The Renin-Angiotensin System in the Brain: Possible Therapeutic Implications for AT1- Receptor Blockers

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Journal of Human Hypertension (2002) 16, S64–S70  2002 Nature Publishing Group All rights reserved 0950-9240/02 $25.00 www.nature.com/jhh The renin-angiotensin system in the brain: possible therapeutic implications for AT1- receptor blockers

J Culman1, A Blume2, P Gohlke1 and T Unger2 1Institute of Pharmacology, Christian-Albrechts-University of Kiel, 24105 Kiel, Germany; 2Institute of Pharmacology and Toxicology, Charite´-Hospital, Humboldt University at Berlin, 10117 Berlin, Germany

Biochemical, physiological and functional studies sug- the blood pressure lowering effects of AT1 receptor gest that the brain renin-angiotensin system (RAS) is blockers. Animal studies have shown that AT1 receptor regulated independently of the peripheral RAS. The antagonists enable endogenous angiotensin II to stimu- classical actions of angiotensin II in the brain include late neuronal regeneration via activation of AT2 recep- blood pressure control, drinking behaviour, natriuresis tors. In animal models, inhibition of the brain RAS and the release of vasopressin into the circulation. At proved to be beneﬁcial with respect to stroke incidence least two subtypes of G-protein coupled receptors, the and outcome. Blockade of brain and cerebrovascular

AT1 and the AT2 receptor, have been identiﬁed. Most of AT1 receptors by AT1 receptor blockers prevents the the classic actions of angiotensin II in the brain are reduction in blood ﬂow during brain ischaemia, reduces mediated by AT1 receptors. The AT2 receptor is involved the volume of ischaemic injury and improves neurologi- in brain development and neuronal regeneration and cal outcome after brain ischaemia. This paper reviews protection. Additionally, AT2 receptors can modulate the actions of angiotensin II and its receptors in the some of the classic angiotensin II actions in the brain. brain, and discusses the possible consequences of AT1 Selective non-peptide AT1 receptor blockers, applied receptor blockade in neuroprotection, neuroregener- systemically, have been shown to inhibit both peripheral ation, cerebral haemodynamics and ischaemia. and brain AT1 receptors. In genetically hypertensive Journal of Human Hypertension (2002) 16, S64–S70. rats, inhibition of brain AT1 receptors may contribute to doi:10.1038/sj.jhh.1001442

Keywords: AT1 receptor blockers; brain; candesartan; neuroprotection; renin-angiotensin system; stroke

Introduction in the brain independently of peripheral sources. Angiotensin II acts on brain structures localised The renin angiotensin system (RAS) has tradition- inside and outside the blood-brain barrier to induce ally been linked to the regulation of the salt and drinking behaviour and natriuresis, stimulate vaso- water homoeostasis. The effector peptide of the pressin release, modulate sympathetic outflow to the RAS, angiotensin II, binds at least to two receptor periphery, and attenuate the baroreceptor reflex. subtypes, referred to as the AT1 and the AT2 recep- Several lines of evidence suggest that inappropriate tors. The classical peripheral actions of angiotensin RAS activity in the brain may contribute to the II, which include vasoconstriction, facilitation of development and maintenance of arterial hyperten- sympathetic transmission and renal salt and water sion. retention are mediated by the AT1 receptor. These During the past 10 years, much has been learnt actions can be regarded as compensatory mech- about neuronal effects of angiotensin II that are not anisms to preserve salt and water, and maintain directly related to the central control of fluid and electrolyte balance and adequate organ perfusion, in electrolyte homoeostasis and the regulation of blood the face of water and salt loss. pressure. In addition, numerous findings have dem- In the last three decades, evidence has accumu- onstrated that angiotensin II, acting via the AT2 lated that angiotensin II can be formed in various receptor, may modulate embryonic development, tissues, such as brain, kidney, adrenal gland, heart tissue regeneration and protection, and initiate pro- and blood vessels. The brain has long been recog- cesses leading to programmed cell death (apoptosis). nised as a site of tissue RAS activity, and it has been There is also substantial evidence that the AT2 firmly established that angiotensin II is synthesised receptor can offset or counteract the effects mediated by the AT1 receptor, for example on cell 1,2 Correspondence: J Culman, MD, Institute of Pharmacology, Chris- proliferation, water intake and blood pressure. tian-Albrechts-University of Kiel, Hospitalstrasse 4, 24105 Kiel, This paper reviews the mechanisms of action and Germany. E-mail: juraj.culman.Ȱpharmakologie.uni-kiel.de function of angiotensin-containing pathways in the The renin-angiotensin system in the brain J Culman et al S65 brain, and possible therapeutic implications of angiotensin II-stained cells were found in the med- inhibiting the brain RAS. ulla was the nucleus of the solitary tract.9

