Integrative role of the lamina terminalis in the regulation of cardiovascular and body fluid homeostasis

The Regulatory Effect of the Paraventricular Nucleus on Hypertension

Hypertension is among the most harmful factors of cardiovascular and cerebrovascular diseases and poses an urgent problem for the development of human society. In addition to previous studies on its pathogenesis focusing on the peripheral sympathetic nervous system, investigating the central causes of high blood pressure involving the neuroendocrine and neuroinflammatory mechanisms of the hypothalamic paraventricular nucleus (PVN) is paramount. This nucleus is considered to regulate the output of neurohormones and sympathetic nerve activity. In this article, we focussed on the neuroendocrine mechanism, primarily exploring the specific contributions and interactions of various neurons and neuroendocrine hormones, including GABAergic and glutamatergic neurons, nitric oxide, arginine vasopressin, oxytocin, and the renin-angiotensin system. Additionally, the neuroinflammatory mechanism in the PVN was discussed, encompassing microglia, reactive oxygen species, inflammatory factors, and pathways, as well as immune connections between the brain and extracerebral organs. Notably, the two central mechanisms involved in the PVN not only exist independently but also communicate with each other, jointly maintaining the hypertensive state of the body. Furthermore, we introduce well-known molecules and signal transduction pathways within the PVN that can play a regulatory role in the two mechanisms to provide a basis and inspire ideas for further research.

The role of the apelin/APJ system in water homeostasis regulation

Water balance in the body is achieved by balancing renal and non-renal water losses with corresponding water intake. It is under the control of both the central nervous system, which integrates many parameters of water and electrolyte balance in the body, including inducing important adaptive behavioral responses, and three hormonal systems: vasopressinergic, renin-angiotensin-aldosterone and apelinergic. A lot of research is devoted to the regulation of water-electrolyte metabolism. However, this process is still quite difficult to understand, especially since more and more of its regulators are being discovered over time. One of them is the hormone apelin, an endogenous ligand for the APJ receptor. As is known, the receptor is highly expressed in many organs, such as the brain, heart, liver and kidneys, lungs, and has multidirectional effects.

This literature review discusses the main characteristics and features of the regulation of these systems in relation to water-electrolyte metabolism, as well as issues of intersystem interaction and modulation of the effects of apelin.

ACE2, Circumventricular Organs and the Hypothalamus, and COVID-19

The SARS-CoV-2 virus gains entry to cells by binding to angiotensin-converting enzyme 2 (ACE2). Since circumventricular organs and parts of the hypothalamus lack a blood-brain barrier, and immunohistochemical studies demonstrate that ACE2 is highly expressed in circumventricular organs which are intimately connected to the hypothalamus, and the hypothalamus itself, these might be easy entry points for SARS-CoV-2 into the brain via the circulation. High ACE2 protein expression is found in the subfornical organ, area postrema, and the paraventricular nucleus of the hypothalamus (PVH). The subfornical organ and PVH are parts of a circuit to regulate osmolarity in the blood, through the secretion of anti-diuretic hormone into the posterior pituitary. The PVH is also the stress response centre in the brain. It controls not only pre-ganglionic sympathetic neurons, but is also a source of corticotropin-releasing hormone, that induces the secretion of adrenocorticotropic hormone from the anterior pituitary. It is proposed that the function of ACE2 in the circumventricular organs and the PVH could be diminished by binding with SARS-CoV-2, thus leading to a reduction in the ACE2/Ang (1-7)/Mas receptor (MasR) signalling axis, that modulates ACE/Ang II/AT1R signalling. This could result in increased presympathetic activity/neuroendocrine secretion from the PVH, and effects on the hypothalamic-pituitary-adrenal axis activity. Besides the bloodstream, the hypothalamus might also be affected by SARS-CoV-2 via transneuronal spread along the olfactory/limbic pathways. Exploring potential therapeutic pathways to prevent or attenuate neurological symptoms of COVID-19, including drugs which modulate ACE signalling, remains an important area of unmet medical need.

Human thirst behavior requires transformation of sensory inputs by intrinsic brain networks

BACKGROUND

To survive and thrive, many animals, including humans, have evolved goal-directed behaviors that can respond to specific physiological needs. An example is thirst, where the physiological need to maintain water balance drives the behavioral basic instinct to drink. Determining the neural basis of such behaviors, including thirst response, can provide insights into the way brain-wide systems transform sensory inputs into behavioral outputs. However, the neural basis underlying this spontaneous behavior remains unclear. Here, we provide a model of the neural basis of human thirst behavior.

RESULTS

We used fMRI, coupled with functional connectivity analysis and serial-multiple mediation analysis, we found that the physiological need for water is first detected by the median preoptic nucleus (MnPO), which then regulates the intention of drinking via serial large-scale spontaneous thought-related intrinsic network interactions that include the default mode network, salience network, and frontal-parietal control network.

CONCLUSIONS

Our study demonstrates that the transformation in humans of sensory inputs for a single physiological need, such as to maintain water balance, requires large-scale intrinsic brain networks to transform this input into a spontaneous human behavioral response.

Molecular neurobiological markers in the onset of sodium appetite

Sodium appetite is a motivational state involving homeostatic behavior, seeking the ingest of salty substances after sodium loss. There is a temporal dissociation between sodium depletion (SD) and the appearance of sodium appetite. However, the responsible mechanisms for this delay remain poorly elucidated. In the present study, we measured the temporal changes at two and 24 h after SD in the gene expression of key elements within excitatory, inhibitory, and sensory areas implicated in the signaling pathways involved in the onset of sodium appetite. In SD rats, we observed that the expression of critical components within the brain control circuit of sodium appetite, including Angiotensin-type-1 receptor (Agtr1a), Oxytocin-(OXT-NP)-neurophysin-I, and serotonergic-(5HT)-type-2c receptor (Htr2c) were modulated by SD, regardless of time. However, we observed reduced phosphorylation of mitogen-activated protein kinases (MAPK) at the paraventricular nucleus (PVN) and increased oxytocin receptor (Oxtr) mRNA expression at the anteroventral of the third ventricle area (AV3V), at two hours after SD, when sodium appetite is inapparent. At twenty-four hours after SD, when sodium appetite is released, we observed a reduction in the mRNA expression of the transient receptor potential channel 1gene (Trpv1) and Oxtr in the AV3V and the dorsal raphe nucleus, respectively. The results indicate that SD exerts a coordinated timing effect, promoting the appearance of sodium appetite through changes in MAPK activity and lower Trpv1 channel and Oxtr expression that trigger sodium consumption to reestablish the hydroelectrolytic homeostasis.

[Physiological role of the apelin receptor: implication in body fluid homeostasis and hyponatremia]

Apelin, a vasoactive neuropeptide, its receptor and arginine-vasopressin (AVP, antidiuretic hormone) are co-localized in magnocellular vasopressinergic neurons. In the kidney, the apelin receptor is present in glomerular arterioles and the collecting duct (CD) where the AVP type 2 (V2-R) receptors are located. Apelin exerts an aquaretic action both by its inhibitory effect on the phasic electrical activity of vasopressinergic neurons and the secretion of AVP into the bloodstream and by its direct actions at the kidney level resulting in an increase in the renal microcirculation and the inhibition of the antidiuretic effect of AVP mediated by V2-R in the CD. Plasma apelin and AVP are conversely regulated by osmotic stimuli in both humans and rodents, showing that apelin is involved with AVP in maintaining body fluid homeostasis. Clinically, in patients with inappropriate antidiuresis syndrome (SIAD), the apelin/AVP balance is altered, which contributes to water metabolism defect. Activation of the apelin receptor by the metabolically stable apelin-17 analog, that increases aqueous diuresis and moderately water intake and gradually corrects hyponatremia, may constitute a new approach for the treatment of SIAD.

