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Déjà vu: angiotensin and stress
Update
TRENDS in Endocrinology and Metabolism
Vol.14 No.6 August 2003
249
| Research Focus
De´ja` vu: angiotensin and stress Ferenc A. Antoni and Christopher J. Kenyon Division of Neuroscience and Molecular Endocrinology Unit, University of Edinburgh, Edinburgh, UK
In the 21st century we wish to believe that the interaction of our genetic background with the stress of life underlies most of our health problems, especially those associated with cardiovascular disease. Doubtless, there are many interrelated connections. A recent study provides evidence of a direct link between the neuroendocrine stress response and the renin–angiotensin – aldosterone system, the major blood-pressure control system of the body. Angiotensin is a hormone with a mission in mammals: its numerous actions in multiple organ systems are geared to retaining sufficient water and salt in the body to keep the systemic circulation of blood going. It works in concert with the hypothalamic neuropeptide vasopressin (antidiuretic hormone) and aldosterone, the major mineralocorticoid synthesized by the zona glomerulosa of the adrenal cortex. The regulation of these hormones is connected to the CNS through the neuroendocrine and autonomic nervous systems, and as such is responsible for one of the major woes of Western postindustrial society – hypertension. Recently published studies [1] have compared the levels of angiotensin (AT) receptor mRNAs (Box 1) in various organs of Wistar Kyoto rats (WKY) with those in spontaneously hypertensive rats (SHR). The main findings were higher levels of AT1A receptor mRNA in the pituitary and adrenal glands of SHR. In parallel, the plasma adrenocorticotrophic hormone (ACTH) and corticosterone responses to intra-arterial angiotensin II (AII; Box 2) were enhanced in the SHR, whereas the plasma levels of aldosterone were identical in the WKY and SHR animals. Because AT1A receptor mRNA is normally barely detectable in the pituitary gland, its appearance, accompanied by a reduction in AT1B receptor mRNA levels, is all the more remarkable. The implication of the data is that enhanced expression of AT1A receptors augments the stress response in SHR. Various methodological issues notwithstanding, these observations have several previous correlates and as such are a useful piece of the jigsaw puzzle that needs to be assembled for understanding the connection between behavioural traits, neuroendocrine control mechanisms and genetically determined hypertension. A fundamental question for the interpretation of such data is what constitutes a good control group for SHRs? Typically, WKY are used, but studies of the hypothalmus – pituitary–adrenal (HPA) axis of inbred rat strains [2–4] Corresponding author: F.A. Antoni ([email protected]). http://tem.trends.com
show that WKYanimals fall into the ‘stress hyporesponsive’ category and by comparison SHR show a tendency for an exaggerated HPA response. However, the response is not highly exaggerated and it is not consistent at all levels of the axis [2 – 4]. Indeed it is important to look carefully at the experimental variables used, the way the animals are brought to the experimenters’ laboratory, as well as the conditions of rearing. Data in the literature show that perinatal events could determine a lifetime ‘programme’ of developing hypertension [5,6] and abnormal pituitary – adrenal responsiveness [7]. By using immobilization and electric shocks to the tail as the stressor, it was concluded that the differences in HPA function in five inbred rat strains (including WKY as well as SHR) were largely manifested by the adrenal cortex [3,4]. Although in vitro studies of adrenocortical cells agree with this [8], monitoring of early-immediate genes as well as other stress-induced genes has indicated that responses in the hypothalamic paraventricular nuclei of SHR are greater than in WKY [9]. Moreover, behavioural analyses have claimed that WKY have traits indicative of mild depression, whereas SHR are ‘hyper’ – possibly a rat model of
