Attenuated fos expression in the caudal ventrolateral medulla of rats with area postrema lesions after isotonic volume expansion

Previous work from this laboratory has shown that isotonic volume expansion in rats increased fos expression in specific brain areas that are thought to be involved in cardiovascular regulation and body fluid homeostasis (Randolph et al., 1997). One of those areas was the area postrema (AP), a brainstem circumventricular organ which is capable of detecting changes in humoral factors and also receives vagal afferent input. Therefore, the present study examined fos expression after volume expansion in rats with lesions of the AP and the subadjacent nucleus tractus solitarius (APX; n = 5) and in rats with sham lesions (APS; n = 3). Rats were infused with 0.15 M NaCl intravenously at a rate equivalent to 10% of their body weight in ten min, and then at 0.5 ml/min to maintain the expansion for a total of two h. Rats then were anesthetized and perfused with paraformaldehyde. The brains were removed, sectioned, and processed for fos immunocytochemistry (Oncogene, AB-5). Comparable numbers of fos-positive nuclei were observed in the caudal nucleus tractus solitarius of rats with APX (80.2 ± 14.6) and rats with APS (109.2 ± 28.4). In contrast, less fos-positive nuclei were observed in the caudal ventrolateral medulla of rats with APX compared to those in rats with APS (7.5 ± 1.7 vs. 16.0 ± 1.6; p<0.05). These results indicate that the activation of the caudal ventrolateral medulla during volume expansion depends on the integrity of the AP and the adjacent nucleus tractus solitarius. Thus, activation of the caudal ventrolateral medulla during volume expansion may be mediated by humoral and/or vagal input integrated at the level of the AP.