5-HT2 and 5-HT3 receptors in the lateral parabrachial nucleus mediate opposite effects on sodium intake.

The present study investigated the role of several 5-HT receptor subtypes in the lateral parabrachial nucleus (LPBN) in the control of sodium appetite (i.e. NaCl consumption). Male Holtzman rats had cannulas implanted bilaterally into the LPBN for the injection of 5-HT receptor agonists and antagonists in conjunction with either acute fluid depletion or 24-h sodium depletion. Following these treatments, access to 0.3 M NaCl was provided and the intakes of saline and water were measured for the next 2 h. Bilateral injections of the 5-HT2A receptor antagonist, ketanserin or the 5-HT2C receptor antagonist, mianserin into the LPBN increased 0.3 M NaCl intake without affecting water intake induced by acute fluid-depletion. Bilateral injections of the 5-HT2B receptor agonist, BW723C86 hydrochloride, had no effect on 0.3 M NaCl or water intake under these conditions. Treatment of the LPBN with the 5-HT2B/2C receptor agonist, 2-(2-methyl-4-clorophenoxy) propanoic acid (mCPP) caused dose-related reductions in 0.3 M NaCl intake after 24 h sodium depletion. The effects of mCPP were prevented by pretreating the LPBN with the 5-HT2B/2C receptor antagonist, SDZSER082. Activation of 5-HT3 receptors by the receptor agonist, 1-phenylbiguanide (PBG) caused dose-related increases in 0.3 M NaCl intake. Pretreatment of the LPBN with the 5-HT3 receptor antagonist, 1-methyl-N-[8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584) abolished the effects of PBG, but LY-278,584 had no effects on sodium or water intake when injected by itself. PBG injected into the LPBN did not alter intake of palatable 0.06 M sucrose in fluid replete rats. The results suggest that activation of the 5-HT2A and 5-HT2C receptor subtypes inhibits sodium ingestion. In contrast, activation of the 5-HT3 receptor subtype increases sodium ingestion. Therefore, multiple serotonergic receptor subtypes in the LPBN are implicated in the control of sodium intake, sometimes by mediating opposite effects of 5-HT. The results provide new information concerning the control of sodium intake by LPBN mechanisms.

Effects of hypotension and fluid depletion on central angiotensin-induced thirst and salt appetite.

We examined the effects of hypotension and fluid depletion on water and sodium ingestion in rats in response to intracerebroventricular infusions of ANG II. Hypotension was produced by intravenous infusion of the vasodilator drug minoxidil (25 microg x kg(-1) x min(-1)) concurrently with the angiotensin-converting enzyme inhibitor captopril (0.33 mg/min) to prevent endogenous ANG II formation. Hypotension increased water intake in response to intracerebroventricular ANG II (30 ng/h) but not intake of 0.3 M NaCl solution and caused significant urinary retention of water and sodium. Acute fluid depletion was produced by subcutaneous injections of furosemide (10 mg/kg body wt) either alone or with captopril (100 mg/kg body wt sc) before intracerebroventricular ANG II (15 or 30 ng/h) administration. Fluid depletion increased water intake in response to the highest dose of intracerebroventricular ANG II but did not affect saline intake. In the presence of captopril, fluid depletion increased intakes of both water and saline in response to both doses of intracerebroventricular ANG II. Because captopril administration causes hypotension in fluid-depleted animals, the results of the two experiments suggest that hypotension in fluid-replete animals preferentially increases water intake in response to intracerebroventricular ANG II and in fluid-depleted animals increases both salt and water intake in response to intracerebroventricular ANG II.

Circulating angiotensin II mediates sodium appetite in adrenalectomized rats.

