title: Fiber connections and synaptic organization of the preoptic retinopetal nucleus in the filefish (Balistidae, Teleostei).
abstract: Neuron cell bodies of the preoptic retinopetal nucleus (PRN), located in the diencephalon of the filefish , Navodon modestus, project axons to the retina. In the present study, the fiber connections and synaptic organization of the preoptic retinopetal nucleus (PRN) were investigated light- and electron-microscopically. The majority of neural cell bodies are located in the rostral half of this rostro-caudally elongated nucleus. Four types of synaptic terminals are distinguishable. The first (L) consists of large, irregularly shaped terminals that contain electron-dense mitochondria and numerous synaptic vesicles. These profiles make asymmetrical multi-synaptic contacts and gap junctions with somata and dendrites. The L terminals are also presynaptic to a second class of terminals (P), which have pleomorphic synaptic vesicles and form synapses onto dendrites. F terminals which have flat synaptic vesicles were also seen PRN. Very few S terminals were also seen in PRN. This type of terminal contains spherical synaptic vesicles of various sizes and a few pale mitochondria. S terminals form asymmetrical synapses with somata, dendrites and P terminals. Following unilateral tectal ablation, degenerating fibers from the lesion were traced into PRN bilaterally, although ipsilateral projections were far more numerous. L terminals exhibit degenerative changes after large tectal resection, whereas S terminals degenerate after contralateral eye enucleation. Therefore, a tecto-PRN-retinal circuit and a reciprocal connection between the retina and PRN have been documented. The similarity between PRN in the filefish and retinopetal nuclei in other classes of vertebrates, especially the isthmo-optic nucleus in birds, is discussed.
mesh_terms: D000818:Animals; D001717:Birds / Q000033:anatomy & histology; D001931:Brain Mapping; D004027:Diencephalon / Q000033:anatomy & histology*; D004525:Efferent Pathways / Q000033:anatomy & histology; D005399:Fishes / Q000033:anatomy & histology*; D012160:Retina / Q000033:anatomy & histology*; D013045:Species Specificity; D013477:Superior Colliculi / Q000033:anatomy & histology; D013687:Telencephalon / Q000033:anatomy & histology; D014795:Visual Pathways / Q000033:anatomy & histology*