Differential effects of brain lesions on thirst induced by the administration of angiotensin-II to the preoptic region, subfornical organ and anterior third ventricle

The neural basis of homeostatic and anticipatory thirst

Water intake is one of the most basic physiological responses and is essential to sustain life. The perception of thirst has a critical role in controlling body fluid homeostasis and if neglected or dysregulated can lead to life-threatening pathologies. Clear evidence suggests that the perception of thirst occurs in higher-order centres, such as the anterior cingulate cortex (ACC) and insular cortex (IC), which receive information from midline thalamic relay nuclei. Multiple brain regions, notably circumventricular organs such as the organum vasculosum lamina terminalis (OVLT) and subfornical organ (SFO), monitor changes in blood osmolality, solute load and hormone circulation and are thought to orchestrate appropriate responses to maintain extracellular fluid near ideal set points by engaging the medial thalamic-ACC/IC network. Thirst has long been thought of as a negative homeostatic feedback response to increases in blood solute concentration or decreases in blood volume. However, emerging evidence suggests a clear role for thirst as a feedforward adaptive anticipatory response that precedes physiological challenges. These anticipatory responses are promoted by rises in core body temperature, food intake (prandial) and signals from the circadian clock. Feedforward signals are also important mediators of satiety, inhibiting thirst well before the physiological state is restored by fluid ingestion. In this Review, we discuss the importance of thirst for body fluid balance and outline our current understanding of the neural mechanisms that underlie the various types of homeostatic and anticipatory thirst.

Angiotensin II disrupts inhibitory avoidance memory retrieval

The brain renin-angiotensin system (RAS) is involved in learning and memory, but the actual role of angiotensin II (A(II)) and its metabolites in this process has been difficult to comprehend. This has been so mainly due to procedural issues, especially the use of multi-trial learning paradigms and the utilization of pre-training intracerebroventricular infusion of RAS-acting compounds. Here, we specifically analyzed the action of A(II) in aversive memory retrieval using a hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs. Rats bilaterally implanted with infusion cannulae aimed to the CA1 region of the dorsal hippocampus were trained in IA and tested for memory retention 24 h later. We found that when given into CA1 15 min before IA memory retention test, A(II), but not angiotensin IV or angiotensin(1-7) induced a dose-dependent and reversible amnesia without altering locomotor activity, exploratory behavior or anxiety state. The effect of A(II) was blocked in a dose-dependent manner by the A(II)-type 2 receptor (AT(2)) antagonist PD123319 but not by the A(II)-type 1 receptor (AT(1)) antagonist losartan. By themselves, neither PD123319 nor losartan had any effect on memory expression. Our data indicate that intra-CA1 A(II) hinders retrieval of avoidance memory through a process that involves activation of AT(2) receptors.

The renin-angiotensin system in the brain: an update 1993

The renin-angiotensin system is considered to be one of the most important hormonal systems in the regulation of blood pressure and body fluid homeostasis. Ever since this system has been demonstrated to be present also in the brain, vast efforts have been made in investigating its central impact and function. The last few years, and especially the development of non-peptidic angiotensin II receptor subtype specific antagonists and the subsequent pharmacological characterization of these subtypes, brought this field of research a large step forward. This progress also might have opened up new avenues of developing highly specific anti-hypertensive drugs and thereby new ways of treating hypertension. This paper intends to provide a summary of the knowledge about the brain renin-angiotensin system accumulated during recent years; an update 1993.

Regional expression of c-fos antigen in the basal forebrain following intraventricular infusions of angiotensin and its modulation by drinking either water or saline

