EFFECT OF ENDOTOXIN ON THE HYPOTHALAMIC-PITUITARY-ADRENAL AXIS IN SHEEP

Endotoxin has been shown to stimulate ACTH and cortisol secretion thro ugh an action at the hypothalamic level. However the nature of hypotha lamic neurohormones, corticotropin-releasing hormone (CRH) and especia lly arginine vasopressin (AVP), involved in that regulation is still c ontroversial, The purpose of this study was to determine the effects o f an acute i.v. endotoxin administration on CRH and AVP secretion into hypophysial portal blood (HPB). The experiment has been performed in sheep since it is possible to collect HPB and quantify CRH and AVP sec retion in this animal under physiological conditions. The release of b oth peptides into HPB was stimulated by endotoxin injection, the incre ase in portal AVP being more pronounced than that of CRH. An initial, transient, increase in jugular AVP concentrations was observed, probab ly due to the activation of magnocellular AVP neurons. In conclusion, our data indicate that the activation of the pituitary-adrenal axis af ter endotoxin injection is associated with an increased release of bot h CRH and AVP into HPB, Magnocellular AVP neurons are initially stimul ated while parvocellular CRH and AVP neurons are stimulated throughout the experiment.