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

Conversion of brain angiotensin II to angiotensin III is critical for pressor response in rats

The present investigation measured the relative pressor potencies of intracerebroventricularly infused ANG II, ANG III, and the metabolically resistant analogs d-Asp(1)ANG II and d-Arg(1)ANG III in alert freely moving rats. The stability of these analogs was further facilitated by pretreatment with the specific aminopeptidase A inhibitor EC33 or the aminopeptidase N inhibitor PC18. The results indicate that the maximum elevations in mean arterial pressure (MAP) were very similar for each of these compounds across the dose range 1, 10, and 100 pmol/min during a 5-min infusion period. However, d-Asp(1)ANG II revealed significantly extended durations of pressor effects before return to base level MAP. Pretreatment intracerebroventricular infusion with EC33 blocked the pressor activity induced by the subsequent infusion of d-Asp(1)ANG II, whereas EC33 had no effect on the pressor response to subsequent infusion of d-Arg(1)ANG III. In contrast, pretreatment infusion with PC18 extended the duration of the d-Asp(1)ANG II pressor effect by about two to three times and the duration of d-Arg(1)ANG III's effect by approximately 10 to 15 times. Pretreatment with the specific AT(1) receptor antagonist losartan blocked the pressor responses induced by the subsequent infusion of both analogs indicating that they act via the AT(1) receptor subtype. These results suggest that the brain AT(1) receptor may be designed to preferentially respond to ANG III, and ANG III's importance as a centrally active ligand has been underestimated.

Subcellular identification of angiotensin I/II- and angiotensin II (AT1)-receptor-immunoreactivity in the central nervous system of rats

To gain insight into generating and transport mechanisms of angiotensin (ANG) in the brain the study was focused on the subcellular localization of ANG II and its AT(1)-receptors in the hypothalamus of rats. The present paper demonstrates ANG II- and AT(1)-receptor-immunolabelling at brain parenchyma vessels and at glial and neuronal structures in the perivascular region. Further, ANG II- and AT(1)-receptor-immunoreactivity is shown at plasma membranes and intracellular structures in the ependyma of the third ventricle. Based upon a conventional horseradish peroxidase technique, combined with the classical substrate 3,3'-diaminobenzidine, a procedure is introduced that will be useful with a variety of antibodies used on glutar- and paraformaldehyde-fixed brain tissue. This technique enables a fast correlation between light and electron microscopical results and might also provide an attractive alternative to colloidal gold-labelling and silver-intensification techniques.

Role of AT2 receptor in the brain in regulation of blood pressure and water intake

The effects of intracerebroventricular (ICV) injection of angiotensin II (ANG II) on blood pressure and water intake were examined with the use of ANG II receptor-deficient mice. ICV injection of ANG II increased systolic blood pressure in a dose-dependent manner in wild-type (WT) mice and ANG type 2 AT(2) receptor null (knockout) (AT(2)KO) mice; however, this increase was significantly greater in AT(2)KO mice than in WT mice. The pressor response to a central injection of ANG II in WT mice was inhibited by ICV preinjection of the selective AT(1) receptor blocker valsartan but exaggerated by the AT(2) receptor blocker PD-123319. ICV injection of ANG II also increased water intake. It was partly but significantly suppressed both in AT(2)KO and AT(1)aKO mice. Water intake in AT(2)/AT(1)aKO mice did not respond to ICV injection of ANG II. Both valsartan and PD-123319 partly inhibited water intake in WT mice. These results indicate an antagonistic action between central AT(1)a and AT(2) receptors in the regulation of blood pressure, but they act synergistically in the regulation of water intake induced by ANG II.

Local renin-angiotensin systems and their interactions with extra-adrenal corticosteroid production

Adrenal aldosterone production is regulated by the renin-angiotensin system (RAS). It is now known that several other tissues are capable of extra-adrenal aldosterone biosynthesis and that these tissues can also generate angiotensin II through local RAS. Therefore, the regulation of local aldosterone production by the local RAS is a distinct possibility. In this review, we present evidence for the existence of such systems in the vascular system, heart and brain. We then discuss the possibility of interactions between the RAS and aldosterone synthesis at the local level and speculate on the possible physiological effects of such systems in these tissues.

Distribution of apelin-synthesizing neurons in the adult rat brain

The peptide apelin originating from a larger precursor preproapelin molecule has been recently isolated and identified as the endogenous ligand of the human orphan G protein-coupled receptor, APJ (putative receptor protein related to the angiotensin receptor AT(1)). We have shown recently that apelin and apelin receptor mRNA are expressed in brain and that the centrally injected apelin fragment K17F (Lys(1)-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe(17)) decreased vasopressin release and altered drinking behavior. Using a specific polyclonal antiserum against K17F for immunohistochemistry, the aim of the present study was to establish the precise topographical distribution of apelin immunoreactivity in colchicine-treated adult rat brain. Immunoreactivity was essentially detected in neuronal cell bodies and fibers throughout the entire neuroaxis in different densities. Cells bodies have been visualized in the preoptic region, the hypothalamic supraoptic and paraventricular nuclei and in the highest density, in the arcuate nucleus. Apelin immunoreactive cell bodies were also seen in the pons and the medulla oblongata. Apelin nerve fibers appear more widely distributed than neuronal apelin cell bodies. The hypothalamus represented, by far, the major site of apelin-positive nerve fibers which were found in the suprachiasmatic, periventricular, dorsomedial, ventromedial nuclei and in the retrochiasmatic area, with the highest density in the internal layer of the median eminence. Fibers were also found innervating other circumventricular organs such as the vascular organ of the lamina terminalis, the subfornical and the subcommissural organs and the area postrema. Apelin was also detected in the septum and the amygdala and in high density in the paraventricular thalamic nucleus, the periaqueductal central gray matter and dorsal raphe nucleus, the parabrachial and Barrington nuclei in the pons and in the nucleus of the solitary tract, lateral reticular, prepositus hypoglossal and spinal trigeminal nuclei. The topographical distribution of apelinergic neurons in the brain suggests multiple roles for apelin especially in the central control of ingestive behaviors, pituitary hormone release and circadian rhythms.

Interaction between paraventricular nucleus and septal area in the control of physiological responses induced by angiotensin II

We determined the effects of losartan (40 nmol) and PD 123319 (40 nmol) (both non-peptides and selective antagonists of the AT1 and AT2 angiotensin receptors, respectively), and [Sar1, Ala8] angiotensin II (ANG II) (40 nmol) (a non-selective peptide antagonist of angiotensin receptors) injected into the paraventricular nucleus (PVN) on the water and salt appetite, diuresis and natriuresis and mean arterial pressure (MAP) induced by administration of 10 nmol of ANG II into the medial septal area (MSA) of male Holtzman rats weighing 250-300 g. The volume of drug solution injected was 0.5 micro l over a period of 10-15 s. The responses were measured over a period of 120 min. ANG II alone injected into the MSA induced an increase in all the above parameters (8.1 +/- 1.2, 1.8 +/- 0.3, and 17.1 +/- 1.0 ml, 217 +/- 25 micro Eq/120 min, and 24 +/- 4 mmHg, respectively, N = 10-12) compared with vehicle-treated rats (1.4 +/- 0.2, 0.6 +/- 0.1, and 9.3 +/- 0.5 ml, 47 +/- 5 micro Eq/120 min, and 4.1 +/- 0.8 mmHg, respectively, N = 10-14). Pretreatment with losartan and [Sar1, Ala8] ANG II completely abolished the water and sodium intake, and the pressor increase (0.5 +/- 0.2, 1.1 +/- 0.2, 0.5 +/- 0.2, and 0.8 +/- 0.2 ml, and 1.2 +/- 3.9, 31 +/- 4.6 mmHg, respectively, N = 9-12), whereas losartan blunted the urinary and sodium excretion induced by ANG II (13.9 +/- 1.0 ml and 187 +/- 10 micro Eq/120 min, respectively, N = 9). Pretreatment with PD 123319 and [Sar1, Ala8] ANG II blocked the urinary and sodium excretion (10.7 +/- 0.8, 9.8 +/- 0.7 ml, and 67 +/- 13 and 57 +/- 17 micro Eq/120 min, respectively, N = 9), whereas pretreatment with PD 123319 partially blocked the water and sodium intake, and the MAP induced by ANG II administration (2.3 +/- 0.3, 1.1 +/- 0.1 ml, and 12 +/- 3 mmHg, respectively, N = 9-10). These results suggest the angiotensinergic effect of the MSA on the AT1 and AT2 receptors of the PVN in terms of water and sodium homeostasis and MAP modulation.

