The distribution of angiotensin II binding sites in rodent brain

Neuroactive substances in the dorsal vagal complex of the medulla oblongata: nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus

The distributions of classical and putative neurotransmitters within somata and fibres of the dorsal vagal complex are reviewed. The occurrence within the dorsal medulla oblongata of receptors specific for some of these substances is examined, and possible functional correlations of the specific neurochemicals with respect to their distribution within the dorsal vagal complex are discussed. Many of the known transmitters and putative transmitters are represented in the dorsal vagal complex, particularly within various subnuclei of the nucleus of the solitary tract, the main vagal afferent nucleus. In a few cases, some of these have been examined in detail, particularly with respect to their possible mediation of cardiovascular or gastrointestinal functions. For example, the catecholamines, substance P and angiotensin II in the nucleus of the solitary tract have all been strongly implicated as playing a role in the central control of cardiovascular function. Other neurotransmitters or putative transmitters may be involved as well, but probably to a lesser extent. Similarly, the roles in the dorsal vagal complex of dopamine, the endorphins and cholecystokinin in control of the gut have been studied in some detail. Future investigations of the distributions of and electrophysiological parameters of neurotransmitters at the cellular level should provide much needed clues to advance our knowledge of the correlations between anatomical distributions of specific neurochemicals and physiological functions mediated by them.

Brain AT1 angiotensin receptor subtype binding: importance of peptidase inhibition for identification of angiotensin II as its endogenous ligand

The existence and localization of brain angiotensin receptors is well established. However, questions regarding the endogenous ligand for brain angiotensin type 1 (AT(1)) receptors necessitates re-examination of brain angiotensin receptor binding studies. To assess the ability of angiotensin II to bind to the brain AT(1) receptor, radioligand binding studies of rat brain AT(1) receptors were performed using both (125)I-angiotensin II and (125)I-sarcosine(1), isoleucine(8) angiotensin II. Determination of binding kinetics and competition by an AT(1) receptor antagonist was carried out to reveal the identity of the membrane binding sites and to identify the bound (125)I-labeled molecules. Initial analysis of (125)I-angiotensin II binding to hypothalamic membranes using an established protocol revealed that a negligible amount of intact radioligand was bound to the membranes. In contrast, binding of (125)I-sarcosine(1), isoleucine(8) angiotensin II was saturable, of high affinity, and primarily as intact radioligand. Sequential addition of four peptidase inhibitors-o-phenanthroline, puromycin, phenymethylsulfonyl fluoride, and glutamate phosphonate-to the assay buffer dramatically increased the binding of (125)I-angiotensin II to rat brain membranes: more than 75% of the bound (125)I was the intact radioligand, and the binding was of high affinity and saturable. Some, but not all, of the binding could be displaced by the AT(1)-selective antagonist losartan. This demonstrates that (125)I-angiotensin II can bind to brain AT(1) receptors and does not require conversion to (125)I-angiotensin III to bind to brain AT(1) receptors.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

Neural plasticity and the brain renin-angiotensin system

The brain renin-angiotensin system mediates several classic physiologies including body water balance, maintenance of blood pressure, cyclicity of reproductive hormones and sexual behaviors, and regulation of pituitary gland hormones. In addition, angiotensin peptides have been implicated in neural plasticity and memory. The present review initially describes the extracellular matrix (ECM) and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases, and tissue inhibitors of metalloproteinases in the maintenance and degradation of the ECM. It is the ECM that appears to permit synaptic remodeling and thus is critical to the plasticity that is presumed to underlie mechanisms of memory consolidation and retrieval. The interrelationship among long-term potentiation (LTP), CAMs, and synaptic strengthening is described, followed by the influence of angiotensins on LTP. There is strong support for an inhibitory influence by angiotensin II (AngII) and a facilitory role by angiotensin IV (AngIV), on LTP. Next, the influences of AngII and IV on associative and spatial memories are summarized. Finally, the impact of sleep deprivation on matrix metalloproteinases and memory function is described. Recent findings indicate that sleep deprivation-induced memory impairment is accompanied by a lack of appropriate changes in matrix metalloproteinases within the hippocampus and neocortex as compared with non-sleep deprived animals. These findings generally support an important contribution by angiotensin peptides to neural plasticity and memory consolidation.

Role of angiotensin II receptors in the regulation of vasomotor neurons in the ventrolateral medulla

1. There is a high density of angiotensin type 1 (AT1) receptors in various brain regions involved in cardiovascular regulation. The present review will focus on the role of AT1 receptors in regulating the activity of sympathetic premotor neurons in the rostral part of the ventrolateral medulla (VLM), which are known to play a pivotal role in the tonic and phasic regulation of sympathetic vasomotor activity and arterial pressure. 2. Microinjection of angiotensin (Ang) II into the rostral VLM (RVLM) results in an increase in arterial pressure and sympathetic vasomotor activity. These effects are blocked by prior application of losartan, a selective AT1 receptor antagonist, indicating that they are mediated by AT1 receptors. However, microinjection of AngII into the RVLM has no detectable effect on respiratory activity, indicating that AT1 receptors are selectively or even exclusively associated with vasomotor neurons in this region. 3. Under normal conditions in anaesthetized animals, AT1 receptors do not appear to contribute significantly to the generation of resting tonic activity in RVLM sympathoexcitatory neurons. However, recent studies suggest that they contribute significantly to the tonic activity of these neurons under certain conditions, such as salt deprivation or heart failure, or in spontaneously hypertensive or genetically modified rats in which the endogenous levels of AngII are increased or in which AT1 receptors are upregulated. 4. Recent evidence also indicates that AT1 receptors play an important role in mediating phasic excitatory inputs to RVLM sympathoexcitatory neurons in response to activation of some neurons within the hypothalamic paraventricular nucleus. The physiological conditions that lead to activation of these AT1 receptor-mediated inputs are unknown. Further studies are also required to determine the cellular mechanisms of action of AngII in the RVLM and its interactions with other neurotransmitters in that region.