We investigated the role of circulating ANG II in sodium appetite after adrenalectomy. Adrenalectomized rats deprived of their main access to sodium (0.3 M NaCl) for 9 h drank 14.1 +/- 1.5 ml of the concentrated saline solution in 2 h of access. Intravenous infusion of captopril (2.5 mg/h) during the last 5 h of sodium restriction reduced sodium intake by 77 +/- 12% (n = 5) without affecting the degree of sodium depletion and hypovolemia incurred during deprivation. Functional evidence indicates that this dose of captopril blocked production of ANG II in the peripheral circulation, but not in the brain; that is, injection of ANG I into the lateral brain ventricle stimulated intake of both water and 0.3 M NaCl. Intravenous infusion of ANG II (starting 10-15 min before 0.3 M NaCl became available) in adrenalectomized, captopril-treated rats restored both sodium intake and blood pressure to values seen in rats not treated with captopril. Longer (20 h) infusions of captopril in 22-h sodium-restricted rats also blocked sodium appetite, but reduced or prevented sodium depletion. Intravenous infusion of ANG II after these long captopril infusions stimulated sodium intake, but intake was less than in controls not treated with captopril. These results indicate that most or all of the sodium appetite of adrenalectomized rats is mediated by circulating ANG II.

Hindbrain serotonin and the rapid induction of sodium appetite.

Both systemically administered furosemide and isoproterenol produce water intake (i.e., thirst). Curiously, however, in light of the endocrine and hemodynamic effects produced by these treatments, they are remarkably ineffective in eliciting intake of hypertonic saline solutions (i.e., operationally defined as sodium appetite). Recent work indicates that bilateral injections of the serotonin receptor antagonist methysergide into the lateral parabrachial nuclei (LPBN) markedly enhance a preexisting sodium appetite. The present studies establish that a de novo sodium appetite can be induced with LPBN-methysergide treatment under experimental conditions in which only water is typically ingested. The effects of bilateral LPBN injections of methysergide were studied on the intake of water and 0. 3 M NaCl following acute (beginning 1 h after treatment) diuretic (furosemide)-induced sodium and water depletion and following subcutaneous isoproterenol treatment. With vehicle injected into the LPBN, furosemide treatment and isoproterenol injection both caused water drinking but essentially no intake of hypertonic saline. In contrast, bilateral treatment of the LPBN with methysergide induced the intake of 0.3 M NaCl after subcutaneous furosemide and isoproterenol. Water intake induced by subcutaneous furosemide or isoproterenol was not changed by LPBN-methysergide injections. The results indicate that blockade of LPBN-serotonin receptors produces a marked intake of hypertonic NaCl (i.e., a de novo sodium appetite) after furosemide treatment as well as subcutaneous isoproterenol.

Effect of hyperosmotic solutions on salt excretion and thirst in rats.

We investigated urinary changes and thirst induced by infusion of hyperosmotic solutions in freely moving rats. Intracarotid infusions of 0.3 M NaCl (4 ml/20 min, split between both internal carotid arteries) caused a larger increase in excretion of Na(+) and K(+) than intravenous infusions, indicating that cephalic sensors were involved in the response to intracarotid infusions. Intravenous and intracarotid infusions of hyperosmotic glycerol or urea (300 mM in 150 mM NaCl) had little or no effect, suggesting the sensors were outside the blood-brain barrier (BBB). Intracarotid infusion of hypertonic mannitol (300 mM in 150 mM NaCl) was more effective than intravenous infusion, suggesting that cell volume rather than Na(+) concentration of the blood was critical. Similarly, intracarotid infusion (2 ml/20 min, split between both sides), but not intravenous infusion of hypertonic NaCl or mannitol caused thirst. Hyperosmotic glycerol, infused intravenously or into the carotid arteries, did not cause thirst. We conclude that both thirst and electrolyte excretion depend on a cell volume sensor that is located in the head, but outside the BBB.

Interactions of the systemic and brain renin-angiotensin systems in the control of drinking and the central mediation of pressor responses.

