Plasticity in synaptic appositions of optic nerve afferents under different lighting conditions.

The influence of constant light and darkness (14 days) on the ultrastructure of synaptic appositions of optic nerve afferents has been studied in the suprachiasmatic nucleus (SCN) of hooded rats. Light exposure causes a general loss of postsynaptic density (PD) material, whereas after constant darkness the postsynaptic densities are thicker and the number of subjunctional bodies is higher. Accordingly, there are changes in the relative number of 'asymmetrical' (excitatory?) and 'symmetrical' (inhibitory?) synaptic appositions in optic and non-optic synapses. This phenomenon seems to reflect an adaptive response of the postsynaptic neurons to a long-term change of input activity.