Hypothalamic Activation of the mTOR Cascade Alters Neuronal Morphology and Function Leading to Altered Circadian Rhythm of Cardiovascular Parameters

The paraventricular nucleus of the hypothalamus (PVN) receives and integrates afferent information from various central regions to regulate sympathetic tone and cardiovascular function. These neurons are largely inhibited as baseline, but activated in response to stress, relaying excitatory signals to increase blood pressure and heart rate. Neuronal activation induces the expression of brain-derived neurotrophic factor (BDNF), a well-known regulator of neuroplasticity in the central nervous system (CNS). We have previously shown that BDNF modulates glutamatergic, GABAergic, and catecholaminergic mechanisms in the PVN, shifting the excitatory-inhibitory (E/I) synaptic balance towards excitation, promoting hypertensive and tachycardic responses. BDNF activates the mammalian target of rapamycin (mTOR) as part of mTOR complex 1 (mTORC1) with regulatory associated protein of mTOR (Raptor). In the CNS, mTOR sets E/I balance and controls dendritic branching and neuronal size. While hypothalamic mTOR signaling is known to regulate whole-organism energy balance, and components of this pathway are present in the PVN, the effect of mTOR signaling in the PVN on cardiovascular regulation is not known. Here, we hypothesize that PVN mTOR signaling modulates the integration of afferent signals and mediates the effects of BDNF. To test this, male Sprague-Dawley (SD) rats received bilateral PVN injections of AAV2 viral vectors expressing BDNF or shRNA against TSC1 (a mTORC1 repressor) to stimulate mTORC1 or shRNA against Raptor to inhibit mTORC1. Scrambled shRNA/GFP vectors served as controls. Blood pressure (mean arterial pressure, MAP) and heart rate (HR) were monitored via telemetry over 4 weeks. Injections were verified, and neuronal morphology and pS6 levels (a marker of mTOR activity) were assessed with immunofluorescence. Activating mTORC1 with TSC1 knockdown increased blood pressure and showed changes in circadian rhythm, leading to significant differences in daytime but not nighttime (daytime MAP: controls: 95.2±0.9, shTSC1: 102.6±1.2 mmHg; nighttime MAP: controls: 102.1±1.4, shTSC1: 105.2±0.9 mmHg). In contrast, BDNF overexpression significantly raised blood pressure and heart rate both during day- (increased MAP by ~32 mmHg and HR by ~68.6 bpm) and nighttime (increased MAP by ~29.4 mmHg and HR by ~2.8 bpm), consistent with our previous findings. Raptor knockdown had no effect alone on either day or nighttime blood pressure but completely abolished BDNF-induced increases in both blood pressure and heart rate. Immunofluorescence analysis showed that soma size and immunoreactivity of pS6 were significantly increased by both TSC1 knockdown and BDNF whereas Raptor knockdown reduced soma size and pS6 intensity relative to controls and counteracted the effects of BDNF. In addition, ongoing patch clamp experiments are conducted on PVN brain slices isolated 4-6 weeks after vector injections to test whether these in vivo changes correlate with altered neuronal and synaptic excitability in PVN neurons. This study demonstrates that PVN mTOR-mediated mechanisms play a significant role in cardiovascular mechanisms and mediate hypertensive responses and neuronal morphological changes induced by BDNF. Supported by R01HL166464. 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.