Vertical organization of neurones accumulating ³H-GABA in visual cortex of rhesus monkey

Abstract

Electrophysiological and pharmacological studies^1–6 indicate that the specific responses of most visual cortical neurones depend on intracortical γ-aminobutyric acid (GABA)-mediated inhibitory processes. GABAergic interneurones have been visualized in all layers of the mammalian cerebral cortex by immunocytochemical methods^7,8 and by high-affinity uptake of exogenous ³H-GABA^9–11. It is recognized that GABA is synthesized and specifically accumulated by aspinous and sparsely spinous stellate cells, but there is no evidence available to indicate whether the laminar distribution of these cells and their axonal projections are related to the known role of GABAergic inhibitory processes in the generation of responses in visual cortical cells. It would therefore be of value to delineate the intracortical projection of the axons of different types of GABA-releasing neurones in regions of cortex where the receptive field properties of the neurones, and their modification by GABA antagonists, are well known. The selective high-affinity uptake of labelled GABA has been useful in delineating GABAergic systems¹²: recently, it has been shown that exo-genous ³H-GABA is specifically taken up and transported retrogradely by axons of neurones thought to be GABA-ergic^13,14. Using microinjections of ³H-GABA into different layers of the monkey visual cortex, we have examined the pattern of labelled neurones. We report here a bimodal distribution of GABA-accumulating neurones after injection into layers V and VI, with one group of neurones around the injection site and the other directly above, in layers II and III. We provide evidence that the latter neurones are non-pyramidal cells, probably labelled by retrograde axonal transport from the deep layers.