LAMINAR DISTRIBUTION AND NEURONAL TARGETS OF GABAERGIC AXON TERMINALSIN CAT PRIMARY AUDITORY-CORTEX (AI)

The form, density, and neuronal targets of presumptive axon terminals (puncta) that were immunoreactive for gamma-aminobutyric acid (GABA) o r its synthesizing enzyme, glutamic acid decarboxylase (GAD), were stu died in eat primary auditory cortex (AI) in the light microscope. High -resolution, plastic-embedded material and frozen sections were used. The chief results were: 1) There was a three-tiered numerical distribu tion of puncta, with the highest density in layer Ia, an intermediate number in layers Ib-IVb, and the lowest concentration in layers V and VI, respectively. 2) Each layer had a particular arrangement: layer I puncta were fine and granular (less than 1 mu m in diameter), endings in layers II-IV were coarser and more globular (larger than 1 mu m), a nd layer V and VI puncta were mixed in size and predominantly small. 3 ) The form and density of puncta in every layer were distinctive. 4) I mmunonegative neurons received, in general, many more axosomatic punct a than immunopositive cells, with the exception of the large multipola r (presumptive basket) cells, which invariably had many puncta in laye rs II-VI. 5) The number of puncta on the perikarya of GABAergic neuron s was sometimes related to the number of puncta in the layer, and in o ther instances it was independent of the layer. Thus, while layer V ha d a proportion of GABAergic neurons similar to layer IV, it had only a fraction of the number of puncta; perhaps the intrinsic projections o f supragranular GABAergic cells are directed toward layer IV, as those of infragranular GABAergic neurons may be. Since puncta are believed to be the light microscopic correlate of synaptic terminals, they can suggest how inhibitory circuits are organized. Even within an area, th e laminar puncta patterns may reflect different inhibitory arrangement s. Thus, in layer I the fine, granular endings could contact preferent ially the distal dendrites of pyramidal cells in deeper layers. The re moteness of such terminals from the spike initiation zone contrasts wi th the many puncta on all pyramidal cell perikarya and the large globu lar endings on basket cell somata. Basket cells might receive feed-for ward disinhibition, pyramidal cells feed-forward inhibition, and GABAe rgic non-basket cells would be the target of only sparse inhibitory ax osomatic input. Such arrangements imply that the actions of GABA on AI neurons are neither singular nor simple and that the architectonic lo cus, laminar position, and morphological identity of a particular neur on must be integrated for a more refined view of its role in cortical circuitry. (C) 1994 Wiley-Liss, Inc.