SINGLE-UNIT RESPONSES OF SEROTONERGIC DORSAL RAPHE NEURONS TO SPECIFIC MOTOR CHALLENGES IN FREELY MOVING CATS

Serotonin has been hypothesized to play an important role in the centr al control of motor function. Consistent with this hypothesis, virtual ly all serotonergic neurons within the medullary nuclei raphe obscurus and raphe pallidus in cats are activated in response to specific moto r challenges.(29) To determine whether the response profile of seroton ergic neurons in the midbrain is similar to that observed in the medul la, the single-unit activity of serotonergic dorsal raphe nucleus cell s was studied during three specific motor activities: treadmill-induce d locomotion, hypercarbia-induced ventilatory response and spontaneous feeding. In contrast to the results obtained for medullary raphe cell s, none of the serotonergic dorsal raphe cells studied (n=26) demonstr ated increased firing during treadmill-induced locomotion. A subset of serotonergic dorsal raphe cells (8/36) responded to the hypercarbic v entilatory challenge with increased firing rates that were directly re lated to the fraction of inspired carbon dioxide, and a non-overlappin g subset of cells (6/31) was activated during feeding. All feeding-on cells demonstrated a rapid activation and de-activation coincident wit h feeding onset and offset, respectively. Although the proportions of serotonergic cells activated by hypercarbia or feeding in the dorsal r aphe nucleus were similar to those found in the medullary raphe, there were several major distinctions in the response characteristics for t he two cell groups. In contrast to the medullary serotonergic neurons, only a minority of dorsal raphe nucleus serotonergic neurons responde d to a motor challenge. Overall, the above results suggest very differ ent roles for the midbrain and medullary serotonergic neurons in respo nse to motor activities. (C) 1997 IBRO.