Regional expression of c-fos antigen in the basal forebrain following intraventricular infusions of angiotensin and its modulation by drinking either water or saline

Fourth ventricle injection of ghrelin decreases angiotensin II-induced fluid intake and neuronal activation in the paraventricular nucleus of the hypothalamus

Ghrelin acts in the CNS to decrease fluid intake under a variety of dipsogenic and natriorexigenic conditions. Previous studies on this topic, however, focused on the forebrain as a site of action for this effect of ghrelin. Because the hindbrain contains neural substrates that are capable of mediating the well-established orexigenic effects of ghrelin, the current study tested the hypothesis that ghrelin applied to the hindbrain also would affect fluid intake. To this end, water and saline intakes were stimulated by central injection of angiotensin II (AngII) in rats that also received injections of ghrelin (0.5μg/μl) into either the lateral or fourth ventricle. Ghrelin injected into either ventricle reduced both water and 1.8% NaCl intake that was stimulated by AngII. The nature of the intake effect revealed some differences between the injection sites. For example, forebrain application of ghrelin reduced saline intake by a reduction in both the number of licking bursts and the size of each licking burst, but hindbrain application of ghrelin had a more selective effect on burst number. In an attempt to elucidate a brain structure in which hindbrain-administered ghrelin and forebrain-administered AngII interact to cause the ingestive response, we used Fos-immunohistochemistry in rats given the treatments used in the behavioral experiments. Although several brain areas were found to respond to either ghrelin or AngII, of the sites examined, only the paraventricular nucleus of the hypothalamus (PVN) emerged as a potential site of interaction. Specifically, AngII treatment caused expression of Fos in the PVN that was attenuated by concomitant treatment with ghrelin. These experiments provide the novel finding that the hindbrain contains elements that can respond to ghrelin and cause decreases in AngII-induced fluid intake, and that direct actions by ghrelin on forebrain structures is not necessary. Moreover, these studies suggest that the PVN is an important site of interaction between these two peptides.

Offspring’s hydromineral adaptive responses to maternal undernutrition during lactation

Early development, throughout gestation and lactation, represents a period of extreme vulnerability during which susceptibility to later metabolic and cardiovascular injuries increases. Maternal diet is a major determinant of the foetal and newborn developmental environment; maternal undernutrition may result in adaptive responses leading to structural and molecular alterations in various organs and tissues, such as the brain and kidney. New nephron anlages appear in the renal cortex up to postnatal day 4 and the last anlages to be formed develop into functional nephrons by postnatal day 10 in rodents. We used a model of undernutrition in rat dams that were food-restricted during the first half of the lactation period in order to study the long-term effects of maternal diet on renal development, behaviour and neural hydromineral control mechanisms. The study showed that after 40% food restriction in maternal dietary intake, the dipsogenic responses for both water and salt intake were not altered; Fos expression in brain areas investigated involved in hydromineral homeostasis control was always higher in the offspring in response to isoproterenol. This was accompanied by normal plasma osmolality changes and typical renal histology. These results suggest that the mechanisms for the control of hydromineral balance were unaffected in the offspring of these 40% food-restricted mothers. Undernutrition of the pups may not be as drastic as suggested by dams’ restriction.

Brain somatostatin receptor 2 mediates the dipsogenic effect of central somatostatin and cortistatin in rats: role in drinking behavior

Intracerebroventricular injection of stable somatostatin (SST) agonists stimulates food and water intake in rats. We investigated the receptor subtype(s) involved in the dipsogenic effect of intracerebroventricular injection of SST agonists, mechanisms of action, and role. In nonfasted and non-water-deprived male rats with chronic intracerebroventricular cannula, intake of water without food or food without water was monitored separately to avoid any interactions compared with intracerebroventricular vehicle. SST-14 and cortistatin (CST-14) (1 μg/rat icv) increased water intake by 3.1- and 2.7-fold, respectively, while both peptides did not alter food intake at 1 h postinjection in the light phase. By contrast, the stable pan-somatostatin agonist ODT8-SST (1 μg/rat icv) increased both water and food intake by 4.9- and 3.7-fold, respectively. S-346-011, a selective receptor 2 (sst2) agonist (1 μg/rat icv) induced water ingestion, while sst1 or sst4 agonist, injected under the same conditions, did not. The sst2 antagonist S-406-028 (1 μg/rat icv) prevented the 1-h water intake induced by intracerebroventricular ODT8-SST and CST-14. Losartan (100 μg/rat icv), an angiotensin receptor 1 (AT1) antagonist, completely blocked the water consumption induced by intracerebroventricular ODT8-SST, whereas intracerebroventricular injection of S-406-028 did not modify the intracerebroventricular ANG II-induced dipsogenic response. The sst2 antagonist reduced by 40% the increase of the 3-h water intake in the early dark phase. These data indicate that SST-14 and CST-14 interact with sst2 to exert a potent dipsogenic effect, which is mediated downstream by angiotensin-AT1 signaling. These data also indicate that sst2 activation by brain SST-14 and/or CST-14 may play an important role in the regulation of drinking behavior.

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

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

Central nitrergic system regulation of neuroendocrine secretion, fluid intake and blood pressure induced by angiotensin-II

BACKGROUND

Nitric oxide (NO) synthesis has been described in several circumventricular and hypothalamic structures in the central nervous system that are implicated in mediating central angiotensin-II (ANG-II) actions during water deprivation and hypovolemia. Neuroendocrine and cardiovascular responses, drinking behavior, and urinary excretions were examined following central angiotensinergic stimulation in awake freely-moving rats pretreated with intracerebroventricular injections of Nω-nitro-L-arginine methyl ester (L-NAME, 40 μg), an inhibitor of NO synthase, and L-arginine (20 ug), a precursor of NO.

RESULTS

Injections of L-NAME or ANG-II produced an increase in plasma vasopressin (VP), oxytocin (OT) and atrial natriuretic peptide (ANP) levels, an increase in water and sodium intake, mean arterial blood pressure and sodium excretion, and a reduction of urinary volume. L-NAME pretreatment enhanced the ANG-II response, while L-arginine attenuated VP and OT release, thirst, appetite for sodium, antidiuresis, and natriuresis, as well as pressor responses induced by ANG-II.

DISCUSSION AND CONCLUSION

Thus, the central nitrergic system participates in the angiotensinergic responses evoked by water deprivation and hypovolemia to refrain neurohypophysial secretion, hydromineral balance, and blood pressure homeostasis.

