Fibers showing serotonin-like immunoreactivity (5-HT-LI) are demonstrated in the gracile, cuneate and external cuneate nuclei of the cat using avidin-biotinylated horseradish peroxidase and indirect fluorescence immunohistochemical methods. 5-HT-LI fibers are located in all cytoarchitectural subdivisions of the gracile and cuneate nuclei but are restricted to the medial half of the external cuneate nucleus. In all nuclei, 5-HT-LI fibers consist of long, varicose strands showing a wide range in the diameters of both varicosities and intervaricose segments. Results support the concept that serotonergic systems may be involved in the processing of non-noxious somatosensory submodalities.
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