Angiotensin and calcium signaling in the pituitary and hypothalamus

1) In the rat pituitary, angiotensin type 1B receptors (AT1B) are located in lactotrophs and corticotrophs. 2) Activation of AT1B receptors are coupled to Gq/11 (Guanine protein coupled receptor, or GPCR); they increase phospholipase beta C (PLC) activity resulting in inositol 1,4,5 triphosphate (InsP3) and diacylglycerol (DAG) formation. A biphasic increase in [Ca2+]i triggered by InsP3 and DAG ensues. 3) As many GPCRs, AT1B pituitary receptors rapidly desensitize. 4) This was observed in the generation of InsP3, the mobilization of intracellular Ca(2+), and in prolactin release. Both homologous and heterologous desensitization was evidenced. 5) Desensitization of the angiotensin II type 1 (AT1) receptor in the pituitary shares similarities and differences with endogenously expressed or transfected AT1 receptors in different cell types. 6) In the pituitary hyperplasia generated by chronic estrogen treatment there was desensitization or alteration in angiotensin II (Ang II) evoked intracellular Ca2+ increase, InsP3 generation, and prolactin release. This correlates with a downregulation of AT1 receptors. 7) In particular, in hyperplastic cells Ang II failed to evoke a transient acute peak in [Ca2+]i, which was replaced by a persistent plateau phase of [Ca2+]i increase. 8) Different calcium channels participate in Ang II induced [Ca2+]i increase in control and hyperplastic cells. While spike phase in control cells is dependent on intracellular stores sensitive to thapsigargin, in hyperplastic cells plateau increase is dependent on extracellular calcium influx. 9) Signal transduction of the AT1 pituitary receptor is greatly modified by hyperplasia, and it may be an important mechanism in the control of the hyperplastic process. 10) In the hypothalamus and brain stem there is a predominant expression of AT1A and AT2 mRNA. 11) Ang II acts at specific receptors located on neurons in the hypothalamus and brain stem to elicit alterations in blood pressure, fluid intake, and hormone secretion. 12) Calcium channels play important roles in the Ang II induced behavioral and endocrine responses. 13) Ang II, in physiological concentrations, can activate AT1 receptors to stimulate both Ca2+ release from intracellular stores and Ca2+ influx from the extracellular space to increase [Ca2+]i in polygonal and stellate astroglia of the hypothalamus and brain stem. 14) In primary cell culture of neurons from newborn rat hypothalamus and brain stem, it has also been determined that Ang II elicits an AT1 receptor mediated inhibition of delayed rectifier K(+) current and a stimulation of Ca2+ current. 15) In primary cell cultures derived from the subfornical organ or the organum vasculosum laminae terminalis of newborn rat pups, Ang II produced a pronounced desensitization of the [Ca2+]i response. 16) Hypothalamic and pituitary Ang II systems are involved in different functions, some of which are related. At both levels Ang II signals through [Ca2+]i in a characteristic way.

Angiotensin peptides as neurotransmitters/neuromodulators in the dorsomedial medulla

1. The present review provides an update on evidence of the neurotransmitter pathways and location of receptors within the nucleus tractus solitarii (NTS) mediating the baroreflex and other haemodynamic actions of angiotensin (Ang) II. 2. A series of studies suggests a significant role for substance P in the acute cardiovascular and carotid sinus chemoreceptor facilitatory actions of AngII in the NTS. The use of antisense oligonucleotides to AT1 receptors indicates both pre- and post-synaptic AngII receptors are likely to be involved in these actions. 3. With respect to baroreceptor reflex actions, it is clear that endogenous AngII impairs the gain for operation of the baroreceptor reflex, because AT1 receptor antagonists facilitate reflex function. This effect is either independent of substance P or involves inhibition of release. Moreover, initial data obtained using antisense oligonucleotides to AT1 receptors suggest that, in the NTS, the effect of endogenous AngII on the baroreceptor reflex is mainly due to presynaptic actions on vagal or carotid sinus afferent fibres. In contrast, the level of endogenous AngII within the NTS appears to have variable effects on activation of cardiopulmonary vagal afferent fibres by phenylbiguanide. These results indicate a divergence of effects of AngII on reflexes evoked by these two different types of sensory input. 4. Use of transgenic rats with alterations in brain angiotensin peptides allowed us to assess the effect of long-term alterations in brain Ang peptides on reflex function. We studied (mRen2)27 transgenic rats (TGR(mRen2)) with high brain medulla AngII levels and transgenic rats with angiotensinogen (Aogen) antisense linked to glial fibrillary acidic protein promoter (TGR(ASrAogen)) with greatly reduced brain Aogen. The reflex evoked by activation of cardiac vagal chemosensitive afferent fibres was enhanced in TGR(ASrAogen), whereas the baroreceptor reflex control of heart rate was attenuated in TGR(mRen2), further confirming a divergence of effects of AngII on these two sensory modalities. 5. The overall results are consistent with a sustained inhibitory effect of AngII on the baroreceptor reflexes, with dose-dependent or activation-dependent effects on cardiac vagal afferent fibre activation. Moreover, alterations in substance P pathways may contribute to the actions of AngII on reflex function.

Angiotensin AT1 receptor signalling pathways in neurons

1. The aim of the present article is to review the intracellular signal transduction pathways that are influenced by the peptide angiotensin (Ang) II, acting via its type 1 (AT1) receptor, in neurons. 2. The AT1 receptors couple to a wide variety of signalling pathways in peripheral tissues, such as kidney, heart and vascular smooth muscle. A similar diversity of signalling mechanisms exists for AT1 receptors in neurons. 3. We outline the known neuronal AT1 receptor signalling pathways as they relate to function. Pathways that couple activation of AT1 receptors to short-term changes in neuronal membrane ionic currents and firing rate will be reviewed. These are different from the pathways that elicit longer-term changes in enzyme activity and gene expression and, ultimately, increases in noradrenaline synthesis. 4. Novel AT1 receptor signalling pathways discovered through gene expression profiling and their potential functional significance have been discussed.

Proadrenomedullin-derived peptides in the paracrine control of the hypothalamo-pituitary-adrenal axis

Adrenomedullin (ADM) and proadrenomedullin N-terminal 20 peptide (PAMP) are widely distributed in various body tissues and organs, including the hypothalamo-pituitary-adrenal (HPA) axis. ADM and PAMP inhibit in vitro release of ACTH from pituitary corticotropes, and findings suggest that this effect may become relevant when an exceedingly high ACTH secretion must be counteracted. ADM directly supresses angiotensin-II- and K+-stimulated aldosterone secretion from ZG cells, acting through calcitonin gene-related peptide (CGRP) type 1 ADM(22-52)-sensitive receptors, the activation of which is likely to impair Ca2+ influx. In contrast, ADM stimulates medullary chromaffin cells to release catecholamines, which in turn enhance aldosterone secretion acting in a paracrine manner. Also this effect of ADM occurs via CGRP1 receptors, which are coupled with the adenylate cyclase-dependent cascade. There is indication that in vivo these two opposite effects of ADM on ZG may interact with each other when normal aldosterone secretion has to be restored. ADM exerts a mitogenic effect on rat ZG, acting via CGRP1 receptors that activate the tyrosine kinase-dependent mitogen-activated protein kinase cascade. These findings, along with the demonstration of a high level of ADM gene expression in adrenocortical adenomas and carcinomas, may suggest a role for ADM as adrenocortical growth stimulator and tumor promoter. PAMP, like ADM, suppresses aldosterone response of ZG cells to Ca2+-dependent agonists, but, in contrast with ADM, it inhibits catecholamine release by adrenal medulla. Both effects of PAMP are mediated by PAMP(12-20)-sensitive receptors, whose signaling mechanism is likely to involve the blockade of voltage-gated Ca2+ channels. The concentrations attained by ADM and PAMP in the blood rule out the possibility that they act as true circulating hormones. Conversely, their content in the hypothalamo-pituitary complex and adrenal gland is consistent with a paracrine mechanism of action, which may play an important role in pathophysiological conditions where the function of the HPA axis has to be reset.

