Drinking behaviour in the cat induced by renin, angiotensin I, II and isoprenaline

Neural populations for maintaining body fluid balance

Fine balance between loss-of water and gain-of water is essential for maintaining body fluid homeostasis. The development of neural manipulation and mapping tools has opened up new avenues to dissect the neural circuits underlying body fluid regulation. Recent studies have identified several nodes in the brain that positively and negatively regulate thirst. The next step forward would be to elucidate how neural populations interact with each other to control drinking behavior.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Participation of opioid peptide (beta-endorphin) and norepinephrine in the control of compound 48/80-induced hypovolemic thirst in the rats

1. Subcutaneous (s.c.) administration of compound 48/80 elicited the increases of water intake, plasma beta-endorphin-like immunoreactivity, hypothalamic 3-methoxy-4-hydroxyphenylethyleneglycol sulfate and Hct in the rats. 2. The s.c. pretreatment of naloxone reduced the compound 48/80-induced water intake but had no effects on other variables. 3. Intracerebroventricular (i.c.v.) injection of naloxone attenuated the compound 48/80- and i.c.v. injected angiotensin II (ANG II)-induced water intake. 4. The hypothalamic norepinephrine metabolism was increased by s.c. injection of compound 48/80 but not by i.c.v. ANG II. 5. The present data suggest the possible involvement of opioid peptide (beta-endorphin) on the compound 48/80- and ANG II-induced thirst. However, it is uncertain whether hypothalamic norepinephrine is involved in the hypovolemic thirst mediated via stimulation of renin-angiotensin system.

Effect of beta-adrenergic antagonists on experimentally induced drinking in female rats

The nonspecific beta-adrenergic antagonist d,l propranolol, the specific beta 1-adrenergic antagonist atenolol, and the specific beta 2-adrenergic antagonist butoxamine were administered intraperitoneally (IP) to ovariectomized female rats in order to determine the role of beta-adrenergic receptors in drinking. D,l propranolol and atenolol administered at doses of 6, 12, and 18 mg/kg significantly attenuated the one-hour water intakes of rats administered angiotensin II (200 micrograms/kg, SC) and the water intakes of rats deprived of water for 24 hours. D propranolol, which has little beta-adrenergic blocking ability, administered at doses of 6 and 12 mg/kg, and butoxamine, administered at doses of 25 and 35 mg/kg, had no significant effects on the water intakes of angiotensin II treated or water deprived rats. Regardless of the dose, d,l propranolol, atenolol, and butoxamine failed to significantly alter the water intakes of rats administered 1.0 M NaCl (10 ml/kg, IP) The results provide evidence that beta 1-adrenergic receptors, but not beta 2-adrenergic receptors, are involved in mediating the increased water intakes induced by angiotensin II and water deprivation. On the other hand the increased water intake due to administration of hypertonic saline does not appear to mediated by beta-adrenergic receptors.

Two inhibitors of angiotensin-converting enzyme, enalapril and captopril, increase salt appetite of rats

Male rats were orally administered an inhibitor of angiotensin-converting enzyme (ACE), N-[(S)-1-(ethoxycarbonyl)-3-phenylpropyl]-1-ala-1-pro maleate (enalapril, MK-0421) at dosage levels of 10, 30, and 90 mg/kg X d. After 2-6 wk of dosing, the rats receiving 30 and 90 mg/kg X d produced large numbers of seminal plugs and had lacerated penises due to licking in an attempt to recover urine. Providing 0.9% saline as the source of drinking water prevented this behavior and subsequent lesions. There were no adverse effects on reproductive performance. A subsequent study showed that enalapril at 5 mg/kg X d po and captopril (another ACE inhibitor) at 25 mg/kg X d po increased NaCl intake in rats. Our results with captopril confirm those of Fregly (1980) and Evered and Robinson (1983) and show that both converting-enzyme inhibitors (enalapril and captopril) increase salt appetite in rats.

Effects of propranolol on induced water intake and on the subfornical organ surface

The chronic effect of propranolol on induced water intake and on the subfornical organ was studied. Propranolol reduced water intake postdehydration. It did not inhibit the increase in plasma renin activity due to dehydration or the dipsogenic response to angiotensin II. Propranolol decreased the subfornical organ large protrusion cells. This result suggests that propranolol impairs the thirst not related to angiotensin II. The subfornical organ changes may indicate that propranolol blocks a beta-adrenergic system originating in ependymal cells.

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.

Effects of chronic intraventricular administration of angiotensin II on drinking behavior and blood pressure

Angiotensin II was continuously infused into the lateral cerebral ventricle of rats, and the effects on daily food and water consumption, urine volume, and aortic blood pressure were studied. All was infused at a rate of 10 ng/hr for seven days, using subcutaneously implanted osmotic minipumps. An intraventricular (IVT) control group was infused with only the saline vehicle, while a third group received AII subcutaneously. IVT AII rats showed a four-fold increase in water consumption, to a mean of 171 ml/day during Days 2-4 of infusion, whereas water intake of the other groups did not change from preinfusion levels. Urine volume showed a similar pattern to water intake, increasing five-fold in the IVT AII group during Days 2-4. These measures declined during the final three days of AII infusion, but significant tolerance was not observed. Food intake decreased markedly in both saline and IVT AII groups after implantation of the pumps, but the latter resumed normal food intake more slowly than the former, and body weight remained below preinfusion levels throughout the AII period. Aortic blood pressure of the IVT AII rats showed a slight, but progressive, rise during the infusion period, but it did not significantly exceed that of the saline rats. These results indicate that continuous, low-level, intraventricular infusion of AII may markedly increase water intake without significantly increasing fluid retention or blood pressure.

Angiotensin-induced drinking: sexual differences

The dipsogenic effect of Angiotensin-II (A-II) in relation to sexual variables was studied. It was found that Angiotensin-II administered SC constitutes a stimulus which induces more drinking in females than in males. Moreover, the adult females show maximum sensitivity to A-II during proestrus. The males and females castrated at birth, and females androgenized at birth, drink similar volumes of water after A-II SC injection. The pattern of stimulated intake is different in the two sexes, and appears to depend upon the development of the rats.

The drinking response of the chicken to peripheral and central administration of angiotensin II

Intravenous injection of Ang II (val5 angiotensin II amide) elicited an immediate drinking response in the domestic fowl which lasted at least 20 minutes. The minimal dosage needed was 300 mug. Intracranial injection of 10 mug Ang II through cannulas implanted in the anterior diencephalon caused a significant increase in water intake. The minimal intracranial dosage of Ang II which evoked drinking was 2.5 mug. Intracranial injection of isotonic KCl inhibited the drinking response induced by intravenously injected Ang II when administered simultaneously. This suggests that drinking caused by both intravenous or intracranial injection of Ang II is activated through identical brain regions. The positive drinking response of the chicken is repeated concecutive intracranial injections of Ang II declined from the first injection through the following ones.

Renin-aldosterone axis in ethanol intoxication and hangover

The renin-aldosterone system was studied in human volunteers during ethanol intoxication and hangover. Plasma renin activity increased more than 100%, when 1.5 - 2.3 g ethanol per kg body weight was ingested over a three hour period. During hangover the increase even exceeded 200%. Plasma aldosterone concentration decreased during ethanol intoxication, but increased greatly during hangover. It is suggested that the stimulation of the renin-aldosterone axis during ethanol intoxication and hangover is due to dehydration and increased activity of the sympathetic nervous system.