Heat Stress Responses in Birds: A Review of the Neural Components

Heat stress is one of the major environmental conditions causing significant losses in the poultry industry and having negative impacts on the world's food economy. Heat exposure causes several physiological impairments in birds, including oxidative stress, weight loss, immunosuppression, and dysregulated metabolism. Collectively, these lead not only to decreased production in the meat industry, but also decreases in the number of eggs laid by 20%, and overall loss due to mortality during housing and transit. Mitigation techniques have been discussed in depth, and include changes in air flow and dietary composition, improved building insulation, use of air cooling in livestock buildings (fogging systems, evaporation panels), and genetic alterations. Most commonly observed during heat exposure are reduced food intake and an increase in the stress response. However, very little has been explored regarding heat exposure, food intake and stress, and how the neural circuitry responsible for sensing temperatures mediate these responses. That thermoregulation, food intake, and the stress response are primarily mediated by the hypothalamus make it reasonable to assume that it is the central hub at which these systems interact and coordinately regulate downstream changes in metabolism. Thus, this review discusses the neural circuitry in birds associated with thermoregulation, food intake, and stress response at the level of the hypothalamus, with a focus on how these systems might interact in the presence of heat exposure.

AT1a-dependent GABAA inhibition in the MnPO following chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) is associated with diurnal hypertension, increased sympathetic nerve activity (SNA), and increases in circulating angiotensin II (ANG II). In rats, CIH increases angiotensin type 1 (AT1a) receptor expression in the median preoptic nucleus (MnPO), and pharmacological blockade or viral knockdown of this receptor prevents CIH-dependent increases in diurnal blood pressure. The current study investigates the role of AT1a receptor in modulating the activity of MnPO neurons following 7 days of CIH. Male Sprague-Dawley rats received MnPO injections of an adeno-associated virus with an shRNA against the AT1a receptor or a scrambled control. Rats were then exposed to CIH for 8 h a day for 7 days. In vitro, loose patch recordings of spontaneous action potential activity were made from labeled MnPO neurons in response to brief focal application of ANG II or the GABAA receptor agonist muscimol. In addition, MnPO K-Cl cotransporter isoform 2 (KCC2) protein expression was assessed using Western blot. CIH impaired the duration but not the magnitude of ANG II-mediated excitation in the MnPO. Both CIH and AT1a knockdown also impaired GABAA-mediated inhibition, and CIH with AT1a knockdown produced GABAA-mediated excitation. Recordings using the ratiometric Cl- indicator ClopHensorN showed CIH was associated with Cl- efflux in MnPO neurons that was associated with decreased KCC2 phosphorylation. The combination of CIH and AT1a knockdown attenuated reduced KCC2 phosphorylation seen with CIH alone. The current study shows that CIH, through the activity of AT1a receptors, can impair GABAA-mediated inhibition in the MnPO and contribute to sustained hypertension.

Recent Advances in Studies on the Role of Neuroendocrine Disorders in Obstructive Sleep Apnea-Hypopnea Syndrome-Related Atherosclerosis

Cardiovascular disease is a common cause of death worldwide, and atherosclerosis (AS) and obstructive sleep apnea-hypopnea syndrome (OSAHS) critically contribute to the initiation and progression of cardiovascular diseases. OSAHS promotes endothelial injury, vascular smooth muscle cell (VSMC) proliferation, abnormal lipid metabolism, and elevated arterial blood pressure. However, the exact OSAHS mechanism that causes AS remains unclear. The nervous system is widely distributed in the central and peripheral regions. It regulates appetite, energy metabolism, inflammation, oxidative stress, insulin resistance, and vasoconstriction by releasing regulatory factors and participates in the occurrence and development of AS. Studies showed that OSAHS can cause changes in neurophysiological plasticity and affect modulator release, suggesting that neuroendocrine dysfunction may be related to the OSAHS mechanism causing AS. In this article, we review the possible mechanisms of neuroendocrine disorders in the pathogenesis of OSAHS-induced AS and provide a new basis for further research on the development of corresponding effective intervention strategies.

Apelin and Vasopressin: The Yin and Yang of Water Balance

Apelin, a (neuro)vasoactive peptide, plays a prominent role in controlling body fluid homeostasis and cardiovascular functions. Experimental data performed in rodents have shown that apelin has an aquaretic effect via its central and renal actions. In the brain, apelin inhibits the phasic electrical activity of vasopressinergic neurons and the release of vasopressin from the posterior pituitary into the bloodstream and in the kidney, apelin regulates renal microcirculation and counteracts in the collecting duct, the antidiuretic effect of vasopressin occurring via the vasopressin receptor type 2. In humans and rodents, if plasma osmolality is increased by hypertonic saline infusion/water deprivation or decreased by water loading, plasma vasopressin and apelin are conversely regulated to maintain body fluid homeostasis. In patients with the syndrome of inappropriate antidiuresis, in which vasopressin hypersecretion leads to hyponatremia, the balance between apelin and vasopressin is significantly altered. In order to re-establish the correct balance, a metabolically stable apelin-17 analog, LIT01-196, was developed, to overcome the problem of the very short half-life (in the minute range) of apelin in vivo. In a rat experimental model of vasopressin-induced hyponatremia, subcutaneously (s.c.) administered LIT01-196 blocks the antidiuretic effect of vasopressin and the vasopressin-induced increase in urinary osmolality, and induces a progressive improvement in hyponatremia, suggesting that apelin receptor activation constitutes an original approach for hyponatremia treatment.

Caspase lesions of PVN-projecting MnPO neurons block the sustained component of CIH-induced hypertension in adult male rats

Obstructive sleep apnea is characterized by interrupted breathing that leads to cardiovascular sequelae including chronic hypertension that can persist into the waking hours. Chronic intermittent hypoxia (CIH), which models the hypoxemia associated with sleep apnea, is sufficient to cause a sustained increase in blood pressure that involves the central nervous system. The median preoptic nucleus (MnPO) is an integrative forebrain region that contributes to blood pressure regulation and neurogenic hypertension. The MnPO projects to the paraventricular nucleus (PVN), a preautonomic region. We hypothesized that pathway-specific lesions of the projection from the MnPO to the PVN would attenuate the sustained component of chronic intermittent hypoxia-induced hypertension. Adult male Sprague-Dawley rats (250-300 g) were anesthetized with isoflurane and stereotaxically injected bilaterally in the PVN with a retrograde Cre-containing adeno-associated virus (AAV; AAV9.CMV.HI.eGFP-Cre.WPRE.SV40) and injected in the MnPO with caspase-3 (AAV5-flex-taCasp3-TEVp) or control virus (AAV5-hSyn-DIO-mCherry). Three weeks after the injections the rats were exposed to a 7-day intermittent hypoxia protocol. During chronic intermittent hypoxia, controls developed a diurnal hypertension that was blunted in rats with caspase lesions. Brain tissue processed for FosB immunohistochemistry showed decreased staining with caspase-induced lesions of MnPO and downstream autonomic-regulating nuclei. Chronic intermittent hypoxia significantly increased plasma levels of advanced oxidative protein products in controls, but this increase was blocked in caspase-lesioned rats. The results indicate that PVN-projecting MnPO neurons play a significant role in blood pressure regulation in the development of persistent chronic intermittent hypoxia hypertension.NEW & NOTEWORTHY Chronic intermittent hypoxia associated with obstructive sleep apnea increases oxidative stress and leads to chronic hypertension. Sustained hypertension may be mediated by angiotensin II-induced neural plasticity of excitatory median preoptic neurons in the forebrain that project to the paraventricular nucleus of the hypothalamus. Selective caspase lesions of these neurons interrupt the drive for sustained hypertension and cause a reduction in circulating oxidative protein products. This indicates that a functional connection between the forebrain and hypothalamus is necessary to drive diurnal hypertension associated with intermittent hypoxia. These results provide new information about central mechanisms that may contribute to neurogenic hypertension.