Angiotensin IV enhances LTP in rat dentate gyrus in vivo

Angiotensins have been shown to play a significant role in a variety of physiological functions including learning and memory processes. Relatively recent evidence supports the increasing importance of angiotensin IV (Ang IV), in many of these functions previously associated only with Ang II, including learning and memory. An interesting hypothesis generated by these results has been that Ang II is a precursor for the production of a more active peptide fragment, Ang IV. Since Ang II impairs learning and memory, when administered directly or released into the hippocampal dentate gyrus, and inhibits long term potentiation (LTP) in medial perforant path-dentate granule cell synapses, as well; it remained to be seen what effects Ang IV had on LTP in these same synapses. Results of this study show clearly that Ang IV significantly enhances LTP, and the enhancement is both dose and time dependent. The following solutions of Ang IV were administered over a five min period, at the end of baseline and before the first tetanus was applied: 2.39, 4.78, and 9.56 nM. An inverted U-type dose related effect was observed. A complex time related effect was observed with a maximum at 5 min, a return to normal LTP at 30 min and a minimum below normal at 90 min, and a return to normal LTP at 120 min. The effects of the 4.78 nM solution were determined at the following intervals between administration and the first tetanus: 5, 15, 30, 60, 90, and 120 min. The enhancement of LTP can be prevented by pretreatment with Divalinal, an Ang IV antagonist, without any effect on normal LTP. Two solutions of Divalinal were used; 5 nM and 5 microM, and the 5 microM was more effective and completely blocked the enhancement of normal LTP. Results were also obtained with 4.78 nM Nle1-Ang IV (Norleucine), an Ang IV agonist. Norleucine was less effective than Ang IV in the enhancement of normal LTP and displayed a similar time course of activity. Both Ang IV and Norleucine produced a significant suppression of normal LTP at 90 min; that remains to be explained. However, the inhibition by Ang IV was dose dependent and was blocked by Divalinal. The fact that the Ang IV enhancement of normal LTP was blocked by losartan, an Ang II AT1 receptor antagonist, is puzzling since Divalinal had no effect on the inhibition of LTP by Ang II.

Pressor and renal effects of intracerebroventricularly administered angiotensins II and III in rats

AIMS

Experiments were performed to assess the effects of intracerebroventricular (ICV) angiotensin (ANG) III on blood pressure and renal function in rats with normal and high sodium intake and to compare these effects with those produced by ICV ANG II.

METHODS

Male Sprague-Dawley rats on a normal sodium (0.3%) diet and a normal sodium diet plus 1% NaCl as drinking water were administered ANG II and ANG III ICV through a chronically implanted cannula. Blood pressure and renal clearance function responses were measured before and during peptide administrations. The effect of ICV ANG III on the renal efferent nerve activity was also evaluated.

RESULTS

ICV injections of ANG II and ANG III at 5 pmol in rats on a normal sodium diet did not significantly alter the blood pressure, but significantly increased renal plasma flow, glomerular filtration rate, urine flow, and absolute and fractional excretions of sodium and potassium. Increased doses of ANG II and III (10, 50 and 100 pmol) significantly increased blood pressure and further enhanced these renal functional indices. Central ANG-III-induced increases in blood pressure and renal functional indices were not significantly different from those produced by ANG II at each corresponding dose. The pressor and renal effects of ANG III were blunted by a specific antagonist, Ile(7)-ANG III. ICV administration of ANG III decreased the renal efferent nerve activity. In rats with dietary NaCl loading, ICV injections of ANG II and III also significantly enhanced renal function.

CONCLUSIONS

Centrally administered ANG III is as potent as ANG II in causing pressor and renal effects in rats on normal and high sodium intake. As ANG II, brain ANG III reduced renal efferent nerve activity which may be partly accounted for the augmented renal function.

Characterization and distribution of angiotensin II receptor subtypes in the mouse brain

We localized and characterized angiotensin II receptor subtypes (AT1 and AT2) in the mouse brain, with the use of autoradiography after incubation with [l25I][Sar1]-angiotensin II or [125I]CGP 42112 and displacement with selective angiotensin AT1 (losartan and candesartan) or angiotensin AT2 (CGP 42112(1) and PD 123319(2)) receptor ligands. In the mouse, the receptor subtype affinity for the different ligands was similar to that of the rat. The receptor subtype distribution was also similar to that in the rat, with some notable exceptions, such as the presence of angiotensin AT1 but not AT2 receptors in the locus coeruleus, and the expression of angiotensin AT1 receptors in the caudate putamen. These results confirm that careful consideration of the specific distribution of receptor subtypes in different species, even those closely related such as the mouse and the rat, should be conducted before meaningful comparisons could be proposed. Our data also form the basis for future studies of mouse models such as those with angiotensin receptor gene deficiencies.

Effects of angiotensin II and IV on geniculate activity in nontransgenic and transgenic rats

Microiontophoretic ejection of angiotensin II and angiotensin IV in the vicinity of geniculate neurons was used to study the effects of these peptides on the discharge rate and the discharge pattern of extracellularly recorded activity. The main aim of the experiments was to study the effects of angiotensins in different strains of rats anesthetized with urethane (normotensive Wistar, normotensive Sprague-Dawley and hypertensive, transgenic (TGR(mREN2)27) rats). Both angiotensins mostly increased the spontaneous activity of angiotensin-sensitive geniculate neurons in all strains. Angiotensin II reduced the number of bursts in most neurons, whereas angiotensin IV significantly enhanced it. Inhibitory effects of angiotensins on spontaneous as well as on light-evoked activity could be effectively blocked by GABA(A) or GABA(B) receptor antagonists. Therefore, it can be supposed that angiotensin-containing afferent fibers innervate both projection and local circuit neurons of the dorsal lateral geniculate nucleus. In addition, angiotensin II suppressed excitation induced by glutamate receptor agonists in most neurons tested. Angiotensin-induced effects could be blocked by specific receptor antagonists. There were no significant differences in the effects of angiotensins in the various strains of rats, except for the latencies of the neuronal responses to the iontophoretic ejection of angiotensins.

