Regional suppression by water intake of c-fos expression induced by intraventricular infusions of angiotensin II

Nedd4-2 up-regulation is associated with ACE2 ubiquitination in hypertension

AIMS

Angiotensin-converting enzyme 2 (ACE2) is a critical component of the compensatory renin-angiotensin system that is down-regulated during the development of hypertension, possibly via ubiquitination. However, little is known about the mechanisms involved in ACE2 ubiquitination in neurogenic hypertension. This study aimed at identifying ACE2 ubiquitination partners, establishing causal relationships and clinical relevance, and testing a gene therapy strategy to mitigate ACE2 ubiquitination in neurogenic hypertension.

METHODS AND RESULTS

Bioinformatics and proteomics were combined to identify E3 ubiquitin ligases associated with ACE2 ubiquitination in chronically hypertensive mice. In vitro gain/loss of function experiments assessed ACE2 expression and activity to validate the interaction between ACE2 and the identified E3 ligase. Mutation experiments were further used to generate a ubiquitination-resistant ACE2 mutant (ACE2-5R). Optogenetics, blood pressure telemetry, pharmacological blockade of GABAA receptors in mice expressing ACE2-5R in the bed nucleus of the stria terminalis (BNST), and capillary western analysis were used to assess the role of ACE2 ubiquitination in neurogenic hypertension. Ubiquitination was first validated as leading to ACE2 down-regulation, and Neural precursor cell-expressed developmentally down-regulated protein 4-2 (Nedd4-2) was identified as a E3 ligase up-regulated in hypertension and promoting ACE2 ubiquitination. Mutation of lysine residues in the C-terminal of ACE2 was associated with increased activity and resistance to angiotensin (Ang)-II-mediated degradation. Mice transfected with ACE2-5R in the BNST exhibited enhanced GABAergic input to the paraventricular nucleus (PVN) and a reduction in hypertension. ACE2-5R expression was associated with reduced Nedd4-2 levels in the BNST.

CONCLUSION

Our data identify Nedd4-2 as the first E3 ubiquitin ligase involved in ACE2 ubiquitination in Ang-II-mediated hypertension. We demonstrate the pivotal role of ACE2 on GABAergic neurons in the maintenance of an inhibitory tone to the PVN and the regulation of pre-sympathetic activity. These findings provide a new working model where Nedd4-2 could contribute to ACE2 ubiquitination, leading to the development of neurogenic hypertension and highlighting potential novel therapeutic strategies.

Sensory Circumventricular Organs, Neuroendocrine Control, and Metabolic Regulation

The central nervous system is critical in metabolic regulation, and accumulating evidence points to a distributed network of brain regions involved in energy homeostasis. This is accomplished, in part, by integrating peripheral and central metabolic information and subsequently modulating neuroendocrine outputs through the paraventricular and supraoptic nucleus of the hypothalamus. However, these hypothalamic nuclei are generally protected by a blood-brain-barrier limiting their ability to directly sense circulating metabolic signals—pointing to possible involvement of upstream brain nuclei. In this regard, sensory circumventricular organs (CVOs), brain sites traditionally recognized in thirst/fluid and cardiovascular regulation, are emerging as potential sites through which circulating metabolic substances influence neuroendocrine control. The sensory CVOs, including the subfornical organ, organum vasculosum of the lamina terminalis, and area postrema, are located outside the blood-brain-barrier, possess cellular machinery to sense the metabolic interior milieu, and establish complex neural networks to hypothalamic neuroendocrine nuclei. Here, evidence for a potential role of sensory CVO-hypothalamic neuroendocrine networks in energy homeostasis is presented.

Mapping brain Fos immunoreactivity in response to water deprivation and partial rehydration: Influence of sodium intake

Water deprivation (WD) followed by water intake to satiety, produces satiation of thirst and partial rehydration (PR). Thus, WD-PR is a natural method to differentiate thirst from sodium appetite. WD-PR also produces Fos immunoreactivity (Fos-ir) in interconnected areas of a brain circuit postulated to subserve sodium appetite. In the present work, we evaluated the effect of sodium intake on Fos-ir produced by WD-PR in brain areas operationally defined according to the literature as either facilitatory or inhibitory to sodium intake. Isotonic NaCl was available for ingestion in a sodium appetite test performed immediately after a single episode of WD-PR. Sodium intake decreased Fos-ir in facilitatory areas such as the lamina terminalis (particularly subfornical organ and median preoptic nucleus), central amygdala and hypothalamic parvocellular paraventricular nucleus in the forebrain. Sodium intake also decreased Fos-ir in inhibitory areas such as the area postrema, lateral parabrachial nucleus and nucleus of the solitary tract in the hindbrain. In contrast, sodium intake further increased Fos-ir that was activated by water deprivation in the dorsal raphe nucleus, another inhibitory area localized in the hindbrain. WD-PR increased Fos-ir in the core and shell of the nucleus accumbens. Sodium intake reduced Fos-ir in both parts of the accumbens. In summary, sodium intake following WD-PR reduced Fos-ir in most facilitatory and inhibitory areas, but increased Fos-ir in another inhibitory area. It also reduced Fos-ir in a reward area (accumbens). The results suggest a functional link between sodium intake and the activity of the hindbrain-forebrain circuitry subserving reward and sodium appetite in response to water deprivation.

Expression of (pro)renin receptor in the human brain and pituitary, and co-localisation with arginine vasopressin and oxytocin in the hypothalamus

(Pro)renin receptor [(P)RR], a specific receptor for renin and prorenin, is a 350 amino acid protein with a single transmembrane domain. In the present study, the expression of (P)RR in the human brain and pituitary, and its co-localisation with arginine vasopressin and oxytocin in the human hypothalamus were studied by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry. Human brain and pituitary tissues were obtained at autopsy from the subjects without neurological or endocrinological disorders. The antiserum against (P)RR was raised in a rabbit by injecting the peptide fragment of human (P)RR corresponding to 224-237 amino acids conjugated with bovine serum albumin. Quantitative RT-PCR showed that (P)RR mRNA was widely expressed in every region of brain examined and pituitary, with the highest expression levels found in the pituitary and frontal lobe. Immunocytochemistry showed that (P)RR was expressed in the paraventricular and supraoptic nuclei of human hypothalami, and in anterior pituitary cells. Immunostaining of serial sections showed that (P)RR was co-localised with arginine vasopressin and oxytocin in the magnocellular neurones of the paraventricular and supraoptic nuclei. The preabsorption of the antibody by the antigen peptide abolished the immunostaining of (P)RR in the human hypothalamus. The present study has shown that (P)RR mRNA is widely expressed in the human brain and pituitary, consistent with the hypothesis that (P)RR is related to the various brain functions, such as cognitive function and brain development. Co-localisation of (P)RR with vasopressin in the hypothalamus raised the possibility that (P)RR may be related to the central control of water-electrolyte metabolism and blood pressure.

Differential effects of water deprivation and rehydration on Fos and FosB/DeltaFosB staining in the rat brainstem

This study examined the effects of dehydration and rehydration with water on Fos and FosB staining in the brainstem of rats. Male rats were water deprived for 48 h (Dehyd, n=7) or 46 h followed by 2 h access to water (Rehyd, n=7). Controls had ad libitum access to water (Con, n=9). Brainstems were stained for Fos and FosB/DeltaFosB using commercially available antibodies. In the nucleus of the solitary tract (NTS), the number of Fos stained neurons was significantly increased by dehydration and increased further following rehydration (Con 5+/-1; Dehyd 22+/-1; Rehyd 48+/-5). The average number of Fos-positive cells in the parabrachial nucleus (PBN) was significantly increased only by rehydration (Con 12+/-2; Dehyd 6+/-2; Rehyd 51+/-4). The area postrema (AP) showed significant increases in Fos staining after dehydration and rehydration (Fos: Con 4+/-1; Dehyd 28+/-3; Rehyd 24+/-3). In the rostral ventrolateral medulla (RVL), Fos staining significantly increased after dehydration and this effect was reduced by rehydration (Con 3+/-1; Dehyd 21+/-2; Rehyd 12+/-1). In contrast, Fos staining in the caudal ventrolateral medulla (CVL) was not significantly influenced following either dehydration or rehydration with water (Con 4+/-1; Dehyd 4+/-1; Rehyd 5+/-1). FosB/DeltaFosB staining in the NTS, AP, and RVL was comparably increased by dehydration and rehydration. In the PBN and CVL, FosB/DeltaFosB staining was not affected by the treatments. Dehydration and rehydration have regionally specific effects on Fos and FosB/DeltaFosB staining in the brainstem.

