W.E. Hoffman

Regional study of cerebral ventricle sensitive sites to angiotensin II.

Angiotensin II injected in small doses into the cerebral ventricles produces an increase in blood pressure and drinking behavior. The site of action for both of these effects was studied in 3 main experiments. (1) The response to several doses of angiotensin delivered to each ventricle was investigated with multiple ventricular cannulation. This revealed that the rostral ventricular system was involved in angiotensin II mediated responses. (2) CSF flow was limited by plugging specific anterior and posterior ventricular regions and then testing for angiotensin II induced drinking and pressor responses. This technique showed that the ventral anterior third ventricle must be reached by the peptide in order to produce either blood pressure or drinking effects. (3) In order to separate pressor components due to vasopressin release and sympathetic activation, hypophysectomized rats were also tested. The experiment showed that the pressor response to intraventricular angiotensin II is due to both sympathetic and pituitary hormonal components and both are dependent on sites sensitive to angiotensin in the anterior third ventricule. The ventral anterior third ventricle or periventricular tissue surrounding it seems to be essential for both blood pressure and drinking responses to intraventricular angiotensin II.

Antidiuretic hormone release and the pressor response to central angiotensin II and cholinergic stimulation

Abstract Intraventricular (IVT) injections of angiotensin II (AII) have been shown to produce a short latency blood pressure increase and antidiuretic hormone (ADH) release in several species of mammals. In addition, IVT injections produce these same responses in the rat. Unanaesthetized hydrated rats have been used as their own bioassay to study if the pressor effect produced by IVT AII is caused by the vasopressin release. The results indicate that intraventricular injections of AII in a dose range of 0.5–500 ng result in dose dependent ADH release of from 0.04 to 3.32 mU, and carbachol in doses from 0.025 ng to 250 ng produces ADH release from 0.29 to 4.01 mU. Intravenous AII or carbachol in the same dose range did not release ADH in detectable amounts. The time course and magnitude of ADH release produced by intraventricular AII or carbachol injections and data from hypophysectomized rats indicates that vasopressin is partially but not totally responsible for the centrally-induced blood pressure increase. The data suggest that central activation of the sympathetic nervous system is also involved in the response. To test this, 11 rats received peripheral sympathectomy with 6-hydroxydopamine. The amount of ADH released to a central All challenge in these animals was not changed. Following sympathectomy, there was an increased sensitivity to vasopressin which could completely account for the pressor responses observed.

Attenuated pressor responses to intracranially injected stimuli and altered antidiuretic activity following preoptic-hypothalamic periventricular ablation

A small bilaterally symmetrical periventricular lesion destroying tissue at the margins of the anteroventral third ventricle (AV3V) produces dramatic effects on body fluid balance and cardiovascular dynamics in the rat. Animals with AV3V lesions that recover from acute adipsia and impaired antidiuresis 7 show (1) depressed drinking responses to hyperosmotic and angiotensin-induced thirst stimuli 4, (2) hypernatremia and hyperosmolality 4, (3) failure to develop experimental renal hypertension and steroid salt hypertension 3, and (4) a reduced pressor response in conscious rats to systemically administered angiotensin but not to norepinephrine 3. Intracranial injections of either angiotensin or hyperosmotic solutions increase both antidiuretic activity and blood pressure in goats and normal rats. The pressor response to intraventricularly applied angiotensin II has been attributed to both the release of antidiuretic hormone and central activation of the sympathetic nervous system (see refs. 6 and 10 for references). The impaired pressor response to systemic angiotensin and the blockade of the development of renal hypertension in rats with AV3V lesions may reflect a disruption of a centrally mediated component of the pressor response to angiotensin. The sustained hypernatremia and hyperosmolality suggest that a persisting disturbance in osmotic regulation of antidiuretic activity and of thirst is produced by the lesion4, s. To te3t these hypotheses, the effects of intraventricular injections of angiotensin and hyperosmotic NaCl on arterial pressure and antidiuretic activity were compared in rats with AV3V lesions and in animals that received sham operations. The experiments were conducted on 30 male Sprague-Dawley derived rats weighing between 250 and 325 g at the start of the experiment. Under ether anesthesia, 22 animals received stereotaxically guided electrolytic lesions (2-3 mA for 15-25 sec)

The role of catecholamines in central antidiuretic and pressor mechanisms.

