The organum vasculosum laminae terminalis: a critical area for osmoreception

Fluid dynamics of life: exploring the physiology and importance of water in the critical illness

Water acknowledged as a vital component for life and the universal solvent, is crucial for diverse physiological processes in the human body. While essential for survival, the human body lacks the capacity to produce water, emphasizing the need for regular ingestion to maintain a homeostatic environment. The human body, predominantly composed of water, exhibits remarkable biochemical properties, playing a pivotal role in processes such as protein transport, thermoregulation, the cell cycle, and acid–base balance. This review delves into comprehending the molecular characteristics of water and its interactions within the human body. The article offers valuable insights into the intricate relationship between water and critical illness. Through a comprehensive exploration, it seeks to enhance our understanding of water’s pivotal role in sustaining overall human health.

The Emergence of TRP Channels Interactome as a Potential Therapeutic Target in Pancreatic Ductal Adenocarcinoma

Integral membrane proteins, known as Transient Receptor Potential (TRP) channels, are cellular sensors for various physical and chemical stimuli in the nervous system, respiratory airways, colon, pancreas, bladder, skin, cardiovascular system, and eyes. TRP channels with nine subfamilies are classified by sequence similarity, resulting in this superfamily's tremendous physiological functional diversity. Pancreatic Ductal Adenocarcinoma (PDAC) is the most common and aggressive form of pancreatic cancer. Moreover, the development of effective treatment methods for pancreatic cancer has been hindered by the lack of understanding of the pathogenesis, partly due to the difficulty in studying human tissue samples. However, scientific research on this topic has witnessed steady development in the past few years in understanding the molecular mechanisms that underlie TRP channel disturbance. This brief review summarizes current knowledge of the molecular role of TRP channels in the development and progression of pancreatic ductal carcinoma to identify potential therapeutic interventions.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

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

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

The osmoresponsiveness of oxytocin and vasopressin neurones: Mechanisms, allostasis and evolution

In the rat supraoptic nucleus, every oxytocin cell projects to the posterior pituitary, and is involved both in reflex milk ejection during lactation and in regulating uterine contractions during parturition. All are also osmosensitive, regulating natriuresis. All are also regulated by signals that control appetite, including the neural and hormonal signals that arise from the gut after food intake and from the sites of energy storage. All are also involved in sexual behaviour, anxiety-related behaviours and social behaviours. The challenge is to understand how a single population of neurones can coherently regulate such a diverse set of functions and adapt to changing physiological states. Their multiple functions arise from complex intrinsic properties that confer sensitivity to a wide range of internal and environmental signals. Many of these properties have a distant evolutionary origin in multifunctional, multisensory neurones of Urbilateria, the hypothesised common ancestor of vertebrates, insects and worms. Their properties allow different patterns of oxytocin release into the circulation from their axon terminals in the posterior pituitary into other brain areas from axonal projections, as well as independent release from their dendrites.

Activation of organum vasculosum neurons and water intake in mice by vasopressin neurons in the suprachiasmatic nucleus

Previous studies have shown that mice housed under 12:12 h light-dark conditions display a pronounced increase in water intake during a 2-hour anticipatory period (AP) near the end of their active period (Zeitgeber Time ZT; ZT21.5-ZT23.5) compared to the preceding basal period (BP, ZT19.5-ZT21.5). This increased water intake during the AP is not associated with physiological stimuli for thirst, such as food intake, hyperosmolality, hyperthermia, or hypovolemia. Denying mice the water intake supplement during the AP causes them to be dehydrated at wake time. These observations suggest that this form of thirst may be driven by the circadian clock and serve to mitigate the dehydrating effect of absence of water intake during sleep. Here we review recent findings showing that this behavior is mediated by vasopressin (VP) containing neurons in the suprachiasmatic nucleus (SCN). SCN VP neurons project to the organum vasculosum lamina terminalis (OVLT) where the activity dependent release of VP causes excitation of thirst-promoting neurons. SCN VP neurons increase their electrical activity during the AP and the resultant release of VP causes an increase in the action potential firing rate of OVLT neurons. Experiments involving optogenetic control of VP release from the axon terminals of SCN neurons indicate that this network mechanism is necessary and sufficient to mediate pre-sleep water intake in mice. These findings provide insight into the output mechanisms that are used by the central clock to generate circadian rhythms, and reveal that the regulation of water intake contributes to osmoregulatory homeostasis during sleep. This article is protected by copyright. All rights reserved.