Genetic knockdown of brain-derived neurotrophic factor in the nervous system attenuates angiotensin II-induced hypertension in mice

Introduction:

Brain-derived neurotropic factor (BDNF) is expressed throughout the central nervous system and peripheral organs involved in the regulation of blood pressure, but the systemic effects of BDNF in the control of blood pressure are not well elucidated.

Materials and methods:

We utilized loxP flanked BDNF male mice to cross with nestin-Cre female mice to generate nerve system BDNF knockdown mice, nestin-BDNF (+/–), or injected Cre adenovirus into the subfornical organ to create subfornical organ BDNF knockdown mice. Histochemistry was used to verify injection location. Radiotelemetry was employed to determine baseline blood pressure and pressor response to angiotensin II (1000 ng/kg/min). Real-time polymerase chain reaction was used to measure the expression of renin–angiotensin system components in the laminal terminalis and peripheral organs.

Results:

Nestin-BDNF (+/–) mice had lower renin–angiotensin system expression in the laminal terminalis and peripheral organs including the gonadal fat pad, and a lower basal blood pressure. They exhibited an attenuated hypertensive response and a weak or similar modification of renin–angiotensin system component expression to angiotensin II infusion. Subfornical organ BDNF knockdown was sufficient for the attenuation of angiotensin II-induced hypertension.

Conclusion:

Central BDNF, especially subfornical organ BDNF is involved in the maintenance of basal blood pressure and in augmentation of hypertensive response to angiotensin II through systemic regulation of the expression of renin–angiotensin system molecules.

Sniffer cells for the detection of neural Angiotensin II in vitro

Neuropeptide release in the brain has traditionally been difficult to observe. Existing methods lack temporal and spatial resolution that is consistent with the function and size of neurons. We use cultured "sniffer cells" to improve the temporal and spatial resolution of observing neuropeptide release. Sniffer cells were created by stably transfecting Chinese Hamster Ovary (CHO) cells with plasmids encoding the rat angiotensin type 1a receptor and a genetically encoded Ca2+ sensor. Isolated, cultured sniffer cells showed dose-dependent increases in fluorescence in response to exogenously applied angiotensin II and III, but not other common neurotransmitters. Sniffer cells placed on the median preoptic nucleus (a presumptive site of angiotensin release) displayed spontaneous activity and evoked responses to either electrical or optogenetic stimulation of the subfornical organ. Stable sniffer cell lines could be a viable method for detecting neuropeptide release in vitro, while still being able to distinguish differences in neuropeptide concentration.

Selectively Inhibiting the Median Preoptic Nucleus Attenuates Angiotensin II and Hyperosmotic-Induced Drinking Behavior and Vasopressin Release in Adult Male Rats

The median preoptic nucleus (MnPO) is a putative integrative region that contributes to body fluid balance. Activation of the MnPO can influence thirst, but it is not clear how these responses are linked to body fluid homeostasis. We used designer receptors exclusively activated by designer drugs (DREADDs) to determine the role of the MnPO in drinking behavior and vasopressin release in response to peripheral angiotensin II (ANG II) or 3% hypertonic saline (3% HTN) in adult male Sprague Dawley rats (250-300 g). Rats were anesthetized with isoflurane and stereotaxically injected with an inhibitory DREADD (rAAV5-CaMKIIa-hM4D(Gi)-mCherry) or control (rAAV5-CaMKIIa-mCherry) virus in the MnPO. After two weeks' recovery, a subset of rats was used for extracellular recordings to verify functional effects of ANG II or hyperosmotic challenges in MnPO slice preparations. Remaining rats were used in drinking behavior studies. Each rat was administered either 10 mg/kg of exogenous clozapine-N-oxide (CNO) to inhibit DREADD-expressing cells or vehicle intraperitoneal followed by a test treatment with either 2-mg/kg ANG II or 3% HTN (1 ml/100-g bw, s.c.), twice per week for two separate treatment weeks. CNO-induced inhibition during either test treatment significantly attenuated drinking responses compared to vehicle treatments and controls. Brain tissue processed for cFos immunohistochemistry showed decreased expression with CNO-induced inhibition during either test treatment in the MnPO and downstream nuclei compared to controls. CNO-mediated inhibition significantly attenuated treatment-induced increases in plasma vasopressin compared to controls. The results indicate inhibition of CaMKIIa-expressing MnPO neurons significantly reduces drinking and vasopressin release in response to ANG II or hyperosmotic challenge.

AT1a influences GABAA-mediated inhibition through regulation of KCC2 expression

The median preoptic nucleus (MnPO) is an integrative site involved in body fluid homeostasis, cardiovascular control, thermoregulation, and sleep homeostasis. Angiotensin II (ANG II), a neuropeptide shown to have excitatory effects on MnPO neurons, is of particular interest with regard to its role in body fluid homeostasis and cardiovascular control. The present study investigated the role of angiotensin type 1a (AT1a) receptor activation on neuronal excitability in the MnPO. Male Sprague-Dawley rats were infused with an adeno-associated virus with an shRNA against the AT1a receptor or a scrambled control. In vitro loose-patch voltage-clamp recordings of spontaneous action potential activity were made from labeled MnPO neurons in response to brief focal application of ANG II or the GABAA receptor agonist muscimol. Additionally, tissue punches from MnPO were taken to asses mRNA and protein expression. AT1a receptor knockdown neurons were insensitive to ANG II and showed a marked reduction in GABAA-mediated inhibition. The reduction in GABAA-mediated inhibition was not associated with reductions in mRNA or protein expression of GABAA β-subunits. Knockdown of the AT1a receptor was associated with a reduction in the potassium-chloride cotransporter KCC2 mRNA as well as a reduction in pS940 KCC2 protein. The impaired GABAA-mediated inhibition in AT1a knockdown neurons was recovered by bath application of phospholipase C and protein kinase C activators. The following study indicates that AT1a receptor activation mediates the excitability of MnPO neurons, in part, through the regulation of KCC2. The regulation of KCC2 influences the intracellular [Cl-] and the subsequent efficacy of GABAA-mediated currents.

Involvement of median preoptic nucleus and medullary noradrenergic neurons in cardiovascular and sympathetic responses of hemorrhagic rats

The infusion of hypertonic saline solution (HSS) is known to be beneficial to the treatment of hypovolemic hemorrhage (HH). The central mechanism of HSS-induced cardiovascular and autonomic recovery of animals subjected to HH remains unclear. Hence, the present study evaluated the involvement of median preoptic nucleus (MnPO) and medullary noradrenergic neurons (A1 and A2) in HSS-induced cardiovascular and sympathetic responses in hemorrhagic rats. The wistar rats were subjected to specific lesion of noradrenergic neurons through the nanoinjections of anti-DβH-saporin into caudal ventrolateral medulla (A1 neurons) and nucleus of the solitary tract (A2 neurons). After recovery, mean arterial pressure (MAP) and renal sympathetic nervous activity were recorded. The HH was performed through blood withdrawal until a MAP of 60 mmHg was attained. In sham rats, HSS infusion (3M NaCl) reestablished MAP without change in HH-induced sympathoinhibition. The muscimol (agonist of GABAA receptor) was nanoinjected in MnPO during HH and MnPO inhibition abolished the recovery of MAP and HSS-induced sympathoinhibition. Simultaneous lesions of A1 and A2 abolished MAP restoration and sympathoinhibition after HSS infusion. These results suggest that the recovery of MAP and HSS-induced sympathoinhibition in hemorrhaged rats depend on intact neural projections from A1 and A2 to MnPO.

