Serotonin Involvement in Physiological Function and Behavior
The primary body of data that has contributed to the view that 5-HT has a general effect on behavior by modulating the tone of nervous system activity comes from studies of the firing rate of serotonergic soma in raphe nuclei . Under quiet waking conditions, serotonergic neurons display a slow, clock-like activity of about 1 to 5 spikes/sec, which shows a gradual decline as the animal becomes drowsy and enters slow-wave sleep. A decrease in the regularity of firing accompanies this overall slowing of neuronal activity. During rapid eye movement (REM) sleep, the activity of these neurons becomes silent. In response to certain types of arousing stimuli, the firing rate of these serotonergic neurons increases. Not surprisingly, such data led to the idea that the activity of serotonergic neurons is related to the level of behavioral arousal/activity. Such data also have contributed to the idea that the activity of serotonergic neurons is associated with motor output since atonia of the major skeletal muscle groups occurs during REM sleep. Also oral—buccal motor activity, such as chewing, biting, licking or grooming, causes a marked increase in the firing rate of a subgroup of serotonergic soma that are also activated by somatosensory stimuli applied to the head, neck and face. However, exposing a cat to environmental stressors, such as a loud noise or seeing a dog, although producing strong sympathetic activation and typical behavioral responses, does not alter the firing rate of serotonergic neurons. Thus, the type of motor activity that activates serotonergic soma seems to be repetitive, like that mediated by central pattern generators. Furthermore, activation of serotonergic transmission inhibits information processing in afferent systems. From all such data, it has been suggested that the serotonergic neuronal system functions at the organismic level to integrate functions needed for behavioral output, that is, facilitation of motor output with suppression of activity in sensory systems irrelevant to the ongoing behavior.
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Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.