The expression of c-fos protein was examined in the basal forebrains of male rats 60 min following intracerebroventricular infusions of 250 pmol angiotensin II. Levels of corticosterone and vasopressin were also measured at the same time point. In animals not allowed access to water after infusion, angiotensin II induced intense c-fos expression in a band of neurons extending throughout the anterior region of the third ventricle region, including the organum vasculosum of the lamina terminalis, the median preoptic nucleus (nucleus medianus) and the subfornical organ. There were also high levels of expression in the hypothalamic supraoptic nucleus and the paraventricular nucleus, particularly its lateral (magnocellular) region, though other, parvicellular areas were also affected. No other area of the hypothalamus was altered. There was increased c-fos expression in the central nucleus of the amygdala and the bed nucleus of the stria terminalis. Allowing rats to drink during the 60-min survival period modified this pattern of response. c-fos was markedly reduced in the supraoptic nucleus and the paraventricular nucleus but not in the other areas examined, including the anterior region of the third ventricle and the amygdala. When water was withheld for 15 min, but then allowed, rats drank the same total volume but c-fos expression was no longer inhibited in either the supraoptic nucleus or paraventricular nucleus. When rats were given 0.9% saline to drink, they ingested about three times as much as water, but angiotensin II-induced c-fos expression was similar to that in rats denied access to water. The pattern was similar following access to 1.8% saline, though levels in the organum vasculosum of the lamina terminalis were reduced. There was a marked correlation between the number of c-fos-positive neurons in the supraoptic nucleus or paraventricular nucleus and plasma levels of corticosterone 60 min after infusion, but not with arginine-vasopressin levels. These experiments show that angiotensin II induces highly localized expression of c-fos in areas known to be concerned with the dipsogenic and endocrine actions of this peptide, and that this pattern is selectively altered by allowing the animal to drink solutions of different tonicity. Immediate-early gene expression is a novel and valuable method of determining the neural response to peptides at the cellular level.

ANP and naloxone reduce postdeprivation drinking after subfornical organ lesions

We tested a report that atrial natriuretic peptide (ANP) injected into, or near, the subfornical organ (SFO) will reduce the water consumption of previously water deprived rats and that suggested ANP acts on neurons in the SFO to bring about this action. We tested this suggestion and the hypothesis that the SFO is involved in the facilitation of drinking produced by opioids. ANP (5 nmol in 4 microliters, IVT) or naloxone (2 mg/ml/kg, SC, or 200 micrograms in 2 microliters, IVT) when given to rats deprived of water for 16 h (SC treatment) or 23 h (IVT treatment) significantly depressed postdeprivation drinking measured at 15 and 60 min. Rats with complete, partial, or control lesions of the SFO, after the same treatment, also showed a significant depression of postdeprivation drinking and, after 23-h deprivation, a significant hyperdipsia. There was no interaction between drug effects and lesion effects (two-factor analysis of variance, Tukey's post-hoc tests). The hyperdipsia declined exponentially and was lost 45-50 days after lesioning. Our results do not support the hypothesis that the SFO is involved in the actions of ANP or of opioids on postdeprivation drinking.

Suppression of drinking but not feeding by central eledoisin and physalaemin in the rat

The tachykinins, eledoisin and physalaemin, given by intracerebroventricular (i.c.v.) injection have been shown to be potent antidipsogenic agents in rats. To evaluate their selectivity of action on rat ingestive behaviors, we compared their effects following i.c.v. injection on the intake of water, of milk containing 3.5 or 15% fat, and of solid food. The two tachykinins inhibited water intake induced by i.c.v. angiotensin II or by cellular dehydration, but did not reduce the intake of 15% fat milk or of solid food. The intake of 3.5% fat milk was inhibited only by the highest dose (1000 ng/rat) of eledoisin which also increased grooming and locomotion. The present findings suggest that in adult rats central eledoisin and physalaemin exert a selective suppressive effect on drinking behavior without affecting feeding.

Factors affecting angiotensin II-induced hypothermia in rats

Systemic administration of angiotensin II (AII) to the rat has previously been shown to induce a dose-dependent, hypothermic response manifested by a fall in colonic temperature (CT), a decrease in heat production and an increase in tail skin temperature (TST). The factors mediating AII-induced hypothermia and their site of action were the subjects of the present investigation. To this end, intracerebroventricular administration of 1 microgram of AII induced a 0.4 degrees C reduction in CT and a 2.4 degrees C increase in TST. In contrast, SC administration of 200 micrograms angiotensin III/kg induced a slight increase in CT but had no affect on TST. Pretreatment with the AII-receptor antagonist, saralasin, at either 1 or 10 micrograms/kg, SC did not affect either the fall in CT or the increase in TST induced by administration of 200 micrograms AII/kg, SC. However, the administration of 100 micrograms saralasin/kg, SC attenuated both the fall in CT and the increase in TST induced by either 100 or 200 micrograms AII/kg. Since both the presynaptic alpha adrenoceptor agonist, clonidine, and the opioid antagonist, naloxone, modulate the pressor and dipsogenic responses to AII, their effects on AII-induced hypothermia were tested. Both clonidine (25 micrograms/kg, SC) and naloxone (1 mg/kg, IP) enhanced the fall in CT. Clonidine lengthened the duration of the increase in TST while naloxone had no effect. Pretreatment with the presynaptic adrenoceptor antagonist, yohimbine (300 micrograms/kg, SC), did not alter the hypothermic response to administration of AII. To determine whether vasodilation of the tail of the rat was mediated by AII-induced prostaglandin release, indomethacin (4 and 6 mg/kg) was administered.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of local cerebral glucose utilization during chronic dehydration in rats

