Brain sodium sensing for regulation of thirst, salt appetite, and blood pressure.

The brain possesses intricate mechanisms for monitoring sodium (Na) levels in body fluids. During prolonged dehydration, the brain detects variations in body fluids and produces sensations of thirst and aversions to salty tastes. At the core of these processes Nax , the brain's Na sensor, exists. Specialized neural nuclei, namely the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), which lack the blood-brain barrier, play pivotal roles. Within the glia enveloping the neurons in these regions, Nax collaborates with Na+ /K+ -ATPase and glycolytic enzymes to drive glycolysis in response to elevated Na levels. Lactate released from these glia cells activates nearby inhibitory neurons. The SFO hosts distinct types of angiotensin II-sensitive neurons encoding thirst and salt appetite, respectively. During dehydration, Nax -activated inhibitory neurons suppress salt-appetite neuron's activity, whereas salt deficiency reduces thirst neuron's activity through cholecystokinin. Prolonged dehydration increases the Na sensitivity of Nax via increased endothelin expression in the SFO. So far, patients with essential hypernatremia have been reported to lose thirst and antidiuretic hormone release due to Nax -targeting autoantibodies. Inflammation in the SFO underlies the symptoms. Furthermore, Nax activation in the OVLT, driven by Na retention, stimulates the sympathetic nervous system via acid-sensing ion channels, contributing to a blood pressure elevation.

Synaptic control of rat magnocellular neurosecretory cells by warm-sensing neurons in the organum vasculosum lamina terminalis.

Increases in core body temperature cause secretion of vasopressin (vasopressin, antidiuretic hormone) to promote water reabsorption and blunt water losses incurred through homeostatic evaporative cooling. Subtypes of transient receptor potential vanilloid (Trpv) channels have been shown to contribute to the intrinsic regulation of vasopressin-releasing magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). However, MNCs in vivo can also be excited by local heating of the adjacent preoptic area, indicating they receive thermosensory information from other areas. Here, we investigated whether neurons in the organum vasculosum lamina terminalis (OVLT) contribute to this process using in vitro electrophysiological approaches in male rats. We found that the majority of OVLT neurons are thermosensitive in the physiological range (36-39°C) and that this property is retained under conditions blocking synaptic transmission. A subset of these neurons could be antidromically activated by electrical stimulation in the SON. Whole cell recordings from SON MNCs revealed that heating significantly increases the rate of spontaneous excitatory postsynaptic currents (sEPCSs), and that this response is abolished by lesions targeting the OVLT, but not by bilateral lesions placed in the adjacent preoptic area. Finally, local heating of the OVLT caused a significant excitation of MNCs in the absence of temperature changes in the SON, and this effect was blocked by inhibitors of ionotropic glutamate receptors. These findings indicate that the OVLT serves as an important thermosensory nucleus and contributes to the activation of MNCs during physiological heating.

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.

Time to drink: Activating lateral hypothalamic area neurotensin neurons promotes intake of fluid over food in a time-dependent manner.

The lateral hypothalamic area (LHA) is essential for ingestive behavior but has primarily been studied in modulating feeding, with comparatively scant attention on drinking. This is partly because most LHA neurons simultaneously promote feeding and drinking, suggesting that ingestive behaviors track together. A notable exception are LHA neurons expressing neurotensin (LHANts neurons): activating these neurons promotes water intake but modestly restrains feeding. Here we investigated the connectivity of LHANts neurons, their necessity and sufficiency for drinking and feeding, and how timing and resource availability influence their modulation of these behaviors. LHANts neurons project broadly throughout the brain, including to the lateral preoptic area (LPO), a brain region implicated in modulating drinking behavior. LHANts neurons also receive inputs from brain regions implicated in sensing hydration and energy status. While activation of LHANts neurons is not required to maintain homeostatic water or food intake, it selectively promotes drinking during the light cycle, when ingestive drive is low. Activating LHANts neurons during this period also increases willingness to work for water or palatable fluids, regardless of their caloric content. By contrast, LHANts neuronal activation during the dark cycle does not promote drinking, but suppresses feeding during this time. Finally, we demonstrate that the activation of the LHANts â†’ LPO projection is sufficient to mediate drinking behavior, but does not suppress feeding as observed after generally activating all LHANts neurons. Overall, our work suggests how and when LHANts neurons oppositely modulate ingestive behaviors.

