The role of angiotensin, AT1 and AT2 receptors in the pressor, drinking and vasopressin responses to central angiotensin.

Angiotensin II (Ang II) given centrally produces an increase in blood pressure and motivation to drink. The physiological mechanisms that mediate the pressor response include release of vasopressin (AVP) and activation of the sympathetic nervous system. Using 2 new Ang II receptor antagonists, we were able to investigate the role of AT1 or AT2 receptors in mediating these effects. Adult male Sprague-Dawley rats were cannulated in the lateral ventricle and 5 days later catheterized in the carotid artery for blood pressure measurements. All experiments were carried out in conscious rats. Three treatments were given intraventricularly (i.v.t.), in 2 microliters artificial cerebrospinal fluid (ACSF) at 30 min intervals: (1) 50 ng Ang II, (2) 0.7 micrograms AT1 antagonist Losartan or 7.0 micrograms AT2 antagonist PD123177, followed by 50 ng Ang II, and (3) 50 ng Ang II, to test for recovery. Blood pressure and drinking measurements were recorded. Also, blood samples for assay of AVP were drawn at 1 or 3 min post-injection in 2 separate groups of rats. We found that both Losartan and PD123177 significantly reduced release of AVP to Ang II 1 min post-injection. Losartan significantly blocked the pressor response (P less than 0.001), while PD123177 had no significant effect. Drinking was also antagonized by Losartan (P less than 0.05) and reduced (n.s.) by PD123177. The results suggest that the pressor response to Ang II (i.v.t.) is predominantly AT1 mediated, while the drinking and AVP responses may be mediated by both receptor subtypes.     

Test of a criterion for selecting intracranial doses of angiotensin receptor blockers

Investigators using intracerebroventricular (ICV) injections of competitive antagonists of angiotensin II (Ang II) to study thirst usually select doses sufficient to block drinking to IV Ang II. We questioned whether this test truly indicates the dose needed under physiological conditions when Ang II-induced hypertension, which inhibits thirst, is not present. Rats were prepared with chronic venous and ICV cannulas, plus femoral arterial cannulas in those used to measure arterial pressure. Captopril (100 mg/kg SC) was given before all experiments to block endogenous Ang II production. The test dose of Ang II, 50 ng/kg/min IV for 1 hr, increased water intake and arterial pressure. We selected an ICV dose of saralasin (Sar1Ala8Ang II), 4 micrograms bolus and 4 micrograms/hr for 75 min, that did not stimulate drinking itself and completely blocked drinking to IV Ang II. This dose of saralasin only partially (45%) reduced drinking to the same dose of Ang II IV when arterial pressure was lowered by giving the vasodilator diazoxide (15 mg/kg IV). Diazoxide itself did not stimulate drinking. These results support our concern that the criterion normally used to select ICV doses of Ang II antagonists probably underestimates the amount needed to inhibit angiotensinergic drinking in hypovolemic or hypotensive animals.     

Clonidine antagonism of angiotensin-related drinking: a central site of action

Administration of either isoproterenol (25 μg/kg, s.c.) or angiotensin II (200 μg/kg, s.c.) induces drinking in rats within 0.5–1 h. This drinking was inhibited by prior administration of the presynaptic α-adrenergic agonist clonidine (12 μg/kg, i.p.). Urine output was enhanced by clonidine in the angiotensin II-, but not the isoproterenol-treated group. Drinking in response to peripheral administration of either angiotensin II or isoproterenol was also inhibited by intracerebroventricular (i.v.t.) administration of clonidine (8 μ/kg). This dose of clonidine also enhanced the urine output after angiotensin II. Further, the drinking induced by i.v.t. administration of angiotensin II, at 4 but not 20 ng/kg was inhibited by peripheral administration of clonidine (12 μg/kg, i.p.). When clonidine was administered i.v.t. prior to i.v.t. injection of either angiotensin II (20 ng/kg) or carbachol (1.2 μg/kg), the drinking response to these dipsogens was attenuated. These results suggest that clonidine may act centrally to attenuate drinking at a site, possibly in the nucleus tractus solitarius, that may be considered a final common pathway for this response.     

