Ultrastructural evidence for serotonin-immunoreactive terminals contacting phrenic motoneurons in the cat.

The innervation of the phrenic motor nucleus in the cat by serotonin-containing neurons has been studied using retrograde tracing combined with immunohistochemistry at the electron microscope level. It was found that phrenic motoneuron cell bodies and dendrites are contacted by serotonin-immunoreactive synaptic terminals. This finding suggests that the activity of phrenic motoneurons is directly affected by serotonergic neurons.

Monoamine synaptic structure and localization in the central nervous system.

The monoamines dopamine, noradrenaline, adrenaline, and serotonin as well as the diamine histamine have a widespread distribution in the central nervous system within synaptic terminals and nonsynaptic varicosities. In certain regions of the central nervous system the monoamines are contained in varicosities that have no synaptic specialization associated with them, suggesting a possible neuromodulatory role for some of the monoamines. The majority of monoamine labelled structures are synaptic terminals which are characterized by the presence of small, clear vesicles (40-60 nm) and large, granular vesicles (70-120 nm) within the terminal.