Box 1. Angiotensin receptors Four angiotensin (AT) receptor genes have been isolated in rodents, AT1A, AT1B, AT2 and AT4. Although coded for by distinct genes, AT1A and AT1B are similar in function and, depending on the target tissue, jointly mediate many effects, including stimulation of aldosterone synthesis by the adrenal gland, thirst mechanisms in the brain, maintenance of vascular tone and regulation of renal function. AT2 receptors are distributed widely in the fetus but have a narrower distribution by adulthood (e.g. forebrain and adrenal medulla) and function broadly in opposition to AT1 receptors. AT1A receptors are highly expressed at vascular, renal and adrenocortical sites, whereas AT1B receptors feature predominantly in the pituitary and adrenal glands. Despite a common mechanism of action, an almost identical amino acid sequence and overlapping patterns of expression, AT1A and AT1B receptors do have subtly different properties. This is exemplified by the phenotypes of AT1A and AT1B null mice. In vivo and in vitro studies have shown that the synthetic glucocorticoid hormone dexamethasone reduces angiotensin receptor protein levels, an effect that in vitro studies suggest is because of a selective decrease in AT1B expression. This negative effect of dexamethasone contrasts markedly with effects on tissues that predominantly express the AT1A subtype. In cardiac and smooth muscle cells, AT1A, but not AT1B mRNA is increased by dexamethasone. The molecular basis for this differential response appears to depend on the activity of putative glucocorticoid response elements within the promoter of each gene. For more details of AT receptors, see Refs [17,18].
250
Update
TRENDS in Endocrinology and Metabolism
Box 2. Assay of pituitary –adrenocortical reactivity Jo¨ hren et al. [1] pre-treated rats with the synthetic glucocorticoid dexamethasone. Subsequently, anaesthesia was induced and maintained with a barbiturate, and a femoral artery was cannulated. An infusion of angiotensin II (AII) into the femoral artery was given over a 30 minute period. Dexamethasone works primarily, if not exclusively, at the pituitary level to inhibit adrenocorticotrophic hormone release. Barbiturates act as an anaesthetic and also suppress the activity of hypothalamic corticotropin releasing-factor (CRF) neurons by enhancing GABA-ergic inhibition. Ideally, one would want to know whether the animals were given oxygen because barbiturates tend to suppress breathing, and also whether body core temperature and systemic blood pressure were monitored and maintained. Assuming that AII was infused at the peak of dexamethasone inhibition, there are two main mechanisms by which it could stimulate pituitary adrenal activity; either by direct action on corticotropes in the adenohypophysis or by mobilization of CRF and vasopressin from the hypothalamus. In both cases, the inhibitory effect of dexamethasone could be overcome at the pituitary level [19].
human attention-deficit hyperactivity disorder [10]. It has been argued that the hyperactive and hypertensive phenotype of SHR versus the no hypertension and low activity of WKY are the extremes of a continuum of HPA responses [2]. Therefore, when investigating SHR versus WKY it is probably advisable to run another group of controls (preferably outbred Wistar rats) as the golden standard. Whatever the nature of the changes in the SHR, the results of Jo¨hren et al. suggest that when perturbed by a high dose of AII, these animals return a greater pituitary adrenocortical response than the WKY. This is attributed to altered pituitary AT1 expression rather than AT1-sensitive sites of glucocorticoid synthesis in the adrenal cortex [11], and implies that AII in the circulation contributes to regulation of the HPA axis in SHR but not in WKY animals. So, is there evidence that the antihypertensive therapies that target the renin – angiotensin system alter HPA function or stressrelated behaviour? The answer to this is no. However, only a few studies with a limited range of variables have been published [12,13]. Interestingly, the AT1 receptor antagonist candesartan did attenuate various parameters of isolation-induced stress in rats [14], supporting the thesis that AT1 receptors cause HPA activation. But given that actions mediated by AT2 receptors could antagonize AT1 actions [15], the effects of any antihypertensive therapy that suppressed the endogenous renin – angiotensin system would be difficult to predict. Similarly, an augmentation of the stress response by systemic AII might only be evident under conditions of AT1A receptor over-abundance, such as that observed in the SHR rats. Jo¨hren et al. have rekindled interest in the role of circulating AII in the HPA axis. Given the significance of angiotensin converting enzyme inhibitors and AT1
http://tem.trends.com
Vol.14 No.6 August 2003
