[35S]GTPγS binding and opioid tolerance and efficacy in mouse spinal cord

► Opioid tolerance and efficacy were examined using the [35S]GTPγS assay. ► Efficacy estimates correlated with historical analgesic efficacy estimates. ► Decreased μ-opioid receptor density produced tolerance in the [35S]GTPγS assay. ► Analgesic tolerance was observed following all treatment protocols. ► These data indicate that tolerance is agonist and response dependent.