Cytoarchitecture and ultrastructure of nucleus prethalamicus, with special reference to degenerating afferents from optic tectum and telencephalon, in a teleost (Holocentrus ascensionis).

Histological structure and neuronal geometry of the nucleus prethalamicus of holocentrid teleosts, which is homologous to the nucleus rotundus of reptiles and birds and to the nucleus lateralis posterior-pulvinar complex of mammals, were studied by means of the Bodian, Nissl, toluidine blue, and Golgi methods. Synaptic terminals were classified electron microscopically, and terminal types originating from the telencephalon and the optic tectum were determined by electron microscopy in degeneration experiments. The nucleus prethalamicus is composed of four layers, in the following order from medial to lateral: a small-cell layer, a plexiform layer, a large-cell layer, and a marginal layer. Six types of terminals (U, L, Sp, Sd, F, and P) were distinguished in the nucleus, and the distribution pattern for each type of terminal was determined by counting its relative number in each layer. Sp terminals make synaptic contacts with small-cell dendrites or somata in the small-cell layer, and degenerate after telencephalic ablations. Sd terminals synapse exclusively with spines of large-cell dendrites in both marginal and large-cell layers, and degenerate after tectal ablations. Because only large neurons have been labeled after HRP injections into the telencephalon (Ito et al., '80, '82; Ebbesson, '80; Murakami et al., '83), it is considered that these neurons relay visual information from the optic tectum onto the telencephalon. It is hypothesized that the small neurons in the nucleus, which receive telencephalic input, might modulate the large neurons' relay function.