Investigation of the mechanisms by which chronic infusion of an acutely subpressor dose of angiotensin II induces hypertension

The mechanisms by which chronic infusion of an initially subpressor low dose of angiotensin II (ANG II) causes a progressive and sustained hypertension remain unclear. In conscious sheep ( n = 6), intravenous infusion of ANG II (2 μg/h) gradually increased mean arterial pressure (MAP) from 82 ± 3 to 96 ± 5 mmHg over 7 days ( P < 0.001). This was accompanied by peripheral vasoconstriction; total peripheral conductance decreased from 44.6 ± 6.4 to 38.2 ± 6.7 ml·min−1·mmHg−1 ( P < 0.001). Cardiac output and heart rate were unchanged. In the regional circulation, mesenteric, renal, and iliac conductances decreased but blood flows were unchanged. There was no coronary vasoconstriction, and coronary blood flow increased. Ganglion blockade (125 mg/h hexamethonium for 4 h) reduced MAP by 13 ± 1 mmHg in the control period and by 7 ± 2 mmHg on day 8 of ANG II treatment. Inhibition of central AT1 receptors by intracerebroventricular infusion of losartan (1 mg/h for 3 h) had no effect on MAP in the control period or after 7 days of ANG II infusion. Pressor responsiveness to incremental doses of intravenous ANG II (5, 10, 20 μg/h, each for 15 min) was unchanged after 7 days of ANG II infusion. ANG II caused no sodium or water retention. In summary, hypertension due to infusion of a low dose of ANG II was accompanied by generalized peripheral vasoconstriction. Indirect evidence suggested that the hypertension was not neurogenic, but measurement of sympathetic nerve activity is required to confirm this conclusion. There was no evidence for a role for central angiotensinergic mechanisms, increased pressor responsiveness to ANG II, or sodium and fluid retention.

Bronchobiliary fistula successfully treated with n-butyl cyanoacrylate via a bronchial approach

A bronchobiliary fistula is an abnormal communication between the biliary tree and the airway that can result in debilitating amounts of bilioptysis, or bile-stained sputum. The authors present an approach for the conservative management of a bronchobiliary fistula in a patient who failed traditional conservative therapy and refused surgical intervention.

Amylin induces natriuresis by a central angiotensin-dependent mechanism

This study provides evidence that amylin acts centrally to increase sodium excretion in the sheep. Amylin was infused at 8 mg/h into a carotid artery (IC), via a lateral ventricle (ICV), intravenously (IV) or intra-renally (IR) into conscious sheep (n=5 per group). Renal sodium excretion increased by at least 3-fold after 1 h of amylin infusion by ICV (66+/-14 to 367+/-35 mmol/min) and IC (78+/-14 to 244+/-22 mmol/min) routes of administration. Amylin infusion IV caused a 1.5-fold increase in sodium excretion while IR infusion did not have a significant effect. The natriuretic effect of ICV infused amylin was blocked by pre-treatment with the angiotensin AT1 receptor antagonist, losartan (1 mg/h). No changes in blood pressure or heart rate were recorded at this dose of amylin by any route of administration. Plasma renin concentration increased (1.32+/-0.22 to 2.55+/-0.73 pmol/Ang I/h; P<0.05) following IR infusion of amylin, and remained unchanged when amylin was infused by the other routes of administration. We conclude that amylin causes changes in sodium excretion in sheep through a central, angiotensin-dependent pathway and that amylin may increase renin secretion by a direct effect on the kidney.

Serotonergic mechanism of the lateral parabrachial nucleus and relaxin-induced sodium intake

