Ovine fetal swallowing response to intracerebroventricular hypertonic saline

In the near-term ovine fetus, systemic hyperosmolality stimulates dipsogenic responses. Putative systemic dipsogens (hypertonicity, angiotensin II) may initiate responses by stimulation of select cerebral circumventricular nuclei lacking a blood-brain barrier. To investigate whether central osmotic-dipsogenic mechanisms are functional in utero, fetal swallowing responses to intracerebroventricular (i.c.v.) hypertonic saline were examined. Five pregnant ewes with singleton fetuses (128 +/- 1 days gestation) were prepared with fetal lateral cerebral ventricle and vascular catheters, electrocortical (ECoG) electrodes, and electromyogram wires on the fetal thyrohyoid muscle, nuchal and thoracic esophagus, and diaphragm and studied for a minimum of 5 days postoperatively. After a 2-h basal period, fetuses received an i.c.v. infusion of artificial cerebrospinal fluid followed by successive 30-min infusions of hypertonic NaCl in artificial cerebrospinal fluid (500 and 700 mosmol/kgH2O). In response to the i.c.v. hypertonic NaCl infusions, fetal swallowing significantly increased (1.4 +/- 0.4 to 3.9 +/- 1.4 and 2.9 +/- 0.5 swallows/min low-voltage ECoG, respectively). Plasma arginine vasopressin levels increased, although the change was not statistically significant (9.1 to 24.2 pg/ml; P = 0.07), and there was no change in fetal plasma osmolality, sodium concentration, or ECoG activity. Together with previous studies, these results indicate that both central and systemic osmotic dipsogenic mechanisms are functional in utero.