Fos immunoreactivity after nonhypotensive and hypotensive hemorrhage in conscious rabbits

Immunocytochemistry for FOS was used as an index of neural activation in response to hemorrhage in conscious adult rabbits. Rabbits were instrumented with an arterial catheter for measurement of blood pressure (MAP) and heart rate (HR) and a venous catheter for blood withdrawal. Three groups of rabbits were tested: control (C), no hemorrhage; nonhypotensive hemorrhage (NH), 25% increase in HR, no change in MAP; and hypotensive hemorrhage (HH), decrease in MAP to <40 mmHg. Pre-hemorrhage MAP and HR values were not different among the groups. The mean blood loss in C, NH and H was 0, 10.5 and 17.3 ml/kg respectively. Hemorrhage increased HR in NH and HH and decreased MAP in HH. After NH FOS expression in the supraoptic nucleus (SON) was increased (100%) compared to C. HH was associated with increased FOS in the SON (177%), the rostral ventrolateral medulla IRVLM, 34%) and the caudal ventrolateral medulla (CVLM, 48%) compared to C. Activation of neurons in the SON and RVLM by HH in conscious rabbits is consistent with data from the rat. The activation of CVLM neurons in HH could be related to the sympathpinhibition characteristic of hypotensive bloodless. In addition, activation of CVLM neurons in HH may be associated with vasopressin release from the SON.