It has been shown that central or peripheral injections of the peptide relaxin induces water intake, not sodium intake in rats. Important inhibitory mechanisms involving serotonin and other neurotransmitters in the control of water and NaCl intake have been demonstrated in the lateral parabrachial nucleus (LPBN). In the present study, we investigated the effects of bilateral injections of methysergide (serotonergic receptor antagonist) into the LPBN on intracerebroventricular (i.c.v.) relaxin-induced water and NaCl intake in rats. Additionally, the effect of the blockade of central angiotensin AT(1) receptors with i.c.v. losartan on relaxin-induced water and NaCl intake in rats treated with methysergide into the LPBN was also investigated. Male Holtzman rats with cannulas implanted into the lateral ventricle (LV) and bilaterally in the LPBN were used. Intracerebroventricular injections of relaxin (500 ng/1 microl) induced water intake (5.1+/-0.7 ml/120 min), but not significant 1.8% NaCl intake (0.5+/-0.4 ml/120 min). Bilateral injections of methysergide (4 microg/0.2 microl) into the LPBN strongly stimulated relaxin-induced 1.8% NaCl intake (34.5+/-10.9 ml/120 min) and slightly increased water intake (10.5+/-4.9 ml/120 min). The pretreatment with i.c.v. losartan (100 microg/1 microl) abolished the effects of i.c.v. relaxin combined with LPBN methysergide on 1.8% NaCl intake (0.5+/-0.4 ml/120 min). Losartan (100 microg/1 microl) also abolished relaxin-induced water intake in rats injected with methysergide into the LPBN (1.6+/-0.8 ml/120 min) or not (0.5+/-0.3 ml/120 min). Losartan (50 microg/1 microl) partially reduced the effects of relaxin. The results show that central relaxin interacting with central angiotensinergic mechanisms induces NaCl intake after the blockade of LPBN serotonergic mechanisms.

Physiological and pathophysiological influences on thirst

Thirst motivates animals to seek fluid and drink it. It is regulated by the central nervous system and arises from neural and chemical signals from the periphery interacting in the brain to stimulate a drive to drink. Our research has focussed on the lamina terminalis and the manner in which osmotic and hormonal stimuli from the circulation are detected by neurons in this region and how that information is integrated with other neural signals to generate thirst. Our studies of osmoregulatory drinking in the sheep and rat have produced evidence that osmoreceptors for thirst exist in the dorsal cap of the organum vasculosum of the lamina terminalis (OVLT) and in the periphery of the subfornical organ, and possibly also in the median preoptic nucleus. In the rat, the hormones angiotensin II and relaxin act on neurons in the periphery of the subfornical organ to stimulate drinking. Studies of human thirst using functional magnetic resonance imaging (fMRI) techniques show that systemic hypertonicity activates the lamina terminalis and the anterior cingulate cortex, but the neural circuitry that connects sensors in the lamina terminalis to cortical regions subserving thirst remains to be determined. Regarding pathophysiological influences on thirst mechanisms, both excessive (polydipsia) and inadequate (hypodisia) water intake may have dire consequences. One of the most common primary polydipsias is that observed in some cases of schizophrenia. The neural mechanisms causing the excessive water intake in this disorder are unknown, so too are the factors that result in impaired thirst and inadequate fluid intake in some elderly humans.

Inlet patch: prevalence, histologic type, and association with esophagitis, Barrett esophagus, and antritis

CONTEXT

Inlet patch is a congenital anomaly of the cervical esophagus consisting of gastric mucosa. Case reports have documented the histologic type and its associated complications.

OBJECTIVE

To report the prevalence and histologic types of inlet patch as well as its association with Barrett esophagus and Helicobacter pylori-associated gastritis.

DESIGN

We reviewed 1821 consecutive pathology reports from endoscopies of the upper gastrointestinal tract between 1995 and 2002 and identified 20 patients with inlet patch. The patients' ages ranged from 16 to 75 years (mean, 55 years). We examined biopsies from these patients of the cervical esophagus, distal esophagus, and antrum that had been stained with hematoxylin-eosin and the Steiner stain. Biopsies from the cervical esophagus composed of nonoxyntic gastric mucosa were also stained for gastrin-producing cells.

RESULTS

In our patient population, inlet patch occurred in 1.1% of all patients in whom an inlet patch had been searched for endoscopically and a biopsy performed. In the inlet patch, oxyntic mucosa was the most common histologic type (11/20), followed by cardiac mucosa (5/20). Four specimens of the inlet patch contained only foveolar epithelium and were therefore considered too superficial to be classified. Twelve of 20 biopsies of the inlet patch were inflamed, and 1 of them was associated with H pylori. Pancreatic acinar tissue was noted in 2 patients; no intestinal metaplasia was found. In the distal esophagus, 4 patients with inlet patch had distal esophagitis, 4 had Barrett esophagus, 5 had oxyntic mucosa, 3 had pancreatic acinar tissue (1 coexisting with oxyntic mucosa), and 5 presented with only unremarkable esophageal squamous mucosa. Antral gastritis was seen in 2 patients, 1 of them with H pylori. The same patient also had H pylori in the inlet patch.

