Fos expresssion in male rats following isotonic volume expansion

Physiological responses to volume expansion include a decrease in sympathetic nervous activity and a decrease in arginine vasopressin (AVP) release. In order to determine the regions of the CNS involved in the response to volume expansion, we used Fos immunocytochemistry to indicate areas of neural activity. Rats (N=8) were volume expanded for 2 hours, infusing an amount of isotonic saline equal to 10% of the animal's body weight in the first ten minutes and maintaining the expansion with 0.5 ml/min thereafter. Control animals (N=4) were infused with isotonic saline for 2 hours at a rate of 0.01 ml/min. Volume expanded animals showed an increase in central venous pressure and a decrease in mean arterial pressure. The volume expanded group exhibited significantly greater Fos expression than the control animals in the caudal ventrolateral medulla (CVLM; 203%, p<0.05), nucleus of the tractus solitarius (NTS; 188%, p<0.05), supraoptic nucleus (SON; 285%, p<0.05), and perinuclear zone of the SON (PNZ; 300%, p<0.05). The PNZ is a region that mediates baroreceptor-induced inhibition of AVP neurons in the SON. The location of the Fos-positive cells in the SON could be related to a selective activation of oxytocin neurons. The diagonal band of Broca (DBB), another area involved in baroreceptor-mediated inhibition of AVP release from the SON, failed to show an increase in Fos. Volume expansion is associated with activation of the CVLM, an area associated with sympathetic inhibition. Volume expansion may activate neurons of the PNZ independent of the DBB.