Pituitary aminopeptidase activities involved in blood-pressure regulation are modified by dietary cholesterol: sex differences

Given that the existence of a local renin-angiotensin system (RAS) in the pituitary and its participation in the regulation of blood pressure and other biological functions are widely accepted, the aim of this work is to analyze the influence of dietary cholesterol on the activity of the enzymes involved in the metabolism of the effector peptides of the renin-angiotensin system (angiotensin II and III) and vasopressin, in the pituitary of male and female mice fed on a cholesterol-enriched diet (1% cholesterol and 0.5% cholic acid). Soluble and membrane-bound pituitary aminopeptidase A (aspartyl- and glutamyl-aminopeptidase), aminopeptidase M (alanyl-aminopeptidase), aminopeptidase B (arginyl-aminopeptidase) and cystinyl-aminopeptidase activities were fluorimetrically measured. In female mice, cholesterol-enriched diet produced a significant increase in soluble aspartyl- and membrane-bound aspartyl- and glutamyl-aminopeptidase activities, and a significant decrease in membrane-bound alanyl-, arginyl- and cystinyl-aminopeptidase activities. In male mice, after feeding the diet, a significant increase in soluble glutamyl- and membrane-bound arginyl-aminopeptidase activities was observed. Our results indicate differential effects of dietary cholesterol on the metabolism of angiotensin II and III and vasopressin in the pituitary of male and female mice.

Neural pathways from the lamina terminalis influencing cardiovascular and body fluid homeostasis

1. The lamina terminalis, a region of the brain with a high concentration of angiotensin AT1 receptors, consists of three distinct nuclei, the median preoptic nucleus, the subfornical organ and organum vasculosum of the lamina terminalis (OVLT). These latter two regions lack a blood-brain and detect changes in plasma angiotensin (Ang) II concentration and osmolality. 2. Efferent neural pathways from the lamina terminalis to the hypothalamic paraventricular and supraoptic nuclei mediate vasopressin secretion in response to plasma hypertonicity and increased circulating levels of AngII. 3. Studies using the neurotropic virus pseudorabies, which undergoes retrograde transynaptic neuronal transport following injection into peripheral sites, show that neurons in the lamina terminalis have efferent polysynaptic neural connections to the peripheral sympathetic nervous system. Some of these neurons have been shown to have polysynaptic connections to the kidney and to express AT1 receptor mRNA. We propose that circulating AngII acts at AT1 receptors in the subfornical organ and OVLT to influence the sympathetic nervous system. It is likely that the neural pathway subserving this influence involves a synapse in the hypothalamic paraventricular nucleus. 4. The lamina terminalis may exert an inhibitory osmoregulatory influence on renin secretion by the kidney. This osmoregulatory influence may be mediated by inhibition of renal sympathetic nerve activity and appears to involve a central angiotensinergic synapse. 5. The lamina terminalis exerts an osmoregulatory influence on renal sodium excretion that is independent of the renal nerves and is probably hormonally mediated.

Ontogeny of angiotensin II type 1 receptor mRNAs in fetal and neonatal rat brain

Studies have demonstrated a specific function of the angiotensin II (Ang II) type 1 receptor (AT(1)) in regulation of adult central cardiovascular, fluid, and pituitary hormone release and a predominant role of the renin-angiotensin system in fetal and neonatal cardiovascular homeostasis. The pattern of brain AT(1) mRNA expression during fetal and neonatal development is currently unknown. We used radiolabeled cRNA probes for in situ hybridization histochemistry to determine the ontogenic development of the two AT(1) subtypes (AT(1a) and AT(1b)) mRNA in rat brain, from 11 days of gestation (E11) to 28 days after birth (P28). No AT(1b) mRNA was detected in the developing brain, whereas AT(1a) mRNA was first detected at E19. The age at which AT(1a) mRNA is first detected varied among different brain areas and expression predominates in areas involved in fluid homeostasis, pituitary hormone release, and cardiovascular regulation, where it persists until P28. AT(1a) mRNA expression is present from E19 onward in the median preoptic nucleus, the vascular organ of the lamina terminalis, the paraventricular nucleus, the periaqueductal gray, the nucleus raphe pallidus, the motor facial nucleus, and very weakly in the nucleus of the solitary tract and the ambiguous nucleus, and at E21 in the subfornical organ, the anterior olfactory nucleus and the piriform cortex. AT(1a) mRNA expression is present after birth in many regions, including the preoptic and lateral hypothalamic areas, the area postrema and medullary reticular nuclei. In conclusion, during brain development, expression of AT(1a) mRNA, appears in late gestation at E19, predominantly in forebrain areas involved in fluid homeostasis and cardiovascular regulation. In contrast, AT(1a) mRNA expression is absent or present only in very small amounts until after birth in many medullary nuclei, known to play an important role in cardiovascular modulation. Our results suggest that, in perinatal life, AT(1a) is involved in fluid and perhaps cardiovascular homeostasis and that the role of Ang II in modulating medullary cardiovascular centers matures later in postnatal life.

Novel role of macrophage migration inhibitory factor in angiotensin II regulation of neuromodulation in rat brain

Previously we determined that angiotensin II (Ang II) activates neuronal AT(1) receptors, located in the hypothalamus and the brainstem, to stimulate noradrenergic pathways. To link Ang II to the regulation of norepinephrine metabolism in neurons cultured from newborn rat hypothalamus and brainstem we have used cDNA arrays for high throughput gene expression profiling. Of several genes that were regulated, we focused on macrophage migration inhibitory factor (MIF), which has been associated with the modulation of norepinephrine metabolism. In the presence of the selective AT(2) receptor antagonist PD123,319 (10 microM), incubation of cultures with Ang II (100 nM; 1-24 h) elicited an increase in MIF gene expression. Western immunoblots further revealed that Ang II (100 nM; 1-24 h) increased neuronal MIF protein expression. This effect was inhibited by the AT(1) receptor antagonist losartan (10 microM), the PLC inhibitor U-73122 (10 or 25 microM), the PKC inhibitor chelerythrine (10 microM), and the Ca(2+) chelator 1,2-bis-[2-aminophenoxy]-ethane-N,N,N',N'-tetraacetic acid tetrakis acetoxymethyl ester (10 microM). Taken together with our observation that MIF is expressed in the terminal fields of noradrenergic neurons (hypothalamus) and that Ang II increases the expression of MIF in this region in vivo, our data may suggest a novel role of Ang II in norepinephrine metabolism.

Chronic AT(1) receptor blockade alters autonomic balance and sympathetic responses in hypertension

In the coarctation hypertension model, we have shown that chronic treatment with losartan causes both normalization of impaired reflex control of heart rate and partial correction of the depressed aortic nerve activity/pressure relationship, even with the persistence of hypertension. In the present study, we analyzed the effects of angiotensin II blockade on the efferent pathways of coarcted and sham-operated groups treated chronically with vehicle or losartan (10 mg/kg per day PO). Hypertension was induced by subdiaphragmatic aortic coarctation, and the treatments lasted 9 days (4 control and 5 experimental days). On day 5, autoregressive power spectral analysis was performed on heart rate recordings made in conscious rats. Other groups were used for sympathetic splanchnic nerve activity recordings made simultaneously with pressure (anesthetized rats) at basal condition and during loading/unloading of baroreceptors. Losartan treatment induced a significant reduction in basal pressure but did not interfere with the development of hypertension (similar pressure increases of 24% and 28% over control values in losartan and vehicle groups, respectively). In vehicle-treated rats, establishment of hypertension was accompanied by a marked change in power spectral density from high- (1.19+/-0.06 Hz, 33+/-6%) to low-frequency components (0,42+/-0.03 Hz, 54+/-6%), with increased low-frequency-to-high-frequency ratio. When compared with sham-operated vehicle-treated rats, there was also increase in the gain of sympathetic activity/pressure relationship, with displacement of lower plateau toward high levels of sympathetic activity. No changes in the power spectral density and sympathetic activity/pressure relationship were observed when hypertension developed in the presence of chronic angiotensin type 1 (AT(1)) receptor blockade. The data suggest that angiotensin II, activated during the establishment of coarctation hypertension, acts via AT(1) receptors to alter sympathovagal balance, facilitating the sympathetic outflow to heart and peripheral circulation during baroreceptors unloading. Data also indicate that the observed effects are not conditioned by preexisting pressure levels.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Angiotensin receptor-like immunoreactivity in adult brain white matter astrocytes and oligodendrocytes