CONCLUSION

Inlet patch occurred in about 1% of our study population. Oxyntic mucosa constituted the most common histologic type; inflammation was common. The H pylori infection of the inlet patch correlated with that of the antrum. None of the inlet patch biopsies showed intestinal metaplasia. Pancreatic acinar tissue occurred with similar frequency in the inlet patch and distal esophagus. Esophagitis was noted in 25% of the patients with inlet patch, and Barrett esophagus was noted in 20%.

Inlet patch: prevalence, histologic type, and association with esophagitis, Barrett esophagus, and antritis

CONTEXT

Inlet patch is a congenital anomaly of the cervical esophagus consisting of gastric mucosa. Case reports have documented the histologic type and its associated complications.

OBJECTIVE

To report the prevalence and histologic types of inlet patch as well as its association with Barrett esophagus and Helicobacter pylori-associated gastritis.

DESIGN

We reviewed 1821 consecutive pathology reports from endoscopies of the upper gastrointestinal tract between 1995 and 2002 and identified 20 patients with inlet patch. The patients' ages ranged from 16 to 75 years (mean, 55 years). We examined biopsies from these patients of the cervical esophagus, distal esophagus, and antrum that had been stained with hematoxylin-eosin and the Steiner stain. Biopsies from the cervical esophagus composed of nonoxyntic gastric mucosa were also stained for gastrin-producing cells.

RESULTS

In our patient population, inlet patch occurred in 1.1% of all patients in whom an inlet patch had been searched for endoscopically and a biopsy performed. In the inlet patch, oxyntic mucosa was the most common histologic type (11/20), followed by cardiac mucosa (5/20). Four specimens of the inlet patch contained only foveolar epithelium and were therefore considered too superficial to be classified. Twelve of 20 biopsies of the inlet patch were inflamed, and 1 of them was associated with H pylori. Pancreatic acinar tissue was noted in 2 patients; no intestinal metaplasia was found. In the distal esophagus, 4 patients with inlet patch had distal esophagitis, 4 had Barrett esophagus, 5 had oxyntic mucosa, 3 had pancreatic acinar tissue (1 coexisting with oxyntic mucosa), and 5 presented with only unremarkable esophageal squamous mucosa. Antral gastritis was seen in 2 patients, 1 of them with H pylori. The same patient also had H pylori in the inlet patch.

CONCLUSION

Inlet patch occurred in about 1% of our study population. Oxyntic mucosa constituted the most common histologic type; inflammation was common. The H pylori infection of the inlet patch correlated with that of the antrum. None of the inlet patch biopsies showed intestinal metaplasia. Pancreatic acinar tissue occurred with similar frequency in the inlet patch and distal esophagus. Esophagitis was noted in 25% of the patients with inlet patch, and Barrett esophagus was noted in 20%.

Vasopressin secretion: osmotic and hormonal regulation by the lamina terminalis

The lamina terminalis, located in the anterior wall of the third ventricle, is comprised of the subfornical organ, median preoptic nucleus (MnPO) and organum vasculosum of the lamina terminalis (OVLT). The subfornical organ and OVLT are two of the brain's circumventricular organs that lack the blood-brain barrier, and are therefore exposed to the ionic and hormonal environment of the systemic circulation. Previous investigations in sheep and rats show that this region of the brain has a crucial role in osmoregulatory vasopressin secretion and thirst. The effects of lesions of the lamina terminalis, studies of immediate-early gene expression and electrophysiological data show that all three regions of the lamina terminalis are involved in osmoregulation. There is considerable evidence that physiological osmoreceptors subserving vasopressin release are located in the dorsal cap region of the OVLT and possibly also around the periphery of the subfornical organ and in the MnPO. The circulating peptide hormones angiotensin II and relaxin also have access to peptide specific receptors (AT(1) and LGR7 receptors, respectively) in the subfornical organ and OVLT, and both angiotensin II and relaxin act on the subfornical organ to stimulate water drinking in the rat. Studies that combined neuroanatomical tracing and detection of c-fos expression in response to angiotensin II or relaxin suggest that both of these circulating peptides act on neurones within the dorsal cap of the OVLT and the periphery of the subfornical organ to stimulate vasopressin release.