DOCA-salt hypertension and the role of the OVLT-sympathetic-gut microbiome axis.

Hypertension is a multifaceted condition influenced by genetic and environmental factors and estimated to cause 9.4 million deaths globally every year. Recently, there has been growing interest in understanding the gut microbe-host interaction in the maintenance of health or disease states, but relatively few studies have shown an association between the gut microbiome and specific types of hypertension. The deoxycorticosterone acetate (DOCA)-salt model of hypertension in rats is known to have a neurogenic component linked to increased sympathetic nervous system activity. As such, our lab has recently shown the hypertensive response in DOCA treated rats requires an intact organum vasculosum of the lamina terminalis (OVLT), a central hypothalamic circumventricular organ. Currently, we hypothesize the OVLT mediates changes in the gut microbiome associated with concomitant hypertension. Herein, we report that the hypertensive effects of DOCA-salt treatment were significantly attenuated throughout the 24-hour day/night cycle in OLVT lesioned rats on days 1, 3, and 9-21 of DOCA treatment compared with sham rats. Increased blood pressure (BP) in DOCA-salt treated rats was accompanied by specific changes in regional gut microbial populations yet was mitigated and offset by lesion of the OVLT. Furthermore, bacterial populations in OVLT-lesioned rats with attenuated hypertension more closely resembled those in normal control rats. We conclude that DOCA-salt hypertension is associated with specific microbiome changes in the gut, and the attenuated hypertensive effects of DOCA-salt in OVLT-lesioned rats is mediated in part through counteracting changes in these bacterial populations.

SLC9A4 in the organum vasculosum of the lamina terminalis is a [Na+] sensor for the control of water intake.

Nax is a brain [Na+] sensor expressed in the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT) in the brain. We previously demonstrated that Nax signals are involved in the control of water intake behavior through the Nax/TRPV4 pathway. Nax gene knockout mice showed significantly attenuated water intake after an intracerebroventricular (ICV) injection of a hypertonic NaCl solution; however, the induction of a certain amount of water intake still remained, suggesting that another unknown [Na+]-dependent pathway besides the Nax/TRPV4 pathway contributes to water intake. In the present study, we screened for novel [Na+] sensors involved in water intake control and identified SLC9A4 (also called sodium (Na+)/hydrogen (H+) exchanger 4 (NHE4)). SLC9A4 is expressed in angiotensin II (Ang II) receptor type 1a (AT1a)-positive neurons in the OVLT. Sodium-imaging experiments using cultured cells transfected with slc9a4 revealed that SLC9A4 was activated by increases in extracellular [Na+] ([Na+]o), but not osmolality. Moreover, the firing activity of SLC9A4-positive neurons was enhanced by increases in [Na+]o and Ang II. slc9a4 knockdown in the OVLT reduced water intake induced by increases in [Na+], but not osmolality, in the cerebrospinal fluid. ICV injection experiments of a specific inhibitor suggested that the increase in extracellular [H+] caused by SLC9A4 activation next stimulates acid-sensing channel 1a (AS1C1a) to induce water intake. Our results thus indicate that SLC9A4 in the OVLT functions as a [Na+] sensor for the control of water intake and that the SLC9A4 signal is independent of the Nax/TRPV4 pathway.

Keeping time in the lamina terminalis: Novel oscillator properties of forebrain sensory circumventricular organs.

Drinking behavior and osmotic regulatory mechanisms exhibit clear daily variation which is necessary for achieving the homeostatic osmolality. In mammals, the master clock in the brain's suprachiasmatic nuclei has long been held as the main driver of circadian (24 h) rhythms in physiology and behavior. However, rhythmic clock gene expression in other brain sites raises the possibility of local circadian control of neural activity and function. The subfornical organ (SFO) and the organum vasculosum laminae terminalis (OVLT) are two sensory circumventricular organs (sCVOs) that play key roles in the central control of thirst and water homeostasis, but the extent to which they are subject to intrinsic circadian control remains undefined. Using a combination of ex vivo bioluminescence and in vivo gene expression, we report for the first time that the SFO contains an unexpectedly robust autonomous clock with unusual spatiotemporal characteristics in core and noncore clock gene expression. Furthermore, putative single-cell oscillators in the SFO and OVLT are strongly rhythmic and require action potential-dependent communication to maintain synchrony. Our results reveal that these thirst-controlling sCVOs possess intrinsic circadian timekeeping properties and raise the possibility that these contribute to daily regulation of drinking behavior.