Mechanosensing in hypothalamic osmosensory neurons

Osmosensory neurons are specialized cells activated by increases in blood osmolality to trigger thirst, secretion of the antidiuretic hormone vasopressin, and elevated sympathetic tone during dehydration. In addition to multiple extrinsic factors modulating their activity, osmosensory neurons are intrinsically osmosensitive, as they are activated by increased osmolality in the absence of neighboring cells or synaptic contacts. This intrinsic osmosensitivity is a mechanical process associated with osmolality-induced changes in cell volume. This review summarises recent findings revealing molecular mechanisms underlying the mechanical activation of osmosensory neurons and highlighting important roles of microtubules, actin, and mechanosensitive ion channels in this process.

Mechanical basis of osmosensory transduction in magnocellular neurosecretory neurones of the rat supraoptic nucleus

Rat magnocellular neurosecretory cells (MNCs) release vasopressin and oxytocin to promote antidiuresis and natriuresis at the kidney. The osmotic control of oxytocin and vasopressin release at the neurohypophysis is required for osmoregulation in these animals, and this release is mediated by a modulation of the action potential firing rate by the MNCs. Under basal (isotonic) conditions, MNCs fire action potentials at a slow rate, and this activity is inhibited by hypo-osmotic conditions and enhanced by hypertonicity. The effects of changes in osmolality on MNCs are mediated by a number of different factors, including the involvement of synaptic inputs, the release of taurine by local glial cells and regulation of ion channels expressed within the neurosecretory neurones themselves. We review recent findings that have clarified our understanding of how osmotic stimuli modulate the activity of nonselective cation channels in MNCs. Previous studies have shown that osmotically-evoked changes in membrane potential and action potential firing rate in acutely isolated MNCs are provoked mainly by a modulation of nonselective cation channels. Notably, the excitation of isolated MNCs during hypertonicity is mediated by the activation of a capsaicin-insensitive cation channel that MNCs express as an N-terminal variant of the transient receptor potential vanilloid 1 (Trpv1) channel. The activation of this channel during hypertonicity is a mechanical process associated with cell shrinking. The effectiveness of this mechanical process depends on the presence of a thin layer of actin filaments (F-actin) beneath the plasma membrane, as well as a densely interweaved network of microtubules (MTs) occupying the bulk of the cytoplasm of MNCs. Although the mechanism by which F-actin contributes to Trpv1 activation remains unknown, recent data have shown that MTs interact with Trpv1 channels via binding sites on the C-terminus, and that the force mediated through this complex is required for channel gating during osmosensory transduction. Indeed, displacement of this interaction prevents channel activation during shrinking, whereas increasing the density of these interaction sites potentiates shrinking-induced activation of Trpv1. Therefore, the gain of the osmosensory transduction process can be regulated bi-directionally through changes in the organisation of F-actin and MTs.

Anatomical organization of the rat organum vasculosum laminae terminalis

The organum vasculosum of the laminae terminalis (OVLT) is a circumventricular organ located along the ventral part of the anterior wall of the third ventricle. Because it lacks a complete blood-brain barrier (BBB), blood-borne signals detected in the OVLT provide the brain with information from the periphery and contribute to the generation of centrally mediated responses to humoral feedback and physiological stressors. Experimental studies on the rat OVLT are hindered by a poor understanding of its precise anatomical dimensions and cellular organization. In this study, we use histological techniques to characterize the spatial outline of the rat OVLT and to examine the location of neurons, astrocytes, tanycytes, and ependymocytes within its confines. Our data reveal that OVLT neurons are embedded in a dense network of tanycyte processes. Immunostaining against the neuronal marker NeuN revealed that neurons are distributed throughout the OVLT, except for a thick midline septum, which comprises densely packed cells of unknown function or lineage. Moreover, the most ventral aspect of the OVLT is devoid of neurons and is occupied by a dense network of glial cell processes that form a thick layer between the neurons and the pial surface on the ventral aspect of the nucleus. Lastly, combined detection of NeuN and c-Fos protein following systemic injection of hypertonic NaCl revealed that neurons responsive to this stimulus are located along the entire midline core of the OVLT, extending from its most anterior ventral aspect to the more caudally located "dorsal cap" region.