Localization of angiotensin-converting enzyme, angiotensin II, angiotensin II receptor subtypes, and vasopressin in the mouse hypothalamus

The hypothalamic angiotensin II (Ang II) system plays an important role in pituitary hormone release. Little is known about this system in the mouse brain. We studied the distribution of angiotensin-converting-enzyme (ACE), Ang II, Ang II receptor subtypes, and vasopressin in the hypothalamus of adult male mice. Autoradiography of binding of the ACE inhibitor [125I]351A revealed low levels of ACE throughout the hypothalamus. Ang II- and vasopressin-immunoreactive neurons and fibers were detected in the paraventricular, accessory magnocellulary, and supraoptic nuclei, in the retrochiasmatic part of the supraoptic nucleus and in the median eminence. Autoradiography of Ang II receptors was performed using [125I]Sar1-Ang II binding. Ang II receptors were present in the paraventricular, suprachiasmatic, arcuate and dorsomedial nuclei, and in the median eminence. In all areas [125I]Sar1-Ang II binding was displaced by the AT1 receptor antagonist losartan, indicating the presence of AT1 receptors. In the paraventricular nucleus [125I]Sar1-Ang II binding was displaced by Ang II (Ki = 7.6 X 10(-9)) and losartan (Ki = 1.4 X 10(-7)) but also by the AT2 receptor ligand PD 123319 (Ki = 5.0 X 10(-7)). In addition, a low amount of AT2 receptor binding was detected in the paraventricular nucleus using [125I]CGP42112 as radioligand, and the binding was displaced by Ang II (Ki = 2.4 X 10(-9)), CGP42112 (Ki = 7.9 x 10(-10)), and PD123319 (Ki = 2.2 x 10(-7)). ACE, Ang II, and AT1 as well as AT2 receptor subtypes are present in the mouse hypothalamus. Our data are the basis for further studies on the mouse brain Ang II system.

Upregulation of angiotensin II AT1 receptors in the mouse nucleus accumbens by chronic haloperidol treatment

The distribution of angiotensin II AT1 and AT2 receptor subtypes were mapped in the mouse brain by in vitro autoradiography. Along with a differing distribution of AT1 and AT2 receptors in the hind brain compared to the rat, moderate densities of AT1 receptors were observed in dopamine-rich regions, namely the caudate putamen and nucleus accumbens, previously observed in the human, but not rat or rabbit. Considering our previous anatomical and functional studies demonstrating an interaction between brain angiotensin II and dopaminergic systems, the effect of chronic treatment with the dopamine antagonist, haloperidol, on AT1 and AT2 receptor levels was investigated in the mouse brain. Haloperidol treatment for 21 days resulted in an increase in angiotensin II AT1 receptor levels in the nucleus accumbens, accompanied by an increase in dopamine D2 receptors, but no change in dopamine D1 receptors. Striatal AT1 receptors did not alter with treatment, nor did AT1 or AT2 receptors in a number of brain regions not associated with dopaminergic systems, such as the median preoptic nucleus, paraventricular hypothalamic nucleus, and nucleus of the solitary tract. The present study suggests that brain angiotensin II-dopamine interactions extend beyond the known effects on the nigrostriatal dopaminergic system, to the mesocorticolimbic dopaminergic system.

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 content within the subfornical organ and organum vasculosum lamina terminalis increases after experimental subarachnoid haemorrhage in rats

Nests of cells within the central nervous system, namely the circumventricular organs (CVOs) which include the subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT), area postrema (AP) and the median eminence (ME) are known to contain not only receptors for angiotensin II (ANG II) but also ANG II itself. Though the significance of this central ANG II network in the pathophysiology of certain conditions like hypertension is well established, there appears to be a lack of knowledge as to how this system might be involved after subarachnoid haemorrhage (SAH). In this study, we have investigated ANG II receptor content change at various circumventricular organs after experimental subarachnoid haemorrhage in rats using a transcervical transclival model. ANG II receptor content was detected by in vivo autoradiography using intracisternal ANG II Sar 1, Ile 8 labelled with iodine (I) 125 both at 30 minutes and 48 hours after the SAH. Serum angiotensin converting enzyme activity was also detected during the time course reflecting the involvement of the peripheral angiotensin system and showed an early rise and a fall after two days. Immunohistochemistry was utilized to show the ANG II-containing cells within the circumventricular organs. SFO and OVLT were found to have a statistically significant increase in ANG II receptor content persisting over two days after the SAH. These alterations in the receptor content of CVOs may indicate their possible role in delayed ischaemic deficits seen after SAH.

Expression of a novel angiotensin II receptor subtype in gerbil brain

Angiotensin II receptors are highly localized in adult gerbil brain. Apparent receptor number is high in subfornical organ, vascular organ of the lamina terminalis, nucleus of the solitary tract, hippocampus, and in the anterior pituitary gland. In the hippocampus, binding is localized to the stratum oriens, radiatum, the lacunar molecular layers of the CA1 subfield, and the molecular layer of the gyrus dentatus, with a medial to lateral and anterior to posterior gradient in receptor expression. Binding is absent from the pyramidal layer of the CA1 subfield and from the granular cell layer of the gyrus dentatus, areas rich in angiotensin IV binding. Characterization in the hippocampus revealed the presence of a high affinity receptor, sensitive to incubation with the guanine nucleotide GTP gamma S, and displaced by angiotensin II = angiotensin III < Sar1-Ile8-angiotensin II, but not by angiotensin IV or other angiotensin fragments, the AT1 receptor antagonist losartan, or the AT2 ligands CGP 42112 or PD 123177. In other brain areas, binding was equally insensitive to displacement by AT1 or AT2 ligands, with the exception of binding in the olfactory bulb, which was totally displaced by CGP 42112 and PD 123177, but not by losartan. In the gerbil, most of the brain and pituitary angiotensin II receptors are different from the AT1, AT2 and AT4 subtypes, and should be considered 'atypical' until further characterization.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

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(3-8) (ANG IV) hippocampal binding: potential role in the facilitation of memory

The present research characterizes a newly discovered ANG II(3-8) (ANG IV) binding site localized in structures associated with memory function (hippocampus, neocortex, cerebellum), as well as other brain stem structures (thalamus, inferior olivary nucleus). This site is not the AT1 or AT2 site that binds angiotensins II (ANG II) and III (ANG III) nor does it bind the nonpeptide AT1 or AT2 receptor antagonists DuP753 and PD123177, respectively. The intracerebroventricular (ICV) infusion of ANG IV was ineffective at inducing drinking in rats as compared with equivalent doses of ANG II and III. Although not as effective as ANG II or ANG III, ICV infusion of ANG IV did provoke a pressor response at the highest dose (100 pmol/min), which appeared to be mediated by ANG II (AT1)-type receptors and not the specific AIV binding site described here. By contrast, the ICV infusion of ANG IV resulted in greater effects upon retention and retrieval of a passive avoidance task as compared with ANG II. Specifically, ANG II was not different from the ICV infusion of artificial cerebrospinal fluid, while ANG IV improved retention and retrieval of this task.

