Colocalization of parvalbumin, calretinin and calbindin D-28k in human cortical and subcortical visual structures

Several studies have demonstrated that three calcium-binding proteins parvalbumin (PV), calbindin D-28k (CB) and calretinin (CR) mark distinct subsets of cortical interneurons. This study demonstrates, in cortical and subcortical visual structures, the coexistence of two calcium-binding proteins in some neuronal subpopulations. The human visual cortex (VC), lateral geniculate nucleus (LGN), lateral inferior pulvinar (LIP) and superior colliculus (SC) were examined by a double-labelling immunocytochemical technique. The VC showed mostly separate populations of PV, CB and CR immunoreactive (-ir) interneurons, but also small populations of double-stained PV+CR and CR+CB neurons, while PV+CB neurons were less frequent. An average of 2.5% of the immunoreactive neurons were double-stained for PV+CR and 7.1% for CR+CB in area 17, while this percentage was slightly higher in association area 18 (3.3 and 7.4%, respectively). In the LGN and LIP, double-stained neurons were scarce, but in the fibre capsule of these nuclei, as well as in the optic radiation (OR) and white matter underlying area 17, both double-stained PV+CR or CR+CB and separate populations of PV-ir, CB-ir and CR-ir neurons and fibres were observed. Unlike the thalamic regions, the SC showed some double-stained PV+CR and CR+CB neurons, scattered both in the superficial and deep layers. These findings are discussed in the light of similar observations recently reported from other regions of the human brain.