I wrote here some time ago about human cells actually making their own morphine - real morphine, the kind that everyone thought was only produced in poppy plants. Now there's a paper in PNAS where various deuterium-labeled precursors of morphine were dosed in rats, and in each case they converted it to the next step in the known biosynthesis. The yields were small, since each compound was metabolically degraded as well, but it appears that rats are capable of all steps of a morphine synthesis from at least the isoquinoline compound tetrahydropapaveroline (THP).

And that's pretty interesting, because it's also been established that rats have small THP in their brains and other tissues - as do humans. And humans, it appears, almost always have trace amounts of morphine in the urine - which leads one to think that our bodies may well, in fact, be making it themselves.

Why that's happening is quite another question, and where the THP comes from is another one. Working under the assumption that all this machinery is not just there for the heck of it, you also wonder if this system could be the source of one or more drug targets (I spoke about that possibility here). What you probably don't want to assume is that these targets would necessarily have to do with pain. We still don't know if there's room to work in here. But it's worth thinking about, if (for no other reason) to remind ourselves that there are plenty of things going on inside the human body that we don't understand at all.