A quantitative study of visual cortex synapses during the postnatal development of dark-reared rats

The method of Aghajanian and Bloom (1967) was applied to the visual cortex of normal and neonatally visually deprived rats. The rats were kept with their mothers in total darkness since birth, in a ventilated and temperature-controlled animal quarter. Controls were rats from the same stock, maintained in a regular 12-h-light/12-h-dark rhythm. The animals were killed at 15, 23, 40, and 65 days of age, and the visual cortices fixed in 4% glutaraldehyde and processed for electron micrography with ethanol-phosphotungstic acid. A total of 6249 synaptic profiles were counted and their numerical density (DS) determined in both conditions. Features of the presynaptic grid were used for classifying the synaptic profiles in: type A [with one presynaptic dense projection (PsDP)]; type B (with two or three PsDPS); and type C (with four or more PsDPS). In the visually deprived rats the DS increases with a rate similar to controls, but the values for each age are slightly lower (P less than 0.01). Type B synapses predominate in the visually deprived group while types A and C are scarcer. The differences found between types were maximal at 65 days of age and the results were highly significant (P less than 0.001). It is concluded that major effects of dark-rearing are manifested when the individual features of the presynaptic grid are considered. It seems that a selected population of synapses is affected by the alteration of the normal epigenetic influence of early visual experience.