The quantitative autoradiographic deoxyglucose method was used to study changes in local cerebral glucose utilization in conscious dehydrated rats. Animals were either given saline to drink or were deprived of water for 5 days. Saline ingestion did not alter the rates of glucose metabolism in any brain region when compared to the rates of glucose metabolism in animals which had free access to water. Glucose utilization was increased by 140%, however, in the pituitary neural lobe. Water deprivation produced both increases and decreases in glucose metabolism, depending on the particular structure. In 20 of 44 brain structures analyzed, there were significant decreases from -18 to -34% in glucose utilization. Four forebrain structures, the subfornical organ, septal triangular nucleus, and hypothalamic paraventricular and supraoptic nuclei, had increases in glucose utilization of 30-73%. The rate of glucose utilization in the pituitary neural lobe was increased by 367% in water-deprived rats. The results demonstrate that metabolic activity is stimulated in some, but not all, of the structures participating in fluid regulation during an intense thirst challenge. Many brain regions have depressed metabolism in chronic severe dehydration.

Selective metabolic stimulation of the subfornical organ and pituitary neural lobe by peripheral angiotensin II

The subfornical organ is a major receptor area for one of the principal stimuli of thirst, the octapeptide, angiotensin II. In conscious water-sated rats, we examined the effects of intravenous infusion of angiotensin II on the rate of glucose utilization in the subfornical organ and in structures anatomically and functionally connected with it. Angiotensin II produced pressor and drinking responses and increased glucose utilization selectively in the subfornical organ and pituitary neural lobe and in no other brain structure. Treatment with the angiotensin II antagonist, sar1-leu8-angiotensin II, before intravenous administration of angiotensin II prevented metabolic stimulation of the subfornical organ and neural lobe. Captopril, an inhibitor of angiotensin-converting enzyme, was administered to homozygous Brattleboro rats, which normally have elevated rates of glucose utilization in the subfornical organ. Captopril reduced subfornical organ glucose metabolism to a level similar to that found in control animals. These results demonstrate that peripheral angiotensin II stimulates glucose metabolism in the subfornical organ under conditions in which it provokes drinking and pressor responses. The findings suggest that circulating angiotensin II is responsible for the high rate of glucose utilization observed in the subfornical organ of Brattleboro rats homozygous for diabetes insipidus.

Drinking behavior following electrical stimulation of the subfornical organ in the rat

Electrical stimulation of the subfornical organ (SFO) through implanted stainless-steel electrodes produced drinking in water-sated rats. Drinking was elicited primarily during the interstimulation (OFF) periods. Water intake in rats with hippocampal electrode placements occurred during both ON and OFF periods at current intensities (24-100 microA) similar to rats with SFO placements. These findings support the hypothesis that the SFO is involved in the central control of fluid balance.

Acute abdominal vagotomy reduces drinking to peripheral but not central angiotensin II

Rats were tested two or three days after bilateral abdominal vagotomy or a laparotomy control procedure for their drinking responses to subcutaneous (1 mg-kg-1) or intracerebroventricular (100 ng) injections of angiotensin II. Vagotomy delayed the initiation of drinking and decreased 60-min water intake after subcutaneous, but not after intracerebroventricular, angiotensin II. This is the shortest postoperative interval in which the decrease in drinking after systemic injection of angiotensin II by abdominal vagotomy has been observed. The failure of vagotomy to decrease the response to intracerebroventricular angiotensin II demonstrates that the deficit after subcutaneous injection was not a nonspecific effect of recent vagotomy. These results, therefore, suggest that the abdominal vagus is necessary for normal drinking in response to circulating angiotensin II. Furthermore, the selective and acute onset of the deficit is consistent with the loss of a specific, rather than tonic facilitatory, vagal mechanism for drinking after elevation of circulating angiotensin II levels. Finally, the results imply that the physiological mechanisms which mediate the drinking responses to central and peripheral angiotensin are not identical.

