Cellular localization of angiotensin type 1 receptor and angiotensinogen mRNAs in the subfornical organ of the rat brain

The Renin Angiotensin System as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a major public health problem, with limited pharmacological options available beyond symptomatic relief. The renin angiotensin system (RAS) is primarily known as a systemic endocrine regulatory system, with major roles controlling blood pressure and fluid homeostasis. Drugs that target the RAS are used to treat hypertension, heart failure and kidney disorders. They have now been used chronically by millions of people and have a favorable safety profile. In addition to the systemic RAS, it is now appreciated that many different organ systems, including the brain, have their own local RAS. The major ligand of the classic RAS, Angiotensin II (Ang II) acts predominantly through the Ang II Type 1 receptor (AT1R), leading to vasoconstriction, inflammation, and heightened oxidative stress. These processes can exacerbate brain injuries. Ang II receptor blockers (ARBs) are AT1R antagonists. They have been shown in several preclinical studies to enhance recovery from TBI in rodents through improvements in molecular, cellular and behavioral correlates of injury. ARBs are now under consideration for clinical trials in TBI. Several different RAS peptides that signal through receptors distinct from the AT1R, are also potential therapeutic targets for TBI. The counter regulatory RAS pathway has actions that oppose those stimulated by AT1R signaling. This alternative pathway has many beneficial effects on cells in the central nervous system, bringing about vasodilation, and having anti-inflammatory and anti-oxidative stress actions. Stimulation of this pathway also has potential therapeutic value for the treatment of TBI. This comprehensive review will provide an overview of the various components of the RAS, with a focus on their direct relevance to TBI pathology. It will explore different therapeutic agents that modulate this system and assess their potential efficacy in treating TBI patients.

AT1 receptors in the subfornical organ modulate arterial pressure and the baroreflex in two-kidney, one-clip hypertensive rats

The subfornical organ (SFO), a forebrain circumventricular organ that lies outside the blood-brain barrier, has been implicated in arterial pressure and baroreflex responses to angiotensin II (ANG II). We tested whether pharmacological inhibition or selective silencing of SFO ANG II type 1 receptors (AT₁R) of two-kidney, one-clip rats with elevated plasma ANG II decreases resting arterial pressure and renal sympathetic nerve activity (RSNA) and/or modulates arterial baroreflex responses of heart rate (HR) and RSNA. Male Sprague-Dawley rats underwent renal artery clipping [2-kidney, 1-clip (2K,1C)] or sham clipping (sham). After 6 wk, conscious rats instrumented with vascular catheters, renal nerve electrodes, and a cannula directed to the SFO were studied. In another set of experiments, rats were instrumented with hemodynamic and nerve radio transmitters and injected with scrambled RNA or silencing RNA targeted against AT₁R. Mean arterial pressure (MAP) was significantly higher in 2K,1C rats. Acute SFO injection with the AT₁R inhibitor losartan did not change MAP in sham or 2K,1C rats. Baroreflex curves of HR and RSNA were shifted rightward in 2K,1C rats. Losartan exerted no effect. SFO AT₁R knockdown did not influence MAP in sham rats but decreased MAP in 2K,1C rats, despite no change in plasma ANG II or resting RSNA. AT₁R knockdown prevented the reduction in maximum gain and slope of baroreflex responses of HR and RSNA; the reduced RSNA response to baroreceptor unloading was partially restored in 2K,1C rats. These findings show that AT₁R activation within the SFO contributes to hypertension and baroreflex dysfunction in 2K,1C rats and highlight the temporal requirement for reversal of these effects.

Potential mechanisms of hypothalamic renin-angiotensin system activation by leptin and DOCA-salt for the control of resting metabolism

The renin-angiotensin system (RAS), originally described as a circulating hormone system, is an enzymatic cascade in which the final vasoactive peptide angiotensin II (ANG) regulates cardiovascular, hydromineral, and metabolic functions. The RAS is also synthesized locally in a number of tissues including the brain, where it can act in a paracrine fashion to regulate blood pressure, thirst, fluid balance, and resting energy expenditure/resting metabolic rate (RMR). Recent studies demonstrate that ANG AT_1A receptors (Agtr1a) specifically in agouti-related peptide (AgRP) neurons of the arcuate nucleus (ARC) coordinate autonomic and energy expenditure responses to various stimuli including deoxycorticosterone acetate (DOCA)-salt, high-fat feeding, and leptin. It remains unclear, however, how these disparate stimuli converge upon and activate this specific population of AT_1A receptors in AgRP neurons. We hypothesize that these stimuli may act to stimulate local expression of the angiotensinogen (AGT) precursor for ANG, or the expression of AT_1A receptors, and thereby local activity of the RAS within the (ARC). Here we review mechanisms that may control AGT and AT_1A expression within the central nervous system, with a particular focus on mechanisms activated by steroids, dietary fat, and leptin.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Proinflammatory cytokines upregulate sympathoexcitatory mechanisms in the subfornical organ of the rat

