In light of the fact that you can soothe pain by injecting morphine (as done to wounded soldiers on the battle field), scientists assumed that there are morphine receptors in the brain. Indeed, in 1973, following significant efforts and research, the receptors for opioids were discovered not only in the brain but also in the digestive system, in the placenta and in other organs.
This discovery caused scientists to ask themselves why indeed are there in the body receptors for substances that should not naturally exist within it. The quest for answers to this question led in 1975-6 to the discovery of the Endorphins (short for Endo-Morphines - internal morphines), by Hans Kosterlitz in a laboratory in the University of Aberdeen in Scotland, and two months later, by Sol Schneider at the John Hopkins University in Baltimore. This group was also involved Prof. R. Siman-Tov from the Weizmann Institute.
The more important Endorphins that were discovered were the Beta-Endorphins - peptides (chains of amino-acids) that act as. Another kind of neurotransmitters – those belonging to the Acetylcholine group) – includes smaller – more compact molecules such as the dopamine serotonin, glutamate and GABA, which have a major role in the mechanism of drug-addiction.
Proof of the existence of natural opioids in the body can also be found in the effect of substances that block the nerve cell receptors - the antagonists. Substances of this kind are Naloxoneand Naltrexone that bind to the receptors of the opioid and block the opioid effect. These materials are used for drug-addiction rehab, to block the enjoyable sensations that the drugs cause. Experiments show that when these substances are injected to laboratory animals, they respond in a much more difficult way to stimulated pain. Since due to the fact that the endorphin can not bind to its receptors the brain does not receive the messages of pain suppression, hence the sharper sensation of pain.