Potentiation of angiotensin II-induced drinking by glucocorticoids is a specific glucocorticoid Type II receptor (GR)-mediated event

Earlier studies showed that pretreatment (3 and 6 h) of rats with the glucocorticoid hormone, dexamethasone, potentiated the drinking response to either central or peripheral administration of angiotensin II (AII). In the present study the specificity and mechanisms of this potentiation were examined. Intraperitoneal (i.p.) injection of rats with the pure glucocorticoid agonist, RU 28362 (0.4-1.6 mg/kg; 3-24 h), resulted in a time- and dose-dependent potentiation of the drinking responses to either peripherally (100 micrograms/kg, s.c.) or centrally (10 ng) injected AII, similar to the effects of dexamethasone. Drinking induced by central injection of carbachol (200 ng) was unaltered by pretreatment with RU 28362, suggesting that potentiation by this compound was specific for AII. The potentiation of AII-induced drinking by either dexamethasone or RU 28362 was completely abolished by pretreatment with the glucocorticoid Type II receptor (GR) antagonist, RU 38486 (2 mg/kg, i.p.), but not by the mineralocorticoid Type I receptor (MR) blocker, mespirenone (2 mg/kg, i.p.). Taken together, these results indicate that the glucocorticoid-induced potentiation of AII-induced drinking is mediated via GR. Associated with the fact that glucocorticoids potentiate AII-induced drinking is the observation that these steroids also potentiate AII-induced urine output. This enhancement of urine output may explain in part the potentiation in drinking behavior. Possible mechanisms are discussed.     

Mineralocorticoid pretreatment enhances angiotensin II-induced neuronal excitation but not salt drinking in male Fischer rats

Central administration of angiotensin (Ang) II stimulates thirst and sodium intake via the AT-1 receptor. Mineralocorticoid pretreatment enhances Ang II-induced drinking of hypertonic salt solutions (i.e. the synergy theory) in Wistar and Sprague-Dawley rats. Electrophysiological experiments using iontophoretic application of Ang II, and the AT-1 receptor specific nonpeptide antagonist losartan, have shown excitation of neurones in the preoptic/medial septum region of urethane anaesthetised male Wistar rats. Deoxycorticosterone acetate (DOCA) pretreatment further enhanced this neuronal excitation to Ang II and reduced the responses to losartan. This generated the hypothesis that DOCA-enhanced Ang II-induced neuronal excitation was necessary for the enhanced salt intake of synergy theory. We tested this hypothesis in Fischer 344 rats that are known to have a low basal salt appetite and reduced sensitivity for i.c.v. Ang II. We compared the effect of DOCA pretreatment on i.c.v. Ang II-induced water and 2% NaCl intake in behaving adult male, Fischer rats, as well as preoptic/medial septum region neuronal responses to Ang II and losartan, using a seven-barrelled micro-iontophoretic electrode sealed to a recording electrode in urethane anaesthetised, male Fischer rats. Two groups were used: one pretreated with DOCA (0.5 mg/day for 3 days) and the other comprising controls, treated with isotonic saline. Ang II applied iontophoretically increased activity in 31% of the spontaneously active neurones. Following DOCA pretreatment, the responsiveness to Ang II (when applied after aldosterone) was increased. By contrast, in the behaving animals, water and 2% NaCl intake in response to i.c.v. Ang II were not enhanced by DOCA pretreatment. These results do not support the working hypothesis but could be interpreted as evidence for the existence of two separately modulated central Ang II systems: one responding to mineralocorticoids with increased neuronal activity and the other responsible for the Ang II-induced sodium appetite in conscious rats.     

