Calretinin immunoreactivity in the anterior olfactory nucleus of the rat

Calretinin-immunoreactive structures in the anterior olfactory nucleus of the rat were studied using a polyclonal antibody, which does not cross-react with the highly homologous calcium-binding protein calbindin-D28k, and the avidin–biotin–peroxidase technique. Calretinin-immunopositive neurons were found in all regions of the anterior olfactory nucleus, with the highest number in the medial subdivision and dorsal transition area. The immunostained neurons, although morphologically heterogeneous, demonstrated typically small size. In addition to neuronal somata, calretinin-immunopositive fibres and terminals, some of them forming basket-like arrangements surrounding immunonegative neurons, were observed. Although calretinin and calbindin-D28k colocalize in several brain regions, and both proteins showed an extensive overlap in the anterior olfactory nucleus, immunostained semithin sections demonstrated that calretinin does not co-localize with calbindin-D28k in this nucleus.