Morphine-3-glucuronide's neuro-excitatory effects are mediated via indirect activation of N-methyl-D-aspartic acid receptors: mechanistic studies in embryonic cultured hippocampal neurones.

UNLABELLED: Indirect evidence indicates that morphine-3-glucuronide (M3G) may contribute significantly to the neuro-excitatory side effects (myoclonus and allodynia) of large-dose systemic morphine. To gain insight into the mechanism underlying M3G's excitatory behaviors, we used fluo-3 fluorescence digital imaging techniques to assess the acute effects of M3G (5-500 microM) on the cytosolic calcium concentration ([Ca(2+)](CYT)) in cultured embryonic hippocampal neurones. Acute (3 min) exposure of neurones to M3G evoked [Ca(2+)](CYT) transients that were typically either (a) transient oscillatory responses characterized by a rapid increase in [Ca(2+)](CYT) oscillation amplitude that was sustained for at least approximately 30 s or (b) a sustained increase in [Ca(2+)](CYT) that slowly recovered to baseline. Naloxone-pretreatment decreased the proportion of M3G-responsive neurones by 10%-25%, implicating a predominantly non-opioidergic mechanism. Although the naloxone-insensitive M3G-induced increases in [Ca(2+)](CYT) were completely blocked by N-methyl-D-aspartic acid (NMDA) antagonists and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate antagonist), CNQX did not block the large increase in [Ca(2+)](CYT) evoked by NMDA (as expected), confirming that M3G indirectly activates the NMDA receptor. Additionally, tetrodotoxin (Na(+) channel blocker), baclofen (gamma-aminobutyric acid(B) agonist), MVIIC (P/Q-type calcium channel blocker), and nifedipine (L-type calcium channel blocker) all abolished M3G-induced increases in [Ca(2+)](CYT), suggesting that M3G may produce its neuro-excitatory effects by modulating neurotransmitter release. However, additional characterization is required. IMPLICATIONS: Large systemic doses of morphine administered to some patients for cancer pain management have been reported to produce myoclonus and allodynia. Indirect evidence implicates the major morphine metabolite, morphine-3-glucuronide (M3G), in these neuro-excitatory side effects. Hence, this study was designed to gain insight into the cellular mechanism responsible for M3G's neuro-excitatory actions.