Synapses of double bouquet cells in monkey cerebral cortex visualized by calbindin immunoreactivity

In the monkey neocortex, immunoreactivity for the 28-kDa vitamin D-dependent calcium binding protein (Calbindin) is contained in a set of GABAergic intrinsic neurons whose small size and laminar locations render them very distinct from a second set of GABAergic intrinsic neurons that show immunoreactivity for another calcium-binding protein, parvalbumin. A conspicuous feature of many calbindin-immunoreactive cells is their possession of long, vertically oriented bundles of immunoreactive processes that descend or ascend vertically through several cortical layers. These are components of the radial fasciculi of the cortex and are here shown by correlative electron microscopic immunocytochemistry to consist of both immunoreactive dendrites and unmyelinated axons. The morphology of the bundles and the parent cells indicates that the cells are classical double bouquet cells. The calbindin-positive axons in the radial fasciculi in the present study formed symmetric synapses on unlabeled dendritic shafts (62%) and spines (38%). Despite the close-packed nature of the immunoreactive axons, relatively few terminals of the same axon converged on a single postsynaptic profile. The postsynaptic profiles were identified in certain cases as side branches of pyramidal cell apical and basal dendrites. Mainstem apical dendrites generally did not receive synapses derived from the calbindin-positive axons. These results indicate that double bouquet cells can be distinguished both by their GABAergic character and by their possession of calbindin immunoreactivity. They are probably major contributors to the vertical flow of inhibitory influences across laminae of the cerebral cortex.