Differential effects of water and saline intake on water deprivation-induced c-Fos staining in the rat

We studied c-Fos staining in adult male rats after 48 h of water deprivation and after 46 h of water deprivation with 2 h of access to water or physiological saline. Controls were allowed ad libitum access to water and physiological saline. For immunocytochemistry, anesthetized rats were perfused with a commercially available antibody for c-Fos. Dehydration significantly increased plasma vasopressin (AVP), osmolality, plasma renin activity (PRA), hematocrit, and sodium concentration and decreased urinary volume. Fos staining was significantly increased in the median preoptic nucleus, organum vasculosum of the lamina terminalis, supraoptic nucleus (SON), and magnocellular and parvocellular paraventricular nucleus (PVN), as well as the area postrema, nucleus of the solitary tract (NTS), and rostral ventrolateral medulla (RVL). Rehydration with water significantly decreased AVP levels and Fos staining in the SON, PVN, and RVL and significantly increased Fos expression in the perinuclear zone of the SON, NTS, and parabrachial nucleus. Rehydration with water was associated with decreased urinary sodium concentration and hypotonicity, and hematocrit and PRA were comparable to levels seen after dehydration. After rehydration with saline, plasma osmolality, hematocrit, and PRA were not different from control, but plasma AVP and urinary sodium concentration were increased. In the SON, Fos staining was significantly increased, with a great percentage of the Fos cells also stained for oxytocin compared with water deprivation. Changes in Fos staining were also observed in the NTS, RVL, parabrachial nucleus, and PVN. Rehydration with water or saline produces differential effects on plasma AVP, Fos staining, and sodium concentration.

The effects of osmotic stimulation and water availability on c-Fos and FosB staining in the supraoptic and paraventricular nuclei of the hypothalamus

We studied the effects of osmotic stimulation on the expression of FosB and c-Fos in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). Adult male rats were divided into two groups that were injected with lidocaine (0.1-0.2 ml sc) followed by either 0.9% or 6% NaCl (1 ml/100 g bw sc). After the NaCl injections, the rats were anesthetized and perfused 2, 6, or 8 h after the injections. Their brains were prepared for immunocytochemistry and stained with FosB and c-Fos antibodies. The number of c-Fos-positive cells was significantly increased only at 2 h in the SON and PVN. In contrast, the number of FosB-positive cells was significantly increased at 6, and 8 h in both the SON and PVN. In a second experiment, the effect of water availability on FosB staining 8 h after injections of 6% NaCl was tested in 3 groups of rats: water ad libitum, rats that had no access to water, and rats that were given water 2 h prior to perfusion. FosB staining was significantly reduced in both the SON and the PVN of rats that had ad libitum water compared to the two water-restricted groups. In the third experiment, rats were injected with either 0.9% NaCl or 6% NaCl and were either given ad libitum access to water or water restricted for 6 h after the injections and perfused 24 h after the saline injections. FosB staining was not increased when water was available ad libitum. FosB staining was significantly increased at 24 h in the rats injected with 6% NaCl when water was restricted. Thus, FosB may continue to influence protein expression in the SON and PVN for at least 24 h following acute osmotic stimulation.

Effects of water deprivation and rehydration on c-Fos and FosB staining in the rat supraoptic nucleus and lamina terminalis region

We studied cFos and FosB staining in the supraoptic nucleus (SON) the organum vasculosum of the lamina terminalis (OVLT) and the median preoptic nucleus (MnPO) in adult male rats after water deprivation (24 h, n = 11; 48 h, n = 12) and water deprivation with rehydration (22 h + water, n = 11; 46 h + water, n = 10). Control rats ( n = 15) had water available ad libitum. Separate sets of serial sections from each brain were processed for immunocytochemistry using primary antibodies against either c-Fos or FosB protein. Plasma osmolality, vasopressin, hematocrit, and plasma proteins were measured in separate groups ( n = 6–7). The number of c-Fos-positive cells in the SON was significantly increased after 24 and 48 h of water deprivation. In contrast, rehydrated groups were not different from control. Water deprivation significantly increased c-Fos staining in both the OVLT and the MnPO, but c-Fos staining was not altered by rehydration. FosB staining in the SON was significantly increased only by 48-h water deprivation, and this effect was significantly decreased by rehydration. In the MnPO and OVLT, FosB staining was significantly increased by water deprivation, and, like c-Fos staining, these increases were not affected by rehydration. Water deprivation significantly increased osmolality and hematocrit, as well as plasma protein and vasopressin concentrations. Plasma measurements from rehydrated rats were not different from control. We conclude that water deprivation and rehydration differentially affect c-Fos and FosB staining in a region-dependent manner.

Potentiation by angiotensin II of spontaneous excitatory postsynaptic currents in rat supraoptic magnocellular neurones

The physiological actions of angiotensin II in the supraoptic (SON) and paraventricular nuclei have been widely demonstrated, including the modulation of firing rate and release of arginine vasopressin and oxytocin. Here, we investigated whether angiotensin II modulates synaptic inputs into the SON. To do this, we measured spontaneous excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) from rat SON neurones in thin slice preparations using the whole-cell patch-clamp technique. Angiotensin II reversibly increased the frequency of spontaneous EPSCs in a dose-related manner without affecting the amplitude, indicating that angiotensin II potentiated EPSCs via a presynaptic mechanism. Angiotensin II-induced potentiation of EPSCs was unaffected in the presence of tetrodotoxin. On the other hand, angiotensin II did not cause significant effects on IPSCs. The potentiation of EPSCs by angiotensin II was potently suppressed by previous exposure to the angiotensin type 1 (AT1) receptor antagonist, losartan. Our results suggest that angiotensin II potentiates the excitatory synaptic inputs into SON neurones, via the AT1 receptors.

Central angiotensin II-induced pressor responses and neural activity in utero and hypothalamic angiotensin receptors in preterm ovine fetus

The central renin-angiotensin system is important in the control of blood pressure in the adult. However, few data exist about the in utero development of central angiotensin-mediated pressor responses. Our recent studies have shown that the application of ANG II into the fetal brain can increase blood pressure at near term. The present study determined fetal blood pressure and heart rate in response to a central application of ANG II in the chronically prepared preterm ovine fetus, determined the action sites marked by c-Fos expression in the fetal central pathways after intracerebroventricular injection of ANG II in utero, and determined angiotensin subtype 1 receptors in the fetal hypothalamus. Central injection of ANG II significantly increased fetal mean arterial pressure (MAP). Adjusted fetal MAP against amniotic pressure was also increased by ANG II. Fetal heart rate was subsequently decreased after the central administration of ANG II and/or the increase of blood pressure. ANG II induced c-Fos expression in the central putative cardiovascular area, the paraventricular nuclei in the brain sympathetic pathway. Application of ANG II also caused intense Fos immunoreactivity in the tractus solitarius nuclei in the hindbrain. In addition, intense angiotensin subtype 1 receptors were expressed in the hypothalamus at preterm. These data demonstrate that central ANG II-related pressor centers start to function as early as at preterm and suggest that the central angiotensin-related sympathetic pathway is likely intact in the control of blood pressure in utero.

