The study cited focusses on the receptor density in the spinal cord only ie says nothing about differential receptor density between the sexes in the brain.Estrogens have been demonstrated to act as a neurotransmitter (or a facilitator of neurotransmission) and upon heterodimerization of opioid receptors of different classes (mu, delta, kappa) including in the limbic system which will produce inter-sex variations, certainly at a histochemical level.
The opioid receptors are complicated and pretty fascinating, and the concept of heterodimerization renders the classic view of straightforward receptor-agonist interaction as rather limited. Heres a good paper which considers this aspect of the opioid receptors:
http://bja.oxfordjournals.org/content/107/1/8.fullI am paranoid of my brain. It thinks all the time, even when I'm asleep. My thoughts assail me. Murderous lechers they are. Thought is the assassin of thought. Like a man stabbing himself with one hand while the other hand tries to stop the blade. Like an explosion that destroys the detonator. I am paranoid of my brain. It makes me unsettled and ill at ease. Makes me chase my tail, freezes my eyes and shuts me down. Watches me. Eats my head. It destroys me.