Hidden Player in Hypertension? Exploring the Role of Angiotensin II Receptor-Associated Protein in the Paraventricular Nucleus of Spontaneously Hypertensive Rats

Dysregulation of Angiotensin II (AngII) signaling via the AngII Type 1 Receptor (AT1R) is implicated in neurogenic hypertension and elevated sympathetic activity. The AngII Receptor-Associated Protein (ATRAP/ Agtr ) regulates AngII signaling by promoting AT1R internalization. Previous studies have demonstrated that enhancing ATRAP expression via lentivirus-mediated gene delivery in the paraventricular nucleus of the hypothalamus (PVN), a key cardiovascular regulatory region of the brain, lowers blood pressure, though not to a normal level, in a model of AngII-induced hypertension. However, whether ATRAP is differentially expressed within the PVN of Spontaneously Hypertensive Rats (SHR), and in which cell types ATRAP are expressed within the brain remains unknown. We hypothesized that ATRAP gene and protein expression are decreased within the PVN of SHRs compared to normotensive Wistar rats (WIS). Using 12-week-old male SHRs and WIS, we measured Agtrap and Agt1r mRNA expression in PVN punches via real-time PCR. ATRAP protein density and cell type localization were analyzed using immunofluorescence (IF) and confocal imaging. We observed no significant differences in Agtrap mRNA expression in PVN punches between groups (p > 0.05). However, there was a marginal increase in Agt1r mRNA expression in SHR PVN punches compared to WIS (WIS: 1.12 ± 0.35 vs. SHR: 4.17 ± 1.07, p = 0.055). Additionally, a significant decrease in ATRAP protein density was detected in the PVN of SHRs compared to WIS (WIS: 31.21 ± 1.03 vs. SHR: 27.27 ± 0.81, p = 0.0048), as measured by immunofluorescence (IF). To investigate whether these differences were localized to subnuclei containing neurons projecting to the rostral ventrolateral medulla (RVLM), which modulate sympathetic activity (parvocellular neurons), we used vasopressin IF as an anatomical marker to identify levels of the parvocellular dorsal cap (PaDC), parvocellular ventromedial (PaVM), lateral magnocellular (LM), and parvocellular posterior (PaPo) subnuclei of the PVN. We found a significant decrease in ATRAP protein expression in the PaDC (WIS: 32.13 ± 0.74 vs. SHR: 25.76 ± 0.82, p < 0.0001) and LM (WIS: 42.25 ± 1.13 vs. SHR: 36.53 ± 1.80, p = 0.0117). In contrast, no significant changes were observed in the PaVM (WIS: 32.43 ± 1.44 vs. SHR: 30.46 ± 1.04, p = 0.2228) or PaPo (WIS: 27.13 ± 1.89 vs. SHR: 23.81 ± 1.98, p = 0.697). To identify the cellular sources containing ATRAP protein, we performed immunohistochemistry for neurons (NeuN), astrocytes (GFAP), and microglia (IBA1), followed by high-magnification full Z-stack confocal imaging. ATRAP protein colocalized with neurons (NeuN IF) and astrocytes (GFAP IF) within the PVN, but was not detected in microglial cells (IBA1 IF). This finding is particularly intriguing, as astrocytes have been implicated as intermediary cell types influencing AngII activity in PVN-RVLM neurons modulating sympathetic output. In summary, our findings suggest that reduced ATRAP protein expression, primarily in neurons and astrocytes, may hinder ATRAP-mediated internalization of AT1R, contributing to the exacerbation of AngII-AT1R signaling. This work was supported by AHA 953524 to VCB. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.