Compensatory synaptic growth in the rod terminals as a sequel to partial photoreceptor cell loss in the retina of chimaeric mice.

In the retina of chimaeric mice of rd and wild-type genotypic combination, selective loss of rd/rd photoreceptor cells, after initial development, leads to a mosaic retina with variable amounts of normal photoreceptor cells present over the retinal surface. In some of the rod terminals of these retinas the synaptic complexes with the second order retinal neurons are seen to contain multiple synaptic ribbons and an increased number of profiles of the postsynaptic elements. These changes are observed only in the rod terminals and not in the cone pedicles. Computer aided three-dimensional reconstruction of the altered synapses shows that these changes result from an increase in the number of synaptic sites, characterized by multiplication of the synaptic ribbons and enlargement of the second order neuronal processes. A quantitative analysis of such synapses, based on serial electron micrographs, shows that these are most frequently located in the retinal regions of the chimaeric individuals that have suffered maximum photoreceptor cell loss. Thus synaptic growth appears to take place as a reaction to the reduction of afferent input to the postsynaptic components. These findings demonstrate persistent synaptic plasticity in the rod terminals of mammalian retina during the maturational phase of late postnatal development. Compensatory synaptic growth in the rod terminals, as recorded here, can have important implications for the maintenance of visual sensitivity in the diseased or ageing retina.