Most of the biological actions of the circulating (a.k.a., the systemic or blood-borne) renin-angiotensin system require the generation of the octapeptide angiotensin (ANG) II from the decapeptide ANG I. In the case of circulating ANG I, the lungs are generally considered the major site for this conversion. The present experiments explored the possibility that under conditions of marked elevations of blood-borne ANG I, the generation of ANG II takes place within brain-associated target tissues, most notably circumventricular organs (CVOs) that lack a blood-brain barrier. The first important result of these experiments demonstrates that intracerebroventricular (i.c.v.) infusion of the converting enzyme inhibitor, captopril, completely blocks the drinking response and significantly attenuates the pressor response produced by systemically infused ANG I. This result indicates that under physiological/pathophysiological conditions associated with large elevations of circulating ANG I, an important part of the biological responses derived from blood-borne ANG may result from local conversion of ANG I to ANG II within specific brain target tissues which have high concentrations of converting enzyme. This local conversion process provides an important mechanism that would act to reinforce the "classic" conversion process which takes place in the lungs thereby delivering more ANG II immediately to central target receptors. The second important finding from these studies showed that drinking produced by systemically infused ANG II was not attenuated by an i.c.v. dose of captopril which was effective in blocking a comparable dipsogenic response induced by i.v. ANG I. This observation suggests that drinking induced by systemic ANG II does not require an intact metabolic cascade within the brain for the formation of ANG II (or ANG II-like effector peptide) from ANG I.

Effects of subfornical organ lesions on acutely induced thirst and salt appetite.

We examined the role of the subfornical organ (SFO) in stimulating thirst and salt appetite using two procedures that initiate water and sodium ingestion within 1-2 h of extracellular fluid depletion. The first procedure used injections of a diuretic (furosemide, 10 mg/kg sc) and a vasodilator (minoxidil, 1-3 mg/kg ia) to produce hypotension concurrently with hypovolemia. The resulting water and sodium intakes were inhibited by intravenous administration of ANG II receptor antagonist (sarthran, 8 micrograms . kg(-1). min(-1)) or angiotensin-converting enzyme inhibitor (captopril, 2.5 mg/h). The second procedure used injections of furosemide (10 mg/kg sc) and a low dose of captopril (5 mg/kg sc) to initiate water and sodium ingestion upon formation of ANG II in the brain. Electrolytic lesions of the SFO greatly reduced the water intakes, and nearly abolished the sodium intakes, produced by these relatively acute treatments. These results contrast with earlier findings showing little effect of SFO lesions on sodium ingestion after longer-term extracellular fluid depletion.

Salt appetite: interaction of forebrain angiotensinergic and hindbrain serotonergic mechanisms.

Methysergide injected bilaterally into the lateral parabrachial nucleus (LPBN) increases NaCl intake in several models of renin-dependent salt appetite. The present study investigated the role of angiotensin Type 1 (AT1) receptors in the subfornical organ (SFO) on this effect. The intake of 0.3 M NaCl and water was induced by combined administration of the diuretic, furosemide (FURO), and the angiotensin-converting enzyme inhibitor, captopril (CAP). Pretreatment of the SFO with an AT1 receptor antagonist, losartan (1 microgram/200 nl), reduced water intake but not 0.3 M NaCl intake induced by subcutaneous FURO+CAP. Methysergide (4 microgram/200 nl) injected bilaterally into the LPBN increased 0.3 M NaCl intake after FURO+CAP. Losartan injected into the SFO prevented the additional 0. 3 M NaCl intake caused by LPBN methysergide injections. These results indicate that AT1 receptors located in the SFO may have a role in mediating an enhanced sodium intake produced by methysergide treatment.

Fos expression in rat brain during depletion-induced thirst and salt appetite.

The expression of Fos protein (Fos immunoreactivity, Fos-ir) was mapped in the brain of rats subjected to an angiotensin-dependent model of thirst and salt appetite. The physiological state associated with water and sodium ingestion was produced by the concurrent subcutaneous administration of the diuretic furosemide (10 mg/kg) and a low dose of the angiotensin-converting enzyme (ACE) inhibitor captopril (5 mg/kg; Furo/Cap treatment). The animals were killed 2 h posttreatment, and the brains were processed for Fos-ir to assess neural activation. Furo/Cap treatment significantly increased Fos-ir density above baseline levels both in structures of the lamina terminalis and hypothalamus known to mediate the actions of ANG II and in hindbrain regions associated with blood volume and pressure regulation. Furo/Cap treatment also typically increased Fos-ir density in these structures above levels observed after administration of furosemide or captopril separately. Fos-ir was reduced to a greater extent in forebrain than in hindbrain areas by a dose of captopril (100 mg/kg sc) known to block the actions of ACE in the brain. The present work provides further evidence that areas of lamina terminalis subserve angiotensin-dependent thirst and salt appetite.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration.

