C-Fos activation in the periaqueductal gray following acute morphine-3β-D-glucuronide or morphine administration

Morphine-3β-D-glucuronide (M3G), a primary morphine metabolite, evokes hyperalgesia in mice and rats and putatively mediates hyperalgesia associated with morphine (MOR) administration. However, M3G does not act via opioid receptors and its locus of activity in the CNS is unknown. Here we assessed the density of neurons immunoreactive for c-Fos, an immediate early gene regulated by neuronal activity, in the periaqueductal gray (PAG), a midbrain region critical to pain modulation, in male CD-1 mice after MOR and M3G exposure. Mice were injected with acute doses of MOR or M3G following a pre-injection of saline (SAL) or the opioid antagonist naltrexone (NTX), perfused 3 h later, and labeled for c-Fos using immunohistochemistry. Labeled image stacks taken from the PAG were then analyzed on a confocal microscope for the number of neurons showing c-Fos expression. Relative to controls, significant but similar increases in the mean density of PAG c-Fos immunoreactive neurons were observed in mice pre-injected with SAL then M3G or morphine. However, NTX pre-injection blocked this increase in MOR but not M3G injected mice. The data demonstrate for the first time a CNS locus for M3G activity. Consistent with previous observations, this M3G activity is not mediated by opioid receptors.