BACKGROUND AND PURPOSE Regulation of mu receptor dynamics such as its trafficking is a possible mechanism underlying opioid tolerance that contributes to inefficient recycling of opioid responses. We aimed to characterize the functional turnover of mu receptors in the noradrenergic nucleus locus coeruleus (LC). EXPERIMENTAL APPROACH We measured opioid effect by single-unit extracellular recordings of LC neurons from rat brain slices. Immunocytochemical techniques were used to evaluate mu receptor trafficking. KEY RESULTS After near-complete, irreversible mu receptor inactivation with beta-funaltrexamine (beta-FNA), opioid effect spontaneously recovered in a rapid and efficaciousmanner. In contrast, alpha(2)-adrenoceptor-mediated effect hardly recovered after receptor inactivation with the irreversible antagonist EEDQ. When the recovery of opioid effect was tested after various inactivating time schedules, we found that the longer the beta-FNA pre-exposure, the less efficient and slower the functional mu receptor turnover became. Interestingly, mu receptor turnover was slower when beta-FNA challenge was repeated in the same cell, indicating constitutive mu receptor recycling by trafficking from a depletable pool. Double immunocytochemistry confirmed the constitutive nature of mu receptor trafficking from a cytoplasmic compartment. The mu receptor turnover was slowed down when LC neuron calcium-or firing-dependent processes were prevented or vesicular protein trafficking was blocked by a low temperature or transport inhibitor. CONCLUSIONS AND IMPLICATIONS Constitutive trafficking of mu receptors from a depletable intracellular pool (endosome) may account for its rapid and efficient functional turnover in the LC. A finely-tuned regulation of mu receptor trafficking and endosomes could explain neuroadaptive plasticity to opioids in the LC.