Estrogen receptor-alpha expression in osmosensitive elements of the lamina terminalis: regulation by hypertonicity

The subfornical organ (SFO), median preoptic nucleus (MnPO), and organum vasculosum lamina terminalis (OVLT), which are associated with the lamina terminalis, are important in the control of body fluid balance. Neurons in these regions express estrogen receptor (ER)-alpha, but whether the ER-alpha neurons are activated by hypertonicity and whether hypertonicity regulates ER-alpha expression are not known. Using fluorescent, double-label immunocytochemistry, we examined the expression of ER-alpha-immunoreactivity (ir) and Fos-ir in control and water-deprived male rats. In control animals, numerous ER-alpha-positive neurons were expressed in the periphery of the SFO, in both the dorsal and ventral MnPO, and in the dorsal cap of the OVLT. Fos-positive neurons were sparse in euhydrated rats but were numerous in the SFO, MnPO, and the dorsal cap of the OVLT after 48-h water deprivation. Most ER-alpha-ir neurons in these areas were positive for Fos, indicating a significant degree of colocalization. To examine the effect of dehydration on ER-alpha expression, animals with and without lesions surrounding the anterior and ventral portion of the 3rd ventricle (AV3V) were water deprived for 48 h. Water deprivation resulted in a moderate increase in ER-alpha-ir in the SFO of sham-lesioned rats (P = 0.03) and a dramatic elevation in AV3V-lesioned animals (P < 0.05). This was probably induced by the significant increase in plasma osmolality in both dehydrated groups (P < 0.001) rather than a decrease in blood volume, because hematocrit was significantly increased only in the dehydrated sham-lesioned animals. Thus these studies implicate the osmosensitive regions of the lamina terminalis as possible targets for sex steroid effects on body fluid homeostasis.     

Circumventricular organs and ANG II-induced salt appetite: blood pressure and connectivity

A lesion of the subfornical organ (SFO) may reduce sodium depletion-induced salt appetite, which is largely dependent on ANG II, and yet ANG II infusions directly into SFO do not provoke salt appetite. Two experiments were designed to address this apparent contradiction. In experiment 1 sustained infusions of ANG II into SFO did not produce a sustained elevation of blood pressure, and neither a reduction of blood pressure alone with minoxidil and captopril nor a reduction of both blood pressure and volume with furosemide and captopril enhanced salt appetite. Infusions of ANG II in the organum vasculosum laminae terminalis (OVLT) did evoke salt appetite without raising blood pressure. In experiment 2 knife cuts of the afferent and efferent fibers of the rostroventral pole of the SFO abolished water intake during an infusion of ANG II into the femoral vein but failed to reduce salt appetite during an infusion of ANG II into the OVLT. We conclude that 1) hypertension does not account for the failure of infusions of ANG II in the SFO to generate salt appetite and 2) the OVLT does not depend on its connectivity with the SFO to generate salt appetite during ANG II infusions.     

Aortic stiffness affects the coronary blood flow response to percutaneous coronary intervention

Chronic subcutaneous infusion of ouabain causes hypertension via central pathways involving angiotensin type 1 (AT(1)) receptor stimulation. The present study assessed plasma and tissue ANG I and II levels as well as AT1 receptor and angiotensin-converting enzyme (ACE) mRNA levels and binding densities by real-time PCR and in vitro autoradiography in relevant brain nuclei and peripheral tissues (heart and kidney) in rats at 1 and/or 2 wk after start of ouabain infusion at 50 microg/day. After 2 wk (but not after 1 wk), blood pressures significantly increased (+15 mmHg). At 2 wk, plasma ANG I and II levels were markedly suppressed by ouabain. In contrast, in the heart and kidneys, ANG I levels were not affected, and ANG II levels tended to decrease, whereas in the hypothalamus ANG II content clearly increased. At 1 wk, no changes in ACE and AT1 receptor densities were seen. After 2 wk, there were significant decreases in AT(1) receptor mRNA and densities in the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and paraventricular nucleus (PVN). ACE densities decreased only in the OVLT and SFO, but ACE mRNA showed more variable responses (decrease in OVLT vs. increase in PVN). In the kidneys, at 2 wk both AT1 receptor and ACE densities were decreased, but mRNA abundance did not change. The heart showed no significant changes. The increase in hypothalamic ANG II content and associated decreases in central AT1 receptor and ACE densities support the involvement of the brain renin-angiotensin system in the central hypertensive mechanism of action of ouabain.     

