A team of international researchers have used a form of electro stimulation called transcranial direct current stimulation (tDCS) to release endrogenous opioids - the human body's most powerful painkillers that are similar in strength to morphine.  

This noninvasive procedure is especially significant and rather scary, as this is an immediate natural high, which can be accessed using damp sponges on your scalp, attached to a 9-volt battery.