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Cardiovascular and Autonomic Modulation by Paraventricular Nucleus of Hypothalamus During Head Down Tilt in Unanesthetized Rats

The exposure to microgravity induces autonomic and vestibular disorders as alterations in cardiovascular function. The paraventricular nucleus of the hypothalamus (PVN) is known as an important center for integrating autonomic and cardiovascular responses as blood volume reflexes. The acute effects promoted by microgravity and the PVN involvement in cardiovascular and autonomic parameters were not evaluated yet. Male wistar rats were anesthetized to guide cannulae implantation to PVN. After 3 days of surgical recovery, femoral artery and vein catheters were implanted for direct recording of blood pressure (BP) and heart rate (HR) in conscious animals to evaluate cardiovascular and autonomic changes in an acute protocol of head down tilt (HDT) in non‐anesthetized rats.During HDT, there was an increase in mean arterial pressure (MAP) (11±1mmHg) and decrease in heart rate (HR) (‐28±5 bpm). Spectral analysis of systolic arterial pressure (SAP) showed an increase in low frequency (LF) component. In addition, HDT induced a reduction in the LF and increase in high frequency (HF) component of the pulse interval (PI). PVN inhibition with muscimol reversed bradycardia and blocked the reduction of the LF and HF increases in PI during HDT. Our results suggest that PVN participates in the cardiovascular compensation during HDT, especially modulating the cardiac responses.