Activity of serotonergic neurons in behaving animals

Brain serotonergic neurons display a distinctive slow and regular discharge pattern in behaving animals. This activity gradually declines across the a rousal-waking sleep cycle, becoming virtually silent during rapid eye movem ent sleep. The activity of these neurons, in both the pontine and medullary groups, is generally unresponsive to a variety of physiological challenges or stressors, However, these neurons are activated in association with inc reased muscle tone/tonic motor activity, especially if the motor activity i s in the repetitive or central pattern generator mode. We interpret these d ata within the following theoretical framework. The primary function of the brain serotonergic system is to facilitate motor output. Concurrently, the system coordinates automatic and neuroendocrine function with the present motor demand, and inhibits information processing in various sensory pathwa ys. Reciprocally, when the serotonin system is briefly inactivated (e.g., d uring orientation to salient stimuli), this disfacilitates motor function a nd disinhibits sensory information processing. If is within this context th at serotonin exerts its well-known effects on pain, feeding, memory, mood, etc. (C) 1999 American College of Neuropsychopharmacology. Published by Els evier Science Inc.