C-fos expression and electrolytic lesions studies reveal activation of the posterior region of Locus Coeruleus during hemorrhage induced hypotension

Bilateral electrolytic lesions in the rat Locus Coeruleus (LC) were made one or seven days before experimentation Four hemorrhage sessions, withdrawing 10% of the blood volume per session, were performed in 5 min intervals in freely moving rats. Blood pressure (BP) was not affected by the lesions and did not drop in the first, but decreased in all subsequent hemorrhages. The decrease in BP in animals with lesion in the anterior LC was similar to the controls. However, animals with lesions in the posterior LC showed an enhanced decrease in BP during the second hemorrhage, in acute and chronic experiments. Expression of Fos protein was studied to investigate the relationship between LC activity and BP changes. Two hours after the second hemorrhage, the brains were removed and processed for Fos immunocytochemistry. Hemorrhage increased the number of Fos immunoreactive neurons mainly in the posterior LC. We conclude that (1) the LC does may not play a role in cardiovascular- control during resting, but seems to mediate compensatory cardiovascular mechanisms in situations of hypovolemia; and (2) the posterior LC, but not the anterior, plays a presser role during hemorrhage. (C) 1998 Elsevier Science B.V. All rights reserved.