Glucocorticoids potentiate the dipsogenic action of angiotensin II

The effect of a short-term, acute treatment with a glucocorticoid, dexamethasone sodium phosphate, on the drinking induced by angiotensin II (AII) was investigated in a series of experiments. Initial studies indicated that a single injection of dexamethasone (700-750 micrograms/kg, i.p.) reduced food intake, body weight and water intake for up to 48 h, but had little effect on blood pressure when it was measured 6 h subsequent to the injection. The drinking elicited by peripherally administered AII (200 micrograms/kg, s.c.) was enhanced if the glucocorticoid (700 micrograms/kg, i.p.) was given 3 h or 6 h prior to the dipsogen. There was no effect of pretreatment with the steroid if the drinking test was delayed by 24 h. The subsequent experiment showed that the glucocorticoid effect on AII-stimulated drinking was dose dependent (100 micrograms-1600 micrograms/kg). The drinking stimulated by intracerebroventricular (i.c.v.) AII (2.5 ng) was enhanced in terms of volume and total duration by prior treatment with dexamethasone, but i.c.v. carbachol (200 ng)-induced drinking remained unaffected. The final study showed that binding of AII to its receptors in five different areas of the rat brain was not affected by prior treatment with dexamethasone.     

Effect of intracerebroventricularly administered octopamines and synephrines on angiotensin II-induced water intake in rats

Recent studies from this laboratory showed that l-m-synephrine (phenylephrine), a metabolite of l-m-octapamine, inhibited the drinking response of rats to peripherally administered angiotensin II. The objective of this investigation was to determine whether the isomers of both octapamine and synephrine could inhibit angiotensin II-induced dipsogenesis in the rat. Of the isomers tested, only d,l-m-octopamine and l-m-synephrine blocked the dipsogenic response to administration of angiotensin II (200 micrograms/kg, SC). The antidipsogenic effect of both d,l-m-octopamine and l-m-synephrine could be blocked by concurrent administration of yohimbine (300 micrograms/kg, IP), an alpha 2-adrenoceptor antagonist. The results indicate that m-octopamine and m-synephrine exert their antidipsogenic effect via alpha 2-adrenoceptors. These studies add to a growing body of data suggesting that activation of alpha 2-adrenoceptors inhibits, while blockade of these receptors enhances, angiotensin II-induced drinking.     

Amastatin potentiation of drinking induced by blood-borne angiotensin: evidence for mediation by endogenous brain angiotensin

Angiotensinergic synapses in the central nervous system (CNS) have been proposed to be involved in drinking induced by both intracerebroventricular (i.c.v.) and peripheral administration of angiotensins. In the present studies, we tested this hypothesis with i.c.v. application of amastatin, an aminopeptidase A inhibitor, to block peptide degradation. Potentiation of i.c.v. angiotensin II (Ang II)-induced drinking responses was observed when amastatin and Ang II were administered. Amastatin did not potentiate drinking to carbachol which demonstrates that the enhancement is specific to peptides. Centrally administered amastatin also potentiated drinking following systematic administration of Asn¹ angiotensin II. (Asn¹ Ang II). The results are consistent with the hypothesis that CNS angiotensin synapses are involved in the dipsogenic response that results from elevated levels of circulating angiotensin.     

Neuroanatomical site of the inhibitory influence of anxiolytic drugs on central serotonergic transmission

The neuroanatomical site of the inhibitory influence of anxiolytics on central serotonergic transmission has been investigated in the rat by studying the effect of systemic or intracerebral administration of these drugs on cerebral serotonin (5-HT) synthesis. Systemic administration of diazepam (3 mg/kg s.c.) or flunitrazepam (1 mg/kg, s.c.) caused a reduction of 5-HT synthesis (as measured by the accumulation of 5-hydroxytryptophan after inhibition of aromatic amino acid decarboxylase) in the hippocampus but not in the cerebral cortex, striatum, cerebellum or spinal cord of the rat. Zopiclone (22 mg/kg, s.c.) decreased the amine synthesis in hippocampus, striatum and prefrontal cortex. The decrease of hippocampal 5-HT synthesis induced by diazepam (5 mg/kg, s.c.) was antagonized by the benzodiazepine antagonist Ro 15-1788 (2 X 30 mg/kg, s.c.) but not by bicuculline (2 X 1 mg/kg, s.c.). Acute cerebral hemitransection or electrolytic lesion of the fasciculus retroflexus did not prevent the ability of diazepam (5 mg/kg, i.p.) to diminish hippocampal 5-HT synthesis. Local infusion of diazepam (15 micrograms) of flurazepam (1.5 micrograms) into the hippocampus of conscious rats (via indwelling cannulae) markedly reduced 5-HT synthesis in this brain area whereas infusion of these drugs into the raphé medianus (origin of the serotonergic afferents to the hippocampus) failed to affect hippocampal 5-HT synthesis. In contrast, local injection of muscimol (25-150 ng) into the raphé medianus reduced 5-HT synthesis in the hippocampus. This effect of muscimol was potentiated by a systemic administration of diazepam or an intra-raphé medianus infusion of flurazepam (at doses or concentrations which exhibited no intrinsic activity). It is concluded from these data that anxiolytic drugs exert an inhibitory influence on hippocampal serotonergic neurons which is mediated primarily via GABA-independent benzodiazepine receptors located in the vicinity of serotonergic nerve terminals.     