Intravenous angiotensin induces brain c-fos expression and vasopressin release in the near-term ovine fetus

The effect of intravenous angiotensin II (ANG II) on fetal brain c-fos expression and arginine vasopressin (AVP) release was studied in the near-term ovine fetus. Fetuses with chronically implanted catheters received an intravenous infusion of ANG II or saline. Fetal plasma AVP concentrations were significantly increased after the peripheral administration of ANG II, with peak levels (3-fold) at 30 min after the intravenous infusion. There was no change in fetal plasma osmolality, sodium, and hematocrit levels between the control and experimental groups or between the periods before and after the infusion of ANG II. Intravenous ANG II administration induced Fos immunoreactivity (Fos-IR) in the circumventricular organs and the median preoptic nucleus of the fetal brain. Fos-IR was also demonstrated in the fetal supraoptic nuclei (SON). Double labeling demonstrated that the AVP-containing neurons in the SON were expressing c-fos in response to intravenous ANG II. These results indicate that the peripheral ANG II in the fetus may play a significant role in stimulating the central hypothalamic-neurohypophysial system during late gestation. It supports the hypothesis that circulating ANG II may act at the fetal AVP neurons in the hypothalamus in body fluid balance via the circumventricular organs, which are situated outside the blood-brain barrier, and the central neural pathway between these two brain structures has been relatively established in utero, at least at near-term.

In utero development of central ANG-stimulated pressor response and hypothalamic fos expression

Central renin-angiotensin system (RAS) is as important as the peripheral RAS in the control of the cardiovascular homeostasis in the adult. However, previous fetal studies on angiotensin II (ANG II)-induced cardiovascular responses focused exclusively on the peripheral side. Thus, few data exist characterizing the in utero development of central angiotensin-mediated pressor responses. The present study determined cardiovascular responses to central application of ANG II in the chronically prepared near-term ovine fetus, and determined the action sites marked by c-fos expression in the fetal hypothalamus following intracerebroventricular (icv) injection of ANG II in utero. ANG II significantly increased fetal systolic, diastolic, and mean arterial pressure (MAP) within 5 min after injection of this peptide into the brain. Adjusted fetal MAP against amniotic pressure was also increased by icv ANG II, associated with increased c-fos in the central putative cardiovascular area--the paraventricular nuclei (PVN). Application of ANG II also induced intense c-fos expression in the supraoptic nuclei (SON), accompanied by a significant increase of fetal plasma vasopressin (AVP) levels, while maternal blood pressure (BP) and plasma AVP concentration were not changed. These results indicate that the central ANG II-mediated pressor response is functional at the last third of gestation, acting at the sites consistent with the cardiovascular neural network in the hypothalamus.

Representations of motivational drives in mesial cortex, medial thalamus, hypothalamus and midbrain

We propose that neural representations of motivational drives, including sexual desire, hunger, thirst, fear, power-dominance, the motivational aspect of pain, the need for sleep, and nurturance, are represented in four areas in the brain. These are located in the medial hypothalamic/preoptic area, the periaqueductal gray matter (PAG) in the midbrain/pons, the midline and intralaminar thalamic nuclei, and in the anterior part of the mesial cortex, including the medial prefrontal and anterior cingulate areas. We attempt to determine the locations of each of these representations within the hypothalamus/preoptic area, periaqueductal gray and cortex, based on the available literature on activation of brain structures by stimuli that evoke these forms of motivation, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on hodological data. We discuss the hierarchical organization of the representations for a given drive, outputs from these representations to premotor structures in the medulla, caudate-putamen, and cortex, and their contributions to involuntary, learned-sequential (operant) and voluntary behaviors.

Water deprivation-induced sodium appetite: humoral and cardiovascular mediators and immediate early genes

Adult rats deprived of water for 24-30 h were allowed to rehydrate by ingesting only water for 1-2 h. Rats were then given access to both water and 1.8% NaCl. This procedure induced a sodium appetite defined by the operational criteria of a significant increase in 1.8% NaCl intake (3.8 +/- 0.8 ml/2 h; n = 6). Expression of Fos (as assessed by immunohistochemistry) was increased in the organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MnPO), subfornical organ (SFO), and supraoptic nucleus (SON) after water deprivation. After rehydration with water but before consumption of 1.8% NaCl, Fos expression in the SON disappeared and was partially reduced in the OVLT and MnPO. However, Fos expression did not change in the SFO. Water deprivation also 1) increased plasma renin activity (PRA), osmolality, and plasma Na+; 2) decreased blood volume; and 3) reduced total body Na+; but 4) did not alter arterial blood pressure. Rehydration with water alone caused only plasma osmolality and plasma Na+ concentration to revert to euhydrated levels. The changes in Fos expression and PRA are consistent with a proposed role for ANG II in the control of the sodium appetite produced by water deprivation followed by rehydration with only water.

Functional relationship between subfornical organ cholinergic stimulation and cellular activation in the hypothalamus and AV3V region

The subfornical organ (SFO) has been suggested to be important for water intake and secretion of vasopressin (AVP). However, the role of the SFO cholinergic mechanism in the control of body fluid regulation is not clear. This study determined the effects of local cholinergic stimulation in the SFO produced by administration of physostigmine on drinking and cellular excitation in the anterior third ventricle (AV3V) region and in the supraoptic and paraventricular nuclei (SON and PVN). The results showed that injection of physostigmine into the SFO induced water intake and c-fos expression in the AV3V area as well as in the AVP containing neurons in the hypothalamus. Pretreatment of the SFO with mecamylamine, a nicotinic receptor antagonist, had no effect on physostigmine induced behavioral and c-fos responses. The muscarinic receptor blocker atropine, however, abolished both drinking and cellular activation after injection of physostigmine into the SFO. Immunostaining experiments demonstrated positive acetyltransferase (ChAT) in the SFO. Intensive ChAT immunoreactivity was located in the cholinergic fibers in the SFO. Together, the results indicate that SFO cholinergic mechanisms are important in co-operation with the AV3V and hypothalamic neurons in the control of thirst and AVP-mediated body fluid homeostasis.

Central angiotensin induction of fetal brain c-fos expression and swallowing activity

The present study examined physiological and cellular responses to central application of ANG II in ovine fetuses and determined the fetal central ANG-mediated dipsogenic sites in utero. Chronically prepared near-term ovine fetuses (130 +/- 2 days) received injection of ANG II (1.5 microg/kg icv). Fetuses were monitored for 3.5 h for swallowing activity, after which animals were killed and fetal brains were perfused for subsequent Fos staining. Intracerebroventricular ANG II significantly increased fetal swallowing in near-term ovine fetuses (1.1 +/- 0.2 to 4.5 +/- 1.0 swallows/min). The initiation of stimulated fetal swallowing activity was similar to the latency of thirst responses (drinking behavior) elicited by central ANG II in adult animals. ANG II evoked increased Fos staining in putative dipsogenic centers, including the subfornical organ, organum vasculosum of the lamina terminalis, and median preoptic nucleus. Intracerebroventricular injection of ANG II also caused c-fos expression in the fetal hindbrain. These results indicate that an ANG II-mediated central dipsogenic mechanism is intact before birth, acting at sites consistent with the dipsogenic neural network. Central ANG II mechanisms likely contribute to fetal body fluid and amniotic fluid regulation.

Plasma osmolality dipsogenic thresholds and c-fos expression in the near-term ovine fetus

In ovine and human pregnancy, fetal swallowing contributes importantly to amniotic fluid homeostasis. Fetal dipsogenic responsiveness to short-term plasma hyperosmolality develops in late gestation, although fetal swallowing is not stimulated in response to long-term plasma osmolality increases (2 to 3%), which typically stimulate adult drinking behavior. To explore the near-term fetal plasma osmolality threshold for swallowing stimulation, we examined the effects of i.v. hypertonic saline-induced subacute increases in plasma hypertonicity on fetal swallowing behavior. Central sites of activation were examined by c-fos expression in putative dipsogenic nuclei. The results demonstrate that subacute 2 to 3% plasma osmolality increases do not stimulate near-term ovine fetal swallowing. However, fetal swallowing activity significantly increased (3 times) after plasma osmolality increased >6% above basal values. Consistent with a specific dipsogenic response, i.v. hypertonic saline induced c-fos expression in the anterior third ventricle region, a putative dipsogenic center, as well as in the fetal hindbrain. The stimulation of fetal swallowing under conditions of higher osmotic stimulation and the correlation with forebrain c-fos expression indicates that near-term fetal osmoregulation mechanisms are functional, although not completely mature. Reduced fetal dipsogenic responsiveness may result from altered osmoreceptor sensitivity, downstream neuronal or synaptic immaturity, or potentially inhibitory actions of stimulated hindbrain nuclei.