Distribution of GnRH and Kisspeptin Immunoreactivity in the Female Llama Hypothalamus.

Llamas are induced non-reflex ovulators, which ovulate in response to the hormonal stimulus of the male protein beta-nerve growth factor (ß-NGF) that is present in the seminal plasma; this response is dependent on the preovulatory gonadotrophin-releasing hormone (GnRH) release from the hypothalamus. GnRH neurones are vital for reproduction, as these provide the input that controls the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland. However, in spontaneous ovulators, the activity of GnRH cells is regulated by kisspeptin neurones that relay the oestrogen signal arising from the periphery. Here, we investigated the organisation of GnRH and kisspeptin systems in the hypothalamus of receptive adult female llamas. We found that GnRH cells exhibiting different shapes were distributed throughout the ventral forebrain and some of these were located in proximity to blood vessels; sections of the mediobasal hypothalamus (MBH) displayed the highest number of cells. GnRH fibres were observed in both the organum vasculosum laminae terminalis (OVLT) and median eminence (ME). We also detected abundant kisspeptin fibres in the MBH and ME; kisspeptin cells were found in the arcuate nucleus (ARC), but not in rostral areas of the hypothalamus. Quantitative analysis of GnRH and kisspeptin fibres in the ME revealed a higher innervation density of kisspeptin than of GnRH fibres. The physiological significance of the anatomical findings reported here for the ovulatory mechanism in llamas is still to be determined.

[Na+] Increases in Body Fluids Sensed by Central Nax Induce Sympathetically Mediated Blood Pressure Elevations via H+-Dependent Activation of ASIC1a.

Increases in sodium concentrations ([Na+]) in body fluids elevate blood pressure (BP) by enhancing sympathetic nerve activity (SNA). However, the mechanisms by which information on increased [Na+] is translated to SNA have not yet been elucidated. We herein reveal that sympathetic activation leading to BP increases is not induced by mandatory high salt intakes or the intraperitoneal/intracerebroventricular infusions of hypertonic NaCl solutions in Nax-knockout mice in contrast to wild-type mice. We identify Nax channels expressed in specific glial cells in the organum vasculosum lamina terminalis (OVLT) as the sensors detecting increases in [Na+] in body fluids and show that OVLT neurons projecting to the paraventricular nucleus (PVN) are activated via acid-sensing ion channel 1a (ASIC1a) by H+ ions exported from Nax-positive glial cells. The present results provide an insight into the neurogenic mechanisms responsible for salt-induced BP elevations.

The Forebrain Thirst Circuit Drives Drinking through Negative Reinforcement.

The brain transforms the need for water into the desire to drink, but how this transformation is performed remains unknown. Here we describe the motivational mechanism by which the forebrain thirst circuit drives drinking. We show that thirst-promoting subfornical organ neurons are negatively reinforcing and that this negative-valence signal is transmitted along projections to the organum vasculosum of the lamina terminalis (OVLT) and median preoptic nucleus (MnPO). We then identify molecularly defined cell types within the OVLT and MnPO that are activated by fluid imbalance and show that stimulation of these neurons is sufficient to drive drinking, cardiovascular responses, and negative reinforcement. Finally, we demonstrate that the thirst signal exits these regions through at least three parallel pathways and show that these projections dissociate the cardiovascular and behavioral responses to fluid imbalance. These findings reveal a distributed thirst circuit that motivates drinking by the common mechanism of drive reduction.

