Osmotic regulation of prolactin secretion in the fetal sheep

The functions of prolactin in the fetus remain speculative. No obvious physiological role has been found for the prolactin present in the fetal or maternal plasma and amniotic fluid compartments. The aim of the present study was to investigate changes in fetal plasma prolactin following intracerebroventricular (i.c.r.) administration to the fetus of artificial cerebrospinal fluid of different tonicities. A lateral ventricle catheter was placed in 11 fetuses at 107-128 days of gestation. Either isotonic artificial cerebrospinal fluid (300 mOsm.1^-1; n = 9), 15% polyethylene glycol (340 mOsm.1^-1, n = 5), or 7% distilled water in isotonic artificial cerebrospinal fluid (270 mOsm.1^-1; n = 9) was infused i.c.v. at 35 μ1.min^-1 for 240 min. At 180 min thyrotropin releasing hormone (TRH) was administered through a fetal jugular catheter. Fetal carotid arterial blood gases, plasma osmolality and concentrations of prolactin, vasopressin (AVP), and norepinephrine (NE) were measured. Administration of hypotonic artificial cerebrospinal fluid produced an increase in fetal plasma prolactin from 46.6 ± 36 ng·ml^-1 at 0 min to 83.3 ± 49 ng.ml^-1 at 180 min (mean ± SEM; P < 0.05). No changes in either AVP or NE were observed. Administration of hypertonic artificial cerebrospinal fluid produced a decrease in plasma prolactin from 85 ± 57 at time 0 to 49 ± 35 at 180 min (P < 0.05). No changes in either AVP or NE were observed. Fetal plasma prolactin, AVP, and NE did not change during control infusion of isotonic artificial cerebrospinal fluid. The response of the fetal pituitary to TRH seemed to be independent of the effects of tonicity on basal prolactin secretion. The administration of 5 μg TRH increased fetal plasma prolactin by 313 ± 60, 398 ± 71 and 278 ± 50% and fetal plasma AVP from 1.22 ± 0.7 to 3.37 ± 2.1; 1.41 ± 0.3 to 2.84 ± 1.0 and from 1.59 ± 0.3 to 3.57 ± 0.9 pg.ml^-1 (mean ± SEM) in the Iso-artificial cerebrospinal fluid, Hypo-artificial cerebrospinal fluid and Hyper-artificial cerebrospinal fluid groups respectively when compared with the concentration at time 0 (P < 0.05). These findings support previously reported data that suggest a role of tonicity in the hypothalamic control of prolactin secretion.