Electrophysiological evidence for multiple sites of actions of angiotensin II for stimulating paraventricular neurosecretory cells in the rat

Chronic angiotensin II infusion attenuates the renal sympathoinhibitory response to acute volume expansion

In this study the hypothesis was tested that chronic infusion of ANG II attenuates acute volume expansion (VE)-induced inhibition of renal sympathetic nerve activity (SNA). Rats received intravenous infusion of either vehicle or ANG II (12 ng. kg(-1). min(-1)) for 7 days. ANG II-infused animals displayed an increased contribution of SNA to the maintenance of mean arterial pressure (MAP) as indicated by ganglionic blockade, which produced a significantly (P < 0.01) greater decrease in MAP (75 +/- 3 mmHg) than was observed in vehicle-infused (47 +/- 8 mmHg) controls. Rats were then anesthetized, and changes in MAP, mean right atrial pressure (MRAP), heart rate (HR), and renal SNA were recorded in response to right atrial infusion of isotonic saline (20% estimated blood volume in 5 min). Baseline MAP, HR, and hematocrit were not different between groups. Likewise, MAP was unchanged by acute VE in vehicle-infused animals, whereas VE induced a significant bradycardia (P < 0.05) and increase in MRAP (P < 0.05). MAP, MRAP, and HR responses to VE were not statistically different between animals infused with vehicle vs. ANG II. In contrast, VE significantly (P < 0.001) reduced renal SNA by 33.5 +/- 8% in vehicle-infused animals but was without effect on renal SNA in those infused chronically with ANG II. Acutely administered losartan (3 mg/kg iv) restored VE-induced inhibition of renal SNA (P < 0.001) in rats chronically infused with ANG II. In contrast, this treatment had no effect in the vehicle-infused group. Therefore, it appears that chronic infusion of ANG II can attenuate VE-induced renal sympathoinhibition through a mechanism requiring AT(1) receptor activation. The attenuated sympathoinhibitory response to VE in ANG II-infused animals remained after arterial barodenervation and systemic vasopressin V(1) receptor antagonism and appeared to depend on ANG II being chronically increased because ANG II given acutely had no effect on VE-induced renal sympathoinhibition.

Modulation of membrane potential and ionic currents by the AT1 and AT2 receptors of angiotensin II

Angiotensin II, the principal effector of the renin-angiotensin system, modulates various ionic currents. Its effects on potassium currents, including outward transient potassium current, the inward or outward rectifiers, as well as Ca(2+)- activated potassium currents, is well described. Other ionic currents, such as voltage-dependent calcium currents, cationic or chloride currents, are also altered by the hormone. All these effects provoke changes in membrane potential, such as modulation of action potential firing or resting membrane potential and control intracellular calcium concentration. Summarized here are the results obtained on these membrane electrical properties using electrophysiological recordings.

Electrophysiological properties of paraventricular magnocellular neurons in rat brain slices: modulation of IA by angiotensin II

Whole-cell patch-clamp recordings obtained from magnocellular neurons of the hypothalamic paraventricular nucleus in brain slice preparations of adult Sprague-Dawley rats have been utilized to examine three outward potassium conductances and the ionic mechanisms through which angiotensin II exerts its neurotransmitter actions within this region. Lucifer Yellow fills showed that neurons from which we recorded had large ovoid cell bodies 11-17 microns wide and 22-35 microns long, as well as 1-3 minimally branched processes, anatomical features in accordance with those previously described for magnocellular neuroendocrine neurons. These neurons had an average resting membrane potential of -58.3 +/- 0.9 (mean +/- S.E.M.) mV, spike amplitude of 92.8 +/- 1.4 mV, and input resistance of 788.9 +/- 50.4 M omega. Most of these cells displayed irregular or continuous spontaneous activity with a mean frequency of 2.44 +/- 0.33 Hz. Voltage-clamp recordings revealed three outward potassium currents; (1) a delayed outward current (IK), (2) a Ca(2+)-dependent outward current (IK(Ca)) and (3) a transient outward current (IA). These currents were classified according to their voltage dependence, inactivation, Ca2+ dependence and pharmacology. The IK was activated by depolarization beyond -40 mV and its amplitude consistently increased with depolarizing steps. The membrane conductance underlying this current was 27.3 +/- 3.8 nS for depolarization to +50 mV. In medium containing 2 mM Ca2+, depolarization to above -20 mV evoked a slowly-activating IK(Ca) which showed minimal inactivation. This current was suppressed in Ca(2+)-free/Co2+ medium and its membrane conductance was also smaller (19.4 +/- 3.5 nS at +50 mV) than that of IK. The IA demonstrated both fast activation and inactivation and was evoked only if depolarizing pulse steps were preceded by conditioning hyperpolarization. The activation threshold was approximately -65 mV and IA amplitude increased in non-linear fashion as test voltage steps became more positive. The 90% maximum of IA conductance was 15.7 +/- 1.1 nS, and was observed at membrane potentials around -15 mV. The reversal potentials of these currents were in accordance with the K+ equilibrium potential. Tetra-ethylammonium reversibly inhibited both the peak and steady-state currents of the IK, while 4-aminopyridine suppressed the IA. Replacement of 2 mM Ca2+ with 2 mM Co2+ in our bath solution or addition of Co2+ into Ca(2+)-free medium reduced the magnitude of IA, revealing the existence of a Co(2+)-sensitive IA. Bath administration of 10(-7) M angiotensin was without significant effect on IK, but resulted in a statistically significant reduction in IA (-31.0 +/- 4.1%) in 12 of 14 paraventricular nucleus cells tested, effects which were not observed following pretreatment with the AT1 receptor antagonist losartan. We conclude that in paraventricular nucleus magnocellular cells, like other CNS neurons, at least three sets of potassium channels contribute to the outward current evoked by depolarization. Our data also demonstrate ionic mechanisms through which angiotensin may act at AT1 receptors to influence the excitability of hypothalamic neuroendocrine cells.