Circadian modulation of osmoregulated firing in rat supraoptic nucleus neurones

The antidiuretic hormone vasopressin (VP) promotes water reabsorption from the kidney and levels of circulating VP are normally related linearly to plasma osmolality, aiming to maintain the latter close to a predetermined set point. Interestingly, VP levels rise also in the absence of an increase in osmolality during late sleep in various mammals, including rats and humans. This circadian rhythm is functionally important because the absence of a late night VP surge results in polyuria and disrupts sleep in humans. Previous work has indicated that the VP surge may be caused by facilitation of the central processes mediating the osmotic control of VP release, and the mechanism by which this occurs was recently studied in angled slices of rat hypothalamus that preserve intact network interactions between the suprachiasmatic nucleus (SCN; the biological clock), the organum vasculosum lamina terminalis (OVLT; the central osmosensory nucleus) and the supraoptic nucleus (SON; which contains VP-releasing neurohypophysial neurones). These studies confirmed that the electrical activity of SCN clock neurones is higher during the middle sleep period (MSP) than during the late sleep period (LSP). Moreover, they revealed that the excitation of SON neurones caused by hyperosmotic stimulation of the OVLT was greater during the LSP than during the MSP. Activation of clock neurones by repetitive electrical stimulation, or by injection of glutamate into the SCN, caused a presynaptic inhibition of glutamatergic synapses made between the axon terminals of OVLT neurones and SON neurones. Consistent with this effect, activation of clock neurones with glutamate also reduced the excitation of SON neurones caused by hyperosmotic stimulation of the OVLT. These results suggest that clock neurones in the SCN can mediate an increase in VP release through a disinhibition of excitatory synapses between the OVLT and the SON during the LSP.

Hypertonicity sensing in organum vasculosum lamina terminalis neurons: a mechanical process involving TRPV1 but not TRPV4

Primary osmosensory neurons in the mouse organum vasculosum lamina terminalis (OVLT) transduce hypertonicity via the activation of nonselective cation channels that cause membrane depolarization and increased action potential discharge, and this effect is absent in mice lacking expression of the transient receptor potential vanilloid 1 (Trpv1) gene (Ciura and Bourque, 2006). However other experiments have indicated that channels encoded by Trpv4 also contribute to central osmosensation in mice (Liedtke and Friedman, 2003; Mizuno et al., 2003). At present, the mechanism by which hypertonicity modulates cation channels in OVLT neurons is unknown, and it remains unclear whether Trpv1 and Trpv4 both contribute to this process. Here, we show that physical shrinking is necessary and sufficient to mediate hypertonicity sensing in OVLT neurons isolated from adult mice. Steps coupling progressive decreases in cell volume to increased neuronal activity were quantitatively equivalent whether shrinking was evoked by osmotic pressure or mechanical aspiration. Finally, modulation of OVLT neurons by tonicity or mechanical stimulation was unaffected by deletion of trpv4 but was abolished in cells lacking Trpv1 or wild-type neurons treated with the TRPV1 antagonist SB366791. Thus, hypertonicity sensing is a mechanical process requiring Trpv1, but not Trpv4.

Involvement of anteroventral third ventricular AMPA/kainate receptors in both hyperosmotic and hypovolemic AVP secretion in conscious rats

The area of the brain called the anteroventral third ventricular region (AV3V) includes three different subtypes of glutamate receptor, as well as neural circuits controlling fluid balance and cardiovascular and neuroendocrine functions. Although our previous data indicate the ability of AV3V N-methyl-d-aspartate (NMDA) and metabotropic receptors to provoke vasopressin (AVP)-releasing, pressor and hyperglycemic responses, the roles of non-NMDA receptors selective for alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate have not been elucidated to date. To address this question, the effects of intracerebral infusion with FWD or NBQX (specific agonist and antagonist for non-NMDA receptors, respectively) on plasma AVP, glucose, osmolality, electrolytes and cardiovascular parameters were examined in conscious rats in the absence or presence of an osmotic or volemic stimulus. When applied topically to AV3V structures such as the median preoptic nucleus, FWD augmented plasma AVP, osmolality, glucose and arterial pressure in a dose-associated fashion. All responses of the variables were abolished by pre-administering NBQX, which exerted no conspicuous effect on any variable except arterial pressure. It was revealed that NBQX administration in AV3V structures such as the median preoptic nucleus and the periventricular nucleus inhibited the rise of plasma AVP in response to intravenous infusion with hypertonic saline or removal of systemic blood through the femoral artery. Elevation of plasma osmolality and sodium evoked by osmotic load, and elevation of plasma osmolality, glucose and angiotensin II and decrease of arterial pressure caused by bleeding, were not significantly affected by NBQX treatment. These results suggest that AV3V non-NMDA receptors, as well as NMDA receptors, may elicit AVP-releasing, pressor and hyperglycemic actions when stimulated in the basal state, and may facilitate AVP secretion under both hyperosmotic and hypovolemic conditions, without contributing to cardiovascular, blood glucose or other responses.

Central osmotic regulation of sympathetic nerve activity

AIM

In this review, we will focus on the central neural mechanisms that couple osmotic perturbations to changes in sympathetic nerve discharge, and the possible impact these actions have in cardiovascular diseases such as arterial hypertension and congestive heart failure.

RESULTS

Changes in extracellular fluid osmolality lead to specific regulatory responses in defence of body fluid and cardiovascular homeostasis. Systemic hyperosmolality is well known to stimulate thirst and the release of antidiuretic hormone. These responses are largely due to osmosensing neurones in the forebrain lamina terminalis and hypothalamus and are critical elements in a control system that operates to restore body fluid osmolality. An equally important, but less characterized, target of central osmoregulatory processes is the sympathetic nervous system.

