A Potent Dipsogenic Action of Homologous Angiotensin II Infused at Physiological Doses in Eels

Hormonal control of drinking behavior in teleost fishes; insights from studies using eels

Marine teleost fishes drink environmental seawater to compensate for osmotic water loss, and the amount of water intake is precisely regulated to prevent dehydration or hypernatremia. Unlike terrestrial animals in which thirst motivates a series of drinking behaviors, aquatic fishes can drink environmental water by reflex swallowing without searching for water. Hormones are key effectors for the regulation of drinking. In particular, angiotensin II and atrial natriuretic peptide are likely candidates for physiological regulators because of their potent dipsogenic and antidipsogenic activities, respectively. In the eel, these hormones act on the area postrema in the medulla oblongata, a circumventricular structure without blood-brain barrier, which then regulates the activity of the glossopharyngeal-vagal motor complex. These motor neurons in the hindbrain innervate the upper esophageal sphincter muscle and other swallowing-related muscles in the pharynx and esophagus for regulation of drinking. Thus, the neural circuitry for drinking in fishes appears to be confined within the hindbrain. This simple mechanism is much different from that of terrestrial animals in which thirst sensation is induced through hormonal actions on the subfornical organ and organum vasculosum of the lamina terminalis that are located in the forebrain. It seems that the neural and hormonal mechanism that regulates drinking behavior has evolved from fishes depending on the availability of water in their natural habitats.

Changes in plasma angiotensin subtypes in Japanese eel acclimated to various salinities from deionized water to double-strength seawater

Our knowledge of complexity of the renin-angiotensin system (RAS) has grown in recent years and various angiotensin peptides including Ang II, Ang III, Ang IV, and Ang (1-7) were found to have specific functions. Using a combination of HPLC and radioimmunoassay (RIA), we established a high resolution method to quantify various angiotensin subtypes in the plasma of eel acclimated to deionized water (dW), freshwater (FW), seawater (SW), and double-strength seawater (DSW). [Asn(1), Val(5)]-Ang II, [Asp(1), Val(5)]-Ang II, [Val(4)]-Ang III, and [Val(3)]-Ang IV are all present in the circulation and both Ang II subtypes were significantly higher in DSW eel. When the eel was transferred from FW to SW, plasma immunoreactive (ir) Ang II concentration increased and its levels were highly correlated to plasma osmolality, suggesting that the elevated plasma osmolality is the major stimulus for activating the RAS during high salinity transfer. To examine the conversion of [Asn(1)] to [Asp(1)] residue in vivo and in vitro, synthetic [Asn(1), Val(5)]-Ang II was injected into the circulation or incubated with plasma, but the production of [Asp(1), Val(5)]-Ang II was insignificant, which implies that the conversion may occur at the angiotensinogen level. An asparaginase assay was further developed for measuring asparaginase activity and the highest activity was in liver in both FW and SW eel. This new method of analysis can be extended to study the endogenous angiotensin ligands in the local RAS. The potential significance of [Asn(1)] to [Asp(1)] conversion on Ang II metabolism and function is discussed.

The area postrema in hindbrain is a central player for regulation of drinking behavior in Japanese eels

It is recognized that fish will drink the surrounding water by reflex swallowing without a thirst sensation. We evaluated the role of the area postrema (AP), a sensory circumventricular organ (CVO) in the medulla oblongata, in the regulation of drinking behavior of seawater (SW) eels. The antidipsogenic effects of ghrelin and atrial natriuretic peptide and hypervolemia and hyperosmolemia (1 M sucrose or 10% NaCl) as well as the dipsogenic effects of angiotensin II and hypovolemia (hemorrhage) were profoundly diminished after AP lesion (APx) in eels compared with sham controls. However, the antidipsogenic effect of urotensin II was not influenced by APx, possibly due to the direct baroreflex inhibition on the swallowing center in eels. When ingested water was drained via an esophageal fistula, water intake increased 30-fold in sham controls but only fivefold in APx eels, suggesting a role for the AP in continuous regulation of drinking by SW eels. After transfer from freshwater to SW, APx eels responded normally with an immediate burst of drinking, but after 4 wk these animals showed a much greater increase in plasma osmolality than controls, suggesting that the AP is involved in acclimation to SW by fine tuning of the drinking rate. Taken together, the AP in the hindbrain of eels plays an integral role in SW acclimation, acting as a conduit of information from plasma for the regulation of drinking, probably without a thirst sensation. This differs from mammals in which sensory CVOs in the forebrain play pivotal roles in thirst regulation.

