Compartmental organization of calretinin in the rat striatum.

The present study examined the compartmental distribution of a calcium-binding protein, calretinin, in the rat striatum. Calretinin-immunoreactive cells were homogeneously scattered throughout the striatum, but calretinin-immunoreactive fibers were clustered as patches in the medial, central and ventral caudoputamen and in the lateral nucleus accumbens. These patches corresponded to striosomes, identified by immunostaining for calbindin-D28 K in adjacent sections. In the medial nucleus accumbens, calretinin-immunoreactive fibers showed diffuse distribution with occasional islands of calretinin-poor zones. These islands contained tightly packed, Nissl-stained cells, which have been previously shown to correspond to mu-opiate receptor-rich patches. Calretinin-positive fibers fill striosomes/patches in the caudoputamen and in the lateral nucleus accumbens and avoid them in the medial nucleus accumbens.