Stereotaxic atlas of the brain of Octodon degus

We present a stereotaxic atlas of the brain of the trumpet-tailed rat or degu (Octodon degus), an hystricomorph rodent native to Chile and one which has become increasingly popular as a research animal, among other things because of its use as a model for diabetic cataracts and its tendency to become hyperglycemic. The atlas contains 38 transverse and two sagittal sections of the brain covering pros-, mes-, and rhombencephalon, as well as diagrams of the brain's surface anatomy. It was constructed from brains of young adult male degus but can be used readily in studies of adult females, since there is no apparent sexual dimorphism in the brain size of this rodent. Ninety percent of 40 experimental lesions used to check the accuracy of the atlas were correctly placed. The fore- and midbrain of the degu are generally more compact than corresponding regions of the brain in the laboratory rat (suborder Myomorpha) and the guinea pig (another hystricomorph). The amygdaloid complex extends further forward in the telencephalon. Major mesencephalic nuclei and fiber tracts are more rostral in position. However, superior and inferior colliculi are much longer in degus than rats. The basic organization of the rhombencephalon is similar in degus and rats, although there are clearcut differences in the length or size of some hindbrain nuclei.

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.

Mobilization of calcium from intracellular store as a possible mechanism underlying the anti-opioid effect of angiotensin II

Angiotensin II (AII), injected intracerebroventricularly, has been shown to antagonize opioid analgesia. The mechanism for this was obscure. In the neuroblastoma X glioma NG 108-15 hybrid cell line, the K(+)-induced increase in [Ca2+]i can be suppressed by the delta opioid agonist [D-Pen2, D-Pen5]enkephalin (DPDPE) at 0.01-1 microM, an effect completely reversed by the opioid antagonist naloxone. Angiotensin II (AII) at concentrations of 0.1 and 1 microM mobilized free Ca2+ from an intracellular pool, and this effect was antagonized by the AII receptor antagonist saralasin. All (1 microM) had no significant effect on the increase in [Ca2+]i induced by K+, but it blocked the suppressive effect of DPDPE on the K(+)-induced [Ca2+]i increase. The results indicate that mobilization of intracellular calcium may underlie the anti-opioid effect of AII.

Renin and angiotensin converting enzyme in elasmobranchs

Renin-like activity (RLA) and angiotensin I converting enzyme-like activity (ACELA), the two key enzymes of the renin-angiotensin system (RAS), were sought in the elasmobranch Scyliorhinus canicula. Renal extracts were desalted in a G-25 and eluted in a G-100 Sephadex column (calibration 15,000-70,000). The fractions were concentrated in a vacuum device. A 48,000-MW fraction incubated with synthetic and porcine angiotensiongen generated angiotensin I estimated by RIA. This same fraction was vasopressor in rats and dogfish. ACELA was sought in gill, heart, liver, spleen, pancreas, intestine, kidney, gonads, brain, skin, and muscle of dogfish using a spectrophotometric assay. The highest level of ACELA was found in the gills followed by spleen, kidney, and brain (33.79 +/- 2.3, 29.56 +/- 1.0, 14.62 +/- 1.0, and 13.80 +/- 2.3 nmol hippurate/min/mg protein, respectively). Intestine, gonads, skin and muscle contained no measurable amounts of ACELA. Captopril inhibited enzymatic activity from all ACELA containing tissues.

Angiotensin II blocks hippocampal long-term potentiation

We have found that injection of angiotensin II (AII) above the hippocampus in the intact rat blocks the induction of long-term potentiation (LTP) in perforant path-stimulated dentate granule cells. A minimum dose of 4.78 pmol AII was required for the complete blockade of LTP and this blockade was entirely prevented if the AII-specific antagonist saralasin was co-injected at a 50-fold molar excess. AII thus appears to act via AII receptors and does not cause non-specific inhibition. The injection of saralasin alone yielded LTP comparable to that obtained when vehicle was injected. Angiotensin III was found to be 40-50 fold less potent than AII in blocking LTP. Both AII and AII receptors of unknown function occur in the hippocampal formation. The results reported here suggest a role for these molecules in the control of hippocampal LTP.

The effect of analogues of angiotensin II on drinking and cardiovascular responses to central angiotensin II in the rat

1. Intracerebroventricular (I.C.V.) infusion (60 ng h-1) of Isoleu5-angiotensin II (Isoleu5--AngII) and des-amine-angiotensin II (des-amine-AngII) in rats caused increased drinking behaviour and an increase in arterial blood pressure. 2. Des-amine-AngII caused similar increases in heart rate and arterial blood pressure as AngII. 3. Previous I.C.V. injection of the antagonists [Leu8]-AngII, des-amine-[Leu8]-AngII and octanoyl-[Leu8]-AngII prevented the increases in heart rate and blood pressure produced by I.C.V. infusion of AngII and caused partial reduction of the dipsogenic response. 4. The three antagonists had no effect on the increase in arterial blood pressure and heart rate caused by des-amine-AngII. The drinking response was reduced by previous injection of [Leu8]-AngII and des-amine-[Leu8]-AngII but not by octanoyl-[Leu8]-AngII. 5. In conclusion, Isoleu5-AngII and des-amine-AngII increase drinking behaviour, arterial blood pressure and heart rate when infused into the cerebral ventricle of rats. The study with the antagonists showed that des-amine-AngII probably binds more strongly to AngII-receptors.