Intracerebroventricular carbachol induces FOS immunoreactivity in lamina terminalis neurons projecting to the supraoptic nucleus

Central application of the non-selective cholinergic receptor agonist, carbachol, induces water intake, vasopressin (VP) release and an acute increase in arterial blood pressure. Forebrain sites, particularly those located along the lamina terminalis (LT) (i.e. the subfornical organ (SFO), organum vasculosum (OV) and the median preoptic nucleus (MePO)) and in the hypothalamus, have been proposed as the major targets for producing the effects induced by intracerebroventricular (i.c.v.) carbachol injections. However, the functional and neuroanatomical relationship among carbachol-activated cells along the LT and hypothalamic areas such as the supraoptic nuclei (SON), is unclear. The present study investigated the i.c.v. carbachol-induced activity of the soma of LT projections which descend from the SFO, OV and MePO and terminate in the region of the SON. Cells along the LT were retrogradely labeled from SON-targeted injections of fluoro-gold, and FOS-immunoreactivity (FOS-ir) was used to assess activation. A significant number of cells in the SFO, OV and MePO were double-labeled for both FOS-ir and fluoro-gold. The FOS labeling in the cells of the LT-associated structures was significantly reduced by pretreatment with the i.c.v. muscarinic antagonist, atropine. Taken together, the results indicate that neurons located in structures located along the LT and projecting to the region of the SON are activated by i.c.v. carbachol and that these receptors are likely to be muscarinic.

The awareness of thirst: proposed neural correlates

The neural and endocrine bases of the generation of thirst are reviewed. Based on this review, a hierarchical system of neural structures that regulate water conservation and acquisition is proposed. The system includes primary sensory-receptive areas; secondary sensory structures (circumventricular organs), which detect levels of hormones, including angiotensin II and vasopressin, which are involved in generating thirst; preoptic and hypothalamic structures; and an area within the ventrolateral quadrant of the periaqueductal gray matter. Hodological and other data are used to determine the hierarchical organization of the system. Based on studies of the effects of lesions to various structures within the hierarchy of the system, it is proposed that the awareness of thirst in rodents is either entirely or predominantly due to neuronal activities in a subsection of the ventrolateral periaqueductal gray matter. It is also hypothesized that the awareness of thirst in primates is due to neuronal activities in both the ventrolateral periaqueductal gray and in a region within the medial prefrontal and anterior cingulate cortex.

Osmotic threshold and sensitivity for vasopressin release and fos expression by hypertonic NaCl in ovine fetus

In adults, hyperosmolality stimulates central osmoreceptors, resulting in arginine vasopressin (AVP) secretion. Near-term fetal sheep have also developed mechanisms to respond to intravascular hypertonicity with stimulation of in utero AVP release. However, prior studies demonstrating fetal AVP secretion have utilized plasma tonicity changes greater than those required for adult osmotically induced AVP stimulation. We sought to examine near-term fetal plasma osmolality threshold and sensitivity for stimulation of AVP secretion and to correlate plasma hormone levels with central neuronal responsiveness. Chronically instrumented ovine fetuses (130 +/- 2 days) and maternal ewes simultaneously received either isotonic or hypertonic intravascular NaCl infusions. Maternal and fetal plasma AVP and angiotensin II (ANG II) levels were examined at progressively increasing levels of plasma hypertonicity. Intravenous hypertonic NaCl gradually elevated plasma osmolality and sodium levels. Both maternal and fetal plasma AVP increased during hypertonicity, whereas ANG II levels were not changed. Maternal AVP levels significantly increased with a 3% increase in plasma osmolality, whereas fetal plasma AVP significantly increased only at higher plasma osmolality levels (over 6%). Thus the slope of the regression of AVP vs. osmolality was greater for ewes than for fetuses (0.232 vs. 0.064), despite similar maternal and fetal plasma osmolality thresholds for AVP secretion (302 vs. 304 mosmol/kg). Hyperosmolality induced Fos immunoreactivity (FOS-ir) in the circumventricular organs of the fetal brain. FOS-ir was also demonstrated in the fetal supraoptic and paraventricular nuclei (SON and PVN), and double labeling demonstrated that AVP-containing neurons in the SON and PVN expressed Fos in response to intravenous NaCl. These results demonstrate that, in the ovine fetus at 130 days of gestation, neuroendocrine responses to cellular dehydration are functional, although they evidence a relatively reduced sensitivity for AVP secretion compared with the adult.

Appearance of central dipsogenic mechanisms induced by dehydration in near-term rat fetus

Cellular dehydration of central osmoreceptors evokes an integration of behavioral (i.e. drinking) and endocrinologic (i.e. arginine vasopressin secretion) responses to maintain body fluid balance. These osmoregulatory mechanisms have been intensely investigated in adult models. However, there has been limited research of the fetal development of neural mechanisms regulating responses to dehydration. Although behavioral and neuroendocrine responses to dehydration have been demonstrated in utero in precocial species (e.g. ovine), there has been no study to date demonstrating that these responses develop before the neonatal period of altricial species (e.g. rat). This study is the first to use the near-term rat fetus to investigate the effects of maternal subcutaneous hypertonic (2 M NaCl) or isotonic (0.15 M NaCl) saline injection on fetal plasma osmolality and brain FOS-immunoreactivity (FOS-ir). Maternal subcutaneous hypertonic saline significantly increased maternal and fetal plasma osmolality to similar levels (328+/-6 and 326+/-6 mosM/kg, respectively). In response to plasma hypertonicity, maternal and fetal brain FOS-ir increased significantly in the regions including the lamina terminalis, and the supraoptic and paraventricular nuclei (SON and PVN) of the hypothalamus. Together, these data indicate that central mechanisms for dipsogenic and arginine vasopressin secretory responses to hypertonicity are present and responsive in the fetal rat brain at near-term gestation. However, differences between fetal and maternal FOS-ir mapping suggest that fetal osmoreceptor development is not yet completed near term.

Drinking and Fos-immunoreactivity in rat brain induced by local injection of angiotensin I into the subfornical organ

Previous studies suggested that angiotensinergic stimulation in the subfornical organ (SFO) has effects on the anterior third ventricle (AV3V) region and the hypothalamus for dipsogenic response and vasopressin release. In this study, Angiotensin I (ANG I) was directly injected into the SFO and this stimulated drinking. Injection of ANG I into the SFO also induced Fos-immunoreactivity (Fos-ir) in the AV3V region and in the vasopressin neurons of the supraoptic and paraventricular nuclei (SON and PVN). Pretreatment of the SFO with either captopril, an ANG converting enzyme inhibitor, or losartan, an AT1 receptor antagonist, abolished both drinking and Fos-ir induced by ANG I. Water intake partially decreased ANG I-induced Fos-ir in the SON and PVN, but not in the other areas. These results indicate that there is an ANG converting system in the SFO and suggest that neurons in the AV3V region and vasopressin cells in the hypothalamus can be regulated by angiotensinergic components in the SFO.