Lesions of the diagonal band of broca enhance drinking in the rat

This study examined the role of the diagonal band of Broca (DBB) in drinking behaviour and vasopressin release. Adult male rats were anaesthetized (pentobarbital 50 mg/kg) and received DBB injections of either ibotenic acid (0.5 microl of 5 micro g/ microl) or vehicle (0.5 microl of phosphate-buffered saline). Although baseline drinking and urine output were not affected, drinking to 30% polyethylene glycol (MW 8000; 1 ml/100 g s.c.) and angiotensin II (0, 1.5 and 3.0 mg/kg s.c.) were significantly increased in ibotenic acid in phosphate-buffered saline (DBBX) rats. Drinking to hypertonic saline (0.9, 4 and 6%; 1 ml/100 g), and water deprivation were not significantly affected. DBBX rats had significantly lower basal heart rates than controls but the cardiovascular responses to infusions of angiotensin II (100 ng/kg/min i.v. for 45 min) were not affected. DBBX rats had significantly higher basal vasopressin, but angiotensin-stimulated vasopressin release was not significantly different. Although the DBB is not involved in basal water intake, it is involved in dipsogenic responses to hypovolemic stimuli and possibly basal autonomic function and basal vasopressin release.     

Bradykinin-induced dipsogenesis in captopril-treated rats

The dipsogenic responsiveness to acute administration of the peptide, bradykinin, was studied in 4 groups of male Sprague-Dawley rats. All groups received captopril (35 mg/kg, IP) 15 minutes prior to the study. The first group then received saline (1 ml/kg, IP) while the remaining three groups received bradykinin at 50, 100 and 200 micrograms/kg, SC, respectively. The rats were placed alone into metabolic cages without food and given a preweighed bottle of tap water. Water intakes and urine outputs were then measured at 0.5, 1.0 and 2.0 hours thereafter. Bradykinin induced a dose-related increase in water intake but had no significant effect on urine output. To assess whether bradykinin might induce drinking by way of angiotensin II (AII) receptors, the AII receptor antagonist. Sar1, Ile8-AII (Sar, 300 and 600 micrograms/kg, SC) was administered to both bradykinin- (200 micrograms/kg, SC) and bradykinin plus captopril-treated (35 mg/kg, IP) groups of rats in a two-way ANOVA statistical design. Bradykinin again stimulated drinking in captopril-treated rats, but the drinking was not blocked significantly by administration of Sar. The results of this study indicate that blockade of the angiotensin I converting enzyme increases the half-life of bradykinin and unmasks its dipsogenic properties. The results suggest further that bradykinin does not appear to induce drinking via interaction with receptors for AII.     

Subfornical organ lesions in rats abolish hyperdipsic effects of isoproterenol and serotonin

Isoproterenol (300 micrograms/ml/kg) and serotonin (2 mg/ml/kg) given SC to rats (n = 27) caused significant drinking (Fisher PLSD, Scheffe F test, Dunnett t) in the 1 and 2 hours after injection. Such drinking was completely prevented in rats later shown to have complete lesions of their subfornical organs (n = 7). In contrast a response not significantly different from the prelesion response was found in rats later given cortical lesions (n = 11) or other lesions which did not damage the subfornical organ (n = 7). We conclude that drinking evoked by SC injection of serotonin and isoproterenol is brought about by peripheral production of angiotensin II. Blood borne angiotensin II in turn stimulates neurons in subfornical organ which initiate the neural organization of a drinking response.     