The sensory circumventricular organs of the mammalian brain

The brain's three sensory circumventricular organs, the subfornical organ, organum vasculosum of the lamina terminalis and the area postrema lack a blood brain barrier and are the only regions in the brain in which neurons are exposed to the chemical environment of the systemic circulation. Therefore they are ideally placed to monitor the changes in osmotic, ionic and hormonal composition of the blood. This book describes their. General structure and relationship to the cerebral ventricles Regional subdivisions Vasculature and barrier properties Neurons, glia and ependymal cells Receptors, neurotransmitters, neuropeptides and enzymes Neuroanatomical connections Functions.

Circulating angiotensin II activates neurones in circumventricular organs of the lamina terminalis that project to the bed nucleus of the stria terminalis

The aim of this study was to determine, in conscious rats, whether elevated concentrations of circulating angiotensin II activate neurones in both the subfornical organ and organum vasculosum of the lamina terminalis (OVLT) that project to the bed nucleus of the stria terminalis (BNST). The strategy employed was to colocalize retrogradely transported cholera toxin B subunit (CTB) from the BNST, with elevated levels of Fos protein in response to angiotensin II. Circulating angiotensin II concentrations were increased by either intravenous infusion of angiotensin II or subcutaneous injection of isoproterenol. Neurones exhibiting Fos in response to angiotensin II were present in the subfornical organ, predominantly in its central core but with some also seen in its peripheral aspect, the dorsal and lateral margins of the OVLT, the supraoptic nucleus and the parvo- and magnocellular divisions of the paraventricular nucleus. Fos-labelling was not apparent in control rats infused with isotonic saline intravenously or injected with either CTB or CTB conjugated to gold particles (CTB-gold) only. Of the neurones in the subfornical organ that were shown by retrograde labelling to project to BNST, approximately 50% expressed Fos in response to isoproterenol. This stimulus also increased Fos in 33% of neurones in the OVLT that project to BNST. Double-labelled neurones were concentrated in the central core of the subfornical organ and lateral margins of the OVLT in response to increased circulating angiotensin II resulting from isoproterenol treatment. These data support a role for circulating angiotensin II acting either directly or indirectly on neurones in subfornical organ and OVLT that project to the BNST and provide further evidence of functional regionalization within the subfornical organ and the OVLT. The function of these pathways is yet to be determined; however, a role in body fluid homeostasis is possible.

The effect of urocortin on ingestive behaviours and brain Fos immunoreactivity in mice

The influence of urocortin (UCN) on ingestive behaviours and brain neural activity, as measured immunohistochemically by the presence of Fos protein, was determined in mice. Rat UCN was administered by continuous intracerebroventricular (ICV) or subcutaneous (SC) infusion. ICV infusion of UCN (100 ng/h, 14 days) transiently reduced daily food and water intakes (days 1-4) but body weight was reduced from day 2 into the post-infusion period. Sodium intake was reduced from day 3 to the end of infusion. SC infusion of UCN caused similar but smaller reductions in food and water intakes and body weight, without change in sodium intake. In separate experiments, Fos immunoreactivity was increased in several brain nuclei known to be involved in the control of body fluid and energy homeostasis, e.g. central nucleus of the amygdala, median preoptic nucleus, bed nucleus of the stria terminalis and arcuate nucleus. Increased Fos expression was similar for ICV and SC infusions when measured on days 2-3 or 6-7 of infusion. In conclusion, increases of brain activity by UCN may be associated with stimulation of adrenocorticotrophic hormone release and sympathetic nervous activity, but increases may also indicate suppression of ingestive behaviours by stimulating central inhibitory mechanisms located in areas known to control body fluid and energy homeostasis.

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.