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.

Effects of subfornical organ lesions on sympathetic nerve responses to insulin.

Although insulin exerts potent excitatory effects on the sympathetic nervous system, the mechanisms of insulin-induced activation remain unclear. To demonstrate a central nervous system site of sympathoexcitation, we recently found that destruction of tissues surrounding the anteroventral third ventricle region abolishes elevations in sympathetic nerve activity to intravenous insulin administration. Anteroventral third ventricle lesions may eliminate sympathoexcitation by destroying cell bodies in the lesioned area or by interrupting fibers of passage from the subfornical organ. To determine whether the lesions abolish sympathetic increases by disrupting efferent fibers from the subfornical organ, we measured lumbar sympathetic activity in anesthetized anteroventral third ventricle-lesioned (n = 4) and subfornical organ-lesioned (n = 12) rats before and during intravenous insulin at 0.13 U/h while maintaining euglycemia. Additional sham-lesioned rats received infusion of insulin (n = 10) and the vehicle for insulin (n = 10). Insulin administration in sham-lesioned rats elevated lumbar activity from 100% to 171 +/- 14% (+/-SE), whereas vehicle infusion did not alter sympathetic activity (100% to 113 +/- 11%). In anteroventral third ventricle-lesioned rats, insulin failed to increase sympathetic nerve activity (100% to 119 +/- 14%). Importantly, rats with subfornical organ lesions had increases in nerve activity that were indistinguishable from increases observed in insulin-infused sham-lesioned rats (100% to 163 +/- 21%). These findings indicate that whereas the anteroventral third ventricle region itself is crucial for sympathoexcitation to insulin, the subfornical organ and fibers originating from the subfornical organ traversing the anteroventral third ventricle area are not essential in mediating elevations in lumbar sympathetic nerve activity to hyperinsulinemia.

Role of renal nerves in sodium depletion-induced salt appetite.

The ingestion of water and 0.3 M NaCl solution and the secretion of key hormones were studied in groups of intact and bilaterally renal-denervated rats after extracellular fluid depletion. Hypovolemia with mild hypotension was produced by subcutaneous injections of the diuretic furosemide (10 mg/kg) followed by injections of the angiotensin-converting enzyme inhibitor captopril (5 mg/kg s.c.). Denervated rats drank significantly less of a concentrated saline solution in response to depletion than intact control rats did, but drank similar amounts of water. Denervated rats finished testing in significantly greater negative water and sodium balance compared with controls. Renal denervation did not impair the secretion of renin and aldosterone or the formation of angiotensin I. The diminished sodium intake of denervated rats is not attributable to reduced water and sodium excretion in response to the hypovolemic protocol. These results indicate that the integrity of the renal nerves is important for the normal elaboration of salt appetite in response to hypovolemia/hypotension.

Rapid elicitation of salt appetite by an intravenous infusion of angiotensin II in rats.

A role for the renal renin-angiotensin system in the direct stimulation of salt appetite in the rat remains controversial because attempts to elicit the behavior by intravenous administration of angiotensin II (ANG II) have been unconvincing. We recently demonstrated that depletion-induced salt appetite was attenuated by selective blockade of peripheral ANG II synthesis with an intravenous dose of converting enzyme inhibitor [captopril (Cap)] that does not block the synthesis of ANG II inside the blood brain barrier. We now show that intravenous ANG II at 30 ng/min rapidly reestablishes salt appetite in Cap-blocked rats. The mean arterial blood pressure (MAP) of unblocked, sodium-depleted rats was normal, but Cap-blocked, depleted rats had low MAP. An intravenous infusion of ANG II in Cap-blocked rats brought MAP into the normal range and elicited water and salt drinking within 90 min. Phenylephrine also normalized MAP but failed to elicit fluid intake in Cap-blocked, sodium-deficient rats. Sodium and water balances tended to be more positive during ANG II than during phenylephrine infusions. Thus circulating ANG II may stimulate both thirst and salt appetite by a direct action on the brain and not by causing natriuresis or by raising the blood pressure.

