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.     

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.     

Natriuretic action of central angiotensin II in conscious rats

The effects of intracerebroventricular (i.c.v.) injections of angiotensin II (ANG II, 10 pg, 100 pg and 10 ng) on renal sodium excretion were investigated in conscious rats instrumented with a chronic urethral catheter. ANG II increased renal sodium excretion dose-dependently with a threshold i.c.v. dose of 10 pg. Only after the highest dose was a concomitant increase in arterial blood pressure and urinary flow observed. The ANG II-induced natriuresis began within 5 min of the i.c.v. injection and lasted for more than 1 h. The angiotensin receptor antagonist saralasin (1 ng, i.c.v.) largely prevented the natriuretic effect of i.c.v. injected ANG II (100 pg). Our results lend further support to the hypothesis that brain ANG II by its potent natriuretic actions may be instrumental in central osmotic control.     

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.     

Effect of central nicotinic activation on drinking behavior

Intracerebroventricular injections of angiotensin II (ANG) and nicotine activate the subfornical organ (SFO), an essential central nucleus for ANG-induced drinking. Nicotine has been, however, reported to induce little drinking behavior. To clarify this paradox, we investigated effects of nicotine and ANG on activity of SFO neurons and drinking behavior. In extracellular recordings many SFO neurons (57%) were excited by the both drugs. The nicotine-induced excitation was transient, whereas the ANG-induced was long-lasting. After intracerebroventricular injection of nicotine, the latency to drinking was dose-dependently shortened, but the drinking volumes were much smaller than those by ANG. These suggest that central nicotinic activation contributes to induction of drinking behavior while drinking volume is small because effects of nicotine on neurons are short-lasting.     

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.     

Effects of drinking and central angiotensin II stimulation on organ blood flow in conscious rats

While the hemodynamic response pattern accompanying feeding behavior has been well characterized, there is less information about the hemodynamic changes associated with drinking. In the present study, we have measured organ blood flows in conscious, unrestrained rats during schedule-induced drinking behavior, using the tracer microsphere technique (diameter of spheres 15 +/- 3 microns; labels: 141Ce, 113Sn). In addition, we determined the hemodynamic response pattern following intracerebroventricular (i.c.v.) injection of 100 ng angiotensin II (ANG II) (a dose known to be dipsogenic) in rats that were not allowed to drink during the experiment. The hemodynamic responses during drinking behavior included (a) significant increases in blood flow through the kidney, stomach and small intestine, (b) a decrease in blood flow through skeletal muscle, and (c) no significant changes in the rest of the organs. ANG II i.c.v. elicited (a) significant decreases in blood flow through the kidney, stomach, small intestine and skin, (b) a significant increase in blood flow through the liver (hepatic artery), and (c) no significant changes in blood flow through the brain, heart, lung (bronchial arteries), colon, skeletal muscle (biceps) and testis. We conclude that spontaneous drinking behavior in rats is associated with a characteristic hemodynamic drinking response, which resembles a classical feeding reaction. In non-drinking rats the hemodynamic response pattern following ANG II i.c.v. was different from the drinking response, providing further evidence, that the behavioral and cardiovascular effects of the neuropeptide can be dissociated.     

Angiotensin II evokes noradrenaline release from the paraventricular nucleus in conscious rats.

In vitro and in vivo experiments have provided indirect evidence that some of the central actions of angiotensin II (ANG II) involve catecholaminergic pathways in the brain. In this study in conscious rats we investigated the effect of stimulation of periventricular ANG II receptors on blood pressure and on catecholamine release (microdialysis and HPLC) from the paraventricular nucleus (PVN), a hypothalamic area thought to be instrumental in the central pressor responses to ANG II through the release of vasopressin into the blood. Intracerebroventricular (i.c.v.) injections of pressor doses of ANG II (1 ng and 100 ng) led to significant dose-dependent increases of the noradrenaline (NA) release in the PVN (1 ng: 30.95 +/- 6.01 to 47.38 +/- 6.79 pg/sample, P less than or equal to 0.01; 100 ng: 32.93 +/- 5.38 to 73.18 +/- 11.4 pg/sample, P less than or equal to 0.01). These changes coincided in extent and duration with the respective pressor responses. A subpressor dose of ANG II (100 pg) did not release catecholamines from the PVN. Dopamine (DA) and the NA and DA, metabolites 3,4-dihydroxyphenylethylglycol and 3,4-dihydroxyphenylacetic acid, were not influenced by i.c.v. injections of ANG II at any dose. Pretreatment with the novel non-peptide ANG II-AT 1 receptor antagonist DuP 753 (5 micrograms, i.c.v.) abolished the effect of 100 ng ANG II on blood pressure and on NA release. Our results show for the first time in vivo that stimulation of periventricular ANG II-AT 1 receptors induces a selective NA release in the PVN. They further support the hypothesis that ANG II engages a noradrenergic pathway in the PVN to release vasopressin.     