Components of the RAS in the brain Angiotensin receptors and signal Angiotensin II in the brain is generated from angio- transduction pathways tensinogen by an enzymatic cascade involving renin and angiotensin converting enzyme (ACE). The In the adult brain, structures related to the regu- expression of renin in the brain has been clearly lation of body fluid homoeostasis and blood press- demonstrated using various biochemical and ure express mainly or exclusively AT1 receptors. As immunohistochemical techniques. The enzyme is already mentioned, the AT1 receptor mediates vir- synthesised in neurones and is present in high con- tually all of the known physiological actions of centrations in nerve terminals. ACE has also been angiotensin II in the brain, such as the regulation found in the synaptosomal fraction of brain tissue of arterial blood pressure and vasopressin release, with high concentrations in the lamina terminalis electrolyte and water balance, thirst and hormone and the circumventricular organs, hypothalamus secretion (Figure 1).10 and some brain stem nuclei. Angiotensinogen is The signalling pathways of the AT1 receptor are mostly found extracellularly, and the mRNA enco- well understood. They include the classic cascades ding for angiotensinogen is predominantly localised activated by G proteins, resulting in increases of in glial cells, although the peptide is also present in intracellular calcium and activation of protein kin- some neuronal populations.3,4 These findings sug- ase C. These signalling pathways are responsible for gest that angiotensin II is formed extracellularly in the generation and mediation of immediate the brain. However, the fact that angiotensin II in responses, such as the release of vasopressin and the brain can be found almost exclusively within oxytocin from the posterior pituitary. Other signal- synaptic vesicles in nerve endings favours an intra- ling pathways initiated by angiotensin II binding to cellular formation of angiotensin II. Another the AT1 receptor are phosphorylation-dependent unsolved question seems to be the lack of abundant reactions, some of which involve increased renin expression in those brain areas where mRNA expression of inducible transcription factors such as encoding for angiotensinogen is highly expressed c-Fos and c-Jun. These signalling cascades are and angiotensin II is present in high concentrations. believed to be involved in AT1 receptor-mediated 11 In general, angiotensin I levels are low in the brain, cell growth. Activation of periventricular AT1 which suggests that angiotensin II can be formed receptors in the rat brain induces expression of tran- directly from angiotensinogen, rather than by the scription factors in all brain regions nuclei involved enzymatic activity of renin. Non-renin, non-ACE in cardiovascular control and osmoregulation.11 pathways involving acid proteases have been A variety of signalling mediators have been 5 described. Alternatively, renin expression may be described for the AT2 receptor. The AT2 receptor strongly controlled to avoid a permanently high seems to be coupled to the Gi protein, and the signal- level of renin activity in brain areas abundant in ling mechanisms involve inhibition of mitogen- angiotensinogen.6 activated protein kinases, alteration in intracellular cGMP levels and inhibition of phosphorylation. Angiotensin pathways in the brain Recent findings indicate that angiotensin II, acting via AT2 receptors, is involved in the regulation of The distribution of angiotensin peptides and their pro- and/or antiapoptotic events (Figure 1). receptors is intimately linked to the brain areas associated with central regulation of fluid and salt homoeostasis and blood pressure control. Angiotensin II is a principal neurotransmitter within the lamina terminalis and its neuronal connections to other brain regions related to central cardiovascular control including the hypothalamic nuclei. Angio- tensin II-stained cell bodies are prominent in the paraventricular and supraoptic nuclei. The neurosecretory cells in these regions, which synthesise vasopressin, lie within a network of angiotensin- immunoreactive fibres and terminals.7,8 In addition, cell groups positively stained for angiotensin II are located in the stria terminalis and in the medial nucleus of the amygdala. Angiotensin receptors have also been found in the hindbrain regions involved in the modulation of sympathetic vaso- Figure 1 Effects mediated by angiotensin II (Ang II) acting on the motor tone, despite the fact that the only site where AT1 and the AT2 receptor. ITF: inducible transcription factors.

