Changes in synaptic density after developmental compression or expansion of retinal input to the superior colliculus

Area of Science:

• Neuroscience
• Developmental Biology
• Synaptic Plasticity

Background:

• The superior colliculus (SC) receives retinal input, and its density can be altered by developmental manipulations.
• Variations in input convergence may be compensated by changes in synaptic features like number, arborization, or density.

Purpose of the Study:

• To investigate how neonatal partial tectal ablation or monocular enucleation affects synaptic length, density, and synapse class proportions in the hamster SC.
• To understand the compensatory mechanisms at the synaptic level in response to altered visual input.

Main Methods:

• Neonatal partial tectal ablation and monocular enucleation were performed in hamsters.
• Synaptic density, length, and class proportions were analyzed in the superficial gray layer of the SC using electron microscopy.

Main Results:

• Monocular enucleation significantly reduced synaptic density in the ipsilateral SC, specifically decreasing the number of retinocollicular (RLP) synapses.
• RLP synapses were larger in monocular enucleates, but no compensatory increase in other synapse types was observed.
• Partial tectal ablation did not affect overall synaptic density or the proportion of synapse types, though RLP synapses were slightly smaller.

Conclusions:

• Synaptic density in the SC may be regulated by the number of incoming inputs, potentially operating at a maximum capacity.
• Decreasing the number of inputs reduces density, suggesting a lack of competition between different terminal classes.
• Postsynaptic cells appear to control the types and numbers of their potential inputs, rather than input terminals competing for space.