Brain Angiotensin II Receptor Subtypes and the Control of Luteinizing Hormone and Prolactin Secretion in Female Rats

The present experiments examined the role of the two recently identified angiotensin II (Ang II) receptor subtypes, AT, and AT(2) , in the central nervous system regulation of luteinizing hormone (LH) and prolactin secretion in estrogen- and progesterone-treated ovariectomized rats. In this animal model, intracerebroventricular (icv) injection of Ang II stimulates LH and inhibits prolactin release. The specific Ang II receptor subtype antagonists losartan (AT(1) ) or PD123177 (AT(2) ) were administered (icv) in various doses (10 ng to 1,000 ng) 10 min prior to icv injection of Ang II (100 ng). Control animals were pretreated with artificial cerebrospinal fluid prior to Ang II administration. Blood samples for LH and prolactin determinations were taken from conscious, freely-moving rats prior to and following injection of the antagonists and Ang II. Water intake was measured. Ang ll-induced water intake was attenuated 62% by 1,000 ng losartan; water intake was not affected by lower doses of losartan or by any dose of PD123177. Ang ll-induced stimulation of LH release was abolished by the 1,000 ng doses of losartan and PD123177 and attenuated by the 500 ng doses of both drugs. Lower doses did not affect Ang ll-induced LH secretion. Ang ll-induced inhibition of prolactin release was significantly reduced by the 1,000 ng doses of both losartan and PD123177. Lower doses of either drug did not affect the Ang II inhibition of prolactin release. Previous studies had shown that Ang II administration into the anterior hypothalamus-medial preoptic (AHPO) area stimulated LH release. This brain area contains AT(1) receptors. To investigate the potential brain site where the AT(2) receptor may influence LH release, Ang II was injected into the locus ceruleus, a brain nucleus which contains predominately the AT(2) receptor subtype. Ang II administration into the locus ceruleus was paired with an injection of artificial cerebrospinal fluid or Ang II into the AHPO area. Injection of Ang II into the AHPO area stimulated LH release. Injection into the locus ceruleus did not affect LH secretion, nor did it modify the rise in LH elicited by administration of Ang II into the AHPO area. Plasma levels of prolactin were not altered by any of these injections. Taken together, these data demonstrate that, in estrogen- and progesterone-treated female rats, icv Ang ll-induced water intake is mediated by the AT, receptor subtype, while Ang ll-induced changes in LH and prolactin secretion appear to be mediated by both the AT(2) and AT(2) receptor subtypes. The latter observations are one of the first suggesting a potential function for the AT(2) subtype in vivo, although the physiological relevance of this observation, as well as the site of action for the effects on LH and prolactin, remain to be established.     

Effects of SFO lesion or captopril on drinking induced by intragastric hypertonic saline

This study examined the hypothesis that the subfornical organ (SFO), a circumventricular organ with both osmosensitive elements and dipsogenic receptors for circulating angiotensin (ANG) II, is important for the water drinking response that follows an intragastric (ig) load of hypertonic NaCl. A 2-ml saline load was administered ig at 300, 900, or 1200 mOsm/kg to rats with sham lesions or lesions of the SFO, and intake was measured periodically for 2 h. Hypertonic loads caused sham-lesioned rats, but not SFO-lesioned rats, to drink earlier in the test or to drink more water than did the isotonic load. Inhibition of ANG II synthesis in unoperated rats with 100 mg/kg of captopril reduced water intake only during the initial 15 min after a gavage of 1200 mOsm/kg saline. Loads of 900 and 1200 mOsm/kg both increased plasma osmolality and sodium concentration by 15 min after gavage without greatly affecting hematocrit or plasma protein concentration. Thus, the SFO is important for the osmotically-induced water drinking response after acute ig administration of hypertonic saline. With the possible exception of the first 15 min, this drinking response is independent of the peripheral synthesis of ANG II.     