receptor antagonists in clinical therapy [16], this topic should play an important role in the revitalization of integrative physiology in the post-genomic era. References 1 Jo¨hren, O. et al. (2003) Differential expression of AT1 receptors in the pituitary and adrenal gland of SHR and WKY. Hypertension 41, 984 – 990 2 Castanon, N. et al. (1993) Psychoneuroendocrine profile associated with hypertension or hyperactivity in spontaneously hypertensive rats. Am. J. Physiol. 265, R1304 – R1310 3 Gomez, F. et al. (1996) Hypothalamic– pituitary – adrenal response to chronic stress in five inbred rat strains: differential responses are mainly located at the adrenocortical level. Neuroendocrinology 63, 327 – 337 4 Gomez, F. et al. (1998) Glucocorticoid negative feedback on the HPA axis in five inbred rat strains. Am. J. Physiol. 274, R420– R427 5 Harrap, S.B. et al. (1990) Brief angiotensin converting enzyme inhibitor treatment in young spontaneously hypertensive rats reduces blood pressure long-term. Hypertension 16, 603 – 614 6 O’Regan, D. et al. (2001) Glucocorticoid programming of pituitary – adrenal function: mechanisms and physiological consequences. Semin. Neonatol. 6, 319– 329 7 Walker, C.D. (1995) Chemical sympathectomy and maternal separation affect neonatal stress responses and adrenal sensitivity to ACTH. Am. J. Physiol. 268, R1281 – R1288 8 Kenyon, C.J. et al. (1993) The role of glucocorticoid activity in the inheritance of hypertension: studies in the rat. J. Steroid Biochem. Mol. Biol. 45, 7 – 11 9 Imaki, T. et al. (1998) Stress-induced changes of gene expression in the paraventricular nucleus are enhanced in spontaneously hypertensive rats. J. Neuroendocrinol. 10, 635 – 643 10 King, J. et al. (2000) Early androgen treatment decreases cognitive function and catecholamine innervation in an animal model of ADHD. Behav. Brain Res. 107, 35 – 43 11 McEwan, P.E. et al. (1999) Control of adrenal cell proliferation by AT1 receptors in response to angiotensin II and low-sodium diet. Am. J. Physiol. 276, E303 – E309 12 Weiner, I. et al. (1994) The effects of chronic administration of ceronapril on the partial-reinforcement extinction effect and latent inhibition in rats. Behav. Pharmacol. 5, 306 – 314 13 Prickaerts, J. et al. (1996) Effects of myocardial infarction and captopril therapy on anxiety-related behaviors in the rat. Physiol. Behav. 60, 43 – 50 14 Armando, I. et al. (2001) Peripheral administration of an angiotensin II AT1 receptor antagonist decreases the hypothalamic – pituitary – adrenal response to isolation stress. Endocrinology 142, 3880 – 3889 15 Opie, L.H. and Sack, M.N. (2001) Enhanced angiotensin II activity in heart failure: reevaluation of the counterregulatory hypothesis of receptor subtypes. Circ. Res. 88, 654– 658 16 Hollenberg, N.K. (1992) Treatment of hypertension – the place of angiotensin-converting enzyme-inhibitors in the nineties. J. Cardiovasc. Pharmacol. 20, S29– S32 17 de Gasparo, M. et al. (2000) International union of pharmacology. XXIII. The angiotensin II receptors. Pharmacol. Rev. 52, 415 – 472 18 Berry, C. et al. (2001) Angiotensin receptors: signaling, vascular pathophysiology, and interactions with ceramide. Am. J. Physiol. Heart Circ. Physiol. 281, H2337 – H2365 19 Lim, M.C. et al. (2002) Post-translational modulation of glucocorticoid inhibition at the pituitary level. Endocrinology 143, 3796 – 3801 1043-2760/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1043-2760(03)00109-7
TRENDS in Endocrinology and Metabolism
Vol.14 No.6 August 2003
249
| Research Focus
De´ja` vu: angiotensin and stress Ferenc A. Antoni and Christopher J. Kenyon Division of Neuroscience and Molecular Endocrinology Unit, University of Edinburgh, Edinburgh, UK
In the 21st century we wish to believe that the interaction of our genetic background with the stress of life underlies most of our health problems, especially those associated with cardiovascular disease. Doubtless, there are many interrelated connections. A recent study provides evidence of a direct link between the neuroendocrine stress response and the renin–angiotensin – aldosterone system, the major blood-pressure control system of the body. Angiotensin is a hormone with a mission in mammals: its numerous actions in multiple organ systems are geared to retaining sufficient water and salt in the body to keep the systemic circulation of blood going. It works in concert with the hypothalamic neuropeptide vasopressin (antidiuretic hormone) and aldosterone, the major mineralocorticoid synthesized by the zona glomerulosa of the adrenal cortex. The regulation of these hormones is connected to