Response of serotonergic caudal raphe neurons in relation to specific motor activities in freely moving cats

Serotonergic neuronal responses during three specific motor activities were studied in nuclei raphe obscurus (NRO) and raphe pallidus (NRP) of freely moving cats by means of extracellular single-unit recordings. Responses to treadmill-induced locomotion were primarily excitatory, with 21 of 24 neurons displaying increased firing rates, directly related to treadmill speed. Individual regression analyses determined three response patterns: maximal activation at low speed (0.25 m/sec), augmentation of neuronal activity only at high treadmill speed (0.77 m/sec), and a linear increase. A smaller fraction of NRO and NRP serotonergic neurons (6 of 27) also responded to hypercarbic ventilatory challenge with increased firing rates. The magnitude of neuronal response was dependent upon the fraction of inspired CO₂ and was related to ventilatory motor output, specifically, inspiratory amplitude. A subgroup of neurons responsive to hypercarbia in wakefulness demonstrated significant reductions in neuronal response to hypercarbia in slow-wave sleep. Finally, unit activity for 12 of 29 cells increased in response to spontaneous feeding, displaying two distinct patterns of neuronal response in relation to onset and termination of feeding: rapid activation and deactivation versus a gradual increase and decrease. More than half of the cells studied under all three conditions were responsive to more than one motor task. These results indicate that serotonergic caudal raphe neurons are responsive to specific motor system challenges, with many neurons responsive to multiple motor tasks, and that the responsiveness of serotonergic neurons to at least one motor task, hypercarbic ventilatory challenge, is state dependent.