Our previous work indicated that the subfornical organ (SFO) is an important brain sensor of blood-borne proinflammatory cytokines, mediating their central effects on autonomic and cardiovascular function. However, the mechanisms by which SFO mediates the central effects of circulating proinflammatory cytokines remain unclear. We hypothesized that proinflammatory cytokines act within the SFO to upregulate the expression of excitatory and inflammatory mediators that drive sympathetic nerve activity. In urethane-anesthetized Sprague-Dawley rats, direct microinjection of tumor necrosis factor (TNF)-α (25 ng) or interleukin (IL)-1β (25 ng) into SFO increased mean blood pressure, heart rate, and renal sympathetic nerve activity within 15 to 20 minutes, mimicking the response to systemically administered proinflammatory cytokines. Pretreatment of SFO with microinjections of the angiotensin II type-1 receptor blocker losartan (1 μg), angiotensin-converting enzyme inhibitor captopril (1 μg) or cyclooxygenase-2 inhibitor NS-398 (2 μg) attenuated those responses. Four hours after the SFO microinjection of TNF-α (25 ng) or IL-1β (25 ng), mRNA for angiotensin-converting enzyme, angiotensin II type-1 receptor, TNF-α and the p55 TNF-α receptor, IL-1β and the IL-1R receptor, and cyclooxygenase-2 had increased in SFO, and mRNA for angiotensin-converting enzyme, angiotensin II type-1 receptor, and cyclooxygenase-2 had increased downstream in the hypothalamic paraventricular nucleus. Confocal immunofluorescent images revealed that immunoreactivity for the p55 TNF-α receptor and the IL-1 receptor accessory protein, a subunit of the IL-1 receptor, colocalized with angiotensin-converting enzyme, angiotensin II type-1 receptor-like, cyclooxygenase-2, and prostaglandin E2 EP3 receptor immunoreactivity in SFO neurons. These data suggest that proinflammatory cytokines act within the SFO to upregulate the expression of inflammatory and excitatory mediators that drive sympathetic excitation.

Functional roles of neuropeptides in cerebellar circuits

Whereas the cerebellum contains 22 different types of neuropeptides as presently known, their expression is generally weak and diffusely dispersed in cerebellar tissues, which often makes their functional significance doubtful. Nevertheless, our knowledge about certain neuropeptides has advanced to the extent that we can figure out their unique functional roles in cerebellar circuits. Throughout the cerebellum, CRF is contained in climbing fibers and its spontaneous release is required for the induction of cerebellar long-term depression (LTD), a cellular mechanism of motor learning. Corticotropin-releasing factor (CRF) is also expressed in the paraventricular nucleus-pituitary system and amygdala-lower brainstem system, both of which are involved in coping responses to stress. In view that motor learning requires stressful efforts for correcting errors in repeated trials, CRF in climbing fibers may imply that the olivocerebellar system is part of a large CRF-operated functional system that acts to cope with various stressors. Orexin, on the other hand, is contained in beaded fibers, which, originating from the hypothalamus, project to various brainstem nuclei and also to the cerebellum, exclusively the flocculus. Currently available evidence suggests that, in fight-or-flight situations, orexinergic neurons switch the state of cardiovascular control systems including the flocculus to secure blood supply to working muscles. Considerable knowledge has also been accumulated about angiotensin II, galanin, and cerebellin, but there is still a gap in defining their unique functional roles in cerebellar circuits.

Angiotensin and NMDA receptors in the median preoptic nucleus mediate hemodynamic response patterns to stress

The brain renin-angiotensin system plays an important role in the regulation of arterial pressure in response to stress, in part due to activation of AT1 receptors in the hypothalamic median preoptic nucleus (MnPO) by endogenous angiotensin II (ANG II). N-methyl-d-aspartate (NMDA) receptors are also involved in the angiotensinergic signaling pathway through the MnPO. We investigated whether AT1 and NMDA receptors in the MnPO are responsible for variable hemodynamic response patterns to stress. Cocaine or startle with cold water evoked a pressor response in Sprague-Dawley rats due, in some rats [vascular responders (VR)], to a large increase in systemic vascular resistance (SVR) and, in other rats [mixed responders (MR)], to small increases in SVR and cardiac output (CO). Microinjection of the GABAA agonist muscimol into the MnPO to block synaptic transmission attenuated the cocaine- or stress-induced increase in SVR and the decrease in CO seen in VR without altering either response in MR. Likewise, administration of either an AT1 receptor antagonist, losartan, or an NMDA receptor antagonist, MK-801, attenuated the increase in SVR and the decrease in CO in VR in response to either cocaine (losartan and MK-801) or startle with cold water (losartan) without altering either response in MR. We propose that the MnPO is responsible for greater SVR responses in VR and that AT1 and NMDA receptors play an important role in greater SVR responses in VR. These data provide additional support for the critical role of the MnPO in cardiovascular responses to stress.

The significance of brain aminopeptidases in the regulation of the actions of angiotensin peptides in the brain

From the outset, the concept of a brain renin-angiotensin system (RAS) has been controversial and this controversy continues to this day. In addition to the unresolved questions as to the means by which, and location(s) where brain Ang II is synthesized, and the uncertainties regarding the functionality of the different subtypes of Ang II receptors in the brain, a new controversy has arisen with respect to the identity of the angiotensin peptide(s) that activate brain AT(1) receptors. While it has been known for some time that Ang III can activate Ang II receptors with equivalent or near-equivalent efficacy to Ang II, it has been proposed that in the brain, only Ang III is active. This proposal, which we have named "The Angiotensin III Hypothesis" states that Ang II must be converted to Ang III in order to activate brain AT(1) receptors. This review examines several aspects of the controversies regarding the brain RAS with a special focus on brain aminopeptidases, studies that either support or refute The Angiotensin III Hypothesis, and the implications of The Angiotensin III Hypothesis for the activity of the brain RAS. It also addresses the need for further research that can test The Angiotensin III Hypothesis and definitively identify the angiotensin peptide(s) that activate brain AT(1) receptor-mediated effects.