Comparative expression of vasopressin and angiotensin type-1 receptor mRNA in rat hypothalamic nuclei: a double in situ hybridization study

Angiotensin II (Ang) injected intracerebroventricularly stimulates neurohypophyseal vasopressin (AVP) release into the peripheral circulation. As we have shown previously, central actions of Ang II in the rat forebrain are mediated by the AT1A receptor subtype. In the present paper, we attempted to clarify the cellular localization of the AT1A receptor mRNA in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei, in order to reappraise the conflicting data on the nature of the angiotensin II receptor involved in Ang induced vasopressin release. For this purpose, double in situ hybridization was performed using a radioactive AT1A receptor riboprobe and a digoxygenin labeled AVP oligoprobe, and immunohistochemical localization of the glial marker glial fibrillary acidic protein (GFAP) on the same brain slice. The results show neuronal expression of AT1A receptor mRNA mainly in dorsal and medial parvocellular parts of the PVN, its localization in some magnocellular PVN neurons and the absence of its expression in AVP producing neurons either in the PVN or in the SON. Thus, while indirect evidence indicates the involvement of the AT1A receptor subtype in the regulation of CRH and oxytocin release, the stimulation of vasopressinergic neurons is likely due to indirect mechanisms, or to a yet unknown type of angiotensin receptor.

Microinjections of angiotensin II into the supraoptic and paraventricular nuclei produce potent antidiureses by vasopressin release mediated through adrenergic and angiotensin receptors

We investigated the effects of angiotensin II (Ang II), microinjected into the supraoptic (SON) and paraventricular (PVN) nuclei of rats, on the urine outflow rate and underlying mechanisms. Ang II produced antidiuretic effects in a dose-dependent manner with ED50 values of 0.1 and 0.05 nmol in the SON and PVN, respectively. [Sar1, Ile8]Ang II at 0.1 nmol diminished the Ang II (0.5 nmol)-induced antidiureses in the SON more markedly than in the PVN. A high dose of [Sar1,Ile8]Ang II, 1 nmol, completely inhibited the effects in both the nuclei. In addition, the Ang II (1 nmol)-induced antidiuretic effects were partially inhibited by phenoxybenzamine (80 nmol) in the SON and by phenoxybenzamine, timolol (100 nmol) and propranolol (100 nmol) in the PVN. The microinjection of Ang II (1 nmol) into both the nuclei, after pretreatment with a vasopressin V1V2-antagonist, d(CH2)5-D-Tyr(Et)VAVP (i.v.) significantly increased the urine outflow rate. These findings suggest that 1) Two mechanisms account for the Ang II receptor-mediated antidiureses resulting from an increase in vasopressin release: direct stimulation on vasopressin-containing neurons and indirect stimulation on them through alpha-adrenoceptors in the SON and alpha- and beta-adrenoceptors in the PVN; 2) The Ang II-induced antidiuretic effect in the SON is slightly less potent than that in the PVN; and 3) Ang II receptors in the nuclei may possibly produce the diureses through mechanisms that are not presently understood.