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.

Different neuronal populations of the rat median preoptic nucleus express c-fos during sleep and in response to hypertonic saline or angiotensin-II

The median preoptic nucleus (MnPN) of the hypothalamus contains sleep-active neurones, and sleep-related Fos-immunoreactivity (IR) in this nucleus is primarily expressed in GABAergic cells. The MnPN also contains cells responsive to hypertonic saline and to angiotensin-II (Ang-II). To clarify functional relationships between MnPN neurones involved in the regulation of sleep and body fluid homeostasis, we examined c-fos expression in the MnPN after administration of hypertonic saline and Ang-II in both spontaneously sleeping and sleep-deprived rats. Systemic administration of hypertonic saline and intracerebroventricular (i.c.v.) injection of Ang-II increased Fos-IR in both spontaneously sleeping and sleep-deprived rats, compared to control animals. To determine if the population of MnPN neurones activated in response to osmotic and hormonal stimuli is similar to or different from neurones activated during sleep, we quantified Fos-IR in MnPN GABAergic neurones in spontaneously sleeping hypertonic saline- and Ang-II-treated rats versus respective control rats. Fos-IR evoked by these treatments occurred primarily (80-85%) in non-GABAergic neurones. Findings of the present study provide evidence that separate populations of MnPN neurones are involved in the regulation of sleep and body fluid homeostasis.

Social recognition memory requires two stages of protein synthesis in mice

Olfactory recognition memory was tested in adult male mice using a social discrimination task. The testing was conducted to begin to characterize the role of protein synthesis and the specific brain regions associated with activity in this task. Long-term olfactory recognition memory was blocked when the protein synthesis inhibitor anisomycin was injected 20 min before, immediately after, or 6 h after sampling. No effect was observed when anisomycin was administered 3 h or 18 h after sampling. Immunohistochemical analysis of Fos expression revealed that sampling-like exposure to a juvenile increased the activity of a subset of cells in the accessory olfactory bulb and the brain areas that are associated with it. Additionally, increased Fos expression was measured in the main olfactory bulb and the piriform cortex, whereas no signs of activation were seen in the cortical nucleus of the amygdala, all components of the main olfactory system. No increases in Fos immunoreactivity were observed after 4 h. Our data suggest that long-lasting olfactory recognition memory requires two stages of protein synthesis. The first stage takes place within 1-2 h and the second stage between 6-7 h after sampling. The first but not the second stage is paralleled by an increase in the number of Fos-immunoreactive cells in brain areas associated with both the main and accessory olfactory systems. It therefore appears that the role of the second stage of protein synthesis in recognition memory depends on the integrity of the first stage of protein synthesis.

ANG III induces expression of inducible transcription factors of AP-1 and Krox families in rat brain

In addition to rapid responses comprising increases in blood pressure, drinking, and stimulation of natriuresis, ANG II induces the expression of transcription factors (TF) in the central nervous system. The ANG II metabolite ANG III (ANG 2–8) has been demonstrated to exert physiological effects similar to those of ANG II. We aimed to determine 1) whether ANG III induces TF expression in the brain, 2) which ANG II (AT) receptor subtype is involved, and 3) whether the two peptides, ANG II and ANG III, differ in their efficacy to stimulate TF expression. ANG II (100 pmol), ANG III (100 pmol), or vehicle was injected into the lateral brain ventricle of conscious rats alone or in combination with the AT1 receptor antagonist losartan (10 nmol), the AT2 receptor antagonist PD-123319 (5 nmol), or the aminopeptidase inhibitor amastatin (10 nmol). Similar to ANG II, ANG III induced the expression of c-Fos, c-Jun, and Krox-24 in four brain regions, subfornical organ, median preoptic area, paraventricular nucleus, and supraoptic nucleus of the hypothalamus, with the same efficacy. This effect was AT1 receptor mediated. Pretreatment with amastatin reduced the expression of TF in response to ANG II, indicating that this expression is partly mediated by ANG III. Interestingly, the AT2 receptor antagonist PD-123319 alone slightly enhanced the expression of c-Fos, c-Jun, and Krox-24 in different populations of neurons of the paraventricular nucleus. These data indicate that different populations of neurons in the paraventricular nucleus are tonically inhibited by AT2 receptors under physiological conditions.

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.

Angiotensin peptides modulatory system: how is it implicated in the control of seizure susceptibility?

Accumulated studies support the concept that angiotensin peptides, ANG II, ANG III, and ANG IV act as neurotransmitters or neuromodulators in specific neuronal pathways in the brain stem, the hypothalamus, and the forebrain. They have been implicated in the regulation of several physiological processes, particularly in excitable brain structures that express high concentration of their receptors. With the help of pharmacological approaches it was shown that angiotensin peptides appear to be anticonvulsant in a variety of experimental seizure models. Thus, ANG II increases the threshold for pentylenetetrazol (PTZ)-, bicuculline-and picrotoxin-induced seizures in mice. It also attenuates the intensity of clonic seizures evoked by PTZ and 3-mercaptopropionic acid and is effective in the maximal electroshock test. Furthermore, ANG II, ANG III, and ANG IV protect against the clonic convulsions in the PTZ kindling model of epilepsy in mice. From the accumulated results it could be assumed that the angiotensin peptides appear to realize their effects acting directly on their receptors (AT(1), AT(2) and AT(4)) and through close interaction with different neurotransmitter/neuromodulator systems as dopamine (DA)-, gamma-aminobutyric acid (GABA)-and adenosine. This may contribute to a new potential use of angiotensin drugs either alone or in combination with other neuroprotective agents acting through the above mentioned systems, thus providing a more rational strategy for the treatment of neurodegenerative disorders such as epilepsy.

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.