Effects of losartan and irbesartan administration on brain angiotensinogen mRNA levels

Losartan, 2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'(1H-tetrazol-5-yl)-biphenil-4-yl)methyl]imidazole, and Irbesartan, 2-n-butyl-3-[(2'-(1H-tetrazol-5-yl)-biphenyl-4-yl)methyl]-1,3-diaza-spiro[4,4]non-1-en-4-one, are two angiotensin AT1 receptor antagonists largely used in human health care as antihypertensive agents. Their ability to cross the blood-brain barrier and to influence the central renin-angiotensin system are widely investigated, but how this brain system responds to the subchronic and chronic block of the angiotensin AT1 receptor is still unknown. Normotensive rats were intragastrically implanted for 7- and 30-day administration, with a dose of 3 and 30 mg/kg body weight. Treatments were shown to influence, in a dose-, time- and brain-area-dependent manner, angiotensinogen mRNA levels in scanned areas. This study showed a general up-regulation of angiotensinogen mRNA expression after 7 days and a widespread down-regulation or basal level of expression after a 30-day administration of two angiotensin AT1 receptor antagonists.

TTF-1, a homeodomain-containing transcription factor, participates in the control of body fluid homeostasis by regulating angiotensinogen gene transcription in the rat subfornical organ

In recent years, it has become increasingly evident that angiotensins synthesized in the brain contribute to regulating body fluid homeostasis. Although angiotensinogen, the unique angiotensin precursor, is produced in the brain, the factors that regulate its gene expression remain unknown. We recently found that TTF-1, a homeodomain-containing transcription factor essential for the development of the fetal diencephalon, is postnatally expressed in discrete areas of the hypothalamus. We now report that the subfornical organ, an important site of angiotensinogen synthesis, is an extra-hypothalamic site of TTF-1 expression. Double in situ hybridization histochemistry demonstrated the presence of TTF-1 mRNA in angiotensinogen-producing cells of the rat subfornical organ. RNase protection assays showed that TTF-1 and angiotensinogen mRNA levels are simultaneously increased in the subfornical organ by water deprivation. The angiotensinogen promoter contains seven presumptive TTF-1 binding motifs, four of which are recognized by the TTF-1 homeodomain. In the C6 glioma cell line, TTF-1 transactivates the angiotensinogen promoter in a dose-dependent manner. This transactivation is abolished by deletion of the TTF-1 binding motif at -125. Intracranial administration of an antisense TTF-1 oligodeoxynucleotide decreased angiotensinogen mRNA in the subfornical organ and dramatically reduced the animal's water intake while increasing urine excretion. Moreover, plasma arginine vasopressin content was decreased by the same treatment. These results demonstrate a novel role for TTF-1 in the regulation of body fluid homeostasis, exerted via the transactivational control of angiotensinogen synthesis in the subfornical organ.

Hormonal and neurotransmitter roles for angiotensin in the regulation of central autonomic function

In this review we present the case for both hormonal and neurotransmitter actions of angiotensin II (ANG) in the control of neuronal excitability in a simple neural pathway involved in central autonomic regulation. We will present both single-cell and whole-animal data highlighting hormonal roles for ANG in controlling the excitability of subfornical organ (SFO) neurons. More controversially we will also present the case for a neurotransmitter role for ANG in SFO neurons in controlling the excitability of identified neurons in the paraventricular nucleus (PVN) of the hypothalamus. In this review we highlight the similarities between the actions of ANG on these two populations of neurons in an attempt to emphasize that whether we call such actions "hormonal" or "neurotransmitter" is largely semantic. In fact such definitions only refer to the method of delivery of the chemical messenger, in this case ANG, to its cellular site of action, in this case the AT1 receptor. We also described in this review some novel concepts that may underlie synthesis, metabolic processing, and co-transmitter actions of ANG in this pathway. We hope that such suggestions may lead ultimately to the development of broader guiding principles to enhance our understanding of the multiplicity of physiological uses for single chemical messengers.

Functional evidence for subfornical organ-intrinsic conversion of angiotensin I to angiotensin II

Using extracellular electrophysiological recording in an in vitro slice preparation, we investigated whether ANG I can be locally converted to the functionally active ANG II within the rat subfornical organ (SFO). ANG I and ANG II (10(-8)-10(-7) M) excited approximately 75% of all neurons tested with both peptides (n = 25); the remainder were insensitive. The increase in firing rate and the duration and the latency of the responses of identical neurons, superfused with equimolar concentrations of ANG I and ANG II, were not different. The threshold concentrations of the ANG I- and ANG II-induced excitations were both 10(-9) M. Inhibition of the angiotensin-converting enzyme by captopril (10(-4) M; n = 8) completely blocked the ANG I-induced excitation, a 10-fold lower dose was only effective in two of four neurons. The AT1-receptor antagonist losartan (10(-5) M; n = 6) abolished the excitation caused by ANG I and ANG II. Subcutaneous injections of equimolar doses of ANG I and ANG II (200 microliters; 2 x 10(-4) M) in water-sated rats similarly increased water intake by 2.4 +/- 0.5 (n = 16) and 2. 7 +/- 0.4 ml (n = 20) after 1 h, respectively. Control rats receiving saline drank 0.07 +/- 0.06 ml under these conditions. Pretreatment with a low dose of captopril (2.3 x 10(-3) M) 10 min before the injection of ANG I caused a water intake of 2.8 +/- 0.5 ml (n = 10), whereas a high dose of captopril (4.6 x 10(-1) M) suppressed the dipsogenic response of ANG I entirely (n = 11). These data provide direct functional evidence for an SFO-intrinsic renin-angiotensin system (RAS) and underline the importance of the SFO as a central nervous interface connecting the peripheral with the central RAS.