Estrogen attenuates the drinking response induced by activation of angiotensinergic pathways from the lateral hypothalamic area to the subfornical organ in female rats

The present study was carried out to investigate whether estrogen modulates the drinking response induced by activation of angiotensinergic neural pathways from the lateral hypothalamic area (LHA) to the subfornical organ (SFO) in the female rats. Microinjection of ANG II (10(-10) M, 0.2 microl) into the LHA caused drinking in 17 out of 26 ovariectomized (OVX) female rats that were treated with propylene glycol (PG) vehicle and in 18 out of 28 OVX female rats that were treated with estrogen benzoate (EB). In both groups, previous injections of the ANG II antagonist saralasin (Sar, 10(-10) M, 0.2 microl) into the SFO significantly attenuated the water intake caused by the ANG II injection, suggesting that the ANG II-induced drinking response may be mediated by the angiotensinergic LHA projections to the SFO. Injections of ANG II (10(-10) M, 0.2 microl) into the SFO elicited drinking in all the animals that demonstrated the drinking response to ANG II injected into the LHA. The amount of water intake caused by either the injection of ANG II into the LHA or the SFO was significantly greater in the PG-treated than in the EB-treated animals. These results suggest that the circulating estrogen may act to attenuate the dipsogenic response induced by activation of the angiotensinergic pathways from the LHA to the SFO.     

Angiotensinergic and noradrenergic mechanisms in the hypothalamic paraventricular nucleus participate in the drinking response induced by activation of the subfornical organ in rats

The present study was done to investigate the contribution of the hypothalamic paraventricular nucleus (PVN) to the drinking response caused by activation of the subfornical organ (SFO) following angiotensin II (ANG II) injections in the awake rat. Microinjection of ANG II into the SFO elicited the drinking response. Previous injections of either saralasin, an ANG II antagonist, or phentolamine, an alpha-adrenoceptor antagonist, bilaterally into the PVN resulted in the significant attenuation of the drinking response to ANG II. Similar injections of any of the beta-adrenoceptor antagonist timolol, the muscarinic antagonist atropine, or saline vehicle into the PVN had no significant effect on the drinking response. In an attempt to clarify the neural mechanisms in the PVN involved in the drinking response to ANG II injected into the SFO, the effect of microinjection of ANG II into the SFO on noradrenaline (NA) release in the PVN was examined using intracerebral microdialysis techniques. The injection of the ANG II, but not saline vehicle, significantly enhanced the NA release in the region of the PVN. These results indicate the involvement of both the angiotensinergic and alpha-adrenergic systems in the PVN in the drinking response caused by angiotensinergic activation of the SFO, and imply that the angiotensinergic projections from the SFO to the PVN may serve to increase NA release which results in mediating water intake.     

Dipsogenic response induced by angiotensinergic pathways from the lateral hypothalamic area to the subfornical organ in rats

Experimental observations in several species have suggested that angiotensinergic neural circuits from the lateral hypothalamic area (LHA) to the subfornical organ (SFO) may participate in the control of drinking behavior in the rat. In an attempt to verify this possibility, experiments were undertaken to investigate whether activation of LHA neurons following microinjection of angiotensin II (ANG II) into the LHA elicits drinking. Injections of ANG II (10(-11) mol) into the LHA caused drinking in 25 out of 36 rats having the tips of cannulas in the LHA. The efficacy of ANG II was potentiated by increasing the dose of the drug. To clarify the contribution of angiotensinergic neurons in the LHA with efferent projections to the SFO to the drinking induced by ANG II, the effects of pretreatment with saralasin (Sar), a specific ANG II antagonist, in the SFO or its surrounding region on the drinking to ANG II were examined. Previous injections of Sar into the SFO significantly reduced the water intake caused by ANG II injected into the LHA, whereas treatment with Sar in the ventral hippocampal commissure (VHC) or third ventricle (3V) was without effect. These findings provide the evidence for the involvement of the angiotensinergic pathways from the LHA to the SFO in the dipsogenic action.     

