FOS RESPONSE OF FETAL SHEEP ANTERIOR CIRCUMVENTRICULAR ORGANS TO OSMOTIC CHALLENGE IN LATE-GESTATION

We hypothesized that the anterior circumventricular organs (ACVO) and the supraoptic (SON) and hypothalamic paraventricular nuclei (PVN), am ong other structures that play a role in sensing extracellular body fl uid volume and composition in postnatal animals las demonstrated by Fo s protein production by the immediate-early gene c-fos), would show si milar activation in fetal sheep during an osmotic challenge. The brain s of 10 fetal sheep 16 treated, 4 controls; 129-131 days of gestationa l age (dGA) = 0.87 gestation] were immunostained for Fos. Seventy-five minutes before tissue collection the dams were given intravenous 20% mannitol(l ml.min(-1).kg(-1) for 10 min). Subsequently, the ACVO, SON, and PVN were scored for the amount of neuronal Fos immunostaining. Th e subfornical organ (SFO; 24.5 +/- 9.0 vs. 1.7 +/- 1.2), the organum v asculosum of the lamina terminalis (OVLT; 26.8 +/- 5.6 vs. 7.0 +/- 2.0 ), the SON (39.8 +/- 3.0 vs. 0.15 +/- 0.1), and the PVN (59.8 +/- 7.9 vs. 0.7 +/- 0.7) had increases (P < 0.05) in the average number of Fos -positive cells per field compared with controls, whereas the median p reoptic nucleus did not. Double immunostaining for Fos and arginine va sopressin (AVP) or oxytocin (OT) indicated that AVP- but not OT-immuno positive neurons in SON and PVN respond to osmotic challenge. These re sults demonstrate that the SFO, OVLT, SON, and PVN are activated by os motic challenge in fetal sheep at 130 dGA.