Most of the physiological effects of brain angiotensins are currently believed to be mediated by angiotensin receptors located principally on neurons. However, numerous studies in vitro have demonstrated the presence of functional angiotensin receptors on brain astrocytes, raising the possibility that glial cells may also participate in mediating the effects of the central renin-angiotensin system. Nevertheless, it is uncertain whether these cells in situ express angiotensin receptors, raising questions about the physiological significance of results observed in cell cultures. We have examined the distribution of angiotensin receptor-like immunoreactivity in glial cells in white matter tracts in the adult CNS, using a panel of antisera to the AT1 and AT2 angiotensin receptors. Antiserum preadsorption and/or Western blot demonstrated the specificity of the antisera in brain tissue. In immunohistochemical experiments, the AT1 antisera selectively labeled AT1-expressing neurons in the piriform cortex, whereas the AT2 antiserum stained cells in the trigeminal motor nucleus, these being nuclei known to express AT1 and AT2 receptors, respectively. Using double-label immunohistochemistry, we observed AT1- and AT2-immunoreactive astrocytes and oligodendrocytes in white matter tracts, which include the rat cerebellar white matter, periventricular white matter, and optic nerve, in addition to the bovine corpus callosum and human subcortical white matter. In contrast, astrocytes in the gray matter region of the cerebral cortex were not found to be angiotensin receptor-like immunoreactive. These results demonstrate the presence of AT1 and/or AT2 angiotensin receptor-like immunoreactivity in brain white matter macroglial cells in situ and support the idea that glial cells may play a more important role in the central renin-angiotensin system than previously thought.

Physiological role of a novel neuropeptide, apelin, and its receptor in the rat brain

Apelin, a peptide recently isolated from bovine stomach tissue extracts, has been identified as the endogenous ligand of the human orphan APJ receptor. We established a stable Chinese hamster ovary (CHO) cell line expressing a gene encoding the rat apelin receptor fused to the enhanced green fluorescent protein, to investigate internalization and the pharmacological profile of the apelin receptor. Stimulation of this receptor by the apelin fragments K17F (Lys1-Phe-Arg-Arg-Gln-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) and pE13F (pGlu5-Arg-Pro-Arg-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) resulted in a dose-dependent inhibition of forskolin-induced cAMP production and promoted its internalization. In contrast, the apelin fragments R10F (Arg8-Leu-Ser-His-Lys-Gly-Pro-Met-Pro-Phe17) and G5F (Gly13-Pro-Met-Pro-Phe17) were inactive. The physiological role of apelin and its receptor was then investigated by showing for the first time in rodent brain: (i) detection of apelin neurons in the supraoptic and paraventricular nuclei by immunohistochemistry with a specific polyclonal anti-apelin K17F antibody; (ii) detection of apelin receptor mRNA in supraoptic vasopressinergic neurons by in situ hybridization and immunohistochemistry; and (iii) a decrease in vasopressin release following intracerebroventricular injection of K17F, or pE13F, but not R10F. Thus, apelin locally synthesized in the supraoptic nucleus could exert a direct inhibitory action on vasopressinergic neuron activity via the apelin receptors synthesized in these cells. Furthermore, central injection of pE13F significantly decreased water intake in dehydrated normotensive rats but did not affect blood pressure. Together, these results suggest that neuronal apelin plays an important role in the central control of body fluid homeostasis.

Nav2/NaG channel is involved in control of salt-intake behavior in the CNS

Na(v)2/NaG is a putative sodium channel, whose physiological role has long been an enigma. We generated Na(v)2 gene-deficient mice by inserting the lacZ gene. Analysis of the targeted mice allowed us to identify Na(v)2-producing cells by examining the lacZ expression. Besides in the lung, heart, dorsal root ganglia, and Schwann cells in the peripheral nervous system, Na(v)2 was expressed in neurons and ependymal cells in restricted areas of the CNS, particularly in the circumventricular organs, which are involved in body-fluid homeostasis. Under water-depleted conditions, c-fos expression was markedly elevated in neurons in the subfornical organ and organum vasculosum laminae terminalis compared with wild-type animals, suggesting a hyperactive state in the Na(v)2-null mice. Moreover, the null mutants showed abnormal intakes of hypertonic saline under both water- and salt-depleted conditions. These findings suggest that the Na(v)2 channel plays an important role in the central sensing of body-fluid sodium level and regulation of salt intake behavior.

Peripheral and central interactions between the renin-angiotensin system and the renal sympathetic nerves in control of renal function

Increases in renal sympathetic nerve activity (RSNA) regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. As increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between RSNA and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, that is, the direct (via specific innervation) and indirect (via angiotensin II) contributions of increased RSNA to the regulation of renal function. The effects of increased RSNA on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with angiotensin-converting enzyme inhibitors or angiotensin II-type AT1 receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated following renal denervation. These interactions can also be extrarenal, that is, in the central nervous system, wherein RSNA and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, the permeable blood-brain barrier of which permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II type AT1 receptor antagonists, into the ventricular system or microinjected into the rostral ventrolateral medulla, are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (e.g., congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and RSNA are involved in influencing the neural control of renal function.

Angiotensin II induces inward currents in subfornical organ neurones of rats

The action of angiotensin II on subfornical organ (SFO) neurones was studied using whole-cell current and voltage-clamp recordings in rat slice preparations. In the current-clamp mode, membrane depolarization in response to angiotensin II was accompanied by an increased frequency of action potentials and an increased membrane conductance. In the voltage-clamp mode, angiotensin II elicited inward currents in a dose-dependent manner. The net angiotensin II-induced inward currents were voltage-independent, with a mean reversal potential of -29.8 +/- 6.2 mV. Amplitudes of the angiotensin II-induced inward currents were decreased during perfusion with a low sodium medium. The angiotensin II-induced inward currents were blocked by the AT1 antagonist losartan, and were partially blocked by the AT2 antagonist PD-123319. Neurones which were sensitive to angiotensin II were found in the peripheral region of the SFO, whereas neurones in the central region were less sensitive to angiotensin II. These results suggest that angiotensin II induces inward currents, with opening of nonselective cation channels through mainly AT1 receptors in a subpopulation of SFO neurones of rats.

Brain angiotensin and body fluid homeostasis

Angiotensinogen, the precursor molecule of the peptides angiotensin I, II, and III, is synthesized in the brain and the liver. Evidence is reviewed that angiotensin II, and possibly angiotensin III, that are generated within the brain act within neural circuits of the central nervous system to regulate body fluid balance. Immunohistochemical studies in the rat brain have provided evidence of angiotensin-containing neurons, especially in the hypothalamic paraventricular nucleus, subfornical organ, periventricular region, and nucleus of the solitary tract, as well as in extensive angiotensin-containing fiber pathways. Angiotensin immunoreactivity is observed by electron microscope in synaptic vesicles in several brain regions, the most prominent of these being the central nucleus of the amygdala. Neurons in many parts of the brain (lamina terminalis, paraventricular and parabrachial nuclei, ventrolateral medulla, and nucleus of the solitary tract) known to be involved in the regulation of body fluid homeostasis exhibit angiotensin receptors of the AT(1) subtype. Pharmacological studies in several species show that intracerebroventricular administration of AT(1) receptor antagonist drugs inhibit homeostatic responses to the central administration of hypertonic saline, intravenous infusion of the hormone relaxin, or thermal dehydration. Responses affected by centrally administered AT(1) antagonists are water drinking, vasopressin secretion, natriuresis, increased arterial pressure, reduced renal renin release, salt hunger, and thermoregulatory adjustments. We conclude that angiotensinergic neural pathways in the brain probably have an important homeostatic function, especially in regard to osmoregulation and thermoregulation, and the maintenance of arterial pressure.