Heterogeneous co-localization of AT1A receptor and Fos protein in forebrain neuronal populations responding to acute hydromineral deficit

The present study investigates co-localization of AT(1A) receptor subtype and Fos protein in neuronal populations of the lamina terminalis (LT) that have been recruited during acute Na(+) and water depletion mediated by furosemide injections. For that purpose, we combined high cellular resolution of in situ hybridization technique to reveal neurons expressing AT(1A) receptor gene (AT(1A) mRNA) with the specificity of Fos protein immunoreactivity as a marker of neuronal activation (Fos-ir). As expected, furosemide treatment dramatically increased the density of Fos-immunoreactive neuronal population in all the regions of the LT compared to control (saline-injected animals). Distribution analysis of Fos-ir neurons and AT(1A) receptor-expressing neurons performed consecutively to furosemide-induced Na(+) and water depletion indicated that double-labeled neurons (AT(1A) mRNA+Fos-ir) represented the majority (67%) of the neuronal population that expressed AT(1A) receptor in the rim of the vascular organ of the lamina terminalis (OVLT). Double-labeled neurons amounted about 60% of the neurons that expressed AT(1A) receptor in the core of the subfornical organ (SFO) and 34% in the periphery of the SFO. In the median preoptic nucleus (MnPO), the density of the double-labeled neuronal population observed in the furosemide-treated animals remained weak compared to the control group of animals. Double-labeled neuronal population estimated in the MnPO of the furosemide-treated group of animals represented 17% of the neurons that express AT(1A) receptor gene. Our results report a heterogeneous distribution of the neuronal populations that co-localize AT(1A) receptor and Fos protein in the lamina terminalis after an acute Na(+) and water depletion. This study gives anatomical support to a direct action of endogenous AngII on c-fos transcription via binding on AT(1A) receptor in specific areas of the circumventricular organs (rim of the OVLT and core of the SFO). In the MnPO, our data indicate that intracellular signaling pathways unlikely couple AT(1A) receptor with c-fos transcription. The expression of Fos protein in this nucleus might be therefore secondary to the recruitment of excitatory inputs different from AngII. This observation underlines the complexity of molecules and neurocircuits in the preoptic region that are involved in the control of acute Na(+) and water deficit.

Induction of c-fos in forebrain circumventricular organs after renal artery stenosis

Experiments were done in the anaesthetized rat to determine the effect of activation of renal receptors following renal arterial occlusion (RAO) on the induction of c-fos in neurons of the lamina terminalis in the forebrain. Following RAO, fos labeled neurons were found in both the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). Transection of the renal nerves ipsilateral to RAO reduced ( approximately 61%) the number of fos labeled neurons in the SFO and prevented the fos labeling in the OVLT. Similarly, administration of the angiotensin II converting enzyme inhibitor enalapril maleate prior to RAO also reduced ( approximately 27%) the number of fos labeled neurons in the SFO to RAO. However, the number of fos labeled neurons was not altered in the OVLT. The number of fos labeled neurons in the SFO of the intact animals after RAO was found to be greater than the algebraic sum of the number of fos labeled neurons in the renal nerve transected and enalapril treated animals. These results suggest that neurons in the SFO are activated by at least two different mechanisms following renal artery occlusion; those involving the activation of afferent renal nerves and those due to changes in circulating levels of angiotensin II. In addition, afferent renal nerve inputs combined with the effect of increased circulating levels of angiotensin II produce a greater activation of the SFO than either input alone. On the other hand, the OVLT appears to be selectively activated by afferent renal nerve inputs following RAO. Taken together, these data suggest that neural inputs from the kidney may play an important role in controlling body fluid balance and arterial pressure (AP) by influencing the activity of forebrain circumventricular organs neurons that function in the detection of blood borne signals associated with changes in extracellular fluid volume.

The brain renin-angiotensin system: location and physiological roles

Angiotensinogen, the precursor molecule for angiotensins I, II and III, and the enzymes renin, angiotensin-converting enzyme (ACE), and aminopeptidases A and N may all be synthesised within the brain. Angiotensin (Ang) AT(1), AT(2) and AT(4) receptors are also plentiful in the brain. AT(1) receptors are found in several brain regions, such as the hypothalamic paraventricular and supraoptic nuclei, the lamina terminalis, lateral parabrachial nucleus, ventrolateral medulla and nucleus of the solitary tract (NTS), which are known to have roles in the regulation of the cardiovascular system and/or body fluid and electrolyte balance. Immunohistochemical and neuropharmacological studies suggest that angiotensinergic neural pathways utilise Ang II and/or Ang III as a neurotransmitter or neuromodulator in the aforementioned brain regions. Angiotensinogen is synthesised predominantly in astrocytes, but the processes by which Ang II is generated or incorporated in neurons for utilisation as a neurotransmitter is unknown. Centrally administered AT(1) receptor antagonists or angiotensinogen antisense oligonucleotides inhibit sympathetic activity and reduce arterial blood pressure in certain physiological or pathophysiological conditions, as well as disrupting water drinking and sodium appetite, vasopressin secretion, sodium excretion, renin release and thermoregulation. The AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and plays a role in memory mechanisms. In conclusion, angiotensinergic neural pathways and angiotensin peptides are important in neural function and may have important homeostatic roles, particularly related to cardiovascular function, osmoregulation and thermoregulation.

Involvement of mesolimbic structures in short-term sodium depletion: in situ hybridization and ligand-binding analyses

Acute treatment with the diuretic furosemide (Lasix) produces a reduction in plasma Na(+) and volume as well as increased thirst and salt appetite. The resulting hypovolemia stimulates the well-known counter-regulatory physiological response from the renin-angiotensin-aldosterone system. However, the neurochemical players underpinning the behavioral responses of thirst and salt appetite are less clear. Previously, we have reported that salt-replete deoxycorticosterone (DOCA) treatment activates mesolimbic structures associated with reward and goal-seeking behavior. The present study was designed to test whether the same brain regions are affected in a salt-depleted state. In experiment 1, two groups of adult male Sprague-Dawley (SD) rats were injected with Lasix (10 mg/rat, s.c.) and 18 h later were allowed access either to 2% NaCl solution ('Lasix+salt') or only to tap water ('Lasixnosalt') for 2 h. For comparison purposes, a third group received an isotonic saline injection instead of Lasix and was allowed access to the 2% salt solution (Vehicle). All groups were permitted 24 h access to tap water. We found no differences in dynorphin-mRNA levels in any striatal and accumbal regions among any of the treatment groups. However, as found previously in DOCA-treated rats, there were increased enkephalin (ENK)-mRNA and decreased dopamine transporter (DAT) binding levels throughout the striatum in Lasix+salt and decreased ENK-mRNA in Lasixnosalt rats versus Vehicle. In experiment 2, the involvement of the ENKergic and/or dopaminergic system was tested in rats divided into the same three groups described in experiment 1. However, before access to salt or water, the Lasix+salt and the vehicle groups were administered either a delta-opioid, naltrindole or a dopamine D(2) antagonist, raclopride. Only the naltrindole-treated rats showed a blunted intake of salt solution. Thus, these findings along with our neurochemical results suggest that mesolimbic enkephalin might impact salt intake through dopaminergic systems.

