Evidence of a direct action of angiotensin II on neurones in the septum and in the medial preoptic area

Brain Angiotensinergic Regulation of the Immune System: Implications for Cardiovascular and Neuroendocrine Responses

OBJECTIVE

The Renin-Angiotensin-Aldosterone System (RAAS) plays a major role in the regulation of cardiovascular functions, water and electrolytic balance, and hormonal responses. We perform a review of the literature, aiming at providing the current concepts regarding the angiotensin interaction with the immune system in the brain and the related implications for cardiovascular and neuroendocrine responses.

METHODS

Appropriate keywords and MeSH terms were identified and searched in Pubmed. Finally, references of original articles and reviews were examined.

RESULTS

Angiotensin II (ANG II), beside stimulating aldosterone, vasopressin and CRH-ACTH release, sodium and water retention, thirst, and sympathetic nerve activity, exerts its effects on the immune system via the Angiotensin Type 1 Receptor (AT 1R) that is located in the brain, pituitary, adrenal gland, and kidney. Several actions are triggered by the binding of circulating ANG II to AT 1R into the circumventricular organs that lack the Blood-Brain-Barrier (BBB). Furthermore, the BBB becomes permeable during chronic hypertension thereby ANG II may also access brain nuclei controlling cardiovascular functions. Subfornical organ, organum vasculosum lamina terminalis, area postrema, paraventricular nucleus, septal nuclei, amygdala, nucleus of the solitary tract and retroventral lateral medulla oblongata are the brain structures that mediate the actions of ANG II since they are provided with a high concentration of AT 1R. ANG II induces also T-lymphocyte activation and vascular infiltration of leukocytes and, moreover, oxidative stress stimulating inflammatory responses via inhibition of endothelial progenitor cells and stimulation of inflammatory and microglial cells facilitating the development of hypertension.

CONCLUSION

Besides the well-known mechanisms by which RAAS activation can lead to the development of hypertension, the interactions between ANG II and the immune system at the brain level can play a significant role.

Vasopressin and angiotensin receptors of the medial septal area of the brain in the control of thirst and salt appetite induced by vasopressin in water-deprived and sodium-depleted rats

In this study we investigated the influence of d(CH(2))(5)-Tyr (Me)-AVP (A(1)AVP) and [Adamanteanacatyl(1),D-ET-D-Tyr(2), Val(4), aminobutyril(6),A(8,9)]-AVP (A(2)AVP), antagonists of V(1) and V(2) arginine(8)-vasopressin (AVP) receptors, respectively, as well as the effects of losartan and CGP42112A, antagonists of angiotensin II (ANGII) AT(1) and AT(2,) receptors, respectively, on water and 0.3 M sodium intake induced by water deprivation or sodium depletion (furosemide treatment) and enhanced by AVP injected into the medial septal area (MSA). A stainless steel cannula was implanted into the medial septal area (MSA) of male Holtzman rats AVP injection enhanced water and sodium intake in a dose-dependent manner. Pretreatment with V(1) antagonist injected into the MSA produced a dose-dependent reduction, whereas prior injection of V(2) antagonist increased, in a dose-dependent manner, the water and sodium responses elicited by the administration of AVP. Both AT(1) and AT(2) antagonists administered into the MSA elicited a concentration-dependent decrease in water and sodium intake induced by AVP, while simultaneous injection of the two antagonists was more effective in decreasing AVP responses. These results also indicate that the increase in water and sodium intake induced by AVP was mediated primarily by MSA AT(1) receptors.

Mineralocorticoid pretreatment enhances angiotensin II-induced neuronal excitation but not salt drinking in male Fischer rats

Central administration of angiotensin (Ang) II stimulates thirst and sodium intake via the AT-1 receptor. Mineralocorticoid pretreatment enhances Ang II-induced drinking of hypertonic salt solutions (i.e. the synergy theory) in Wistar and Sprague-Dawley rats. Electrophysiological experiments using iontophoretic application of Ang II, and the AT-1 receptor specific nonpeptide antagonist losartan, have shown excitation of neurones in the preoptic/medial septum region of urethane anaesthetised male Wistar rats. Deoxycorticosterone acetate (DOCA) pretreatment further enhanced this neuronal excitation to Ang II and reduced the responses to losartan. This generated the hypothesis that DOCA-enhanced Ang II-induced neuronal excitation was necessary for the enhanced salt intake of synergy theory. We tested this hypothesis in Fischer 344 rats that are known to have a low basal salt appetite and reduced sensitivity for i.c.v. Ang II. We compared the effect of DOCA pretreatment on i.c.v. Ang II-induced water and 2% NaCl intake in behaving adult male, Fischer rats, as well as preoptic/medial septum region neuronal responses to Ang II and losartan, using a seven-barrelled micro-iontophoretic electrode sealed to a recording electrode in urethane anaesthetised, male Fischer rats. Two groups were used: one pretreated with DOCA (0.5 mg/day for 3 days) and the other comprising controls, treated with isotonic saline. Ang II applied iontophoretically increased activity in 31% of the spontaneously active neurones. Following DOCA pretreatment, the responsiveness to Ang II (when applied after aldosterone) was increased. By contrast, in the behaving animals, water and 2% NaCl intake in response to i.c.v. Ang II were not enhanced by DOCA pretreatment. These results do not support the working hypothesis but could be interpreted as evidence for the existence of two separately modulated central Ang II systems: one responding to mineralocorticoids with increased neuronal activity and the other responsible for the Ang II-induced sodium appetite in conscious rats.

