Animal experiment:

Mice[1]Male Kunming mice (20.0±1.0 g) are used. In order to investigate the chronic effect of Melatonin on opioid receptor agonist analgesia, 18, 17, 16 and 21 mice are treated with Melatonin (0, 0.5, 1 and 2.5 mg/kg, respectively) twice daily for 4 days. On the 5th day, 8, 8, 8 and 9 mice are administered with Endomorphin-1 18 μg/mouse (corresponding to 0, 0.5, 1 and 2.5 mg/kg Melatonin, respectively), while 10, 9, 8 and 12 mice are administered with Deltorphin I 20 μg/mouse (corresponding to 0, 0.5, 1 and 2.5 mg/kg Melatonin, respectively). Subsequently tail-flick latency is measured at 10 min interval within 60 min[1].

Deltorphin I is a δ-opioid receptor agonist with high affinity and selectivity.

Twice daily administration of Deltorphin I (20μg/mouse) for 4 days produces tolerance to Deltorphin I analgesia, as shown by the decrease in the analgesic response. The peak analgesic response to Deltorphin I (20 μg/mouse) at 10 min after injections is decreased from 8.36±0.28 s (the 1st day) to 4.53±0.14 s (the 4th day) markedly. Concurrent treatment of Melatonin (0.5, 1 and 2.5 mg/kg) and Deltorphin I (20 μg/mouse) twice daily for 4 days can attenuate the tolerance to Deltorphin I analgesia (P<0.05,<0.05 and<0.05), and this effect is dose dependent[1].

References:
[1]. Dai X, et al. Melatonin attenuates the development of antinociceptive tolerance to delta-, but not to mu-opioid receptor agonist in mice. Behav Brain Res. 2007 Aug 22;182(1):21-7.