This seems to be a very special opioid agonist, because it does not bind to any of the ordinary opioid receptors, but instead the ORL-1 receptor. Read the article at https://www.thevespiary.org/rhodium/Rhodium/pdf/orl1.agonist.pdf (https://www.thevespiary.org/rhodium/Rhodium/pdf/orl1.agonist.pdf)
Compound 13 is the agonist, the others are antagonists.
This is a nice review of the different kind of opioid receptors:
http://www.tocris.com/opioidreview.htm (http://www.tocris.com/opioidreview.htm)
There are also links to cannabinoid, dopamine, GABA, ... receptors - with a lot of refs.