Glutamate decarboxylase (GAD)‐immunoreactive varicosities were found around cell bodies of nonimmunoreactive and immunoreactive neurons in the cat's visual cortex; they also occurred along apical dendrites and axon initial segments of pyramidal neurons. By examination in the electron microscope of structures first identified in the light microscope, it was established that the GAD‐immunoreactive varicosities were boutons in symmetrical synaptic contact with pyramidal cells in layers II‐IV. More than 90% of 142 boutons surrounding the cell bodies of 20 pyramidal neurons were immunoreactive for GAD. Since such a high proportion of the axosomatic boutons are GAD‐immunoreactive, it is likely that the terminals of basket cells are included in this population and so the basket cell probably uses γ‐aminobutyrate as a transmitter, as suggested by previous authors. Almost all the 68 boutons in symmetrical contact with the axon initial segments of six pyramidal neurons could be shown to be GAD‐immunoreactive, which makes it very likely that the boutons of axoaxonic cells contain GAD‐immunoreactivity. This was established unequivocally for an individual Golgi‐impregnated axoaxonic cell by combining Golgi impregnation and immunocytochemistry in the same sections: A Golgi‐impregnated axoaxonic cell whose cell body was in layer II gave rise to numerous terminal segments, some of which were examined in the electron microscope after gold‐toning. These boutons were in synaptic contact with axon initial segments and not only contained the Golgi precipitate but were also immunoreactive for GAD. It is concluded that the axoaxonic cell in the visual cortex uses γ‐aminobutyrate as a transmitter. An individual axoaxonic cell in layer II/III was filled with horseradish peroxidase by intracellular iontophoresis. The very extensive local axonal field was composed of 330 terminal bouton rows in layer II/III and a sparse descending collateral projection to infragranular layers. A computer‐assisted reconstruction of the axonal field in three dimensions revealed the following: The main output of the cell is to pyramidal neurons that lie deeper than the soma; the axonal arborization occupies an area of 400 μm in the anteroposterior axis and extends 200 μm along the mediolateral axis; the terminal bouton rows in layer II/III form clusters about 50 μm wide running approximately at right angles to the border between areas 17 and 18, with an intercluster interval of about 100 μm. These findings suggest that the terminals of an individual axoaxonic cell could be contained within one ocular dominance column but that there may be inhomogeneities in the weighting of the axoaxonic input to pyramidal cells in the supragranular layers. The possible functions of GABAergic axoaxonic cells are discussed, including the ideas that they may be involved in controlling the output of pyramidal neurons in superficial layers not only to contralateral cortical areas but also to deeper layers of the ipsilateral cortex, and that they may be involved in imposing a functionally significant spatial pattern on groups of pyramidal neurons.
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