Acute sodium deficit triggers plasticity of the brain angiotensin type 1 receptors

The brain renin-angiotensin system (bRAS) is involved in the control of hydromineral balance. However, little information is available on the functional regulation of the bRAS as a consequence of sodium deficit in the extracellular fluid compartments. We used a pharmacological model of acute Na+ depletion (furosemide injections) to investigate changes of a major component of the bRAS, the hypothalamic angiotensin type 1A (AT(1A)) receptors. Furosemide induced a rapid and long-lasting expression of the AT(1A) mRNA in the subfornical organ, the median preoptic nucleus (MnPO), and the parvocellular division of the paraventricular nucleus (pPVN). Na+ depletion increased the number of cells expressing AT(1A) mRNA in the pPVN, but not in the MnPO. The enhancement of AT(1A) mRNA expression was associated with an increase in AT(1) binding sites in all the regions studied. It is of interest that in the paraventricular nucleus, the majority of the neurons expressing AT(1A) mRNA also showed an increase in metabolic activity (Fos-related antigen immunoreactivity [FRA-ir]). By contrast, in the MnPO, we observe two distinct cell populations. Our data demonstrated that an acute Na+ deficit induced a functional regulation of the hypothalamic AT(1A) receptors, indicating that these receptors are subject to plasticity in response to hydromineral perturbations.

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.

Involvement of adrenoceptors in the angiotensin II-induced expression of inducible transcription factors in the rat forebrain and hypothalamus

Angiotensin II (Ang II) acts as a neuromodulator/neurotransmitter in specific brain nuclei involved in the regulation of blood pressure and volume homeostasis. It also induces a highly differentiated transcription factor expression in these nuclei. We investigated whether adrenoceptors, which modulate other central actions of angiotensin II like the vasopressin release, also play a role in the AT1 receptor-mediated expression of the transcription factors (TF) c-Fos, c-Jun and Krox-24 in the rat brain. Ang II, injected intracerebroventricularly, induced the expression of c-Fos, c-Jun and Krox-24 in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Pretreatment with the alpha 1-adrenoceptor antagonist, prazosin, significantly inhibited the Ang II-induced transcription factor expression in the SON and PVN. The alpha 2-adrenoceptor antagonist, yohimbine, also reduced Ang II-stimulated transcription factors significantly in both nuclei. This inhibition was mainly localized in vasopressinergic magnocellular neurons in both nuclei. The beta-adrenoceptor antagonist, propranolol, did not influence the Ang II-induced expression of TF. Our results show that both, Ang II-induced vasopressin release and transcription factor expression, involve the same neuronal connections in the brain, implicating that the signal transduction pathways leading to the two different effects are at least to a certain degree convergent.

Hypothalamic and zona incerta neurons in sheep, initially only responding to the sight of food, also respond to the sight of water after intracerebroventricular injection of hypertonic saline or angiotensin II

Extracellular single-unit recordings were made from neurons in the lateral hypothalamus (LH) or zona incerta (ZI) of conscious sheep. A small population of neurons (12/83) were found which responded with increased firing rate when the animal looked at food but did not respond when the sheep looked at water. The effects of rapidly inducing intense thirst by the intracerebroventricular (i.c.v.) injection of hypertonic (0.85 M) saline or 200 ng of angiotensin II, or a mixture of the two dipsogenic stimuli, on the response of neurons initially responding only to the sight of food were investigated. Following i.c.v. injection of the dipsogenic stimuli the neurons began to respond strongly to the sight of water. The results demonstrated that changing the animal's motivational state alters the response of some neurons in the LH and ZI and suggests that the neuronal response is influenced by the animal's dominant need at the time of testing.

Activation of kidney-directed neurons in the lamina terminalis by alterations in body fluid balance

This study was undertaken to determine if neurons in the lamina terminalis, previously identified as projecting to the kidney (35), were responsive to alterations in stimuli associated with fluid balance homeostasis. Neurons in the lamina terminalis projecting to the kidney were identified by the retrograde transynaptic transport of Bartha's strain of pseudorabies virus in anesthetized rats. Rats were also exposed to 24-h water deprivation, intravenous hypertonic saline, or intracerebroventricular ANG II. To determine if "kidney-directed" neurons were activated following each stimulus, brain sections that included the lamina terminalis were examined immunohistochemically for viral antigen and Fos protein. With the exception of ANG II in the subfornical organ, all regions of the lamina terminalis contained neurons that were significantly activated by water deprivation, hypertonic saline, and ANG II. These results provide evidence for a neural substrate, which may underpin some of the effects of hypertonic saline and ANG II on renal function thought to be mediated through the lamina terminalis.

Effects of hypotension and fluid depletion on central angiotensin-induced thirst and salt appetite

We examined the effects of hypotension and fluid depletion on water and sodium ingestion in rats in response to intracerebroventricular infusions of ANG II. Hypotension was produced by intravenous infusion of the vasodilator drug minoxidil (25 microg x kg(-1) x min(-1)) concurrently with the angiotensin-converting enzyme inhibitor captopril (0.33 mg/min) to prevent endogenous ANG II formation. Hypotension increased water intake in response to intracerebroventricular ANG II (30 ng/h) but not intake of 0.3 M NaCl solution and caused significant urinary retention of water and sodium. Acute fluid depletion was produced by subcutaneous injections of furosemide (10 mg/kg body wt) either alone or with captopril (100 mg/kg body wt sc) before intracerebroventricular ANG II (15 or 30 ng/h) administration. Fluid depletion increased water intake in response to the highest dose of intracerebroventricular ANG II but did not affect saline intake. In the presence of captopril, fluid depletion increased intakes of both water and saline in response to both doses of intracerebroventricular ANG II. Because captopril administration causes hypotension in fluid-depleted animals, the results of the two experiments suggest that hypotension in fluid-replete animals preferentially increases water intake in response to intracerebroventricular ANG II and in fluid-depleted animals increases both salt and water intake in response to intracerebroventricular ANG II.

