The interneuronal nature of GABAergic neurons in the lateral geniculate nucleus of the rhesus monkey: a combined HRP and GABA-immunocytochemical study.

Neurons immunoreactive to a GABA antibody, located in the retrogradely labeled segment of LGN of the rhesus monkey obtained after massive injections of HRP in a sector of the striate cortex, were examined for HRP label in plastic semithin (1 micron) sections of LGN. In a total of 691 GABA(+) neurons sampled from the 6 main geniculate layers none was seen to contain HRP grains. In contrast, the vast majority of GABA(-) neurons were densely labeled with HRP reaction product. It is concluded from these results that GABAergic cells in macaque LGN do not project to the striate cortex, as previous studies have shown for equivalent cells in cat LGN, and therefore reasons are given to consider these cells as interneurons in this nucleus. In addition, several GABA(-) neurons which were also unlabeled with HRP were observed isolated in the midst of populations of neurons homogeneously labeled with this enzyme. These cells were mostly located at or near the interlaminar zones and some of them were substantially larger than the neighboring geniculostriate relay cells in the parvocellular subdivision of LGN. For these characteristics, these unlabeled somata are considered to correspond to the geniculate neurons projecting to prestriate cortical visual areas in the macaque described in several studies and which, the present results suggest, do not branch collateral axons to the striate cortex.