There are almost certainly some issues with the Rat Park experiment, but even exercise alone has been shown to reduce binding availability at the mu opioid receptor in humans, probably as a result of beta-endorphin being released: http://www.ncbi.nlm.nih.gov/pubmed/18296435. It wouldn't be a huge leap to hypothesize that this could decrease exogenous opioid consumption.
Interestingly enough, there's also a genetic mutation that's at least loosely associated with alcoholism, a possible increased response to naturally-produced beta-endorphin, and a need for more opioids to achieve equivalent pain relief:
Interestingly enough, there's also a genetic mutation that's at least loosely associated with alcoholism, a possible increased response to naturally-produced beta-endorphin, and a need for more opioids to achieve equivalent pain relief:
http://www.pnas.org/content/95/16/9608.full.pdf
http://www.snpedia.com/index.php/Rs1799971