A study of tachykinin-immunoreactive neurons in monkey cerebral cortex

Immunocytochemical methods were used to localize tachykinin-like immunoreactivity within neurons of the monkey cerebral cortex. Three primary antibodies were used: polyclonal antisera raised against fragments of substance P and substance K that excluded the carboxyl termini of these peptides, and a monoclonal antibody that recognized the carboxyl terminus of the tachykinin family. Each antibody stained 2 populations of cortical nonpyramidal neurons: (1) A small number of large, intensely stained cells that give rise to long, coarsely beaded processes; (2) a relatively large number of small, lightly stained cells that are embedded in dense plexuses of stained punctate profiles. The large, dark cells are present in a superficial band that includes layers II and III, and in a deep band that includes layer VI and the subjacent white matter. The smaller, pale cells are present in the middle layers of cortex (layers IV and/or V). Colocalization studies indicate that virtually all the small tachykinin-immunoreactive neurons also display GABA immunoreactivity. The larger cells are not GABA-positive, but display both somatostatin-like and neuropeptide Y-like immunoreactivity. The immunocytochemically stained beaded processes and punctate profiles from plexuses that vary in density and laminar distribution among different areas of monkey cortex. The coarsely beaded processes form a basic quadrilaminar pattern, with relatively dense plexuses in layers I and VI and in 2 middle layers, usually III and V. However, this pattern varies considerably from area to area. Electron microscopically, the large cells contain a rich collection of cytoplasmic organelles, particularly Golgi complex, while the small cells contain relatively few organelles. Both types of cells, including large neurons in the white matter, receive symmetric and asymmetric synaptic contacts on their somata and proximal dendrites. The numbers of these axosomatic contacts are low. Virtually all synaptic contacts formed by immunoreactive terminals possess symmetric membrane thickenings. In 2 areas examined in detail (areas 2 and 4), pyramidal cell somata and dendrites are the major targets of the immunoreactive synaptic terminals.