Influence of arginine vasopressin receptors and angiotensin receptor subtypes on the water intake and arterial blood pressure induced by vasopressin injected into the lateral septal area of the rat

In this study we investigated the influence of d(CH2)5-Tyr(Me)-[Arg8]vasopressin (AAVP) and [adamanteanacetyl1,0-ET-d-Tyr2,Val4,aminobutyryl6,Arg8,9]-[Arg8]vasopressin (ATAVP), which are antagonists of vasopressin V1 and V2 receptors, and the effects of losartan, a selective angiotensin AT1 receptor antagonist, and CGP42112A, a selective AT2 receptor antagonist, injected into the lateral septal area (LSA) on thirst and hypertension induced by [Arg8]vasopressin (AVP). AAVP and ATAVP injected into the LSA reduced the drinking responses elicited by injecting AVP into the LSA. Both the AT1 and AT2 ligands administered into the LSA elicited a concentration-dependent decrease in the water intake induced by AVP injected into the LSA, but losartan was more effective than CGP42112A. The increase in MAP, due to injection of AVP into the LSA, was reduced by prior injection of AAVP from 18 +/- 1 to 6 +/- 1 mm Hg. Losartan injected into the LSA prior to AVP reduced the increase in MAP to 7 +/- 0.8 mm Hg. ATAVP and CGP42112A produced no changes in the pressor effect of AVP. These results suggest that the dipsogenic effects induced by injecting AVP into the LSA were mediated primarily by AT1 receptors. However, doses of losartan were more effective when combined with CGP42112A than when given alone, suggesting that the thirst induced by AVP injections into LSA may involve activation of multiple AVP and angiotensin II receptor subtypes. The pressor response of AVP was reduced by losartan and by AAVP. CGP42112A and ATAVP did not change the AVP pressor response. These results suggest that facilitator effects of AVP on water intake are mediated through the activation of V1 receptors and that the inhibitory effect requires V2 receptors. The involvement of AT1 and AT2 receptors can be postulated. Based on the present findings, we suggest that the AVP in the LSA may play a role in the control of water and arterial blood pressure balance.

Interaction between supraoptic nucleus and septal area in the control of water, sodium intake and arterial blood pressure induced by injection of angiotensin II

We investigated the effects of injection into the supraoptic nucleus (SON) of losartanand PD 123319 (nonpeptide AT(1) and AT(2)-angiotensin II [ANG II] receptor antagonists, respectively); d(CH(2))(5)-Tyr(Me)-AVP (AVPA; an arginine-vasopressin [AVP] V(1) receptor antagonist), FK 409 (a nitric oxide [NO] donor), and N(W)-nitro-l-arginine methyl ester (l-NAME; an NO synthase inhibitor) on water intake, sodium chloride 3% (NaCl) intake and arterial blood pressure induced by injection of ANG II into the lateral septal area (LSA). Male Holtzman rats (250-300 g) were implanted with cannulae into SON and LSA unilaterally. The drugs were injected in 0.5 microl over 30-60 s. Controls were injected with a similar volume of 0.15 M NaCl. ANG II was injected at a dose of 10 pmol. ANG II antagonists and AVPA were injected at doses of 80 nmol. FK 409 and l-NAME were injected at doses of 20 and 40 microg, respectively. Water and NaCl intake was measured over a 2-h period. Prior administration of losartan into the SON decreased water and NaCl intake induced by injection of ANG II. While there was a decrease in water intake, ANG II-induced NaCl intake was significantly increased following injection of AVPA. FK 409 injection decreased water intake and sodium intake induced by ANG II. l-NAME alone increased water and sodium intake and induced a pressor effect. l-NAME-potentiated water and sodium intake induced by ANG II. PD 123319 produced no changes in water or sodium intake induced by ANG II. The prior administration of losartan or AVPA decreased mean arterial pressure (MAP) induced by ANG II. PD 123319 decreased the pressor effect of ANG II to a lesser degree than losartan. FK 409 decreased the pressor effect of ANG II while l-NAME potentiated it. These results suggest that both ANG II AT(1) and AVP V(1) receptors and NO within the SON may be involved in water intake, NaCl intake and the pressor response were induced by activation of ANG II receptors within the LSA. These results do not support the involvement of LSA AT(2) receptors in the mediation of water and NaCl intake responses induced by ANG II, but influence the pressor response.

