Our previous studies have shown that the Hering-Breuer (HB) reflex prolongation of expiratory duration exhibits a biphasic temporal adaptation pattern demonstrating habituation and desensitization in vagal and pontine pathways (Siniaia et al., J. Physiol. 523:479, 2000), which are similar to the on- and off-transients respectively of a second-order differentiator (Young, Siniaia & Poon, SFN 2001 Meeting). To elucidate the role of the pontine pathway in the adaptation of the HB reflex, we obtained extracellular recordings of pontine neuronal activity simultaneously with phrenic nerve recording during electrical vagal stimulation (1.5xT, 80 Hz, 1 min) in urethane-anesthetized, bi-vagotomized and artificially ventilated rats. Neurons with tonic and respiratory modulated activity were found within or near the parabrachialis (PB) and Kölliker-Fuse (KF) complex. Most tonic neurons in KF (n=11) were depressed during and/or shortly after vagal stimulation that elicited the characteristic biphasic adaptation of the...