The central effects of thyrotropin-releasing hormone on the breathing movements and electrocortical activity of the fetal sheep.

The fetal respiratory and electrocortical effects of thyrotropin-releasing hormone (TRH) administered into the lateral cerebral ventricles, have been investigated in chronically catheterized unanesthetized fetal sheep at 125-140 days of gestation. Stimulatory effects on fetal breathing movements were seen at doses as low as a lug bolus. TRH given as a 5-micrograms bolus followed by a 10 micrograms/h infusion for 2 h induced a rapid switch to significantly faster, deeper, and continuous fetal breathing movements, while the electrocorticogram remained episodic. Fetal breathing movements did not stop during hypoxia. TRH given as a 2-micrograms bolus followed by a 4 micrograms/h infusion or as a 5-micrograms bolus followed by a 5 micrograms/h infusion induced the same stimulation of FBMs, but breathing essentially remained episodic, state related and inhibited by hypoxia. As hypothermia presumably induces a surge in TRH secretion at birth it is possible that TRH has some role in the switch from fetal to postnatal breathing patterns.