Cholera toxin-B subunit blocks excitatory effects of opioids on sensory neuron action potentials indicating that GM1 ganglioside may regulate G s-linked opioid receptor functions

Abstract

In a previous study, we demonstrated that cholera toxin-A subunit, as well as the whole toxin, selectively blocks opioid-induced prolongation of the Ca 2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons, indicating mediation of this excitatory effect by G s-linked opioid receptors. The present study shows that pretreatment of DRG neurons with the B subunit of cholera toxin (1–10 ng/ml; 15min) can also block μ/gd and κ opioid-induced APD prolongation, but not shortening. Since the B subunit binds selectively to GM1 ganglioside located on the cell surface, these results suggest that this ganglioside may regulate G s-linked excitatory opioid receptor functions in DRG neurons. Possible contamination of purified B subunit preparations of cholera toxin with traces of the more potent A subunit was eliminated by heating the stock solution to 56 °C for 20 min. Exposure of DRG neurons to an affinity-purified anti-GM1 antiserum also blocked opioid-induced APD prolongation, providing further evidence that GM1 ganglioside may play an essential role in excitatory opioid modulation of the action potential of these cells. The blockade by cholera toxin-B subunit and anti-GM1 antibodies of opioid-induced APD prolongation is best accounted for by the following hypothesis: CTX-B interferes with an endogenous GM1 ganglioside component of the excitatory, but not inhibitory, opioid receptor complex on DRG neurons that may allosterically regulate coupling of the receptors via G s to adenylate cyclase/cyclic adenosine monophosphate-dependent ionic conductances.