Neurotransmitter-evoked reversal of in vitro apnea. A, Control activity in 3 mm [K+] (left, top trace) spontaneously arrested after 156 min in a mPBC[500/−0.70]SD-P1.5 slice (right). Bath application of DHPG reactivated bursting activity very similar to control, which again stopped within 9 min of wash. A comparable rhythm was also elicited by TRH. Note that, in contrast to DHPG, bursting continued for almost 2 h after TRH washout. B, C, In the initial phases of bath application, DHPG (B) (10 μm; mPBC[500/−0.70]SD-P1.5) or TRH (C) (100 nm; mPBC[500/−0.70]W-P2) evoked tonic activity, as evidenced by a shift and increased thickness in the baseline of the integrated VRC signals. D, E, Bar graphs show that the efficacy of DHPG (D) and TRH (E) in reactivating inspiratory rhythm is strongly dose dependent. Note that 1 μm DHPG and 0.5 nm TRH evoke a rhythm similar to that observed in control solution. The bars represent means ± SEM; values indicate the number of preparations tested.