Identification of vasopressin in the subfornical organ region: effects of dehydration.

The subfornical organ (SFO), a neuroendocrine structure implicated in saltwater homeostasis, contains secretory structures histochemically similar to those in the neurohypophysis. Because of these morphological similarities, we compared levels of vasopressin (vp) in the SFO area and the adjacent hippocampal commissure-fornix (HC-F) of normally hydrated and 48 h water-deprived (WD) rats. VP in the SFO region from normally hydrated rats was 44 +/- 5 pg/mg protein, 3.5 +/- 0.4 ng/g wet weight or 3.6 +/- 0.4 pg/SFO. These concentrations increased (P less than 0.05) about twofold after WD. The content of VP, ng/g wet weight, in HC-F was higher (P less than 0.05) than the SFO area and also increased (P less than 0.05) after WD. VP was detected in other fiber tracts, anterior commissure (AC) and fornix (F), but was unchanged by WD. Changes in hormone observed in the SFO and HC-F regions were therefore not part of a generalized increase of VP in the brain, nor can they be ascribed to elevated plasma levels. Thus, VP changes in the SFO region may be functionally significant and related to an SFO endocrine role in hydration. VP in fiber tracts (F, AC) unassociated with the hypothalamo-hypophysial system and unchanged after WD may suggest an unidentified role of this hormone in the central nervous system.