The neurochemical characterisation of hypothalamic pathways projecting polysynaptically to brown adipose tissue in the rat

The identification of leptin and a range of novel anorectic and orexigenic peptides has focussed attention on the neural circuitry involved in the genesis of food intake and the reflex control of thermogenesis. Here, the neurotropic virus pseudorabies has been utilised in conjunction with the immunocytochemical localisation of a variety of neuroactive peptides and receptors to better define the pathways in the rat hypothalamus directed polysynaptically to the major thermogenic endpoint, brown adipose tissue. Infected neurones were detected initially in the stellate ganglion, then in the spinal cord followed by the appearance of third-order premotor neurones in the brainstem and hypothalamus. Within the hypothalamus these were present in the paraventricular nucleus, lateral hypothalamus, perifornical region, and retrochiasmatic nucleus. At slightly longer survival times virus-infected neurones appeared in the arcuate nucleus and dorsomedial hypothalamus. Neurones in the retrochiasmatic nucleus and in the adjacent lateral arcuate nucleus which project to the brown adipose tissue express cocaine- and amphetamine-regulated transcript, pro-opiomelanocortin and leptin receptors. Neurones in the lateral hypothalamus, a site traditionally associated with the promotion of feeding, project to brown adipose tissue and large numbers of these contained melanin-concentrating hormone and orexin A and B. These data provide part of an anatomical framework which subserves the regulation of energy expenditure.

"Tannenbaum" Teflon stents versus traditional polyethylene stents for treatment of malignant biliary stricture

BACKGROUND

Premature stent clogging is the major limitation with plastic stents used in the treatment of malignant biliary structures. A pilot study suggested improved duration of patency of the Tannenbaum stent compared with polyethylene stents. The aim of this prospective, multicenter randomized trial was to compare the Tannenbaum Teflon stent with a conventional polyethylene endoprosthesis (Cotton-Leung biliary stent set) for the treatment of malignant biliary strictures.

METHODS

Patients over age 18 years with symptoms caused by nonhilar malignant biliary strictures were enrolled. Patients were randomized to receive a 10F Tannenbaum or polyethylene stent after a guidewire was passed beyond the stricture. One hundred six patients (mean age 72 years and 71 years, respectively) were enrolled (54 Tannenbaum, mean age 72 years; 52 polyethylene, mean age 71 years).

RESULTS

Tannenbaum and polyethylene stent placement was successful in, respectively, 100% and 96% of procedures without complications. The mean (SD) 90-day stent patency of the Tannenbaum stent was 67% (7%) compared with 73% (7%) for the polyethylene stents.

CONCLUSIONS

The present study demonstrated no difference in ease of implantation or stent patency between Tannenbaum and polyethylene stents.

Circulating relaxin acts on subfornical organ neurons to stimulate water drinking in the rat

Relaxin, a peptide hormone secreted by the corpus luteum during pregnancy, exerts actions on reproductive tissues such as the pubic symphysis, uterus, and cervix. It may also influence body fluid balance by actions on the brain to stimulate thirst and vasopressin secretion. We mapped the sites in the brain that are activated by i.v. infusion of a dipsogenic dose of relaxin (25 microg/h) by immunohistochemically detecting Fos expression. Relaxin administration resulted in increased Fos expression in the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus, and magnocellular neurons in the supraoptic and paraventricular nuclei. Ablation of the SFO abolished relaxin-induced water drinking, but did not prevent increased Fos expression in the OVLT, supraoptic or paraventricular nuclei. Although ablation of the OVLT did not inhibit relaxin-induced drinking, it did cause a large reduction in Fos expression in the supraoptic nucleus and posterior magnocellular subdivision of the paraventricular nucleus. In vitro single-unit recording of electrical activity of neurons in isolated slices of the SFO showed that relaxin (10(-7) M) added to the perfusion medium caused marked and prolonged increase in neuronal activity. Most of these neurons also responded to 10(-7) M angiotensin II. The data indicate that blood-borne relaxin can directly stimulate neurons in the SFO to initiate water drinking. It is likely that circulating relaxin also stimulates neurons in the OVLT that influence vasopressin secretion. These two circumventricular organs that lack a blood-brain barrier may have regulatory influences on fluid balance during pregnancy in rats.

