Immunocytochemical localization of mu-opioid receptor in the rat caudate-putamen.

A guinea pig antibody against a C-terminal peptide of rat mu-opioid receptor (MOR) was produced to examine the distribution of MOR in the rat caudate-putamen (CP). The anti-peptide antibody recognized a protein of M(r) 69,000 in Triton X-100 extract of rat brain and in the membrane fraction of MOR-expressing culture cells. Intense MOR-like immunoreactivity (LI) was observed in island-like areas of the CP. Some MOR-LI was located on the cell bodies and dendrites of CP neurons. Double immunofluorescence study revealed that the intensely MOR-immunoreactive areas showed weak calbindin-LI, surrounded by intensely calbindin-positive regions. The results indicate that MOR-LI is enriched in the 'patches' of the neostriatal mosaic compartmentation.