Central AII evokes a normal sodium appetite in the Fischer rat, but its low spontaneous sodium intake may be related to reduced excitation and increased inhibition in septo-preoptic AII neurons

Fischer rats show a low or absent basal salt appetite and a reduced intake of salt solutions in response to peripherally administered angiotensin II (AII) when compared to other strains. We investigated spontaneous sodium intake, and sodium intake after intracerebroventricular (i.c.v.) AII and losartan, and septo-preoptic neuronal responses to AII and losartan, in age-matched male Fischer and Wistar rats. Spontaneous intake of 1.8% NaCl was lower in Fischers, but i.c.v. injection of 10 pmol AII produced similar 2 h intakes in a 2 h test period. Iontophoretic application of AII and losartan onto neurons in the septo-preoptic continuum revealed differences between the two strains of rat. In the Fischer rats only 11% of the spontaneously active neurons were sensitive to locally applied AII compared to approximately 30% in the Wistar. Local application of losartan produced neuronal inhibition in Fischer rats but neuronal excitation in Wistars. The central AII system appears to be regulated differently in these two strains, and may be related to the differences in their spontaneous sodium intake, but not to AII aroused sodium appetite.

Role of angiotensin II and vasopressin receptors within the supraoptic nucleus in water and sodium intake induced by the injection of angiotensin II into the medial septal area

In this study we investigated the effects of the injection into the supraoptic nucleus (SON) of non-peptide AT1- and AT2-angiotensin II (ANG II) receptor antagonists, DuP753 and PD123319, as well as of the arginine-vasopressin (AVP) receptor antagonist d(CH2)5-Tyr(Me)-AVP, on water and 3% NaCl intake induced by the injection of ANG II into the medial septal area (MSA). The effects on water or 3% NaCl intake were assessed in 30-h water-deprived or in 20-h water-deprived furosemide-treated adult male rats, respectively. The drugs were injected in 0.5 microliter over 30-60 s. Controls were injected with a similar volume of 0.15 M NaCl. Antagonists were injected at doses of 20, 80 and 180 nmol. Water and sodium intake was measured over a 2-h period. Previous administration of the AT1 receptor antagonist DuP753 into the SON decreased water (65%, N = 10, P < 0.01) and sodium intake (81%, N = 8, P < 0.01) induced by the injection of ANG II (10 nmol) into the MSA. Neither of these responses was significantly changed by injection of the AT2-receptor antagonist PD123319 into the SON. On the other hand, while there was a decrease in water intake (45%, N = 9, P < 0.01), ANG II-induced sodium intake was significantly increased (70%, N = 8, P < 0.01) following injection of the V1-type vasopressin antagonist d(CH2)5-Tyr(Me)-AVP into the SON. These results suggest that both AT1 and V1 receptors within the SON may be involved in water and sodium intake induced by the activation of ANG II receptors within the MSA. Furthermore, they do not support the involvement of MSA AT2 receptors in the mediation of these responses.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

The neuronal role of angiotensin II in thirst, sodium appetite, cognition and memory

Within the past two decades, a great deal has been learnt about the renin-angiotensin system in the brain. The renin-angiotensin system is one of the best-studied enzyme-neuropeptide systems in the brain. The diversity of localization of this peptide throughout the brain has implied a variety of potential functions. Besides its classical role in the regulation of blood pressure and body-fluid homeostasis, it has more subtle functions involving complex mechanisms such as learning and memory. The profound effects on behaviour produced by angiotensin are of broad interest to neuroscientists. The mechanisms of action differ depending on whether angiotensin is locally synthesized and whether regulation is governed by neural or metabolic inputs impinging on the neurones. Its central action is mediated through peptidergic receptors present on neurones. The description of the receptor subtypes AT1 and AT2 for angiotensin II and the development of non-peptidic specific angiotensin receptor subtype antagonists have opened a new area in this field of research. The AT1 site, which preferentially binds to angiotensin II and angiotensin III, appears to mediate the classical angiotensin functions concerned with maintenance of blood pressure and body-fluid control. In addition, most of the behavioural effects described so far are linked with AT1, although so-called psychotropic effects are presumed to be mediated by receptor systems other than the known specific angiotensin receptors. In fact, evidence for the existence of such receptors with high-affinity binding has been reported. The central action of angiotensin II mediated by AT2 is as yet unclear. Most reports concerning this receptor subtype suggest a role in differentiation and development, since the number of binding sites is higher in fetal and young rats than in adults. Furthermore, the neuronal effect of angiotensin II in the inferior olivary nucleus which is blocked specifically by AT2 antagonists suggests an involvement in motor control. Over the next few years we should find answers to many of the questions currently unanswered about angiotensin function and, given the rapid progress in research on this neuropeptide, it may serve as a model for the action of peptides on neuronal function in general.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.