Effects of central oxytocin receptor blockade on water and saline intake,mean arterial pressure,and c-Fos expression in rats

Central injection of ANG II has been proposed to have dual effects on salt appetite including a direct stimulatory effect and an indirect inhibitory effect through an activation of central oxytocinergic neurons. The inhibition was demonstrated by pretreating rats with central ornithine vasotocin (OVT; oxytocin antagonist) 30 min before a central ANG II injection. The OVT pretreatment produced a large increase in ANG II-induced saline intake. The present paper reports a failure to replicate that influential experiment. However, we also report for the first time that OVT by itself: 1) provokes drinking of both water and saline solution with a latency almost as short as that produced by ANG II; 2) produces a mild pressor response; and 3) increases c-Fos expression in the organum vasculosum laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Oxytocin activity may provide an inhibitory control of drinking responses as has been suggested, but the inhibition is tonic and includes both water and saline drinking. Inhibition of this tonic activity may stimulate drinking by increasing neural activity in the OVLT and MnPO.     

Central actions of angiotensin in cardiovascular control: multiple roles for a single peptide.

Angiotensin II (ANG II) acts peripherally as a hormone, with actions on the vasculature, adrenals, and kidney. In addition, certain actions of ANG II in the central nervous system are directed toward cardiovascular control and fluid volume homeostasis. Dense binding sites for ANG II are found at circumventricular organs, which apparently have the ability to relay information to cardiovascular centers via neural circuitry. Microinjection of ANG II into the subfornical organ (SFO) or area postrema (AP) produces site-specific increases in blood pressure. In addition, electrophysiological studies demonstrate profound effects of ANG II, acting at the SFO, on activity of neurohypophysial neurons and release of oxytocin and vasopressin, which can be antagonized by ANG II blockers or attenuated by SFO lesions. Evidence from microinjection, electrophysiological, and lesion studies indicate a complex interaction between central sites involved in mechanisms of cardiovascular control: the SFO, AP, organum vasculosum of the lamina terminalis, and paraventricular and supraoptic nuclei of the hypothalamus. Not only is ANG II a humoral messenger in this central scenario, but evidence suggests it acts as a neurotransmitter or neuroendocrine substance within specific CNS pathways, suggesting multiple roles for this peptide in central cardiovascular control.     

Decreased monoamine release in the median preoptic area following ventricular treatment with the angiotensin II antagonist saralasin in normotensive and spontaneously hypertensive rats

Previous findings have shown that some of the neurons in the median preoptic nucleus (MnPO) receive both catecholaminergic inputs from the brainstem and angiotensinergic inputs from the subfornical organ (SFO), and that alterations in the function of the brain renin-ANG system are implicated in hypertension, especially in spontaneous hypertensive rats (SHR). In an attempt to clarify the action of these inputs on MnPO neurons and to find the difference in the action between normotensive Wistar-Kyoto (WKY) rats and SHR, we used microdialysis to investigate the effects of injections of saralasin (Sar), an angiotensin II (ANG II) antagonist, into the third ventricle (3V) on monoamine release in the MnPO area of awake WKY and SHR. The content of noradrenaline (NA) in the MnPO area was significantly higher in SHR. No significant differences were observed between WKY and SHR in the concentrations of dopamine (DA) and of its two metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). In both WKY and SHR, Sar (Sar, 5 microg in 1 microl, three injections at 2-h intervals) injected into the 3V significantly decreased the extracellular concentrations of NA, DOPAC and HVA in the MnPO area. The decreases were much greater in SHR than in WKY rats. Similar injections of saline vehicle had no significant effect on the extracellular levels of NA, DA and the metabolites. These results suggest that central angiotensinergic circuits may serve to increase NA and DA release in the MnPO area, and support that a disorder in the ANG system may contribute, in part, to the elevated blood pressure of SHR.     