The angiotensinogen gene is expressed in both astrocytes and neurons in murine central nervous system

Two transgenic mouse models were used to examine the cellular localization of angiotensinogen (AGT) in the brain. The first model was previously described in detail and consists of a human AGT genomic transgene containing all exons and introns of the gene and 1. 2 kb of the 5' flanking DNA. The second model contains a fusion between 1.2 kb of HAGT 5' flanking DNA and the beta-gal reporter gene which exhibits a similar pattern of tissue-specific expression to the HAGT transgene. Expression of both transgenes qualitatively mirrors the expression of endogenous AGT. Double staining of transgenic mouse brain sections with X-gal and GFAP revealed that a majority of beta-gal activity was localized to astrocytes in almost all brain areas. However, both beta-gal activity as identified by X-gal, and HAGT mRNA as detected by in situ hybridization, were also found in neurons in restricted areas of the brain, including the mesencephalic trigeminal nucleus (meV), subfornical organ (SFO) and the external lateral parabrachial nucleus (elPB). The expression of these transgenes provides the first convincing evidence for AGT gene expression in neurons in the brain. We further report by angiotensin II (Ang-II) immunostaining in rat brains after selective lesioning, that Ang-II is likely involved in a neuronal pathway from the PB to the amygdala (Ce). Finally, we performed double-labeling, first by retrograde labeling of HRP injected into the Ce, and then by X-gal on PB neurons in beta-gal transgenic mice, and identified doubly labeled neurons. Based on these results, we propose that AGT is generated in neurons in the elPB, transported to the Ce and converted into Ang-II locally to exert is biological functions.

Brain mechanisms of mammalian fluid homeostasis: insights from use of immediate early gene mapping

A comprehensive review of the literature through mid-1997 is presented on the application of immediate early gene mapping to problems related to brain mechanisms of fluid homeostasis and cardiovascular regulation in mammals. First, the basic mechanisms of fluid intake and the principles and pitfalls of immediate early gene mapping are briefly introduced. Then, data from several principal paradigms are reviewed. These include fluid deprivation and intracellular dehydration, both of which are associated with thirst and water intake. The contributions of peripheral sodium receptors, and of both hindbrain and forebrain integrative mechanisms are evaluated. Extracellular dehydration, and associated aspects of both thirst and sodium appetite are then reviewed. The contributions of both structures along the lamina terminalis and the hypothalamic magnocellular neurosecretory groups figure prominently in most of these paradigms. Effects of hypotension and hypertension are discussed, including data from the endogenous generation and the exogenous application of angiotensin II. Lastly, we summarize the contribution of the early gene mapping technique and consider briefly the prospects for new advances using this method.

Plasma neuropeptide-Y levels, monoamine metabolism, electrolyte excretion and drinking behavior in children with attention-deficit hyperactivity disorder

Against a background of (a) increased drinking behavior in children with attention-deficit hyperactivity disorder (ADHD); (b) the parallel between some behaviors associated with ADHD and hypertension; (c) the use of the spontaneously hypertensive rat as a model for ADHD; and (d) similarities in the changes of neuropeptide Y (NPY) and catecholamine in studies of hypertension and drinking, NPY, catecholamines and electrolyte balance were compared in the plasma and urine of healthy children and those with ADHD. Drinking was monitored during 3 h of neuropsychological tests over 2 days in 14 ADHD and nine healthy children. Patients drank four times as much water and showed twice the levels of NPY found in controls. In controls there were positive and in patients there were negative relationships for NPY with drinking and restless behavior. Patients' plasma levels of norepinephrine (NE) and epinephrine were slightly elevated, but urinary levels of NE and the serotonin metabolite were markedly increased. Urinary excretion rates for sodium (not potassium), phosphate and especially calcium were decreased in patients even after covarying for less urine production in the ADHD group. NPY levels were inversely related to calcium excretion and drinking was inversely related to circulating sodium. Increases of drinking and circulating NPY in ADHD children and decreased electrolyte excretion may reflect a common disturbance in metabolic homeostasis.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Effects of intracerebroventricular dizocilpine (MK801) on dehydration-induced dipsogenic responses, plasma vasopressin and c-fos expression in the rat forebrain

This study determines the interaction between glutamate receptors and dehydration-induced drinking, vasopressin (AVP) release, plasma osmolality and c-fos expression in the brain of conscious rats. The NMDA receptor antagonist dizocilpine (100 nmol infused into the cerebral ventricles) suppressed drinking following either 22 h water deprivation or intragastric injection of hypertonic saline (1.5 M), attenuated the increased plasma vasopressin induced by dehydration, but had no effects on peripheral hyperosmolality caused by either water deprivation or injections of hypertonic saline. Dizocilpine had no inhibitory effects on feeding after 24 h food deprivation. Dizocilpine also suppressed c-fos expression induced by dehydration in the median preoptic nucleus (MPN), the supraoptic and paraventricular nuclei (SON and PVN), but did not influence c-fos expression in the subfornical organ (SFO). The non-NMDA receptor antagonists CNQX (400 nmol) or DNQX (60 nmol) affected neither the animals' drinking nor c-fos expression induced by dehydration. Double staining showed that suppression of c-fos expression following dizocilpine occurred in the NMDA R1 receptor containing neurons in the hypothalamus. These results suggest that the NMDA-type glutamate receptors may be involved in dehydration induced dipsogenic and neuroendocrinological responses. They complement our earlier findings that dizocilpine also attenuates drinking and c-fos expression following intraventricular infusions of angiotensin II.

Central renin injections: effects on drinking and expression of immediate early genes

This study investigated the drinking response and the expression of Fos- and Egr-1-immunoreactivity (Fos-ir; Egr-1-ir) in the brain induced by endogenous angiotensin generated by intracerebroventricular (i.c.v.) injection of renin. Renin induced Fos-ir in the subfornical organ (SFO), median preoptic (MnPO), supraoptic and paraventricular nuclei (SON and PVN), area postrema (AP), nuclei of the solitary tract (NTS) and lateral parabrachial nuclei (LPBN). Renin-induced Egr-1-ir exhibited a similar pattern of distribution as that observed for Fos-ir. The dose of i.c.v. renin that induced expression of immediate early gene (IEG) product immunoreactivity also produced vigorous drinking. When renin-injected rats were pretreated with captopril, an angiotensin converting enzyme inhibitor, drinking was blocked. With the same captopril pretreatment, both Fos- and Egr-1-ir in the SFO, MnPO, SON, PVN, AP and LPBN were also significantly reduced.

Calcium channels mediate angiotensin II-induced drinking behaviour and c-fos expression in the brain

It is widely accepted that calcium ions are critically important in both short- and/or long-lasting responses of neurons to a stimulus. We have shown previously that NMDA receptors play a role in dipsogenic responses and c-fos expression induced by intracerebroventricular (i.c.v.) infusion of angiotensin II (Ang II). Since NMDA receptors are known to be linked to receptor-operated calcium channels, this study determined whether voltage dependent calcium channels are also involved in Ang II-induced behavioural (drinking) and endocrine responses as well as c-fos expression. The antidipsogenic actions of three L-type calcium channel antagonists, nifedipine, diltiazem and verapamil on Ang II-induced drinking behaviour were studied. These bind to the dihydropyridine, phenylalkylamine and benzothiazepine sites respectively. Rats (Lister-hooded) pre-treated i.c.v. with either 25 or 100 microg nifedipine, followed by 25 pmol Ang II, drank significantly less water than controls during the first 15 min after infusion. However, rats pre-treated with i.c.v. 100 microg diltiazem or verapamil showed no change in Ang II-induced drinking behaviour. The antidipsogenic actions of N- and P-type calcium channel antagonists omega-conotoxin GVIA and omega-conotoxin MVIIC were also evaluated. Rats pre-treated with 5 pmol or 20 pmol omega-conotoxin GVIA did show a slight but not significant suppression of water intake, particularly after the higher dose. Rats pre-treated with omega-conotoxin MVIIC drank almost the same amount of water as those pre-treated with saline. Nifedipine was found to suppress both Ang II-induced corticosterone release and c-fos expression in the following areas: organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MNPO), hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). The results described in this paper provide evidence that calcium channels play important roles in the Ang II-induced behavioural and endocrine responses, and in the expression of the immediate-early gene c-fos. This suggests that an L-type calcium channel may participate both short- and longer-term neuronal actions of Ang II.

