The inspiratorily activated preganglionic tracheobronchial vagal motoneurones (I-TVMs) in the nucleus ambiguus, which predominate in the central control of the tracheobronchial smooth muscles, receive dense inputs from terminals containing thyrotropin-releasing hormone (TRH).TRH microinjection into the nucleus ambiguus caused constriction of the tracheal smooth muscles however the neuronal and synaptic mechanisms are unknown.In this study, the I-TVMs were retrogradedly labeled from the extrathoracic trachea of newborn rats, identified in rhythmically firing brainstem slices, and the effects of TRH were examined using patch-clamp.Under voltage clamp TRH (100 nmol L-1) caused a slow excitatory inward current, enhanced the excitatory inputs and evoked a distinct oscillatory pattern of the baseline current.The slow excitatory inward current and the oscillatory pattern were unaffected by blockade of chemical synapses.Gap junction blocker carbenoxolone (100 μmol L-1) prevented the oscillatory pattern without affecting the slow excitatory inward current and the enhancement of the excitatory inputs.Tetrodotoxin (1 μmol L-1), a blocker of both the voltage-dependent sodium transient and persistent sodium current, riluzole (20 μmol L-1), a blocker of the persistent the sodium current, and cadmium chloride (200 μmol L-1), a non-selective blocker of the voltage-dependent calcium channels, each blocked a major proportion of the slow excitatory inward current and prevented the oscillatory pattern.Under current clamp TRH caused a slow depolarization and continuous or oscillatory firing.These results demonstrate that TRH excited the I-TVMs presynaptically via enhancement of the excitatory inputs, and postsynaptically via a slow excitatory inward current and a gap junction-mediated oscillatory pattern.