Functional evidence of paraventricular nucleus involvement in cardiovascular and autonomic modulation in response to acute microgravity (head-down tilt) in unanesthetized rats.

Exposure to microgravity induces autonomic and vestibular disorders such as alterations in cardiovascular function. The paraventricular nucleus of the hypothalamus (PVN) is known to be an important center for integrating autonomic and cardiovascular responses as blood volume reflexes. The acute effects promoted by microgravity and PVN involvement in cardiovascular and autonomic parameters have not yet been evaluated. Male Wistar rats were anesthetized to facilitate cannulae implantation in the PVN. After 3 days of surgical recovery, femoral artery and vein catheters were implanted for direct recording of blood pressure and heart rate (HR) in conscious animals to evaluate cardiovascular and autonomic changes in an acute protocol of head-down tilt (HDT) in nonanesthetized rats. During HDT, there was an increase in mean arterial pressure (11 ± 1 mmHg, P < 0.05) and a decrease in HR (-28 ± 5 bpm, P < 0.05). Spectral analysis of systolic arterial pressure showed an increase in the low-frequency (LF) component. In addition, HDT induced a reduction in the LF component and an increase in the 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. These results suggest that the PVN participates in the cardiovascular compensation during HDT, especially modulating cardiac responses.

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.

Altered baroreflex and autonomic modulation in monosodium glutamate-induced hyperadipose rats.

We aimed to examine the cardiovascular function by tonic and baroreflex alterations in obese rats induced by monosodium glutamate (MSG). Neonatal male Wistar rats were injected with MSG (4 mg/g body weight) or equimolar saline (control, C). At 90 days, all rats were anesthetized for catheterization of the femoral artery for mean arterial pressure (MAP) and heart rate (HR) recordings in the conscious state. After baseline, we performed IV treatment with hexamethonium (25 mg/kg), or atropine (1 mg/kg) or propranolol (3 mg/kg). We also performed the spectral analysis of heart rate variability (HRV) and baroreflex sensitivity. Baseline comparison showed that obese rats are hypertensive compared with control (C=110±2 mmHg; MSG=: 123±3 mmHg, P<0.05). After ganglionic blockade with hexamethonium the differences in MAP between control and obese rats disappeared. Beta adrenergic blockade with propranolol induced a greater decrease in heart rate compared with control. The analysis of HRV showed that obese rats have increased modulation by both components of the autonomic nervous system compared with control rats. The baroreflex gain showed increased sensitivity for the parasympathetic component in the obese rats (C=-2.41±0.25; MSG=-3.34±0.23 bpm/mmHg) compared with control. Our data suggest that both components of autonomic cardiac tonus and the parasympathetic component of the baroreflex sensitivity are increased in the MSG obese rat. It is possible that the parasympathetic alterations observed in these MSG obese rats may have originated from central areas of cardiovascular control.

Involvement of the paraventricular nucleus (PVN) of hypothalamus in the cardiovascular alterations to head up tilt in conscious rats.

We evaluated the involvement of paraventricular nucleus (PVN) in the changes in mean arterial pressure (MAP) and heart rate (HR) during an orthostatic challenge (head up tilt, HUT). Adult male Wistar rats, instrumented with guide cannulas to PVN and artery and vein catheters were submitted to MAP and HR recording in conscious state and induction of HUT. The HUT induced an increase in MAP and HR and the pretreatment with prazosin and atenolol blocked these effects. After inhibition of neurotransmission with cobalt chloride (1 mM/100 nl) into the PVN the HR parameters did not change, however we observed a decrease in MAP during HUT. Our data suggest the involvement of PVN in the brain circuitry involved in cardiovascular adjustment during orthostatic challenges.