C-FOS expression in the rat brain in response to substance P and neurokinin B

Substance P, the principal neurokinin peptide in the mammalian brain and the natural ligand for the NK(1) tachykinin receptor, plays an integrative role in the regulation of cardiovascular, neuroendocrine and behavioural responses to stress. In rats, stimulation of periventricular NK(1) receptors in the forebrain induces a distinct pattern of cardiovascular responses which is accompanied by intense grooming behaviour. Ligands for NK(3) receptors induce a different pattern of cardiovascular and behavioural responses which comprises an increased release of vasopressin from the posterior pituitary and wet-dog shakes behaviour. To define the brain areas in the rat which respond to stimulation of forebrain NK(1) and NK(3) receptors and participate in the generation of these responses, the induction of c-Fos immunoreactivity was examined in brains following intracerebroventricular injections of substance P and neurokinin B in conscious rats. Stimulation of central NK(1) receptors by substance P (25, 100 and 500 pmol) injected into the lateral ventricle elicited grooming behaviour (face washing and hind limb grooming) and resulted in a marked c-Fos expression in the paraventricular, dorsomedial and parabrachial nuclei and in the medial thalamus. At 25 pmol, substance P did not significantly increase c-Fos expression, at 100 pmol, maximal c-Fos activation was induced in all four brain regions which responded to the peptide. Intracerebroventricular pretreatment of rats with the selective and high-affinity, non-peptide NK(1) receptor antagonist, RP 67580 (500 pmol), but not with its inactive enantiomer, RP 68651, completely abolished the behavioural response to substance P and reduced the substance P-induced c-Fos expression in all brain areas to nearly control levels. Intracerebroventricular injection of the natural ligand for NK(3) receptors, neurokinin B (500 pmol), elicited wet-dog shakes behaviour and activated c-Fos expression in localized regions of the forebrain including the organum vasculosum laminae terminalis, subfornical organ, median preoptic nucleus, paraventricular, supraoptic and anterior hypothalamic nuclei, medial thalamus and in the ventral tegmental area. These results demonstrate that the neurokinins, substance P and neurokinin B, induce specific and different patterns of c-Fos expression in distinct regions of the rat brain. Brain areas which selectively responded to substance P have been traditionally linked to the central regulation of cardiovascular and neuroendocrine reactions to stress or involved in the processing of nociceptive responses. On the other side, brain areas activated by neurokinin B are known to be involved in the central regulation of blood pressure, water and salt homeostasis or control of behaviour.

Basic organization of projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis in adult rat brain

The organization of axonal projections from the oval and fusiform nuclei of the bed nuclei of the stria terminalis (BST) was characterized with the Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracing method in adult male rats. Within the BST, the oval nucleus (BSTov) projects very densely to the fusiform nucleus (BSTfu) and also innervates the caudal anterolateral area, anterodorsal area, rhomboid nucleus, and subcommissural zone. Outside the BST, its heaviest inputs are to the caudal substantia innominata and adjacent central amygdalar nucleus, retrorubral area, and lateral parabrachial nucleus. It generates moderate inputs to the caudal nucleus accumbens, parasubthalamic nucleus, and medial and ventrolateral divisions of the periaqueductal gray, and it sends a light input to the anterior parvicellular part of the hypothalamic paraventricular nucleus and nucleus of the solitary tract. The BSTfu displays a much more complex projection pattern. Within the BST, it densely innervates the anterodorsal area, dorsomedial nucleus, and caudal anterolateral area, and it moderately innervates the BSTov, subcommissural zone, and rhomboid nucleus. Outside the BST, the BSTfu provides dense inputs to the nucleus accumbens, caudal substantia innominata and central amygdalar nucleus, thalamic paraventricular nucleus, hypothalamic paraventricular and periventricular nuclei, hypothalamic dorsomedial nucleus, perifornical lateral hypothalamic area, and lateral tegmental nucleus. Moderately dense inputs are found in the parastrial, tuberal, dorsal raphé, and parabrachial nuclei and in the retrorubral area, ventrolateral division of the periaqueductal gray, and pontine central gray. Light projections end in the olfactory tubercle, lateral septal nucleus, posterior basolateral amygdalar nucleus, supramammillary nucleus, and nucleus of the solitary tract. These and other results suggest that the BSTov and BSTfu are basal telencephalic parts of a circuit that coordinates autonomic, neuroendocrine, and ingestive behavioral responses during stress.

Mineralocorticoid treatment attenuates activation of oxytocinergic and vasopressinergic neurons by icv ANG II

Central oxytocin (OT) neurons limit intracerebroventricular (icv) ANG II-induced NaCl intake. Because mineralocorticoids synergistically increase ANG II-induced NaCl intake, we hypothesized that mineralocorticoids may attenuate ANG II-induced activation of inhibitory OT neurons. To test this hypothesis, we determined the effect of deoxycorticosterone (DOCA; 2 mg/day) on icv ANG II-induced c-Fos immunoreactivity in OT and vasopressin (VP) neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus and also on pituitary OT and VP secretion in male rats. DOCA significantly decreased the percentage of c-Fos-positive (%c-Fos+) OT neurons in the SON and PVN, both in the magnocellular and parvocellular subdivisions, and the %c-Fos+ VP neurons in the SON after a 5-ng icv injection of ANG II. DOCA also significantly reduced the %c-Fos+ OT neurons in the SON after 10 ng ANG II and tended to attenuate 10 ng ANG II-induced OT secretion. However, the %c-Fos+ OT neurons in DOCA-treated rats was greater after 10 ng ANG II, and DOCA did not affect the %c-Fos+ OT neurons in the PVN nor VP secretion or c-Fos immunoreactivity in either the SON or PVN after 10 ng ANG II. DOCA also did not significantly alter the effect of intraperitoneal (ip) cholecystokinin (62 microg) on %c-Fos+ OT neurons or of ip NaCl (2 ml of 2 M NaCl) on the %c-Fos+ OT and VP neurons. These findings indicate that DOCA attenuates the responsiveness of OT and VP neurons to ANG II without completely suppressing the activity of these neurons and, therefore, support the hypothesis that attenuation of OT neuronal activity is one mechanism by which mineralocorticoids enhance NaCl intake.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Brain angiotensin and body fluid homeostasis