Angiotensin II responsiveness of rat paraventricular and subfornical organ neurons in vitro

The responsiveness of neurons in the hypothalamic paraventricular nucleus to angiotensin II was investigated using extracellular single unit recording techniques in rat brain slices. Bath application of angiotensin II at a concentration of 3 x 10(-7) M for 2-5 min resulted in excitatory responses in 50.4% of 141 paraventricular cells tested. The mean increase in firing rate was 2.12 +/- 0.20 (mean +/- S.E.M.) spikes/s, which represents a mean increase in activity of 149.8 +/- 16.5%. Angiotensin II-sensitive neurons usually displayed irregular, phasic, or very slow spontaneous activity, with the majority of these neurons located in the magnocellular region. Under physiological blockade of synaptic transmission with low Ca2+/high Mg2+ medium, neuronal responses to this peptide remained in 12 (92.3%) of 13 cells tested. Application of three successive doses of angiotensin II ranging from 3 x 10(-9)-3 x 10(-7) M showed that neuronal responses were dose-dependent with an estimated threshold of 10(-8) M. In comparison with angiotensin III, angiotensin II not only stimulated more paraventricular cells, but usually induced larger excitatory responses. Angiotensin II subtype 1 receptor antagonist losartan completely blocked angiotensin II responsiveness in each of 14 paraventricular cells tested whereas PD 123319, an angiotensin II subtype 2 receptor antagonist, exhibited a partial inhibitory effect in about one half of another 13 cells. In addition, single unit in vitro subfornical organ recordings demonstrate that angiotensin II evokes greater excitatory responses than in the paraventricular nucleus and that these effects are abolished by losartan application. These results support the hypothesis that within both the paraventricular nucleus and subfornical organ angiotensin II is a bioactive peptide which modulates neuronal activity and thus may exert significant control over neuroendocrine and autonomic functions.

Angiotensin II actions in paraventricular nucleus: functional evidence for neurotransmitter role in efferents originating in subfornical organ

Angiotensin II (ANG) has been suggested to be the neurotransmitter utilised by subfornical organ (SFO) efferents projecting to the paraventricular nucleus (PVN). The PVN has been shown to be involved in mediating the cardiovascular response elicited by electrical stimulation of SFO. The possible role of ANG as a neurotransmitter in these pathways has been examined in the present study. The cardiovascular effects of ANG microinjection into the PVN were examined in urethane anaesthetized, male Sprague-Dawley rats. Microinjection of 20 ng or 50 ng ANG into PVN resulted in mean increases in blood pressure of 12.8 +/- 0.6 mmHg (P < 0.0005), and 16.2 +/- 1.4 mmHg (P < 0.0001) respectively, without effect on heart rate. These responses were significantly attenuated following systemic administration of losartan, an ANG type 1 receptor (AT1) antagonist (Control, +12.8 +/- 0.6 mmHg; post-losartan, +5.6 +/- 1.7 mmHg), but were unaffected by the AT2 receptor antagonist, PD123319 (Control, +10.8 +/- 1.6 mmHg; post-PD123319, +11.6 +/- 2.4 mmHg). Initial and later components of the biphasic pressor response elicited by electrical stimulation of SFO (200 microA, 10 Hz, 1 ms pulse width, 10 s) were also significantly attenuated by losartan, but unaffected by PD123319. The short latency increase in mean arterial pressure was 16.6 +/- 2.3 mmHg in comparison to a post-losartan increase of 9.3 +/- 1.6 mmHg (P < 0.001). Similarly, the secondary response consisted of a control increase of 9.6 +/- 1.3 mmHg and a post-losartan increase of 3.4 +/- 0.9 mmHg (P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Functional evidence that the angiotensin antagonist losartan crosses the blood-brain barrier in the rat

Losartan is a novel nonpeptidergic antagonist of angiotensin (ANG) II subtype 1 (AT1) receptors, which effectively lowers blood pressure in high-renin hypertensive rat and blocks the pressor response to systemic ANG II. It is well known that high densities of ANG II receptors exist in the hypothalamic paraventricular nucleus (PVN). In addition, activation of putative angiotensinergic afferents to the PVN originating in subfornical organ (SFO) elevates blood pressure and facilitates the activity of PVN neurons. We report here that systemic administration of losartan (3 mg/kg) significantly attenuates the pressor response to electrical stimulation of SFO. The excitatory responses of PVN neurons to SFO stimulation or local pressure microinjection of ANG II were also significantly inhibited in 58.8% and 88.9% of PVN cells, respectively, by intravenous administration of losartan. These pharmacological effects were rapid and reversible, and were accompanied by little change of basal arterial blood pressure or spontaneous neuronal activity. These observations suggest that systemic losartan crosses the blood-brain barrier (BBB) and acts at AT1 receptors within the PVN.

The pathogenesis of DOCA-salt hypertension

Deoxycorticosterone acetate (DOCA) is an agent commonly used to induce hypertension in experimental animals. This form of hypertension is dependent on altered regulation of central pressor mechanisms including the brain renin-angiotensin system. Additionally, there are characteristic changes involving the cardiovascular system and baroreflex responses. This review will discuss aspects of the pathogenesis of DOCA hypertension and the effect of various antihypertensive agents on the development of this form of hypertension.