Fos protein is required for the re-expression of angiotensin II type 1 receptors in the nucleus tractus solitarii after baroreceptor activation in the rat

We evaluated in Sprague--Dawley rats the hypothesis that Fos protein induced by baroreceptor activation in the nucleus tractus solitarii participates in transcriptional regulation of the expression of angiotensin receptor genes. Reverse transcription-polymerase chain reaction revealed that baroreceptor activation elicited by sustained hypertension resulted in a transient decrease in angiotensin II subtype 1, but not subtype 2, receptor messenger RNA, in the dorsomedial medulla, including the nucleus tractus solitarii. There was subsequently a transitory reduction in the pressor response elicited by microinjection bilaterally of angiotensin II (40 pmol) into the nucleus tractus solitarii, followed by an increase in c-fos messenger RNA and Fos immunoreactivity at the same nucleus. Both the re-expression of angiotensin II subtype 1 receptor messenger RNA and restoration of pressor response to angiotensin II after baroreceptor activation were significantly blunted by bilateral application into the nucleus tractus solitarii of an antisense oligonucleotide (50 pmol) that targets against the initiation codon of c-fos messenger RNA. Control pretreatment with the corresponding sense oligonucleotide (50 pmol), or an antisense c-fos oligonucleotide that targets against a different portion of the coding sequence of the c-fos messenger RNA (50 pmol), was ineffective. At the receptor level, the angiotensin II-induced pressor response was antagonized by the subtype 1 receptor antagonist losartan (1.6 nmol), but not by the subtype 2 receptor antagonist PD-123319 (1.6 nmol). These findings suggest that sustained hypertension down-regulates angiotensin II subtype 1 receptors at both messenger RNA and functional expression levels in the nucleus tractus solitarii. Furthermore, Fos protein induced in the nucleus tractus solitarii by baroreceptor activation may play a permissive role in the transcriptional regulation of the re-expression of this subtype of angiotensin receptors.

Identification of efferent neural pathways from the lamina terminalis activated by blood-borne relaxin

The ovarian hormone relaxin, in addition to its role in pregnancy, exerts an action on the brain to influence oxytocin and vasopressin secretion, water drinking, and cardiovascular function. Intravenous (i.v.) infusion of relaxin causes an acute water drinking response, confirming its role as a dipsogenic hormone. The aim of this study was to determine whether neurones in the lamina terminalis, which project to the hypothalamic paraventricular and supraoptic nuclei, are activated by elevated levels of circulating relaxin in conscious rats. Immunocytochemistry combined with retrograde neuronal tracing with cholera toxin B subunit conjugated to cholera toxin B (CTB-gold) was used to identify populations of neurones responding with elevated cells of Fos protein to i.v. relaxin administration and which project to these specific hypothalamic sites. Neurones exhibiting Fos were present in the outer parts of the subfornical organ (SFO), the dorsal part of the organum vasculosum (OVLT), the supraoptic nucleus and the paraventricular nucleus. These did not occur in control rats with i.v. infusions of isotonic saline. Approximately 90% of neurones concentrated in the outer parts of the SFO and in the dorsal OVLT showed both retrogradely transported CTB-gold and Fos in response to i.v. infusion of relaxin. These data support a role for relaxin acting on the brain to regulate body fluid and electrolyte homeostasis by activating neural pathways subserving water drinking, vasopressin and oxytocin secretion.

Angiotensin III: a central regulator of vasopressin release and blood pressure

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for type 1 and type 2 AngII receptors. Both peptides, injected intracerebroventricularly, cause similar increases in vasopressin release and blood pressure. Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarizes new insights into the predominant role of brain AngIII in the control of vasopressin release and blood pressure and underlines the fact that brain aminopeptidase A, the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension.

Tonic suppression of spontaneous baroreceptor reflex by endogenous angiotensins via AT(2) subtype receptors at nucleus reticularis ventrolateralis in the rat

We evaluated the role of endogenous angiotensins at the rostral nucleus reticularis ventrolateralis (NRVL) in the modulation of spontaneous baroreceptor reflex (BRR) response and the subtype of angiotensin receptors involved using rats anesthetized and maintained with pentobarbital sodium. Bilateral microinjection of angiotensin II (ANG II) or its active metabolite angiotensin III (ANG III) (5, 10, or 20 pmol) into the NRVL significantly suppressed the spontaneous BRR response, as represented by the magnitude of transfer function between systemic arterial pressure and heart rate signals. The inhibitory effect of ANG III (20 pmol) was discernibly reversed by coadministration with its peptide antagonist, [Ile(7)]ANG III (1.6 nmol), or the nonpeptide AT(2) receptor antagonist, PD-123319 (1.6 nmol), but not by the nonpeptide AT(1) receptor antagonist, losartan (1.6 nmol). On the other hand, the peptide antagonist, [Sar(1), Ile(8)]ANG II (1.6 nmol) or both non-peptide antagonists appreciably reversed the suppressive action of ANG II (20 pmol). Whereas losartan produced minimal effect, blocking the endogenous activity of the angiotensins by microinjection into the bilateral NRVL of PD-123319, [Sar(1), Ile(8)]ANG II or [Ile(7)]ANG III elicited significant enhancement of the spontaneous BRR response. We conclude that under physiologic conditions both endogenous ANG II and ANG III may exert a tonic inhibitory modulation on the spontaneous BRR response by acting selectively on the AT(2) subtype receptors at the NRVL.

The peripheral renin-angiotensin system is not involved in the hypertension of sheep exposed to prenatal dexamethasone

1. Fetal exposure to an adverse intrauterine environment has been linked with cardiovascular and metabolic disease later in life. We have shown previously, in sheep, that brief exposure (48 h) to maternally administered dexamethasone (0.28 mg/kg per day) at 27 days of gestation (prenatal treatment group (PTG) 1; term approximately 150 days), but not at 64 days of gestation (PTG2), produced hypertensive offspring at 40 months of age. The present study aimed to determine whether the elevated blood pressure in these sheep was associated with an altered peripheral renin-angiotensin system (RAS). 2. Measurements of the basal levels of the RAS components (renin, angiotensinogen, angiotensin (Ang) I, angiotensin- converting enzyme (ACE), AngII and Ang-(1-7)) were made. In addition, we studied the effect of a peripherally administered AngII type 1 (AT1) receptor antagonist (irbesartan at 1.02 mg/kg per h) on mean arterial pressure (MAP) over 4.5 h. 3. There was no significant difference in basal plasma concentrations of the components of the RAS measured between control (n = 7) and PTG1 (n = 5) or PTG2 (n = 6) animals. The MAP in PTG1 was significantly higher than in the control group during both vehicle infusion and AT1 receptor blockade. The effect of 4.5 h irbesartan (1.02 mg/kg per h) infusion on blood pressure was similar between the groups. 4. In conclusion, intrauterine exposure for 48 h to maternally administered dexamethasone at 27 days of gestation caused elevated blood pressure in adult sheep that does not appear to be associated with an alteration in the peripheral RAS.

Brain angiotensin receptors and sympathoadrenal regulation during insulin-induced hypoglycemia

Simultaneous blockade of systemic AT1 and AT2 receptors or converting enzyme inhibition (CEI) attenuates the hypoglycemia-induced reflex increase of epinephrine (Epi). To examine the role of brain AT1 and AT2 receptors in the reflex regulation of Epi release, we measured catecholamines, hemodynamics, and renin during insulin-induced hypoglycemia in conscious rats pretreated intracerebroventricularly with losartan, PD-123319, losartan and PD-123319, or vehicle. Epi and norepinephrine (NE) increased 60-and 3-fold, respectively. However, the gain of the reflex increase in plasma Epi (Deltaplasma Epi/Deltaplasma glucose) and the overall Epi and NE responses were similar in all groups. The ensuing blood pressure response was similar between groups, but the corresponding bradycardia was augmented after PD-123319 (P < 0.05 vs. vehicle) or combined losartan and PD-123319 (P < 0.01 vs. vehicle). The findings indicate 1) brain angiotensin receptors are not essential for the reflex regulation of Epi release during hypoglycemia and 2) the gain of baroreceptor-mediated bradycardia is increased by blockade of brain AT2 receptors in this model.

