Synaptic reorganization in the lateral geniculate nucleus of the adult cat following chronic decortication.

The corona radiata containing all geniculo-cortical and cortico-geniculate fibers was transected in adult cats. One year after the operation synaptic architecture of laminae A and A1 of the decorticated LGN was investigated utilizing electron-microscopy and Golgi-EM. It was found that synaptic glomeruli became embedded in heavily gliotic neuropile, which contained the dendrites of surviving relay cells in addition to retinal axons and interneuronal processes. In contrast to the intact LGN, where the relay cell dendrites are exclusively postsynaptic, many morphologically identified relay neuronal dendrites of the decorticated LGN also contained large, round synaptic vesicles associated with distinct presynaptic sites. In 60% of all glomeruli, the large presynaptic dendrites of relay neurons occupied a central position and were postsynaptic to retinal axons and interneuronal dendrites/axons, but were presynaptic to other relay dendrites or interneuronal dendritic profiles. Occasional dendro-dendritic synapses between relay cell processes could be observed in the extraglomerular neuropile. Relay cell presynaptic dendrites formed unequivocal Gray I-type synapses, in accordance with the supposed excitatory character of relay cell output. Since sprouting of extrinsic or intrinsic axons could not be observed after decortication, it is proposed that the partial synaptic reorganization of the LGN was primarily due to the compensatory " axonization " of relay cell dendrites. It is further suggested that the electrophysiologically demonstrated expansion of retinal receptive fields in the chronically decorticated LGN was accomplished by the formation of dendro-dendritic synapses between relay cells and interneurons.