Respiratory effects of TRH in preterm rabbits.

The respiratory activity in newborn preterm (29 days gestation) rabbits was studied after administration of thyrotropic releasing hormone. Intraperitoneal injection induced an increase in respiratory frequency (f) and a decrease in tidal volume (VT) resulting in a slight increase in pulmonary ventilation (VE). These effects were seen in parallel to a decrease in expiratory time (TE) and respiratory time (TTOT). An increase in the TI/TTOT ratio but (unaffected) VT/TI ratio indicates that thyrotropic releasing hormone affects "respiratory timing" mechanisms rather than "inspiratory drive." The changes in respiratory parameters are most probably due to an effect on the central respiratory controlling centers in the brain stem.