Oxytocin-receptor-expressing neurons in the parabrachial nucleus regulate fluid intake

Brain regions that regulate fluid satiation are not well characterized, yet are essential for understanding fluid homeostasis. We found that oxytocin-receptor-expressing neurons in the parabrachial nucleus of mice (Oxtr^PBN neurons) are key regulators of fluid satiation. Chemogenetic activation of Oxtr^PBN neurons robustly suppressed noncaloric fluid intake, but did not decrease food intake after fasting or salt intake following salt depletion; inactivation increased saline intake after dehydration and hypertonic saline injection. Under physiological conditions, Oxtr^PBN neurons were activated by fluid satiation and hypertonic saline injection. Oxtr^PBN neurons were directly innervated by oxytocin neurons in the paraventricular hypothalamus (Oxt^PVH neurons), which mildly attenuated fluid intake. Activation of neurons in the nucleus of the solitary tract substantially suppressed fluid intake and activated Oxtr^PBN neurons. Our results suggest that Oxtr^PBN neurons act as a key node in the fluid satiation neurocircuitry, which acts to decrease water and/or saline intake to prevent or attenuate hypervolemia and hypernatremia.

The authors show that oxytocin-receptor-expressing neurons in the parabrachial nucleus are key regulators of fluid homeostasis that suppress fluid intake when activated, but do not decrease food intake after fasting or salt intake after salt depletion.

The neural basis of homeostatic and anticipatory thirst

Water intake is one of the most basic physiological responses and is essential to sustain life. The perception of thirst has a critical role in controlling body fluid homeostasis and if neglected or dysregulated can lead to life-threatening pathologies. Clear evidence suggests that the perception of thirst occurs in higher-order centres, such as the anterior cingulate cortex (ACC) and insular cortex (IC), which receive information from midline thalamic relay nuclei. Multiple brain regions, notably circumventricular organs such as the organum vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), monitor changes in blood osmolality, solute load and hormone circulation and are thought to orchestrate appropriate responses to maintain extracellular fluid near ideal set points by engaging the medial thalamic-ACC/IC network. Thirst has long been thought of as a negative homeostatic feedback response to increases in blood solute concentration or decreases in blood volume. However, emerging evidence suggests a clear role for thirst as a feedforward adaptive anticipatory response that precedes physiological challenges. These anticipatory responses are promoted by rises in core body temperature, food intake (prandial) and signals from the circadian clock. Feedforward signals are also important mediators of satiety, inhibiting thirst well before the physiological state is restored by fluid ingestion. In this Review, we discuss the importance of thirst for body fluid balance and outline our current understanding of the neural mechanisms that underlie the various types of homeostatic and anticipatory thirst.

Neuronal circuits involved in osmotic challenges

The maintenance of plasma sodium concentration within a narrow limit is crucial to life. When it differs from normal physiological patterns, several mechanisms are activated in order to restore body fluid homeostasis. Such mechanisms may be vegetative and/or behavioral, and several regions of the central nervous system (CNS) are involved in their triggering. Some of these are responsible for sensory pathways that perceive a disturbance of the body fluid homeostasis and transmit information to other regions. These regions, in turn, initiate adequate adjustments in order to restore homeostasis. The main cardiovascular and autonomic responses to a change in plasma sodium concentration are: i) changes in arterial blood pressure and heart rate; ii) changes in sympathetic activity to the renal system in order to ensure adequate renal sodium excretion/absorption, and iii) the secretion of compounds involved in sodium ion homeostasis (ANP, Ang-II, and ADH, for example). Due to their cardiovascular effects, hypertonic saline solutions have been used to promote resuscitation in hemorrhagic patients, thereby increasing survival rates following trauma. In the present review, we expose and discuss the role of several CNS regions involved in body fluid homeostasis and the effects of acute and chronic hyperosmotic challenges.

Role of angiotensin-converting enzyme 1 within the median preoptic nucleus following chronic intermittent hypoxia

Sustained hypertension is an important consequence of obstructive sleep apnea. An animal model of the hypoxemia associated with sleep apnea, chronic intermittent hypoxia (CIH), produces increased sympathetic nerve activity (SNA) and sustained increases in blood pressure. Many mechanisms have been implicated in the hypertension associated with CIH, including the role of ΔFosB within the median preoptic nucleus (MnPO). Also, the renin-angiotensin system (RAS) has been associated with CIH hypertension. We conducted experiments to determine the possible association of FosB/ΔFosB with a RAS component, angiotensin-converting enzyme 1 (ACE1), within the MnPO following 7 days of CIH. Retrograde tract tracing from the paraventricular nucleus (PVN), a downstream region of the MnPO, was used to establish a potential pathway for FosB/ΔFosB activation of MnPO ACE1 neurons. After CIH, ACE1 cells with FosB/ΔFosB expression increased colocalization with a retrograde tracer that was injected unilaterally within the PVN. Also, Western blot examination showed ACE1 protein expression increasing within the MnPO following CIH. Chromatin immunoprecipitation (ChIP) assays demonstrated an increase in FosB/ΔFosB association with the ACE1 gene within the MnPO following CIH. FosB/ΔFosB may transcriptionally target ACE1 within the MnPO following CIH to affect the downstream PVN region, which may influence SNA and blood pressure.

Role of the Vasopressin/Apelin Balance and Potential Use of Metabolically Stable Apelin Analogs in Water Metabolism Disorders

Apelin, a (neuro)vasoactive peptide, plays a prominent role in controlling body fluid homeostasis and cardiovascular functions. In animal models, experimental data demonstrate that intracerebroventricular injection of apelin into lactating rats inhibits the phasic electrical activity of arginine vasopressin (AVP) neurons, reduces plasma AVP levels, and increases aqueous diuresis. In the kidney, apelin increases diuresis by increasing the renal microcirculation and by counteracting the antidiuretic effect of AVP at the tubular level. Moreover, after water deprivation or salt loading, in humans and in rodents, AVP and apelin are conversely regulated to facilitate systemic AVP release and to avoid additional water loss from the kidney. Furthermore, apelin and vasopressin secretion are significantly altered in various water metabolism disorders including hyponatremia and polyuria-polydipsia syndrome. Since the in vivo half-life of apelin is in the minute range, metabolically stable apelin analogs were developed. The efficacy of these lead compounds for decreasing AVP release and increasing both renal blood flow and diuresis, make them promising candidates for the treatment of water retention and/or hyponatremic disorders.

