Numbers and proportions of GABA-immunoreactive neurons in different areas of monkey cerebral cortex

The number and proportion of neurons displaying GABA immunoreactivity were determined for 50-micron-wide columns through the thickness of 10 areas of monkey cerebral cortex, including the precentral motor area (area 4), 3 cytoarchitectonic fields of the first somatic sensory area (areas 3b, 1, and 2), 2 areas of parietal association cortex (areas 5 and 7), the first and second visual areas (areas 17 and 18), area 21 of the temporal lobe, and areas of the orbital and lateral frontal cortex. Methods of fixation and immunocytochemical processing were designed to maximize the number of stained cells in 15-micron-thick frozen sections and 1-micron-thick plastic sections. In 8 of the 10 areas the number and proportion of GABA-immunoreactive neurons per 50-micron-wide column were found to be the same (34–43 cells/column; 25% of the total neuronal population). Areas 17 and 3b differed. Area 17 contained 50% more GABA-immunoreactive neurons (52–66 cells/column) but more than twice the total number of neurons, so that the GABA cells made up less than 20% of the total. In 3 monkeys, the number and proportion of GABA- positive neurons per 50-micron-wide column in area 3b were smaller than in adjacent areas of sensorimotor cortex (26–42 cells/column; 19–22%). In 2 other monkeys, the number and proportion (34–43 cells/column; 24– 26%) were the same as in adjacent areas. Despite the similarity among most areas of monkey cortex, within some areas, the number of GABA- positive neurons per 50-micron-wide column varied as much as 30%. These variations form a significant, repeating pattern only in area 18, where narrow bands (150–200 micron wide) of relatively few stained cells alternated with either narrow or wide bands (600–700 micron wide) in which columns contained more cells. The GABA-immunoreactive neurons were unevenly distributed across layers, with every area containing large numbers and proportions of stained cells in layer II, and every area but area 4 displaying a second concentration in the principal thalamocortical recipient layers. In area 4, the number of GABA- positive neurons declined sharply from layer II to layer III and remained low through layer VI. For areas displaying the greatest intra- areal variability, only 1 or 2 layers contributed significantly to that variability (layer IV in area 3b, layers III and V in area 18, and layers II and III in area 17).(ABSTRACT TRUNCATED AT 400 WORDS)