Journal of Human Hypertension The renin-angiotensin system in the brain J Culman et al S66 Angiotensin receptor antagonists treatment with irbesartan is short-lasting (up to 6 h). We have also demonstrated that telmisartan can Saralasin, the first specific peptide antagonist of penetrate the blood-brain barrier in a dose- and angiotensin receptors, interacts with both angioten- time-dependent manner to inhibit centrally sin receptor subtypes. Its therapeutic potential was mediated effects of angiotensin II. Compared with limited by its short duration of action, poor oral losartan and irbesartan, telmisartan is more lipho- bioavailability and partial agonist activity. In the philic and this may explain its greater potency at late 1980s and early 1990s, a number of selective, brain AT1 receptors following systemic appli- 15 non-peptide antagonists of AT1 receptors were cation. developed. These compounds selectively block the In our studies, candesartan produced the most AT1 receptor, while leaving the AT2 receptor unaf- effective inhibition of brain AT1 receptors. Candes- fected. The selective non-peptide AT1 receptor artan produced a complete, 24-h, blockade of cen- antagonists have been proven to be safe and very trally mediated angiotensin II effects at doses 5–10 effective agents in the treatment of hypertension and times lower than the losartan or irbesartan doses other cardiovascular diseases. required to inhibit central angiotensin II activity.16 Several selective and high-affinity AT1 receptor Chronic systemic treatment with candesartan also antagonists are currently available, including cande- dose-dependently decreased AT1 binding in brain sartan, eprosartan, irbesartan, losartan, telmisartan areas localised outside, as well as inside, the blood- and valsartan. These compounds differ pharmaco- brain barrier.17 The effective and long-lasting block- logically in terms of their affinity for the AT1 recep- ade of brain AT1 receptors displayed by candesartan tor and in the duration of the receptor blockade. In may be attributed to the insurmountable antagon- in vitro binding studies, candesartan has a remark- ism, characterised by tight binding and slow dis- ably high affinity for the AT1 receptor, compared sociation from the receptor. This suggests that sys- with other antagonists. In general, the inhibition of temically administered candesartan may exhibit a AT1 receptors by these antagonists is competitive long duration of action in the brain. and reversible. However, some AT1 receptor antagonists, such as candesartan, have been found to dis- The brain RAS: cardiovascular effects sociate extremely slowly from the receptor. Slow dissociation seems to be sufficient to produce insur- and hypertension mountable inhibition since insurmountable antag- Stimulation of angiotensin receptors in the brain onism is characterised by a long-lasting inhibition results in vasopressin release, modulation of sympa- of the receptor by the antagonist. In contrast to can- thoadrenal activity and inhibition of the barorecep- desartan, losartan is a surmountable antagonist, tor reflex.6 An increase in blood pressure can be whereas EXP3174, the active metabolite of losartan, observed when angiotensin II is injected into the lat- valsartan and irbesartan exhibit different degrees of eral ventricle or distinct forebrain areas. Angioten- 12 insurmountable inhibition. Relative to other AT1 sin II injected into the cerebral ventricles primarily receptor antagonists, candesartan exerts a long- activates neurons in the circumventricular organs, lasting blockade of the AT1 receptor. which project directly or indirectly to vasopressin- While the antihypertensive effects of AT1 receptor synthesising neurones in the paraventricular and antagonists have mainly been ascribed to inhibition supraoptic nuclei. The pressor response to intracere- of AT1 receptors in the periphery, evidence has broventricular angiotensin II is therefore initially 18 accumulated that inhibition of central AT1 receptors mediated by vasopressin. Angiotensin II acts as a may also contribute to the effects of these com- neurotransmitter in some of the neuronal circuits pounds on blood pressure.13 Furthermore, effective innervating the neurosecretory cells in the hypothal- 9 and long-lasting blockade of central AT1 receptors amic paraventricular and supraoptic nuclei. AT1 may protect against stroke and provide therapeutic receptors predominate in these hypothalamic benefits for patients suffering from brain ischaemia regions19 and mediate the pressor response to the (see below). peptide. The mechanisms involved in vasopressin In our laboratory, we have studied the access of secretion following intracerebroventricular adminis- systemically administered AT1 receptor antagonists tration of angiotensin II comprise noradrenaline to brain AT1 receptors. Both losartan and irbesartan, release from nerve endings and stimulation of administered intravenously or orally, dose- postsynaptic ␣-adrenoceptors localised on dependently attenuated central effects of angioten- vasopressin-synthesising neurones in the paraven- sin II (increases in mean arterial pressure, drinking tricular and supraoptic nuclei.7 and the release of vasopressin). At up to 60 min after While the role of noradrenaline and ␣-adrenocep- systemic application, irbesartan appeared to pen- tors in the paraventricular and supraoptic nuclei in etrate the blood-brain barrier more effectively than the mediation of vasopressin release following injec- losartan. However, even with high doses, complete tion of angiotensin II into the cerebral ventricles has inhibition of central angiotensin II effects was never been firmly established, the mechanisms respon- achieved with either compound.14 Moreover, the sible for vasopressin release after direct stimulation inhibition of brain AT1 receptors following systemic of AT1 receptors by angiotensin II in these regions

