Effects of monocular deprivation on geniculocortical synapses in the cat

In monocularly deprived (MD) cats, many cells in the lateral geniculate nucleus (LGN) but few cells in the visual cortex respond to input from the deprived eye, suggesting that the connections to visual cortex from the deprived geniculate laminae may have been disrupted. I have examined these connections in MD cats by using electron microscopic autoradiography of visual cortex after injections of tritiated lysine into single laminae of LGN. After injections into either deprived or experienced laminae, there was label over terminals that contained mitochondria and round synaptic vesicles and that made asymmetric contacts with dendritic profiles. However, the terminals of deprived afferents differed from those of experienced afferents. They were 25% smaller, contained 33% fewer mitochondria, were more likely to make synapses that were presynaptically convex (and thus, perhaps, immature), and synapsed onto smaller spines. These morphological changes were greater for afferents to upper layer IV than for afferents to lower layer IV. The geniculocortical synapses from deprived laminae were also reduced in number. To correct for variations in injection size and for a probable reduction in protein synthesis by cells in the deprived laminae, I computed the ratio of labeled synaptic terminals to labeled myelinated axons. Injections into the deprived laminae labeled 43% fewer synaptic terminals per labeled myelinated axon than did injections into the experienced lamina. The finding that the synaptic terminals of deprived afferents are both abnormal morphologically and fewer in number can help to explain the reduced effectiveness of the deprived eye in driving cortical cells.