Angiotensin II modulation of glutamate excitation of locus coeruleus neurons

The effects of iontophoretically applied angiotensin II (AII) have been tested on intracellularly recorded locus coeruleus neurons in an in vitro brain slice preparation. In most neurons, AII strongly depressed the depolarizing effect of L-glutamate in the absence of other effects on membrane properties. This action was specific, in that AII had no effect on depolarizations caused by current injection or application of acetylcholine, and it was blockable by the AII antagonist saralasin. These results appear to demonstrate a potent and previously unreported neuromodulatory action of AII in the central nervous system.

Locations and properties of angiotensin II-responsive neurones in the circumventricular region of the duck brain

1. In brain slice preparations from the hypothalamus of domestic ducks, single-unit activity was recorded extracellularly to investigate location and properties of angiotensin II (AngII)-responsive neurones in various periventricular regions. 2. When exposing the slice to 10(-7) M-AngII in the perfusion medium, more than 65% of the neurones recorded in the subfornical organ (SFO) were activated (49 out of 75) and none inhibited. In the magnocellular (MC) region of the paraventricular nucleus (PVN) only four out of eighty-one neurones were influenced by AngII; one was inhibited and three were activated. In the anterior third ventricle region (A3V) two out of twenty-one neurones were activated by AngII. In the dorsal periventricular (PeV) region, one out of thirty-seven neurones was activated and one inhibited. The changes in firing rate of AngII-responsive neurones at comparable doses of AngII were generally large in the SFO and A3V but were small in neurones from the MC and PeV regions. 3. Analysis of AngII-responsive SFO neurones consistently revealed a dose-dependent stimulation with a threshold at 10(-9) M-AngII. The AngII antagonist 1Sar-8Ile-AngII (4 x 10(-7) to 10(-6) M) caused reversible, complete or partial suppression of responsiveness to 10(-7) M-AngII. Synaptic blockade with a medium low in Ca2+ and high in Mg2+ did not abolish AngII responsiveness in eight out of ten SFO neurones tested. 4. Angiotensin III affected neither AngII-responsive nor AngII-insensitive neurones. When eighteen AngII-responsive neurones were exposed to hypertonic stimulation (+20 to +30 mosmol/kg) by adding NaCl to the perfusion medium, only one neurone was stimulated and two were inhibited. 5. The results indicate that: (a) the SFO is a specific target for circulating AngII; (b) although neurones in the A3V responsive to AngII are rare, the pronounced excitation of those which were found suggest that neurones in this region might serve as targets for AngII acting from the brain side; (c) neurones in the MC region do not seem to function as direct AngII targets; (d) neuronal AngII responsiveness in the duck's hypothalamus seems to be specific inasmuch as activation by AngII (i) is readily blocked by an AngII antagonist, (ii) cannot be induced by AngIII, and (iii) is not associated, as a rule, with responsiveness to hypertonic stimulation.

Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

Pretreatment of rats with prazosin (PRA), an alpha 1-adrenergic receptor blocker, abolished the increased rate of learning of conditioned avoidance responses stimulated by intracerebroventricular angiotensin II (AII) administration. Yohimbine (YOH), an alpha 2-receptor blocker, reversed the effect of AII. PRA did not affect, and YOH abolished, the improvement of recall of a passive avoidance behavior caused by AII. The stereotypies produced by apomorphine (APO) and amphetamine (AMP) were enhanced by AII. PRA changed neither stereotypy, but it abolished the AII effect in both cases. YOH did not alter APO stereotypy and abolished the enhancement of that behavior caused by AII. YOH increased AMP stereotypy and had an additive effect with AII. No significant changes of exploratory motor activity were caused by PRA, YOH, or their combination, with AII. These findings indicate that functioning alpha 1- and alpha 2-adrenergic receptors are necessary for the facilitation of learning by AII, while only alpha 2-receptors appear to be involved in AII improvement of recall. The central dopaminergic system may in part be responsible for the modulation by PRA and YOH of the effects of AII on learning and recall.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)

Angiotensin II may mediate excitatory neurotransmission from the subfornical organ to the hypothalamic supraoptic nucleus: an anatomical and electrophysiological study in the rat

In the rat, it has been proposed that angiotensin II (AII) neurons in the subfornical organ, a midline circumventricular structure, participate in the activation of hypothalamic neurosecretory neurons and promote a rise in plasma vasopressin and oxytocin. In this study, we observed AII-immunoreactive fibers coursing throughout the supraoptic nucleus as well as in other magnocellular cell groups of the hypothalamus. Moreover, following retrograde transport of Fast blue deposited within the supraoptic nucleus, cell counts in our best case revealed that 40% of AII-immunoreactive neurons in subfornical organ contained Fast blue, and 46% of the retrogradely labeled subfornical organ cells contained AII. In separate electrophysiological studies, post-stimulus histograms from 18 of 28 supraoptic neurons displayed a 30-55% reversible reduction in the excitation evoked by an electrical stimulus in the subfornical organ during local pressure applications of 100 microM to 1 mM saralasin. In 2 of 14 other cells, tubocurare (100 microM) produced only a 10% reduction in subfornical organ excitation. These observations indicate that AII may mediate an excitatory input to supraoptic neurons from the subfornical organ.