Integrative role of the lamina terminalis in the regulation of cardiovascular and body fluid homeostasis.

1. Cardiovascular and body fluid homeostasis depends upon the activation and co-ordination of reflexes and behavioural responses. In order to accomplish this, the brain receives and processes both neural and chemical input. Once in the brain, information from sources signalling the status of the cardiovascular system and body fluid balance travels, and is integrated, throughout a widely distributed neural network. Recent studies using neuroanatomical and functional techniques have identified several key areas within this neural network. One major processing node is comprised of structures located along the lamina terminalis. 2. Structures associated with the lamina terminalis include the median preoptic nucleus (MePO) and two sensory circumventricular organs (SCVO), the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). Current evidence indicates that blood-borne signals, such as angiotensin II (AngII), reach SCVO (e.g. SFO) where they are transduced. This information is then carried via neural pathways to brain nuclei (e.g. MePO) where it is integrated with other inputs, such as those derived from systemic arterial blood pressure and volume receptors. 3. Because of their receptive and integrative functions, lamina terminalis structures are essential for the normal control of hormone release (e.g. vasopressin), sympathetic activation and behaviours (thirst and salt appetite), which collectively contribute to maintenance of cardiovascular and body fluid homeostasis.

Lateral parabrachial nucleus and serotonergic mechanisms in the control of salt appetite in rats.

This study investigated the effects of bilateral injections of serotonergic receptor agonist and antagonist into the lateral parabrachial nucleus (LPBN) on the ingestion of water and 0.3 M NaCl induced by intracerebroventricular angiotensin II (ANG II) or by combined subcutaneous injections of the diuretic furosemide (Furo) and the angiotensin-converting enzyme inhibitor captopril (Cap). Rats had stainless steel cannulas implanted bilaterally into the LPBN and into the left lateral ventricle. Bilateral LPBN pretreatment with the serotonergic 5-HT1/5-HT2 receptor antagonist methysergide (4 micrograms/200 nl each site) increased 0.3 M NaCl and water intakes induced by intracerebroventricular ANG II (50 ng/microliter) and 0.3 M NaCl intake induced by subcutaneous Furo + Cap. Pretreatment with bilateral LPBN injections of a serotonergic 5-HT2A/2C receptor agonist DOI (5 micrograms/200 nl) significantly reduced 0.3 M NaCl intake induced by subcutaneous Furo + Cap. Pretreatment with methysergide or DOI into the LPBN produced no significant changes in the water intake induced by subcutaneous Furo + Cap. These results suggest that serotonergic mechanisms associated with the LPBN may have inhibitory roles in water and sodium ingestion in rats.

Role of peripheral angiotensin in salt appetite of the sodium-deplete rat.

Lines of evidence indicate that the importance of peripherally derived angiotensin II as a stimulus for salt appetite in rats has been underestimated. First, a series of observations is consistent with the idea that peripherally derived angiotensin acts at circumventricular organs of the brain to stimulate salt appetite following sodium depletion. Second, recent experiments show that depletion-induced salt appetite is abolished by the i.v. infusion of converting-enzyme inhibitor (captopril) at a dose that totally prevents the formation of angiotensin II within the peripheral circulation. This same dose of converting-enzyme inhibitor does not penetrate the blood-brain barrier to affect the actions of centrally derived angiotensin. These findings suggest that angiotensin II of peripheral origin is critical for the expression of salt appetite following extracellular fluid depletions. Together, these two lines of evidence suggest that the role of circulating angiotensin II in the stimulation of salt appetite in the rat should be re-examined.

Sensory mechanisms in the behavioral control of body fluid balance: thirst and salt appetite.

