Central respiratory stimulation produced by thyrotropin-releasing hormone in the cat

Thyrotropin-releasing hormone (TRH) was administered intracerebroventricularly and it's effects on respiration were evaluated in the alpha-chloralose anesthetized cat. Respiratory activity was measured using a Fleisch pneumotachograph to monitor tracheal airflow. TRH (0.28-28 nmol) caused an elevation in respiratory minute volume which was due to an increase in respiratory rate with no effect on tidal volume. The site of TRH-induced tachypnea was in the hindbrain as both injections into the cisterna magna and the fourth ventricle produced similar effects. No changes in respiratory activity were seen when TRH injection was restricted to the lateral and third ventricles (forebrain). Furthermore, systemic administration of TRH (28 nmol) produced no significant respiratory effects. The active analogue, [3-Me-His2]-TRH (2.7 nmol) produced the same respiratory effects as TRH. The inactive analogue, TRH free acid (28-280 nmol), caused no significant change in respiratory activity. The data suggest that TRH interacts with a specific receptor in the hindbrain of the cat to affect respiration.