Opioid induced histamine release

First we look at what exactly is opioid mechanism: http://www.opioids.com/receptors/index.html

Well, I come across this cute question about opioid. When an opioid compound, especially a µ-receptor agonist (such as morphine), is administered in response to chronic pain, this causes the release of histamine in neurons. This leads to the activation of histamine H2 receptors, which play a role in the relief of pain.

Well, take a look at this: http://www.rch.org.au/pharmacy/drugs/index.cfm?doc_id=2113
It seems that it's really opioid fault for inducing histamine release. I find it quite strange as activating histamine can cause pain, doesn't it? How come it is the histamine, especially H2 receptor involved in relieving the pain? Article: Enhanced antinociceptive effects of morphine in histamine H2 receptor gene knockout mice

I shall leave the question mark here. One thing of interest is if in the exam, the other 4 options seem illogical, pick something which you think is logical, though you may or may not know the thing you are going to pick.