Angiotensinogen, the precursor molecule of the peptides angiotensin I, II, and III, is synthesized in the brain and the liver. Evidence is reviewed that angiotensin II, and possibly angiotensin III, that are generated within the brain act within neural circuits of the central nervous system to regulate body fluid balance. Immunohistochemical studies in the rat brain have provided evidence of angiotensin-containing neurons, especially in the hypothalamic paraventricular nucleus, subfornical organ, periventricular region, and nucleus of the solitary tract, as well as in extensive angiotensin-containing fiber pathways. Angiotensin immunoreactivity is observed by electron microscope in synaptic vesicles in several brain regions, the most prominent of these being the central nucleus of the amygdala. Neurons in many parts of the brain (lamina terminalis, paraventricular and parabrachial nuclei, ventrolateral medulla, and nucleus of the solitary tract) known to be involved in the regulation of body fluid homeostasis exhibit angiotensin receptors of the AT(1) subtype. Pharmacological studies in several species show that intracerebroventricular administration of AT(1) receptor antagonist drugs inhibit homeostatic responses to the central administration of hypertonic saline, intravenous infusion of the hormone relaxin, or thermal dehydration. Responses affected by centrally administered AT(1) antagonists are water drinking, vasopressin secretion, natriuresis, increased arterial pressure, reduced renal renin release, salt hunger, and thermoregulatory adjustments. We conclude that angiotensinergic neural pathways in the brain probably have an important homeostatic function, especially in regard to osmoregulation and thermoregulation, and the maintenance of arterial pressure.

Glucocorticoids potentiate central actions of angiotensin to increase arterial pressure

Experiments were performed to determine if glucocorticoids potentiate central hypertensive actions of ANG II. Male Sprague-Dawley rats were treated for 3 days to 3 wk with corticosterone (Cort). Experiments were performed in conscious rats that had previously been instrumented with arterial and venous catheters and an intracerebroventricular guide cannula in a lateral ventricle. Baseline arterial pressure (AP) was greater in Cort-treated rats than in control rats (119 ± 2 vs. 107 ± 1 mmHg, P < 0.01). Microinjection of ANG II intracerebroventricularly produced a significantly larger increase in AP in Cort-treated rats than in control rats. For example, at 30 ng ANG II, AP increased by 23 ± 1 and 16 ± 2 mmHg in Cort-treated and control rats, respectively ( P < 0.01). Microinjection of an angiotensin type 1 receptor antagonist significantly decreased AP (−6 ± 2 mmHg) and heart rate (−26 ± 7 beats/min) in Cort-treated but not control rats. Increases in AP produced by intravenous administration of ANG II were not different between control and Cort-treated rats. Intravenous injections of ANG II antagonist had no significant effects on mean AP or heart rate in control or Cort-treated rats. Therefore, a sustained increase in plasma Cort augments the central pressor effects of ANG II without altering the pressor response to peripheral administration of the hormone.

Partial aortic ligature induces selective long-term c-fos like immunoreactivity in the organum vasculosum of the lamina terminalis, medial preoptic area and choroid plexus in the rat

Partial aortic ligature causes an increase in water and sodium intake. Circumventricular brain regions are known to be involved in the regulation of these processes. In this work we use c-fos-like immunoreactivity to detect active areas involved in the long-term control of increased water and sodium intake due to partial aortic ligature. A significant increase in water intake was found on the first day after the induction, while natriophilia was observed on the fourth day. c-fos-like immunoreactivity was found selectively in the subfornical organ, the organum vasculosum of the lamina terminalis, the medial preoptic area, and the choroid plexus of the third ventricle. Present results provide further evidence for the involvement of circumventricular organs and the preoptic area in the regulation of hydromineral balance. Moreover, they suggest a maintained and long-term regulation of sodium intake by these same brain areas.

Corticosterone differentially modulates expression of corticotropin releasing factor and arginine vasopressin mRNA in the hypothalamic paraventricular nucleus following either acute or repeated restraint stress

Exposing rats to repeated restraint stress induces well-characterized adaptations in the expression of either corticotropin-releasing factor (CRF) or arginine-vasopressin (AVP) mRNA in the parvocellular neurons of the hypothalamic paraventricular nucleus (PVN). The effects of regulating corticosterone levels on this adaptation was studied in male rats. In intact rats, acute restraint stress increased the expression of CRF mRNA whilst AVP mRNA expression was no different to control. Repeated exposure resulted in habituation of CRF expression, whereas AVP mRNA increased above that seen in either non stressed or acutely stressed animals. In adrenalectomised rats with replacement pellets of corticosterone that replicated blood levels approximating to the daily trough (mean levels 37--65 ng/mL), basal CRF expression levels were raised, but the response to acute stress was still observed. However, the habituation seen in normal animals that had been repeatedly stressed was prevented, so that CRF mRNA levels continued to be raised after repeated stress. By contrast, the AVP response to both acute and repeated stress was unaltered in these low-dose corticosterone-treated rats compared with controls. Higher dose pellets, which resulted in blood levels around those of the daily maximum (mean 118--141 ng/mL) had the opposite effects. There was no change compared to intact rats in the expression of CRF mRNA following either acute or repeated stress, but the expected increase in AVP following repeated restraint was prevented. These experiments show that corticosterone has important modulating effects on the adaptive pattern of both CRF and AVP mRNA expression in the parvocellular PVN. The 'set-point' of corticosterone differs; for CRF, experiencing higher levels is necessary for subsequent adaptation to repeated restraint to occur, whereas for AVP a return to lower levels is necessary to allow this peptide to respond to repeated stress.

Estrogen increases angiotensin II-induced c-Fos expression in the vasopressinergic neurons of the paraventricular nucleus in the female rat

Previous studies in female rats have shown that estrogen treatment attenuates angiotensin II (AngII)-induced water intake. The mechanism underlying this attenuation may be decreased responsiveness to AngII, as revealed by a reduction in AngII binding to the angiotensin type 1 (AT1) receptor in the subfornical organ (SFO). It has not been determined whether these changes in receptor binding translate into changes in neuronal activity that, in turn, may influence behavior. Therefore, an estrogen-modulated change in neuronal pathways relevant to AngII-induced water intake was tested in ovariectomized (OVX) female rats using immunohistochemistry for the immediate early gene c-Fos as a marker for neuronal activation. Third cerebroventricular injection of AngII (6 ng) induced intense c-Fos immunoreactivity in forebrain regions associated with fluid intake, including the organum vasculosum of the lamina terminalis, the median preoptic nucleus, the SFO, the supraoptic nucleus and the paraventricular nucleus (PVN). Forty-eight-hour estradiol (10 microg) administration to OVX female rats increased AngII-induced c-Fos labeling in the lateral magnocellular neurons of the PVN by 30% as compared to vehicle-treated controls. Double labeling neurons in the PVN with c-Fos and either vasopressin or oxytocin antisera revealed that estrogen increased AngII-induced c-Fos expression by 28%, specifically in vasopressinergic neurons. Such changes in neuronal activation may explain the estrogen modulation of AngII-induced water intake that has been previously reported; it may be due to increased water retention to maintain plasma osmolality or to induction of a pressor response.