Solubilization and partial characterization of angiotensin II receptors from rat brain

Rat brain angiotensin II (Ang II) receptors were solubilized with a yield of 30-40% using the synthetic detergent 3[(3-cholamidopropyl)dimethylammonio)]-1-propanesulfonate. Kinetic analysis employing the high-affinity antagonist 125I-Sar1,Ile8-Ang II indicated that the solubilized receptors exhibited the same properties as receptors present within intact brain membranes. Furthermore, there was a positive correlation (r = 0.99) between the respective pIC50 values of a series of agonist and antagonists competing for 125I-Sar1,Ile8-Ang II labeled binding sites in either solubilized or intact membranes. Moreover, covalent labeling of 125I-Ang II to solubilized receptors with the homo-bifunctional cross-linker disuccinimidyl suberate, followed by gel filtration, revealed one major and one minor binding peak with apparent molecular weights of 64,000 and 115,000, respectively. Two binding proteins of comparable molecular weights (i.e., 112,000 and 60,000) were also identified by covalent cross-linking of 125I-Ang II to solubilized brain membranes followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. In contrast, only the smaller molecular mass binding protein was observed when solubilized membranes were labeled with the antagonist 125I-Sar1,Ile8-Ang II prior to gel filtration, and chromatofocusing of antagonist labeled sites revealed only one peak with an isoelectric point of 6.2. The successful solubilization of these binding sites should facilitate continued investigation of Ang II receptors in the brain.

Sulfhydryl reducing agents distinguish two subtypes of angiotensin II receptors in the rat brain

Two angiotensin II receptor subtypes were distinguished in the rat brain using in vitro receptor autoradiography based on the differential effects of sulfhydryl reducing agents on 125I-sarcosine1,isoleucine8 angiotensin II binding in various brain nuclei. At several nuclei, e.g. the hypothalamus, circumventricular organs and the dorsal medulla, 125I-sarcosine1,isoleucine8 angiotensin II binding was strongly inhibited by 30 mM beta-mercaptoethanol or 5 mM dithiothreitol, whereas at other nuclei, e.g. the lateral septum, colliculi, locus coeruleus and medial amygdala, sulfhydryl reducing agents had either little effect on radioligand binding or enhanced the binding. The distribution of the sulfhydryl reducing agent inactivated subtype corresponds exactly with the distribution of DuP 753 sensitive (designated as AII alpha) 125I-sarcosine1,isoleucine8 angiotensin II binding sites25. The subtype not inhibited by sulfhydryl reducing agents corresponds with the DuP 753 insensitive (designated as AII beta) sites in the brain25. The sulfhydryl reducing agent effect on brain angiotensin II receptor subtypes is similar to that seen in angiotensin II receptor subtypes in peripheral tissues. These observations indicate that many previous studies of brain angiotensin II receptor binding that included 5 mM dithiothreitol in the assay medium overlooked the sulfhydryl reducing agent inactivated (AII alpha) receptor subtype.

Angiotensin II effects on the cytosolic free Ca2+ concentration in N1E-115 neuroblastoma cells: kinetic properties of the Ca2+ transient measured in single fura-2-loaded cells

The effect of angiotensin II on the cytosolic free Ca2+ concentration was measured in single mouse neuroblastoma N1E-115 cells loaded with fura-2. Angiotensin II induced a transient concentration-dependent increase in Ca2+ and also increased the production of inositol polyphosphates. The Ca2+ increase did not require extracellular Ca2+ and was unaffected by pretreatment with pertussis toxin. These data suggest that angiotensin II increased Ca2+ by an inositol trisphosphate-mediated release of intracellular Ca2+ following activation of phospholipase C via a pertussis toxin-insensitive guanine nucleotide binding protein. Similar results were obtained with bradykinin. The angiotensin II- or bradykinin-induced increase in Ca2+ occurred after a concentration-dependent latent period. Low concentrations of agonist elicited a small increase in Ca2+ following a variable lag that sometimes exceeded 1 min, whereas at maximally effective angiotensin II concentrations a larger, more rapid increase in Ca2+ occurred without a measurable delay. In some cells, oscillatory increases in Ca2+ were induced by angiotensin II and bradykinin. Possible mechanisms to explain the concentration dependency of the latent period and the oscillatory nature of the increases of Ca2+ are discussed. These results indicate that the mouse neuroblastoma N1E-115 cell represents a useful model for studying the signal response transduction mechanisms regulating the effects of angiotensin II in neuronal cells.

Increased blood pressure induced by central application of aminopeptidase inhibitors is angiotensinergic-dependent in normotensive and hypertensive rat strains

Two aminopeptidase inhibitors, amastatin (AM) and bestatin (BE), were employed in 3 strains of rats, spontaneously hypertensive (SHR), Wistar-Kyoto (WKY), and Sprague-Dawley (SD), to investigate the central angiotensinergic system. The results indicate that intracerebroventricular (i.c.v.) injections of AM and BE induced pressor elevations in all 3 strains of rats. In order to test for the possibility of spillage into peripheral vasculature, members from all 3 strains were peripherally infused with AM, BE, or 0.15 NaCl via jugular vein catheters. The SHRs were significantly more responsive to the aminopeptidases than the normotensive strains, however their overall pressor responses were only 33% of those to i.c.v. infusion. Next, in order to test the notion that these aminopeptidase inhibitors are having their effect via the central angiotensinergic system, and not some other peptidergic system, the specific angiotensin receptor antagonist, Sar1, Thr8-AII (sarthran) was employed. Intracerebroventricular pretreatment with sarthran prevented subsequent pressor responses to i.c.v. AM and BE in members of all 3 strains, thereby suggesting that these aminopeptidase inhibitors are having their effect via the central angiotensinergic system.