Journal of Human Hypertension The renin-angiotensin system in the brain J Culman et al S67 are poorly understood. Studies have failed to dem- Neuronal regeneration and tissue repair onstrate expression of AT1 receptors in vasopressin- synthesising neurones in the magnocellular part of The recognition that angiotensin II, acting via AT2 the paraventricular nucleus, although injection of receptors, may modulate tissue regeneration and angiotensin II into this region elicits an immediate protection, differentiation and processes leading to 10 increase in circulating vasopressin, which can be programmed cell death represents the most excit- completely inhibited by prior injection of losart- ing discovery during the last decade. Since it has an.19,20 become apparent that activation of AT2 receptors In addition to the forebrain sites of action, angio- plays a role in wound healing and repair in periph- tensin II can act in the cardiovascular centres of the eral tissues, the attention of investigators has turned medulla to control blood pressure. The area post- to the processes occurring after neuronal injury. rema, the nucleus of the solitary tract and the rostral Adult neurones do not usually reinnervate their tar- ventrolateral medulla appear to be the principal get regions after injury. However, if they are sup- sites of action. All of these brain regions contain plied with growth-promoting substances, they can generate new processes over longer distances and high densities of AT1 receptors. The area postrema, which receives its blood supply from the vertebral reinnervate their target regions. Recent studies have demonstrated that following sciatic nerve injury, arteries, is considered to monitor blood-borne angio- mRNA encoding for the AT receptor is up-regulated tensin II; microinjection of the peptide into the area 2 in sciatic nerve segments, and that this coincides postrema produces a pressor response.21 Angioten- with the successful regeneration of nerve fibres.32 In sin II has also been reported to modify the baroreflex the in vivo optic nerve crush model, angiotensin II by acting in the nucleus of the solitary tract. Interest- induced a concentration-dependent outgrowth of ingly, microinjection of angiotensin II into the neurites; this effect was also mediated by the AT2 nucleus of the solitary tract causes a depressor receptor, since the regeneration process was paral- response at low doses, and a pressor response at leled by a time-dependent increase in AT2 receptor high doses. Blood-borne angiotensin II can modify mRNA expression in the retina and the crushed the baroreflex via neural pathways from the area optic nerve.33 These findings point to an unexpected postrema to the nucleus of the solitary tract.22 Exci- role of angiotensin II and the AT2 receptor in the tation of the rostral ventrolateral medulla produces regeneration processes following neuronal injury. an increase in blood pressure due to activation of Moreover, these data provide direct evidence that sympathetic vasoconstrictor nerves, tachycardia and stimulation of AT2 receptors can promote axonal the release of catecholamines from the adrenal med- regeneration in vivo after neuronal lesions. ulla.22–26 An overactive brain RAS has been implicated in the development and maintenance of high blood Ischaemic stroke pressure in spontaneously hypertensive rats (SHR). Ischaemic stroke results from a transient or perma- In SHR, the central RAS is activated, as demon- nent reduction or interruption of cerebral blood strated by increased formation of angiotensin II and 4 flow. Both neuronal necrosis and apoptosis can be increased AT1 receptor expression in the brain. Sur- observed after brain ischaemia. Some brain cells are prisingly, results concerning the blood pressure low- lost immediately, whereas cells in the penumbra ering effects of AT1-receptor inhibition in the brain (the area surrounding the ischaemic focus) are sever- are equivocal. In conscious adult SHR, acute or ely damaged and remain in a compromised state for chronic inhibition of brain AT1 receptors with losar- several hours. Brain damage continues for days after tan or candesartan failed to lower blood press- the ischaemic event. 27,28 ure. In contrast, Kamitani et al reported a Cerebral ischaemia also leads to alteration in gene reduction of high blood pressure after blockade of expression, which is under the control of inducible 29 brain AT1 receptors in nephrectomised SHR. Simi- transcription factors. These transcription factors larly, antisense inhibition of mRNA encoding for the may provide a direct link between extracellular sig- AT1 receptor in the brain reduced hypertension in nals activating signal transduction pathways and the SHR.13 Furthermore, normotensive rats developed initiation of intracellular molecular and metabolic hypertension following in vivo gene transfer of events leading to delayed neuronal death. human ACE.30 The exact mechanisms by which During the past decade, a number of studies have increased RAS activity in the brain contributes to indicated that the brain RAS may be involved in the the development of hypertension are not known. initiation and regulation of processes occurring dur- Recent findings indicate that angiotensin II-induced ing and after brain ischaemia. Treatment with ACE suppression of the afferent feedback from barorecep- inhibitors or AT1 receptor antagonists has been tors may represent one of the mechanisms by which reported to prevent the occurrence of stroke in SHR elevated central angiotensin II activity could lead to or salt-loaded Dahl salt-sensitive rats, or to reduce 31 hypertension. Central effects of AT1 receptor the infarction volume after carotid artery blockers may thus contribute, at least partly, to the occlusion.34,35 It is generally considered that ACE effects of these compounds on blood pressure. inhibitors protect against brain ischaemia primarily

