Opposing effects of 5-hydroxytryptamine on two types of medullary inspiratory neurons with distinct firing patterns

1. Activity of inspiratory neurons was recorded extracellularly from the caudal portion of the nucleus ambiguous (0-3.5 mm rostral to the obex) in decerebrated, spontaneously breathing cats. Using a micropressure ejection method, we tested the responsiveness of the inspiratory neurons to direct applications of serotonin (5-HT) and noradrenaline (NA) in comparison with applications of glutamate and control artificial cerebrospinal fluid (ACSF) by means of a multibarreled micropipette. 2. We made detailed examinations of 52 inspiratory neurons that were excited by glutamate but did not react to control ACSF. Those inspiratory neurons were further classified into two subgroups based on the differences in firing patterns: inspiratory neurons with an augmenting firing pattern ["augmenting I units" (22/52)] and inspiratory neurons with a decrementing firing pattern ["decrementing I units" (30/52)]. 3. Although application of NA produced predominantly inhibitory effects on both the decrementing (22/30) and augmenting I units (20/22), application of 5-HT resulted in distinct or opposing effects on these two types of inspiratory neurons: the decrementing I units (25/30, 83%) were excited by 5-HT, whereas the augmenting I units (17/22, 77%) were inhibited by 5-HT. 4. The excitation of the decrementing I units with 5-HT was characterized by a long onset-latency of response and a prolonged recovery process. The increase in firing rate occurred not only during the inspiratory active phase but also during the expiratory phase.(ABSTRACT TRUNCATED AT 250 WORDS)