the CNS through the neuroendocrine and autonomic nervous systems, and as such is responsible for one of the major woes of Western postindustrial society – hypertension. Recently published studies [1] have compared the levels of angiotensin (AT) receptor mRNAs (Box 1) in various organs of Wistar Kyoto rats (WKY) with those in spontaneously hypertensive rats (SHR). The main findings were higher levels of AT1A receptor mRNA in the pituitary and adrenal glands of SHR. In parallel, the plasma adrenocorticotrophic hormone (ACTH) and corticosterone responses to intra-arterial angiotensin II (AII; Box 2) were enhanced in the SHR, whereas the plasma levels of aldosterone were identical in the WKY and SHR animals. Because AT1A receptor mRNA is normally barely detectable in the pituitary gland, its appearance, accompanied by a reduction in AT1B receptor mRNA levels, is all the more remarkable. The implication of the data is that enhanced expression of AT1A receptors augments the stress response in SHR. Various methodological issues notwithstanding, these observations have several previous correlates and as such are a useful piece of the jigsaw puzzle that needs to be assembled for understanding the connection between behavioural traits, neuroendocrine control mechanisms and genetically determined hypertension. A fundamental question for the interpretation of such data is what constitutes a good control group for SHRs? Typically, WKY are used, but studies of the hypothalmus – pituitary–adrenal (HPA) axis of inbred rat strains [2–4] Corresponding author: F.A. Antoni ([email protected]). http://tem.trends.com
show that WKYanimals fall into the ‘stress hyporesponsive’ category and by comparison SHR show a tendency for an exaggerated HPA response. However, the response is not highly exaggerated and it is not consistent at all levels of the axis [2 – 4]. Indeed it is important to look carefully at the experimental variables used, the way the animals are brought to the experimenters’ laboratory, as well as the conditions of rearing. Data in the literature show that perinatal events could determine a lifetime ‘programme’ of developing hypertension [5,6] and abnormal pituitary – adrenal responsiveness [7]. By using immobilization and electric shocks to the tail as the stressor, it was concluded that the differences in HPA function in five inbred rat strains (including WKY as well as SHR) were largely manifested by the adrenal cortex [3,4]. Although in vitro studies of adrenocortical cells agree with this [8], monitoring of early-immediate genes as well as other stress-induced genes has indicated that responses in the hypothalamic paraventricular nuclei of SHR are greater than in WKY [9]. Moreover, behavioural analyses have claimed that WKY have traits indicative of mild depression, whereas SHR are ‘hyper’ – possibly a rat model of
Box 1. Angiotensin receptors Four angiotensin (AT) receptor genes have been isolated in rodents, AT1A, AT1B, AT2 and AT4. Although coded for by distinct genes, AT1A and AT1B are similar in function and, depending on the target tissue, jointly mediate many effects, including stimulation of aldosterone synthesis by the adrenal gland, thirst mechanisms in the brain, maintenance of vascular tone and regulation of renal function. AT2 receptors are distributed widely in the fetus but have a narrower distribution by adulthood (e.g. forebrain and adrenal medulla) and function broadly in opposition to AT1 receptors. AT1A receptors are highly expressed at vascular, renal and adrenocortical sites, whereas AT1B receptors feature predominantly in the pituitary and adrenal glands. Despite a common mechanism of action, an almost identical amino acid sequence and overlapping patterns of expression, AT1A and AT1B receptors do have subtly different properties. This is exemplified by the phenotypes of AT1A and AT1B null mice. In vivo and in vitro studies have shown that the synthetic glucocorticoid hormone dexamethasone reduces angiotensin receptor protein levels, an effect that in vitro studies suggest is because of a selective decrease in AT1B expression. This negative effect of dexamethasone contrasts markedly with effects on tissues that predominantly express the AT1A subtype. In cardiac and smooth muscle cells, AT1A, but not AT1B mRNA is increased by dexamethasone. The molecular basis for this differential response appears to depend on the activity of putative glucocorticoid response elements within the promoter of each gene. For more details of AT receptors, see Refs [17,18].
