control of hydromineral balance, focusing on the major neural and endocrine systems implicated in the control of water and sodium balance.

Time-Course of Neuroendocrine Changes and Its Correlation with Hypertension Induced by Ethanol Consumption

UNLABELLED Ethanol (ETOH) consumption has been associated with endocrine and autonomic changes, including the development of hypertension. However, the sequence of pathophysiological events underlying the emergence of this effect is poorly understood. AIMS This study aimed to establish a time-course correlation between neuroendocrine and cardiovascular changes contributing to the development of hypertension following ETOH consumption. METHODS Male adult Wistar rats were subjected to the intake of increasing ETOH concentrations in their drinking water (first week: 5%, second week: 10%, third and fourth weeks: 20% v/v). RESULTS ETOH consumption decreased plasma and urinary volumes, as well as body weight and fluid intake. Furthermore, plasma osmolality, plasma sodium and urinary osmolality were elevated in the ETOH-treated rats. ETOH intake also induced a progressive increase in the mean arterial pressure (MAP), without affecting heart rate. Initially, this increase in MAP was correlated with increased plasma concentrations of adrenaline and noradrenaline. After the second week of ETOH treatment, plasma catecholamines returned to basal levels, and incremental increases were observed in plasma concentrations of vasopressin (AVP) and angiotensin II (ANG II). Conversely, plasma oxytocin, atrial natriuretic peptide, prolactin and the hypothalamus-pituitary-adrenal axis components were not significantly altered by ETOH. CONCLUSIONS Taken together, these results suggest that increased sympathetic activity may contribute to the early increase in MAP observed in ETOH-treated rats. However, the maintenance of this effect may be predominantly regulated by the long-term increase in the secretion of other circulating factors, such as AVP and ANG II, the secretion of both hormones being stimulated by the ETOH-induced dehydration.

HYPOTHALAMIC COCAINE- AND AMPHETAMINE-REGULATED TRANSCRIPT AND CORTICOTROPHIN RELEASING FACTOR NEURONS ARE STIMULATED BY EXTRACELLULAR VOLUME AND OSMOTIC CHANGES

Several studies suggest that hypothalamic cocaine- and amphetamine-regulated transcript (CART) may interact with the hypothalamic-pituitary-adrenal (HPA) axis in the control of neuroendocrine function and may also participate in cardiovascular regulation. Therefore, this study aimed to evaluate, in experimental models of isotonic (I-EVE) and hypertonic (H-EVE) extracellular volume expansion and water deprivation (WD), the activation of CART- and corticotrophin releasing factor (CRF)-immunoreactive neurons, as well as the relative expression of CART and CRF mRNAs in the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Both H-EVE (0.30M NaCl, 2mL/100g of body weight, in 1 minute) and 24 hours of WD significantly increased plasma sodium concentrations, producing, respectively, either an increase or a decrease in extracellular volume. I-EVE (0.15M NaCl, 2mL/100g of body weight, in 1 minute) evoked a significant increase in the circulating volume accompanied by unaltered plasma concentrations of sodium. CART-expressing neurons of both magnocellular and parvocellular hypothalamic divisions were activated to produce Fos in response to H-EVE but not in response to I-EVE. Furthermore, increased expression of CART mRNA was found in the PVN of H-EVE but not I-EVE rats. These data show for the first time that EVE not only activates hypothalamic CRF neurons but also increases CRF mRNA expression in the PVN. In contrast, WD increases the number of CART-immunoreactive neurons activated to produce Fos in the PVN and SON but does not change the number of neurons double labeled for Fos and CRF or expression of CRF mRNA in the PVN. These findings provided new insights into the participation of CART in diverse processes within the PVN and SON, including its possible involvement in activation of the HPA axis and cardiovascular regulation in response to changes in extracellular volume and osmolality.

Mapping and signaling of neural pathways involved in the regulation of hydromineral homeostasis

Several forebrain and brainstem neurochemical circuitries interact with peripheral neural and humoral signals to collaboratively maintain both the volume and osmolality of extracellular fluids. Although much progress has been made over the past decades in the understanding of complex mechanisms underlying neuroendocrine control of hydromineral homeostasis, several issues still remain to be clarified. The use of techniques such as molecular biology, neuronal tracing, electrophysiology, immunohistochemistry, and microinfusions has significantly improved our ability to identify neuronal phenotypes and their signals, including those related to neuron-glia interactions. Accordingly, neurons have been shown to produce and release a large number of chemical mediators (neurotransmitters, neurohormones and neuromodulators) into the interstitial space, which include not only classic neurotransmitters, such as acetylcholine, amines (noradrenaline, serotonin) and amino acids (glutamate, GABA), but also gaseous (nitric oxide, carbon monoxide and hydrogen sulfide) and lipid-derived (endocannabinoids) mediators. This efferent response, initiated within the neuronal environment, recruits several peripheral effectors, such as hormones (glucocorticoids, angiotensin II, estrogen), which in turn modulate central nervous system responsiveness to systemic challenges. Therefore, in this review, we shall evaluate in an integrated manner the physiological control of body fluid homeostasis from the molecular aspects to the systemic and integrated responses.

Anandamide modulates the neuroendocrine responses induced by extracellular volume expansion

The aim of the present study was to evaluate the effects of intracerebroventricular administration of anandamide (AEA), an inhibitor of fatty acid amide hydrolase activity (URB597) and a CB1 receptor (CB1R) antagonist (AM251) on the homeostatic responses elicited by extracellular volume expansion (EVE) in male adult rats. Pretreatment with AEA (100 ng/4 μL) significantly reduced the effect of hypertonic (H‐) EVE on plasma concentrations of prolactin (PRL), oxytocin (OT) and corticosterone, but not vasopressin (AVP). Administration of URB597 (20 μg/5 μL) alone significantly reduced PRL, OT, AVP and corticosterone in the H‐EVE group. Conversely, URB597 and AEA had no significant effect on basal hormone concentrations. Pretreatment with AM251 (200 ng/2 μL) potentiated OT but did not change AVP plasma levels in the H‐EVE group. Hypertonic EVE significantly increased AVP and OT mRNA expression in the supraoptic nucleus (SON), an effect that was blunted in AEA‐pretreated rats. Pretreatment with AEA did not change the percentage of vasopressinergic or oxytocinergic neurons colocalizing c‐Fos in the SON, but increased nitrate concentrations in the median eminence of animals subjected to H‐EVE. The present data suggest that: (i) vasopressinergic and oxytocinergic neurons may be differentially affected by AEA; (ii) activation of CB1R may restrain the response of the neurohypophyseal system (NHS) to EVE; (iii) the hypothalamic–pituitary–adrenal axis, PRL and the NHS may still be sensitive to AEA after EVE, with these effects probably not dependent on AEA metabolism; and (iv) AEA and nitric oxide could interact in vivo as modulators to directly control stress‐induced responses.