Mu receptor-serotonin link in opioid induced hyperactivity in mice.

Opioid induced locomotor excitation in mice was studied. Both morphine and fentanyl increased spontaneous locomotor activity (SLA). Pentazocine produced a significant inhibition of SLA and also blocked the effects of subsequent morphine and fentanyl, thereby underscoring the importance of mu receptors for the opioid induced enhancement of SLA. Serotonergic receptor blockade with cyproheptadine or depletion with fenfluramine blocked, while uptake blockade with clomipramine potentiated the excitatory effect of fentanyl. Thus a mu receptor-serotonin link for the expression of opioid induced locomotor excitation becomes evident.