Synaptic transmission in the degenerating lateral geniculate nucleus an ultrastructural and electrophysiological study

Abstract

The functional and anatomical changes taking place at the main (optic) geniculate synapses after bilateral eye enucleation were studied using neurophysiological and ultrastructural techniques. Cessation of synaptic transmission in the lateral geniculate nucleus (LGN) took place between 48 and 72 hours after enucleation in two cats, and between 72 and 96 hours in five cats. During the period in which synaptic transmission was still preserved, the study of the discharges to orthodromic stimulation in the optic radiation and the analysis of the recovery cycle of the main geniculate neurons both indicated that a progressive deterioration of synaptic efficiency was taking place. The ultrastructural study of the main synapses of the LGN revealed, immediately after cessation of transmission, that the synaptic terminals were in the filamentous stage of degeneration. The gradual decrease in number of synaptic vesicles in the optic terminals of the LGN correlates well with the progressive deterioration of synaptic efficiency. It is suggested that these phenomena were due to a decrease in the amount of available transmitter for release at the arrival of each presynaptic spike. Possibly, the rate of transmitter synthesis was also altered.