Sodium appetite: species and strain differences and role of renin-angiotensin-aldosterone system

The characteristics of the appetite for NaCl in humans differ in some aspects from those in other species. The mechanisms of appetite for NaCl have been studied in detail in two species, rats and sheep. We review the treatments known to induce an appetite for NaCl in rats, with special reference to differences among strains in their spontaneous preference for NaCl solution. The current view of the mechanism is critically appraised, with particular emphasis on the role of angiotensin II, mineralocorticoids, cerebroventricular sodium transport, and the relation between preference for NaCl and the concentration of sodium in saliva. The appetite for NaCl in rodents other than rats is considered next, and reveals that mice, hamsters and gerbils are reluctant to ingest NaCl either spontaneously or after treatment with several of the natriorexigenic stimuli that are effective in rats. The characteristics of the appetite for NaCl in non-rodent species, including sheep, rabbit, dog, and non-human primates, are then described. We discuss some of the possible differences in mechanism that might account for this behavioral diversity among species.

Area postrema and alcohol: effects of area postrema lesions on ethanol self-administration, pharmacokinetics, and ethanol-induced conditioned taste aversion

An investigation was carried out to test the hypothesis that the area postrema (AP) may detect ethanol as a blood-borne toxin and thereby mediate aversive postingestinal effects of the drug. These aversive effects in turn may impose an upper limit on the amount of drug that can be consumed. In Experiment 1 rats had continuous access to water and a 4% ethanol solution. Animals with AP lesions drank more ethanol than sham-operated controls. No differences in drug disposition or metabolism were found when these rats were injected with ethanol and blood ethanol levels were measured. If the AP lesions resulted in increased ethanol drinking because of a reduction in the aversive effects of the drug, then the lesions might also be expected to attenuate a conditioned taste aversion induced by ethanol. In Experiment 2, groups of lesioned and sham-operated rats drank a novel tasting fluid and then were given i.p. injections of ethanol (0.9, 1.2, or 1.6 g/kg) or vehicle. Similar degrees of aversion to the taste of the fluid developed in both the lesioned and the sham-control groups. Since the AP lesion did not result in the attenuation of the ethanol-induced conditioned taste aversion, it was suggested that the AP may not mediate the aversive consequences of ethanol and that the increased ethanol self-administration observed in Experiment 1 may be due to other effects of the lesion.

Elevated salt appetite and brain binding of angiotensin II in mineralocorticoid-treated rats

Angiotensin II (Ang II) and aldosterone levels increase with sodium deficiency, promoting sodium conservation and arousing a salt appetite in rats. The mechanism(s), by which these two hormones interact to produce salt appetite is not known. The experiments reported here tested the possibility that increased mineralocorticoids change the number and/or affinity of Ang receptors in the brain. Rats were given a series of deoxycorticosterone acetate (DOCA) injections (500 micrograms/day, s.c., for 4 days) which are known to produce a salt appetite when given in conjunction with an intracerebroventricular injection of Ang. The binding of 125I-Ang II to membranes prepared from the septal-anteroventral third ventricular region was then examined. DOCA treatment resulted in a significant increase in the number of Ang binding sites (Bmax) with no change in binding affinity (Kd). The binding of 125I-Ang II was then investigated in membranes prepared from 12 other brain regions as well as the pituitary and adrenal gland, showing that the increase in binding capacity occurred in only a few specific brain regions. A third experiment verified that the DOCA treatment used here was sufficient to arouse a salt appetite when combined with a single intracerebroventricular injection of Ang II. The mechanism that underlies the production of salt appetite by aldosterone and Ang II may at least partially consist of mineralocorticoid-induced increases in the number of Ang receptors in discrete brain regions.

Localization of angiotensin II receptor binding in rabbit brain by in vitro autoradiography

Binding of 125I-[Sar1,Ile8] angiotensin II (AII) to sections of brains from both wild and laboratory rabbits was determined by in vitro autoradiography. In the forebrain, specific high density binding was observed in the olfactory bulb, organum vasculosum of the lamina terminalis (OVLT), subfornical organ, median eminence, lateral septum, median preoptic nucleus and hypothalamic paraventricular, supraoptic and arcuate nuclei. In the midbrain, binding of the radioligand was observed in the interpeduncular and parabrachial nuclei, in the locus coeruleus, and ventrolateral pons. In the hind brain, there was dense binding of 125I-[Sar1,Ile8] AII to the nucleus of the solitary tract (NTS) and to both rostral and caudal parts of the reticular formation of the ventrolateral medulla oblongata. Weaker specific binding of the radioligand to the molecular layer of the cerebellum, to the nucleus of the spinal trigeminal tract, dorsal motor nucleus of the vagus, area postema, and to a band of tissue connecting the NTS to the ventrolateral medulla was also observed. Binding of the ligand to circumventricular organs such as the OVLT, subfornical organ, and median eminence suggests that these are sites in the brain of the rabbit at which blood-borne AII may exert influences on the central regulation of fluid balance and pituitary hormone secretion, although AII of neuronal origin could also act at these sites. Binding of the radioligand in several other brain regions suggests that angiotensin II of cerebral origin may be involved in a number of different aspects of brain function in the rabbit. The finding of dense binding in the NTS and ventrolateral medulla, which are involved in autonomic activity and are also sites of catecholamine-containing neurons, raises the possibility of angiotensin interaction with these neurons and involvement in autonomic function.

Comparison of 125I-angiotensin III and 125I-angiotensin II binding to rat brain membranes

The binding of 125I-angiotensin III (125I-ANG III) to rat brain membranes was examined and compared with that of 125I-angiotensin II (125I-ANG II). Degradation of each ligand, as monitored by HPLC, was effectively inhibited using fragments of ANG III and ANG II known to have little affinity for angiotensin binding sites. Three classes of 125I-ANG III-binding sites were observed based on affinity (KD = 0.13, 1.83, and 10.16 nM) and capacity (Bmax = 1.30, 18.41, and 67.2 fmol/mg protein, respectively). Two classes of 125I-ANG II-binding sites of high affinity (KD = 0.11 and 1.76 nM) and low capacity (Bmax = 1.03 and 18.86 fmol/mg protein, respectively) were also identified. Cross-displacement studies confirmed that the two highest-affinity 125I-ANG III-binding sites and the 125I-ANG II-binding sites were the same. On the other hand, the binding of 125I-ANG III to the low-affinity 125I-ANG III-binding site could not be inhibited with ANG II. These data imply that previously measured differences in the biological potency of cerebroventricularly applied ANG III and ANG II probably do not result from differential binding of these peptides to central angiotensin receptors.

