Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

FURO/CAP: a protocol for sodium intake sensitization

We investigated if a history of FURO/CAP, a protocol that increases brain angiotensin II (ANG II), sensitizes or enhances sodium intake. A subcutaneous injection of the diuretic furosemide (FURO, 10mg/kg) was combined with a converting enzyme inhibitor captopril (CAP, 5mg/kg) to induce a short latency stimulated sodium (0.3M NaCl) and water intake in a 2h FURO/CAP test. Repeated injections of only FURO/CAP, separated by one-week interval, enhanced stimulated and spontaneous (daily) sodium intake. Stimulated fluid intake was completely suppressed when FURO/CAP was combined with two intraperitoneal injections of the ANG II type-1 receptor antagonist losartan (10, 20, or 40 mg/kg each) given within 1h prior to the FURO/CAP test. Losartan reduced by only 35% the FURO/CAP-induced natriuresis. A history of FURO/CAP, FURO/CAP+losartan (all doses), or vehicle produced similar stimulated fluid intake when all animals received only FURO/CAP in the third final FURO/CAP test. However, a history of vehicle or FURO/CAP+losartan 10mg/kg precluded the enhancement in spontaneous sodium intake after the third final FURO/CAP. The FURO/CAP combined with losartan (all doses) also precluded the spontaneous sodium intake enhancement in the weeks that preceded the third final FURO/CAP test. A history of only FURO/CAP, but not vehicle, also enhanced water deprivation-induced sodium appetite. The results suggest that a history of FURO/CAP enhances stimulated and spontaneous sodium intake, as well as water deprivation-induced sodium appetite, and reinforce the role of ANG II as a peptide that mediates long-term effects on behavior.

Central AII evokes a normal sodium appetite in the Fischer rat, but its low spontaneous sodium intake may be related to reduced excitation and increased inhibition in septo-preoptic AII neurons

Fischer rats show a low or absent basal salt appetite and a reduced intake of salt solutions in response to peripherally administered angiotensin II (AII) when compared to other strains. We investigated spontaneous sodium intake, and sodium intake after intracerebroventricular (i.c.v.) AII and losartan, and septo-preoptic neuronal responses to AII and losartan, in age-matched male Fischer and Wistar rats. Spontaneous intake of 1.8% NaCl was lower in Fischers, but i.c.v. injection of 10 pmol AII produced similar 2 h intakes in a 2 h test period. Iontophoretic application of AII and losartan onto neurons in the septo-preoptic continuum revealed differences between the two strains of rat. In the Fischer rats only 11% of the spontaneously active neurons were sensitive to locally applied AII compared to approximately 30% in the Wistar. Local application of losartan produced neuronal inhibition in Fischer rats but neuronal excitation in Wistars. The central AII system appears to be regulated differently in these two strains, and may be related to the differences in their spontaneous sodium intake, but not to AII aroused sodium appetite.

Effects of central oxytocin receptor blockade on water and saline intake, mean arterial pressure, and c-Fos expression in rats

Central injection of ANG II has been proposed to have dual effects on salt appetite including a direct stimulatory effect and an indirect inhibitory effect through an activation of central oxytocinergic neurons. The inhibition was demonstrated by pretreating rats with central ornithine vasotocin (OVT; oxytocin antagonist) 30 min before a central ANG II injection. The OVT pretreatment produced a large increase in ANG II-induced saline intake. The present paper reports a failure to replicate that influential experiment. However, we also report for the first time that OVT by itself: 1) provokes drinking of both water and saline solution with a latency almost as short as that produced by ANG II; 2) produces a mild pressor response; and 3) increases c-Fos expression in the organum vasculosum laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Oxytocin activity may provide an inhibitory control of drinking responses as has been suggested, but the inhibition is tonic and includes both water and saline drinking. Inhibition of this tonic activity may stimulate drinking by increasing neural activity in the OVLT and MnPO.

Glial angiotensinogen regulates brain angiotensin II receptors in transgenic rats TGR(ASrAOGEN)

TGR(ASrAOGEN)680, a newly developed transgenic rat line with specific downregulation of astroglial synthesis of angiotensinogen, exhibits decreased brain angiotensinogen content associated with a mild diabetes insipidus and lower blood pressure. Autoradiographic experiments were performed on TGR(ASrAOGEN) (TG) and Sprague-Dawley (SD) control rats to quantify AT(1) and AT(2) receptor-binding sites in different brain nuclei and circumventricular organs. Dose-response curves for drinking response to intracerebroventricular injections of ANG II were compared between SD and TG rats. In most of the regions inside the blood-brain barrier [paraventricular nucleus (PVN), piriform cortex, lateral olfactory tract (LOT), and lateral preoptic area (LPO)], AT(1) receptor binding (sensitive to CV-11974) was significantly higher in TG compared with SD. In contrast, in the circumventricular organs investigated [subfornical organ (SFO) and area postrema], AT(1) receptor binding was significantly lower in TG. AT(2) receptors (binding sensitive to PD-123319) were detected at similar levels in the inferior olive (IO) of both strains. Angiotensin-binding sites sensitive to both CV-11974 and PD-123319 were detected in the LPO of SD rats and specifically upregulated in LOT, IO, and most notably PVN and SFO of TG. The dose-response curve for water intake after intracerebroventricular injections showed a higher sensitivity to ANG II of TG (EC(50) = 3.1 ng) compared with SD (EC(50) = 11.2 ng), strongly suggesting that the upregulation of AT(1) receptors inside the blood-brain barrier of TG rats is functional. Finally, we showed that downregulation of angiotensinogen synthesized by astroglial cells differentially regulates angiotensin receptor subtypes inside the brain and in circumventricular organs.