Mechanisms for induction of drinking with special reference to angiotensin II

The Japanese quail drinks water vigorously after water deprivation, haemorrhage and administration of hypertonic saline solution. Most avian species responded to angiotensin II (AII) by drinking, but carnivorous birds and those originating in arid regions were insensitive. The receptive sites for AII were the subfornical organ (SFO) and the preoptic area (POA) in the Japanese quail. Catecholaminergic fibers proceed from the POA to the SFO. Dipsogenic information generated by AII at the POA is transferred to the SFO through the catecholaminergic nerve fibres. Plasma AII increased following dehydration and haemorrhage and returned to a normal level immediately after rehydration. Following dehydration, arginine vasotocin, aldosterone and corticosterone increased in plasma as well as AII. A single intraperitoneal injection of AII induced increases of arginine vasotocin, aldosterone and corticosterone in plasma. It seems that AII functions as a trigger for release of these other hormones during dehydration.

Intracranial injection parameters which affect angiotensin II-induced drinking

Three intracranial injection parameters, injectate concentration at equivalent doses, rupture or bypass of the lateral ventricle by cannula in reaching the site, and multiple injections per animal, were studied to assess their effects on the drinking behavior elicited by angiotensin II at the lateral preoptic area (LPOA). Half of the animals were implanted with 23 gauge cannulae which penetrated the lateral ventricle en route to the site. The remaining animals received cannulae that were angled laterally to bypass the ventricle. In ventricular animals, the more concentrated injectate increased the total water intake over a 30 min period and affected the pattern of drinking through time. Animals with cannulae that penetrated the ventricle en route to the LPOA drank significantly more than the animals whose cannulae missed the ventricle. In all cases, no significant difference in drinking response was found between the first and second injections received by each rat. These results indicate that standardized intracranial chemical injection methods are needed for comparison of experiments utilizing this technique.

Evidence of a direct action of angiotensin II on neurones in the septum and in the medial preoptic area

Angiotensin II (AII) was microiontophoretically applied on neurones located in the septum and the medial preoptic area (MPOA). All the septal neurones sensitive to AII (15/37) responded by an inhibition to the peptide application. Of 44 units tested in the MPOA 21 cells (48%) were sensitive to AII and responded either by an increase (11/21) or decrease (10/21) in their firing. The specificity of these responses were ascertained by simultaneous application of the antagonist Sar-Ile-Angiotensin II. These data suggest that Angiotensin II acts directly on neurones of the septum and medial preoptic area, structures implicated in the control of drinking behaviour.

Involvement of catecholaminergic nerve fibers in angiotensin II-induced drinking in the Japanese quail, Coturnix coturnix japonica

Monamine distribution in a septohypothalamic area was investigated in the Japanese quail using a histochemical fluorescence method. This area includes the subfornical organ (SFO) and the preoptic area (POA) which are inferred dipsogenic receptor sites for angiotensin II (AII) in the Japanese quail. Nerve fibers showing yellow-green fluorescence were found between the POA and the SFO. Thwy traversed from the POA to the SFO, and some fibers seemed to terminate on the neurons in the SFO. After a low dose of reserpine, a considerable number of fluorescent perikarya were found in the POA. These fibers and perikarya appeared to be of primary catecholamine judging from the fluorescence color. Following transection of these fibers, fluorescence disappeared from the fibers located on the SFO side of the transection plane, while it became a little more intense on the POA side. After transection, microinjection of AII into the POA was no longer effective in induction of drinking. On the other hand, sham operation or transection in areas other than between the POA and the SFO produced only minute changes in those fluorescent fibers and had little effect on the dipsogenic potency of AII injected into the POA. These results suggest that information of AII perceived at the POA is transferred to the SFO via those primary catecholamine-containing nerve fibers, which effect induced drinking.