Differential regulation of angiotensinogen and AT1A receptor mRNA within the rat subfornical organ during dehydration

The present study describes the differential rostro-caudal patterning of angiotensinogen (AoGen) and AT1A receptor mRNAs in the rat SFO using specific and validated oligodeoxynucleotide probes for in situ hybridization. Highest levels of AoGen-specific gene expression were observed in the rostral region of the SFO with gradually decreasing intensity towards the caudal region of this sensory circumventricular organ lacking blood-brain barrier function. AoGen-related hybridization signals proved to be specifically prominent above cells in lateral aspects of the SFO, surrounding septal venules. Maximal expression of the AT1A receptor-specific gene, on the other hand, could be detected in the neuron-enriched, ventro-medial core region and dorsal annulus of the SFO, with low-intensity hybridization signals in its rostral and caudal parts. Water deprivation for 48 h, leading to extracellular hypertonic hypovolemia with elevated circulating AngII concentrations within the physiological range, caused a significant increase in AoGen-specific hybridization signals in the rostral and medial SFO regions. AT1A receptor gene expression and AngII receptor binding were markedly stimulated in the medial and caudal regions of the SFO (core and annulus) as compared to euhydrated animals. These data indicate, that mild dehydration differentially up-regulates AoGen- and AT1A receptor-specific mRNA formation as well as AT1 receptor binding in distinct regions of the SFO, and supports the involvement of different cellular subgroups in the expression of two major components of the central nervous renin-angiotensin system in this sensory circumventricular organ.

Actions of amylin on subfornical organ neurons and on drinking behavior in rats

Amylin, a peptide hormone secreted by pancreatic beta-cells after food intake, contributes to metabolic control by regulating nutrient influx into the blood, whereas insulin promotes nutrient efflux and storage. We now report that amylin activates neurons in the subfornical organ (SFO), a structure in which the lack of a functional blood-brain barrier and the presence of a high density of amylin receptors may render it accessible and sensitive to circulating amylin. In an in vitro slice preparation of the rat SFO, 73% of 78 neurons were excited by superfusion with rat amylin (10(-8)-10(-7) M); the remainder were insensitive. The threshold concentration for the excitatory response of amylin was <10(-8) M and thus similar in potency to a previously reported excitatory effect of ANG II on the same neurons. The excitatory effect of amylin was completely blocked by coapplication of the selective amylin receptor antagonist AC-187 (10(-6)-10(-5) M) but was not affected by losartan (10(-5) M). Subcutaneous injections of 40 nmol of amylin significantly increased water intake in euhydrated rats, as did an equimolar dose of ANG II, which is a well-described SFO-mediated effect of circulating ANG II. These results point to the SFO as a sensory central nervous target for amylin released systemically in response to metabolic changes. Furthermore, we suggest that amylin release during food intake may stimulate prandial drinking.

Angiotensin II-induced calcium signalling in neurons and astrocytes of rat circumventricular organs

The subfornical organ and organum vasculosum laminae terminalis represent neuroglial circumventricular organ structures bordering the anterior third cerebral ventricle. Owing to the absence of the blood-brain barrier, the cellular elements of the subfornical organ and the organum vasculosum laminae terminalis can be reached by circulating messenger molecules transferring afferent information. As demonstrated for the control of extracellular fluid composition, the circulating hormone angiotensin II acts on these sensory circumventricular organs to induce drinking, elevated peripheral resistance and neurohypophyseal hormone release via interaction with membrane-spanning receptor proteins. To characterize the cell-specific distribution of angiotensin II receptors within the circumventricular organs, primary cell cultures derived from the subfornical organ or organum vasculosum laminae terminalis of five- to six-day-old rat pups were used to measure alterations in intracellular calcium at the single cell level. Neurons and astrocytes were identified by immunocytochemical staining for specific marker proteins. Bath application of angiotensin II (10(-10)-10(-6) M) dose-dependently induced calcium transients in neurons (19.6%) and astrocytes (15.7%), and angiotensin II threshold concentrations to elicit intracellular calcium signalling proved to be one order of magnitude higher in astrocytes as compared to neurons (10(-9) M). At angiotensin II concentrations higher than 10(-7) M, pronounced desensitization of the angiotensin II receptor occurred. Employing the angiotensin II receptor antagonists losartan (DUP-753; AT1-receptor) and PD-123319 (AT2-receptor), exclusive expression of the AT1 receptor subtype coupled to intracellular calcium concentration signalling could be demonstrated for neurons and astrocytes. In all cells examined, the angiotensin II-evoked increase in intracellular calcium concentrations could be fully suppressed in the absence of extracellular calcium. Co-activation by angiotensin II and other agents (vasopressin, its fragment 8-arginine-vasopressin(4-9), oxytocin, endothelin) was indicated for subfornical organ neurons and organum vasculosum laminae terminalis astrocytes.

Structure of the subfornical organ: a review

In this review, the light microscopic and fine structural characteristics of neurons, axons, dendrites, glial cells, and capillaries and their topography within the subfornical organ are summarized, with an emphasis on recent findings. Structure-function relationships are discussed whenever possible and put into perspective in a concluding section.