CONCLUSION

Understanding the neurobiology of sympathetic responses to changes in osmolality has important implications for body fluid and cardiovascular physiology. By stabilizing osmolality, vascular volume is preserved and thereby relatively normal levels of cardiac output and arterial pressure are maintained.

Nitrergic modulation of vasopressin, oxytocin and atrial natriuretic peptide secretion in response to sodium intake and hypertonic blood volume expansion

The central nervous system plays an important role in the control of renal sodium excretion. We present here a brief review of physiologic regulation of hydromineral balance and discuss recent results from our laboratory that focus on the participation of nitrergic, vasopressinergic, and oxytocinergic systems in the regulation of water and sodium excretion under different salt intake and hypertonic blood volume expansion (BVE) conditions. High sodium intake induced a significant increase in nitric oxide synthase (NOS) activity in the medial basal hypothalamus and neural lobe, while a low sodium diet decreased NOS activity in the neural lobe, suggesting that central NOS is involved in the control of sodium balance. An increase in plasma concentrations in vasopressin (AVP), oxytocin (OT), atrial natriuretic peptide (ANP), and nitrate after hypertonic BVE was also demonstrated. The central inhibition of NOS by L-NAME caused a decrease in plasma AVP and no change in plasma OT or ANP levels after BVE. These data indicate that the increase in AVP release after hypertonic BVE depends on nitric oxide production. In contrast, the pattern of OT secretion was similar to that of ANP secretion, supporting the view that OT is a neuromodulator of ANP secretion during hypertonic BVE. Thus, neurohypophyseal hormones and ANP are secreted under hypertonic BVE in order to correct the changes induced in blood volume and osmolality, and the secretion of AVP in this particular situation depends on NOS activity.

Hepatic denervation does not affect plasma vasopressin response to intragastric hypertonic saline in conscious rats

Peripheral osmoreceptors monitor dietary NaCl and modify central nervous system and renal sympathetic nervous system activity accordingly. Experimental evidence suggests that these responses are dependent on the hepatic nerves. Peripheral osmoreceptors also modify arginine vasopressin (AVP) secretion. However, although hepatic denervation reportedly blunts activation of both supraoptic and paraventricular hypothalamic neurons after intraportal NaCl infusion, the role of the hepatic nerves in the AVP release has not been directly examined. The present study tests the hypothesis that the hepatic nerves modify AVP release in response to intragastric NaCl infusion. Wistar-Kyoto rats (WKY) received either hepatic denervation or a sham operation. Intragastric NaCl infusion significantly elevated plasma AVP in both sham-operated WKY and hepatic-denervated WKY, and the responses were not different between these groups. Second, previous studies suggest that both AVP secretion and baroreflexes are blunted in spontaneously hypertensive rats (SHR), deficits that contribute to the observed hypertension in SHR. We hypothesized that SHR also have a blunted peripheral osmoreceptor reflex and that this contributes to NaCl-sensitive hypertension. In contrast to our prediction, in SHR intragastric NaCl infusion induced an increase in plasma AVP that was similar to that in the WKY groups. Thus, although hepatic osmoreceptors are important for chronic regulation of arterial pressure, renal sympathetic nervous system activity, and the activity of hypothalamic neurons, they do not appear to influence plasma AVP concentration in response to intragastric NaCl.

Osmoreceptors in the central nervous system

Osmoreceptors regulate sodium and water balance in a manner that maintains the osmotic pressure of the extracellular fluid (ECF) near an ideal set point. In rats, the concerted release of oxytocin and vasopressin, which is determined by the firing rate of magnocellular neurosecretory cells (MNCs), plays a key role in osmoregulation through the effects of natriuresis and diuresis. Changes in excitatory synaptic drive, derived from osmosensitive neurons in the organum vasculosum lamina terminalis (OVLT), combine with endogenously generated osmoreceptor potentials to modulate the firing rate of MNCs. The cellular basis for osmoreceptor potentials has been characterized using patch-clamp recordings and morphometric analysis in MNCs isolated from the supraoptic nucleus of the adult rat. In these cells, stretch-inactivated cationic channels transduce osmotically evoked changes in cell volume into functionally relevant changes in membrane potential. The experimental details of these mechanisms are reviewed in their physiological context.