Water versus NaCl intake by rats administered certain dipsogens acutely

Isotonic saline was ingested in a greater volume than water when any one of three dipsogenic agents [L-5-hydroxytryptophan (25 mg/kg, SC), isoproterenol (25 micrograms/kg, SC), and angiotensin II (100 micrograms/kg, SC)] was administered acutely to rats given either isotonic saline or water to drink. These results are consistent with, but not exclusive to, the hypothesis that feedback inhibition of ingested isotonic saline on further fluid intake is less than that of water. Additional studies showed that the dipsogenic response to angiotensin II (200 micrograms/kg, SC) decreased as the concentration of NaCl solution offered to drink increased beyond 0.15 M in rats given either water or NaCl solution to drink. This suggests the possibility that the intensity of taste of the NaCl solutions may be a factor in their intake under these conditions, although the intake of these salt solutions by untreated controls did not reflect this. When the rats were allowed to choose between either 0.15 or 0.25 M NaCl solution and water, administration of angiotensin II (either 100 or 200 micrograms/kg, SC) increased water intake in preference to the NaCl solution. These results suggest that the rat, under these conditions, has a preference for water over NaCl solution.     

Effects of increased circulating angiotensin II (AII) on fluid exchange and binding of AII in the brain

Previous studies have shown that elevated levels of circulating angiotensin II (AII) can influence the binding capacity of this peptide for its receptors in peripheral tissues, but the effect of increased circulating levels of AII on its receptors in the brain has not been well-defined. In the present study, the effect of chronic subcutaneous infusions of AII on: (1) the binding of AII to neuronal membranes from the diencephalon (hypothalamus, thalamus and septum) (HTS) of the brain; (b) water intake and urine output, (c) blood pressure, and (d) their interrelationships was evaluated in rats. Significant increases in daily water intake and urine output accompanied chronic infusions of AII at a rate of 125 ng/kg/min. Both blood pressure and the concentration of aldosterone in plasma were also elevated in these rats. The acute dipsogenic response to either central (10 ng) or peripheral (100 micrograms/kg, SC) administration of AII was also tested both in controls and in rats receiving chronic infusions of AII at a rate of either 40 or 125 ng/kg/min, and no differences were observed. Analysis of the HTS region of the brain revealed a significant increase in the specific binding of AII in AII-infused rats compared to controls. Scatchard analysis of the specific binding of AII to its receptors in the HTS of rats treated with 40 ng AII/kg/min for 6 days revealed a significant increase in the number of binding sites for AII compared to controls (Bmax 12.13 vs. 8.79 fmol/mg protein), but no change in binding affinity (Kd).(ABSTRACT TRUNCATED AT 250 WORDS)     

Rat brain angiotensin II receptors: Effects of intracerebroventricular angiotensin II infusion

Angiotensin II (Ang II) was infused into a lateral cerebral ventricle of male Sprague-Dawley rats and its effects on blood pressure, water balance and specific [125I]Ang II binding to brain and adrenal tissues were studied. The infusion was maintained at a rate of 500 ng/microliter/h for 6 days using subcutaneously implanted osmotic minipumps. A control group was infused intracerebroventricularly (i.c.v.) with 0.9% saline at a rate of 1 microliter/h for 6 days. Angiotensin II treated rats showed a four-fold increase in water intake and urine volume and a moderate increase in blood pressure; these effects were not observed in rats given saline i.c.v. There was no significant difference in [125I]Ang II binding site density or binding affinity in either the hypothalamus-thalamus-septum-midbrain (HTSM) or the brainstem between Ang II-treated and saline-treated groups. In addition, [125I]Ang II binding sites in the adrenals were also unaffected by i.c.v. infusion of Ang II. The results suggest that brain Ang II receptors are unresponsive to increased Ang II levels in cerebrospinal fluid.     