Increased mitochondrial superoxide in the brain, but not periphery, sensitizes mice to angiotensin II-mediated hypertension

Angiotensin II (AngII) elicits the production of superoxide (O₂^•−) from mitochondria in numerous cell types within peripheral organs and in the brain suggesting a role for mitochondrial-produced O₂^•− in the pathogenesis of hypertension. However, it remains unclear if mitochondrial O₂^•− is causal in the development of AngII-induced hypertension, or if mitochondrial O₂^•− in the absence of elevated AngII is sufficient to increase blood pressure. Further, the tissue specific (i.e. central versus peripheral) redox regulation of AngII hypertension remains elusive. Herein, we hypothesized that increased mitochondrial O₂^•− in the absence of pro-hypertensive stimuli, such as AngII, elevates baseline systemic mean arterial pressure (MAP), and that AngII-mediated hypertension is exacerbated in animals with increased mitochondrial O₂^•− levels. To address this hypothesis, we generated novel inducible knock-down mouse models of manganese superoxide dismutase (MnSOD), the O₂^•− scavenging antioxidant enzyme specifically localized to mitochondria, targeted to either the brain subfornical organ (SFO) or peripheral tissues. Contrary to our hypothesis, knock-down of MnSOD either in the SFO or in peripheral tissues was not sufficient to alter baseline systemic MAP. Interestingly, when mice were challenged with chronic, peripheral infusion of AngII, only the MnSOD knock-down confined to the SFO, and not the periphery, demonstrated an increased sensitization and potentiated hypertension. In complementary experiments, over-expressing MnSOD in the SFO significantly decreased blood pressure in response to chronic AngII. Overall, these studies indicate that mitochondrial O₂^•− in the brain SFO works in concert with other AngII-dependent factors to drive an increase in MAP, as elevated mitochondrial O₂^•− alone, either in the SFO or peripheral tissues, failed to raise baseline blood pressure.

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Mitochondrial O₂^•− has been implicated as a primary contributor to hypertension.

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Novel models with altered MnSOD expression utilized to influence mitochondrial O₂^•−.

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Knock-down of MnSOD alone is not sufficient to alter systemic hemodynamics.

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Knock-down of MnSOD in the brain SFO, but not periphery, exacerbates hypertension.

Impact of dehydration on the forebrain preoptic recess walls in the mudskipper, Periophthalmus modestus: a possible locus for the center of thirst

The forebrain lamina terminalis has not yet been examined for the role of osmosensing in teleosts, although the thirst center is well known to be present in this vascular permeable forebrain region in mammals. Here, we examined vascular permeability and neuronal responsiveness to dehydration in the lamina terminalis of the mudskipper, a euryhaline goby. Evans blue and N-hydroxysulfosuccinimide-biotin both bind to blood proteins, and are impermeable to the blood-brain barrier. Intraperitoneal injection of these probes stained the walls of the preoptic recess (PR) of the third ventricle, indicating increased vascular permeability in this region. When mudskippers kept in isotonic brackish water (ca. 11 psu) were challenged to seawater (ca. 34 psu) for 3 h, body water content showed a 1 % decrease, compared with mudskippers without hypertonic challenge. Simultaneously, the number of immunohistochemically identified cFos-expressing neurons in the anterior parvocellular preoptic nucleus (PPa) of the PR walls increased in a site-specific manner by approximately 1.6-fold compared with controls. Thus, these findings indicate that PPa neurons are activated, following dehydration in mudskippers. Taken together, the vascularly permeable PR walls may be involved in osmosensing, as in the mammalian thirst center.

Acute hypothalamo-pituitary-adrenal axis response to LPS-induced endotoxemia: expression pattern of kinin type B1 and B2 receptors

Bradykinin (BK) and des-Arg9-BK are pro-inflammatory mediators acting via B2 (B2R) and B1 (B1R) receptors, respectively. We investigated the role of B2R and B1R in lipopolysaccharide (LPS)-induced hypothalamo-pituitary-adrenal (HPA) axis activation in SD rats. LPS given intraperitoneally (ip) up-regulated B1R mRNA in the hypothalamus, both B1R and B2R were up-regulated in pituitary and adrenal glands. Receptor localization was performed using immunofluorescence staining. B1R was localized in the endothelial cells, nucleus supraopticus (SON), adenohypophysis and adrenal cortex. B2R was localized nucleus paraventricularis (PVN) and SON, pituitary and adrenal medulla. Blockade of B1R prior to LPS further increased ACTH release and blockade of B1R 1 h after LPS decreased its release. In addition, we evaluated if blockade of central kinin receptors influence the LPS-induced stimulation of hypothalamic neurons. Blockade of both B1R and B2R reduced the LPS-induced c-Fos immunoreactivity in the hypothalamus. Our data demonstrate that a single injection of LPS induced a differential expression pattern of kinin B1R and B2R in the HPA axis. The tissue specific cellular localization of these receptors indicates that they may play a crucial role in the maintenance of body homeostasis during endotoxemia.

Anatomical organization of the rat organum vasculosum laminae terminalis

The organum vasculosum of the laminae terminalis (OVLT) is a circumventricular organ located along the ventral part of the anterior wall of the third ventricle. Because it lacks a complete blood-brain barrier (BBB), blood-borne signals detected in the OVLT provide the brain with information from the periphery and contribute to the generation of centrally mediated responses to humoral feedback and physiological stressors. Experimental studies on the rat OVLT are hindered by a poor understanding of its precise anatomical dimensions and cellular organization. In this study, we use histological techniques to characterize the spatial outline of the rat OVLT and to examine the location of neurons, astrocytes, tanycytes, and ependymocytes within its confines. Our data reveal that OVLT neurons are embedded in a dense network of tanycyte processes. Immunostaining against the neuronal marker NeuN revealed that neurons are distributed throughout the OVLT, except for a thick midline septum, which comprises densely packed cells of unknown function or lineage. Moreover, the most ventral aspect of the OVLT is devoid of neurons and is occupied by a dense network of glial cell processes that form a thick layer between the neurons and the pial surface on the ventral aspect of the nucleus. Lastly, combined detection of NeuN and c-Fos protein following systemic injection of hypertonic NaCl revealed that neurons responsive to this stimulus are located along the entire midline core of the OVLT, extending from its most anterior ventral aspect to the more caudally located "dorsal cap" region.

The biopsychology of salt hunger and sodium deficiency

Sodium is a necessary dietary macromineral that tended to be sparsely distributed in mankind's environment in the past. Evolutionary selection pressure shaped physiological mechanisms including hormonal systems and neural circuits that serve to promote sodium ingestion. Sodium deficiency triggers the activation of these hormonal systems and neural circuits to engage motivational processes that elicit a craving for salty substances and a state of reward when salty foods are consumed. Sodium deficiency also appears to be associated with aversive psychological states including anhedonia, impaired cognition, and fatigue. Under certain circumstances the psychological processes that promote salt intake can become powerful enough to cause "salt gluttony," or salt intake far in excess of physiological need. The present review discusses three aspects of the biopsychology of salt hunger and sodium deficiency: (1) the psychological processes that promote salt intake during sodium deficiency, (2) the effects of sodium deficiency on mood and cognition, and (3) the sensitization of sodium appetite as a possible cause of salt gluttony.