Distribution of angiotensin type-1 receptor messenger RNA expression in the adult rat brain

Angiotensin II and angiotensin III in the brain exert their various effects by acting on two pharmacologically well-defined receptors, the type-1 (AT1) and the type-2 (AT2) receptors. Receptor binding autoradiography has revealed the dominant presence of AT1 in brain nuclei involved in cardiovascular, body fluid and neuroendocrine control. The cloning of the AT1 complementary DNA has revealed the existence of two receptor subtypes in rodents, AT1A and AT1B. Using specific riboprobes for in situ hybridization, we have previously shown that the AT1A messenger RNA is predominantly expressed in the rat forebrain; in contrast the AT1B subtype predominates in the anterior pituitary. Using a similar technical approach, the aim of the present study was to establish the precise anatomical localization of cells synthetising the AT1A receptor in the adult rat brain. High AT1A messenger RNA expression was found in the vascular organ of the lamina terminalis, the median preoptic nucleus, the subfornical organ, the hypothalamic periventricular nucleus, the parvocellular parts of the paraventricular nucleus, the nucleus of the solitary tract and the area postrema, in agreement with previous autoradiographic studies, describing a high density of AT1 binding sites in these nuclei. In addition, AT1A messenger RNA expression was detected in several brain areas, where no AT1 binding was reported previously. Thus, we identify strong expression of AT1A messenger RNA expression in scattered cells of the lateral parts of the preoptic region, the lateral hypothalamus and several brainstem nuclei. In none of these structures was the AT1B messenger RNA detectable at the microscopic level. In conclusion, it is suggested that angiotensins may exert their central effects on body fluid and cardiovascular homeostasis mainly via the AT1A receptor subtype.

Localization of angiotensin II AT1 receptor-like immunoreactivity in catecholaminergic neurons of the rat medulla oblongata

There exist at least two distinct subtypes of angiotensin II receptors in the brain, namely the AT1 and AT2 subtypes. The high density of angiotensin II AT1 receptors is present in the medulla oblongata. The AT1 subtype of angiotensin II receptors mainly mediates central cardiovascular events. In the present study a polyclonal antibody against the angiotensin II AT1 receptor and a monoclonal antibody against tyrosine hydroxylase were employed to evaluate the possible presence of angiotensin II AT1 receptor-like immunoreactivity in the catecholaminergic neurons of the rat medulla oblongata by means of the double colour immunofluorescence technique. A weak, diffuse cytoplasmic angiotensin II AT1 receptor-like immunoreactivity was observed in almost all the catecholaminergic cell bodies of the A2, C1, C2 and C3 cell groups, except those of the A1 cell group containing moderately intense, diffuse cytoplasmic angiotensin II AT1 receptor-like immunoreactivity, occasionally found in the noradrenergic dendrites of the A1 cell group. There was a higher density of the angiotensin II AT1 receptor-like immunoreactive profiles in the A2 cell group area than in other catecholaminergic cell group areas. In addition, the angiotensin II AT1 receptor-like immunoreactivity was seen in non-catecholaminergic neurons. The present results provide evidence for the existence of the specific angiotensin II AT1 receptor-like immunoreactivity in the noradrenergic and adrenergic neurons of the rat medulla oblongata known to have a cardiovascular role. Thus, the findings support the view that angiotensin II AT1 receptors in the medulla oblongata participate in cardiovascular control and indicate a cellular substrate for the documented interaction between the angiotensin II and adrenergic transmission lines in cardiovascular function at the level of the nucleus tractus solitarii.

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.

Endothelin-1-like immunoreactivity in human atherosclerotic coronary tissue: a detailed analysis of the cellular distribution of endothelin-1

Endothelin (ET) is a very potent vasoconstrictor peptide, which was originally reported to be produced by endothelial cells and to act locally in a paracrine fashion to regulate vascular tone. Recent studies have demonstrated that endothelin-1 (ET-1) not only is produced by endothelial cells, but is also present in non-endothelial cells of atherosclerotic lesions. The present study was therefore designed to characterize the cell type and distribution of ET-expressing cells in different areas of human atherosclerotic coronary plaques, obtained by directional atherectomy of 30 patients. In addition, ET-1 messenger RNA (mRNA) distribution was studied in human atherosclerotic plaque tissue by in situ hybridization (ISH). The strongest ET-1-like immunoreactivity (ET-1-IR) was present in all cell-rich areas of 27 plaques. In fibrotic areas of 27 tissue samples, ET-1-IR was found in 44 per cent (12/27). ET expression was most prevalent in foamy macrophages (MPs, HAM 56-positive) and myofibroblasts (MFBs, alpha-actin-positive) in the vicinity of necrotic areas with signs of previous intraplaque haemorrhage. By contrast, ET-1-IR was weak and inconsistently found in MPs (11/27; 40 per cent) and MFBs (12/27; 44 per cent) in fibrous areas. Luminal endothelial cells (Ulex europeus agglutinin reaction-positive, UEA) exhibited strong ET-1-IR, whereas endothelial cells of intraplaque microvessels demonstrated inconsistent staining for ET-1. ISH revealed that ET mRNA is produced locally in intimal MPs showing strong ET-1-IR. These findings demonstrate that ET-1 is produced by human MPs, the principal inflammatory cell type in atherosclerosis, suggesting a role for ET-1 in the chronic inflammation associated with complicated atherosclerosis.

Expression of AT1A and AT1B angiotensin II receptor messenger RNA in forebrain of 2-wk-old rats

The gene expression of angiotensin II receptor subtypes AT1A and AT1B was localized in the forebrain of 2-wk-old rats by in situ hybridization histochemistry and compared with [125I]Sar1-angiotensin II binding patterns. AT1A receptor mRNA was expressed in circumventricular organs, in hypothalamic nuclei like the paraventricular nucleus, in the lateral olfactory tract, in the basolateral amygdaloid and anterior olfactory nuclei, and in the piriform cortex. No AT1B receptor mRNA was detected in these areas. AT1A and AT1B receptor mRNA was detected in the hippocampus, cingulate cortex, and choroid plexus. No forebrain area studied expressed AT1B receptor mRNA exclusively. Most often, a good match for receptor mRNA and binding was found. In some areas, apparent mismatches suggested receptor formation elsewhere (median eminence) or receptor presence in local neuronal circuits (hippocampus, cingulate, and piriform cortex). Our results support the hypothesis that AT1A receptors are involved in the classical central functions of angiotensin II. Both AT1A and AT1B receptors may play roles in cortical and limbic system function, particularly early in development.