250
Update
TRENDS in Endocrinology and Metabolism
Box 2. Assay of pituitary –adrenocortical reactivity Jo¨ hren et al. [1] pre-treated rats with the synthetic glucocorticoid dexamethasone. Subsequently, anaesthesia was induced and maintained with a barbiturate, and a femoral artery was cannulated. An infusion of angiotensin II (AII) into the femoral artery was given over a 30 minute period. Dexamethasone works primarily, if not exclusively, at the pituitary level to inhibit adrenocorticotrophic hormone release. Barbiturates act as an anaesthetic and also suppress the activity of hypothalamic corticotropin releasing-factor (CRF) neurons by enhancing GABA-ergic inhibition. Ideally, one would want to know whether the animals were given oxygen because barbiturates tend to suppress breathing, and also whether body core temperature and systemic blood pressure were monitored and maintained. Assuming that AII was infused at the peak of dexamethasone inhibition, there are two main mechanisms by which it could stimulate pituitary adrenal activity; either by direct action on corticotropes in the adenohypophysis or by mobilization of CRF and vasopressin from the hypothalamus. In both cases, the inhibitory effect of dexamethasone could be overcome at the pituitary level [19].
human attention-deficit hyperactivity disorder [10]. It has been argued that the hyperactive and hypertensive phenotype of SHR versus the no hypertension and low activity of WKY are the extremes of a continuum of HPA responses [2]. Therefore, when investigating SHR versus WKY it is probably advisable to run another group of controls (preferably outbred Wistar rats) as the golden standard. Whatever the nature of the changes in the SHR, the results of Jo¨hren et al. suggest that when perturbed by a high dose of AII, these animals return a greater pituitary adrenocortical response than the WKY. This is attributed to altered pituitary AT1 expression rather than AT1-sensitive sites of glucocorticoid synthesis in the adrenal cortex [11], and implies that AII in the circulation contributes to regulation of the HPA axis in SHR but not in WKY animals. So, is there evidence that the antihypertensive therapies that target the renin – angiotensin system alter HPA function or stressrelated behaviour? The answer to this is no. However, only a few studies with a limited range of variables have been published [12,13]. Interestingly, the AT1 receptor antagonist candesartan did attenuate various parameters of isolation-induced stress in rats [14], supporting the thesis that AT1 receptors cause HPA activation. But given that actions mediated by AT2 receptors could antagonize AT1 actions [15], the effects of any antihypertensive therapy that suppressed the endogenous renin – angiotensin system would be difficult to predict. Similarly, an augmentation of the stress response by systemic AII might only be evident under conditions of AT1A receptor over-abundance, such as that observed in the SHR rats. Jo¨hren et al. have rekindled interest in the role of circulating AII in the HPA axis. Given the significance of angiotensin converting enzyme inhibitors and AT1
http://tem.trends.com
Vol.14 No.6 August 2003
