Distribution of activated neurons in the rabbit brain following a volume load

Immunohistochemical detection of the protein, Fos, was used to identify neurons in the brain activated following a volume load. The plasma expanders, Haemaccel and 6% dextran, were infused intravenously in conscious rabbits for 60 min. Compared to control animals both stimuli significantly increased right atrial pressure but had no effect on blood pressure. Heart rate was significantly elevated with dextran only. Volume expansion with Haemaccel also reduced renal sympathetic nerve activity by about 50% from the pre-infusion resting level. Ninety minutes after the start of the infusion, the rabbits were perfusion fixed and the distribution of Fos-positive cell nuclei was examined. Following Haemaccel infusion there were significant increases in the number of Fos-positive cell nuclei in the organum vasculosum of the lamina terminalis, parvocellular paraventricular nucleus and in specific rostrocaudal levels of the nucleus tractus solitarius and ventrolateral medulla. Following dextran similar effects were observed in the medulla but Fos-positive cell nuclei were not significantly elevated above controls in the forebrain. After Haemaccel or dextran areas such as the supraoptic nucleus, the magnocellular paraventricular nucleus, the bed nucleus of the stria terminalis, diagonal band of Broca and amygdala either did not produce Fos or were not consistently different from the control group. The results suggest that specific brain regions, that are known to be important in cardiovascular control, are activated by a volume load. These areas are likely to play an important role in the reflex responses initiated by that particular stimulus.