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Although morphine is still obtained from natural sources, there are now many chemical substances available with similar analgesic, sedative and mood-elevating effects.
Most are synthetic although some are derived from morphine (codeine is methylmorphine and diamorphine is diacetylmorphine).
Opioids work by binding to receptors found in the brain, spinal cord and other nervous tissue which are normally activated by endogenous enkephalins and endorphins.
There is considerable debate over the nature of these, but there is general acceptance of the presence of μ, κ and δ receptors (pronounced mu, kappa and delta), and there may be subtypes of each category .
The naturally occurring substances tend to be called .