The GABA-immunoreactive neurons in the interlaminar regions of the cat lateral geniculate nucleus: light and electron microscopic observations.

GABA-immunoreactive cells located in the interlaminar zone between the A and A1 laminae of cat LGN were studied at the LM and EM levels. The mean perikaryal size of these neurons was larger than that of GABA-immunoreactive cells in the A-laminae of LGN. Interlaminar GABA+ cells examined in plastic semithin sections of LGN after massive injections of HRP in the striate and extrastriate visual cortex were not retrogradely labeled with reaction products (as previously reported for the GABA+ cells in the laminar regions of LGN) suggesting that these cells do not project to the visual cortex. Serial EM analysis of two partially reconstructed interlaminar GABA+ cells showed that they receive synaptic inputs from RLD terminals of axon collaterals of geniculo-cortical relay cells, from cortical (RSD) terminals, from inhibitory (F) axon terminals, and from other undetermined terminals, but not from retinal (RLP) axon terminals. These data suggest that the GABAergic cells in the interlaminar zones of LGN participate as interneurons in recurrent inhibitory circuits in LGN. The synaptic inputs to these cells and ultrastructural features, notably somatic spines and dendrites oriented predominantly orthogonal to the projection lines in LGN, are similar to those of neurons of the perigeniculate nucleus.