Four types of neuron in layer IVab of cat cortical area 17 accumulate 3H-GABA

Roughly 10% of the neurons in layer IVab of cat area 17 accumulate exogenous 3H-gamma-aminobutyric acid (GABA) but how many types of neuron comprise this population was unknown. We characterized these neurons by partial reconstruction of their somas from serial electron microscope autoradiograms and distinguished four types. GABA 1 was large (greater than 16.5 micron) and dark with a dense distribution of synaptic terminals, substantial geniculate input to the soma, and a moderate accumulation of GABA. GABA 2 was small (less than 13 micron) and pale, also with a dense distribution of terminals but without evidence of somatic geniculate input, and a moderate accumulation of GABA. GABA 3 was radially fusiform (20 micron X 8 micron) with varicose dendrites, a sparse distribution of synaptic terminals, and a heavy accumulation of GABA. GABA 4 was medium in size (15 micron) with a moderate distribution of synaptic terminals and a heavy accumulation of GABA. Reasons are presented for believing that each of these four categories of GABA-accumulating neuron represents a fundamental cell type.