Effect of angiotensin II and peptide YY on cerebral and circumventricular blood flow

Angiotensin II and peptide YY (PYY) are putative neuro/humoral agents acting at several circumventricular regions. These peptides also constrict cerebral vessels. We examined the effect of acute intravenous infusion of saline, angiotensin II and peptide YY on local cerebral blood flow (14C-iodoantipyrine autoradiography) in the circumventricular and non-circumventricular brain regions of 17 conscious rats. No reductions in brain blood flow (28 regions) were observed although angiotensin II and PYY infusion elevated arterial blood pressure 15-25% without influencing heart rate, suggesting an increase in peripheral resistance. However, local blood flow was dependent on the peptide infused. During PYY infusion, blood flow was rather constant in the 20 non-circumventricular regions examined whereas an increase in blood flow and a slight decrease in cerebrovascular resistance occurred in the circumventricular regions. The area postrema exhibited the most pronounced changes--an elevation in blood flow of 44 +/- 11% and a reduction in resistance of 20 +/- 5% in comparison to that in control animals. During angiotensin II infusion, local cerebral blood flow was similar to that in controls and local cerebrovascular resistance was elevated. Thus, the local cerebral circulatory response to peptide administration was dependent on the location of the region examined (circumventricular or non-circumventricular) and on the vasoactive peptide infused.

Water intake of Djungarian and Syrian hamsters treated with various dipsogenic stimuli

Djungarian and Syrian hamsters were found to exhibit robust water intake following water deprivation, and in response to hyperosmotic and hypovolumetric challenges to body fluids following injections of hypertonic NaCl and polyethylene glycol, respectively. Water intake was not stimulated following peripheral injections of isoproterenol, serotonin, or angiotensins II or III. Both hypovolema and isoproterenol activated the renin-angiotensin system. This profile of drinking responses in hamsters is similar to that reported previously for mice and degus.

Regulation of angiotensin II binding sites in the subfornical organ and other rat brain nuclei after water deprivation

1. Binding sites for angiotensin II have been localized in forebrain and brain-stem areas of water-deprived and control Sprague-Dawley rats, employing autoradiography with computerized microdensitometry. 2. Angiotensin II receptor sites were identified in the organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus, median preoptic nucleus, area postrema, nucleus of the solitary tract, and inferior olive. 3. After dehydration a significant increases in the concentration of angiotensin II receptors was detected only in the subfornical organ. Although there was an increased concentration of angiotensin II binding sites in the organum vasculosum of the lamina terminalis, the median preoptic nucleus, and the paraventricular nucleus after dehydration, these changes did not reach statistical significance. Other brain nuclei investigated did not show differences in angiotensin II binding sites in the dehydrated rats compared to controls. 4. These results indicate that angiotensin II receptors in the subfornical organ may play an important role in fluid homeostasis during dehydration.

Osmotic stress, plasma renin activity, and spermatogenesis in Vipera aspis

Circulating electrolytes (Na+, K+), plasma renin-like activity, testosterone, and testis morphology were investigated in early summer during the spermatogenic progressive phase in Vipera aspis subjected to sodium loading and sodium depletion. After sodium loading, plasma sodium and plasma testosterone levels were significantly elevated compared with those of controls, while plasma renin-like activity was depressed, spermiogenesis was increased, the epithelium lining the epididymis was very thick, and the Leydig cells were hypertrophied. After sodium depletion, plasma sodium and plasma testosterone levels were significantly depressed and plasma renin-like activity was significantly elevated. Spermiogenesis seemed to be slightly regressed: the epithelium lining the epididymis was very thin, and the lumen was devoid of spermatozoa. The Leydig cells were hardly visible. All the data strongly suggest that osmotic stress affects gonadal activity in the snake. V. aspis.

Improved immunohistochemical staining of angiotensin II in rat brain using affinity purified antibodies

Recent immunohistochemical studies that have sought to detect angiotensin II/III (AII/AIII) immunoreactive material in the brain have been forced to rely on a small number of antisera because most AII/AIII antibodies have unexplainably proved unsuitable for immunohistochemistry. Although extremely useful tools, these antisera have suffered from high background staining. The purpose of this study was to re-examine and characterize the staining using the most popular AII/AIII antiserum (Denise) before and after purification on an AII CH-sepharose affinity column. The use of crude AII/AIII antiserum resulted in the staining of large varicosities and cell bodies. Fibres were all but invisible owing to extensive background staining. In contrast, the purified antibodies yielded little background staining and produced a discrete staining of AII/AIII fibres with small varicosities in the paraventricular-hypophysial pathway and of cell bodies of large hypothalamic neurones. In addition punctate staining demarcated the perikarya of some neurones and resembled boutons containing immunoreactive AII/AIII. Biochemical and histochemical analysis of the crude antiserum, the affinity purified antibodies and other fractions off the sepharose column demonstrated that a large portion of the total staining (various types of background) seen with crude antiserum and column fractions was not to AII/AIII or several angiotensin-derived fragments. Furthermore, successful preabsorption blanks for the purified antibodies could only be achieved with AII coupled through its N-terminal, suggesting that these purified antibodies reacted best with conjugated angiotensin in the fixed tissue. In total the results of this study indicate that the background staining seen with crude antiserum is not to AII/AIII. The use of affinity purified antibodies greatly enhances resolution, enabling one to visualise even small fibres in rats not treated with colchicine, and should improve our ability to develop accurate maps of central angiotensinergic pathways.

Effects of angiotensin II on [3H]noradrenaline release and phosphatidylinositol hydrolysis in the parietal cortex and locus coeruleus of the rat

Angiotensin II (ANGII) (3-100 nM) facilitated the potassium-evoked (22.5 mM) release of [3H]-noradrenaline ([3H]NA) from slices of parietal cortex in a concentration-dependent manner, but did not significantly alter the release of [3H]NA evoked in a similar manner from locus coeruleus slices. The facilitatory action of ANGII was blocked by saralasin (0.1-3 microM). Neither nimodipine (10-30 microM) nor phenylmethylsulphonyl fluoride (1 mM) altered either [3H]NA release or the facilitatory action of ANGII in the parietal cortex. Carbachol (0.01-3 mM) and raised potassium (22.5 mM), but not ANGII (3-100 nM), stimulated the production of inositol phosphates in parietal cortex slices. The potassium-evoked increase in inositol phosphate production was unaffected by ANGII (3-100 nM). In the locus coeruleus, ANGII (3-100 nM) did not stimulate inositol phosphate production. The mechanism underlying the ANGII facilitation of [3H]NA release from the parietal cortex does not appear to involve either nimodipine-sensitive calcium channels, or, as far as we have been able to determine, the release of calcium from intracellular stores following the breakdown of phosphoinositides.