Effect of central angiotensin II on body weight gain in young rats

Systemic infusion of ANG II decreases body weight and food intake and increases energy expenditure. We recently reported that young rats receiving a 1-week intracerebroventricular (i.c.v.) infusion of angiotensin II (ANG II) exhibited decreased body weight compared to control. The aim of the present investigation was to determine if chronic i.c.v. infusion of ANG II also decreases food intake and increases energy expenditure. Young rats were infused with i.c.v. 0.9% saline or ANG II (16.7 or 4.2 ng/min) for at least 10 days and body weight and food intake were monitored daily. Pair-fed rats had the same daily food intake as the ANG II-infused rats. The i.c.v. ANG II decreased body weight gain and food intake. The decrease in weight gain was greater than in the pair-fed groups. The expression of mRNA for uncoupling protein-1 (UCP-1) in BAT was increased significantly in the ANG II-infused rats compared to the pair-fed animals. Subcutaneous infusion of ANG II at the same doses used for i.c.v. infusion had no effect on body weight or food intake. The expression of CRH mRNA in the paraventricular nucleus was not increased in the ANG II-infused rats. These data are consistent with the idea that i.c.v. ANG II decreases body weight gain in young rats, in part, by decreasing food intake and, in part, by increasing thermogenesis (although via a CRH-independent mechanism). This central effect of ANG II may contribute to or complement the effect of peripheral ANG II on body weight.     

Reduced dipsogenic response induced by angiotensin II activation of subfornical organ projections to the median preoptic nucleus in estrogen-treated rats

The present study was carried out to investigate whether estrogen modulates the drinking response induced by activation of angiotensinergic neural pathways from the subfornical organ (SFO) to the median preoptic nucleus (MnPO). Microinjection of angiotensin II (ANG II, 10(-10) M, 0.2 microl) into the SFO elicited drinking in ovariectomized (OVX) female rats that were treated with either propylene glycol (PG) vehicle or estrogen benzoate (EB). The amount of water intake induced by the ANG II injection was significantly greater in the PG-treated than in the EB-treated animals. In both groups of female rats, previous injections of saralasin (Sar, 10(-10) M, 0.2 microl), a specific ANG II antagonist, into the MnPO resulted in the significant attenuation of the drinking response to ANG II, showing that the ANG II-induced drinking response may be mediated in part by the angiotensinergic SFO projections to the MnPO. Injections of ANG II (10(-10) M, 0.2 microl) into the MnPO caused drinking in both groups, while no significant difference was found between the groups in the amount of water intake. These results suggest that increases in the circulating level of estrogen may attenuate the drinking response induced by ANG II activation of the SFO projections to the MnPO.     

Angiotensin II-induced noradrenaline release from anterior hypothalamus in conscious rats: a brain microdialysis study.

Interaction with aminergic transmitter substances has been implicated in the central actions of angiotensin II (ANG II). We used the novel technique of brain microdialysis in conscious rats to investigate whether ANG II influences the release of endogenous catecholamines (CA) from the anterior hypothalamus (AH). Intracerebroventricular (i.c.v.) administration of ANG II (1 ng and 100 ng) increased mean arterial pressure. ANG II at 1 ng had no effect on the release of noradrenaline (NA) from the AH but ANG II at 100 ng significantly increased NA release. Intracerebroventricular pretreatment with the ANG II-receptor antagonist sarilesin (Sar1, Ile8-ANG II; 3 micrograms) prevented the ANG II-induced NA release. The release of the intraneuronal NA and dopamine (DA) metabolites 3,4-dihydroxyphenylethyl glycol (DOPEG) and 3,4-dihydroxyphenylacetic acid (DOPAC) from the AH was not altered by i.c.v. ANG II. Our results provide the first in vivo evidence for NA release from the hypothalamus induced by periventricular ANG II receptor stimulation. They support the hypothesis that hypothalamic noradrenergic mechanisms are involved in the central actions of this peptide.     

