Serotonergic innervation of the isthmo-optic nucleus of the pigeon centrifugal visual system. An immunocytochemical electron microscopic study

• Published in: Brain research Vol. 924; no. 1; pp. 127 - 131
• Main Authors: Rio, J.P, Repérant, J, Miceli, D, Medina, M, Kenigfest-Rio, N
• Format: Journal Article
• Language: English
• Published: London Elsevier B.V 04.01.2002; Amsterdam Elsevier; New York, NY
• Subjects: 5-HT immunocytochemistry; Animals; Aves; Biological and medical sciences; Columbidae - anatomy & histology; Columbidae - metabolism; Eye and associated structures. Visual pathways and centers. Vision; Fundamental and applied biological sciences. Psychology; Immunohistochemistry; Isthmo-optic nucleus; Microscopy, Electron; Neural Inhibition - physiology; Presynaptic Terminals - metabolism; Presynaptic Terminals - ultrastructure; Raphe Nuclei - metabolism; Raphe Nuclei - ultrastructure; Serotonin - metabolism; Synaptic Transmission - physiology; Synaptic Vesicles - metabolism; Synaptic Vesicles - ultrastructure; Tegmentum Mesencephali - metabolism; Tegmentum Mesencephali - ultrastructure; Ultrastructure; Vertebrates: nervous system and sense organs; Vision, Ocular - physiology; Visual Pathways - metabolism; Visual Pathways - ultrastructure; 5-HT immunocytochemistry; Isthmo-optic nucleus; Ultrastructure; Neurotransmitters, modulators, transporters, and receptors; Serotonin; Immunocytochemistry; Innervation; Central nervous system; Anatomy; Pigeon; Isthmic nucleus; Visual system; Vertebrata; Visual pathway; Neurotransmitter; Aves; Brain (vertebrata)
• ISSN: 0006-8993; 1872-6240
• DOI: 10.1016/S0006-8993(01)03262-0

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.