Dizocilpine maleate, an N-methyl-D-aspartate antagonist, inhibits dipsogenic responses and C-Fos expression induced by intracerebral infusion of angiotensin II

The interactions between dizocipline, an N-methyl-D-aspartate open channel antagonist, and the induction of water drinking and c-fos expression by intracerebroventricular (i.c.v.) infusion of angiotensin II have been studied. Pretreating male rats with i.c.v. dizocilpine maleate (100 or 300 nmol) or tenocyclidine (150 nmol), both non-competitive N-methyl-D-aspartate antagonists, inhibited the subsequent dipsogenic response to i.c.v. angiotensin II (125 or 50 pmol, 5-10 min later). Dizocilpine also decreased the angiotensin II-evoked expression of c-fos in the median preoptic nucleus, supraoptic nucleus and the medial (parvicellular) and lateral (magnocellular) parts of the hypothalamic paraventricular nucleus, as well as in the nucleus of the solitary tract and the lateral parabrachial nucleus. Double staining showed that suppression of c-fos expression occurred in N-methyl-D-aspartate R1 receptor containing neurons in the hypothalamus. Pretreating rats with any of three competitive glutamate antagonists (2-amino-5-phosphonopentanoic acid, 60 or 160 nmol; gamma-D-glutamylglyine, 400 nmol; (DL-3/(R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, 0.1 nmol) or the glycine site antagonist 7-chlorokynurenic acid had no effects on angiotensin II-induced drinking. Neither did pretreating rats with the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid antagonist 6-cyano-7-nitroquinoxaline-2,3-dione [two infusions, 30 min (240 nmol) and 5 min (160 nmol) before angiotensin II]. To eliminate cross-reactivity of dizocilpine with nicotinic receptors, animals were pretreated with nicotinic acetylcholine blocker mecamylamine (250 nmol, i.c.v.), but this had no effect on angiotensin II-dependent drinking or c-fos expression. These results suggest that an N-methyl-D-aspartate-type glutamate receptor is implicated in the dipsogenic and cellular responses to i.c.v. angiotensin II, and point to the existence of a novel set of interactions between excitatory amino acids and this neuropeptide.

Effects of DOCA pretreatment on neuronal sensitivity and cell responsiveness to angiotensin II, in the bed nucleus of the stria terminalis in the rat

A previous study has shown that DOCA pretreatment altered the responsiveness of neurons to microiontophoretic administration of angiotensin II (AII) and aldosterone (Aldo). This result coincided with an increase in activity in the septo-preoptic region and a decrease in activity of the central nucleus of the amygdala. The latter region is anatomically linked to the bed nucleus of the stria terminalis (BNST). Single unit activity was recorded in the BNST in response to iontophoretic application of AII, non-peptide AII-receptor antagonists or Aldo in DOCA-pretreated and in non-pretreated rats. DOCA-pretreatment significantly decreased the responsiveness to AII (28 cells (18.5%) vs. 8 cells (14.0%) u = 0.018 for excitation and 3 cells (8.6%) vs. 0 cells 0%, u = 0.011 for inhibition, P < 0.05) and to Aldo (24 cells (21.4%) vs. 4 cells (10.2%), u = 0.026 for excitation, and 3 cells (2.6%) vs. 0 cells, u = 0.009 for inhibition, P < 0.05) of the neurons localised in the BNST. A significant decrease was found in the inhibitory responses to iontophoretic application of losartan, an AII type-1 receptor (AT-1) antagonist (u = 0.042, P < 0.05). No significant differences were recorded with iontophoretic application of PD 123319, a specific AII-type-2 (AT-2) receptor antagonist. Therefore AT-1 receptors are likely responsible for the decreased responsiveness of the BNST correlated with the decrease in the activity within the amygdala.

Studying the central actions of angiotensin using the expression of immediate-early genes: expectations and limitations

Intracerebral infusions of angiotensin II (Ang-II) elicit a widespread but discrete expression of c-fos (and other immediate-early genes: IEGs) in the basal forebrain and brainstem. This has given us a new approach to the study of the central actions of Ang-II (and other peptides). It is unlikely that the dipsogenic response to Ang-II is directly related to c-fos expression in the AV3V, since the onset of behaviour occurs much before that of gene expression, and suppression of drinking by preloading rats with water does not alter Ang-II-induced c-fos in this part of the brain. Whether endocrine or cardiovascular actions are directly related to c-fos needs to be established. Water intake inhibits c-fos in the SON and PVN; whether this is a direct interaction between Ang-II and altered osmolality on magnocellular neurons or a secondary event of an action elsewhere (e.g., in the AV3V) remains uncertain. AV3V (median preoptic nucleus) lesions also inhibit Ang-II-evoked c-fos in the SON and PVN as well as in brainstem Ang-II sensitive sites such as the NTS and parabrachial nucleus, suggesting that the AV3V may be a nodal site for the distributed action of Ang-II. The rapid dipsogenic effects of Ang-II may involve glutamate receptors, since i.c.v. dizocilpine (an NMDA open channel antagonist) reduces both drinking and c-fos expression after i.c.v. Ang-II. This relatively new IEG technique acts as an anatomical and temporally specific marker of neuronal response to Ang-II, and has already added to knowledge about the central actions of Ang-II at the level of the neuron. Combining this approach with other methods allows cellular events in the brain to be related to the functional effects of Ang-II and its adaptive role in regulating water and salt metabolism.

Complex activation of inducible transcription factors in the brain of normotensive and spontaneously hypertensive rats following central angiotensin II administration

The effects of intracerebroventricular (i.c.v.) injections of angiotensin II (Ang II) on the expression of inducible transcription factors (ITF) (c-Fos, FosB, c-Jun, JunB, JunD, Krox-20 and Krox-24) in the brain of conscious rats were assessed immunohistochemically using polyclonal antisera. Ang II (1, 10, 100 ng) induced after 90 min a dose-dependent expression of c-Fos, FosB, c-Jun, JunB and Krox-24, which was confined to four specific brain areas, namely the subfornical organ (SFO), median preoptic area (MnPO), paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the above-mentioned regions, JunD exhibited a high basal staining which was not visibly altered by Ang II. Krox 20 was not induced by AnG II. FosB was only induced 4 h after i.c.v. injection of 100 ng Ang II in the MnPO and PVN. The Ang II-AT1 receptor antagonist, losartan, applied i.c.v. 5 min prior to Ang II (100 ng, i.c.v.) prevented the Ang II-induced ITF expression. In spontaneously hypertensive rats (SHR) but not in Wistar rats with nephrogenic hypertension due to aortic banding (WIab), the Ang II-induced expression of c-Fos, and c-Jun was enhanced in all four areas when compared to normotensive Wistar Kyoto (WKY)- and Wistar (WI) rats. The Ang II-induced expression of Krox-24 in the SFO, MnPO and PVN in SHR was also significantly increased when compared to WKY, WI and WIab rats. Our data demonstrate that a stimulation of periventricular Ang II-AT1 receptors induces a temporally and spatially highly differentiated expression pattern of ITFs restricted to four distinct regions of the forebrain involved in blood pressure regulation and body fluid homeostasis. The points to a strictly regulated expression of target genes in the respective regions. The enhanced Ang II-induced expression of ITFs in SHR compared to normotensive controls is not due to elevated blood pressure itself, since it was not observed in secondary hypertensive rats WIab. Thus, the increased sensitivity to Ang II in SHR appears to be genetically determined. The target genes regulated by Ang II-induced ITFs will have to be identified.

