Visualization of chandelier cell axons by parvalbumin immunoreactivity in monkey cerebral cortex

Antibodies directed against the calcium-binding protein parvalbumin label a subpopulation of γ-aminobutyric acid-releasing neurons in the cerebral cortex that is thought to have particular metabolic and physiological properties. The chandelier cell is a well-characterized morphological type of γ-aminobutyric acid-releasing cortical interneuron, the axon of which possesses very distinctive terminal portions located around the initial axon segments of pyramidal cells. In the pre- and postcentral gyri of the monkey, we found that these distinctive terminal portions of chandelier cell axons were immuncytochemically stained for parvalbumin in a manner that reveals their complete structure. The chandelier cell axons were identified light-microscopically as short, vertically oriented rows of parvalbumin-positive puncta (PV-Rs). The PV-Rs varied in both length and complexity and were located beneath unstained pyramidal cells. PV-Rs were very numerous in layers II-III, where most pyramidal cells appeared to have a PV-R beneath them. Fewer PV-Rs were found in deeper layers, and in layer VI PV-Rs were rare. With EM all PV-Rs could be seen to form multiple synaptic contacts of the symmetrical type on the initial segments of pyramidal cell axons. Parvalbumin immunoreactivity can therefore be used as a reliable marker for chandelier cell axons.