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1 Perinatal Research Laboratory, Department of Obstetrics and Gynecology, Harbor-University of California Los Angeles Medical Center, Torrance, California 90502; and 2 Laboratory for Pregnancy and Newborn Research, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853
We hypothesized that neural structures, involved
in sensing extracellular body fluid composition in adult animals during
an osmotic challenge, would show similar patterns of activation in fetal sheep. Eight adult sheep [4 hypertonic saline-treated
adults (HYP-A), 4 isotonic saline-treated adults] and six
near-term fetal sheep [3 hypertonic saline-treated fetuses
(HYP-F), 3 isotonic saline-treated fetuses; 130 days gestation]
were prepared with vascular and intraperitoneal catheters. Seventy-five
minutes before tissue collection, hypertonic (1.5 M) or isotonic saline
was infused via an intraperitoneal catheter to adult (18 ml/kg) or
fetal sheep (6 ml/kg). Brains were examined for patterns of neuronal
activation (demonstrated by Fos protein expression). HYP-A and HYP-F
demonstrated similar acute increases in plasma osmolality (~10
mosmol/kgH2O) and comparable
patterns of Fos expression 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 revealed 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.
immunocytochemistry; circumventricular organs; thirst; swallowing; arginine vasopressin secretion
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