Effect of intraventricular infusion of an angiotensin II antagonist on 125I-angiotensin II binding in rats

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.

Pharmacological characterization of angiotensin II AT(2) receptor subtype heterogeneity in the rat adrenal cortex and medulla

Adrenal angiotensin II (AII) receptors have been pharmacologically and structurally divided into two main subtypes, AT(1) and AT(2). Radioligand receptor binding assays with(125)I-sarcosine(1), isoleucine(8) angiotensin II ((125)I-SI AII) in the presence of losartan, an AT(1) selective ligand, and PD123177 an AT(2) selective ligand, indicated that the AT(1) subtype was predominant in membrane homogenates of the rat adrenal cortex (AT(1) Bmax=649 ± 62 fmol/mg protein; AT(2) Bmax=237 ± 29 fmol/mg protein). In membrane homogenates of the adrenal medulla, the AT(2) subtype was predominant (AT(1) Bmax=55 ± 5 fmol/mg protein; AT(2) Bmax=109 ± 29 fmol/mg protein). Overall 58% of the(125)I-SI AII binding in the rat adrenal was to the AT(1) subtypes, and 42% was to the AT(2) subtypes. The outer cortex contained 59% of the AH receptor binding sites in the adrenal, while the medulla accounted for the remaining 41%. The affinity of the AT(1) binding sites in membrane homogenates of the cortex and medulla (K( D )=672 ± 123 pM and 573 ± 85 pM, respectively) was not significantly different. The affinity for(125)I-SH AII of AT(2) binding sites in membrane homogenates was higher than that of AT, binding sites. The affinity for(125)I-SI All of AT(2) binding sites in membrane homogenates of the outer cortex (K( D )=265 ± 35 pM) was significantly less than that in the medulla (K( D )=133 ± 11 pM).In vitro receptor autoradiography also demonstrated that the AT(2) subtype in frozen sections of the cortex had a lower affinity (K( D )=1512 ± 191 pM) than that in the medulla (K( D )=867 ± 72 pM). The heterogeneous affinity of adrenal AT(2) binding sites may indicate existence of multiple AT(2) receptor subtypes in the rat adrenal.

Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

Pretreatment of rats with prazosin (PRA), an alpha 1-adrenergic receptor blocker, abolished the increased rate of learning of conditioned avoidance responses stimulated by intracerebroventricular angiotensin II (AII) administration. Yohimbine (YOH), an alpha 2-receptor blocker, reversed the effect of AII. PRA did not affect, and YOH abolished, the improvement of recall of a passive avoidance behavior caused by AII. The stereotypies produced by apomorphine (APO) and amphetamine (AMP) were enhanced by AII. PRA changed neither stereotypy, but it abolished the AII effect in both cases. YOH did not alter APO stereotypy and abolished the enhancement of that behavior caused by AII. YOH increased AMP stereotypy and had an additive effect with AII. No significant changes of exploratory motor activity were caused by PRA, YOH, or their combination, with AII. These findings indicate that functioning alpha 1- and alpha 2-adrenergic receptors are necessary for the facilitation of learning by AII, while only alpha 2-receptors appear to be involved in AII improvement of recall. The central dopaminergic system may in part be responsible for the modulation by PRA and YOH of the effects of AII on learning and recall.

Monoamine depletion does not alter angiotensin II binding sites in the rat brain

6-Hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT) was administered in the lateral cerebral ventricles of rats. These respective treatments caused reductions in norepinephrine (NE) and 5-hydroxytryptamine (5-HT) levels in the hypothalamus-thalamus-septum-midbrain (HTSM; 62% and 44%) and brainstem (57% and 26%), respectively. Catecholamine depletion in 6-OHDA-treated rats resulted in a 55% decrease in the pressor response to ICV Ang II which was not associated with any significant change in the binding of 125I-sarcosine1, isoleucine8 Ang II to Ang II binding sites in the HTSM or brainstem. The radioligand binding was not significantly affected by the 5,7-DHT treatment. These results indicate that brain Ang II binding sites are not predominantly localized on NE- or 5-HT-containing nerve terminals. They do not, however, preclude the existence of a small proportion of Ang II binding sites on these neuronal populations.

Angiotensin II-(3-8)-hexapeptide affects motor activity, performance of passive avoidance and a conditioned avoidance response in rats

Angiotensin II-(3-8)-hexapeptide, at the dose of 1 nmol given intracerebroventricularly, only slightly less than angiotensin II (the same dose and route) stimulated exploratory locomotor behaviour in an open field and electromagnetic motimeter. Both peptides considerably enhanced stereotyped behaviour produced by apomorphine and amphetamine. Angiotensin II-(3-8)-hexapeptide improved recall in a passive avoidance situation as well as angiotensin II. The 3-8 C-terminus of angiotensin II enhanced acquisition of active avoidance nearly as effectively as the complete peptide. The results indicate that the effectiveness of equimolar doses of angiotensin II-(3-8)-hexapeptide and angiotensin II in improving processes related to learning and memory in rats is almost identical and thus must be independent of specific angiotensin receptors in brain to which the hexapeptide binds with about 1000 times lower affinity than angiotensin II. The stimulation of stereotypy, a dopamine-controlled behaviour, by the peptides points to the possibility of dopaminergic mediation of their psychotropic effects.

Neuronal localization of specific angiotensin II binding sites in the rat inferior olivary nucleus

Adult male Sprague-Dawley rats were treated with 3-acetylpyridine, a neurotoxin selective for the inferior olivary nucleus. Following treatment, the rats exhibited deficits in locomotor behavior indicative of destruction in the inferior olivary nucleus. The rats were sacrificed 3 weeks later, and the binding of 125I-sarcosine, isoleucine angiotensin II to brain homogenates and slide-mounted sections of brainstem was determined. Treatment with 3-acetylpyridine significantly decreased specific 125I-sarcosine, isoleucine angiotensin II binding in homogenates of posteroventral brainstem (containing the inferior olivary nucleus) by approximately 50%. Homogenates of dorsal brainstem and hypothalamus-thalamus-midbrain showed no significant changes in specific binding. Treatment with 3-acetylpyridine did not significantly alter the radioligand binding affinity which was determined in the hypothalamus-thalamus-midbrain. Autoradiographic analysis of 125I-sarcosine, isoleucine angiotensin II binding in the brainstem sections indicated that specific angiotensin II binding sites in the inferior olivary nucleus were virtually eliminated by the 3-acetylpyridine treatment. In addition, a comparatively small, but significant, decrease in specific 125I-sarcosine, isoleucine angiotensin II binding occurred in the solitary tract nucleus/dorsal vagal motor nucleus complex. These results indicate that specific angiotensin II binding sites in the inferior olivary nucleus occur exclusively on neuronal perikarya and/or dendrites.