 Other areas where they have been located include the external plexiform layer of the olfactory bulb, the nucleus accumbens, in several layers of the cerebral cortex, and in some of the nuclei of the amygdala, as well as the nucleus of the solitary tract.Some MORs are also found in the intestinal tract.Activation of these receptors inhibits peristaltic action which causes constipation, a major side effect of μ agonists.