Sodium intake influences hemodynamic and neural responses to angiotensin receptor blockade in rostral ventrolateral medulla

To determine the effects of physiological alterations in endogenous angiotensin II activity on basal renal sympathetic nerve activity (RSNA) and its arterial baroreflex regulation, angiotensin II type 1 receptor antagonists were microinjected into the rostral ventrolateral medulla of anesthetized rats consuming a low, normal, or high sodium diet that were instrumented for simultaneous measurement of arterial pressure and RSNA. Plasma renin activity was increased in rats fed a low sodium diet and decreased in those fed a high sodium diet. Losartan (50, 100, and 200 pmol) decreased heart rate and RSNA (but not mean arterial pressure) dose-dependently; the responses were significantly greater in rats fed a low sodium diet than in those fed a high sodium diet. Candesartan (1, 2, and 10 pmol) decreased mean arterial pressure, heart rate, and RSNA dose-dependently; the responses were significantly greater in rats fed a low sodium diet than in those fed a normal or high sodium diet. [D-Ala(7)]Angiotensin-(1-7) (100, 200, and 1000 pmol) did not affect mean arterial pressure, heart rate, or RSNA in rats fed either a low or a high sodium diet. In rats fed a low sodium diet, candesartan reset the arterial baroreflex control of RSNA to a lower level of arterial pressure, and in rats with congestive heart failure, candesartan increased the arterial baroreflex gain of RSNA. Physiological alterations in the endogenous activity of the renin-angiotensin system influence the bradycardic, vasodepressor, and renal sympathoinhibitory responses to rostral ventrolateral medulla injection of antagonists to angiotensin II type 1 receptors but not to angiotensin-(1-7) receptors.

Central inhibition of AT1receptors by eprosartan--in vitro autoradiography in the brain

All components of the renin-angiotensin system have been demonstrated in the brain and AT1 receptors have been localized in brain areas involved in central cardiovascular regulation. It is currently unclear whether AT1 receptor antagonists, which are increasingly used in the treatment of arterial hypertension and chronic heart failure, have the potential to mediate action via the central renin-angiotensin system. Therefore, we tested the in vivo access of the non-peptide AT1 receptor antagonist, eprosartan (30 and 60 mg per kg of body weight (BW) for 4 weeks, i.p. administered by osmotic minipumps), to angiotensin II receptors in the rat brain by in vitro autoradiography with 125I- (Sar1- Ile8) angiotensin II as a ligand. Eprosartan significantly increased plasma renin activity by four-fold and six-fold at doses of 30 and 60 mg x kg(-1), respectively (P< 0.05 vs CTRL). In the brain, eprosartan produced a dose-dependent inhibition of AT receptor binding in the median cerebral artery ( 850 +/- 249 and 650 +/- 106 vs 1072 +/- 116 dpm x mm(-2) of CTRL; P< 0.05). Furthermore, eprosartan inhibited angiotensin II receptor binding in discrete brain areas, which express exclusively, or predominantly, AT1 receptors both outside and within the blood-brain barrier, such as the paraventricular nucleus ( 180 +/- 47 and 130 +/- 18 vs 545 +/- 99 dpm x mm(-2)of CTRL; P< 0.05), the subfornical organ ( 106 +/- 26 and 112 +/- 17 vs 619 +/- 256 dpm x mm(-2)of CTRL; P< 0.05), and the organum vasculosum laminae terminalis ( 461 +/- 110 and 763 +/- 136 vs 1033 +/- 123 dpmx mm(-2)of CTRL; P< 0.05). These results emphasize that eprosartan readily crosses the blood-brain barrier in vivo and selectively inhibits binding to AT1 receptors in specific brain nuclei. The modulation of central regulatory mechanisms might contribute to AT1 receptor antagonists overall therapeutic efficacy in cardiovascular disease.

Gene expression profiling of rat brain neurons reveals angiotensin II-induced regulation of calmodulin and synapsin I: possible role in neuromodulation

Angiotensin (Ang II) activates neuronal AT(1) receptors located in the hypothalamus and the brainstem and stimulates noradrenergic neurons that are involved in the control of blood pressure and fluid intake. In this study we used complementary DNA microarrays for high throughput gene expression profiling to reveal unique genes that are linked to the neuromodulatory actions of Ang II in neuronal cultures from newborn rat hypothalamus and brainstem. Of several genes that were regulated, we focused on calmodulin and synapsin I. Ang II (100 nM; 1-24 h) elicited respective increases and decreases in the levels of calmodulin and synapsin I messenger RNAs, effects mediated by AT(1) receptors. This was associated with similar changes in calmodulin and synapsin protein expression. The actions of Ang II on calmodulin expression involve an intracellular pathway that includes activation of phospholipase C, increased intracellular calcium, and stimulation of protein kinase C. Taken together with studies that link calmodulin and synapsin I to axonal transport and exocytotic processes, the data suggest that Ang II regulates these two proteins via a Ca(2+)-dependent pathway, and that this may contribute to longer term or slower neuromodulatory actions of this peptide.

Differentiation of brain angiotensin type 1a and 1b receptor mRNAs: A specific effect of dehydration

The objective was to examine the effect of dehydration on the expression of the angiotensin type 1 (AT(1)) receptor subtype mRNAs in mice by using an in situ hybridization method. The method used free-floating brain sections with (35)S-labeled probes specific for the untranslated 5' (AT(1a)) and 3' (AT(1b)) regions. AT(1a) and AT(1b) mRNA levels in the subfornical organ (SFO) and anterior third ventricle (AV3V) were quantified by using a phosphor-imaging system. Emulsion autoradiography with cresyl violet counterstaining was used to show cellular expression. Adult male C57BL mice (25 to 30 g) were given water ad libitum or were deprived of water for 48 hours. Dehydration produced increases in plasma osmolality (349+/-6 versus 314+/-4 mOsm/kg) and hematocrit (58+/-2% versus 47+/-1%). In situ hybridization showed that there was expression of AT(1a) and of AT(1b) mRNA in SFO and AV3V. Dehydration produced an increase in AT(1a) mRNA in both regions, with no changes noted for AT(1b). AT(1a) mRNA was increased in the AV3V region from 0.3+/-0.2 to 0.7+/-0.2 muCi/g and in the SFO from 0.6+/-0.3 to 1.0+/-0.2 muCi/g. These results provide information regarding the localization and physiological importance of a subset of angiotensin receptors that are important in volume and blood pressure regulation. AT(1a) and AT(1b) mRNAs showed a similar pattern of expression in rostral forebrain osmosensitive regions. However, osmotic/volume stimulation with dehydration produced specific activation of AT(1a) receptors. This verifies the role of AT(1a) receptors in volume regulation but raises a question concerning the physiological role of the AT(1b) subtype.

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.

Glial angiotensinogen regulates brain angiotensin II receptors in transgenic rats TGR(ASrAOGEN)

TGR(ASrAOGEN)680, a newly developed transgenic rat line with specific downregulation of astroglial synthesis of angiotensinogen, exhibits decreased brain angiotensinogen content associated with a mild diabetes insipidus and lower blood pressure. Autoradiographic experiments were performed on TGR(ASrAOGEN) (TG) and Sprague-Dawley (SD) control rats to quantify AT(1) and AT(2) receptor-binding sites in different brain nuclei and circumventricular organs. Dose-response curves for drinking response to intracerebroventricular injections of ANG II were compared between SD and TG rats. In most of the regions inside the blood-brain barrier [paraventricular nucleus (PVN), piriform cortex, lateral olfactory tract (LOT), and lateral preoptic area (LPO)], AT(1) receptor binding (sensitive to CV-11974) was significantly higher in TG compared with SD. In contrast, in the circumventricular organs investigated [subfornical organ (SFO) and area postrema], AT(1) receptor binding was significantly lower in TG. AT(2) receptors (binding sensitive to PD-123319) were detected at similar levels in the inferior olive (IO) of both strains. Angiotensin-binding sites sensitive to both CV-11974 and PD-123319 were detected in the LPO of SD rats and specifically upregulated in LOT, IO, and most notably PVN and SFO of TG. The dose-response curve for water intake after intracerebroventricular injections showed a higher sensitivity to ANG II of TG (EC(50) = 3.1 ng) compared with SD (EC(50) = 11.2 ng), strongly suggesting that the upregulation of AT(1) receptors inside the blood-brain barrier of TG rats is functional. Finally, we showed that downregulation of angiotensinogen synthesized by astroglial cells differentially regulates angiotensin receptor subtypes inside the brain and in circumventricular organs.