Alpha-adrenergic mediation of the tail skin temperature response to naloxone in morphine-dependent rats

Studies were undertaken to evaluate the role of central noradrenergic neurons in the tail skin temperature (TST) surge that accompanies morphine withdrawal in the rat. A 5 degrees C increase in TST and a 1-2 degrees C decrease in rectal temperature (Tr) was observed following administration of a dose of naloxone HCl (NAL, 1 mg/kg, s.c.) which precipitated withdrawal in morphine-dependent rats. Intracerebroventricular (i.c.v.) injection of clonidine HCl, a partial alpha 2-adrenergic agonist did not alter TST in morphine-dependent animals. However, clonidine (10 or 50 micrograms/rat, i.c.v.) given 10 min prior to the administration of NAL completely blocked the TST response to the opiate antagonist in the morphine-dependent animals. Although NAL and clonidine reduced Tr to a similar extent in morphine-dependent rats, their effects were not additive when the drugs were administered sequentially. Treatment with the alpha-adrenergic antagonist phentolamine (11.9 or 60 micrograms/rat, i.c.v.) failed to alter TST when administered alone, but the highest dose significantly reduced the TST response to naloxone in the morphine-dependent rat. In addition, phentolamine, at high doses only, moderately reduced Tr, but the alpha-adrenergic antagonist failed to modify the decline in Tr associated with NAL-precipitated morphine withdrawal. Collectively, these data indicate that brain noradrenergic neurons play a role in the TST surge which accompanies NAL-precipitated morphine withdrawal, and that the TST and Tr responses can be dissociated in the morphine-dependent rat.     

Kappa-opioid antagonist strongly attenuates drinking of genetically polydipsic mice.

Effects of opioid antagonists on the genetic polydipsia of the STR/N strain of mice were investigated. Naltrexone (0.5-5.0 mg/kg) injected subcutaneously before dark period attenuated spontaneous drinking for the first 3 h after injection only in the inbred polydipsic mice (STR/N), whose water intake was 5 times that of controls (non-polydipsic mutant, STR/1N, and Swiss/Webster mice). The highest dose (5 mg/kg) of naltrexone administration reduced drinking also during the next 3-6 h period and overnight feeding. Cerebroventricular (i.c.v.) injection of naltrexone, 1.0 and 2.5 micrograms (per mouse), suppressed drinking only in the polydipsic mice, while the higher dose (5.0 micrograms) attenuated drinking and feeding of both the polydipsic mice and their controls. However, i.c.v. injection of specific kappa-receptor antagonist, nor-binaltorphimine (nor-BNI, 0.5-2.5 micrograms), suppressed drinking only in the polydipsic strain of mice at one-half dose of that needed for naltrexone. Furthermore, even a higher dose of nor-BNI administration was without effect on food intake in all strains. These findings suggest that the central opioid system plays an important role in causing the polydipsia in the STR/N mice, probably through the kappa-opioid receptor.     

Elevated salt appetite and brain binding of angiotensin II in mineralocorticoid-treated rats

Angiotensin II (Ang II) and aldosterone levels increase with sodium deficiency, promoting sodium conservation and arousing a salt appetite in rats. The mechanism(s), by which these two hormones interact to produce salt appetite is not known. The experiments reported here tested the possibility that increased mineralocorticoids change the number and/or affinity of Ang receptors in the brain. Rats were given a series of deoxycorticosterone acetate (DOCA) injections (500 micrograms/day, s.c., for 4 days) which are known to produce a salt appetite when given in conjunction with an intracerebroventricular injection of Ang. The binding of 125I-Ang II to membranes prepared from the septal-anteroventral third ventricular region was then examined. DOCA treatment resulted in a significant increase in the number of Ang binding sites (Bmax) with no change in binding affinity (Kd). The binding of 125I-Ang II was then investigated in membranes prepared from 12 other brain regions as well as the pituitary and adrenal gland, showing that the increase in binding capacity occurred in only a few specific brain regions. A third experiment verified that the DOCA treatment used here was sufficient to arouse a salt appetite when combined with a single intracerebroventricular injection of Ang II. The mechanism that underlies the production of salt appetite by aldosterone and Ang II may at least partially consist of mineralocorticoid-induced increases in the number of Ang receptors in discrete brain regions.