Angiotensin acts at the subfornical organ to increase plasma oxytocin concentrations in the rat

We have examined the effects of systemic angiotensin II (AII) on plasma oxytocin (OXY) concentrations in freely moving male Sprague-Dawley rats. We have also examined the role of the subfornical organ (SFO) as a CNS site at which circulating AII acts to influence secretion of this neurohypophysial peptide. OXY concentrations were measured by radioimmunoassay in plasma samples obtained by drawing blood samples through indwelling atrial catheters. In SFO intact animals (n = 8) AII infusion (1.0 microgram/kg/min) resulted in increases in plasma OXY concentrations from baseline values of 6.8 +/- 2.5 pg/ml to postinfusion concentrations of 44.9 +/- 11.9 pg/ml. In a second series of experiments electrolytic lesions were placed in the region of the SFO prior to testing the effects of AII infusion on OXY concentrations. Two further experimental groups were thus established according to the histologically verified location of lesions in either the rostral or caudal SFO. In the caudal SFO lesioned group AII infusion resulted in increases in plasma OXY concentrations from control values of 6.9 +/- 1.4 pg/ml to postinfusion levels of 45.1 +/- 9.8 pg/ml. These changes were not significantly different from the SFO intact group. In contrast rostral SFO lesions resulted in significantly elevated basal concentrations of OXY (17.4 +/- 3.4 pg/ml, n = 6) while postinfusion concentrations were found to be 22.8 +/- 4.9 pg/ml indicating that AII infusion was without effect following such lesions. These data are in accordance with the hypothesis that circulating AII acts at the SFO to influence SFO efferents which in turn activate OXY secreting neurons in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. These neuroendocrine cells then release this peptide into the systemic circulation from the posterior pituitary.

Localization and characterization of angiotensin II receptor binding and angiotensin converting enzyme in the human medulla oblongata

Angiotensin II receptor and angiotensin converting enzyme distributions in the human medulla oblongata were localised by quantitative in vitro autoradiography. Angiotensin II receptors were labelled with the antagonist analogue 125I-[Sar1, Ile8] AII while angiotensin converting enzyme was labelled with 125I-351A, a derivative of the specific converting enzyme inhibitor, lisinopril. Angiotensin II receptor binding and angiotensin converting enzyme are present in high concentrations in the nucleus of the solitary tract, the dorsal motor nucleus of vagus, the rostral and caudal ventrolateral reticular nucleus, and in a band connecting the dorsal and ventral regions. In the rostral and caudal ventrolateral reticular nucleus, angiotensin II receptors are distributed in a punctate pattern that registers with neuronal cell bodies. The distribution and density of these cell bodies closely resemble those of catecholamine-containing neurones mapped by others. In view of the known interactions of angiotensin II with both central and peripheral catecholamine-containing neurons of laboratory animals, the current anatomical findings suggest similar interactions between these neuroactive compounds in the human central nervous system. The presence of angiotensin II receptors and angiotensin converting enzyme in the nucleus of the solitary tract, dorsal motor nucleus of vagus, and rostral and caudal ventrolateral reticular nucleus demonstrates sites for central angiotensin II to exert its known actions on vasopressin release and autonomic functions including blood pressure control. These data also suggest a possible interaction between angiotensin II and central catecholeminergic systems.

Comparative neuroanatomy of angiotensin II receptor localization in the mammalian hypothalamus

1. The distribution of angiotensin II (AII) receptor binding sites in the hypothalamus of rat, rabbit, sheep and human was determined by in vitro autoradiography using 125I-[Sar1,Ile8]-AII as radioligand. 2. High receptor binding levels were observed in the continuum of tissue comprising the anterior wall of the third ventricle, including the subfornical organ, the median pre-optic nucleus and the organum vasculosum of the lamina terminalis. 3. High levels of binding sites were also found in the paraventricular and supra-optic nuclei, the median eminence and the arcuate nucleus. 4. These findings demonstrate sites in the hypothalamus of rat, rabbit, sheep and human where AII could exert its known actions on fluid and electrolyte balance, pituitary hormone release and cardiovascular function.