Topological and histological description of preoptic area and hypothalamus in cardinal tetra Paracheirodon axelrodi (Characiformes: Characidae)

Topological and histological descriptions of the preoptic area and hypothalamus of the cardinal tetra Paracheirodon axelrodi were performed. Standard histological paraffin sections were used and stained with Nissl technique, and plastic sections for high-resolution optic microscopy (HROM). The preoptic area showed some differences related to the location of the magnocellular preoptic nucleus (PM) and the size of the neurons in this region, as they were the biggest in all the preoptic area. Additionally, within the preoptic area, the different structures that comprise the organum vasculosum of the lamina terminalis (OVLT) were identified and characterized. The hypothalamus could be subdivided in three regions - the ventral, the dorsal and the caudal hypothalamic regions - neuron morphology, size and staining pattern were similar in all of them.(AU)

ESUMEN Se realizó la descripción topológica e histológica del área preóptica e hipotálamo en el Neón cardenal Paracheirodon axelrodi. Se usaron cortes obtenidos con tecnicas histológicas estándar, coloreados con técnica de Nissl y secciones en resina con microscopía óptica de alta resolución (MOAR). El área preóptica muestra algunas diferencias relacionadas con la localización del núcleo preóptico magnocelular (PM) y el tamaño de algunas neuronas en esta región, puesto que estas eran las más grandes de toda el área preoptica. Adicionalmente, dentro del área preóptica, fue posible identificar y caracterizar las diferentes estructuras que componen el órgano vasculoso de la lámina media (OVLM). El hipotálamo puede sudividirse en tres zonas: la zona hipotalámica ventral, la zona hipotalámica dorsal y la zona hipotalámica caudal. La morfología de las neuronas de los núcleos que comprenden las diferentes zonas del hipotálamo tiene tamaño, forma y coloración similar.(AU)

Differential responses of the vasotocin 1a receptor (V1aR) and osmoreceptors to immobilization and osmotic stress in sensory circumventricular organs of the chicken (Gallus gallus) brain.

Past studies have shown that the avian vasotocin 1a receptor (V1aR) is involved in immobilization stress. It is not known whether the receptor functions in osmotic stress, and if sensory circumventricular organs may be involved. An experiment was designed with four treatment groups including a 1h immobilization acute stress (AS) group, an unstressed acute control (AC), a third given an intraperitoneal (ip) hypertonic saline injection (HS) and isotonic saline controls (IC) administered ip. One set of chick brains was perfused for immunohistochemistry while a second was sampled for quantitative RT-PCR. Plasma corticosterone (CORT) and arginine vasotocin (AVT) concentrations were significantly increased in the immobilized and hypertonic saline groups (p<0.01) compared to controls. Intense staining of the V1aR occurred throughout the organum vasculosum of the lamina terminalis (OVLT) and subseptal organ (SSO)/subfornical organ (SFO). The immunostaining allowed the boundaries of the two circumventricular organs (CVOs) to be described for the first time in avian species. Both treatment groups showed marked morphological changes in glia within the OVLT and SSO/SFO. The avian V1aR, angiotensin II type 1 receptor (AT1R), and transient receptor potential vanilloid receptor 1 (TRPV1) mRNA levels were increased in the SSO/SFO in hypertonic saline treated birds compared to isotonic controls. In contrast, the latter two genes (AT1R and TRPV1) were significantly decreased in the OVLT of birds subjected to hyperosmotic stress, while all three genes were significantly up-regulated after immobilization. Taken together, results show a possible differential function for the same receptors in two anatomically adjacent CVOs.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states.

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

The OVLT initiates the fall in arterial pressure evoked by high dose lipopolysaccharide: evidence that dichotomous, dose-related mechanisms mediate endotoxic hypotension.

This study tested the hypothesis that lipopolysaccharide (LPS) lowers arterial pressure through two different mechanisms depending on the dose. Previously, we found that a low hypotensive dose of LPS (1mg/kg) lowers arterial pressure by activating vagus nerve afferents. Here we report that hypotension evoked by high dose LPS (15mg/kg) can be prevented by injecting lidocaine into the OVLT but not by vagotomy or inactivation of the NTS. The hypotension produced by both LPS doses was correlated with elevated extracellular norepinephrine concentrations in the POA and prevented by blocking alpha-adrenergic receptors. Thus, initiation of endotoxic hypotension is dose-related, mechanistically.

