In previous studies, we showed that low (nM) concentrations of opioids prolong the action potential duration (APD) of many mouse dorsal root ganglion (DRG) neurons via G2-linked excitatory opioid receptors, whereas micromolar opioid levels shorten the APD via Gi/ G0-linked inhibitory receptors. In addition, cholera toxin-B subunit (CTX-B) selectively blockss opioid- but not forskolin-induced prolongation of the APD in DRG neurons. Since CTX-B binds with selective high affinity to GM1 ganglioside located on the cell surface, the results suggest that GM1 plays an essential role in regulating excitatory opioid receptor functions. This hypothesis was tested by treating DRG neurons in mouse DRG-cord explants with exogenous gangliosides and determining whether the efficacy of opioid agonists in prolonging the APD is enhanced. The threshold concentration of the opioids, dynorphin(1-13) and morphine required to prolong the APD in many DRG neurons was markedly decreased from nM to fM levels after bath exposure to 10 nM to 1 μM GM1 ganglioside for <5 min. In contrast, GM2 and GM3 gangliosides and asialo-GM1 ganglioside were ineffective, even when DRG neurons were exposed to high concentrations (1-10 μM) for periods > 1 h. Although GD1a, GD1b and GQ1b gangliosides appeared to be as effective as GM1 when tested at μM concentrations for 15 min, tests at lower concentrations, shorter periods, and/or at lower temperature (24° vs 34°C), showed that they were significantly less effective than GM1. Our data suggest that the enhanced excitatory opioid responsiveness of DRG neurons exposed to GD1a, GD1b or GQ1b gangliosides may have been due to their rapid metabolic conversion to GM1 by endogenous neuraminidase following initial incorporation into the plasma membrane. Specificity was also evidenced by tests showing that after pretreatment of DRG neurons with 1 μM GM1, but not with the other gangliosides, concentrations of CTX-B which regularly blocked opioid-induced APD prolongation were no longer effective.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Clinical Neurology
- Developmental Biology