A comparison of brain angiotensin II receptors during lactation and diestrus of the estrous cycle in the rat

Angiotensin type 1a receptors on corticotropin-releasing factor neurons contribute to the expression of conditioned fear

Although generally associated with cardiovascular regulation, angiotensin II receptor type 1a (AT1a R) blockade in mouse models and humans has also been associated with enhanced fear extinction and decreased post-traumatic stress disorder (PTSD) symptom severity, respectively. The mechanisms mediating these effects remain unknown, but may involve alterations in the activities of corticotropin-releasing factor (CRF)-expressing cells, which are known to be involved in fear regulation. To test the hypothesis that AT1a R signaling in CRFergic neurons is involved in conditioned fear expression, we generated and characterized a conditional knockout mouse strain with a deletion of the AT1a R gene from its CRF-releasing cells (CRF-AT1a R((-/-)) ). These mice exhibit normal baseline heart rate, blood pressure, anxiety and locomotion, and freeze at normal levels during acquisition of auditory fear conditioning. However, CRF-AT1a R((-/-)) mice exhibit less freezing than wild-type mice during tests of conditioned fear expression-an effect that may be caused by a decrease in the consolidation of fear memory. These results suggest that central AT1a R activity in CRF-expressing cells plays a role in the expression of conditioned fear, and identify CRFergic cells as a population on which AT1 R antagonists may act to modulate fear extinction.

Selective C1 Lesioning Slightly Decreases Angiotensin II Type I Receptor Expression in the Rat Rostral Ventrolateral Medulla (RVLM)

Cardiovascular homeostasis is regulated in large part by the rostral ventrolateral medulla (RVLM) in mammals. Projections from the RVLM to the intermediolateral column of the thoracolumbar spinal cord innervate preganglionic neurons of the sympathetic nervous system causing elevation of blood pressure and heart rate. A large proportion, but not all, of the neurons in the RVLM contain the enzymes necessary for the production of epinephrine and are identified as the C1 cell group. Angiotensin II (Ang II) activates the RVLM acting upon AT1 receptors. To assess the proportion of AT1 receptors that are located on C1 neurons in the rat RVLM this study employed an antibody to dopamine-beta-hydroxylase conjugated to saporin, to selectively destroy C1 neurons in the RVLM. Expression of tyrosine hydroxylase immunoreactive neurons in the RVLM was reduced by 57 % in the toxin injected RVLM compared to the contralateral RVLM. In contrast, densitometric analysis of autoradiographic images of (125)I-sarcosine(1), isoleucine(8) Ang II binding to AT1 receptors of the injected side RVLM revealed a small (10 %) reduction in AT1-receptor expression compared to the contralateral RVLM. These results suggest that the majority of AT1 receptors in the rat RVLM are located on non-C1 neurons or glia.

Distribution of non-AT1, non-AT2 binding of 125I-sarcosine1, isoleucine8 angiotensin II in neurolysin knockout mouse brains

The recent identification of a novel binding site for angiotensin (Ang) II as the peptidase neurolysin (E.C. 3.4.24.16) has implications for the renin-angiotensin system (RAS). This report describes the distribution of specific binding of ¹²⁵I-Sarcosine¹, Isoleucine⁸ Ang II (¹²⁵I-SI Ang II) in neurolysin knockout mouse brains compared to wild-type mouse brains using quantitative receptor autoradiography. In the presence of p-chloromercuribenzoic acid (PCMB), which unmasks the novel binding site, widespread distribution of specific (3 µM Ang II displaceable) ¹²⁵I-SI Ang II binding in 32 mouse brain regions was observed. Highest levels of binding >700 fmol/g initial wet weight were seen in hypothalamic, thalamic and septal regions, while the lowest level of binding <300 fmol/g initial wet weight was in the mediolateral medulla. ¹²⁵I-SI Ang II binding was substantially higher by an average of 85% in wild-type mouse brains compared to neurolysin knockout brains, suggesting the presence of an additional non-AT₁, non-AT₂, non-neurolysin Ang II binding site in the mouse brain. Binding of ¹²⁵I-SI Ang II to neurolysin in the presence of PCMB was highest in hypothalamic and ventral cortical brain regions, but broadly distributed across all regions surveyed. Non-AT₁, non-AT₂, non-neurolysin binding was also highest in the hypothalamus but had a different distribution than neurolysin. There was a significant reduction in AT₂ receptor binding in the neurolysin knockout brain and a trend towards decreased AT₁ receptor binding. In the neurolysin knockout brains, the size of the lateral ventricles was increased by 56% and the size of the mid forebrain (−2.72 to +1.48 relative to Bregma) was increased by 12%. These results confirm the identity of neurolysin as a novel Ang II binding site, suggesting that neurolysin may play a significant role in opposing the pathophysiological actions of the brain RAS and influencing brain morphology.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

AT₁ angiotensin II receptor and novel non-AT₁, non-AT₂ angiotensin II/III binding site in brainstem cardiovascular regulatory centers of the spontaneously hypertensive rat