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.

Angiotensin type 1a receptors in the forebrain subfornical organ facilitate leptin-induced weight loss through brown adipose tissue thermogenesis.

OBJECTIVE: Elevations in brain angiotensin-II cause increased energy expenditure and a lean phenotype. Interestingly, the metabolic effects of increased brain angiotensin-II mimic the actions of leptin, suggesting an interaction between the two systems. Here we demonstrate that angiotensin-type 1a receptors (AT1aR) in the subfornical organ (SFO), a forebrain structure emerging as an integrative metabolic center, play a key role in the body weight-reducing effects of leptin via brown adipose tissue (BAT) thermogenesis. METHODS: Cre/LoxP technology coupled with targeted viral delivery to the SFO in a mouse line bearing a conditional allele of the Agtr1a gene was utilized to determine the interaction between leptin and SFO AT1aR in metabolic regulation. RESULTS: Selective deletion of AT1aR in the SFO attenuated leptin-induced weight loss independent of changes in food intake or locomotor activity. This was associated with diminished leptin-induced increases in core body temperature, blunted upregulation of BAT thermogenic markers, and abolishment of leptin-mediated sympathetic activation to BAT. CONCLUSIONS: These data identify a novel interaction between angiotensin-II and leptin in the control of BAT thermogenesis and body weight, and highlight a previously unrecognized role for the forebrain SFO in metabolic regulation.

Involvement of brain cytokines in zymosan-induced febrile response.

This study compared the involvement of interleukin-1ß (IL-1ß), IL-6, and tumor necrosis factor-α (TNF-α) within the central nervous system (CNS) in the febrile response induced by zymosan (zym) and lipopolysaccharide (LPS). In addition, we investigated whether zym could activate important regions related to fever; namely, the vascular organ of the laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Intraperitoneal injection of zym (1, 3, and 10 mg/kg) induced a dose-related increase in core temperature. Zym (3 mg/kg) also reduced tail skin temperature, suggesting the activation of heat conservation mechanisms, as expected, during fever. LPS increased plasma levels of TNF-α measured at 1 h, IL-1ß measured at 2 h, and IL-6 measured at 3 h after injection. Zym increased circulating levels of IL-6 but not those of TNF-α or IL-1ß at the same time points. In addition, an intracerebroventricular injection of antibodies against TNF-α (2.5 µg) and IL-6 (10 µg) or the IL-1 receptor antagonist (160 ng) reduced the febrile response induced by zym and LPS. Zym (100 µg/ml) also increased intracellular calcium concentration in the OVLT and MnPO from rat primary neuroglial cultures and increased release of TNF-α and IL-6 into the supernatants of these cultures. Together, these results suggest that TNF-α, IL-1ß, and IL-6 within the CNS participate in the febrile response induced by zym. However, the time course of release of these cytokines may be different from that of LPS. In addition, zym can directly activate the brain areas related to fever.

Possible crosstalk between leptin and prolactin during pregnancy.

Rodents exhibit leptin resistance and high levels of prolactin/placental lactogens during pregnancy. A crosstalk between prolactin and leptin signaling has been proposed as a possible mechanism to explain the changes in energy balance during gestation. However, it remains unclear if specific neuronal populations co-express leptin and prolactin receptors. Therefore, our present study was undertaken to identify in the mouse brain prolactin-responsive cells that possibly express the leptin receptor (LepR). In addition, we assessed the leptin response in different brain nuclei of pregnant and nulliparous mice. We used a LepR-reporter mouse to visualize LepR-expressing cells with the tdTomato fluorescent protein. Prolactin-responsive cells were visualized with the immunohistochemical detection of the phosphorylated form of the signal transducer and activator of transcription-5 (pSTAT5-ir). Notably, many neurons that co-expressed tdTomato and pSTAT5-ir were observed in the medial preoptic area (MPA, 27-48% of tdTomato cells), the retrochiasmatic area (34-51%) and the nucleus of the solitary tract (NTS, 16-24%) of prolactin-treated nulliparous mice, pregnant mice and prolactin-treated leptin-deficient (ob/ob) mice. The arcuate nucleus of the hypothalamus (8-22%), the medial tuberal nucleus (11-15%) and the ventral premammillary nucleus (4-10%) showed smaller percentages of double-labeled cells among the groups. Other brain nuclei did not show significant percentages of neurons that co-expressed tdTomato and pSTAT5-ir. Late pregnant mice exhibited a reduced leptin response in the MPA and NTS when compared with nulliparous mice; however, a normal leptin response was observed in other brain nuclei. In conclusion, our findings shed light on how the brain integrates the information conveyed by leptin and prolactin. Our results corroborate the hypothesis that high levels of prolactin or placental lactogens during pregnancy may directly interfere with LepR signaling, possibly predisposing to leptin resistance.

