Neuronal uptake and laminar distribution of tritiated aspartate, glutamate, gamma-aminobutyrate and glycine in the prestriate cortex of squirrel monkeys: Correlation with levels of cytochrome oxidase activity and their uptake in area 17

Abstract

The neuronal uptake and laminar distribution of cortically injected tritium-labeled gamma-aminobutyrate (GABA), aspartic acid, glutamate and glycine was examined in the prestriate cortex of squirrel monkeys. The intent of this investigation was not to examine the role of these amino acids as neurotransmitters, but to correlate the distribution of tritium-labeled neurons with their levels of cytochrome oxidase activity. A comparison of the number of these labeled neurons was made between the metabolically active “puff” and the less active “nonpuff” regions. In addition, these results were contrasted with the findings in area 17. With each tritiated amino acid tested, labeled neurons that had either high or low levels of cytochrome oxidase activity were present in all laminae. However, the density of labeled neurons varied between lamina for a given amino acid as well as between different amino acids. While many neurons that were cytochrome oxidase-reactive were also tritium-labeled, cytochrome oxidase activity was not a prerequisite for the sequestering of tritium label. In fact, many of the labeled neurons exhibited relatively low levels of cytochrome oxidase activity. Similar to area 17, few aspartate- or glutamate-labeled neurons were present in laminae II–III. The number of labeled neurons for both amino acids increased in laminae IV–VI, with the greatest increase observed in laminae V–VI. Gamma-aminobutyrate-labeled neurons were more prevalent in laminae I and upper II than in the other laminae, whereas in area 17, a greater proportion of the labeled neurons were found in laminae V–VI. With the exception of the uppermost laminae, where GABA-labeled neurons were more abundant, the number of glycine-labeled neurons was significantly greater throughout most laminae than with the other amino acids examined. The density of glycine-labeled neurons in lamina IV, however, was significantly less than the number observed in lamina III even though lamina III was farther away from the injection site which was at the boundary between laminae V–VI. Glycine-labeled neurons were, on average, larger than those labeled with any other amino acid. Similar to area 17, more GABA- and glycine-labeled neurons were observed within the puff regions than in nonpuff regions. No puff/nonpuff differences were observed in the distribution of leucine-injected controls. Labeled neurons for each amino acid included stellate-, fusiform- and pyramidal-shaped cells, each of varying sizes. However, outside the intensely labeled injection sites, no GABA-labeled pyramidal cells were observed. With each amino acid tested, the cytochrome oxidase-reactive/tritium-labeled neurons were larger than their unreactive counterparts. Laminar size variations in labeled neurons occurred between laminae, as well as between the amino acids. In general, the labeled neurons found in lamina IV were smaller than in the other laminae. As in area 17, GABA- and glycine-labeled neurons had distinct uptake patterns, which in turn were different from the pattern exhibited by both aspartate and glutamate.