Mediation of dehydration-induced peptidergic gene expression in the rat lateral hypothalamic area by forebrain afferent projections

We have previously shown in dehydrated rats that cellular levels of the mRNAs encoding the precursor peptides for corticotropin-releasing hormone and neurotensin/neuromedin N significantly increase in a restricted region of the lateral hypothalamic area (Watts, 1992, Brain Res. 581:208-216). The experiments reported here address the role that forebrain osmosensitive cells groups or regions associated with autonomic regulation play in developing this mRNA response. The first experiment showed that unilateral knife cuts placed between the rostral forebrain and the lateral hypothalamic area (LHA) will unilaterally attenuate the mRNA response in the LHA to dehydration. In a second experiment, small injections of the retrograde tracer Fluorogold into the region of the LHA containing these mRNAs revealed a direct input from the osmosensitive median preoptic nucleus and subfornical organ and from the fusiform nucleus of the bed nuclei of the stria terminalis, which is part of a complex of cell groups associated with autonomic regulation. We found that at least 30% of the neurons in the median preoptic nucleus and subfornical organ and 14% of the neurons in the fusiform nucleus of the bed nuclei of the stria terminalis that project to the LHA responded to a rapid increase in plasma osmolality with increased c-fos mRNA levels. In the final experiment, injections of Fluorogold into the LHA were made simultaneously with ipsilateral rostral knife cuts. Here the numbers of neurons accumulating Fluorogold in the median preoptic nucleus, subfornical organ, and the fusiform nucleus were all significantly decreased concomitantly with attenuated mRNA responses in the LHA to dehydration. We conclude that the LHA receives direct and functional projections from the median preoptic nucleus, subfornical organ, and the fusiform nucleus. These projections appear capable of mediating a substantial part of the response of peptidergic mRNAs in the LHA to dehydration.

2-Mercaptoacetate and 2-deoxy-D-glucose induce Fos-like immunoreactivity in rat brain

2-Deoxy-D-glucose (2-DG) and 2-mercaptoacetate (MA) are antimetabolic drugs that selectively antagonize glucose and fatty acid utilization, respectively, and stimulate feeding. Fos immunohistochemistry was employed to identify brain neurons activated by these drugs and to assess the role of the vagus nerve in the drug effects. Remote intravenous infusions of both MA and 2-DG induced Fos-like immunoreactivity (Fos-li) in specific brain sites, but the pattern was different for the two drugs. Mercaptoacetate induced Fos-li in the nucleus of the solitary tract (NTS), the central subnucleus of the lateral parabrachial nucleus (1PBN), the central nucleus of the amygdala (CNA, lateral part) and the dorsal motor nucleus of the vagus (DMV). Induction of Fos-li in the brain by MA was totally abolished by vagotomy. 2-Deoxy-D-glucose also induced Fos-li in the NTS, CNA (lateral part) and DMV, as well as in the external 1PBN subnucleus, locus coeruleus, paraventricular and supraoptic hypothalamic nuclei, and in scattered cells throughout the diencephalon. Induction of Fos-li by 2-DG was not blocked by vagotomy. Results suggest that 2-DG's effects on Fos-li are mediated by a direct central action, whereas MA's effects are mediated by peripheral sensory neurons. Thus, availability of glucose and fatty acids influences the activity of specific brain sites by different neural mechanisms. The correlation of Fos-immunoreactive sites with sites where lesions have been shown to cause deficits in MA- and 2-DG-induced feeding indicates that c-fos expression defines in part the central pathways involved in the metabolic control of feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

c-fos may code for a common transcription factor within the hypothalamic neural circuits involved in osmoregulation

Conscious rats were given an intraperitoneal (i.p.) injection of isotonic or hypertonic saline, and killed 10-240 min later. In the hypothalamus of hypertonic saline-injected rats, c-fos-mRNA positive cells were mainly restricted to the supraoptic and paraventricular nuclei and to structures associated with the lamina terminalis of the third ventricle, including in particular the subfornical organ, the organum vasculosum of the lamina terminalis (OVLT) and the median preoptic nucleus. These structures comprise the proposed anterior hypothalamic 'osmoreceptor complex' for regulation of vasopressin release. The time course of the appearance and disappearance of c-fos mRNA signals was similar in all regions. Thus c-fos protein (Fos) may be a common transcription factor in the hypothalamic neural circuits involved in osmoregulation.

Angiotensin and sodium interaction in the organum cavum pre-lamina terminalis: electrophysiological and drinking responses

Recording from single neurons in the rostral juxta-cerebroventricular areas, previous investigations in this laboratory failed to find neurons responding to iontophoretic application of both angiotensin II and Na+ in an additive or in a synergistic way. We are now describing such neurons found along the walls of the organum cavum pre-lamina terminalis (O.C.P.L.T.), a recently described structure. In another experiment, angiotensin II microinjections into this hollow O.C.P.L.T. in freely moving rats elicited drinking responses that were blocked whenever the vehicle contained no Na+. Although a critical concentration of Na+ seems to play a permissive role in the dispogenic action of angiotensin within the O.C.P.L.T., various concentrations of NaCl with angiotensin II did not alter the drinking response to angiotensin II itself, and hypertonic NaCl was not dipsogenic by itself in this area.

