Pre‐hypertensive molecular alterations in the paraventricular nucleus of the hypothalamus during angiotensin‐II hypertension

Abstract

The paraventricular nucleus of the hypothalamus (PVN) plays a critical role in the development of hypertension in response to elevated levels of angiotensin‐II (Ang‐II). Upstream circumventricular organs, regions lacking a blood‐brain‐barrier, sense circulating Ang‐II and in turn influence endocrine and autonomic control via efferent projections to the PVN. Despite progress in unraveling the function of the PVN in hypertension development, the underlying signaling mechanisms remain unclear. Notably, the majority of investigations have examined alterations in the PVN during established hypertension, which limits our understanding of temporal changes that may occur prior to elevations in arterial blood pressure. Based on this, we investigated key molecular targets in the PVN during the pre‐hypertensive phase of Ang‐II hypertension. C57Bl/6J male mice were implanted with subcutaneous osmotic minipumps for chronic delivery of low dose Ang‐II (600 ng/kg/min). Brains were harvested for immunohistochemistry analysis of target molecules (n=3–4/group) at baseline (day 0) and pre‐hypertensive (days 3, 5, 7) timepoints. Histological analysis of synaptophysin, a marker for synaptic terminal innervation, indicated a progressive increase in immunoreactivity that peaked by ~10% within the first week of Ang‐II infusion (day 7 fold change: 1.1± 0.03 density/area, p<0.05); indicative of increased synaptic input to the PVN prior to elevations in arterial blood pressure. Previous findings have demonstrated that circulating arginine vasopressin protein (AVP) is elevated during chronic Ang‐II hypertension. Consistent with this, AVP immunoreactivity in the PVN was elevated as early as day 3 of Ang‐II infusion (day 3 fold change: 1.7 ± 0.09 density/area, p<0.05), and this response was sustained for up to 7 days (day 5 fold change: 1.4 ± 0.02; day 7 fold change: 1.3 ± 0.03 density/area, both p<0.05). Interestingly, we also observed synaptic end AVP immunoreactivity in the circumventricular subfornical organ very early during Ang‐II infusion (e.g. day 3 fold change: 1.2 ± 0.02 density/area, p<0.05), suggesting a possible feedback loop between the PVN and subfornical organ in AVP regulation. Unlike AVP, PVN oxytocin immunoreactivity was not changed during Ang‐II hypertension (e.g. day 7 fold change: 1.01 ± 0.08 density/area, p>0.05). Collectively, these findings indicate early molecular changes within the PVN prior to the development of Ang‐II hypertension including: 1) Gradual synaptic innervation of the PVN during the first week of Ang‐II infusion; 2) Very early Ang‐II associated elevations in PVN AVP expression, but not oxytocin; and 3) Potential alterations in the innervation of hypothalamic AVP fibers to the circumventricular subfornical organ in response to Ang‐II.Support or Funding Information19CDA34630010 (to Jin Kwon Jeong) and 1R01HL141393 (to Colin N. Young)