OVLT lesion decreases basal arterial pressure and the chronic hypertensive response to AngII in rats on a high-salt diet.

We have reported that lesion of the organum vasculosum of the lamina terminalis (OVLT) has no effect on basal levels of mean arterial pressure (MAP) but abolishes the hypertensive effects of angiotensin II (AngII) in rats consuming a normal-salt diet. These results suggest that the OVLT does not contribute to regulation of MAP under conditions of normal salt intake, but it is an important brain site for the hypertensive actions of AngII. The OVLT has been proposed as a major sodium sensor in the brain and the hypertensive effects of AngII are exacerbated by high-salt intake. Therefore, the objective of this study was to investigate the role of the OVLT during AngII-induced hypertension in rats fed a high-salt diet. Male Sprague-Dawley rats underwent sham (Sham; n = 9) or OVLT lesion (OVLTx; n = 8) surgery and were placed on a high-salt (2% NaCl) diet. MAP was measured by radio telemetry during three control days, 10 days of AngII infusion (10 ng/kg/min, i.v.), and a 3-day recovery period. MAP was significantly lower in OVLTx (97 ± 2 mmHg) compared to Sham (106 ± 1 mmHg) rats during the control period (P < 0.05). Moreover, the chronic pressor response to AngII was markedly attenuated in OVLTx rats. MAP increased 58 ± 3 mmHg in Sham rats by Day 10 of AngII compared to a 40 ± 7 mmHg increase in OVLTx rats (P < 0.05). We conclude that (1) the OVLT regulates the basal levels of MAP in rats consuming a high-salt and (2) the OVLT is an important brain site of action for the pathogenesis of AngII-salt hypertension in the rat. Supported by HL076312.

ENaC γ-expressing astrocytes in the circumventricular organs, white matter, and ventral medullary surface: sites for Na+ regulation by glial cells.

Using a double immunofluorescence procedure, we report the discovery of a novel group of fibrous astrocytes that co-express epithelial sodium channel (ENaC) γ-subunit protein along with glial acidic fibrillary protein (GFAP). These cells are concentrated along the borders of the sensory circumventricular organs (CVOs), embedded in the white matter (e.g., optic nerve/chiasm, anterior commissure, corpus callosum, pyramidal tract) and are components of the pia mater. In the CVOs, a compact collection of ENaC γ-immunoreactive glial fibers form the lamina terminalis immediately rostral to the organum vasculosum of the lamina terminalis (OVLT). Astrocyte processes can be traced into the median preoptic nucleus - a region implicated in regulation of sodium homeostasis. In the subfornical organ (SFO), ENaC γ-GFAP astrocytes lie in its lateral border, but not in the ventromedial core. In the area postrema (AP), a dense ENaC γ-GFAP glial fibers form the interface between the AP and nucleus tractus solitarius; this area is termed the subpostremal region. Antibodies against the ENaC α- or ß-subunit proteins do not immunostain these regions. In contrast, the antibodies against the ENaC γ-subunit protein react weakly with neuronal cell bodies in the CVOs. Besides affecting glial-neural functions in the CVOs, the astrocytes found in the white matter may affect saltatory nerve conduction, serving as a sodium buffer. The ENaC γ-expressing astrocytes of the ventral medulla send processes into the raphe pallidus which intermingle with the serotoninergic (5-HT) neurons found in this region as well as with the other nearby 5-HT neurons distributed along ventral medullary surface.