The Distribution of M4 Muscarinic Acetylcholine Receptors in the Islands of Calleja and Striatum of Rats and Cynomolgus Monkeys

The distribution of m4 muscarinic acetylcholine receptors, and their relation to a number other markers, was examined using immunocytochemical techniques. Staining in the dorsal striatum tended to be more pronounced in the striosomal than the matrix compartment of both rats and cynomolgus monkeys. Within the ventral striatum, immunoreactivity was more pronounced within the olfactory tubercle and the shell region of the nucleus accumbens than in the nucleus accumbens core and was especially marked within the lateral striatal stripe. Modest staining was also seen in the external plexiform layer of the olfactory bulb. By far, the most intense staining in the forebrain of both rats and cynomolgus monkeys was found in islands of Calleja, where it appeared to be a selective marker for the core or hilus regions of the islands, or an analogous region found adjacent to them. The core regions of different islands appear to be continuous with each other so as to form a complex three-dimensional structure, which is largely encased by layers of granule cells. The neuronal elements in the islands of Calleja, which express m4 receptors, remain to be identified, but it is unlikely that cholinergic neurons are a major locus of these receptors. Although there are certain similarities between the islands of Calleja and other components of the striatal complex, the current studies emphasize the extent to which the islands are unique in terms of their architecture and chemical anatomy.