A method for the identification of pseudorabies virus protein and angiotensin AT(1A) receptor mRNA expression in the same CNS neurons

Neurotropic viruses have been used over the last 10 years to map the distribution of chains of synaptically connected neurons in the CNS. The peptide content of infected neurons has been determined in a number of cases immunohistochemically. However, it has been unclear whether specific mRNA can be assessed in virus-infected neurons. We have established a technique which enables the identification of viral protein and mRNA in the same neuron. In the present study pseudorabies virus retrogradely transported from the kidney was localised using immunohistochemistry and mRNA for the angiotensin II AT(1A) receptor was detected by hybridisation histochemistry. Virus protein was visualised using an immunohistochemical procedure with diaminobenzidine as the chromogen and the same sections were exposed to radioactively labelled ((35)S) riboprobes, hybridising the angiotensin II AT(1A) receptor. The combination of these two approaches resulted in the identification of neurons shown to project polysynaptically to the kidney and express AT(1A) mRNA. These data provide neuroanatomical support for previous physiological observations that ablation of the lamina terminalis and administration of losartan, the AT(1) receptor antagonist, blocks the inhibition of renal sympathetic nerve activity following centrally injected Ang II in rats and sheep [5].

Neural pathways from the lamina terminalis influencing cardiovascular and body fluid homeostasis

1. The lamina terminalis, a region of the brain with a high concentration of angiotensin AT1 receptors, consists of three distinct nuclei, the median preoptic nucleus, the subfornical organ and organum vasculosum of the lamina terminalis (OVLT). These latter two regions lack a blood-brain and detect changes in plasma angiotensin (Ang) II concentration and osmolality. 2. Efferent neural pathways from the lamina terminalis to the hypothalamic paraventricular and supraoptic nuclei mediate vasopressin secretion in response to plasma hypertonicity and increased circulating levels of AngII. 3. Studies using the neurotropic virus pseudorabies, which undergoes retrograde transynaptic neuronal transport following injection into peripheral sites, show that neurons in the lamina terminalis have efferent polysynaptic neural connections to the peripheral sympathetic nervous system. Some of these neurons have been shown to have polysynaptic connections to the kidney and to express AT1 receptor mRNA. We propose that circulating AngII acts at AT1 receptors in the subfornical organ and OVLT to influence the sympathetic nervous system. It is likely that the neural pathway subserving this influence involves a synapse in the hypothalamic paraventricular nucleus. 4. The lamina terminalis may exert an inhibitory osmoregulatory influence on renin secretion by the kidney. This osmoregulatory influence may be mediated by inhibition of renal sympathetic nerve activity and appears to involve a central angiotensinergic synapse. 5. The lamina terminalis exerts an osmoregulatory influence on renal sodium excretion that is independent of the renal nerves and is probably hormonally mediated.

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.

Influence of ruminal water-loading on renal sodium excretion and water intake following feeding in sheep

We investigated the effect of ruminal water loading before feeding on the natriuretic and drinking responses that follow feeding. Six sheep fed 800 g of chaff drank 1360 +/- 150 mL during the 5 h immediately following feeding and increased renal Na excretion. Plasma Na concentration increased by 4 mmol L (-1) and plasma osmolality by 9 mosmol kg (-1) within 1.5 h and remained elevated. A rumen load of water administered before feeding prevented the increases in plasma Na and osmolality without affecting feeding. The natriuresis, water drinking and vasopressin secretion in response to feeding were abolished. Total sodium excreted during the experiment was halved in water-loaded animals compared with untreated animals (30.4 +/- 2.1 mmol (-1) cf. 63.8 +/- 2.9 mmol-1; P < 0.01). Ruminal loading with isotonic saline caused a 33% reduction in postprandial drinking, however, reducing cerebrospinal fluid NaCl concentration abolished postprandial drinking and natriuresis. Intravenous infusion of isotonic dextran appeared to delay the onset of water intake without changing the total volume of water drunk, suggesting a role of plasma volume in initiating drinking. We conclude from the data that central osmoregulatory mechanisms that include increased sodium excretion as well as thirst and vasopressin release are activated following food intake by sheep.

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.

