Differential responses of oxytocin and vasopressin neurons to the osmotic and stressful components of hypertonic saline injections: a Fos protein double labeling study.

Expression of Fos protein, detected immunocytochemically, was used to assess the relative responses of supraoptic nucleus (SON) oxytocin- (OX) and vasopressin- (VP) containing neurons to the osmotic vs. the osmotic plus stressful components of intraperitoneal hypertonic saline injections. The percentage of SON neurons showing Fos-like immunoreactivity (Fos-ir) was quantified for rats receiving general anesthesia only, anesthesia 1 h prior to either isotonic or hypertonic saline injection or no anesthesia prior to hypertonic injection. Hypertonic saline injection with and without anesthesia induced Fos-ir in 66% and 77% of SON neurons, respectively, whereas isotonic saline with anesthesia and anesthesia alone resulted in 15% and 13%, respectively, of cells showing Fos-ir. Double labeling for Fos-ir and either OX-ir or VP-ir resulted in quantitatively different responses to hypertonic injections with and without anesthesia in OX-ir and VP-ir neurons. The VP-ir neuronal response was similar under the two conditions: 49% and 48% of VP cells displaying Fos-ir with and without prior anesthesia, respectively. By contrast, a higher percentage of OX-ir neurons was found to exhibit Fos-ir without (68%) than with (53%) anesthesia. Thus, a greater percentage of neurons was induced to express Fos-ir when the stressful components of the hypertonic injection were unattenuated by anesthesia, and this difference was entirely due to increased numbers of responding OX neurons. These data indicate that, under these experimental conditions, SON OX neurons respond in larger numbers to the osmotic components of hypertonic saline injections and have a greater responsiveness than do VP neurons to the stressful components.