Nervous kidney. Interaction between renal sympathetic nerves and the renin-angiotensin system in the control of renal function

Increases in renal sympathetic nerve activity regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. Because increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between the renal sympathetic nerves and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, for example, the direct (by specific innervation) and indirect (by angiotensin II) contributions of increased renal sympathetic nerve activity to the regulation of renal function. The effects of increased renal sympathetic nerve activity on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with ACE inhibitors or angiotensin II-type AT(1)-receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated after renal denervation. These interactions can also be extrarenal, for example, in the central nervous system, wherein renal sympathetic nerve activity and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, whose permeable blood-brain barrier permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II-type AT(1)-receptor antagonists into the ventricular system or microinjected into the rostral ventrolateral medulla are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (eg, congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and renal sympathetic nerve activity are involved in influencing the neural control of renal function.

AT-1 receptor and phospholipase C are involved in angiotensin III modulation of hypothalamic noradrenergic transmission

1. We previously reported that angiotensin III modulates noradrenergic neurotransmission in the hypothalamus of the rat. In the present work we studied the effects of angiotensin III on norepinephrine release and tyrosine hydroxylase activity. We also investigated the receptors and intracellular pathways involved in angiotensin III modulation of noradrenergic transmission. 2. In rat hypothalamic tissue labeled with [3H]norepinephrine 1, 10, and 100 nM and 1 microM losartan (AT1 receptor antagonist) had no effect on basal neuronal norepinephrine release, whereas 10 and 100 nM and 1 microM losartan partially diminished norepinephrine secretion evoked by 25 mM KCl. The AT2 receptor antagonist PD 123319 showed no effect either on basal or evoked norepinephrine release. The increase in both basal and evoked norepinephrine output induced by 1 microM angiotensin III was blocked by 1 microM losartan, but not by 1 microM PD 123319. 3. The phospholipase C inhibitor 5 microM neomicin inhibited the increase in basal and evoked norepinephrine release produced by 1 microM angiotensin III. 4. Tyrosine hydroxylase activity was increased by 1 microM angiotensin III and this effect was blocked by 1 microM LST and 5 microM neomicin, but not by PD 123319. On the other hand, 1 microM angiotensin III enhanced phosphatidyl inositol hydrolysis that was blocked by 1 microM losartan and 5 microM neomicin. PD 123319 (1 microM) did not affect ANG III-induced phosphatidyl inositol hydrolysis enhancement. 5. Our results confirm that angiotensin III acts as a modulator of noradrenergic transmission at the hypothalamic level through the AT1-phospholipase C pathway. This enhancement of hypothalamic noradrenergic activity suggests that angiotensin III may act as a central modulator of several biological processes regulated at this level by catecholamines, such as cardiovascular, endocrine, and autonomic functions as well as water and saline homeostasis.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

KA1-like kainate receptor subunit immunoreactivity in neurons and glia using a novel anti-peptide antibody

Functional kainate receptors can be formed by various combinations of subunits with low (GluR5, GluR6 and GluR7) or high affinity (KA1 and KA2) for kainate. The precise contribution of each subunit to native receptors, as well as their distribution within the central nervous system (CNS) is still unclear. Here, we describe the presence of KA1-like immunoreactivity in both neurons and glial cells of the CNS, using a newly developed antiserum to a specific carboxy terminus epitope of the KA1 subunit. Intense immunoreactivity was observed in the CA3 area of the rat hippocampus. Electron microscopy revealed that immunostaining was present in dendritic structures postsynaptic to commissural-associational fibers, rather than in those contacted by mossy fiber terminals. We also observed immunostaining of CA1 pyramidal cell apical dendrites. In the cerebral cortex, KA1-like immunostaining was observed in many pyramidal neuron somata, mainly in layer V, and along their apical dendrites. A subset of gamma-amino-butyric acidic cells were also intensely stained. In the cerebellum, the antiserum selectively stained Purkinje cell somata and their dendrites as well as Bergmann glial processes. Other types of macroglia were also labeled by the KA1 antiserum. Thus, optic nerve oligodendrocytes both in vitro and in situ and cultured astrocytes were densely stained. Our results indicate that KA1-type subunits are more widely distributed throughout the CNS than previously thought. This newly developed antiserum may help to clarify the properties of kainate receptors containing KA1 or KA1-type subunits within the normal and pathological brain.

Expression of the genes encoding the vasopressin-activated calcium-mobilizing receptor and the dual angiotensin II/vasopressin receptor in the rat central nervous system

The distributions of two newly discovered receptors, the vasopressin-activated calcium-mobilizing receptor (VACM-1) and the dual angiotensin II/vasopressin receptor (AII/AVP), in the central nervous system (CNS) of the rat were determined using reverse transcriptase-polymerase chain reaction and in situ hybridization. The sequence of the rat VACM-1 cDNA was determined and found very homologous to the rabbit and human sequences. Both VACM-1 and AII/AVP receptor genes were widely expressed in the brain, but differed according to the cell type studied. Glial cells were very faintly labelled. The epithelial cells of the choroid plexuses, the ependymal cells and the pia mater were all labelled. Both genes were most active in neurones throughout the CNS. VACM-1 and AII/AVP receptors were detected in neurones previously shown to possess V1a and V1b vasopressin receptors, and/or the AT1 and AT2 angiotensin II receptors in many brain areas. This was the case for the magnocellular neurones of the supraoptic and paraventricular nuclei of the hypothalamus. We suggest that the VACM-1 and AII/AVP receptors may account for the V2-like responses to vasopressin by these neurones which lack a genuine V2 vasopressin receptor.

Increased central AT(1)-receptor activation, not systemic vasopressin, sustains hypertension in ANP knockout mice

We tested the hypothesis that hypertension in atrial natriuretic peptide (ANP) knockout mice is caused in part by disinhibition of angiotensin II-mediated vasopressin release. Inactin-anesthetized F(2) homozygous ANP gene-disrupted mice (-/-) and wild-type (+/+) littermates were surgically prepared for carotid arterial blood pressure measurement (ABP) and background intravenous injection of physiological saline or vasopressin V(1)-receptor antagonist (Manning compound, 10 ng/g body wt) and subsequent intracerebroventricular (left lateral ventricle) injection of saline (5 microl) or ANP (0.5 microg) or angiotensin II AT(1)-receptor antagonist losartan (10 microg). Only (-/-) showed significant decrease in ABP after intracerebroventricular ANP or losartan. Both showed significant hypotension after intravenous V(1) antagonist, but there was no difference between their responses. We conclude that 1) vasopressin contributes equally to ABP maintenance in ANP-disrupted mice and wild-type controls; 2) permanently elevated ABP in ANP knockouts is associated with increased central nervous angiotensin II AT(1)-receptor activation; 3) disinhibition of central nervous angiotensin II AT(1) receptors in ANP-deficient animals does not lead to a significant increase in the importance of vasopressin as a mechanism for blood pressure maintenance.

Angiotensin AT1 receptors in the preoptic area negatively modulate the cardiovascular and ACTH responses induced in rats by intrapreoptic injection of prostaglandin E2

We previously reported that brain angiotensin II type 2 (AT2) receptors contribute to the hyperthermia induced by intrahypothalamic (intrapreoptic (i.p.o.)) administration of prostaglandin E2 (PGE2) in rats. The present study was carried out to investigate the role of angiotensin II (ANG II) receptors in the cardiovascular and adrenocorticotropic hormone (ACTH) responses induced in rats by i.p.o. injection of PGE2. PGE2 (100 ng) produced marked increases in blood pressure, heart rate, and plasma ACTH concentration. These changes were significantly enhanced by i.p.o. treatment with an AT1-receptor antagonist, losartan, while an AT2-receptor antagonist, CGP 42112A, had no effect. In contrast, losartan, but not CGP 42112A, reduced the pressor and ACTH responses to i.p.o. injection of a large dose of "exogenous" ANG II (25 ng). These results suggest that while "endogenous" ANG II exerts inhibitory effects on both the cardiovascular and the ACTH responses to i.p.o. PGE2 by way of preoptic AT1-receptors, a large dose of exogenous ANG II produces effects opposite to those induced by the endogenous ANG II that is released locally and in small amounts by i.p.o. PGE2.