Spontaneously hypertensive rats (SHR) have an activated brain angiotensin system that contributes to the elevation of blood pressure in this animal model. Physiological and pharmacological studies suggest that hyperactivation of brain AT₁ angiotensin receptors is a major pathophysiological factor. Consistent with these observations, radioligand binding studies indicate widespread up-regulation of brain angiotensin receptors in SHR. One key brainstem site in which AT₁ receptor stimulation appears to contribute to the elevated blood pressure in SHR is the rostral ventrolateral medulla (RVLM). However, no quantitative comparison of AT₁ receptor binding in the RVLM has been made in SHR versus normotensive rats. A novel, non-AT₁, non-AT₂ binding site, specific for angiotensins II and III, has recently been discovered in the brain. To determine if radioligand binding to either AT₁ receptors or this novel angiotensin binding site is altered in the RVLM and other caudal brainstem regions of SHR, a quantitative densitometric autoradiographic comparison of radioligand binding in SHR versus normotensive Wistar-Kyoto rats was made. In both the RVLM and caudal ventrolateral medulla (CVLM) as well as dorsomedial medulla (DMM), there was increased expression of AT₁ receptor binding in SHR (13%, 9%, and 23%, respectively). Conversely, expression of the novel, non-AT₁, non-AT₂, angiotensin II and III binding site was decreased in the RVLM and DMM of SHR (37% and 13%, respectively). This increased AT₁ receptor binding in the RVLM may contribute to the hypertension of SHR. Reduced radioligand binding to the novel, non-AT₁, non-AT₂, angiotensin binding site in the RVLM of SHR may indicate a role for this binding site to reduce blood pressure via its interactions with angiotensins II and III.

Angiotensin II in dorsomedial hypothalamus modulates cardiovascular arousal caused by stress but not feeding in rabbits

The dorsomedial hypothalamus (DMH) is critically implicated in the cardiovascular response to emotional stress. This study aimed to determine whether the DMH is also important in cardiovascular arousal associated with appetitive feeding behavior and, if so, whether locally released angiotensin II and glutamate are important in this arousal. Emotional (air-jet) stress and feeding elicited similar tachycardic (+51 and +45 beats/min, respectively) and pressor (+16 and +9 mmHg, respectively) responses in conscious rabbits. Bilateral microinjection of GABAA agonist muscimol (500 pmol) into the DMH, but not nearby hypothalamic regions, attenuated pressor and tachycardic responses to air-jet by 56–63% and evoked anorexia. Conversely, stimulation of the DMH with the glutamate analog kainic acid (250 pmol) elicited hypertension (+25 mmHg) and tachycardia (+114 beats/min) and activated feeding behavior. Local microinjection of a glutamate receptor antagonist, kynurenic acid (10 nmol), decreased pressor responses to stress and eating by 46 and 72%, respectively, without affecting feeding behavior. Bilateral microinjection of a selective AT1-receptor antagonist, candesartan (500 pmol), into the DMH, but not nearby sites, attenuated pressor and tachycardic stress responses by 31 and 33%, respectively. Candesartan did not alter feeding behavior or circulatory response to feeding. These results indicate that, in addition to its role in mediating stress responses, the DMH may be important in regulating cardiovascular arousal associated with feeding. Local glutamatergic inputs appear to regulate cardiovascular response to both stress and feeding. Conversely, angiotensin II, acting via AT1 receptors, may selectively modulate, in the DMH, cardiovascular response to stress, but not feeding.

Brain angiotensinergic mediation of enhanced water consumption in lactating rats

The mechanism by which lactating rats increase fluid consumption to meet the demands of milk production is unknown. Because ANG II is the most potent dipsogenic stimulus known, this study examined whether angiotensinergic signaling plays a role in enhanced drinking in lactating rats. ANG II administered intracerebroventricularly caused a significantly greater dipsogenic response in lactating rats than in control rats, suggesting that dipsogenic responsivity to ANG II is enhanced in the brains of lactating rats. The angiotensin type 1 (AT1) ANG II receptor subtype antagonist SKF-108566, also given intracerebroventricularly, caused a significant reduction in water consumption in lactating rats, whereas it did not significantly affect water intake in control rats. In contrast, stimulation of drinking by the muscarinic agonist carbachol, also administered intracerebroventricularly, did not differ between lactating and control rats. Inhibition of drinking by the muscarinic antagonist atropine also did not differ significantly between lactating and control rats. These results suggest that the increased drinking in lactating rats involves an increased responsivity to ANG II in neurons that mediate dipsogenesis, as well as an enhancement in the amount of angiotensinergic input to these ANG II-responsive neurons.

Lactation decreases angiotensinogen mRNA expression in the midcaudal arcuate nucleus of the rat brain

In lactating rats, ANG II receptor binding in the arcuate nucleus (ARH) and median eminence is decreased. To further evaluate brain angiotensinergic activity during lactation, we assessed angiotensinogen (AON) mRNA by in situ hybridization in forebrains of day 10 or 11 postpartum lactating and diestrous rats. AON mRNA was abundantly expressed in the ARH, preoptic, suprachiasmatic, supraoptic, paraventricular, and dorsomedial hypothalamic nuclei, and other regions, similar to that reported in male rat brains. AON mRNA levels were decreased 27% in the midcaudal ARH of lactating rats but did not differ between lactating or diestrous rats in any of the other brain areas examined. Immunofluorescence for AON and glial fibrillary acidic protein or tyrosine hydroxylase confirmed that the AON immunoreactivity in the ARH was limited to astrocytes. Confocal microscopy revealed close appositions of AON-positive astrocytes to dopaminergic neurons in the ARH. The decrease in AON mRNA in the midcaudal ARH during lactation coupled with decreased ARH ANG II receptor binding suggests that lactating rats are less subject to ANG II-mediated inhibition of prolactin secretion.