Journal of Human Hypertension The renin-angiotensin system in the brain J Culman et al S68 by reducing blood pressure. However, several lines of evidence suggest that additional effects may also be involved. ACE inhibitors have been reported to exert beneficial effects on the metabolic and circulatory derangement following cerebral ischaemia in SHR, and to improve the recovery from cerebral ischaemia in normotensive rats.36,37 Autoregulation of cerebral blood flow is impaired during brain ischaemia, and the neuroprotective effects of ACE inhibitors may be at least partly related to the normalisation of cerebrovascular autoregulation. In humans, cerebral blood flow is similar in normotensive and hypertensive individuals, due to an increased cerebrovascular resistance in hypertensive subjects. As a result of this increased resistance, the arteries display hypertrophy and lose the capability to dilate. The impaired dilatation of cerebral vessels at lower blood pressures results in a Figure 2 Beneficial effects of a long-term blockade of brain AT1 shift of the upper and lower limits of autoregulation receptors, achieved by intracerebroventricular (i.c.v.) infusion of irbesartan or by systemic, subcutaneous (s.c.) treatment with can- to the right. ACE inhibitors have been shown to shift desartan over a 5-day period before the induction of focal cerebral

the limits of cerebral autoregulation to lower blood ischaemia in normotensive rats. Pretreatment with the AT1 recep- pressure levels in normotensive rats and in SHR.38 tor antagonists improves neurological outcome and the recovery from cerebral ischaemia. Recent findings indicate that AT1 receptor blockers produce similar effects. Acute administration of the AT1 receptor blocker candesartan, at a dose that did ischaemia, markedly reduces the expression of the not influence baseline cerebral blood flow, shifted transcription factors c-Fos and c-Jun in the parietal the autoregulation curve towards lower blood press- and piriform cortices on the ligated side of the brain, ure in both normotensive rats and SHR.39 It has been and completely abolishes ischaemia-induced c-Fos 43 further demonstrated that chronic treatment of expression in the hippocampus. The AT1 receptor stroke-prone SHR with candesartan reduces the inci- blockers were infused into the cerebral ventricles dence of stroke in these rats without affecting over a 5-day period before the induction of ischae- blood pressure.40 mia, at a dose which effectively inhibited brain but 41 43 Nishimura et al attempted to clarify the role of not vascular AT1 receptors. In further studies we AT1 receptors in the control of cerebral blood flow showed that systemic pretreatment of normotensive in genetically hypertensive rats during cerebral rats with candesartan, at doses which did not affect ischaemia. Candesartan was used in these studies the arterial blood pressure, improved the recovery because of its potency at the AT1 receptor and from cerebral ischaemia and reduced the volume of because it had previously been reported to com- ischaemic injury (Figure 2; Groth et al unpublished pletely normalise the cerebrovascular autoregulat- results). Together, these findings demonstrate that 39 ory curve in these rats. Pretreatment of SHR with AT1 receptor blockers may improve the recovery candesartan for 2 weeks resulted in almost complete from stroke by restoration of blood flow after ischae- inhibition of AT1 receptors in the cerebral arteries mia and by blocking the biochemical and metabolic and in the brain areas involved in the central regu- changes at the ischaemic cascade level. lation of cerebral blood flow and the sympathetic We have also investigated the role of AT2 recep- system. In SHR exposed to temporary carotid tors during and after focal brain ischaemia. occlusion followed by reperfusion, candesartan Although long-term inhibition of brain AT2 recep- reduced the volume of injury predominantly in the tors did not affect the recovery from stroke, it pre- 41 affected cortical areas. The proposed mechanisms vented the beneficial effects of AT1 receptor block- involve increasing vasodilation and preventing the ade (Culman et al, unpublished results). AT1 decrease in blood flow in the marginal zone of receptor blockade exposes AT2 receptors to ischaemia. The observed reductions in cerebral increased concentrations of angiotensin II. Acti- oedema immediately after carotid occlusion, and in vation of AT2 receptors in brain tissue that has the infarction volume, probably resulted from the undergone ischaemic injury may initiate neuro- normalisation of cerebrovascular autoregulation in regenerative events or induce apoptosis in severely the marginal ischaemic zone.42 damaged neurones. Both these effects are important 44 AT1 receptor blockers may also improve recovery for the recovery from stroke. from stroke by mechanisms independent of cerebrovascular autoregulation or blood pressure reduction. Outlook We have demonstrated that long-term blockade of brain AT1 receptors by irbesartan and losartan Intensive research over the past 10 years has yielded improves neurological outcome of focal cerebral new insights into the role of angiotensin receptors