Activation of renal afferent pathways following furosemide treatment. I. Effects Of survival time and renal denervation

Three experiments were performed to determine whether renal afferent pathways were activated by the diuretic drug, furosemide. It was hypothesized that activated neurons of the renal afferent pathway would express the protein product Fos of the c-fos immediate early gene and be identified by immunocytochemical staining for Fos in the cell nucleus. In the first two experiments, rats were injected with either furosemide (5 mg) or vehicle solution (sterile isotonic saline) and sacrificed either 1.75 h (short-survival experiment) or 3.5 h (long-survival experiment) after injection. In both experiments, the furosemide-treated rats had significantly more Fos-positive cell nuclei than vehicle-treated rats in the subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT), supraoptic nuclei (SON), and magnocellular region of the paraventricular nuclei (PVN) - areas previously shown to be activated by hypovolemia or peripheral angiotensin. In the short-survival experiment, the furosemide-treated rats had more Fos-positive cell nuclei in the nucleus of the solitary tract (NTS) and in the dorsal horn of the spinal cord at spinal levels T(11), T(12), and T(13). In contrast, furosemide treatment did not produce more Fos-positive cell nuclei in the NTS and dorsal horn of the spinal cord in the long-survival experiment. These results suggest that the activation of the SFO, OVLT, SON and PVN may be via a different mechanism than that of NTS or spinal cord dorsal horn. Based upon our previous work, we hypothesized that the NTS and spinal cord dorsal horn labeling was due to activation of sympathetic afferents originating in the kidney and labeling in forebrain structures was due to stimulation by angiotensin generated by renal renin release. To test this hypothesis, a third experiment was devised that was identical to the short-survival experiment, except that all rats had bilateral renal denervation surgery 1 week previously. In this experiment, furosemide administration increased the number of Fos-positive cells in the SFO, OVLT, SON and PVN, but not in the caudal thoracic spinal cord or NTS. These results together with the results of first two experiments lend support to our hypothesis that furosemide-induced neuronal activation in the thoracic spinal cord and NTS is due to activation of second- and/or third-order neurons of a renal sympathetic afferent pathway. Furosemide-induced activation in the SFO, OVLT, SON and PVN does not depend on renal innervation. It is hypothesized that activation in these forebrain regions depends on the action of angiotensin II that is generated after furosemide treatment. Our results indicate that both a hormonal pathway and a renal sympathetic afferent pathway conduct information from the kidney to the central nervous system (CNS) after furosemide treatment.

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.

Expression of Fos immunoreactivity in rat brain during dehydration: effect of duration and timing of water deprivation

Water deprivation induces expression of the immediate early gene c-fos in specific brain regions, most likely as a result of the activation of cells that are responsive to changes in osmolality and/or blood volume. We hypothesized that the magnitude of c-fos expression would be a function of both the duration of water deprivation and the time of day at which the deprivation started. This study was designed to examine the pattern of Fos-like immunoreactivity (FLI) following water deprivation in rats under normal light/dark conditions (nLD) and reverse light/dark conditions (rLD). Rats were deprived of water but not food either for 0, 5, 16, 24 or 48 h. As expected, hematocrit ratio (HCT), osmolality (OSM), plasma renin activity (PRA) and weight loss increased as a function of duration of water deprivation. In non-deprived rats (0 h), very little FLI was observed in most brain regions. The number of cells showing FLI increased with duration of water deprivation in the supraoptic nucleus (SON), paraventricular nucleus (PVN), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO) and subfornical organ (SFO) in both nLD and rLD conditions. However, the pattern of FLI differed between nLD and rLD conditions. Compared to corresponding nLD groups after 5 or 24-h water deprivation, rLD groups had significantly more FLI in SON and PVN, and higher PRA and HCT. Also, weight loss and FLI in the MnPO were greater after 5 h, and FLI in the SFO was greater after 24 h under rLD compared to nLD conditions. Our findings indicate that the magnitude of c-fos expression, and change in weight and plasma parameters were a function of both the duration of water deprivation and the time of day at which the deprivation started. This may result from ingestion of food early in the deprivation periods during the rLD tests, thus producing greater change in osmolality and blood volume.

c-Fos induction in spinal cord neurons after renal arterial or venous occlusion

Experiments were done in the anesthetized rat to identify the dorsal root ganglia (DRG) and the spinal cord segments that contain neurons activated by either renal venous occlusion (RVO) or by renal arterial occlusion (RAO). Fos induction, detected immunohistochemically in DRG and the spinal cord neurons, was used as a marker for neuronal activation. RVO induced Fos immunoreactivity in neurons in the DRG of spinal segments T8-L2 on the side ipsilateral to that of occlusion. The largest number of Fos-labeled neurons was found in the T11 DRG. In the spinal cord the largest number of Fos-labeled neurons was found in the ipsilateral dorsal horn of spinal segments T11-T12, predominantly in a cluster near the dorsomedial edge of laminae I-II. A few additional Fos-labeled neurons were observed in laminae IV and V. After RAO Fos-labeled neurons were found in the ipsilateral DRG of spinal segments similar to those observed to contain neurons after RVO. However, most of the Fos-labeled neurons were observed within the T12-L1 DRG. In the spinal cord Fos-labeled neurons were scattered throughout lamina I-II of the ipsilateral dorsal horn of spinal segments T8-L2, although the largest number was observed at the T13 level. Additionally, a distinct cluster of Fos-labeled neurons was observed predominantly in the region of the ipsilateral intermediolateral cell column, although a few neurons were found scattered throughout the nucleus intercalatus, central autonomic areas, and laminae IV and V of the cord bilaterally. No Fos labeling was observed in the complementary contralateral DRG or dorsal horns after either RVO or RAO. In addition, renal nerve transection prevented Fos labeling in the ipsilateral DRG and dorsal horns after RVO or RAO. Taken together, these data suggest that functionally different renal afferent fibers activate DRG neurons that may have distinct projections in the spinal cord.