Functional role of brain AT1 and AT2 receptors in the central angiotensin II pressor response

Intracerebroventricular (i.c.v.) angiotensin II (ANG II) increase vascular resistance and elicits a pressor response characterized by sympathetic nervous system activation (SNS component) and increased vasopressin (VP) secretion (VP component). This study examines the role of brain AT1 and AT2 ANG II receptors in mediating the pressor and renal hemodynamic effects of i.c.v. ANG II in conscious Sprague-Dawley rats. Mean arterial pressure, heart rate and renal vascular resistance responses to i.c.v. ANG II (100 ng in 5 μl) were determined 10 min after i.c.v. injection of either the AT1 receptor antagonist, DuP 753 (1.0, 2.5, 5.0, 10.0 μg), the AT2 receptor ligand, PD 123319 (3.5 × [10⁻⁶, 10⁻⁴, 10⁻², 10⁰ μg), or both. In control rats, i.c.v. DuP 753 prevented the pressor response and the increase in renal vascular resistance that occurred following i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 was without affect. When the VP- and SNS components were studied individually, by preventing the SNS component with intravenous (i.v.) chlorisondamine or the VP component with a V1 receptor antagonist (i.v.) similar results were obtained; DuP 753 prevented the SNS component and significantly reduced the VP component. These results indicate that both central ANG II pressor components are mediated primarily by brain AT1 receptors. However, doses of DuP 753 were more effective when combined with 3.5 μg of PD 123319 than when given alone (P < 0.05), suggesting that the pressor effects of i.c.v. ANG II may involve activation of multiple ANG II receptor subtypes.     

Effects of lidocaine injections into the lateral parabrachial nucleus on dipsogenic and pressor responses to central angiotensin II in rats

This study investigated the effects of bilateral injections of the local anesthetic, lidocaine, into the lateral parabrachial nucleus (LPBN) on the dipsogenic and pressor responses induced by intracerebroventricular (i.c.v.) injection of angiotensin II (ANG II). Centrally injected ANG II (50 ng/1 μl) induced water intake (10.2 ± 0.8 ml/h) and pressor responses (22 ± 1mmHg). Prior bilateral injection of 10% lidocaine (200 nl) into the LPBN increased the water intake (14.2 ± 1.4 ml/h), but did not change the pressor response (17 ± 1 mmHg) to i.c.v. ANG II. Lidocaine alone injected into the LPBN also induced a pressor response (23 ± 3 mmHg). These results showing that bilateral LPBN injection of lidocaine increase water intake induced by i.c.v. ANG II are consistent with electrolytic and neurotoxic lesion studies and suggest that the LPBN is associated with inhibitory mechanisms controlling water intake induced by ANG II. These results also provides evidence that it is feasible to reversibly anesthetize this brain area to facilitate fluid-related ingestive behavior.     

Enhanced effects of central angiotensin II on cardiovascular and drinking responses in inbred polydipsic (STR/N) mice

STR/N, an inbred strain of mice, is known to exhibit extreme polydipsia and polyuria. The objective of this study was to investigate the possible reasons for polydipsia. First, comparisons were made between STR/N mice and control mice from the Institute of Cancer Research (ICR) concerning daily drinking, urinary excretion, and basal cardiovascular function. Then, since angiotensin II (ANG II) is a potent stimulus for drinking behavior, we investigated the effects of intracerebroventricular (i.c.v.) administration of ANG II on cardiovascular and water intake responses. Daily water intake, food intake, urinary volume, and urinary electrolytes (Na and K) excretion were larger in STR/N mice than in ICR mice, and the basal blood pressure was significantly lower in STR/N mice than in ICR mice. The i.c.v. administration of ANG II (10 pmol/per mouse) resulted in increased mean arterial blood pressure (MAP) and water intake in both STR/N and ICR mice, but the changes in MAP were significantly larger in STR/N mice than in ICR mice. These results suggest that polydipsia in STR/N mice is at least partially attributable to high sensitivity of central ANG II receptors and low MAP.     

