The combined effects of unilateral enucleation and rearing in a ?dim? red light on synapse-to-neuron ratios in the rat superior colliculus

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Abstract
Rats whose right eyes were enucleated on day 1 after birth and nonenucleated rats were raised in either “light” or “dark” (red light) conditions from birth until 39 days of age. This resulted in 4 groups of animals: light‐reared nonenucleated, light‐reared enucleated, dark‐reared nonenucleated, and dark‐reared enucleated. At 39 days of age, the animals were killed by perfusion with 2.5% sodium cacodylate buffered glutaraldehyde. The superior colliculi were dissected out and processed for embedding in resin. Stereological procedures at the light and electron microscopical levels were used to estimate the synapse‐to‐neuron ratios in the superficial layers of these colliculi. Light‐reared, nonenucleated rats had about 1,850 synapses‐per‐neuron in both the right and left superior colliculi. Rearing nonenucleated rats in the dark reduced this value to about 1,200. Enucleated rats reared in the light showed a differential response in the 2 colliculi. Thus, the contralateral (to the enucleated eye) colliculi showed a decrease, whereas the ipsilateral colliculi showed an increase in the synapse‐to‐neuron ratio compared with light‐reared, nonenucleated rats. When enucleated rats were reared in the red light, there was a decrease in the ratio in both colliculi, although the extent of this decrease was more marked in the contralateral than the ipsilateral colliculi. However, the decrease in the contralateral colliculi was not significantly greater than that observed in the corresponding colliculi from dark‐reared, nonenucleated rats. These results provide useful information on the combined and separate effects of unilateral enucleation at around birth and dark (red light) rearing during early life on the interneuronal connectivity of both the ipsi‐ and contralateral superior colliculi of rats. They also show the vast importance of visual stimulation for the normal development of the subcortical visual centers.