Synaptic organization of the nucleus dorsolateralis anterior thalami in the Japanese quail ( Coturnix coturnix japonica)

Abstract

The nucleus dorsolateralis anterior thalami (DLA) of birds is the homologue of the mammalian dorsal lateral geniculate nucleus 6,16. The positions of terminals from the retina and visual Wulst upon identified relay neurons in the DLA were examined in Japanese quail with both light and electron microscopic techniques. Injection of horseradish peroxidase (HRP) into the visual Wulst showed that relay neurons projecting ipsilaterally or contralaterally were located in a rostrolateral subdivision (DLAlr) and in Zones A and B of a lateral subdivision (DLL) of the DLA. Removal of the contralateral eye resulted in dense terminal degeneration in the DLAlr and moderate terminal degeneration in Zones A and B. By contrast, lesions in the visual Wulst produced dense degenerating terminals in Zones A and B of the DLL. The somata and proximal dendrites of relay neurons or terminals from the retina in the DLA were identified electron microscopically following HRP injection into the visual Wulst or optic nerve, respectively. Terminals from the retina contained spherical vesicles, glycogen granules, and mitochondria with widely spaced cristae. Terminals from the retina made synaptic contact with proximal dendrites and somata of HRP-labeled relay neurons. Presynaptic dendrites formed symmetric synaptic contact with dendrites of relay neurons. Synaptic glomeruli were observed in the DLAlr that involved dendrites of relay neurons, terminals from the retina and presynaptic dendrites. Lesions of the visual Wulst resulted in degeneration of small terminals with spherical vesicles. These terminals were not involved in the synaptic glomeruli of the DLA, but made asymmetric contacts with spine of unidentified neurons and with terminals of presynaptic dendrites.