Journal of Human Hypertension The renin-angiotensin system in the brain J Culman et al S69 in neuronal tissue. AT2 receptors have been impli- 13 Gyurko R, Wielbo D, Phillips MI. Antisense inhibition cated in processes occurring in neuronal tissue dur- of AT1 receptor mRNA and angiotensinogen mRNA in ing development, regeneration or repair, but the the brain of spontaneously hypertensive rats reduces actual contribution of these receptors to the regu- hypertension of neurogenic origin. Regul Peptides lation of events associated with these processes 1993; 49: 167–174. 14 Culman J et al. Effects of systemic treatment with remains to be clearly defined. As effects mediated irbesartan and losartan on central responses to by AT2 receptors generally counteract those angiotensin II in conscious, normotensive rats. Eur J mediated by AT1 receptors, the net effect of angio- Pharmacol 1999; 367: 255–265. tensin II in neuronal tissue may therefore depend on 15 Gohlke P et al.AT1 receptor antagonist telmisartan the numbers and expression of both receptor types. administered peripherally inhibits central responses to The neurotrophic actions of AT2 receptor stimu- angiotensin II in conscious rats. J Pharmacol Exp Ther lation may provide a basis for the design of new 2001; 298:62–70. therapeutic strategies in the failure of axonal regen- 16 Gohlke P et al. Effects of orally applied candesartan eration or in the treatment of neurodegenerative dis- cilexetil on central responses to angiotensin II in con- orders and stroke. Similarly, the finding that AT scious rats. J Hypertens 2002; 20:1–10. 1 17 Nishimura Y, Ito T, Hoe K-L, Saavedra J. Chronic peri- receptor blockers cross the blood-brain barrier and pheral administration of the angiotensin II AT recep- improve neurological outcome and prevent brain 1 tor antagonist Candesartan blocks brain AT1 receptors. injury following brain ischaemia may provide a Brain Res 2000; 871:29–38. basis for new therapeutic strategies in the preven- 18 Unger T et al. Differential effects of central angiotensin tion and treatment of stroke. The potential clinical II and substance P on sympathetic nerve activity in relevance of these findings is underlined by the conscious rats. Circ Res 1985; 56: 563–575. 19 Lenkei Z, Palkovits M, Corvol P, Lorens-Corte`sC. increasing use of AT1 receptor blockers in antihypertensive treatment and the prevention of end-organ Expression of angiotensin type-1 (AT1) and type-2 damage related to hypertension. (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review. Frontiers Neuroendocrinol 1997; 18: 383–439. References 20 Veltmar A et al. Involvement of adrenergic and angio- tensinergic receptors in the paraventricular nucleus in 1 Unger T. The angiotensin type 2 receptor: variations the angiotensin II-induced vasopressin release. J on an enigmatic theme. J Hypertens 1999; 17: 1775– Pharmacol Exp Ther 1992; 262: 1253–1260. 