Water versus salty taste and Iontophoretic ANGII responses of septopreoptic neurons in dehydrated and euhydrated awake rats

Little is known of the influence of gustatory, particularly salt, input on neurons of the forebrain and if the same neurons are sensitive to hydromineral balance humoral stimuli. In awake, nonpremedicated rats we recorded the activity of spontaneously active neurons in the preoptic/anterior hypothalamic area of dehydrated and euhydrated rats while allowing them to ingest water or a hypertonic salt solution (1.6% NaCl) administered to the tongue. The hormones angiotensin and aldosterone, both implicated in hydromineral balance, were applied by iontophoresis to the same neurons. In the dehydrated rats, 27% (15/55) of the spontaneously active neurons responded to a liquid (either water or the NaCl) applied to the tongue; in the euhydrated rats 23% (18/78) responded to the same stimuli. In the dehydrated rats, however, 33% (5/15) of the responding neurons were inhibited when the NaCl solution was applied to the tongue compared with only 5% (1/18) in the euhydrated rats. Iontophoretic application of angiotensin increased the spontaneous activity in 21% of those neurons tested that responded to taste. These results suggest that the state of hydration of an animal is able to change the neuronal response to substances applied to the tongue. Furthermore, it appears that these gustatory-sensitive neurons may also be related to hydromineral balance regulation since they are able to respond to angiotensin.

Tonic neuronal inhibition by AII revealed by iontophoretic application of Losartan, a specific antagonist of AII type-1 receptors

Short-term low-dose mineralocorticoid pretreatment enhances subsequent neuronal activity in the medial septum/preoptic region and in the stria terminalis/posterior amygdala of urethane anaesthetised male Wistar rats and sensitises these neurons to angiotensin II (AII). We have investigated the effect of iontophoretic application of Losartan, a specific nonpeptidergic AII type-1 receptor antagonist, on the background activity of spontaneously active neurons in these regions using a seven-barrelled microiontophoretic electrode sealed to a recording electrode. The influence of Losartan on the effects of iontophoretically applied AII in deoxycorticosterone acetate (DOCA) pretreated and nonpretreated rats was also investigated. Iontophoretically applied Losartan was observed to block the excitatory effect of AII in some neurons. In other spontaneously active neurons Losartan was seen to stimulate (or inhibit) immediately, this effect being greater in nonpretreated than in DOCA pretreated rats. Losartan was also observed to provoke persistent excitation of some spontaneously active neurons only in the nonpretreated rats. These results suggest that there exists a tonic inhibition by AII on the neurons in this area of the forebrain and that there may exist at least two subtypes of the AII type-1 receptor.

The renin-angiotensin system in the brain: an update 1993

The renin-angiotensin system is considered to be one of the most important hormonal systems in the regulation of blood pressure and body fluid homeostasis. Ever since this system has been demonstrated to be present also in the brain, vast efforts have been made in investigating its central impact and function. The last few years, and especially the development of non-peptidic angiotensin II receptor subtype specific antagonists and the subsequent pharmacological characterization of these subtypes, brought this field of research a large step forward. This progress also might have opened up new avenues of developing highly specific anti-hypertensive drugs and thereby new ways of treating hypertension. This paper intends to provide a summary of the knowledge about the brain renin-angiotensin system accumulated during recent years; an update 1993.

Angiotensin II in the spinal cord of the rat and its sympatho-excitatory effects

Immunohistochemical studies have shown there is a dense angiotensin-like immunoreactivity of terminals in the sympathetic region of the thoracic and lumbar spinal cord. In the present study measurements were made of the concentration of angiotensin in the spinal cord of rats using radioimmunoassay following two different extraction procedures. These gave concentrations of angiotensin as mean of 108 and 161 pg.g-1 tissue wet weight. Angiotensin II given intrathecally or microinjected into the spinal cord caused an increase in postganglionic sympathetic nerve activity which was blocked by prior application of saralasin. Angiotensin III was without effect. Intracellular recordings from sympathetic preganglionic neurones in-vitro in slices of neonate rat spinal cord showed that angiotensin II produced an increase of excitability of the neurones by a slow depolarisation without the generation of action potentials. This effect still occurred in the presence of TTX. Angiotensin II also could increase synaptic activity, both EPSPs and IPSPs as well as a synaptically induced slow depolarisation being observed suggesting that presympathetic interneurones are also sensitive to the peptide. The evidence indicates that if angiotensin is released from nerve terminals surrounding sympathetic neurones it will enhance the gain of the neurone so that it could more easily be discharged by other excitatory inputs.

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

Angiotensin II modulation of glutamate excitation of locus coeruleus neurons

The effects of iontophoretically applied angiotensin II (AII) have been tested on intracellularly recorded locus coeruleus neurons in an in vitro brain slice preparation. In most neurons, AII strongly depressed the depolarizing effect of L-glutamate in the absence of other effects on membrane properties. This action was specific, in that AII had no effect on depolarizations caused by current injection or application of acetylcholine, and it was blockable by the AII antagonist saralasin. These results appear to demonstrate a potent and previously unreported neuromodulatory action of AII in the central nervous system.

