Cholera toxin-A subunit blocks opioid excitatory effects on sensory neuron action potentials indicating mediation by Gs-linked opioid receptors.
Our previous studies indicated that opioid-induced prolongation of the Ca2+ component of the action potential duration (APD) in dorsal root ganglion (DRG) neurons is mediated by excitatory opioid receptors that are coupled to cyclic AMP-dependent voltage-sensitive ionic conductances. In the present study, DRG neurons were treated with cholera toxin (CTX), or with the A subunit of CTX, in order to determine if these excitatory opioid receptors are positively coupled via the GTP-binding protein Gs to the adenylate cyclase/cyclic AMP system. In contrast, inhibitory opioid receptors have been shown to be linked to pertussis toxin-sensitive Gi/Go regulatory proteins that mediate APD shortening responses. After pretreatment of DRG-spinal cord explants with remarkably low concentrations of CTX-A (1 pg/ml-1 ng/ml; greater than 15 min) or whole toxin (1 pg/ml-1 microgram/ml) the APD prolongation elicited in DRG neurons by 1-10 nM delta/mu (DADLE) or kappa (U-50,488H) opioids was blocked (29 out of 30 cells), whereas APD shortening by microM opioid concentrations was unaffected. Opioid-induced APD prolongation was blocked even when the initial treatment with CTX or CTX-A alone did not prolong the APD. The blocking effects of CTX and CTX-A were reversed in tests made 2 h after return to control medium. The mechanisms underlying the unusually potent blocking effects of CTX and CTX-A on opioid excitatory modulation of the APD of DRG neurons require correlative biochemical analyses.(ABSTRACT TRUNCATED AT 250 WORDS)
Shen
K F
KF
Dept. of Neuroscience, Albert Einstein College of Medicine, Yeshiva University, Bronx, NY 10461.
Crain
S M
SM
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology*; D016308:Enkephalin, Leucine-2-Alanine / Q000494:pharmacology*; D005333:Fetus; D019204:GTP-Binding Proteins / Q000502:physiology*; D005727:Ganglia, Spinal / Q000502:physiology; D046911:Macromolecular Substances; D051379:Mice; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology*; D009924:Organ Culture Techniques; D011957:Receptors, Opioid / Q000187:drug effects / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*
D046911:Macromolecular Substances; D011957:Receptors, Opioid; D016308:Enkephalin, Leucine-2-Alanine; D002772:Cholera Toxin; D019204:GTP-Binding Proteins