The risks of respiratory depression are thus diminished. Heroin mimics the action of natural chemicals, endorphins, produced by the body in response to pain. Endorphins are small-chain peptides that activate our endogenous opiate receptors. The highest density of opiate receptors is found in the limbic system. Their activation produces feelings of happiness, relaxation, fearlessness and tolerance to pain.
Endorphins are hundreds or even thousands of times more potent than morphine on a molar basis.
Their potency means their concentrations in vivo are low. Endorphins are also involved in respiration, nausea, vomiting, pain modulation, hormonal regulation and itching. Opioid drugs also act in these limbic brain regions. Yet except at very high doses, the opioids don't block the pain messages themselves. Rather, they change the subjective experience of the pain.