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

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

Ibotenate lesion of the medial hypothalamus alters the salt intake and pressor responses to activation of the median preoptic nucleus in rats

We investigated the influence of ibotenic acid lesions of the medial hypothalamus (MH) on salt appetite and arterial blood pressure responses induced by angiotensinergic and adrenergic stimulation of the median preoptic nucleus (MnPO) of rats. Previous injection of the adrenergic agonists norepinephrine, clonidine, phenylephrine, and isoproterenol into the MnPO of sham MH-lesioned rats caused no change in the sodium intake induced by ANG II. ANG II injected into the MnPO of MH-lesioned rats increased sodium intake compared with sham-lesioned rats. Previous injection of clonidine and isoproterenol increased, whereas phenylephrine abolished the salt intake induced by ANG II into the MnPO of MH-lesioned rats. Previous injection of norepinephrine and clonidine into the MnPO of sham MH-lesioned rats caused no change in the mean arterial pressure (MAP) induced by ANG II. Under the same conditions, previous injection of phenylephrine increased, whereas isoproterenol reversed the increase in MAP induced by angiotensin II (ANG II). ANG II injected into the MnPO of MH-lesioned rats induce a decrease in MAP compared with sham-lesioned rats. Previous injection of phenylephrine or norepinephrine into the MnPO of MH-lesioned rats induced a negative MAP, whereas pretreatment with clonidine or isoproterenol increased the MAP produced by ANG II injected into the MnPO of sham- or MH-lesioned rats. These data show that ibotenic acid lesion of the MH increases the sodium intake and pressor responses induced by the concomitant angiotensinergic, α₂ and β adrenergic activation of the MnPO, whereas α₁ activation may have opposite effects. MH involvement in excitatory and inhibitory mechanisms related to sodium intake and MAP control is suggested.     

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.     

Role of angiotensin in body fluid homeostasis of mice: effect of losartan on water and NaCl intakes

It is known that mice injected peripherally with ANG II do not show a drinking response but that cFos immunoreactivity (ir) is induced in brain regions similar to those in rats. We now show in Crl:CD1(ICR) mice that peripheral injection of the ANG II type 1 receptor antagonist losartan was sufficient to prevent this induction of Fos-ir in the subfornical organ (SFO). Injection of ANG II into the lateral cerebral ventricle produced a robust water intake in mice and induced Fos-ir in SFO, as well as in median preoptic (MnPO) and paraventricular (PVN) nuclei. Peripheral injection of losartan blocked this drinking response and prevented the induction of Fos-ir in each of these brain regions. Hypovolemia produced by polyethylene glycol (PEG) produced a robust water intake but no evidence of sodium appetite, and it induced Fos-ir in SFO, MnPO, and PVN. Peripheral injection of losartan did not affect this drinking response. Fos-ir induced by PEG in SFO and MnPO was reduced by treatment with losartan, while that induced in the PVN was further increased by losartan. Sodium depletion with furosemide and low-sodium diet produced a strong sodium appetite and induced Fos-ir in SFO and MnPO. Treatment with losartan completely blocked the sodium appetite, as well as the induction of Fos-ir in these brain regions. These data indicate that endogenous production of ANG II and action at forebrain receptors is critically involved in depletion-related sodium appetite in mice. The absence of an effect of losartan on PEG-induced drinking suggests the critical involvement of other factor(s) such as arterial or venous baroreceptor input, and we discuss how this factor could also explain why peripheral ANG II is not dipsogenic in mice.     

A new angiotensinergic system in the CNS of the rat

The use of two different polyclonal, affinity-purified, monospecific antibodies to ANG II (called BODE and BODE 1) revealed dissimilar distribution of ANG II immunoreactivity within the rat central nervous system (CNS). The ANG II-like material detected using BODE was concentrated in the neurosecretory hypothalamic nuclei, in the inner layer of the median eminence and in the posterior lobe of the pituitary. In contrast, the BODE 1 antibody did not stain the hypothalamic-neurohypophysial angiotensinergic system, and the staining pattern was much more broadly distributed throughout the CNS. BODE 1 is the first antibody that can be used to verify the locations of endogenous angiotensin and their receptor sites in the CNS. The diverse distribution of the ANG II-like material detected by the two antibodies provides strong evidence for the existence of at least two different angiotensinergic systems in the CNS.     

Modulation of noradrenaline release in the median preoptic area by GABAergic inputs from the organum vasculosum of the lamina terminalis in the rat

Previous observations have shown that gamma-aminobutyric acid (GABA) receptor mechanisms modulate the release of noradrenaline (NA) in the median peptic nucleus (MnPO). The present study was carried out to investigate whether neural inputs from the organum vasculosum of the lamina terminalis (OVLT) to the MnPO are involved in the GABAergic modulation of NA release in the MnPO area using in vivo microdialysis techniques. In urethane-anesthetized rats, electrical stimulation (5 and 10 microA, 10Hz) of the OVLT region, but not its surrounding region, significantly enhanced dialysate NA concentration in the MnPO area. The enhancement in the NA level caused by the OVLT region stimulation was significantly increased by perfusion with either bicuculline (10 microM), a GABA(A) receptor antagonist, or phaclofen (10 microM), a GABA(B) receptor antagonist, through a microdialysis probe. The amount of the antagonist-induced increase was much greater in the phaclofen-treated group than in the bicuculline-treated group. These results show that the OVLT region may exert both excitatory and inhibitory influences on the release of NA in the MnPO area, and imply that the inhibitory influence may be mediated through GABA(B) receptors rather than GABA(A) receptors.     

