After a month, the structure of the brains of these rats ADAPTS to increased dopamine levels, showing fewer of a certain type of dopamine receptor than they used to have and more opioid receptors.

While receptor affinity determines the amount of opioid needed to occupy a given percentage of receptors, it is the intrinsic activity of the opioid that determines its analgesic efficacy.

Conclusion: Melatonin can change the responses of pain neurons to pain stimulation via opioid receptor in the LHb, which might be one of analgesic mechanisms by MEL.