The Relationship Between GABA Immunoreactivity and Labelling by Local Uptake of [3H]GABA in the Striate Cortex of Monkey

An antiserum to GABA was used in the macaque monkey to determine whether neurons that accumulate exogenously applied [3H]GABA in vivo are also immunoreactive for GABA. Following the injection of [3H]GABA into different laminae of striate cortex in two untreated animals and in one animal treated with amino-oxyacetic acid, selective accumulation of the labelled amino acid was demonstrated in perikarya by autoradiography. Radiographically labelled neurons (n, 519) and their unlabelled neighbours were tested in consecutive 0.5 micron thick sections by immunocytochemistry for GABA immunoreactivity. Injection of [3H]GABA did not increase the number of neurons showing GABA immunoreactivity. On the contrary many of the cells that accumulated [3H]GABA were immunonegative. These neurons were mostly located in layers IVC and VA following [3H]GABA injection into layers II-III, and in layers upper III and II following injection into layers V and VI. A comparison of the position of these neurons with known local projection patterns in the striate cortex of monkey suggests that GABA-immunonegative neurons may nevertheless become labelled by [3H]GABA if most of their local axon terminals fall within the injection site. The interlaminar projection of GABA-immunopositive neurons, which probably contain endogenous GABA, could be deduced from the position of the [3H]GABA injection site that leads to their autoradiographic labelling. Although the present study confirmed our previous results on the interlaminar connections of neurons that accumulate [3H]GABA, it demonstrated that [3H]GABA labelling alone may not be a sufficient criterion to assess the GABAergic nature of neurons in the striate cortex of monkey.