Abstract Angiotensin II (AII), when delivered intraventricularly (IVT) produces an increase in blood pressure and release of antidiuretic hormone (ADH). In the experiments reported here we have investigated the role of central catecholamines in these responses. After central 6-hydroxydopamine (6-OHDA) treatment IVT, AII was less effective in producing a blood pressure increase or ADH release. Similar responses to IVT carbachol were also inhibited. Pressor responses to IVT phenylephrine were not significantly changed after 6-OHDA treatment although ADH release was decreased. In further analysis, both a dopamine blocker, haloperidol, and phentolamine, an α-adrenergic blocker inhibited the pressor response but not ADH release to IVT AII. Both effects may be explained by α-adrenergic blockade. Dopamine IVT alone was ineffective in producing either ADH release or a blood pressure increase and phenylephrine in high doses produced both effects. We conclude that noradrenergic mechanisms may be important as a common mediator of central sympathetic outflow. The ADH release produced by AII and carbachol may be by direct action of periventricular receptors which can be damaged non-specifically by 6-OHDA.

Scanning electron microscope study of the rat subfornical organ

Abstract The subfornical organ (SFO), which has been studied for many years, has no known function, but recent experiments on the rat SFO suggest that the organ may play a role in the reception of angiotensin II, leading to drinking. The surface morphology of the rat SFO has, therefore, become important and a scanning electron microscope study of the SFO has been made. Rats were perfused systemically or ventricularly. The SFOs of 5 adult rats showed that at least 3 zones can be distinguished. Zone 1, including the anterior stalk, is covered by cilia. Zone 2, which consists of about two-thirds of the surface, is virtually without cilia and the polygonal cell boundaries of the ependymal cells could be seen. Zone 3 is more posterior and contains (i) 3 types of cilia arrangements — ‘line’, ‘single’ and ‘circular’, (ii) numerous ‘bumpy’ surfaces, (iii) possible evidence of granular secretion, and (iv) entrances and exits of blood vessels. All the zones have numerous microvilli-like structures on the surface but at points these are in greater profusion. The 3 different zones suggest different functional significance. The conformities and the differences between the rat SFO and the rabbit SFO are discussed.

The effect of subfornical organ lesions and ventricular blockade on drinking induced by angiotensin II.

The role of the subfornical organ (SFO) as the unique receptor site for the drinking behavior induced by intracranial injections of angiotensin II (AII) was investigated. It was found that: (1) drinking in response to intraventricular (IVT) injections of AII was reduced in 6 rats but was unchanged after 80-100% damage of the SFO in 4 cases; (2) reduction of drinking to lateral ventricular application of AII was seen with no apparent SFO damage in 4 rats; (3) recovery of the AII induced drinking deficit was consistently observed within a short time interval (14 days), even in those animals with complete SFO lesions: (4) the presence of ventricular debris was correlated with deficits in water intake to IVT angiotensin injections. In a second experiment artificial blockade of the ventricular space was produced by a plugging technique. Plugging the anterior third ventricle simulated the effects of SFO lesioning. It was concluded that the SFO is not a unique receptor area since the ventral anterior third ventricle is also sensitive for AII (IVT) induced drinking. If the SFO is a receptor site for AII circulating in the CSF it is probably not the only periventricular receptor site. Access of AII to the anterior ventral third ventricle appears to be essential for inducement of drinking.

A pressor response associated with drinking in rats.