Valvular interstitial cells express angiotensinogen and cathepsin D, and generate angiotensin peptides

Cells capable of de novo angiotensin (Ang)II generation in the heart remain unidentified. High-density angiotensin converting enzyme (ACE) binding has been localized to sites of high collagen turnover, such as heart valve leaflets and their valvular interstitial cells (VIC). VIC express ACE mRNA and their membrane-bound ACE utilizes AngI as substrate. Whether VIC also express angiotensinogen (Ao) and an aspartyl protease, and whether they generate AngI and II de novo, is presently unknown. We sought to address these questions in serum-deprived cultured VIC. Ao, renin and cathepsin D (Cat-D) mRNA expression was addressed by RT-PCR. Production of Ao, AngI and AngII peptides were measured in VIC-culture media by radioimmunoassay (RIA). Immunoreactive Cat-D was detected by immunofluorescein labeling and Western blotting. Cat-D and renin activities were determined by spectrofluorometric and autoradiographic methods and AngI generation by RIA. Results showed (a) expression of Ao and Cat-D both at mRNA and protein levels; (b) AngI and AngII peptides in culture media; (c) acceleration of AngII production by exogenous AngI (1 nmol/l), which was blocked by lisinopril (0.1 mumol/l); (d) that dexamethasone (0.1 mumol/l) increased AngII production; (e) a 46 kDa immunoreactive Cat-D protein by Western blotting; (f) aspartyl protease activity, using chromogenic and 125I-labeled Ao as substrates, inhibited by pepstatin-A; and (g) the absence of renin mRNA and activity. It is concluded that at both the mRNA and protein levels, cultured VIC express Ao and Cat-D, and can generate AngI and AngII peptides by the action of a non-renin protease Cat-D and ACE, respectively. VIC therefore appear to represent a constitutive nonendothelial cell found in adult rat heart valve leaflets, which are capable of de novo Ang peptide generation.

Intracisternally applied angiotensin II does not excite reticulospinal vasomotor neurons in anesthetized rats

We examined whether vasomotor neurons in the rostroventrolateral reticular nucleus of the medulla oblongata might be responsible for an acute increase in arterial pressure, elicited by application of angiotensin II in the central nervous system, as suggested by others. In urethane-pentobarbital-anesthetized and ventilated rats, intracisternal administration of angiotensin II (1-30 nmol, infused over a period of 30 s) produced a dose-dependent pressor response, which was abolished by intracisternal application of [Sar1, Thr8]angiotensin II (100 nmol), an angiotensin II receptor antagonist. The pressor response, however, was neither preceded by nor associated with increased discharges of vasomotor neurons with slow- and fast-conduction axons in the rostroventrolateral reticular nucleus and of lumbar sympathetic chain and renal sympathetic nerves. Intravenous injections of [beta-mercapto-beta, beta-cyclopentamethylenepropinyl1,-O-Et-Tyr2, Val4, Arg8]vasopressin, a vasopressin receptor antagonist, largely abolished the central angiotensin II-induced pressor response, while a blockade of ganglionic transmission with hexamethonium and disruption of descending sympathoexcitatory output were ineffective. We conclude that central administration of angiotensin II, under the experimental conditions and at the doses, evokes an acute pressor response largely through the release of vasopressin, not by exciting vasomotor and sympathetic neurons.

Angiotensin II receptor subtypes in the duck subfornical organ: an electrophysiological and receptor autoradiographic investigation

The pharmacology of angiotensin II (AngII) receptors was investigated in the brain of ducks using receptor autoradiographic and electrophysiological methods. Using 125I[Val5]AngII as a ligand, specific binding was observed in sections of the duck adrenal gland and in several brain areas involved in body fluid homeostasis. Displacement studies using the same antagonists as used for classifying mammalian AngII receptor subtypes revealed that the rank order of potencies in competition with AngII receptors in the adrenal gland and in the subfornical organ was: AngII > CGP-42112A > losartan > PD-123319. Electrophysiological recordings from spontaneously active neurons of duck SFO slices revealed that the majority of neurons could be excited by AngII (10(-7) M). The excitatory effect of AngII could be partially inhibited by CGP-42112A (10(-5) M), which proved to be more effective than equimolar losartan and far more effective than PD-123319. These data suggest that the neuronal AngII receptors in the SFO are pharmacologically distinct from the mammalian AT1- and AT2-receptors. Further, central AngII receptors of ducks share common pharmacological characteristics with AngII receptors in the duck adrenal gland and peripheral organs of other bird species.

