Synapses between interneurons in the lateral geniculate nucleus of monkeys.

Electron microscopic examination of the monkey lateral geniculate nucleus, pars dorsalis (LGNd) reveals the occurrence of synaptic contacts between profiles belonging to interneurons (I-cells). Almost all combinations are observed, namely, axodendritic, dendrodendritic, dendrosomatic and somatodendritic, the most frequent being the dendrodentritic synapses between the presynaptic dendrites characteristic of I-cells. Quantitative analysis of 5 samples, each consisting of 200 mum2 of net LGNd neuropil, shows that presynaptic membrane specializations present in I-cell axonal and dendritic elements amount to a mean of 3.73 mum or 8.16% of the surface of such profiles. Only 61% of this extent is in contact with principal cells (P-cells), and an unexpectedly high 39% engages other I-cell profiles. A tentative segregation of axonal and dendritic endings revealed the following significant mean differences: dendritic terminals are more numerous and larger; axonal profiles have more of their surface occupied by synaptic sites and each contact is longer. Findings suggest the existence of a network of interconnected interneurons which are presumably inhibitory in nature. Such an arrangement can provice a certain measure of anisotropic disinhibition which may be responsible for specific transformations occurring in the LGNd depending upon the size and velocity of the stimulus as well as of the degree of synchronicity of temporal patterns.