Summary When angiotensin II is injected into the cerebral ventricles of the rat, drinking and a short latency pressor response are produced. We have found that the pressor response has two components. The first component is always associated with angiotensin injections. The second component, however, is associated with drinking behavior. If water is not available, the second pressor component is absent. We have investigated this drinking-associated pressor response and find that it is not only seen when angiotensin is the stimulus, but also in normal or carbachol-induced drinking. The effect is not due to head position during drinking or hypoxia but it is mediated by the sympathetic nervous system since it is abolished by alpha-adrenergic blockage. We are grateful for the technical assistance of Judy Phipps and to Dr. J. Farber for advice. This work was supported by Program Project Grant No. HI07007 to W. E. H., NSF Grant No. BNS75-16364 to M. I. P. and Grant No. MRIS 7737.03 from the Veterans Administration to Phillip G. Schmid, M.D.

SENSITIVE SITES IN THE BRAIN FOR THE BLOOD PRESSURE AND DRINKING RESPONSES TO ANGIOTENSIN II

Publisher Summary The integration of water balance, which is essential for terrestrial beings, is a function of the brain. Angiotensin offers the opportunity to unravel the complex circuitry involved by leading to the receptors from which neural circuits emanate into the brain that control thirst, antidiuretic hormone (ADH) release, and sympathetic activation of blood pressure. The chapter discusses that angiotensin II receptors are not scattered all over the brain but are in circumscribed versions close to the ventricles. They are available to either blood-borne angiotensin, as in the subfornical organ and area postrema, or they are available to cerebrospinal fluid (CSF)-borne angiotensin in the ventral anterior third ventricle. In view of the proximity of the paraventricular neurons to the ventricles, angiotensin in the CSF is able to act on ADH secreting cells directly. There is some neurophysiological evidence to support this view for supraoptic nuclei. Angiotensin II produces its effects by simultaneously acting on closely proximated receptors that are the internal points of origin for different neural pathways.

Evidence for sar1-ala8-angiotensin crossing the blood cerebrospinal fluid barrier to antagonize central effects of angiotensin II.

Injections of angiotensin II into the cerebral ventricles of the rat produces both a drinking and a pressor response. We have measured both responses simultaneously in conscious animals. The effect of saralasin acetate (P113), a specific angiotensin II competitive antagonist, has been studied on these angiotensin II induced responses. The results show that: (1) P113 given intravenously (i.v.) in doses of 500 ng/min or 1800 ng/min has no observable effect on 50 ng angiotensin given intraventricularly (IVT). At 72 mug/min, however, there was a 55% reduction in the drinking and pressor responses to 50 ng angiotensin II (IVT); (2) 500 ng of P113 given IVT abolished the effects of 50 ng angiotensin II also given IVT and (3) P113 given i.v. at all doses antagonized the pressor effects of angiotensin II (i.v.) responses. The data indicate that both the drinking and pressor responses to angiotensin II (IVT) injections are centrally mediated and show that when a high enough dose of P113 is given peripherally the central effects of angiotensin II can be reduced. This suggests that a fraction of the P113 injected i.v. may pass across the blood-CSF barrier. Since P113 has a similar structure to angiotensin II the results have implications for studies in which high peripheral doses of angiotensin II are used.

Independent receptors for pressor and drinking responses to central injections of angiotensin II and carbachol

Angiotensin II and carbachol when injected in the brain ventricles of the rat produce similar responses of an increase in blood pressure and drinking behavior. The question of whether these effects are produced by independent receptors or via a cholinergic circuit is debatable for the drinking behavior and evidence is lacking for the blood pressure effect. We have used a chronic rat preparation for recording blood pressure and drinking at the same time during intraventricular injections (i.v.t.) of both angiotensin and carbachol and i.v.t. or intravenous infusions of appropriate antagonists. The results show that drinking and blood pressure response to angiotensin II can be blocked by P113 (500 ng 1.v.t.) an angiotensin antagonist; they are not blocked by atropine (10 mug i.v.t.) a cholinergic antagonist; carbachol effects, however, are not blocked by P113, but are totally blocked by atropine (10 mug i.v.t.), At high doses of atropine there is inhibition of both agents but this probably represents a general inhibition. The hormone and cholinomimetic administered together interact and both are inhibited by adrenergic stimulation. We conclude from these experiments that angiotensin and carbachol act upon independent receptors in the brain to produce blood pressure and drinking responses but at some point they share common, central effector pathways.