Effect of blood withdrawal and angiotensin II on prolactin release in the tilapia, Oreochromis mossambicus

Repeated blood withdrawal (5% of estimated blood volume at 0, 1, 4, 8, 24, 48 and 76 h) from tilapia acclimated to fresh water (FW) resulted in a marked increase in plasma levels of prolactin (PRL) during the first 8 h, reaching a peak above 300 ng/ml after 4 h. The increase in plasma PRL levels was significant except for the level after 72 h. A slight but significant decrease in plasma osmolality was observed at all time points after the blood withdrawal. Repeated blood withdrawal from fish acclimated to seawater (SW) resulted in a marked increase in plasma osmolality after 4 and 8 h. A significant increase was observed in plasma growth hormone (GH) in the fish in SW until the end of the experiment, but there was no change in plasma PRL. Plasma levels of cortisol were significantly higher in the fish in SW than in those in FW during the first 24 h. Blood withdrawal resulted in a significant reduction in hematocrit values in both FW- and SW-adapted fish, suggesting hemodilution. In a separate experiment, a single blood withdrawal (20% of total blood) stimulated drinking after 5 h, regardless of whether the fish were held in FW or SW. Plasma PRL level was also elevated following a single blood withdrawal in the fish acclimated to FW, but not in the fish in SW. Intraperitoneal injection of ANG II (1.0 microg/g) into the fish in FW significantly increased plasma PRL levels after 1 h. Activation of the renin-angiotensin system after blood withdrawal and the dipsogenic action of angiotensin II (ANG II) are well established in fish. The reduction in plasma osmolality after repeated blood withdrawal in FW and the increased osmolality in SW suggest that blood volume is restored, at least in part, by drinking environmental water. These results suggest that the marked increase in PRL concentration after blood withdrawal from the fish in FW is due, at least in part, to a facilitative effect between ANG II and reduced plasma osmolality.

Central effects of various ligands on drinking behavior in eels acclimated to seawater

Intracranial injection of eel angiotensin II (eANG II, 5x10(-13)-5x10(-8) mol), acetylcholine (ACh, 5x10(-12)-5x10(-9) mol), substance P (5x10(-10) mol) and isoproterenol (a beta-adrenoceptor agonist, 5x10(-11)-5x10(-9) mol) enhanced water intake in the seawater eel. The effects of eANG II, ACh and isoproterenol were dose-dependent. By contrast, water intake was inhibited by intracranial injection of eel atrial natriuretic peptide (eANP, 5x10(-13)-5x10(-10) mol), serotonin (5-HT, 5x10(-12)-5x10(-8) mol), ghrelin (5x10(-12)-5x10(-10) mol), gamma-amino butyric acid (GABA, 5x10(-11)-5x10(-8) mol), prolactin (PRL, 5x10(-10)-5x10(-9) mol), arginine vasotocin (AVT, 5x10(-12) mol), vasoactive intestinal peptide (VIP, 5x10(-11) mol), noradrenaline (5x10(-9) mol l(-1)) and phenylephrine (alpha-adrenoceptor agonist, 5x10(-11)-5x10(-9) mol). The inhibitory effects of eANP, 5-HT, ghrelin, GABA, PRL and phenylephrine were dose-dependent. The intracranial stimulatory effect of eANG II was relatively long-lasting compared with the intravenous effect. The stimulatory effect of intravenous eANG II disappeared immediately, and was followed by an inhibition, which could be well explained by an increase in eANP secretion from the atrium.

