Serotonin neurons and their physiological roles.

Anatomical and functional aspects of serotonin (5HT) neurons in the central nervous system are described with special reference to their neurohumoral function. The central 5HT neurons show morphological features characteristics of the modulating nervous system: a small number of cell bodies and extremely broad innervation. These may play some part in EEG arousal through the divergent projection. Being most active during the waking state, the 5HT neurons are responsible for the onset of sleep. 5HT released during waking may initiate the humoral, as well as neuronal mechanisms leading to sleep. Although the 5HT dependent hypogenic structure is highly localized in the anterior hypothalamus, the effect seems to be due to neurohumoral action of 5HT. The neurohumoral function of the 5HT neuron is suggested by the ultrastructure of 5HT terminals almost devoid of a synaptic structure (less than 10%). Intraventricular 5HT fibers also represent a substrate supporting the neurohumoral function. A strong activity of monoamine oxidase (MAO) is found in intercellular spaces in the arcuate nucleus region of the hypothalamus, where a high 5HT immunoreactivity is seen in the young rat before MAO activity appears. It is likely that 5HT liberated in the cerebrospinal fluid is drained through these intercellular channels.