Effects of Hyperosmolality on Hypothalamic Astrocytic Area, mRNA Expression and Glutamate Balance In Vitro.

During prolonged dehydration, body fluid homeostasis is challenged by extracellular fluid (ECF) hyperosmolality, which induce important functional changes in the hypothalamus, in parallel with other effector responses, such as the activation of the local renin-angiotensin system (RAS). Therefore, in the present study we investigated the role of sodium-driven ECF hyperosmolality on glial fibrillary acid protein (GFAP) immunoreactivity and protein expression, membrane capacitance, mRNA expression of RAS components and glutamate balance in cultured hypothalamic astrocytes. Our data show that hypothalamic astrocytes respond to increased hyperosmolality with a similar decrease in GFAP expression and membrane capacitance, indicative of reduced cellular area. Hyperosmolality also downregulates the transcript levels of angiotensinogen and both angiotensin-converting enzymes, whereas upregulates type 1a angiotensin II receptor mRNA. Incubation with hypertonic solution also decreases the immunoreactivity to the membrane glutamate/aspartate transporter (GLAST) as well as tritiated-aspartate uptake by astrocytes. This latter effect is completely restored to basal levels when astrocytes previously exposed to hypertonicity are incubated under isotonic conditions. Together with a direct effect on two important local signaling systems (glutamate and RAS), these synaptic rearrangements driven by astrocytes may accomplish for a coordinated increase in the excitatory drive onto the hypothalamic neurosecretory system, ultimately culminating with increased AVP release in response to hyperosmolality.

The type-1 cannabinoid receptor modulates the hydroelectrolytic balance independently of the energy homeostasis during salt load.

Hydroelectrolytic imbalances, such as saline load (SL), trigger behavioral and neuroendocrine responses, such as thirst, hypophagia, vasopressin (AVP) and oxytocin (OT) release and hypothalamus­pituitary­adrenal (HPA) axis activation. To investigate the participation of the type-1 cannabinoid receptor (CB1R) in these homeostatic mechanisms,male adult Wistar rats were subjected to SL (0.3MNaCl) for four days. SL induced not only increases in the water intake and plasma levels of AVP, OT and corticosterone, as previously described, but also increases in CB1R expression in the lamina terminalis, which integrates sensory afferents, aswell as in the hypothalamus, the main integrative and effector area controlling hydroelectrolytic homeostasis. A more detailed analysis revealed that CB1R-positive terminals are in close apposition with not only axons but also dendrites and secretory granules of magnocellular neurons, particularly vasopressinergic cells. In satiated and euhydrated animals, the intracerebroventricular administration of the CB1R selective agonist ACEA (0.1 µg/5 µL) promoted hyperphagia, but this treatment did not reverse the hyperosmolality-induced hypophagia in the SL group. Furthermore, ACEA pretreatment potentiated water intake in the SL animals during rehydration as well as enhanced the corticosterone release and prevented the increase in AVP and OT secretion induced by SL. The same parameters were not changed by ACEA in the animals whose daily food intake was matched to that of the SL group (Pair-Fed). These data indicate that CB1Rs modulate the hydroelectrolytic balance independently of the food intake during sustained hyperosmolality and hypovolemia.

The endocannabinoid system and the neuroendocrine control of hydromineral balance.

Endocannabinoids (ECBs) are ubiquitous lipophilic agents, and this characteristic is consistent with the wide range of homeostatic functions attributed to the ECB system. There is an increasing number of studies showing that the ECB system affects neurotransmission within the hypothalamic neurohypophyseal system. We provide an overview of the primary roles of ECBs in the modulation of neuroendocrine function and, specifically, in the control of hydromineral homeostasis. Accordingly, the general aspects of ECB-mediated signalling, as well as the specific contributions of the central component of the ECB system to the integration of behavioural and endocrine responses that control body fluid homeostasis, are discussed.

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.

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.

Cannabinoid CB1 receptor mediates glucocorticoid effects on hormone secretion induced by volume and osmotic changes.

