Distribution and density of GABA cells in intralaminar and adjacent nuclei of monkey thalamus.

The density of GABA-immunoreactive neurons was examined in the intralaminar and certain principal relay nuclei of the macaque monkey thalamus. Counts were made in 10-microns-thick frozen sections and in 1-micron-thick plastic sections and stereological formulae applied to obtain an accurate assessment of the volumetric density of GABA cells in the nuclei. It was found that GABA-immunoreactive cells account for at least 27% of the cells per unit area of all the thalamic nuclei investigated and up to as much as 50% by unit volume. The intralaminar nuclei show only slightly fewer GABA cells than the principal relay nuclei. Previous reports of their absence or relatively low numbers in the intralaminar nuclei probably stem from their smaller size, relatively weaker immunoreactive staining and from failure to apply stereometric formulae that reveal the density of neurons per volume of tissue. These results suggest that the cellular elements of the intralaminar nuclei are not fundamentally different from those of the principal relay nuclei of the thalamus.