Antidipsogenic effects of eel bradykinins in the eel Anguilla japonica

A peptide with bradykinin (BK)-like immunoreactivity was isolated from an incubate of heat-denatured eel plasma with porcine pancreatic kallikrein. The purified peptide had the following amino acid sequence: Arg-Arg-Pro-Pro-Gly-Ser-Trp-Pro-Leu-Arg. This decapeptide, named eel [Arg(0)]BK, was identical to two previously identified BK homologs from cod and trout. High conservation of the BK sequence among distant teleost species suggests an important function in this vertebrate group. Bolus intra-arterial injections of eel [Arg(0)]BK, BK, and [Arg(0)]-des-Arg(9)-BK (1-10 nmol/kg) caused significant (P < 0.05) inhibition of drinking in seawater-adapted eels. The potency of the inhibition was ranked in the following order: [Arg(0)]BK > [Arg(0)]-des-Arg(9)-BK = BK. The BK peptides also produced an immediate, transient increase followed by a sustained increase in arterial blood pressure and an initial decrease followed by an increase in heart rate. Strong tachyphylaxis occurred for the cardiovascular effect but not for the antidipsogenic effect. The order of the potency of the cardiovascular actions, [Arg(0)]BK > BK > [Arg(0)]-des-Arg(9)-BK, was different from that of the antidipsogenic action. Slow infusions of eel [Arg(0)]BK in the dose range 1-1,000 pmol x kg(-1) x min(-1) produced concentration-dependent inhibition of drinking without changes in arterial pressure, plasma osmolality, and hematocrit. At the infusion rate of >100 pmol x kg(-1) x min(-1), plasma concentrations of angiotensin II, a potent dipsogenic hormone in eels, increased, suggesting an interaction of the kallikrein-kinin and renin-angiotensin systems. In mammals, BK is dipsogenic and vasodepressor, so that our data demonstrate opposite effects on fluid and cardiovascular regulation of BK in the eel and suggest a new physiological role for the kallikrein-kinin system in teleost fish.

Circulating angiotensins in the river lamprey, Lampetra fluviatilis, acclimated to freshwater and seawater: possible involvement in the regulation of drinking

Plasma angiotensin levels were measured for the first time in a cyclostome, the river lamprey. With the demonstration that angiotensins are present in the circulation, the possibility of a physiological role in the regulation of drinking was re-examined. Angiotensin II and III concentrations and plasma osmolalities were significantly higher in lampreys acclimated to 28 ppt seawater than in those acclimated to freshwater. No changes were found in angiotensin II and III levels 4 h after transfer from freshwater to 50% seawater, although plasma osmolality had started to rise by this time. There was a suggestion that plasma angiotensin II levels might be related to osmolality in the transfer experiment. Injection of Asp(1)Val(5)- or Asn(1)Val(5)-angiotensin II (40-169 microg/kg body wt.) did not stimulate drinking in freshwater-acclimated lampreys, even when they were still capable of drinking. The angiotensin-converting enzyme inhibitor captopril and the smooth muscle relaxant papaverine both reduced drinking rate in 50% seawater-acclimated lampreys. The data do not provide direct evidence for the involvement of the renin-angiotensin system in the control of drinking behaviour in the lamprey. Indirect evidence from the captopril effect is suggestive, but could have other explanations.

Role of the renin-angiotensin system in drinking of seawater-adapted eels Anguilla japonica: a reevaluation

The role of ANG II, a potent dipsogenic hormone, in copious drinking of seawater eels was examined. SQ-14225 (SQ), an angiotensin-converting enzyme inhibitor, infused intra-arterially at 0.01-1 microgram. kg(-1). min(-1), depressed drinking and arterial blood pressure in a dose-dependent manner. The inhibition was accompanied by a small decrease in plasma ANG II concentration, which became significant at 1 microgram. kg(-1). min(-1). After the infusate was changed back to the vehicle, the depression of drinking and arterial pressure continued for >2 h, although plasma ANG II concentration rebounded above the level before SQ infusion. By contrast, infusion of anti-ANG II serum (0.01-1 microgram. kg(-1). min(-1)) did not suppress drinking and arterial pressure, although plasma ANG II concentration decreased to undetectable levels. Plasma atrial natriuretic peptide and plasma osmolality, which influence drinking rate in eels, did not change during SQ or antiserum infusions. These results suggest that the renin-angiotensin system plays only a minor role in the vigorous drinking observed in seawater eels. The results also suggest that the antidipsogenic and vasodepressor effects of SQ in seawater eels are not due solely to the inhibition of ANG II formation in plasma.