Lesions of the MPO or AV3V: influences on fluid intake

Electrolytic lesions in the MPO of rats had no significant effects on ad lib food and water intake, but impaired the drinking response to 1 M NaCl. Large MPO lesions also produced a persistent increase in plasma osmolality. In Experiment 2, we depleted neurons from the MPO of rats by iontophoretic application of the neurotoxin kainic acid (KA) which destroys nerve cell bodies without damage to fibers of passage. KA-induced neuron depletion in the MPO of rats significantly reduced the drinking response to 1.0 M saline, to 30% PG, and to 30 micrograms/kg isoproterenol. Ad lib water intake and drinking responses to food or water deprivation, to low concentrations (0.5 M) of hypertonic saline, to low concentrations (10% or 20%) of PG, and to systemic administration of 1.5 mg/kg angiotensin II were within the normal range. In Experiment 3, rats with electrolytic lesions that were strictly confined to the tissue immediately surrounding the wall of the anteroventral portion of the third ventricle (AV3V), without invading the MPO displayed normal ad lib food and water intake and plasma osmolality as well as drinking responses to water deprivation, hypertonic saline (0.5 or 1.0 M), angiotensin II (1.5 mg/kg) and isoproterenol (30 micrograms/kg).

Adipsia-polydipsia induced by simultaneous lesion of the medioventral septum and anteroventral third ventricle area in the rat

The medioventral septal area (MVS) and also the tissue surrounding the periventricular preoptic-hypothalamic region (AV3V) of male rats, were destroyed by mean of electrolytic lesions. Before and after the lesions, daily water and food intakes, diuresis, body weight, urine osmolarity, and sodium and potassium excretion were determined. Rats with simultaneous AV3V-MVS lesions showed a biphasic pattern of drinking behavior characterized by a first period of adipsia followed by another period of polydipsia. During the first period of adipsia and except for the first two days, postlesion rats were able to reduce total urine volume but failed to produce an appropriate concentrated urine. During the polydipsia period, on the contrary, rats increased urine output and decreased urine osmolarity in a parallel fashion. Immediately after the lesion, food intake was decreased but recovered to pre-lesion levels gradually. By contrast, body weight was decreased during the entire period of the experiment. Sodium but not potassium excretion showed a significant increase from the 9th to the 20th day postlesion. The results suggest that the AV3V and MVS are part of a circuitry subserving the control of water intake and electrolyte balance.

Actions of angiotensin on area postrema of the rat

Previous studies have reported that rats drink more saline after area postrema has been removed. The results presented here indicate that prolonged administration of angiotensin II into area postrema of unrestrained rats at 4 pmol/h also caused them to drink more saline. They drank more when angiotensin was released in the anterolateral part of the organ than when it was released anteromedially. Diurnal variation of drinking was not disordered. Dose-response curves showed that rats lacking area postrema drank more saline in response to systemic angiotensin than sham operated animals. The very large 'spontaneous' consumption of saline by rats lacking area postrema was not diminished by saralasin, an angiotensin antagonist. It is concluded that area postrema is a site where systemic angiotensin can act to promote sodium consumption: and that although removing area postrema increases the sensitivity of the drinking response to systemic angiotensin, this enhanced sensitivity is not the cause of the increased sodium consumption.

Distribution of [125I]angiotensin II binding sites in the rat brain: a quantitative autoradiographic study

Angiotensin II receptors have been localized by quantitative autoradiography in the rat central nervous system after labeling with [125I]angiotensin II. A highly discrete distribution of these receptors was found throughout the rat brain. The highest density was seen in regions of the medulla, hypothalamus and circumventricular organs where angiotensin II could potentially produce cardiovascular, dipsogenic and neuroendocrine responses. The distribution of angiotensin II receptors correlates relatively well with the previously reported distribution of angiotensin immunoreactive nerve terminals as well as areas determined by various physiological techniques to be sensitive to angiotensin II. Finally, the anatomical localization of angiotensin II receptor populations has revealed several areas of the brain where the effects of this peptide have not been investigated. Many of these nuclei are involved in the transmission and processing of somatic and visceral sensory information. These results suggest a broader role for the central renin-angiotensin system in modulating several types of sensory input.

Median preoptic neurones are sensitive to blood pressure changes induced by peripheral angiotensin II

The activity of 50 histologically identified neurones of the median preoptic area (MPO) was recorded simultaneously with blood pressure monitoring. The responsiveness of these neurones was investigated with increasing doses (25-100 ng/ml/kg) of intravenously applied angiotensin II (ANG II). Mean blood pressure rose in a dose-dependent manner (between 3.9 +/- 0.5 and 18.6 +/- 2.6 mm Hg) whereas the rise in firing frequency of MPO neurones (between 47 and 105% of spontaneous discharge) was not dose-related. The latency of neuronal response decreased according to the dose applied: at the lowest dose of angiotensin it was longer (105 +/- 20 s) than at the highest dose (26 +/- 8 s). These findings suggest that MPO neurones are sensitive to ANG II-haemodynamic changes.