Localization of immunoreactive prolactin in ependyma and circumventricular organs of rat brain

Summary Immunoreactive prolactin (IMP) has been localized in the male rat brain using the soluble peroxidase-anti-peroxidase (PAP) technique. In normal untreated animals, reaction product was seen in choroid plexus (CP) and in ependymal cells of the ventricular lining with heaviest concentrations of positively staining cells in the 3rd ventricle near the subcommisural organ (SCO), in the lateral ventricles near the subfornical organ (SFO), and in the 4th ventricle near the area postrema (AP). IMP was also present in numerous ependymal cells resembling tanycytes in the cerebral aqueduct, central canal of the spinal cord at the level of the AP, the organum vasculosum of the lamina terminalis (OVLT) and the floor of the infundibular recess. Immunoreactive cells resembling neurons were localized within the substance of the AP, SCO, and OVLT. IMP was also present in fibers of the zona externa of the median eminence and infundibular stalk; a few cells of the pars tuberalis contained reaction product. Hypophysectomized rats and bromocriptine-treated rats exhibited a similar staining pattern except that bromocriptine treatment eliminated IMP from most CP cells. Hypophysectomy, bromocriptine or estrogen treatment enhanced staining for IMP in cells of the pars tuberalis; estrogen treatment or hypophysectomy produced an increase in the number and distribution of immunoreactive cells as well as increased density of reaction product in cells of the medial habenular nucleus. The functional relevance of prolactin in these locations in the brain, the possible routes of transport of prolactin from the pituitary gland to the central nervous system, and the strong suggestion of extra-pituitary sites of synthesis of a prolactin-like hormone are discussed.     

Activation of Organum Vasculosum of Lamina Terminalis,Median Preoptic Nucleus,and Medial Preoptic Area in Anticipation of Nursing in Rabbit Pups

Rhythmic feeding in rabbit pups is a natural model to study food entrainment because, similar to rodents under a schedule of food restriction, these animals show food-anticipatory activity (FAA) prior to daily nursing. In rodents, several brain systems, including the orexinergic system, shift their activity to the restricted feeding schedule, and remain active when subjects are hungry. As the lamina terminalis and regions of the preoptic area participate in the control of behavioral arousal, it was hypothesized that these brain regions are also activated during FAA. Thus, the effects of daily milk ingestion on FOS protein expression in the organum vasculosum of lamina terminalis (OVLT), median preoptic nucleus (MnPO), and medial preoptic area (MPOA) were examined using immunohistochemistry before and after scheduled time of nursing in nursed and fasted subjects. Additionally, FOS expression was explored in orexin (ORX) cells in the lateral hypothalamic area and in the supraoptic nucleus (SON) because of their involvement in arousal and fluid ingestion, respectively. Pups were entrained by daily nursing, as indicated by a significant increase in locomotor behavior before scheduled time of nursing in both nursed and fasted subjects. FOS was significantly higher in the OVLT, MnPO, and MPOA at the time of nursing, and decreased 8?h later in nursed pups. In fasted subjects, this effect persisted in the OVLT, whereas in the MnPO and MPOA, values did not drop at 8?h later, but remained at the same level or higher than those at the time of scheduled nursing. In addition, FOS was significantly higher in ORX cells during FAA in nursed pups in comparison with 8?h later, but in fasted subjects it remained high during most fasting time points. Additionally, OVLT, SON, and ORX cells were activated 1.5?h after nursing. We conclude that the OVLT, MnPO, and MPOA, but not SON, may participate in FAA, as they show activation before suckling of periodic milk ingestion, and that sustained activation of the OVLT, MnPO, and MPOA by fasting may contribute to the high arousal state associated with food deprivation. In agreement with this, ORX cells also remain active after expected nursing, which is consistent with reports in other species.     

