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 compensatory responses to blood loss. Male Sprague-Dawley rats were prepared with bilateral ibotenate lesions of the parvicellular PVH (PVHx; n = 5) or with sham lesions (Sham; n = 8). After >10 days recovery, hemorrhage was performed by gradual withdrawal of 16 ml/kg blood over 34 min via an indwelling femoral arterial catheter while the rats were conscious and unrestrained. Basal serum corticosterone levels, plasma renin concentration (PRC), mean arterial pressure, and heart rate did not differ between PVHx and Sham, whereas basal hematocrit was lower in PVHx than Sham (40 +/- 1 vs. 44 +/- 1; P < 0.05). After hemorrhage, corticosterone increased fourfold in Sham (P < 0.001) but did not increase significantly in PVHx. However, the blood pressure, heart rate, PRC, and hemodilution responses to hemorrhage were the same in Sham and PVHx during both the normotensive (7-13 ml/kg blood loss) and hypotensive (16 ml/kg blood loss) phases. In conclusion, the parvicellular PVH is essential for the corticosterone response, but not for the cardiovascular or renin responses to blood loss.