Journal of Neuroscience, Vol 2, 1499-1506, Copyright © 1982 by Society for Neuroscience
The ultrastructural localization of serotonin immunoreactivity within the nucleus of the solitary tract of the cat
B Maley and R Elde
Using a modification of the peroxidase-antiperoxidase technique, serotonin
immunoreactivity was localized at the ultrastructural level in the nucleus
of the solitary tract of the cat. Structures containing serotonin
immunoreactivity included unmyelinated axons, varicosities (0.5 to 2
micrometers in diameter), and synaptic terminals. The serotonin-containing
synaptic terminals were found less frequently than axons or varicosities.
Within unmyelinated axons and varicosities, the immunoreactivity was
associated mainly with large granular vesicles (80 to 150 nm). While large
granular vesicles were found in all immunoreactive structures, greater
numbers were observed in axons and nonsynaptic varicosities. Serial
sections of several nonsynaptic serotonin-immunoreactive varicosities
indicated the lack of synaptic specializations associated with these
structures. In a typical section, only one or two granular vesicles were in
synaptic terminals which contained numerous small clear vesicles.
Serotonin-immunoreactive terminals formed asymmetrical contacts with
dendrites and spines. No synaptic contacts involving immunoreactive
terminals were found on cell bodies or other axonal structures.
Serotonin-containing neuronal perikarya within the nucleus of the solitary
tract were never observed. The abundance of nonsynaptic varicosities
containing large granular vesicles suggests a possible neurohumoral role
for serotonin within the feline nucleus of the solitary tract. This is
discussed in relation to previous reports concerning the paucity of genuine
synaptic contacts involving serotonin in other regions of the central
nervous system. The presence of serotonin-immunoreactive terminals in the
nucleus of the solitary tract also suggests its function as a putative
neurotransmitter.