Central Fos expression in fetal and adult sheep after intraperitoneal hypertonic saline

We hypothesized that neural structures, involved in sensing extracellular b ody fluid composition in adult animals during an osmotic challenge, would s how similar patterns of activation in fetal sheep. Eight adult sheep [4 hyp ertonic saline-treated adults (HYP-A), 4 isotonic saline-treated adults] an d six near-term fetal sheep [3 hypertonic saline-treated fetuses (HYP-F), 3 isotonic saline-treated fetuses; 130 days gestation] were prepared with va scular and intraperitoneal catheters. Seventy-five minutes before tissue co llection, hypertonic (1.5 M) or isotonic saline was infused via an intraper itoneal catheter to adult (18 ml/kg) or fetal sheep (6 ml/kg). Brains were examined for patterns of neuronal activation (demonstrated by Fos protein e xpression). HYP-A and HYP-F demonstrated similar acute increases in plasma osmolality (similar to 10 mosmol/kgH(2)O) and comparable patterns of Fos ex pression within the organum vasculosum of the lamina terminalis (HYP-A, 67 +/- 2 vs. HYP-F, 63 +/- 6; means +/- SE) and hypothalamic supraoptic (SON; HYP-A, 107 +/- 8 vs. HYP-F, 102 +/- 7) and paraventricular nuclei (PVN; HYP -A, 71 +/- 18 vs. HYP-F, 124 +/- 19). Fewer activated neurons were detected in HYP-A vs. HYP-F within the subfornical organ (HYP-A, 33 +/- 8 vs. HYP-F , 91 +/- 17) and median preoptic nucleus (HYP-A, 33 +/- 5 vs. HYP-F, 70 +/- 6). In adults and fetuses, counterstaining for arginine vasopressin reveal ed that neurons within the SON and PVN respond to osmotic challenge. These findings demonstrate that central osmoregulatory centers in adult and near- term fetal sheep are similarly activated by osmotic challenge.