The endogenous opioid system consists of 3 families of opioid peptides, β-endorphin, enkephalins, and dynorphins, and their receptors. Opioid peptides and their receptors have widespread, but selective, distribution in the central and peripheral nervous systems, particularly in circuits involved in pain modulation, reward, responses to stress, and autonomic control. It’s believed that one class of opioids (endorphins) is particularly involved with feelings of euphoria during sex, while the other two classes have roles in inhibition of sexual desire and dopamine release.
Animal
Activation of mu opioid receptors in the medial preoptic area following copulation in male rats
Evidence for changes in brain enkephalin contents associated to male rat sexual activity
IRS2-Akt pathway in midbrain dopamine neurons regulates behavioral and cellular responses to opiates