Behavioural, autonomic and endocrine responses associated with C-fos expression in the forebrain and brainstem after intracerebroventricular infusions of endothelins

Endothelins are a range of peptides (endothelin-1, endothelin-2, and endothelin-3) well known to act peripherally as powerful cardiovascular-regulating agents. Recently, they have been shown to be localized in CSN, where they may act as central neurotransmitters. A variety of putative roles has been ascribed to them in the CNS. To identify those regions of the brain capable of responding to these peptides, the expression of c-fos (an immediate-early gene), has been used to map patterns of activation following intracerebroventricular (i.c.v.) infusions of endothelins in Lister-hooded rats. This has been correlated with changes in heart rate, core temperature and plasma corticosterone levels. Endothelin-3 i.c.v. (50 pmol) decreased both heart rate and core temperature (both recorded by telemetry). This effect lasted for about 30-45 min. Endothelin-1 (10 pmol) or endothelin-3 (50 pmol) i.c.v. induced c-fos expression in the specific regions in the forebrain and brainstem. Strong expression was found in the septum, bed nucleus of the stria terminalis, parvicellular paraventricular nucleus, the central nucleus of the amygdala, dorsal motor nucleus of the vagus and solitary nucleus. There was less marked c-fos expression in other areas of the basal forebrain, such as the organum vasculosum of the lamina terminals, median preoptic nucleus, supraoptic nucleus and the magnocellular. There are two classes of endothelin receptor (A and B). An endothelin-A receptor antagonist, BQ-123, abolished c-fos expression in all structures in the forebrain and brainstem following endothelin-1 infusions. However, an endothelin-B agonist (TetraAla endothelin-1) did not induce discernible c-fos expression in the forebrain or brainstem. These results suggest that the endothelin-A receptor is responsible for endothelin-dependent c-fos induction in the brain. Interactions between endothelins and angiotensin II were also studied. The pattern of c-fos induced by endothelin-3 and angiotensin II was different (particularly in the anteroventral region of the third ventricle). Furthermore, prior infusions of endothelin-3 interfered with the expression of c-fos induced by subsequent angiotensin II, and also suppressed the latter's dipsogenic effect. These results show that endothelin-3 and angiotensin II interact at both behavioural and cellular levels, and that endothelins may play significant roles in the central control of fluid balance and autonomic activity.

Effects of unilateral or bilateral lesions within the anteroventral third ventricular region on c-fos expression induced by dehydration or angiotensin II in the supraoptic and paraventricular nuclei of the hypothalamus

This paper reports the effects of AV3V lesions on the pattern of c-fos induced by 24 h dehydration. As expected, bilateral electrolytic lesions within the AV3V region (the ventral median preoptic nucleus) suppressed water intake following 24 h water deprivation. C-fos expression was also suppressed in the supraoptic (SON) and (less completely) in the paraventricular (PVN) nuclei, but not in the subfornical organ (SFO). Unilateral lesions of the AV3V region suppressed c-fos expression in the ipsilateral SON, but this selective ipsilateral effect was less in the PVN. The SFO was again unaffected. Unilateral lesions also suppressed c-fos expression in the ipsilateral SON and PVN (to a lesser degree) following intraventricular infusions of angiotensin II (250 pmol). These results suggest that the cellular response of supraoptic neurons to osmotic stimuli require inputs from the AV3V region, but that this is less absolute for the PVN; that the projection from the ventral AV3V area to the SON is ipsilateral, but that to the PVN may be less lateralised. Activation of the SFO by dehydration is not dependent upon the integrity of the ventral AV3V region. These results are closely comparable to the effects of similar lesions on c-fos expression following intraventricular infusions of angiotensin II, and suggest that the effect of dehydration on forebrain c-fos expression may be related to the central actions of angiotensin II.

Medullary neurons activated by angiotensin II in the conscious rabbit

Previous studies have shown that angiotensin II (Ang II) can activate cardiovascular neurons within the medulla oblongata via an action on specific receptors. The purpose of this study was to determine the distribution of neurons within the medulla activated by infusion of Ang II into the fourth ventricle of conscious rabbits, using the expression of Fos, the protein product of the immediate early gene c-fos as a marker of neuronal activation. Experiments were done in both intact and barodenervated animals. In comparison with a control group infused with Ringer's solution alone, in both intact and barodenervated animals, fourth ventricular infusion of Ang II (4 to 8 pmol/min) induced a significant increase in the number of Fos-positive neurons in the nucleus of the solitary tract and in the rostral, intermediate, and caudal parts of the ventrolateral medulla. Double-labeling for Fos and tyrosine hydroxylase immunoreactivity showed that 50% to 75% of Fos-positive cells in the rostral, intermediate, and caudal ventrolateral medulla and 30% to 40% of Fos-positive cells in the nucleus of the solitary tract were also positive for tyrosine hydroxylase in both intact and barodenervated animals. The distribution of Fos-positive neurons corresponded very closely to the location of Ang II receptor binding sites as previously determined in the rabbit. The results indicate that medullary neurons activated by Ang II are located in discrete regions within the nucleus of the solitary tract and ventrolateral medulla and include, in all of these regions, both catecholamine and noncatecholamine neurons.

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

1. Cardiovascular and body fluid homeostasis depends upon the activation and co-ordination of reflexes and behavioural responses. In order to accomplish this, the brain receives and processes both neural and chemical input. Once in the brain, information from sources signalling the status of the cardiovascular system and body fluid balance travels, and is integrated, throughout a widely distributed neural network. Recent studies using neuroanatomical and functional techniques have identified several key areas within this neural network. One major processing node is comprised of structures located along the lamina terminalis. 2. Structures associated with the lamina terminalis include the median preoptic nucleus (MePO) and two sensory circumventricular organs (SCVO), the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). Current evidence indicates that blood-borne signals, such as angiotensin II (AngII), reach SCVO (e.g. SFO) where they are transduced. This information is then carried via neural pathways to brain nuclei (e.g. MePO) where it is integrated with other inputs, such as those derived from systemic arterial blood pressure and volume receptors. 3. Because of their receptive and integrative functions, lamina terminalis structures are essential for the normal control of hormone release (e.g. vasopressin), sympathetic activation and behaviours (thirst and salt appetite), which collectively contribute to maintenance of cardiovascular and body fluid homeostasis.

The association of thirst, sodium appetite and vasopressin release with c-fos expression in the forebrain of the rat after intracerebroventricular injection of angiotensin II, angiotensin-(1-7) or carbachol

The effect intracerebroventricular injections of angiotensin II (0.1 nm), angiotensin-(1-7) (1 or 100 nm) and carbachol (500 ng) on c-fos expression was examined in the forebrain of Lister hooded rats. Intense staining of the c-Fos protein was found in the median preoptic nucleus, organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus and supraoptic nucleus after angiotensin II and carbachol Angiotensin II caused significantly more c-fos expression in the ventral median preoptic nucleus and organum vasculosum of the lamina terminalis than carbachol, whereas in the paraventricular and supraoptic nuclei this was reversed, with carbachol having a greater effect on c-fos expression in these areas. Angiotensin-(1-7), however, only induced c-Fos protein in the organum vasculosum of the lamina terminalis and median preoptic nucleus with the number and the intensity of staining of the nuclei significantly less in both areas than after angiotensin II or carbachol. Separate groups of Lister rats were given i.c.v. injections of the same substances at the same doses, but excluding the lower dose of angiotensin-(1-7), and the intakes of water and 1.8% NaCl over 60 min were measured. Angiotensin II stimulated intakes of both water and NaCl. The effect on water intake was almost immediate (<1 min), whereas NaCl intake did not usually start until at least 5 min after injection. Over 60 min, water (12.4 +/- 1.0 ml) and NaCl (4.2 +/- 0.9 ml) intakes were significantly greater than water (1.1 +/- 0.2 ml) and NaCl (0.6 +/- 0.5 ml) intakes of the controls. Carbachol caused less drinking than angiotensin II, the water intake over 60 min being significantly less (4.8 +/- 0.7 ml) and the latency of response greater (>5 min). Carbachol, unlike angiotensin II, had little effect on NaCl intake (0.7 +/- 0.4 ml). Angiotensin-(1-7) had no effect on water (1.1 +/- 0.3 ml) or NaCl (0.3 +/- 0.3 ml) intakes. The plasma levels of vasopressin were measured after i.c.v. injection of the same three substances in the same doses, again excluding the lower dose of angiotensin-(1-7), in further groups of rats. Angiotensin II and carbachol caused an approximate five-fold increase in plasma vasopressin levels compared to cerebrospinal fluid-injected rats, but angiotensin-(1-7) had no effect on vasopressin release. Therefore, three compounds with widely differing effects on thirst, sodium appetite and vasopressin release induce distinctive patterns of c-fos protein expression in the forebrain. By combining experimental approaches in this way it is possible to determine areas of the brain which are involved in certain behavioural and endocrine responses.

