Chronic morphine-treated sensory ganglion neurons remain supersensitive to the excitatory effects of naloxone for months after return to normal culture medium: an in vitro model of 'protracted opioid dependence'.

Chronic morphine-treated dorsal-root ganglion (DRG) neurons in DRG/spinal cord explant cultures were previously shown to become supersensitive to the excitatory effects of remarkably low concentrations of the opioid agonists, morphine and dynorphin, and the opioid antagonist, naloxone. The present study demonstrates that this opioid excitatory supersensitivity of chronic morphine-treated DRG neurons (1 microM for > 1 week) is retained for periods > 3 months after return to control culture medium. Acute application of femtomolar dynorphin, as well as nanomolar naloxone, to the treated neurons after months in control medium evoked characteristic prolongation of the action potential duration (APD), as occurs in cells tested during or shortly after chronic opioid exposure. The threshold concentrations for eliciting these excitatory effects in naive DRG neurons are > 1000-fold higher. Furthermore, treatment of micromolar morphine-sensitized neurons with 1 nM etorphine (which is a potent excitatory opioid receptor antagonist) for I week prior to return to control medium blocked further expression of opioid excitatory supersensitivity when tested after an additional 1-7 weeks in culture. These results provide a unique in vitro model system for analyses of some of the cellular mechanisms underlying protracted opioid dependence in vivo.