Pyramidal neurons are immunopositive for peptides, but not GABA, in the temporal cortex of the macaque monkey (Macaca fascicularis)

Abstract

Areas 20, 21 and 22 of the temporal neocortex of the macaque monkey (Macaca fascicularis) were studied with immunocytochemical and electron-microscopic techniques to localise neurons immunoreactive to the neuropeptides vasoactive intestinal polypeptide, substance P and somatostatin, and to γ-aminobutyric acid (GABA). GABAergic neurons were found in all cortical layers, but especially in layers II, IV and VI. They were all of non-pyramidal morphology, comprising small round cells, and bipolar or multipolar forms. Presumed GABAergic axon terminals were also common. Peptidergic neurons were also found in all layers, but they consisted of cells of many morphological types, including pyramidal cells. Compared with previous descriptions in other cortical areas and in other animals, we find a greater proportion of peptidergic temporal cortical neurons compared to the GABAergic population. The immunopositive neurons were easily recognisable ultrastructurally from non-reactive neurons by the dense labelling of the cytoplasm and nucleus. Immunopositive and negative neuronal somata were often contiguous, providing evidence for the specificity of the immune reaction. Stem dendrites were often labelled for a short distance from the soma, and other strongly reacting dendritic segments were found in the neuropil, as were labelled axons. Neurons labelled for GABA had features typical of non-pyramidal cells, but neuropeptides were also found in cells with pyramidal characteristics.