The osmoreceptor complex in the rat: evidence for interactions between the supraoptic and other diencephalic nuclei

1. Experiments were undertaken to provide evidence for the existence of a circuit of neuronal interconnections between the supraoptic nucleus (SON), the ventral anteroventral third ventricular region (including the organum vasculosum of the lamina terminalis; ventral AV3V) and the median preoptic nucleus (MnPO), and to determine the importance of these connections in the osmotic control of the neuronal activity of the SON. Extracellular recordings were made in the urethane-anaesthetized male rat from neurones in one of these three sites, while the other two sites were electrically stimulated. 2. During recording from the SON, electrical stimulus pulses applied either to the ventral AV3V or to the MnPO were followed by orthodromic excitation (OD+) or initial short-duration inhibition followed by long-duration excitation (OD- +) of most SON neurones (44/48). The latency of OD+ or OD+ component of OD- + response produced by electrical stimulation of the MnPO was significantly (paired t test, P less than 0.01) shorter than that by the stimulation of the ventral AV3V. None of the neurones we recorded in the SON was activated antidromically by stimulation of either the ventral AV3V or the MnPO. Pressure injection of lidocaine (10%, 50 nl) into the MnPO reversibly depressed the OD+ effect after stimulation of the ventral AV3V in all the SON neurones tested (11/11), while injection of lidocaine into the ventral AV3V did not affect the OD+ effect after stimulation of the MnPO in most neurones (7/9). Both types of observation are consistent with the presence of an excitatory input to SON through the MnPO. 3. Pressure injection of lidocaine into both the ventral AV3V and the MnPO reversibly blocked the activation of SON neurones following an I.P. injection of 1.5 M-NaCl (1 ml) (ventral AV3V 11/11; MnPO, 10/10 cells tested). Injection of lidocaine at both sites, however, did not prevent activation of SON neurones by hypovolaemia (2 ml of blood was withdrawn through a cannula in the right atrium: ventral AV3V, 4/5; MnPO, 4/4 cells tested). The integrity of connections in the ventral AV3V and MnPO thus appeared to be essential for osmotic activation of the SON. 4. Of the 119 ventral AV3V neurones which were tested for their response to electrical stimulation of the SON, forty-nine neurones showed orthodromic excitation (OD+; n = 33) or initial inhibition followed by excitation (OD- +; n = 16). Thirty of the forty-nine OD+ or OD- + neurones also showed antidromic excitation (AD) after electrical stimulation of the MnPO.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of an osmotic stimulus on the release of neurohypophysial hormones in the cat: preferential release of vasopressin with a possible involvement of the area postrema

Abstract In cats anaesthetized with intravenous chloralose, the injection of 0.05 to 0.4 ml 1.54 M NaCl solution (hypertonic saline, HS) into a lateral cerebral ventricle caused a large release of vasopressin. The concentration of vasopressin greatly exceeded that of oxytocin in the same samples of plasma. Vasopressin was also released when HS was injected into the fourth ventricle and into the cisterna magna from which there is no access in the cat to the ventricles, but it was less effective by these routes than when injected into a lateral ventricle in the same cat. This suggests a possible action of HS on circumventricular organs related to the third ventricle but also indicates an additional site of action reached from the subarachnoid space which would give access to the ventral and dorsal surfaces of the brainstem. Vasopressin was not released on topical application of HS to the 'nicotine sensitive area' on the ventral surface of the brainstem where nicotine acts to release vasopressin without oxytocin. Vasopressin, however, was released without detectable oxytocin on topical appliction of HS to the dorsal surface of the brainstem either outside the fourth ventricle or to the floor of the ventricle at its distal extremity, in the region of the obex. A possible site where HS acts to cause a preferential release of vasopressin on injection into a lateral ventricle is the area postrema, a circumventricular organ which impinges on the walls of the fourth ventricle at the obex. Preferential release of vasopressin might then be mediated by a selective neural input, possibly through the nucleus of the tractus solitarius, from osmoreceptors in the area postrema to the vasopressin-secreting cells in the supraoptic and paraventricular nuclei.

