Rapid-onset "need-free" sodium appetite after lesions of the dorsomedial medulla.

Previous studies indicate the dorsomedial hindbrain is important in the maintenance of fluid balance. Lesions centering on the area postrema (AP) result in enhanced ad libitum intake of concentrated saline solutions on a 24-h basis as well as enhanced excretion of water and electrolytes. Additionally, rats with lesions of the AP are reported to have an exaggerated behavioral response to dipsogenic challenges that mimic depletion of the extracellular fluid compartment. In the studies reported here, we have observed that rats with lesions of the AP and immediately adjacent nucleus of the solitary tract consume large quantities of concentrated saline solutions within minutes of presentation. This intake does not appear to be related to sodium loss or depletion because the lesioned rats are in a positive sodium balance and the appetite persists after bilateral nephrectomy. These data, in conjunction with earlier studies, suggest that the dorsomedial medulla, including the AP, acts to prevent overconsumption of water and sodium that could result in increased extracellular fluid volume.