Page 11 - 75_03

VOL. 75 (3), 389-418, 2009 CENTRAL AND PERIPHERAL ENDOGENOUS MORPHINE

conditions, and in tissue cultures similarly depleted of any trace of
morphine, showed that de novo endogenous morphine synthesis is a
real phenomenon.

b) Synthesis of endogenous morphine

Definitive proof of the existence of endogenous morphine
required demonstration of the de novo synthesis of the compound in
mammalian cells. To do this, various intermediates in the synthesis
of morphine were sought (Figure 2). Morphine and codeine were
the first to be characterised (by mass spectrometry) in bovine
hypothalamus, and then in rat brain extracts (59). Moreover, the
injection of salutaridine, thebaine and codeine (three known
intermediates in the plant synthetic pathway) led to rises in
morphine levels in the rat brain (60). The conversion of reticulin to
salutaridine by a cytochrome P-450 (an essential step in formation
of the morphine skeleton) was also detected in rat liver (61) and
then in microsome extracts from porcine liver tissue. It is interesting
to note that higher levels of morphine and codeine can be detected
in the urine of patients with Parkinson’s disease on L-DOPA (62);
in the brain, L-DOPA is decarboxlated to generate dopamine, an
intermediate in the morphine synthetic pathway (Figure 2).
Moreover, it was shown in vivo that mice whose tyrosine hydroxylase
gene had been knocked out (tyrosine hydroxylase being necessary
for dopamine formation) did not produce any endogenous morphine
(63): morphine synthesis appears to depend on the presence of
dopamine.

Incontrovertible proof that mammals synthesise morphine was
provided by Zenk et al. in 2004 and 2005: when the cell line
SH-SY5Y (derived from a human neuroblastoma) was cultured
in the presence of 18O2, the group were able to isolate a series of
radiolabelled intermediates in the morphine synthetic pathway, as
well as radiolabelled morphine [Figure 2 (64-66)]. Experiments with
different radiolabelled precursors made it possible to define the
entire pathway.

Most steps in the mammalian morphine synthetic pathway are
the same as those in plants. The pathway begins with the

397