Thirst and vasopressin secretion following central administration of angiotensin II in rats with lesions of the septal area and subfornical organ

Various dipsogenic stimuli, including peripheral and central administration of angiotensin II, have been shown to be capable of releasing vasopressin from the neurohypophyseal system. Studies were carried out in the rat to investigate whether the septal area, which contains a high concentration of angiotensin-sensitive cells and has neural connections with hypothalamic vasopressin-secreting neurons, mediated the stimulatory effect produced by angiotensin II on vasopressin release. Rats with electrolytic lesions in the region of the septal area had increased daily water consumption and urine output when these lesions included the medioventral or lateral nuclei of the septal forebrain, but not when the lesion involved the subfornical organ. No difference was observed in drinking responses following water deprivation or intracerebroventricular injection of angiotensin II in all experimental groups. In addition, the impaired ability to maintain water homeostasis (polyuro-polydipsic syndrome) of septal-lesioned rats was associated septal-lesioned rats was associated with decreased levels of circulating radioimmunoassayable vasopressin. Furthermore, the vasopressin release which occurred in response to intracerebroventricular angiotensin II in normal controls, sham-lesioned and subfornical organ-lesioned rats was significantly attenuated in rats with electrolytic lesion of the medioventral or lateral septal area. Since cells in the lateral septal area are excited by iontophoretic application of angiotensin II, the present data might be consistent with the hypothesis that the stimulatory effect produced by central administration of angiotensin II on vasopressin release rests upon the integrity of the lateral septal area.

Alterations of local cerebral glucose utilization during chronic dehydration in rats

The quantitative autoradiographic deoxyglucose method was used to study changes in local cerebral glucose utilization in conscious dehydrated rats. Animals were either given saline to drink or were deprived of water for 5 days. Saline ingestion did not alter the rates of glucose metabolism in any brain region when compared to the rates of glucose metabolism in animals which had free access to water. Glucose utilization was increased by 140%, however, in the pituitary neural lobe. Water deprivation produced both increases and decreases in glucose metabolism, depending on the particular structure. In 20 of 44 brain structures analyzed, there were significant decreases from -18 to -34% in glucose utilization. Four forebrain structures, the subfornical organ, septal triangular nucleus, and hypothalamic paraventricular and supraoptic nuclei, had increases in glucose utilization of 30-73%. The rate of glucose utilization in the pituitary neural lobe was increased by 367% in water-deprived rats. The results demonstrate that metabolic activity is stimulated in some, but not all, of the structures participating in fluid regulation during an intense thirst challenge. Many brain regions have depressed metabolism in chronic severe dehydration.

Selective metabolic stimulation of the subfornical organ and pituitary neural lobe by peripheral angiotensin II

The subfornical organ is a major receptor area for one of the principal stimuli of thirst, the octapeptide, angiotensin II. In conscious water-sated rats, we examined the effects of intravenous infusion of angiotensin II on the rate of glucose utilization in the subfornical organ and in structures anatomically and functionally connected with it. Angiotensin II produced pressor and drinking responses and increased glucose utilization selectively in the subfornical organ and pituitary neural lobe and in no other brain structure. Treatment with the angiotensin II antagonist, sar1-leu8-angiotensin II, before intravenous administration of angiotensin II prevented metabolic stimulation of the subfornical organ and neural lobe. Captopril, an inhibitor of angiotensin-converting enzyme, was administered to homozygous Brattleboro rats, which normally have elevated rates of glucose utilization in the subfornical organ. Captopril reduced subfornical organ glucose metabolism to a level similar to that found in control animals. These results demonstrate that peripheral angiotensin II stimulates glucose metabolism in the subfornical organ under conditions in which it provokes drinking and pressor responses. The findings suggest that circulating angiotensin II is responsible for the high rate of glucose utilization observed in the subfornical organ of Brattleboro rats homozygous for diabetes insipidus.

Vasopressin release to central and peripheral angiotensin II in rats with lesions of the subfornical organ

Angiotensin II (Ang II), peripherally or centrally administered, increases plasma vasopressin concentrations in the rat. Peripherally injected Ang II was unable to effect the release of vasopressin in rats with subfornical organ (SFO) lesions. In contrast, a normal increase of plasma vasopressin levels was induced by centrally injected Ang II. These results suggest that peripherally administered Ang II elicits antidiuretic hormone (ADH) release by stimulating receptors in the SFO, whereas centrally administered Ang II acts at receptors outside the SFO.

Angiotensin II immunoreactivity in the neural afferents and efferents of the subfornical organ of the rat

Angiotensin II (AII) immunoreactive cells and fibers were identified in the subfornical organ (SFO) of the rat. Cells were distributed in an annulus around the periphery of the SFO and were most visible in the Brattleboro rat treated with colchicine. Fibers were observed in a plexus, located centrally within the ring of cells, and knife-cuts suggested that they arise from parent cell bodies lying outside of the SFO. Studies combining immunohistochemistry with retrograde transport identified the perifornical zone of the lateral hypothalamus, the rostral zona incerta, and the nucleus reuniens of the thalamus as the source of AII-stained inputs to the SFO, and the region of the median preoptic nucleus as a recipient of AII-immunoreactive SFO efferents. It is suggested that these biochemically defined connections of the SFO participate in the central neural control of fluid balance.