Paraventricular nucleus neurons projecting to the dorsomedial medulla are influenced by systemic angiotensin

Activation of subfornical organ (SFO) efferents has been reported to increase the excitability of paraventricular nucleus (PVN) neurons projecting to cardiovascular centres in the medulla. Such findings suggest a role for these neural projections in the cardiovascular effects of systemic angiotensin II (AII) acting at the SFO. The present studies examine the effects of peripheral AII (50-500 ng) on the activity of PVN neurons antidromically identified as projecting to the dorsomedial medulla (DM). A total of 68 neurons were tested, of which 25% were found to be specifically activated by AII, and 25% inhibited as a consequence the increased blood pressure accompanying administration of this peptide. The activity of the remaining neurons tested was unaffected by AII. These studies demonstrate that only a small proportion of PVN neurons projecting to the DM, which have been shown to be influenced by SFO efferents, are also activated by systemic AII. Such data do however, support a significant role for these neurons in the cardiovascular functions of SFO.

The hypothalamic-angiotensin system: location and functional considerations

Improved immunohistochemical and quantitative microiontophoretic methods were used to characterise angiotensinergic and angiotensin-sensitive neurones in the paraventricular nucleus (PVN) of the rat. The results can be summarised as follows: 1) Angiotensinogen was found in PVN neurones, astrocytes in the diencephalon which make putative contacts with microvessels, and in cells of the choroid plexus. 2) Affinity-purified angiotensin II/III antibodies were used to locate immunoreactive AII/III in large PVN neurones and their fibre tracts which project either caudally or ventrally to the neurohypophysis. 3) Quantitative microiontophoretic studies showed that PVN neurones are more sensitive to angiotensin II than to angiotensin II. 4) Iontophoretic co-application of the selective aminopeptidase inhibitors bestatin and amastatin, together with angiotensin II and angiotensin III produced results consistent with a central role for angiotensin III.

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

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

Cellular organization of the brain renin-angiotensin system

A model of intracellular Ang II formation (Figure 1) implies that angiotensinogen neurons exist and that CNS Ang II acts both as a neurotransmitter as well as a neurohormone. Such a mechanism is consistent with the immunocytochemical localization of a fraction of brain Ang II in neurosecretory vesicles. To date, several dozen peptide neurotransmitters and neurohormones have been studied. Those assigned to peptidergic systems follow the generalized pathway of biosynthesis shown in Figure 1. In peptidergic systems, a prohormone and all of its processing enzymes are synthesized in the rough endoplasmic reticulum of a cell and move into the Golgi apparatus (Figure 1: #1-3). In the Golgi the prohormone and processing enzymes are packaged into the same vesicle (#3). These secretory vesicles then migrate toward the plasma membrane, frequently via axonal or dendritic projections to terminals. Within these cytoplasmic vesicles and prior to release, the processing enzymes are activated (#4) and the prohormone enzymatically processed, yielding the active peptide (#5-6). Only then do the vesicles fuse with the plasma membrane (in a calcium-dependent process), releasing their contents (#7-8). Once released, the active peptide migrates across the extracellular space and interacts with specific cell surface receptors to initiate a response (#9). Finally, receptor-bound peptide degradation is initiated by receptor-mediated endocytosis (#10-11). For angiotensin peptides to be produced intracellularly, the cell must present only one secretory pathway for Golgi packaging of renin and angiotensinogen; otherwise current theories of protein sorting would predict that these two proteins would be segregated even if synthesized within the same cell. Small quantities of co-packaged renin and angiotensinogen occurring via "spill-over" between compartments seems an unsatisfactory process for a regulated hormone system. Figure 2, depicting an extracellular mechanism for producing Ang II in the brain, has also been proposed. The mechanism of extracellular angiotensin formation is consistent with the molecular information encoded within the component proteins, known mechanisms of protein secretio, well-defined systemic renin-angiotensin enzymatic cascades, and demonstration of all the components of the renin-angiotensin system in the extracellular compartments of the brain. This model (Figure 2) allows independently coordinated gene expression and synthesis of renin (#1R), angiotensinogen (#1A), and angiotensin-converting enzyme (# 1C) in the same or different cells.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Localization of angiotensin II receptors along the anteroventral third ventricle area of the rat brain

Autoradiographic techniques were utilized to localize and to quantify angiotensin II (ANG) binding sites in rat forebrain. Specific, localized ANG binding sites were demonstrated in midline sagittal sections, corresponding to the entire anteroventral third ventricle (AV3V) area, including the nucleus preopticus medianus and the subependymal area of the anterior third ventricle from the nucleus preopticus medianus to the organon vasculosum laminae terminalis. A continuous band of ANG receptors extended dorsally from the nucleus preopticus medianus along the subependymal area of the third ventricle to the organon subfornicalis. Scatchard analysis performed with consecutive sections from single animals revealed a single class of high-affinity ANG receptors in both the organon subfornicalis and the organon vasculosum laminae terminalis. In addition, ANG receptors were localized in areas anatomically and physiologically related to the AV3V area, including the nuclei paraventricularis and periventricularis and the eminentia mediana. These results support the idea that ANG may act as both a hormone and a neurotransmitter in the central regulation of fluid balance and cardiovascular function, and suggest that the circumventricular organs are the most likely sites for an interaction between the peripheral and central ANG systems.