Neural mechanisms subserving central angiotensinergic influences on plasma renin in sheep

The mechanisms and brain regions subserving the suppression of plasma renin concentration caused by intracerebroventricular (ICV) infusion of angiotensin II were studied in sodium-depleted sheep. Infusion of angiotensin II (3 microg/h for 1 hour) into the lateral ventricle reduced plasma renin from 4.3+/-0.4 to 1.6+/-0.2 pmol angiotensin I/mL per hour at 1 hour after the commencement of infusion. This change persisted for at least another 90 minutes and was blocked by concomitant ICV infusion of the AT(1) antagonist losartan (1 mg/h). Arterial pressure did not change, but plasma vasopressin secretion was increased. ICV infusion of losartan (1 mg/h) significantly increased plasma renin in sodium-depleted sheep. The reduction of plasma renin concentration in response to either ICV angiotensin II or hypertonic NaCl (0.75 mol/L at 1 mL/h) and the increase in response to ICV losartan was prevented in sheep in which the lamina terminalis of the brain had been ablated. Lesions in the median eminence (MEL), which blocked the increased plasma vasopressin levels, did not prevent suppression of plasma renin in response to ICV angiotensin II. However, bilateral renal denervation largely blocked this inhibition of plasma renin concentration but not the increased plasma renin resulting from ICV infusion of losartan in sodium-depleted sheep. The results show that AT(1) receptors, probably located in the lamina terminalis, mediate a central inhibitory influence of angiotensin II on renin secretion. This inhibition of renin release is probably due to a reduction in activity of renal sympathetic nerves innervating the juxtaglomerular apparatus of the kidney.

Identification of efferent neural pathways from the lamina terminalis activated by blood-borne relaxin

The ovarian hormone relaxin, in addition to its role in pregnancy, exerts an action on the brain to influence oxytocin and vasopressin secretion, water drinking, and cardiovascular function. Intravenous (i.v.) infusion of relaxin causes an acute water drinking response, confirming its role as a dipsogenic hormone. The aim of this study was to determine whether neurones in the lamina terminalis, which project to the hypothalamic paraventricular and supraoptic nuclei, are activated by elevated levels of circulating relaxin in conscious rats. Immunocytochemistry combined with retrograde neuronal tracing with cholera toxin B subunit conjugated to cholera toxin B (CTB-gold) was used to identify populations of neurones responding with elevated cells of Fos protein to i.v. relaxin administration and which project to these specific hypothalamic sites. Neurones exhibiting Fos were present in the outer parts of the subfornical organ (SFO), the dorsal part of the organum vasculosum (OVLT), the supraoptic nucleus and the paraventricular nucleus. These did not occur in control rats with i.v. infusions of isotonic saline. Approximately 90% of neurones concentrated in the outer parts of the SFO and in the dorsal OVLT showed both retrogradely transported CTB-gold and Fos in response to i.v. infusion of relaxin. These data support a role for relaxin acting on the brain to regulate body fluid and electrolyte homeostasis by activating neural pathways subserving water drinking, vasopressin and oxytocin secretion.

Neurons in the lamina terminalis which project polysynaptically to the kidney express angiotensin AT1A receptor

The retrograde transynaptic transport of pseudorabies virus was used in conjunction with hybridisation histochemistry for the angiotensin II AT1A receptor, to characterise neurons in the lamina terminalis projecting to the kidney. These data demonstrate that some neurons in the lamina terminalis, that project polysynaptically to the kidney, may be responsive to angiotensin II.

Efferent neural projections of angiotensin receptor (AT1) expressing neurones in the hypothalamic paraventricular nucleus of the rat

Angiotensin II acts within the hypothalamic paraventricular nucleus (PVN) to help mediate a number of autonomic and endocrine responses. Evidence is sparse in regard to the particular neuronal cell groups that exhibit angiotensin II type 1 receptors within the PVN, and does not exist in relation to specified efferent neuronal populations in the nucleus. In the present experiments, retrogradely transported neuronal tracers were utilized in conjunction with immunohistochemistry using a well characterized polyclonal antibody raised against a decapeptide sequence at the carboxy terminus of the AT1 receptor, to determine whether it is preferentially distributed amongst different efferent populations within the PVN. The AT1 receptor is not associated with neurones in the PVN that project axons to the spinal cord, dorsomedial or ventrolateral medulla but coexists strongly with neurones in the anterior parvocellular division of the nucleus which direct axons to the median eminence. Such neurones often contain corticotropin releasing factor. These findings highlight the role that angiotensin II and AT1 receptors in the PVN may play in the mediation of responses to stress.