NASA: The review focuses on the mechanisms of action and interaction of afferent signaling pathways involved in the mediation of the behavioral responses to thirst and sodium appetite. Water volume in intracellular and extracellular body fluid compartments, experimental manipulation of cellular and extracellular, and autonomic and endocrine responses to hypovolemia are explained. The roles of the renal renin-angiotensin system in behavioral control of fluid balance and of angiotensin II in isoproterenol-induced drinking are explored. Studies in dogs provide a basis for understanding hypotension and angiotensin II-induced drinking. The functions of hypovolemia, hypotension, and the renin-angiotensin system in the control of sodium appetite are examined.^ieng

Endocrine changes associated with a rapidly developing sodium appetite in rats.

Simultaneous administration of the diuretic furosemide (10 mg/kg) and a low dose of the angiotensin-converting enzyme (ACE) inhibitor captopril (5 mg/kg) results in short-latency thirst and sodium appetite (i.e., the rapid ingestion of water and NaCl solution). To elucidate potential mechanisms for mediating this behavior, changes in plasma levels of key hormones involved in fluid intake and balance were characterized in rats subjected to this treatment protocol. Rats treated jointly with furosemide and low-dose captopril had exaggerated increases in plasma renin activity and angiotensin I but equivalent increases in plasma aldosterone compared with rats treated with either agent alone. Treatment with furosemide plus low-dose captopril increased plasma vasopressin but not plasma oxytocin. The administration of a higher dose of captopril (100 mg/kg) with furosemide, a combination of drugs that does not stimulate fluid intake (29), further increased plasma renin activity and angiotensin I but prevented the rise in plasma vasopressin. The results support the hypothesis that thirst and salt appetite generated by this protocol depend on angiotensin II formed within brain circumventricular organs rather than the systemic circulation.

Effects of sinoaortic baroreceptor denervation on depletion-induced salt appetite.

This study examined the effects of denervation of the carotid sinus and aortic arch baroreceptors on sodium depletion-induced salt appetite in rats. Depletion of extracellular fluid began with two injections of furosemide (10 mg/kg sc), 30 min apart, and removal of all ambient sodium from the cages. Water and sodium-deficient chow were available overnight. The next morning, access to 0.3 M NaCl was provided, and intakes of water and saline were measured every 30 min for 2 h and again at 24 h. Three such tests were administered. Groups of sham and sinoaortic baroreceptor-denervated (SAD) rats had equivalently negative water and sodium balances before access to saline the next morning. Sham-denervated rats drank sufficient saline in 2 h to achieve positive sodium balance and subsequently ingested sodium in excess of need for another 22 h. SAD rats drank half as much saline as sham-denervated rats by 2 and 24 h and remained in negative balance at 2 h. Sham-denervated rats increased their intakes of saline solution with repeated tests, but SAD rats did not. These findings suggest that arterial baroreceptors provide critical neural input for the normal expression of salt appetite.

The role of beta1 and beta2 adrenoceptors in isoproterenol-induced drinking.

The present study examined the contribution of beta1 and beta2 adrenoceptor activation to drinking behavior and the stimulation of plasma renin activity produced by the mixed beta adrenoceptor agonist, isoproterenol. The stimulation of drinking by beta adrenoceptor activation could occur via two independent pathways; by either directly stimulating renal beta1 adrenoceptors on the juxtaglomerular cells to release renin or by stimulating vascular beta2 adrenoceptors that would decrease blood pressure and activate afferent neural and humoral mechanisms. Selective pharmacological antagonism of each adrenoceptor type was achieved by administering atenolol (2.5 mg/kg), a beta1 adrenoceptor antagonist, or ICI 118,551 (1 mg/kg), a beta2 adrenoceptor antagonist, before treatment with isoproterenol (25 micrograms/kg). Neither adrenoceptor mechanism alone could account for all of the water intake or stimulation of plasma renin activity due to isoproterenol treatment. Cardiovascular recordings confirmed the selectivity of the antagonists to their respective receptor subtypes, with atenolol blocking the beta1 adrenoceptor-mediated heart rate increases and ICI 118,551 blocking the beta 2 adrenoceptor-mediated depressor response to isoproterenol. The results provide evidence that the stimulation of both beta1 and beta2 adrenoceptors by isoproterenol acts in a synergistic manner to induce drinking and renin-angiotensin system activation.