Activation of the hypothalamic paraventricular nucleus by forebrain hypertonicity selectively increases tonic vasomotor sympathetic nerve activity

We recently reported that mean arterial pressure (MAP) is maintained in water-deprived rats by an irregular tonic component of vasomotor sympathetic nerve activity (SNA) that is driven by neuronal activity in the hypothalamic paraventricular nucleus (PVN). To establish whether generation of tonic SNA requires time-dependent (i.e., hours or days of dehydration) neuroadaptive responses or can be abruptly generated by even acute circuit activation, forebrain sympathoexcitatory osmosensory inputs to PVN were stimulated by infusion (0.1 ml/min, 10 min) of hypertonic saline (HTS; 1.5 M NaCl) through an internal carotid artery (ICA). Whereas isotonic saline (ITS; 0.15 M NaCl) had no effect (n = 5), HTS increased (P < 0.001; n = 6) splanchnic SNA (sSNA), phrenic nerve activity (PNA), and MAP. Bilateral PVN injections of muscimol (n = 6) prevented HTS-evoked increases of integrated sSNA and PNA (P < 0.001) and attenuated the accompanying pressor response (P < 0.01). Blockade of PVN NMDA receptors with d-(2R)-amino-5-phosphonovaleric acid (AP5; n = 6) had similar effects. Analysis of respiratory rhythmic bursting of sSNA revealed that ICA HTS increased mean voltage (P < 0.001) without affecting the amplitude of inspiratory or expiratory bursts. Analysis of cardiac rhythmic sSNA likewise revealed that ICA HTS increased mean voltage. Cardiac rhythmic sSNA oscillation amplitude was also increased, which is consistent with activation of arterial baroreceptor during the accompanying pressor response. Increased mean sSNA voltage by HTS was blocked by prior PVN inhibition (muscimol) and blockade of PVN NMDA receptors (AP5). We conclude that even acute glutamatergic activation of PVN (i.e., by hypertonicity) is sufficient to selectively increase a tonic component of vasomotor SNA.

Median preoptic nucleus mediates the cardiovascular recovery induced by hypertonic saline in hemorrhagic shock

Changes in plasma osmolarity, through central and peripheral osmoreceptors, activate the median preoptic nucleus (MnPO) that modulates autonomic and neuroendocrine adjustments. The present study sought to determine the participation of MnPO in the cardiovascular recovery induced by hypertonic saline infusion (HSI) in rats submitted to hemorrhagic shock. The recordings of mean arterial pressure (MAP) and renal vascular conductance (RVC) were carried out on male Wistar rats (250–300 g). Hemorrhagic shock was induced by blood withdrawal over 20 min until the MAP values of approximately 60 mmHg were attained. The nanoinjection (100 nL) of GABA_A agonist (Muscimol 4 mM; experimental group (EXP)) or isotonic saline (NaCl 150 mM; control (CONT)) into MnPO was performed 2 min prior to intravenous overload of sodium through HSI (3 M NaCl, 1.8 mL/kg, b.wt.). Hemorrhagic shock reduced the MAP in control (62 ± 1.1 mmHg) and EXP (61 ± 0.4 mmHg) equipotently. The inhibition of MnPO impaired MAP (CONT: 104 ± 4.2 versus EXP: 60 ± 6.2 mmHg) and RVC (CONT: 6.4 ± 11.4 versus EXP: -53.5 ± 10.0) recovery 10 min after HSI. The overall results in this study demonstrated, for the first time, that the MnPO plays an essential role in the HSI induced resuscitation during hypovolemic hemorrhagic shock.

Allosteric modulation of β-arrestin-biased angiotensin II type 1 receptor signaling by membrane stretch

It has recently been appreciated that the angiotensin II type 1 receptor (AT1R), a prototypic member of the G protein-coupled receptor superfamily, also functions as a mechanosensor. Specifically, mechanical stretch activates the AT1R to promote downstream signaling mediated exclusively by the multifunctional scaffold protein, β-arrestin, in a manner consistent with previously identified β-arrestin-biased ligands. However, the ligand-independent mechanism by which mechanical stretch promotes β-arrestin-biased signaling remains unknown. Implicit in the concept of biased agonism (i.e. the ability of an agonist to activate a subset of receptor-mediated signaling pathways) is the notion that distinct active conformations of the receptor mediate differential activation of signaling pathways. Here we determined whether mechanical stretch stabilizes distinct β-arrestin-activating conformations of the AT1R by using β-arrestin2-biased agonists as conformational probes in pharmacological and biophysical assays. When tested at cells expressing the AT1R fused to β-arrestin (AT1R-β-arrestin2), we found that osmotic stretch increased the binding affinity and potency of the β-arrestin-biased agonist TRV120023, with no effect on the balanced agonist AngII. In addition, the effect of osmotic stretch on ERK activation was markedly augmented in cells expressing the AT1R-β-arrestin2 fusion compared with the wild type AT1R and completely blocked in cells expressing the AT1R-Gq fusion. Biophysical experiments with an intramolecular BRET β-arrestin2 biosensor revealed that osmotic stretch and TRV120023 activate AT1Rs to stabilize β-arrestin2 active conformations that differ from those stabilized by the AT1R activated by angiotensin II. Together, these data support a novel ligand-independent mechanism whereby mechanical stretch allosterically stabilizes specific β-arrestin-biased active conformations of the AT1R and has important implications for understanding pathophysiological AT1R signaling.

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.

The role of the hypothalamic paraventricular nucleus and the organum vasculosum lateral terminalis in the control of sodium appetite in male rats

Angiotensin II (AngII) and aldosterone cooperate centrally to produce a robust sodium appetite. The intracellular signaling and circuitry that underlie this interaction remain unspecified. Male rats pretreated with both deoxycorticosterone (DOC; a synthetic precursor of aldosterone) and central AngII exhibited a marked sodium intake, as classically described. Disruption of inositol trisphosphate signaling, but not extracellular-regulated receptor kinase 1 and 2 signaling, prevented the cooperativity of DOC and AngII on sodium intake. The pattern of expression of the immediate early gene product cFos was used to identify key brain regions that may underlie this behavior. In the paraventricular nuclei (PVN) of the hypothalamus, DOC pretreatment diminished both AngII-induced cFos induction and neurosecretion of oxytocin, a peptide expressed in the PVN. Conversely, in the organum vasculosum lateral terminalis (OVLT), DOC pretreatment augmented cFos expression. Immunohistochemistry identified a substantial presence of oxytocin fibers in the OVLT. In addition, when action potentials in the PVN were inhibited with intraparenchymal lidocaine, AngII-induced sodium ingestion was exaggerated. Intriguingly, this treatment also increased the number of neurons in the OVLT expressing AngII-induced cFos. Collectively, these results suggest that the behavioral cooperativity between DOC and AngII involves the alleviation of an inhibitory oxytocin signal, possibly relayed directly from the PVN to the OVLT.

Activation of the renin-angiotensin system, specifically in the subfornical organ is sufficient to induce fluid intake

Increased activity of the renin-angiotensin system within the brain elevates fluid intake, blood pressure, and resting metabolic rate. Renin and angiotensinogen are coexpressed within the same cells of the subfornical organ, and the production and action of ANG II through the ANG II type 1 receptor in the subfornical organ (SFO) are necessary for fluid intake due to increased activity of the brain renin-angiotensin system. We generated an inducible model of ANG II production by breeding transgenic mice expressing human renin in neurons controlled by the synapsin promoter with transgenic mice containing a Cre-recombinase-inducible human angiotensinogen construct. Adenoviral delivery of Cre-recombinase causes SFO-selective induction of human angiotensinogen expression. Selective production of ANG II in the SFO results in increased water intake but did not change blood pressure or resting metabolic rate. The increase in water intake was ANG II type 1 receptor-dependent. When given a choice between water and 0.15 M NaCl, these mice increased total fluid and sodium, but not water, because of an increased preference for NaCl. When provided a choice between water and 0.3 M NaCl, the mice exhibited increased fluid, water, and sodium intake, but no change in preference for NaCl. The increase in fluid intake was blocked by an inhibitor of PKC, but not ERK, and was correlated with increased phosphorylated cyclic AMP response element binding protein in the subfornical organ. Thus, increased production and action of ANG II specifically in the subfornical organ are sufficient on their own to mediate an increase in drinking through PKC.

