The Neuropeptides, the brain's own endogenous narcotics, include Beta endorphin. There is a huge release of beta endorphins (and dopamine) at the time of orgasm. CAT scan studies show that just thinking about orgasm will increase beta endorphin levels. This is not true, to the same degree, with alcohol and drugs but seems to be a key factor in sexual compulsivity. The Enkephalins are another form of the brain's own opiates, which act on the μ receptor sites. [F5] . Another drug that has a huge affinity for the μ receptor sites is marijuana. The brain's endogenous opiates and marijuana all increase dopamine via the μ receptors. So if you're pouring tons of beta endorphin onto your μ receptor sites, your brain is pouring out tons of dopamine - and that's the payoff - the feel good factor. This is why I could take methamphetamine, lay down and take a nap - because I was just bringing my dopamine back to a normal level.
Alcohol increases dopamine by directly stimulating it in an area we're going to look at. Cocaine does it by blocking the re-uptake of dopamine. Marijuana increases dopamine directly and also via the μ receptor sites. All the narcotics increase dopamine through the μ receptor site.