Zone-specific localization of cytochrome P45011B1 in human adrenal tissue by PCR-derived riboprobes

Cytochrome P45011B1 (11 beta-hydroxylase) was detected in the human adrenal cortex and in human adenomas by in situ hybridization methods. Specific riboprobes were generated by in vitro transcription of 11 beta-hydroxylase--specific synthetic oligonucleotides with attached T7 and SP6 polymerase promotors. [35S]- and digoxigenin-labeled riboprobes were hybridized to sections of an aldosterone-producing adenoma (APA), the non-tumour portion of the corresponding adrenal gland, and two adenomas not related to hyperaldosteronism using standard protocols and varying washing conditions. After exposure of the radiolabeled sections to X-ray film, the signals were quantified and compared by statistical tests. Following autoradiography or immunohistochemical detection of the digoxigenin cytochrome P45011B1 mRNA was clearly localized in the zona fasciculata/reticularis of non-tumour portion of an human adrenal with an APA. Zona glomerulosa, medulla and connective tissue were free of label. As revealed by the semi-quantitative analysis, 11 beta-hydroxylase mRNA signals in the APA were significantly lower than those in the attached non-tumour portion and the other two adenomas. The results confirm known observations on the occurrence of cytochrome P45011B1 in the adrenal cortex of other species, but show, contrary to several immunohistochemical studies, that the enzyme is obviously not expressed in the zona glomerulosa.

Effects of human renin in the vasculature of rats transgenic for human angiotensinogen

Transgenic rats, which express the human angiotensinogen gene, provide a unique model for studying local vascular effects of human renin. We examined the cleavage of human angiotensinogen to angiotensin I (Ang I) by human renin and its inhibition by a human renin inhibitor in an isolated perfused hindlimb preparation from such rats. Perfusion resulted in the sustained release of human angiotensinogen, which decreased from 19.4 to 11.8 pmol/mL over 45 minutes. Active human renin at doses of 3, 10, and 30 ng/mL perfusate for 15 minutes increased Ang I release from undetectable levels (mean +/- SEM) to 31.9 +/- 3.3, 147.1 +/- 26.2, and 206.4 +/- 17.1 fmol/mL, respectively, by 9 minutes. In separate experiments aimed at the quantification of renin-induced vasoconstriction, captopril decreased the perfusion pressure and lowered Ang II concentrations to nondetectable levels, whereas Ang I values increased sharply. When renin (30 ng/mL) was infused for 15 minutes, renin values in the perfusate decreased to barely detectable levels within minutes after termination of the infusion. However, Ang I values remained high for at least 30 minutes thereafter. The addition of a human renin inhibitor during renin infusion caused Ang I values to promptly decrease within minutes to undetectable levels. Hindlimbs from non-transgenic control rats released no detectable amounts of Ang I, with or without human renin. Finally, by in situ hybridization we documented the presence of human angiotensinogen message in the vessels of the hindlimb. We conclude that renin acts on angiotensinogen at a site in the vascular wall. The cleavage depends on renin and not on other lysosomal proteases.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-localization of angiotensin II and gamma-aminobutyric acid in axon terminals in the rat subfornical organ

Angiotensin II (Ag II) and gamma-aminobutyric acid (GABA) in the subfornical organ have been implicated in drinking and cardiovascular responses to changes in circulating hormones. We combined immunogold silver labeling of Ag II with immunoperoxidase detection of GABA to determine whether there might be common cellular sites for their physiological actions in this circumventricular region. Electron microscopy showed that numerous terminals in central portions of the rat subfornical organ contained both Ag II and GABA immunoreactivity. These terminals as well as others exclusively labeled for AgII or GABA formed mainly symmetric, inhibitory type synapses with unlabeled dendrites. Immunogold-silver aggregates recognizing AgII were often detected near non-synaptic portions of the plasma membrane with or without apparent association with large dense core vesicles. In contrast, the GABA immunoperoxidase labeling was most intensely localized to membranes of small clear vesicles which were aggregated near the presynaptic junction. Our results indicate that in rat subfornical organ, neuronal AgII may modulate the inhibitory postsynaptic responses to GABA following release from single axon terminals.

The angiotensin receptor subtype AT1A predominates in rat forebrain areas involved in blood pressure, body fluid homeostasis and neuroendocrine control

Subtypes of the angiotensin II (Ang II) type-1 (AT1) receptor are probably involved in distinct actions of the peptide, since their distribution in peripheral organs and regulation of their gene expression are different. We investigated the distribution of AT1A and AT1B receptor subtype mRNAs in the rat forebrain and pituitary using sensitive cRNA probes for in situ hybridization. High level of AT1A receptor mRNA expression is observed in the subfornical organ (SFO) and in the anterior hypothalamus, particularly the periventricular tissue surrounding the anterior portion of the 3rd ventricle (AV3V), which contains the organum vasculosum of the lamina terminalis (OVLT), the median preoptic nucleus and the preoptic periventricular nucleus as well as in the hypothalamic periventricular nucleus and in the parvocellular part of the paraventricular nucleus (PVN). Moderate to strong AT1A labeling was found in the anterior olfactory nucleus, the piriform cortex and the nucleus of the lateral olfactory tract. Very low AT1B receptor mRNA expression was found in the SFO and the PVN. In contrast, strong AT1B receptor mRNA expression coincided with low AT1A receptor mRNA expression in the anterior pituitary. Labeling was cytoplasmic at the light microscopic level. We thus suggest that the AT1A receptor is responsible for the central actions of Ang II in the rat forebrain whereas direct actions of Ang II on the anterior pituitary are mediated by the AT1B receptor subtype.

The brain renin-angiotensin system: molecular mechanisms of cell to cell interactions

The components of the Renin-Angiotensin System (RAS) have been found to be expressed in the brain. Angiotensinogen, the high molecular weight precursor of the system, is widely distributed and expressed in areas not related to control of blood pressure and body fluid homeostasis as well. It has been shown that it is regulated by steroid hormones independently from the liver and that it is also regulated in a different manner in several brain areas. Angiotensin II, the effector peptide of the system, may be generated in the brain via the classical pathway, using renin and angiotensin converting enzyme or directly from angiotensinogen by cathepsin G or tonin. N-terminal peptides of angiotensin II have been found in several brain areas with ANG (1-7) involved in vasopressin release however without influence on blood pressure and with ANG III acting as potent as ANG II. Transgenic animals may be used to study the pathophysiology of an activated brain RAS.

