ROLE OF PARAVENTRICULAR NUCLEUS PARVICELLULAR NEURONS IN THE COMPENSATORY RESPONSES TO GRADED HEMORRHAGE

The goal of this study was to determine the role of the parvicellular component of the paraventricular hypothalamic nucleus (PVH) in the com pensatory responses to blood loss. Male Sprague-Dawley rats were prepa red with bilateral ibotenate lesions bf the parvicellular PVH (PVHx; n = 5) or with sham lesions (Sham; n = 8). After >10 days recovery, hem orrhage was performed by gradual withdrawal of 16 ml/kg blood over 34 min via an indwelling femoral arterial catheter while the rats were co nscious and unrestrained. Basal serum corticosterone levels, plasma re nin concentration (PRC), mean arterial pressure, and heart rate did no t differ between PVHx and Sham, whereas basal hematocrit was lower in PVHx than Sham (40 +/- 1 vs. 44 +/- 1; P < 0.05). After hemorrhage, co rticosterone increased fourfold in Sham (P < 0.001) but did not increa se significantly in PVHx. However, the blood pressure, heart rate, PRC , and hemodilution responses to hemorrhage were the same in Sham and P VHx during both the normotensive (7-13 ml/kg blood loss) and hypotensi ve (16 ml/kg blood loss) phases, In conclusion, the parvicellular PVH is essential for the corticosterone response, but not for the cardiova scular or renin responses to blood loss.