Role of the hypothalamus in the control of atrial natriuretic peptide release

Stimulation of the region antero-ventral to the third cerebral ventricle (AV3V) by a cholinergic drug, carbachol, and lesions of the AV3V have been demonstrated in previous studies to either augment or decrease sodium excretion, respectively. Atrial natriuretic peptide (ANP) dramatically increases renal sodium excretion and has been localized to brain areas previously shown to be involved in control of sodium excretion. Consequently, to evaluate a possible role of brain ANP in evoking the changes in renal sodium excretion that follow stimulations or lesions of the AV3V, we determined the effect of injection of carbachol into the AV3V of rats on the concentration of plasma ANP and its content in several neural tissues, the pituitary gland, lungs, and atria. Conversely, the effect of lesions in the AV3V on plasma ANP and the content of the polypeptide in the various organs was determined. Injection of carbachol into the AV3V produced the expected natriuresis, which was accompanied within 20 min by a dramatic rise in the plasma ANP concentration and a rise in ANP content in the medial basal hypothalamus, the neurohypophysis, and particularly the anterior hypophysis but without alterations in the content of ANP in the lungs or the right or left atrium. Conversely, there was a dramatic decline in plasma ANP at both 24 and 120 hr after the AV3V lesions had been placed. This was accompanied by a slight decline in the content of the peptide in the lungs. There was no change in its content in the right atrium at 24 hr after lesions, but there was a significant increase at 120 hr. There was a small decline in the content in the left atrium at 24 hr, followed by a rebound to slightly elevated levels at 120 hr. These small changes contrasted sharply with the dramatic decline in content of the peptide in the medial basal hypothalamus, median eminence, neurohypophysis, choroid plexus, anterior hypophysis, and olfactory bulb. These declines persisted or became greater at 120 hr; except in the olfactory bulb in which the decline was no longer significant. The dramatic increase in plasma ANP after carbachol stimulation of the AV3V that was accompanied by marked elevations in content of the peptide in basal hypothalamus and neuro- and adenohypophysis suggests that the natriuresis resulting from this stimulation is brought about at least in part by release of ANP from the brain. Conversely, the dramatic decline in plasma ANP after AV3V lesions was accompanied by very dramatic declines in content of ANP in these same structures, which suggests that the previously shown decrease in sodium excretion obtained after these lesions may be at least in part due to a decrease in release of ANP from the brain. In view of the much larger quantities of the peptide stored in the atria, it is still possible that changes in atrial release may contribute to the alterations in plasma ANP observed after stimulation or ablation of the AV3V region; however, these results suggest that the dramatic changes in plasma ANP that followed these manipulations may be due to altered release of the peptide from brain structures as well as the atria and lungs.

The supraoptic nucleus: afferents from areas involved in control of body fluid homeostasis

Physiological evidence indicates that the supraoptic nucleus may be an important integrating region for information relating to body fluid homeostasis. It is known that the supraoptic nucleus receives neural influences from brain receptive zones for plasma osmolality and angiotensin II, as well as from relay centers for blood pressure and blood volume. It is also known that these influences interact to modulate vasopressin release from the supraoptic nucleus. Therefore, a detailed investigation of the neurochemical afferents to the supraoptic nucleus from regions of the lamina terminalis and the brainstem was undertaken. Injection of a fluorescent retrograde tracer, doxorubicin, into the supraoptic nucleus was combined with histochemistry of angiotensin II and catecholamines. Following supraoptic nucleus injection, retrograde label was found in forebrain neurons of the subfornical organ, median preoptic nucleus, and organum vasculosum of the lamina terminals. Some labeled cells in the subfornical organ and organum vasculosum of the lamina terminalis were also found to contain angiotensin II immunoreactivity. In the brainstem, retrograde label was found in neurons of the A1, A2 and A6 cell groups. Many of these cells were also found to contain catecholamine fluorescence or tyrosine hydroxylase immunoreactivity. Corroboration of the A2 projection was obtained by lesions of this nucleus, which reduced catecholamine fluorescence in the supraoptic nucleus. These findings provide an anatomical basis for the functional observations that the supraoptic nucleus plays a key integrative role in the maintenance of body fluid homeostasis.

The vasopressin gene is expressed prior to regulation in the supraoptic nuclei of fetal rats

The development of the regulation of vasopressin (prepropressophysin) mRNA in the supraoptic nuclei (SON) of rats was studied using quantitative in situ hybridization. On day 21 of gestation, vasopressin mRNA levels in the SON were increased by osmotic stimulation, with significant positive correlation between vasopressin mRNA levels and plasma osmolality. On day 19 of gestation, vasopressin mRNA levels in the SON were not significantly increased by osmotic stimulation, and no correlation between vasopressin mRNA levels and plasma osmolality was noted. The results suggest that the mechanisms for osmotic regulation of vasopressin mRNA levels develop between days 19 and 21 of gestation, a time course consistent with the initiation of afferent innervation of the SON.

