The classes of neurons accumulating exogenously injected, tritiated gamma-aminobutyric
acid [( 3H]GABA) were studied in the supragranular layers in the primary auditory
field of the adult cat. The size, laminar locus, and somatodendritic profiles of labeled
neurons were studied light microscopically in frozen- or Vibratome-sectioned, 30 micron
thick material, and in semithin, 1-2 micron thick, plastic-embedded high-resolution
autoradiographic preparations. The chief goals of the study were to determine which
types of cells could be identified as accumulating [3H]GABA in layers I, II and III,
and to establish possible relationships between these cells and neurons described
in Golgi studies of these layers, and the neurons found, in parallel investigations
of the connections of the primary auditory field, to participate as ipsilateral corticocortical
and commissural cells of origin. The principal findings are: that neurons in every
layer in the primary auditory field take up tritiated gamma-aminobutyric acid; that
their Nissl-counterstained somata have a smaller average area, and a smaller range
of areas, than do the unlabeled cells; that more than one type of labeled neuron-as
defined by somatic size and shape, height:width ratios, and nuclear membrane morphology-could
be identified in each layer; that none of the labeled neurons had a soma with a pyramidal
configuration; that the labeled cells are comparable in size, shape, and laminar distribution
to some populations of non-pyramidal ipsilateral corticocortical cells of origin in
layers II and III, and perhaps to certain classes of commissurally projecting, layer
III non-pyramidal neurons; and finally, that only a rather small proportion-perhaps
10% or less, except in layer I-of the supragranular cells appear to accumulate labeled
material. With regard to the identity of particular classes of neurons accumulating
silver grains above background in the individual layers, in layer I, 2 of the 4 types
of neurons characterized in Golgi preparations take up gamma-aminobutyric acid and
the remaining 2 types may also, and the relative number of labeled cells appears to
be higher than in the other layers; in layer II, 2 of the 9 varieties are labeled,
and 4 other types may also be; and in layer III, 2 of the 11 types take up gamma-aminobutyric
acid, and 5 other varieties may as well. Three types of non-pyramidal layer II cells
that project ipsilaterally from AI to the second auditory cortical field, AII, possibly
accumulate gamma-aminobutyric acid; 3 types of commissural non-pyramidal cells of
origin linking AI to AI appear to be labeled by gamma-aminobutyric acid.(ABSTRACT
TRUNCATED AT 400 WORDS)