1786. 21 Lowes VL, McLean LE, Kasting NW, Ferguson AW. 2 Lucius R et al. Beyond blood pressure: new roles for Cardiovascular consequence of microinjection of vaso- angiotensin II. Cell Mol Life Sci 1999; 56: 1008–1019. pressin and angiotensin II in the area postrema. Am J 3 Unger T et al. Brain angiotensin: pathways and phar- Physiol 1993; 262: R625–R631. macology. Circulation 1988; 77 (Suppl I): 1–40. 22 Muratami H et al. Brain angiotensin and circulatory 4 Saavedra JM. Brain and pituitary angiotensin. Endocr control. Clin Exp Pharmacol Physiol 1996; 23: 458– Rev 1992; 13: 329–380. 464. 5 Saye JA, Ragsdale V, Carey RM, Peach MJ. Localization 23 Chalmers J et al. Central neurons and neurotransmit- of angiotensin peptide-forming enzymes of 3T3-F442A ters in the control of blood pressure. Clin Exp adipocytes. Am J Physiol 1993; 264: C1570–C1576. Pharmacol Physiol 1994; 21: 819–829. 6 Phillips MI, Sumners C. Angiotensin II in central ner- 24 Head GA. Role of AT1 receptors in the central control vous system physiology. Regul Peptides 1998; 78:1– of sympathetic vasomotor function. Clin Exp 11. Pharmacol Physiol 1996; Suppl 3: S93–S98. 7 Culman J et al. Angiotensin as neuromodulator/ 25 Dampney RAL, Hirooka Y, Potts PD, Head GA. Func- neurotransmitter in central control of body fluid and tions of angiotensin peptides in the rostral ventrolat- electrolyte homoeostasis. Clin Exp Hypertens 1995; 17: eral medulla. Clin Exp Pharmacol Physiol 1996; Suppl 281–293. 3: S105–S111. 8 Ganten D, Lang RE, Lehmann E, Unger T. Brain 26 Allen AM et al.AT1-receptors in the central nervous angiotensin: on the way to becoming a well studied system. JRAAS 2001; 2 (Suppl 1): S95–S101. neuropeptide system. Biochem Pharmacol 1984; 33: 27 DePasquale MJ, Fossa AA, Holt WF, Mangiapane ML. 3523–3528. Central DuP 753 does not lower blood pressure in 9 Lind RW, Swanson LW, Ganten D. Organization of spontaneously hypertensive rats. Hypertension 1992; angiotensin II immunoreative cells and fibers in the rat 19: 668–671. central nervous system. Neuroendocrinology 1985; 40: 28 Bunting MW, Widdop RE. Lack of centrally-mediated 2–24. antihypertensive effect following acute or chronic 10 De Gasparo M et al. International union of Pharma- central treatment with AT1-receptor antagonists in cology. XXIII. The angiotensin II receptors. Pharmacol spontaneously hypertensive rats. Br J Pharmacol 1995; Rev 2000; 52: 415–472. 116: 3181–3190. 11 Blume A, Herdegen T, Unger T. Angiotensin peptides 29 Kamitani A et al. The effects of central administration and inducible transcription factors. J Mol Med 1999; of angiotensin II type-1 receptor antagonist, CV-22974, 77: 339–357. in nephrectomized spontaneously hypertensive rats. 12 Vauquelin G, Fierens F, Verheijen I, Vanderheyden P. Clin Exp Pharmacol Physiol 1994; 21: 271–276.

