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

Our recent studies have focused on the functional roles
of endogenous morphine in the central nervous system (78).
In experiments on the human neurone line SH-SY5Y [which is
used to study neuronal secretion (81)], immunohistochemical
analysis showed morphine/M6G/M3G (Figure 4) colocalizing with
chromogranin A (CGA, a granular marker) in vesicle-like organelles
with a dense core observed in certain neurons.

Figure 4. Confocal laser micrograph showing SH-SY5Y cells immunostained for
morphine. Colocalisation highlighted in yelow.

Morphine and M6G were purified from SH-SY5Y cells and
analysed by mass spectrometry. The identification of M6G pointed to
the presence of UGT2B7 which is the main enzyme known to be able
to convert morphine into this metabolite, and experiments based on
RT-PCR and Western blotting showed, for the first time, that UGT2B7
was being expressed in these cells. Our experiments showed that
morphine secretion by SH-SY5Y cells following nicotine-induced
depolarisation was Ca2+-dependent. Patch-clamping experiments
showed that morphine and M6G could both induce naloxone-
dependent electrophysiological responses at low concentrations (10-10-
10-9M).

In order to extend the observations made in the in vitro model,
we looked for endogenous morphine in various parts of the brain of
mice [which are reported not to produce M6G (82)] using microscopy

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