The present study provides the first in vivo evidence that the cannabinoid CB(1) receptor mediates the effects of dexamethasone on hormone release induced by changes in circulating volume and osmolality. Male adult rats were administered with the CB(1) receptor antagonist rimonabant (10 mg/Kg, p.o.), followed or not in 1 hour by dexamethasone (1 mg/Kg, i.p.). Extracellular volume expansion (EVE, 2 mL/100 g of body weight, i.v.) was performed 2 hours after dexamethasone or vehicle treatment using either isotonic (I-EVE, 0.15 mol/L) or hypertonic (H-EVE, 0.30 mol/L) NaCl solution. Five minutes after EVE, animals were decapitated and trunk blood was collected for all plasma measurements. Rimonabant potentiated oxytocin (OT) secretion induced by H-EVE and completely reversed the inhibitory effects of dexamethasone in response to the same stimulus. These data suggest that glucocorticoid modulation of OT release is mediated by the CB(1) receptor. Although dexamethasone did not affect vasopressin (AVP) secretion induced by H-EVE, the administration of rimonabant potentiated AVP release in response to the same stimulus, supporting the hypothesis that the CB(1) receptor regulates AVP secretion independently of glucocorticoid-mediated signalling. Dexamethasone alone did not affect atrial natriuretic peptide (ANP) release stimulated by I-EVE or H-EVE. However, pretreatment with rimonabant potentiated ANP secretion induced by H-EVE, suggesting a possible role for the CB(1) receptor in the control of peripheral factors that modulate cardiovascular function. Rimonabant also reversed the inhibitory effects of dexamethasone on H-EVE-induced corticosterone secretion, reinforcing the hypothesis that the CB(1) receptor may be involved in the negative feedback exerted by glucocorticoids on the activity of the hypothalamic-pituitary-adrenal axis. Collectively, the results of the present study indicate that the CB(1) receptor modulates neurohypophyseal hormone secretion and systemic factors, such as corticosterone and ANP, thus participating in homeostatic responses to altered extracellular volume and plasma tonicity.

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.

CB(1) 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.

Central actions of glucocorticoids in the control of body fluid homeostasis: review.

The involvement of the hypothalamic-pituitary-adrenal axis in the control of body fluid homeostasis has been extensively investigated in the past few years. In the present study, we reviewed the recent results obtained using different approaches to investigate the effects of glucocorticoids on the mechanisms of oxytocin and vasopressin synthesis and secretion in response to acute and chronic plasma volume and osmolality changes. The data presented here suggest that glucocorticoids are not only involved in the mechanisms underlying the fast release but also in the transcriptional events that lead to decreased synthesis and secretion of these neuropeptides, particularly oxytocin, under diverse experimental conditions of altered fluid volume and tonicity. The endocannabinoid system, through its effects on glutamatergic neurotransmission within the hypothalamus and the nuclear factor kappaB-mediated transcriptional activity, seems to be also involved in the specific mechanisms by which glucocorticoids exert their central effects on neurohypophyseal hormone synthesis and secretion.

Central actions of glucocorticoids in the control of body fluid homeostasis: review

The involvement of the hypothalamic-pituitary-adrenal axis in the control of body fluid homeostasis has been extensively investigated in the past few years. In the present study, we reviewed the recent results obtained using different approaches to investigate the effects of glucocorticoids on the mechanisms of oxytocin and vasopressin synthesis and secretion in response to acute and chronic plasma volume and osmolality changes. The data presented here suggest that glucocorticoids are not only involved in the mechanisms underlying the fast release but also in the transcriptional events that lead to decreased synthesis and secretion of these neuropeptides, particularly oxytocin, under diverse experimental conditions of altered fluid volume and tonicity. The endocannabinoid system, through its effects on glutamatergic neurotransmission within the hypothalamus and the nuclear factor κB-mediated transcriptional activity, seems to be also involved in the specific mechanisms by which glucocorticoids exert their central effects on neurohypophyseal hormone synthesis and secretion.

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.

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.