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.

Differential stimulation of c-fos expression in hypothalamic nuclei of the rat brain during short-term heat acclimation and mild dehydration

Activation of central nervous structures involved in the perception and integration of thermo- and osmoregulatory signals was investigated in the Sabra rat. Male rats were either non-treated (C-E), water-deprived for 24 h (C-D), short-term acclimated to 34 degrees C for two days (STHA-E) or subjected to both stimuli (STHA-D). Immunoreactivity for c-Fos protein (Fos-IR) as marker for neuronal activation was quantified in (extra-)hypothalamic structures: organum vasculosum laminae terminalis (OVLT); subfornical organ (SFO); medial (MPA), ventromedial preoptic (VMPO) and lateral hypothalamic (LHA) areas; median preoptic (MnPO), magnocellular supraoptic (SON) and paraventricular (mPVN) nuclei; limbic lateral septal (LS) and thalamic paraventricular (PV) nuclei. Compared to C-E rats, dehydration markedly increased Fos-IR exclusively in neurons of the OVLT, SFO and MnPO known to be involved in osmoreception, in the mPVN and SON, and to a minor extent in the VMPO. The VMPO, MPA, LHA and LS-important (extra-)hypothalamic sites for the perception and integration within the thermoregulatory control circuit-exhibited intense elevation of Fos-IR upon short-term heat acclimation. Of all (extra-)hypothalamic structures involved in central osmoregulation, only the MnPO revealed heat-induced Fos-IR in numerous cells located preferentially in its rostral component. Thus, the MnPO proved to be activated during both thermal and osmotic stimulations applied separately. Subjected to the combined stress (STHA-D), most brain structures investigated showed striking Fos-IR due to thermally enhanced osmotic stimulation, with additive effects demonstrated in the MnPO. The data support differential central activation of c-fos expression due to thermal or osmotic stimulations, with the MnPO acting as putative integrative center for both autonomic control circuits.

Evaluation of the ability of irbesartan to cross the blood-brain barrier following acute intragastric treatment

The present study evaluated in functional tests the ability of the angiotensin AT1 receptor antagonist irbesartan, 2-n-butyl-3-[(2'-(1H-tetrazol-5-yl)-biphenyl-4-yl)methyl]-1,3-d iaza-spiro[4,4]non-1-en-4-one, in comparison to losartan, 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'(1H-tetrazol-5-yl) bi-phenyl-4-yl)methyl]imidazole, to cross the blood-brain barrier following acute intragastric administration. Two tests were used: the dipsogenic response to intracerebroventricular injection of angiotensin II, and Na+ intake in response to adrenalectomy. In normotensive rats, irbesartan reduced the dipsogenic response to angiotensin II, 10 pmol per rat, at the dose of 90 mg/kg, but not at lower doses. Losartan significantly reduced angiotensin II-induced drinking at 30 mg/kg, but not at a lower dose. In spontaneously hypertensive rats, irbesartan reduced the response to angiotensin II at 50 mg/kg, but not at lower doses, while losartan significantly inhibited angiotensin II-induced drinking even at 10 mg/kg. In adrenalectomized rats, the intake of 2% NaCl was inhibited by the intragastric administration of losartan 30 or 50 mg/kg, while irbesartan did not reduce it in doses up to 50 mg/kg. The results of the present study consistently indicate that after acute intragastric administration, the ability of irbesartan to cross the blood-brain barrier is lower than that of losartan.

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

Fos expression in rat brain during depletion-induced thirst and salt appetite

The expression of Fos protein (Fos immunoreactivity, Fos-ir) was mapped in the brain of rats subjected to an angiotensin-dependent model of thirst and salt appetite. The physiological state associated with water and sodium ingestion was produced by the concurrent subcutaneous administration of the diuretic furosemide (10 mg/kg) and a low dose of the angiotensin-converting enzyme (ACE) inhibitor captopril (5 mg/kg; Furo/Cap treatment). The animals were killed 2 h posttreatment, and the brains were processed for Fos-ir to assess neural activation. Furo/Cap treatment significantly increased Fos-ir density above baseline levels both in structures of the lamina terminalis and hypothalamus known to mediate the actions of ANG II and in hindbrain regions associated with blood volume and pressure regulation. Furo/Cap treatment also typically increased Fos-ir density in these structures above levels observed after administration of furosemide or captopril separately. Fos-ir was reduced to a greater extent in forebrain than in hindbrain areas by a dose of captopril (100 mg/kg sc) known to block the actions of ACE in the brain. The present work provides further evidence that areas of lamina terminalis subserve angiotensin-dependent thirst and salt appetite.

Water-deprived white-footed mice express c-fos on a day/night cycle graded according to the duration of deprivation

Mammals respond to electrolyte and water imbalance by a variety of neural and endocrine mechanisms that regulate water and salt intake and loss. We used the expression of c-fos and Fos-related antigens to indicate neuronal activation in hypothalamic neurons of members of an outbred laboratory population of white-footed mice (Peromyscus leucopus) deprived of water for biologically reasonable periods of time (6-18 h). We examined Fos-like immunoreactivity (Fos-LIR) in the supraoptic nucleus (SON) and paraventricular nucleus (PVN). During the dark period, when these animals are normally active, 6 h of water deprivation produced near-maximal increases in the number of cells positive for Fos-LIR in the SON and PVN. In contrast, during the light period, when these mice are normally inactive and do not have access to water, 6 h of water deprivation only slightly affected Fos-LIR. During the day, it required as much as 12 h of water deprivation to produce increases in Fos-LIR cells approaching those achieved at night. Plasma osmolarity was directly related to the number of Fos-LIR cells. In addition, mice lost weight more rapidly at night than during the day when water-deprived, and also recovered that lost weight more rapidly when access to water was returned. Our results show (1) that biologically reasonable levels of water restriction (and resulting changes in blood osmolarity) induce changes in Fos-LIR in this wild mouse species, and (2) that these mice have a daily cycle of sensitivity to water deprivation that is demonstrated by both behavioral, psychological and immunohistological assessment of reactions to water deprivation.

Structure of the subfornical organ: a review

In this review, the light microscopic and fine structural characteristics of neurons, axons, dendrites, glial cells, and capillaries and their topography within the subfornical organ are summarized, with an emphasis on recent findings. Structure-function relationships are discussed whenever possible and put into perspective in a concluding section.

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.