Aminopeptidase A inhibitors as potential central antihypertensive agents

Overactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several experimental models, such as spontaneously hypertensive rats and transgenic mice expressing both human renin and human angiotensinogen transgenes. We recently reported that, in the murine brain, angiotensin II (AngII) is converted to angiotensin III (AngIII) by aminopeptidase A (APA), whereas AngIII is inactivated by aminopeptidase N (APN). If injected into cerebral ventricles (ICV), AngII and AngIII cause similar pressor responses. Because AngII is metabolized in vivo into AngIII, the exact nature of the active peptide is not precisely determined. Here we report that, in rats, ICV injection of the selective APA inhibitor EC33 [(S)-3-amino-4-mercaptobutyl sulfonic acid] blocked the pressor response of exogenous AngII, suggesting that the conversion of AngII to AngIII is required to increase blood pressure (BP). Furthermore, ICV injection, but not i.v. injection, of EC33 alone caused a dose-dependent decrease in BP by blocking the formation of brain but not systemic AngIII. This is corroborated by the fact that the selective APN inhibitor, PC18 (2-amino-4-methylsulfonyl butane thiol), administered alone via the ICV route, increases BP. This pressor response was blocked by prior treatment with the angiotensin type 1 (AT(1)) receptor antagonist, losartan, showing that blocking the action of APN on AngIII metabolism leads to an increase in endogenous AngIII levels, resulting in BP increase, through interaction with AT(1) receptors. These data demonstrate that AngIII is a major effector peptide of the brain RAS, exerting tonic stimulatory control over BP. Thus, APA, the enzyme responsible for the formation of brain AngIII, represents a potential central therapeutic target that justifies the development of APA inhibitors as central antihypertensive agents.

Activation of the AT(2) receptor of angiotensin II induces neurite outgrowth and cell migration in microexplant cultures of the cerebellum

Microexplant cultures from three-day-old rats were used to investigate whether angiotensin II (Ang II), through its AT(1) and AT(2) receptors, could be involved in the morphological differentiation of cerebellar cells. Specific activation of the AT(2) receptor during 4-day treatment induced two major morphological changes. The first was characterized by increased elongation of neurites. The second change was cell migration from the edge of the microexplant toward the periphery. Western blot analyses and indirect immunofluorescence studies revealed an increase in the expression of neuron-specific betaIII-tubulin, as well as an increase in expression of the microtubule-associated proteins tau and MAP2. These effects were demonstrated by co-incubation of Ang II with 1 microM DUP 753 (AT(1) receptor antagonist) or with 10 nM CGP 42112 (AT(2) receptor agonist) but abolished when Ang II was co-incubated with 1 microM PD 123319 (AT(2) receptor antagonist), indicating that differentiation occurs through AT(2) receptor activation and that the AT(1) receptor inhibits the AT(2) effect. Taken together, these results demonstrate that Ang II is involved in cerebellum development for both neurite outgrowth and cell migration, two important processes in the organization of the various layers of the cerebellum.

Involvement of central angiotensin receptors in stress adaptation

The present study examined the effects of acute and chronic neurogenic stressors on the expression of two distinct angiotensin receptors in two stress-related brain nuclei: angiotensin type 1A receptor in the paraventricular nucleus of the hypothalamus and angiotensin type 2 receptor in the nucleus locus coeruleus. Male Wistar rats were divided into four experimental groups. The first two groups were subjected once to an acute 90-min immobilization or air-jet stress session, respectively. The other two groups were subjected to 10 days of daily 90-min immobilization sessions and, on the 11th day, one group was exposed to an additional 90-min immobilization and the other to a single air-jet stress (heterotypic but still neurogenic) session. In each group, rats were perfused before stress (0 min), immediately following stress (90 min) or 150, 180, 270 or 360 min (and 24 h in chronic immobilization) after the beginning of the last stress session. Basal expression of both angiotensin receptor subtype 1A and angiotensin receptor subtype 2 messenger RNA was minimal in non-stressed animals. Acute immobilization as well as air-jet stress induced similar patterns (time-course and maximal values) of angiotensin receptor subtype 1A messenger RNA expression in the paraventricular nucleus. Angiotensin receptor subtype 1A messenger RNA expression increased 90-150 min after the beginning of the stress and returned to basal levels by 360 min. Chronic stress immobilization slightly modified the pattern, but not maximal values of angiotensin receptor subtype 1A messenger RNA expression to further immobilization (homotypic) or air-jet stress (heterotypic). Acute immobilization and air-jet stress sessions induced similar locus coeruleus-specific angiotensin receptor subtype 2 messenger RNA expression. This expression increased 90 min following the onset of the stress session and remained elevated for at least 360 min. Chronic immobilization stress increased angiotensin receptor subtype 2 messenger RNA expression to levels comparable to those observed in acute stress conditions. Novel acute exposure to neurogenic stressors did not further increase these levels in either homotypic (immobilization) or in heterotypic (air-jet stress) conditions. These results suggest that central angiotensin receptors are targets of regulation in stress; therefore, stress may modulate angiotensin function in the paraventricular nucleus and locus coeruleus during chronic exposure to neurogenic stressors.

A comparison of brain angiotensin II receptors during lactation and diestrus of the estrous cycle in the rat

During lactation there are many dramatic alterations in the hypothalamic-pituitary (HP) axis, as well as an increased demand for food and water. The renin-angiotensin system (RAS) is one of the major mediators of the HP axis. This study examined the receptors for ANG II in the rat brain during lactation and diestrus. Compared with diestrus, lactating rats had significant decreases in ANG II receptor binding in several forebrain regions, most notably in the arcuate nucleus/median eminence, dorsomedial hypothalamic nucleus (DMH), and lateral hypothalamic area (LHA). In contrast, there was an increase in ANG II receptor binding in the preoptic area during lactation. These significant changes in ANG II binding in the brain during lactation support the hypothesis that changes in the RAS may contribute to the dramatic changes in the HP axis during lactation. In addition, the significant reduction in ANG II binding in the DMH and LHA may be indicative of a role in the regulation of food intake, a function only recently associated with the RAS.

Signals from the AT2 (angiotensin type 2) receptor of angiotensin II inhibit p21ras and activate MAPK (mitogen-activated protein kinase) to induce morphological neuronal differentiation in NG108-15 cells

In a previous study, we had shown that activation of the AT2 (angiotensin type 2) receptor of angiotensin II (Ang II) induced morphological differentiation of the neuronal cell line NG108-15. In the present study, we investigated the nature of the possible intracellular mediators involved in the AT2 effect. We found that stimulation of AT2 receptors in NG108-15 cells resulted in time-dependent modulation of tyrosine phosphorylation of a number of cytoplasmic proteins. Stimulation of NG108-15 cells with Ang II induced a decrease in GTP-bound p21ras but a sustained increase in the activity of p42mapk and p44mapk as well as neurite outgrowth. Similarly, neurite elongation, increased polymerized tubulin levels, and increased mitogen-activated protein kinase (MAPK) activity were also observed in a stably transfected NG108-15 cell line expressing the dominant-negative mutant of p21ras, RasN17. These results support the observation that inhibition of p21ras did not impair the effect of Ang II on its ability to stimulate MAPK activity. While 10 microM of the MEK inhibitor, PD98059, only moderately affected elongation, 50 microM PD98059 completely blocked the Ang II- and the RasN17-mediated induction of neurite outgrowth. These results demonstrate that some of the events associated with the AT2 receptor-induced neuronal morphological differentiation of NG108-15 cells not only include inhibition of p21ras but an increase in MAPK activity as well, which is essential for neurite outgrowth.