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.

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Experience over the past 40 years in the field of neuroscience has taught us that analyzing neuronal activity (extracellularly recorded action potentials or single-unit activity) represents one of the most important methods for elucidating the functional or behavioral significance of brain systems in experimental animals. Research in this area was inspired by the pioneering work of two laboratories. In the late 1950s and 1960s, a series of elegant single-unit studies by Hubel and Wiesel delineated the operating characteristics and function of visual system neurons in cats and monkeys. Also in the 1960s, Evarts demonstrated the value of adapting this technique to behaving animals in order to study the function of various structures within the motor system of monkeys. Our work in this area was made feasible by an extraordinary series of studies by Aghajanian in the late 1960s and early 1970s. He and his colleagues demonstrated that one could identify and reliably record the single-unit activity of brain serotonergic (5-HT), noradrenergic, and dopaminergic neurons in rats.Single-unit studies of the brain 5-HT system in behaving animals, including our own, have been conducted almost exclusively in the domestic cat. In this article we review the past 20 years of our research in this area, including our most recent studies of the activity of medullary 5-HT neurons in relation to autonomic regulation. Finally, we propose a theoretical framework for interpreting these data, and indicate how our results are compatible with the rest of this field. BASIC ANATOMYThe 5-HT system is primitive in a number of different ways (Jacobs and Azmitia 1992). First, the basic plan for

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