Huntington's chorea: selective depletion of activity of angiotensin coverting enzyme in the corpus striatum

The activity of angiotensin converting enzyme, which transforms the relatively inactive decapeptide angiotensin I to the active octapeptide angiotensin II by removal of an L-histidyl-L-leucine residue, has been assayed in numerous region of the calf brain and of the brains of humans with Huntington's chorea and controls. In calf brain there are pronounced regional variations in enzyme activity, with highest activity in the globus pallidus and area postrema. In human brain, enzyme activity is highest in the corpus striatum, with similar levels in the caudate, putamen, and globus pallidus. Converting enzyme activity is reduced by 83 to 92% in the globus pallidus in Huntington's chorea. The caudate and putamen of choreic patients display 62 to 69% reductions in enzyme activity. Converting enzyme activity in two cerebral cortical regions from choreic brains is not significantly different from control.

Self-stimulation of the subfornical organ and lateral hypothalamus: differential effects of atropine and methysergide

The effects of cholinergic blockade of neurons by atropine or serotonergic blockade by methysergide was investigated in rats responding for brain-stimulation reward. Bipolar stimulating electrodes were placed either in the subfornical organ (SFO) or the lateral hypothalamus (LH). Atropine sulphate and methysergide significantly suppressed self-stimulation of the SFO but not of the LH, suggesting that cholinergic and serotonergic neurons are involved in brain-stimulation reward associated with this site.

The role of mesencephalic structures in thirst induced by centrally administered angiotensin II

(1) In 27 animals microinjection of 25--100 ng of angiotensin II through chronic cannulae implanted in the preoptic region initiated drinking and in subsequent acute experiments influenced the spontaneous discharge rate of single neurons in the ipsi-lateral mesencephalon. Of 148 neurons for which recordings were made, 52 (35%) increased their frequency of spike potentials following administration of angiotensin II, 2 (1%) showed inhibition and 94 (64%) showed no change in firing rate. (2) In another series of 44 animals, unilateral or bilateral lesions of the midbrain ventral tegmentum or reticular formation were found to have little or no effect on water intake elicited by the microinjection of angiotensin II into the preoptic region. (3) In contrast to the effects of tegmental and reticular lesions, unilateral lesions located dorsally and laterally to the mammillary peduncle, in the area of passage of the medial forebrain bundle, significantly attenuated the dipsogenic response to either contralateral or ipsilateral injections of angiotensin II into the preoptic region. With bilateral lesions this effect was permanent. (4) Since the more caudal lesions were relatively ineffective in disrupting the elicited drinking, it is suggested that signals from angiotensin II receptors in the preoptic region are transmitted along pathways which diverge in the midbrain. (5) The possibility of a forebrain-hypothalamus-midbrain circuit mediating thirst initiated by activation of the renin-angiotensin system is discussed.

Body fluid changes which influence drinking in the water deprived rat

1. After overnight deprivation of water both the cellular and extracellular fluid volumes are significantly reduced in the rat. 2. In the rat with functional kidneys oral, intragastric or intravenous preloads of 10 ml. water reduce the total water intake after 1 hr by 64-69%. These preloads restore plasma osmolality to pre-deprivation levels but have little effect on plasma volume. 3. In the same rats if the plasma volume is restored with an oral, intragastric or intravenous preload of 10 ml. of an isotonic balanced salt solution which has little effect on osmolality, drinking is significantly reduced by 20-26%. The reduction of drinking correlated with the volume of the preload of balanced salt. 4. Plasma analysis shown that 1 hr after an oral preload of 10 ml. isotonic balanced salt solution, the extracellular fluid volume of the deprived rats is restored to pre-deprivation levels but osmolality is unchanged. Three hr after the balanced salt preload, extracellular fluid volume is still at pre-deprivation levels and there has been a slight decrease in osmolality due to excretion of salt. 5. In rats which had been nephrectomized or had the ureters ligated so there could be no renal modification of the preloads, the effects of the preloads of water and balanced salt are the same as in rats with intact kidneys. 6. The results indicate that after water deprivation in the rat, changes in both the cellular and extracellular fluid compartment are stimuli to drinking.

Evidence that the preoptic region is a receptive site for the dipsogenic effects of angiotensin II

Drinking elicited by administering angiotensin II (ANG II) to the preoptic region with cannulae passing through the lateral ventricles was attenuated significantly when the ventricles or subfornical organ were pretreated with saralasin acetate (Sar1-Ala8-angiotensin II). If the cannulae in the preoptice region were angled to bypass the lateral ventricles water intake elicited by ANG II was less and pretreating the cerebral ventricles with saralasin acetate did not reduce the drinking response. The results suggest that the preoptic region may be a receptive site for ANG II in addition to the subfornical organ and/or cerebral ventricles.