GABA—Peptide Neurons of the Primate Cerebral Cortex

The presence in the mammalian cerebral cortex of the classical neurotransmitter, γ-aminobutyric acid (GABA), and its synthesizing enzyme, glutamic acid decarboxylase (GAD), has been known for a relatively long time (Awapara et al. 1950; Roberts and Fenkel, 1950; Albers and Brady, 1959). In recent years, immunocytochemistry has revealed the existence of a large population of neurons immunoreactive for both GAD and GABA in the cortex of a wide variety of mammals (Ribak, 1978; Hendry and Jones, 1986; Emson and Hunt, 1981; Hendrickson et al. 1981; Peters et al. 1982; Hendry et al. 1983a; Houser et al. 1983b, 1985; Bear et al. 1985; Lin et al. 1985). Recent quantitative assessments in the monkey cortex indicate that approximately 25% of the neuronal population in any cortical area is GABA- or GAD-immunoreactive (Hendry et al. 1987). The point has also been made (Jones and Hendry, 1986; Fig. 1) that probably all the morphological varieties of intrinsic cortical neurons, except the population of small, putatively excitatory, dendritic-spine-covered neurons of layer IV, are GABA-immunoreactive. The several varieties of pyramidal neurons in the cortex are undoubtedly excitatory also and a good case can be made for their use of glutamate as a transmitter (Cotman et al. 1981; Streit, 1984; Donoghue et al. 1985).