Endogenous codeine and morphine are stored in specific brain neurons.

Codeine and morphine have been detected in mammalian brain by radioimmunoassay (RIA), and in brain and other tissues by gas-chromatography/mass-spectrometry (GCMS) in different laboratories. It has been also shown that rat liver can synthesize the skeleton of the morphine molecule, thus suggesting that this alkaloid, which is the prototype of mu-receptor agonists, plays a physiological role in brain. We report the presence of morphine-like immunoreactive compounds inside the cell body, fibers and terminals of neurons in different brain areas. Moreover, neurons localized in the same brain areas were capable of accumulating and storing [3H]morphine slowly infused intracerebroventricularly (i.c.v.) through an osmotic minipump.