Changes in pericytic expression of NG2 and PDGFRB and vascular permeability in the sensory circumventricular organs of adult mouse by osmotic stimulation

The blood–brain barrier (BBB) is a barrier that prevents free access of blood‐derived substances to the brain through the tight junctions and maintains a specialized brain environment. Circumventricular organs (CVOs) lack the typical BBB. The fenestrated vasculature of the sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows parenchyma cells to sense a variety of blood‐derived information, including osmotic ones. In the present study, we utilized immunohistochemistry to examine changes in the expression of NG2 and platelet‐derived growth factor receptor beta (PDGFRB) in the OVLT, SFO and AP of adult mice during chronic osmotic stimulation. The expression of NG2 and PDGFRB was remarkably prominent in pericytes, although these angiogenesis‐associated proteins are highly expressed at pericytes of developing immature vasculature. The chronic salt loading prominently increased the expression of NG2 in the OVLT and SFO and that of PDGFRB in the OVLT, SFO and AP. The vascular permeability of low‐molecular‐mass tracer fluorescein isothiocyanate was increased significantly by chronic salt loading in the OVLT and SFO but not AP. In conclusion, the present study demonstrates changes in pericyte expression of NG2 and PDGFRB and vascular permeability in the sensory CVOs by chronic osmotic stimulation, indicating active participation of the vascular system in osmotic homeostasis. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

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

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

Dehydration and fluid balance: central effects of angiotensin

Central effects of dehydration are stimulated by osmotic stimuli, the reduced input of volume receptors, and angiotensin II. The subfornical organ (SFO) and organum vasculosum laminae terminalis (OVLT) have become accepted as putative receptor sites for angiotensin II in the brain. The exact quantitative relationship between the hours of water deprivation and the amount of angiotensin generated peripherally and whether that amount is sufficient to induce thirst centrally have not been established, but there is no question that when animals are dehydrated their angiotensin levels rise and the animals are thirsty. Attempts to block centrally the contribution of angiotensin II to thirst have been variable and cholinergic inputs have to be blocked at the same time. Various stimuli for thirst interact in a parallel fashion, and when one stimulus is blocked the other stimuli are still effective. Plasma angiotensin II may induce natural thirst, but how it enters the brain still remains to be explained. Although the SFO and OVLT have no blood-brain barrier, the blood supply to these organs acts as a limited perfusion system whereby blood-borne proteins cannot diffuse far from the capillary bed. A second set of receptors is found on the ventricular surface of the OVLT, as shown by fluorescence labeled angiotensin II. The connection between the SFO and OVLT was cut by discrete knife cuts. Drinking to angiotensin II intraventricularly was not significantly altered but the pressor response was reduced by 50%. These results can be explained by a circuit for drinking passing down below the level of the knife cut and a separate pressor pathway passing dorsally through the area that was cut by the knife. Thirst and pressor neural circuits beginning with angiotensin receptors could explain some of the data accumulated with the AV3V syndrome that occurs when the OVLT and nucleus medianas are destroyed.     

The anterior wall of the third cerebral ventricle and homeostatic responses to dehydration

Within the anterior wall of the third cerebral ventricle, structures are found which have been implicated in the regulation of fluid and electrolyte balance. These structures include the subfornical organ (SFO), preoptic medianus nucleus (PMN) and the organum vasculosum of the lamina terminalis (OVLT). In sheep, the OVLT rises from the ventricular floor over the optic chiasma and occupies most of the midline ventricular wall up to the level of anterior commissure. It contains a plexus of blood vessels at its base which possess fenestrated endothelial cells, and appears to lack ependyma. The SFO of sheep bulges into the third ventricle above the anterior commissure and the PMN is situated between the SFO and OVLT, surrounding the rostral edge of the midline anterior commissure. Like most mammals, water deprivation in sheep results in hypertonicity of body fluids, thirst and graded increase in plasma concentration of vasopressin (AVP). Dehydration also causes a natriuresis in these animals. In sheep with combined ablation of OVLT/PMN tissue, the volume of water drunk, the increases in plasma vasopressin (AVP) level, and the natriuresis in response to dehydration were considerably attenuated, and extreme hypernatremia resulted. Additionally, ablation of OVLT/PMN tissue almost abolished water drinking and AVP secretion in response to systemic infusion of hypertonic NaCl, but did not diminish AVP secretion in response to haemorrhage. In other animals, the OVLT and PMN were individually ablated. While partial osmoregulatory deficits were observed in each case, these deficits were smaller than those observed with combined OVLT/PMN ablation. In contrast to these results, the homeostatic responses to dehydration were not diminished in sheep with combined SFO/PMN lesions.(ABSTRACT TRUNCATED AT 250 WORDS)