Opiorphin increases blood pressure of conscious rats through renin-angiotensin system (RAS)

Human opiorphin is a recently identified endogenous pentapeptide, encoded by ProL1 multigenes family that contributes to cardiovascular modulation. The aim of this study was to evaluate the effect of opiorphin through intravenous injection (i.v.) on mean arterial pressure (MAP) regulation. To investigate the bioactivity of opiorphin, a rat cannulation model was developed for MAP measurement and blood sampling. In our present study, opiorphin (200-700 nmol/kg) increased MAP in dose-related and time-dependent manner in conscious rats, which associated highly with the elevation of angiotensin II (AngII) levels in serum. Furthermore, the MAP elevation induced by opiorphin was completely blocked by AngII receptor antagonist valsartan and partially attenuated by angiotensin-converting enzyme inhibitor captopril. Finally, we tested the effect of opiorphin in hypoxia condition, which exhibited that opiorphin reversed hypoxia induced hypotension in conscious rats. Taken together, these results indicated that opiorphin may play an important role in the modulation of blood pressure through AngII dependent pathway, which may help future development of potent clinical therapeutics for emergency treatment.

Angiotensinergic and cholinergic receptors of the subfornical organ mediate sodium intake induced by GABAergic activation of the lateral parabrachial nucleus

Bilateral injections of the GABA(A) agonist muscimol into the lateral parabrachial nucleus (LPBN) induce 0.3 M NaCl and water intake in satiated and normovolemic rats, a response reduced by intracerebroventricular (icv) administration of losartan or atropine (angiotensinergic type 1 (AT₁) and cholinergic muscarinic receptor antagonists, respectively). In the present study, we investigated the effects of the injections of losartan or atropine into the subfornical organ (SFO) on 0.3M NaCl and water intake induced by injections of muscimol into the LPBN. In addition, using intracellular calcium measurement, we also tested the sensitivity of SFO-cultured cells to angiotensin II (ANG II) and carbachol (cholinergic agonist). In male Holtzman rats with cannulas implanted bilaterally into the LPBN and into the SFO, injections of losartan (1 μg/0.1 μl) or atropine (2 nmol/0.1 μl) into the SFO almost abolished 0.3M NaCl and water intake induced by muscimol (0.5 nmol/0.2 μl) injected into the LPBN. In about 30% of the cultured cells of the SFO, carbachol and ANG II increased intracellular calcium concentration ([Ca²⁺](i)). Three distinct cell populations were found in the SFO, i.e., cells activated by either ANG II (25%) or carbachol (2.6%) or by both stimuli (2.3%). The results suggest that the activation of angiotensinergic and cholinergic mechanisms in the SFO is important for NaCl and water intake induced by the deactivation of LPBN inhibitory mechanisms with muscimol injections. They also show that there are cells in the SFO activated by both angiotensinergic and cholinergic stimuli, perhaps those involved in the responses to muscimol into the LPBN.

Median preoptic nucleus and subfornical organ drive renal sympathetic nerve activity via a glutamatergic mechanism within the paraventricular nucleus

The paraventricular nucleus (PVN) of the hypothalamus is involved in the neural control of sympathetic drive, but the precise mechanism(s) that influences the PVN is not known. The activation of the PVN may be influenced by input from higher forebrain areas, such as the median preoptic nucleus (MnPO) and the subfornical organ (SFO). We hypothesized that activation of the MnPO or SFO would drive the PVN through a glutamatergic pathway. Neuroanatomical connections were confirmed by the recovery of a retrograde tracer in the MnPO and SFO that was injected bilaterally into the PVN in rats. Microinjection of 200 pmol of N-methyl-d-aspartate (NMDA) or bicuculline-induced activation of the MnPO and increased renal sympathetic activity (RSNA), mean arterial pressure, and heart rate in anesthetized rats. These responses were attenuated by prior microinjection of a glutamate receptor blocker AP5 (4 nmol) into the PVN (NMDA - ΔRSNA 72 ± 8% vs. 5 ± 1%; P < 0.05). Using single-unit extracellular recording, we examined the effect of NMDA microinjection (200 pmol) into the MnPO on the firing activity of PVN neurons. Of the 11 active neurons in the PVN, 6 neurons were excited by 95 ± 17% (P < 0.05), 1 was inhibited by 57%, and 4 did not respond. The increased RSNA after activation of the SFO by ANG II (1 nmol) or bicuculline (200 pmol) was also reduced by AP5 in the PVN (for ANG II - ΔRSNA 46 ± 7% vs. 17 ± 4%; P < 0.05). Prior microinjection of ANG II type 1 receptor blocker losartan (4 nmol) into the PVN did not change the response to ANG II or bicuculline microinjection into the SFO. The results from this study demonstrate that the sympathoexcitation mediated by a glutamatergic mechanism in the PVN is partially driven by the activation of the MnPO or SFO.

Hypertonicity sensing in organum vasculosum lamina terminalis neurons: a mechanical process involving TRPV1 but not TRPV4

Primary osmosensory neurons in the mouse organum vasculosum lamina terminalis (OVLT) transduce hypertonicity via the activation of nonselective cation channels that cause membrane depolarization and increased action potential discharge, and this effect is absent in mice lacking expression of the transient receptor potential vanilloid 1 (Trpv1) gene (Ciura and Bourque, 2006). However other experiments have indicated that channels encoded by Trpv4 also contribute to central osmosensation in mice (Liedtke and Friedman, 2003; Mizuno et al., 2003). At present, the mechanism by which hypertonicity modulates cation channels in OVLT neurons is unknown, and it remains unclear whether Trpv1 and Trpv4 both contribute to this process. Here, we show that physical shrinking is necessary and sufficient to mediate hypertonicity sensing in OVLT neurons isolated from adult mice. Steps coupling progressive decreases in cell volume to increased neuronal activity were quantitatively equivalent whether shrinking was evoked by osmotic pressure or mechanical aspiration. Finally, modulation of OVLT neurons by tonicity or mechanical stimulation was unaffected by deletion of trpv4 but was abolished in cells lacking Trpv1 or wild-type neurons treated with the TRPV1 antagonist SB366791. Thus, hypertonicity sensing is a mechanical process requiring Trpv1, but not Trpv4.

Normal brain anatomy on magnetic resonance imaging

The exquisite detail provided by brain magnetic resonance imaging scans can make interpretation simultaneously straightforward and complicated, particularly to the novice. For this reason, it is essential to become familiar with normal structures before describing the pathologic condition. This article serves as a practical reference point to further enhance knowledge of the intracranial anatomy.

The role of the brain renin-angiotensin system in hypertension: implications for new treatment

Hypertension affects 26% of adults and is in constant progress related to increased incidence of obesity and diabetes. One-third of hypertensive patients may be successfully treated with one antihypertensive agent, one-third may require two agents and in the remaining patients will need three agents for effective blood pressure (BP) control. The development of new classes of antihypertensive agents with different mechanisms of action therefore remains an important goal. Brain renin-angiotensin system (RAS) hyperactivity has been implicated in hypertension development and maintenance in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III have similar affinities for type 1 (AT1) and type 2 (AT2) Ang II receptors. Following intracerebroventricular (i.c.v.) injection, Ang II and Ang III similarly increase arginine-vasopressin (AVP) release and BP. Blocking the brain RAS may be advantageous as it simultaneously (1) decreases sympathetic tone and consequently vascular resistance, (2) decreases AVP release, reducing blood volume and vascular resistance and (3) blocks angiotensin-induced baroreflex inhibition, decreasing both vascular resistance and cardiac output. However, as Ang II is converted to Ang III in vivo, the exact nature of the active peptide is not precisely determined. We summarize here the main findings identifying AngIII as one of the major effector peptides of the brain RAS in the control of AVP release and BP. Brain AngIII exerts a tonic stimulatory effect on BP in hypertensive rats, identifying brain aminopeptidase A (APA), the enzyme generating brain Ang III, as a potentially candidate target for hypertension treatment. This has led to the development of potent orally active APA inhibitors, such as RB150--the prototype of a new class of centrally acting antihypertensive agents.

