Paraventricular nucleus of hypothalamus participates in the sympathetic modulation and spontaneous fluctuation of baroreflex during head up tilt in unanesthetized rats

The autonomic nervous system is importantly involved in the maintenance of arterial pressure during orthostatic challenges. However, little is known about the specific central areas involved in these cardiovascular compensations. It has been proposed that the paraventricular nucleus of the hypothalamus (PVN) is involved in cardiovascular reflex responses related to blood volume. Our hypothesis is that PVN is involved in autonomic modulation during an orthostatic challenge (head up tilt, HUT). Adult male Wistar rats, instrumented with guide cannulas to the PVN and femoral artery and vein catheters were submitted to mean arterial pressure (MAP) and heart rate (HR) recordings in conscious state. After baseline parameters the rats were submitted to HUT. The spectral analysis during HUT showed an increase in low-frequency oscillation of systolic arterial pressure (SAP) (LF: 14.21±2.73-32.44±8.43 mmHg(2)) and pulse interval (PI) (LF: 14.05±4.25-51.79±10.64 n.u.) and a decrease in high-frequency oscillation (HF; 84.52±4.82-47.49±10.30 n.u.). Previous bilaterally microinjection of cobalt chloride (1 mM/100 nl), a calcium channel blocking agent, into the PVN decreased LF oscillations of SAP (LF: 32.44±8.43-13.23±1.87 mmHg(2)) as well as in PI (LF: 12.38±3.76-5.03±1.20 ms(2)). Muscimol microinjection (40 mM), a GABAA agonist, decreased LF component of PI oscillations (LF: 51.79±10.64-25.76±5.34 n.u.). The baroreflex gain was not altered by HUT, but during tilt, with PVN previously inhibited by muscimol or cobalt chloride, the gain was reduced. Our data suggest that the PVN participates in the brain circuitry involved in autonomic adjustment during orthostatic challenges.