Abstract
The ultrastructural features of serotonergic fibers, terminals and synaptic contacts were studied with the pre-embedding immunocytochemical method in the isthmo-optic nucleus of the pigeon centrifugal visual system. The 5-HT immunoreactive (-ir) profiles were diffusely distributed and their density was low. The labeled axons were thin and unmyelinated (mean diameter=0.21±0.03 μm) though a few larger myelinated axons were observed (mean diameter=0.51±0.07 μm). The 5-HT-ir terminals or varicosities were small (diameter=0.71±0.54 μm) and contained small agranular synaptic vesicles (diameter=28.5±6.9 nm) and large granular vesicles (diameter=102.2±19.5 nm). The latter only constituted approximately 1% of the total profiles containing synaptic vesicles in the isthmo-optic nucleus. In single thin sections, only 5% of the 5-HT-ir varicosities exhibited an active asymmetrical zone synapsing upon dendritic profiles of centrifugal visual neurons. Calculations indicated that 17% of these 5-HT-ir varicosities were actually engaged in junctional synaptic relationships, whereas the remaining (83%) were nonjunctional. The data suggest that, within the isthmo-optic nucleus, 5-HT acts both at synaptic junctions (wiring transmission) and at a distance via the extracellular space (volume transmission). These 5-HT afferents could thus modulate the activity of the retinopetal neurons and visual information processing.
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