Effect of peptidyl-dipeptidase inhibitors in experimental convulsions in mice

The anticonvulsant effect of compounds that inhibit peptidyl-dipeptidase (PDP) on bicuculline (BIC)- and strychnine (STRYC)-induced seizures was assessed after intracerebroventricular (ICV) or intraperitoneal (IP) administration in Swiss albino mice. STRYC-induced seizures were delayed by ICV injections and high IP doses of captopril, but not by ICV or IP injections of enalapril or by lower doses of captopril (0.1 mg/kg and 1 mg/kg IP). BIC-induced seizures were not suppressed by ICV or IP injections of either compound; on the contrary, captopril and enalapril exhibited proconvulsant effects when given IP or ICV by shortening the time of onset of tonic seizures and death. Results indicate that the anticonvulsant effect of captopril against STRYC-induced seizures is not mediated by central gamma-aminobutyric acid (GABA) receptors or by the inhibition of PDP.

Angiotensin II sensitive neurons in the supraoptic nucleus, subfornical organ and anteroventral third ventricle of rats in vitro

The angiotensin II (AII) sensitivity of neurons in the supraoptic nucleus (SON), subfornical organ (SFO) and the region near the anteroventral part of the third ventricle (AV3V) was investigated using extracellular recording in the rat brain slice preparation by adding AII (10(-10)-10(-6) M) to the perfusion medium. Forty seven (44%) of 106 SON neurons, 62 (66%) of 94 SFO neurons and 28 (33%) of 86 AV3V neurons were excited by AII. One cell was inhibited by AII in the SON and one in the SFO. The threshold concentration to evoke responses in the SON neurons was approximately 10(-9) M, but neurons in the SFO and AV3V showed clear excitatory responses to AII at 10(-10) M. In the SON, 18 (40%) of 45 phasic firing neurons (putative vasopressin neurons) and 29 (48%) of 61 nonphasic firing neurons (including putative oxytocin neurons) were excited by AII. The excitatory effect of AII was reversibly antagonized by a specific antagonist saralasin and persisted after synaptic blockade in medium with low [Ca2+] and high [Mg2+]. We conclude that AII can stimulate both vasopressin and oxytocin release, acting directly upon SON neurons and also that both the SFO and AV3V are important receptive sites for AII (although the SFO is relatively more sensitive) which contributes SON input and modulates release of these hormones.

Distribution of [125I]angiotensin II binding sites in the rat brain: a quantitative autoradiographic study

Angiotensin II receptors have been localized by quantitative autoradiography in the rat central nervous system after labeling with [125I]angiotensin II. A highly discrete distribution of these receptors was found throughout the rat brain. The highest density was seen in regions of the medulla, hypothalamus and circumventricular organs where angiotensin II could potentially produce cardiovascular, dipsogenic and neuroendocrine responses. The distribution of angiotensin II receptors correlates relatively well with the previously reported distribution of angiotensin immunoreactive nerve terminals as well as areas determined by various physiological techniques to be sensitive to angiotensin II. Finally, the anatomical localization of angiotensin II receptor populations has revealed several areas of the brain where the effects of this peptide have not been investigated. Many of these nuclei are involved in the transmission and processing of somatic and visceral sensory information. These results suggest a broader role for the central renin-angiotensin system in modulating several types of sensory input.

Quantitative distribution of angiotensin II binding sites in rat brain by autoradiography

Angiotensin II binding sites were localized and quantified in individual brain nuclei from single rats by incubation of tissue sections with 1 nM 125I-[Sar1]-angiotensin II, [3H]-Ultrofilm autoradiography, computerized microdensitometry and comparison with 125I-standards. High angiotensin II binding was present in the circumventricular organs (organon vasculosum laminae terminalis, organon subfornicalis and area postrema), in selected hypothalamic nuclei (nuclei suprachiasmatis, periventricularis and paraventricularis) and in the nucleus tractus olfactorii lateralis, the nucleus preopticus medianus, the dorsal motor nucleus of the vagus and the nucleus tractus solitarii. High affinity (KA from 0.3 to 1.5 X 10(9) M-1) angiotensin II binding sites were demonstrated in the organon subfornicalis, the nucleus tractus solitarii and the area postrema after incubation of consecutive sections from single rat brains with 125I-[Sar1]-angiotensin II in concentrations from 100 pM to 5 nM. These results demonstrate and characterize brain binding sites for angiotensin II of variable high affinity binding both inside and outside the blood-brain barrier.