The effects of the aminopeptidase inhibitors amastatin and bestatin on angiotensin-evoked neuronal activity in rat brain

During a recent comparison of iontophoretically applied angiotensin II (AII) and angiotensin III (AIII) in the paraventricular nucleus of the rat, we observed that the response latency for AIII was much shorter than that for AII. This suggested that AII may have to be converted to AIII before it becomes active. To test this hypothesis we performed 3 experiments. (1) We examined the effects of bestatin, an aminopeptidase B inhibitor, on the activity of applied AII and AIII. (2) Next, we monitored the effects of amastatin, a specific aminopeptidase A inhibitor, on the action of co-applied AII or AIII. (3) And, finally, we examined the response to the aminopeptidase-resistant analog Sar1-AII, both applied alone and in combination with AII or AIII. Bestatin, while having no activity of its own, dramatically enhanced the actions of both AII and AIII. Amastatin, on the other hand, had little effect on AII's action and diminished or totally blocked AII-dependent activity. Like bestatin, amastatin had no effect alone. Sar1-AII reduced spontaneous activity of angiotensin-sensitive neurons and inhibited the actions of AII and AIII in a reversible manner. The same cells were also blocked by the recognized angiotensin antagonist Sar1, Ile8-AII. In total these results strongly support the notion that AII must be converted to AIII in the brain before it is activated.

Inhibition of hypertension and salt intake by oral taurine treatment in hypertensive rats

Effects of oral treatment with taurine on fluid intakes produced by renin were assessed in spontaneously hypertensive rats of the Okamoto strain (SHR). Renin injected into the preoptic area increased water intake and evoked salt (2.7% NaCl solution) intake, and angiotensin II injected into this area increased water intake, but not salt intake, in both SHR and control normotensive Wistar-Kyoto rats (WKY). The salt intake elicited by renin, but not water intake produced by renin or angiotensin II, was potentiated in SHR. These effects of renin and angiotensin II on fluid intakes were antagonized by previous administration of taurine or gamma-aminobutyric acid into the cerebral ventricles in both strains. When SHR received water containing 3% taurine from 32 to 105 days of age, development of hypertension was inhibited. Renin administered into the preoptic area at 105 days of age caused an increase in salt intake, but the increase was markedly inhibited by the oral administration of taurine as well. These results show that salt appetite produced by centrally administered renin is exaggerated in SHR and that development of hypertension as well as renin-induced salt appetite in SHR is inhibited by dietary taurine.

Synthesis and expression of functional angiotensin II receptors in Xenopus oocytes injected with rat brain mRNA

Xenopus laevis oocytes were injected with poly(A)+ mRNA isolated from rat brain and superfused in a medium containing either serotonin, angiotensin II or bradykinin. Applications of serotonin or angiotensin II to injected oocytes elicited, in a dose-dependent manner, changes in membrane potential. The angiotensin II receptor was desensitized fairly rapidly in the continued presence of the agonist. No response was obtained with bradykinin. The selectivity of the angiotensin II-induced response was demonstrated by the finding that the angiotensin II antagonist [( Sar1,Ala8]angiotensin II, saralasin) blocked the angiotensin II-induced response. It is concluded that an appropriate fraction of brain mRNA is capable of directing the synthesis and correct insertion of functional angiotensin II receptors in the Xenopus oocyte membrane.

Delayed cerebroventricular metabolism of [125I]angiotensins in the spontaneously hypertensive rat

This study was designed to evaluate the hypothesis that impaired brain angiotensin signal termination contributes to the sustained blood pressure elevations noted in the genetically hypertensive rat model of human essential hypertension. A technique that combined the intracerebroventricular injection of [125I]angiotensins, followed by focused microwave fixation to stop all peptidase activity and subsequent HPLC analyses, was used for determining half-lives of [125I]angiotensin II and [125I]angiotensin III in the ventricular space. The results indicate that the spontaneously hypertensive rat evidenced significantly longer half-lives for intracerebroventricularly injected [125I]angiotensin II over those measured for the Wistar-Kyoto and Sprague-Dawley normotensive rat strains: 45.0, 27.2, and 25.0 s, respectively. This was also true for intracerebroventricularly administered [125I]angiotensin III: 19.5, 11.4, and 9.0 s, respectively. These results support the notion that a dysfunction in central aminopeptidase activity in the spontaneously hypertensive rat may result in prolonged half-lives of endogenously synthesized angiotensins II and III, which are known to serve as ligands at central angiotensin receptors responsible for the control of cardiovascular function. The extended half-lives of these ligands may contribute to the sustained elevations in blood pressure observed in this animal model.

Nodose ganglionectomy reduces angiotensin II receptor binding in the rat brainstem

Angiotensin II (Ang II) receptor binding sites in the dorsomedial medulla of intact and unilaterally nodose ganglionectomized rats were identified and characterized using 125I-sarcosine,isoleucine Ang II. This radioligand bound saturably and with high affinity to rat brain homogenates and to sections of rat brainstem. Specific (1 microM angiotensin II displaceable) binding of 125I-sarcosine,isoleucine Ang II was displaced by angiotensin analogues with a potency order similar to that described for angiotensin II receptors. Unilateral nodose ganglionectomy caused a reduction in Ang II receptor binding in the medial solitary tract nucleus, dorsal motor nucleus of the vagus, and area postrema ipsilateral to the lesioned ganglion. This observation suggests that Ang II receptors in the dorsomedial medulla may be located on axon terminals of vagal afferents and cell bodies of vagal efferents.