Sensitization to pressor effects by repeated central injections of vasopressin in conscious rats

Arginine vasopressin (AVP) injected intracerebroventricularly (i.c.v.) in the nanogram range elicits increases in mean arterial blood pressure (MAP), heart rate (HR) and efferent sympathetic nerve activity (SpNA) via central V1 AVP receptor stimulation. In this study in conscious rats we investigated, whether the cardiovascular and sympathetic responses can be augmented by repeated central applications of AVP, as has been previously shown for the convulsive responses to higher i.c.v. doses of the peptide. The AVP-induced pressor (0.1 and 1.0 ng) and the SpNA (0.1 ng) responses were significantly enhanced by a second AVP challenge 24 h after the first injection. With higher doses of the peptide (3 ng), the blood pressure responses were not different between two subsequent injections, but barrel rotation occurred in 21% of the animals upon the second challenge. The pressor responses to a threshold i.c.v. dose of 1 ng angiotensin II (ANG II) were not enhanced upon a second ANG II challenge. Our results demonstrate that AVP, unlike ANG II, can sensitize central mechanisms leading to increased MAP and SpNA responses.     

Sensitivity to angiotensin II of neurons in the circumventricular organs of polydipsic inbred mice

The polydipsic inbred mice, STR/N, are known to possess an extremely strong appetite for drinking but no abnormality in the vasopressin system and renal functions. In brain slice preparations the sensitivity of neurons in the anteroventral region of the third ventricle (AV3V) and the subfornical organ (SFO) to angiotensin II (ANG II) was investigated using extracellular recordings in the STR/N and its control, Swiss/Webster (S/W) mice. In the AV3V, less proportion of neurons (15 out of 168; 9%) of the STR/N than that in the S/W (49/206; 24%) was excited by ANG II added to the medium. In the SFO, a proportion of neurons excited by ANG II was again lower in the STR/N (27/104; 26%) than in the S/W mice (64/118; 54%). The threshold concentration of ANG II for excitation of the AV3V and SFO neurons was, however, similar for both strains, 10⁻⁹ M or less. Only one neuron in the SFO of a S/W mouse was inhibited by ANG II application. The excitatory effect of ANG II on AV3V and SFO neurons of both strains of mice persisted under synaptic blockade and was reversibly antagonized by an ANG II antagonist, saralasin. Such differences in sensitivity to ANG II of neurons in the SFO and AV3V, the regions thought to be involved in drinking behavior, suggest the involvement, at least in part, of the central angiotensin system in the polydipsia of the STR/N mice.     

Effects of intracerebroventricular losartan on angiotensin II-mediated pressor responses and c-fos expression in near-term ovine fetus

The renin-angiotensin system plays an important role in cardiovascular control. Intracerebroventricular (i.c.v.) angiotensin (ANG) II causes a reliable pressor response in the fetus at 90% gestation. To determine the roles of brain AT1 and AT2 receptors in this response, the effects of the central AT1 and AT2 receptor antagonists losartan and PD123319 were investigated in chronically prepared near-term ovine fetuses. Losartan at 0.5 mg/kg (i.c.v.) abolished central ANG II-induced pressor responses. High-dose losartan (5 mg/kg, i.c.v.) showed a potentiation of the pressor response to i.c.v. ANG II, accompanied by bradycardia. Associated with the pressor responses, c-fos expression in the cardiovascular controlling areas was significantly different between the low and high doses of losartan. These areas included the subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis, and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, lateral parabrachial nuclei in the hindbrain. Low-dose losartan markedly reduced c-fos in these areas after i.c.v. ANG II, while the high-dose losartan together with ANG II elicited a much stronger FOS-immunoreactivity in these areas than that induced by i.c.v. ANG II alone. This is a novel finding, that c-fos expression in the brain can be both activated and inhibited under the same condition. Central ANG II-induced fetal pressor responses were not altered by PD123319 (0.8 mg/kg). These results indicate that i.c.v. losartan at a high and a low dose has strikingly different effects on central ANG II-induced pressor responses in fetuses at late gestation, and that the AT1 mechanism plays an important role in fetal cardiovascular regulation.