On the plasticity of the cerebellar renin-angiotensin system: localization of components and effects of mechanical perturbation

This study focuses on the renin-angiotensin system (RAS) in the cerebellar cortex and changes within this system after mechanically induced cerebellar injury. Using radioactive and non-radioactive in situ hybridization and immunocytochemistry angiotensinogen mRNA, angiotensinogen, angiotensin II and, for the first time, N-terminal angiotensin fragment (1-7) immunoreactivities, respectively, were demonstrated in the rat cerebellum. Angiotensinogen mRNA and angiotensinogen immunoreactivity (IR) were both present in glial cell populations of all layers, especially in the Purkinje and granular cell layers and within the cerebellar nuclei. Angiotensin II IR was demonstrated in glial cell populations in all layers using a monoclonal angiotensin II antibody, while with a polyclonal angiotensin II antiserum (Denise) some Purkinje cell bodies were labelled. After lesioning the cerebellar cortex mechanically by an injection cannula a strong increase in angiotensinogen gene expression as well as in angiotensin II and angiotensin (1-7) immunoreactivities were observed in the glial cell populations. Furthermore, putative Bergmann glial processes, as indicated from the morphological appearance became strongly angiotensin II and angiotensinogen immunoreactive in the region close to the mechanically induced lesion. It could inter alia be demonstrated for the first time using confocal laser microscopy of ANG II IR and GFAP IR that ANG II in vivo in the intact cerebellar cortex is present in astroglial processes in the molecular layer and presumably secreted into the extracellular space in form of small spheric bodies and/or taken up by other cell types. In contrast, the N-terminal fragment angiotensin (1-7) IR was restricted to the glial cell populations and appeared only after the lesion event. Thus, it is suggested that the cerebellar RAS shows marked changes in response to mechanically induced lesions. The expression of angiotensinogen as well as the production of angiotensinogen IR and angiotensin II like IR is even after mechanical lesion restricted to astrocytes, i.e., cerebellar astrocytes and putative Bergmann glial cells, and in case of immunoreactivities it spreads to the radially oriented Bergmann glial processes in the molecular layer.

Localization of changes in immediate early genes in brain in relation to hydromineral balance: intravenous angiotensin II

Immediate early genes, detected by Fos- and Jun-like immunoreactivity (FLI, JLI), were induced in discrete regions of the rat brain by intravenous infusion of angiotensin II (Ang II) at dipsogenic doses. The regions included subfornical organ (SFO), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO), supraoptic nucleus (SON), and the magnocellular part of the paraventricular hypothalamus (PVH). These responses were sustained for up to 6 h of infusion. In SFO, FLI was induced preferentially in the posterior part, while JLI occurred in more central regions. Cerebroventricular (ICV) injection of the Ang II type 1 receptor (AT-1) antagonist, losartan potassium, completely prevented the FLI induced by Ang II in these brain regions. ICV injection of the Ang II type 2 receptor (AT-2) antagonist, PD 123319, did not reduce Ang II-induced FLI in SFO, OVLT and MnPO, but markedly attenuated the activation in SON and PVH. To determine whether SFO is the primary site for transduction of the circulating Ang II signal, electrolytic lesions were made in or rostral to the SFO. Rats with complete lesions showed a complete absence of Ang-induced FLI in SON and PVH. The data are discussed in terms of functional mapping of the brain regions activated by circulating Ang II and neural circuitry for water intake, including the possible role of AT-2 receptors in PVH and SON.

Photochemically induced focal cerebral ischemia in rat: time dependent and global increase in expression of basic fibroblast growth factor mRNA

Induction of basic fibroblast growth factor (bFGF) mRNA expression was studied in a Rose bengal induced focal cerebral ischemia during a time course of 2, 4, 24, 72 h and 7 days. Focal cerebral ischemia induced by Rose bengal resulted in a global upregulation in bFGF gene expression at the 24 h time-interval. This upregulation in bFGF gene expression was due to an upregulation in glial bFGF expression in most of the areas studied as seen by means of non-radioactive in situ hybridization in combination with immunocytochemistry for glial fibrillary acidic protein. However, in the piriform cortex a putative neuronal upregulation of bFGF could be detected by combination of non-radioactive in situ hybridization, immunohistochemistry for glial fibrillary acidic protein and nuclear staining with Neutral red. Semiquantitative data concerning bFGF mRNA expression were obtained by use of computer-assisted microdensitometry and revealed substantial increases in bFGF mRNA expression in the cingulate cortex, the neostriatum, a 1 mm marginal zone close to the external capsule and the olfactory tubercle at bregma levels 1 to 2 mm rostral to the lesion. No changes in bFGF gene expression were seen in field CA1 of Ammon's horn on the lesioned side and in dentate gyrus at bregma levels between -2.12 to -3.30 mm. We observed significant changes in bFGF upregulation in the caudate putamen, the piriform cortex and the amygdaloid region and the frontoparietal cortex at bregma levels -2.12 to -3.30 mm. These data indicate that photochemically induced focal cerebral ischemia leads to an early and global response in bFGF gene expression, which is due to an upregulation mainly in astrocytes. The observed widespread upregulation of the bFGF gene transcription rostral and caudal to the lesion is suggested to be due in part to neuronal glutaminergic connections between the areas investigated and in part due to increases in extracellular fluid signals (volume transmission).

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.