Regional suppression by lesions in the anterior third ventricle of c-fos expression induced by either angiotensin II or hypertonic saline

Angiotensin II (250 pmol) infused into the cerebral ventricles of male rats induces the expression of c-fos in the subfornical organ, supraoptic and paraventricular nuclei of the hypothalamus, as well as in the lateral parabrachial nucleus, locus coeruleus and the nucleus of the solitary tract in the brainstem. Electrolytic lesions of the anteroventral third ventricle, principally the subcommissural (ventral) median preoptic nucleus, inhibited the dipsogenic response to i.c.v. angiotensin II and also suppressed c-fos expression in supraoptic nucleus, paraventricular nucleus, lateral parabrachial nucleus, locus coeruleus and nucleus of the solitary tract but not in the subfornical organ or dorsal median preoptic nucleus. The stimulating effect of i.c.v. angiotensin II on corticosterone was also reduced. Median preoptic nucleus lesions also suppressed the expression of c-fos following i.v. infusions of 6 micrograms angiotensin II in supraoptic nucleus and paraventricular nucleus but not in subfornical organ, dorsal median preoptic nucleus, lateral parabrachial nucleus, locus coeruleus and nucleus of the solitary tract. Median preoptic nucleus lesions reduced the dipsogenic effects of an intragastric infusion of hypertonic (1.5 M) saline and suppressed c-fos expression in supraoptic nucleus and paraventricular nucleus compared to sham-lesioned rats. However, c-fos expression was unaltered in subfornical organ, dorsal median preoptic nucleus lesions had no effect on the increased corticosterone induced by hypertonic saline. Subfornical organ lesions did not alter dipsogenic responses to i.c.v. angiotensin II, nor was the i.c.v. angiotensin II-induced expression of c-fos suppressed in the basal forebrain. These experiments show that the ventral median preoptic nucleus (but not the subfornical organ), part of the anteroventral third ventricle, is critical for the expression of c-fos in more caudal areas of the brain following i.c.v. angiotensin II. c-fos expression in supraoptic nucleus and paraventricular nucleus following i.v. angiotensin II is also dependent on an intact median preoptic nucleus, suggesting that supraoptic nucleus and paraventricular nucleus activation may be dependent on the median preoptic nucleus, and that suppression following i.c.v. infusions is not due to mechanical obstruction to infused peptide. However, there is a clear separation of the effects of i.c.v. and i.v. angiotensin II on brainstem structures. The median preoptic nucleus (but not the subfornical organ) seems essential for activation following the former but not the latter, suggesting alternative mechanisms for the effect of i.v. angiotension II on the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)

Autoradiographic identification of brain angiotensin IV binding sites and differential c-Fos expression following intracerebroventricular injection of angiotensin II and IV in rats

A unique angiotensin binding site specific for the hexapeptide, angiotensin II(3-8) (AngIV), has been previously reported by our laboratory in the guinea pig brain and is presently described in the rat brain. This angiotensin receptor subtype has been termed AT4 and is prominently distributed in cerebral cortex, piriform cortex, hippocampus, habenulae, colliculi, septum, periaqueductal gray, several thalamic nuclei, the arcuate nucleus of the hypothalamus and cerebellum. In the second part of the present investigation, separate groups of rats received i.c.v. injections of angiotensin II (AngII), AngIV or artificial cerebrospinal fluid (aCSF) and were euthanized 2 h later for the purpose of evaluating for brain c-Fos expression. After i.c.v.-injected AngIV, Fos-like immunoreactivity was present in the hippocampus and piriform cortex. This immunoreactivity was unaffected by i.c.v. pretreatment with the AT1 angiotensin receptor antagonist DuP 753 (losartan) or the AT2 receptor ligand PD123177 but was blocked by the AT4 angiotensin receptor antagonist, divalanal-AngIV. I.c.v. injection of AngII resulted in Fos-like immunoreactivity in the dorsal third and lateral ventricles, subfornical organ, lateral hypothalamus and amygdala. Pretreatment with losartan or PD123177 significantly interfered with this AngII-induced immunoreactivity while divalanal-AngIV did not. These results indicate that in both guinea pig and rat brains the AT4 receptor has a distribution different than that previously reported for AT1 and AT2 receptor subtypes. The c-Fos expression results suggest that different brain neuronal pathways are activated by i.c.v. injection of AngII and AngIV.

Pregastric and postabsorptive inhibitory effects of water on angiotensin-induced Fos in rat brain

Intraventricularly (ICV)-injected Ang II activates cells in several forebrain structures of rats, including SFO, OVLT, PVN and SON, and produces a spectrum of physiological and behavioral responses. It has been reported that the Fos-like immunoreactivity (FLI; a marker of cell activation) that is induced in PVN and SON by exogenous Ang II is prevented by allowing rats to ingest water after injection. The present study tests whether this inhibitory effect of water is due to either preabsorptive (oral) or postingestive factors, or both. Rats received cerebroventricular injection of Ang II (100 ng) and were killed 1 h later. One group received no water after the injection, another group drank water, a third group drank water but had it drain from a gastric fistula (sham drinking), and a fourth group received an intragastric infusion of water. In confirmation of a previous study, rats allowed to drink water after Ang II showed an almost complete absence of FLI in SON and PVN, compared with those not allowed access to water. Both sham drinking and intragastrically-infused rats showed less FLI in SON and PVN than nondrinkers. The reduction was greater in the infused group than the sham drinkers, but was not as great as in those allowed to drink water and absorb it. This suggests that both orally-mediated as well as postabsorptive factors can inhibit Ang II-induced FLI by impinging upon cells in the SON and PVN. In contrast, Ang II-induced FLI in OVLT and SFO was not affected by any of the oral or gastric water conditions.

Fos induced in brain of spontaneously hypertensive rats by angiotensin II and co-localization with AT-1 receptors

The induction of Fos-like immunoreactivity (FLI) by peripheral administration of angiotensin II (Ang II) was used to determine whether central activation was greater in spontaneously hypertensive rats (SHR) than in normotensive WKY and outbred Wistar controls. FLI was induced in the same brain regions (circumventricular organs and neurosecretory hypothalamic cell groups) in all three groups of rats, but the FLI in several of these regions was markedly less in WKY than in either SHR or Wistar. This reduced responsiveness in supraoptic and paraventricular nuclei was selective to Ang II, because the FLI induced in these nuclei by hypertonic NaCl did not differ between groups. We also report that a considerable number of cells in the SON and PVH expressing FLI to these stimuli show immunostaining with an antibody to the AT-1 Ang II receptor. These data indicate that central angiotensinergic pathways may be more sensitive in SHR than WKY, and that WKY are less sensitive than outbred Wistars.

c-fos expression in the paraventricular nucleus of the hypothalamus following intracerebroventricular infusions of neuropeptide Y

Intracerebroventricular (i.c.v.) infusions of neuropeptide Y (NPY) (2500 pmol) induced c-fos protein in the paraventricular nucleus (PVN) of intact male rats 60 min later. The greatest expression was observed in the dorsal (parvicellular) region of the PVN; there were intermediate levels in the lateral (magnocellular) and lowest ones in the medial (parvicellular) regions. Allowing rats to eat during the post-infusion interval did not modify this pattern of c-fos expression. Depriving rats of food for either 24 or 48 h did not induce recognisable expression of c-fos in the PVN, and allowing 24 h-deprived rats to eat also had no effect on PVN c-fos. Plasma insulin was increased by i.c.v. NPY, and raised still further in rats that were allowed to eat following NPY infusions. However, plasma glucose was not altered by either treatment. Food-deprived rats had low levels of insulin, but unaltered blood glucose, compared to controls. These results show that NPY can induce c-fos expression in both parvicellular and magnocellular areas of the PVN. The pattern of expression within the PVN seems to differ from that induced by other peptides, such as angiotensin II, vasopressin and corticotropin-releasing factor, suggesting that distinct populations of neurons are activated by different peptides within the complex structure of the PVN. Food deprivation does not induce c-fos expression within the PVN, though other studies have shown that NPY levels and release are both increased, so there is no simple relation between current energy state, blood levels of either glucose or insulin and c-fos expression within the PVN.