receptor antagonists in clinical therapy [16], this topic should play an important role in the revitalization of integrative physiology in the post-genomic era. References 1 Jo¨hren, O. et al. (2003) Differential expression of AT1 receptors in the pituitary and adrenal gland of SHR and WKY. Hypertension 41, 984 – 990 2 Castanon, N. et al. (1993) Psychoneuroendocrine profile associated with hypertension or hyperactivity in spontaneously hypertensive rats. Am. J. Physiol. 265, R1304 – R1310 3 Gomez, F. et al. (1996) Hypothalamic– pituitary – adrenal response to chronic stress in five inbred rat strains: differential responses are mainly located at the adrenocortical level. Neuroendocrinology 63, 327 – 337 4 Gomez, F. et al. (1998) Glucocorticoid negative feedback on the HPA axis in five inbred rat strains. Am. J. Physiol. 274, R420– R427 5 Harrap, S.B. et al. (1990) Brief angiotensin converting enzyme inhibitor treatment in young spontaneously hypertensive rats reduces blood pressure long-term. Hypertension 16, 603 – 614 6 O’Regan, D. et al. (2001) Glucocorticoid programming of pituitary – adrenal function: mechanisms and physiological consequences. Semin. Neonatol. 6, 319– 329 7 Walker, C.D. (1995) Chemical sympathectomy and maternal separation affect neonatal stress responses and adrenal sensitivity to ACTH. Am. J. Physiol. 268, R1281 – R1288 8 Kenyon, C.J. et al. (1993) The role of glucocorticoid activity in the inheritance of hypertension: studies in the rat. J. Steroid Biochem. Mol. Biol. 45, 7 – 11 9 Imaki, T. et al. (1998) Stress-induced changes of gene expression in the paraventricular nucleus are enhanced in spontaneously hypertensive rats. J. Neuroendocrinol. 10, 635 – 643 10 King, J. et al. (2000) Early androgen treatment decreases cognitive function and catecholamine innervation in an animal model of ADHD. Behav. Brain Res. 107, 35 – 43 11 McEwan, P.E. et al. (1999) Control of adrenal cell proliferation by AT1 receptors in response to angiotensin II and low-sodium diet. Am. J. Physiol. 276, E303 – E309 12 Weiner, I. et al. (1994) The effects of chronic administration of ceronapril on the partial-reinforcement extinction effect and latent inhibition in rats. Behav. Pharmacol. 5, 306 – 314 13 Prickaerts, J. et al. (1996) Effects of myocardial infarction and captopril therapy on anxiety-related behaviors in the rat. Physiol. Behav. 60, 43 – 50 14 Armando, I. et al. (2001) Peripheral administration of an angiotensin II AT1 receptor antagonist decreases the hypothalamic – pituitary – adrenal response to isolation stress. Endocrinology 142, 3880 – 3889 15 Opie, L.H. and Sack, M.N. (2001) Enhanced angiotensin II activity in heart failure: reevaluation of the counterregulatory hypothesis of receptor subtypes. Circ. Res. 88, 654– 658 16 Hollenberg, N.K. (1992) Treatment of hypertension – the place of angiotensin-converting enzyme-inhibitors in the nineties. J. Cardiovasc. Pharmacol. 20, S29– S32 17 de Gasparo, M. et al. (2000) International union of pharmacology. XXIII. The angiotensin II receptors. Pharmacol. Rev. 52, 415 – 472 18 Berry, C. et al. (2001) Angiotensin receptors: signaling, vascular pathophysiology, and interactions with ceramide. Am. J. Physiol. Heart Circ. Physiol. 281, H2337 – H2365 19 Lim, M.C. et al. (2002) Post-translational modulation of glucocorticoid inhibition at the pituitary level. Endocrinology 143, 3796 – 3801 1043-2760/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1043-2760(03)00109-7