Electrophysiological effects of angiotensin II on cultured mouse spinal cord neurons

The effects of angiotensin II (AII) on the membrane properties of cultured spinal neurons were investigated using electrophysiological methods. In 26% of neurons tested AII induced changes in membrane potential and input resistance which varied according to the concentration of applied peptide. At low concentrations (10(-6) M), AII increased input resistance by an ionic mechanism which appears to involve a reduction in Cl- conductance. At higher concentration (10(-4) M), AII evoked depolarization associated with a decrease in input resistance. This response appears to depend on an increase in Na+ conductance. Our observations indicate that AII can have multiple effects on neuronal membrane properties dependent on the concentration of applied peptide.

Localization of angiotensin II binding sites in rat septum by autoradiography

The distribution of septal [125I]angiotensin II (Ang II) binding sites localized by light microscopic autoradiography was compared with the distribution of septal immunoreactive Ang II-containing nerve terminals localized by immunohistochemistry. Within the septum, both the [125I]Ang II binding sites and Ang II-like immunoreactive terminals were restricted to the lateral septal nucleus. The close parallel between the distribution of [125I]Ang II binding sites and immunoreactive Ang II nerve terminals in the lateral septum suggests that central Ang II receptors may be functionally linked to neuronally released Ang II.

Neuroendocrine mechanisms mediating fluid intake during the estrous cycle

Gonadal steroids appear to influence fluid-electrolyte homeostasis through behavioral as well as renal mechanisms. The marked fluctuations in drinking behavior observed during the estrous cycle of the female rat may be due to an interaction between estrogen and the dipsogenic peptide hormone, angiotensin II, at the level of basal forebrain receptors. The preoptic region in particular may play an important integrative role in the maintenance of extracellular fluid balance in synchrony with the estrous cycle, since it contains receptors for angiotensin and estrogen. Prolactin may also directly participate in mechanisms of extracellular thirst, while an exact role for vasopressin has yet to be established. Recent studies also suggest that estrogens may influence body fluid regulation by interacting with several neurotransmitters, including serotonin, dopamine and noradrenaline.

Binding of angiotensins to rat brain tissue: structure activity relationship

The binding of 3H-angiotensin II to a synaptosome-enriched fraction of the subcortical part of rat brain was studied. In this fraction specific high-affinity binding sites for angiotensin II were demonstrated. The binding sites were saturated at a ligand concentration of 2 X 10(-9) M. Scatchard analysis revealed a single class of binding sites with an apparent maximal binding capacity of 14 fmoles/mg of protein and an equilibrium dissociation constant, KD, of 0.9 X 10(-9) M. The specific binding at the KD concentration amounted to 59% of the total binding and was reversible. The association and dissociation rate constants (k1 and k-1) were 0.0212 nM-1 min-1 and 0.0196 min-1, respectively. Binding was dependent on both incubation time and tissue concentration in the incubation mixture. Angiotensins with biological activity in the brain, i.e., angiotensins I, II, III, and the fragments (3-8) and (4-8) competed with 3H-angiotensin II for the binding sites with IC50's of 9 X 10(-8) M, 2 X 10(-9) M, 4 X 10(-9) M, 4 X 10(-7) M and 4 X 10(-6), respectively. In the presence of 1 mM of the converting enzyme inhibitor SQ 14,225 the IC50 for angiotensin I was 2 X 10(-7) M. Competition by the biologically active fragment angiotensin (5-8) could not be demonstrated. The latter peptide, however, was highly metabolized during the incubation under the assay conditions used. The binding potency of the various angiotensins paralleled their dipsogenic and pressor potency. The present data indicate the possible physiological involvement of these binding sites as specific receptors in the actions of angiotensins in the brain.

Chronic infusion of angiotensin II into the olfactory bulb elicits an increase in water intake

Chronic infusion of angiotensin II (ANG II) into the olfactory bulb (OB) elicited a moderate dipsogenesis which occurred only during the dark phase, essentially doubling the water-to-food intake ratio. Removal of the food from the ANG II-OB group reduced water consumption to the level of the saline-infused/food-deprived controls. These experiments suggest that ANG II may interact with the OB to alter the normal water/food relationship.

Angiotensin-induced drinking and the effect of bilateral nephrectomy

The induction of drinking by the intraventricular administration of angiotensin II has been attributed to a specific interaction with receptors in the medial preoptic-anterior hypothalamic border. These studies have been pharmacological in nature, involving exogenous peptide. In experiments reported elsewhere the present authors have shown that nephrectomy resulted in a regionally selective increase in levels of brain angiotensinogen, the putative prohormone of any CNS angiotensin peptides. Nephrectomy also markedly perturbs fluid and ion homeostasis. A study was therefore made of the effect of bilateral nephrectomy on spontaneous and angiotensin-induced drinking behavior. A deficit in spontaneous drinking following nephrectomy was observed, but it was noted that this related directly to diminished food intake. Angiotensin II-induced drinking was markedly potentiated following nephrectomy, indicative of possible up-regulation of angiotensin receptors in selected regions of the rat brain. The results are consistent with a physiological role for angiotensin in regulating fluid intake and volume homeostasis.