The neurophysin-containing innervation of the forebrain of the mouse

The oxytocinergic and vasopressinergic innervation of the forebrain of normal mice was studied immunocytochemically by use of a set of mouse monoclonal anti-neurophysins applied to serial vibratome sections. The extensive hypothalamic and extra-hypothalamic location of these neuropeptides was revealed, with, or without colchicine pretreatment. Magnocellular perikarya immunoreactive for either oxytocin-neurophysin or vasopressin-neurophysin were concentrated mainly: in the anterior commissural nucleus; in various subdivisions of the paraventricular nucleus; in a profuse array in the periventricular region; in the supraoptic nucleus including its retrochiasmatic division; in various accessory nuclei; and as a number of cells scattered throughout the preoptic and hypothalamic regions. Extensive groups of parvocellular neurons, containing only vasopressin-neurophysin, were located in the suprachiasmatic nucleus including a ventromedial division, in the bed nucleus of the stria terminalis, and in the medial amygdaloid nucleus. Perikarya in the magnocellular nuclei were of generally similar size distribution and there was no evidence that distinct populations of magnocellular and parvicellular neurons, separable on the basis of size, had been labelled within these nuclei. Within the paraventricular nucleus, however, neurons in the posterior part were smaller than those located more anteriorly, and the cells containing oxytocin-neurophysin were slightly smaller than those containing vasopressin-neurophysin. Within the generally similar size distribution, magnocellular neurons of the anterior commissural nucleus were the largest. During processing, shrinkage of the tissue and immunolabeled cells had occurred. The immunocytochemical procedure delineated neuronal processes, in particular dendrites, very effectively. The dendrites were shown to project for far greater distances than is generally recognized, some were of a characteristic corkscrew-like morphology, and most were oriented in a well-defined pattern. Many dendrites of paraventricular neurons passed medially than caudally towards and then along the third ventricle. Most dendrites of supraoptic neurons, in particular those containing vasopressin-neurophysin, had an extensive anteroposterior course beneath the pia of the base of the brain. The axons containing oxytocin- and vasopressin-neurophysin were shown to take rather different paths from the paraventricular nucleus towards the median eminence.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurophysiology of body fluid homeostasis

1. Oxytocin as well as vasopressin is released in rats following systemic osmotic stimulation. There is evidence that, in the rat, oxytocin is a natriuretic hormone. 2. Osmotically-induced activation of oxytocin and vasopressin cells and osmotically-induced hormone secretion are diminished by ablation of tissue in the region anterior and ventral to the third ventricle (AV3V region). 3. The nature and identity of the osmoreceptors subserving oxytocin and vasopressin release are discussed.

Endocrine components of body fluid homeostasis

1. Linear relationships between plasma osmolality and thirst and vasopressin secretion are described in conscious dogs. 2. During water deprivation, natriuresis occurs which ameliorates the rise in plasma osmolality. 3. Increases in plasma osmolality prevent the stimulation of aldosterone secretion by angiotensin II.

Regional increase of cyclic GMP by atrial natriuretic factor in rat brain: markedly elevated response in spontaneously hypertensive rats

Atrial natriuretic factor (ANF)-responsive areas in rat brain were examined by measuring ANF-stimulated cyclic GMP production in rat brain slice preparations. The medulla oblongata, thalamus, and pituitary gland responded most sensitively, the septum, hypothalamus, pons, midbrain and olfactory bulb responded moderately, but neocortex, cerebellum, striatum and hippocampus were unresponsive to ANF. The most responsive regions in spontaneously hypertensive rats brains showed 2 to 5 times higher cyclic GMP production than those from the control Wistar-Kyoto rats. These findings provide evidence for biological action of ANF on brain tissues, and indicate the action of ANF produced in the brain.

Atrial natriuretic factor inhibits dehydration- and angiotensin II-induced water intake in the conscious, unrestrained rat

Peptides isolated from atrial extracts possess potent natriuretic and diuretic activities. In general, these peptides, called atrial natriuretic factors (ANFs), oppose the actions of the water-conservatory peptides angiotensin II and vasopressin and are released from the heart in response to atrial stretch as a consequence of increased venous return. The recent description of ANF-like immunoreactivity in brain regions associated with the control of water intake suggested a role for these peptides in the neurogenic mechanisms of thirst. Intracerebroventricular (third ventricle) infusion of 1.0 or 2.0 nmol of ANF in conscious, overnight-dehydrated rats significantly inhibited subsequent water intake over a 2-hr test period. Intravenous infusion of 2.0 nmol, but not 1.0 nmol, of ANF resulted in a similar inhibitory action, suggesting that ANF released from the heart might act centrally to inhibit water intake by an action at one or more of the circumventricular organs. Water intake induced by central infusion of angiotensin II (9.6 and 25 pmol) in normally hydrated rats was significantly inhibited by prior infusion of 2.0 nmol of ANF. Water intake induced by higher doses of angiotensin II was not altered significantly by prior infusion of ANF. These results indicate a possible physiologic role for ANF in the hypothalamic control of water intake and reveal that the cardiac peptides can act centrally, as well as peripherally, to assist in the normalization of extracellular fluid volume.