The locus coeruleus of the Japanese monkey (Macaca fuscata) does not express mu-opioid receptor-like immunoreactivity

It is well known that locus coeruleus (LC) of the rat shows intense mu-opioid receptor-like immunoreactivity (MOR-LI). In the course of our study on the distribution of MOR-LI in the brain of the Japanese monkey (Macaca fuscata), however, no MOR-LI was found in the LC although the distribution pattern of MOR-LI in other regions of the lower brainstem of the monkey was essentially the same as that observed in the rat. It was also found that immunoreactivity for Met-enkephalin, the most potent endogenous ligand for MOR, was intense in the rat LC, but very weak, if any, in the monkey LC. MOR may not be expressed in the monkey LC.