An indispensable role of secretin in mediating the osmoregulatory functions of angiotensin II

Fluid balance is critical to life and hence is tightly controlled in the body. Angiotensin II (ANGII), one of the most important components of this regulatory system, is recognized as a dipsogenic hormone that stimulates vasopressin (VP) expression and release. However, detailed mechanisms regarding how ANGII brings about these changes are not fully understood. In the present study, we show initially that the osmoregulatory functions of secretin (SCT) in the brain are similar to those of ANGII in mice and, more important, we discovered the role of SCT as the link between ANGII and its downstream effects. This was substantiated by the use of two knockout mice, SCTR(-/-) and SCT(-/-), in which we show the absence of an intact SCT/secretin receptor (SCTR) axis resulted in an abolishment or much reduced ANGII osmoregulatory functions. By immunohistochemical staining and in situ hybridization, the proteins and transcripts of SCT and its receptor are found in the paraventricular nucleus (PVN) and lamina terminalis. We propose that SCT produced in the circumventricular organs is transported and released in the PVN to stimulate vasopressin expression and release. In summary, our findings identify SCT and SCTR as novel elements of the ANGII osmoregulatory pathway in maintaining fluid balance in the body.

Hyperosmotic activation of CNS sympathetic drive: implications for cardiovascular disease

Evidence now indicates that exaggerated sympathetic nerve activity (SNA) significantly contributes to salt-sensitive cardiovascular diseases. Although CNS mechanisms that support the elevation of SNA in various cardiovascular disease models have been intensively studied, many mechanistic details remain unknown. In recent years, studies have shown that SNA can rise as a result of both acute and chronic increases of body fluid osmolality. These findings have raised the possibility that salt-sensitive cardiovascular diseases could result, at least in part, from direct osmosensory activation of CNS sympathetic drive. In this brief review we emphasize recent findings from several laboratories, including our own, which demonstrate that neurons of the forebrain organum vasculosum laminae terminalis (OVLT) play a pivotal role in triggering hyperosmotic activation of SNA by recruiting neurons in specific regions of the hypothalamus, brainstem and spinal cord. Although OVLT neurons are intrinsically osmosensitive and shrink when exposed to extracellular hypertonicity, it is not yet clear if these processes are functionally linked. Whereas acute hypertonic activation of OVLT neurons critically depends on TRPV1 channels, studies in TRPV1(-/-) mice suggest that acute and long-term osmoregulatory responses remain largely intact. Therefore, acute and chronic osmosensory transduction by OVLT neurons may be mediated by distinct mechanisms. We speculate that organic osmolytes such as taurine and possibly novel processes such as extracellular acidification could contribute to long-term osmosensory transduction by OVLT neurons and might therefore participate in the elevation of SNA in salt-sensitive cardiovascular diseases.

Increased hypothalamic signal transducer and activator of transcription 3 phosphorylation after hindbrain leptin injection

Reduction of food intake and body weight by leptin is attributed largely to its action in the hypothalamus. However, the signaling splice variant of the leptin receptor, LRb, also is expressed in the hindbrain, and leptin injections into the fourth cerebral ventricle or dorsal vagal complex are associated with reductions of feeding and body weight comparable to those induced by forebrain leptin administration. Although these observations suggest direct hindbrain action of leptin on feeding and body weight, the possibility that hindbrain leptin administration also activates the Janus kinase/signal transducer and activator of transcription 3 (STAT3) signaling in the hypothalamus has not been investigated. Confirming earlier work, we found that leptin produced comparable reductions of feeding and body weight when injected into the lateral ventricle or the fourth ventricle. We also found that lateral and fourth ventricle leptin injections produced comparable increases of STAT3 phosphorylation in both the hindbrain and the hypothalamus. Moreover, injection of 50 ng of leptin directly into the nucleus of the solitary tract also increased STAT3 phosphorylation in the hypothalamic arcuate and ventromedial nuclei. Increased hypothalamic STAT3 phosphorylation was not due to elevation of blood leptin concentrations and the pattern of STAT3 phosphorylation did not overlap distribution of the retrograde tracer, fluorogold, injected via the same cannula. Our observations indicate that even small leptin doses administered to the hindbrain can trigger leptin-related signaling in the forebrain, and raise the possibility that STAT3 phosphorylation in the hypothalamus may contribute to behavioral and metabolic changes observed after hindbrain leptin injections.

Centrally administered ghrelin potently inhibits water intake induced by angiotensin II and hypovolemia in rats

Ghrelin is a potent, centrally acting orexigenic hormone. Recently, we showed that centrally administered ghrelin is a potent antidipsogenic hormone in 24-h water deprived rats. In this study, we examined the effect of intracerebroventricular (icv) injection of ghrelin on angiotensin II (AII)-induced water intake in rats. We also examined the effects of icv injection of ghrelin on drinking induced by intraperitoneal injection of an isotonic polyethylene glycol (PEG) solution that causes isotonic hypovolemia. Water intake induced by the icv injection of AII or ip injection of PEG was significantly reduced after icv injection of ghrelin, although food intake was stimulated by the hormone. The drinking induced by AII was also inhibited by the icv administration of 4alpha-phorbol 12, 13-didecanoate, an agonist of the osmosensitive TRPV4 channel. This study showed that ghrelin is a potent antidipsogenic peptide by antagonizing general dipsogenic mechanisms including those activated by AII and hypovolemia in rats.

Metabotropic glutamate receptors in median preoptic neurons modulate neuronal excitability and glutamatergic and GABAergic inputs from the subfornical organ

Cardiovascular and behavioral responses to circulating angiotensin require intact connectivity along the upper lamina terminalis joining the subfornical organ (SFO) with the median preoptic nucleus (MnPO). In the present study on MnPO neurons, we used whole cell patch-clamp recording techniques in brain slice preparations to evaluate the influence of metabotropic glutamate receptor (mGluR) agonists on modulating their intrinsic excitability and SFO-evoked glutamatergic and GABAergic postsynaptic currents. In 22/36 cells, bath application of a mGluR group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced a TTX-resistant inward current coupled with decrease in a membrane K(+) conductance but also a possible increase in a nonselective cationic conductance. By contrast, 27/49 cells responded to a mGluR group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) with a TTX-resistant outward current and increase in membrane conductance that reversed around -95 mV, suggesting opening of K(+) channels. None of 19 cells responded to the mGluR group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4). Agonists for all mGluR groups suppressed SFO-evoked excitatory postsynaptic currents and significantly increased paired-pulse ratios, implying a presynaptic mechanism. Only the mGluR group II agonist significantly reduced SFO-evoked inhibitory postsynaptic currents and caused an increase in paired-pulse ratios. These results suggest a complexity of pre- and postsynaptic mGluRs are available to modulate rapid neurotransmission along the upper lamina terminalis from SFO to MnPO.