Neuronal responses in subfornical organ and other regions to angiotensin II applied by various routes

Effects of Angiotensin II (AII) and of signals of hydromineral deficiencies on neuronal firing in the subfornical organ (SFO) and the surrounding regions of the rat were examined. AII was applied in three ways: electrophoresis, intracarotid injection, or intracerebroventricular injection. Seventeen SFO neurons (52%) were facilitated by electrophoretic AII and one (3%) was inhibited (AII-responding neurons). Neurons in other regions (cerebral cortex, nucleus triangularis septi, hippocampus, nucleus periventricularis) were also facilitated (23%) or inhibited (14%). Prostaglandin (PG)F2, a universal vasoconstrictor, produced an effect similar to that by AII on neuronal activity of the SFO and surrounding regions, and this effect was antagonized by (NO2-), a vasoplegic agent. Intracarotid injection of AII caused biphasic facilitation of SFO activity. The second increase correlates with changes in blood pressure. Intraventricular injection of AII caused drastic and long lasting excitation of SFO activity. Simultaneous intraventricular application of NO2- blocked the AII effect on SFO neurons but not on blood pressure. Hypovolemia or cerebrospinal fluid withdrawal that might cause mechanical stimulation of circumventricular organs increased SFO neuronal activity. These results are compatible with the vasoconstriction hypothesis of an indirect effect of AII through change in diameter of the vasa that surround neurons.

Effect of transection of subfornical organ efferent projections on vasopressin release induced by angiotensin or isoprenaline in the rat

Transection of subfornical organ efferents in the rat prevented the vasopressin release in response to intravenous angiotensin II infusion or following a small dose of the beta-sympathomimetic amine isoprenaline (30 micrograms/kg i.m.). In contrast, this lesion had no effect on vasopressin release after hypertonic saline injection or a high dose of isoprenaline (480 micrograms/kg i.m.). We conclude that blood-borne angiotensin II induces vasopressin release by acting on the subfornical organ; depending on the dose of isoprenaline, activation of the endogenous renin-angiotensin system may mediate isoprenaline-induced vasopressin release.

Vasopressin-reversible increase in angiotensin-converting enzyme in specific hypothalamic nuclei of Brattleboro rats

The activity of the angiotensin-converting enzyme (ACE) (kininase II, EC 3.4.15.1) was examined in 5 discrete hypothalamic nuclei of rats lacking vasopressin (homozygous Brattleboro rats, DI, di/di) and their corresponding controls (heterozygous Brattleboro rats, HZ, di/+, and Long Evans, LE, +/+ rats), with and without hormonal replacement with arginine-vasopressin (AVP). DI rats showed a vasopressin-reversible increased ACE activity when compared with LE controls, HZ rats showing intermediate activity. These changes occurred only in the supraoptic and periventricular hypothalamic nuclei, and were absent in other hypothalamic areas studied, including the paraventricular nucleus. These results provide biochemical evidence in support of previous anatomical and physiological data, for an interaction between the brain vasopressin and angiotensin systems in discrete hypothalamic nuclei, and suggest that vasopressin could regulate the formation of brain angiotensin II by modulating the activity of the converting enzyme.

Angiotensin-converting enzyme in discrete areas of the rat forebrain and pituitary gland

With the use of a sensitive radioisotopic method we have examined the activity of the angiotensin-converting enzyme (ACE, E.C. 3.4.15.1) in specific nuclei of the rat forebrain and in the anterior, intermediate and posterior lobes of the pituitary gland of the rat. We reported that ACE activity is heterogeneously distributed in the rat forebrain, with a 200-fold difference between the lowest and the highest values. Highest enzyme activities were found in the subfornical organ and in the posterior lobe of the pituitary gland. High ACE activity was also detected in the intermediate and anterior lobes of the pituitary gland, the caudate nucleus, and the medial habenular nucleus. Substantial activity also existed in the globus pallidus, the median eminence, the supraoptic and paraventricular nuclei, the lateral habenular nucleus and the organon vasculosum laminae terminalis. Our results demonstrate that one of the components of the renin-angiotensin system, the angiotensin-converting enzyme, is highly localized to a few discrete brain structures and the pituitary gland. These findings suggest that angiotensin II could be formed locally in some of these structures, supporting previous immunohistochemical data.