Serotonergic innervation of the isthmo-optic nucleus of the pigeon centrifugal visual system. An immunocytochemical electron microscopic study.
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 microm) though a few larger myelinated axons were observed (mean diameter=0.51+/-0.07 microm). The 5-HT-ir terminals or varicosities were small (diameter=0.71+/-0.54 microm) 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.
Rio
J P
JP
INSERM U 106, Neuromorphologie: Développement, Evolution, Hôpital de la Salpêtrière, 47, Bd. de l'Hôpital, 75651 Paris Cedex, France.
Repérant
J
J
Miceli
D
D
Medina
M
M
Kenigfest-Rio
N
N
D000818:Animals; D010856:Columbidae / Q000033:anatomy & histology / Q000378:metabolism*; D007150:Immunohistochemistry; D008854:Microscopy, Electron; D009433:Neural Inhibition / Q000502:physiology; D017729:Presynaptic Terminals / Q000378:metabolism* / Q000648:ultrastructure; D011903:Raphe Nuclei / Q000378:metabolism* / Q000648:ultrastructure; D012701:Serotonin / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology; D013572:Synaptic Vesicles / Q000378:metabolism / Q000648:ultrastructure; D013681:Tegmentum Mesencephali / Q000378:metabolism* / Q000648:ultrastructure; D014785:Vision, Ocular / Q000502:physiology; D014795:Visual Pathways / Q000378:metabolism* / Q000648:ultrastructure
D012701:Serotonin