Angiotensin-converting enzyme blockade by Captopril changes angiotensin II receptors and angiotensinogen concentrations in the brain of SHR-sp and WKY rats

Angiotensin II-sensitive neurons in the brain of spontaneously hypertensive rats (SHR-sp) and of Wistar Kyoto rats (WKY) treated with the angiotensin-converting enzyme inhibitor Captopril were investigated for possible differences at receptor sites. Furthermore, the concentrations of angiotensinogen and renin were measured in different brain regions of these animals by biochemical assay. The higher receptor sensitivity of septal neurons to angiotensin II which existed in SHR-sp as compared to WKY was diminished by Captopril. Angiotensinogen concentrations were lower in the anterior hypothalamus but not in the septum of SHR-sp as compared to WKY. Captopril increased the level in both strains. Renin concentrations did not differ in SHR-sp and WKY. Chronic treatment with Captopril induced an increase of about 20% in septum and hypothalamic regions of SHR-sp and WKY rats. Whether these changes are causally linked to the hypertension in SHR-sp remains to be investigated.

Increased sensitivity of neurons to angiotensin II in SHR as compared to WKY rats

Angiotensin II (ANG II)-sensitive septal neurons in the brain of stroke-prone spontaneously hypertensive rats (SHR-sp) and of normotensive Wistar-Kyoto rats (WKY) were investigated for possible differences at receptor sites. ANG II, the competitive ANG II-antagonist saralasin, and acetylcholine (ACh), were applied microiontophoretically onto neurons of the lateral septal area. ANG II-evoked neuronal firing which was specifically inhibited by saralasin occurred at a significant lower threshold in SHR-sp (23%) and showed an extended postactivity (340%) as compared to the age-matched WKY controls. In contrast, the activity due to ACh remained similar in both strains.

Angiotensin-converting enzyme in discrete areas of the rat forebrain and pituitary gland

With the use of a sensitive radioisotopic method we have examined the activity of the angiotensin-converting enzyme (ACE, E.C. 3.4.15.1) in specific nuclei of the rat forebrain and in the anterior, intermediate and posterior lobes of the pituitary gland of the rat. We reported that ACE activity is heterogeneously distributed in the rat forebrain, with a 200-fold difference between the lowest and the highest values. Highest enzyme activities were found in the subfornical organ and in the posterior lobe of the pituitary gland. High ACE activity was also detected in the intermediate and anterior lobes of the pituitary gland, the caudate nucleus, and the medial habenular nucleus. Substantial activity also existed in the globus pallidus, the median eminence, the supraoptic and paraventricular nuclei, the lateral habenular nucleus and the organon vasculosum laminae terminalis. Our results demonstrate that one of the components of the renin-angiotensin system, the angiotensin-converting enzyme, is highly localized to a few discrete brain structures and the pituitary gland. These findings suggest that angiotensin II could be formed locally in some of these structures, supporting previous immunohistochemical data.

On the separation of functions mediated by the AV3V region

Knife-cuts were used to separate the disruptive effects on fluid balance that are produced by electrolytic lesions of the anteroventral third ventricle (AV3V) region. It was observed that vertical cuts of the dorsal stalk of the subfornical organ (SFO) produced none of these effects. Horizontal cuts between the SFO and the anterior commissure produced neither of the acute effects of AV3V lesions (adipsia and diuretic weight loss) but they did mimic AV3V lesions in disrupting drinking responses to peripherally injected angiotensin and hypertonic saline. In contrast, horizontal cuts between the anterior commissure and the organum vasculosum of the lamina terminalis (OVLT) did not reduce drinking responses to angiotensin but they did cause a large weight loss during the 24 hours following surgery. It is suggested that these ventral cuts severed neural connections between the medial septum and the ventral medial preoptic area in producing the large weight loss. Together with findings from other experiments, these findings support the hypothesis that distinct neural elements mediate the various functions that are disrupted by lesions of the AV3V region.

Brain renin

Although the brain contains cathepsins at high concentrations which exhibit a non-specific renin-like activity at acidic pH, the presence of specific renin in the brain has been demonstrated by characterizing its specific properties. Renin was separated from cathepsin by affinity chromatography on casein-Sepharose. Brain renin showed neutral pH optima for the reaction to generate angiotensin I. The presence of inactive prorenin was also found. The isoelectric points of brain renin were significantly lower differences from that of renal or plasma renin. Immunohistochemical studies demonstrated a wide-spread localization of renin in many different regions. Angiotensin II, the final product of the prohormone-to-hormone conversion reaction mediated by renin and angiotensin converting enzyme, was found to exist in the same cell as renin by immunohistochemical studies of brain sections and with cloned and cultured neuroblastoma cells. This is the first demonstration of the mechanism of peptide hormone formation in neuronal cells. Similar intracellular formation was demonstrated in gonadotrophs of adenohypophysis. Coexistence of renin and angiotensin II was demonstrated in some cells. Electrophysiological studies have shown that angiotensin II functions to disinhibit the inhibition of neuronal response to electrical stimuli in the hippocampus.