NTS catecholamine cell recruitment by hemorrhage and hypoxia

IMMUNOLABELLING for Fos and tyrosine hydroxylase was used to determine the patterns of activation of nucleus tractus solitarius catecholamine cells in response to graded levels of hemorrhage (0, 4, 8, 12 and 16 ml/kg) and sys temic hypoxia (21, 14, 12, 10 and 8% O-2) in conscious rats. Both stimuli e licited graded catecholamine cell recruitment with thresholds of 8 ml/kg an d 12% O-2. The majority of responsive neurons were A2 noradrenergic rather than C2 adrenergic cells. After hemorrhage most Fos-positive catecholamine cells were found below obex whereas most hypoxia-responsive cells were rost ral to obex. These distinctive patterns of catecholamine cell recruitment m ay explain the differences in neuroendocrine responses to these stimuli. (C ) 1999 Lippincott Williams & Wilkins.