Insurmountable AT1 receptor antagonism: the need for 30 Nakamura S et al. Activation of brain angiotensin sys- different antagonist binding states of the receptor. tem by in vivo human angiotensin-converting enzyme Trends Pharmacol Sci 2001; 22: 343–344. gene transfer in rats. Hypertension 1999; 34: 302–308.

Journal of Human Hypertension The renin-angiotensin system in the brain J Culman et al S70 31 Paton JFR, Kasparov S. Sensory channel speciﬁc 39 Vraamark T, Waldemar G, Strandgaard S, Paulson OB. modulation in the nucleus of the solitary tract. J Angiotensin receptor antagonist CV-11974 and cer- Autonom Nerv System 2000; 80: 117–129. ebral blood ﬂow autoregulation. J Hypertens 1995; 13: 32 Gallinat S et al. Sciatic nerve transection evokes last- 755–761.

ing up-regulation of angiotensin AT2 and AT1 receptor 40 Inada Y et al. Protective effects of candesartan cilexetil mRNA in adult rat dorsal root ganglia and sciatic (TCV-116) against stroke, kidney dysfunction and car- nerves. Mol Brain Res 1998; 57: 111–122. diac hypertrophy in stroke-prone spontaneously

33 Lucius R et al. The angiotensin II type 2 (AT2) receptor hypertensive rats. Clin Exp Hypertens 1997; 19: promotes axonal regeneration in the optic nerve of 1079–1099.

adult rats. J Exp Med 1998; 188: 661–670. 41 Nishimura Y, Ito T, Saavedra JM. Angiotensin II AT1 34 von Lutterotti N et al. Angiotensin II receptor antagon- blockade normalizes cerebrovascular autoregulation ist delays renal damage and stroke in salt-loaded Dahl and reduces cerebral ischemia in spontaneously salt-sensitive rats. J Hypertens 1992; 10: 949–957. hypertensive rats. Stroke 2000; 31: 2478–2486. 35 Stier CT Jr, Adler LA, Levien S, Chandler PN. Stroke 42 Saavedra JM, Ito T, Nishimura Y. The role of angioten-

prevention by losartan in stroke-prone spontaneously sin II AT1 receptors in the regulation of the central hypertensive rats. J Hypertens 1993; 11 (Suppl): 537– blood ﬂow and brain ischaemia. JRAAS 2001; 2 (Suppl 542. 1): S102–S109.

36 Werner C et al. Captopril improves neurologic out- 43 Dai W-J et al. Blockade of central angiotensin AT1 come from incomplete cerebral ischemia in rats. improves neurological outcome and reduces Stroke 1991; 22: 910–914. expression of AP-1 transcription factors after focal 37 Sadoshima S et al. Angiotensin converting enzyme brain cerebral ischemia in rats. Stroke 1999; 30: inhibitors attenuate ischemic brain metabolism in 2391–2399. hypertensive rats. Stroke 1993; 24: 1561–1567. 44 Dirnagl U, Iadecola C, Moskowitz MA. Pathobiology 38 Paulson OB et al. Role of angiotensin in autoregulation of ischaemic stroke: an integrated review. Trends of cerebral blood ﬂow. Circulation 1988; 77: I55-I58. Neurosci 1999; 22: 391–397.

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