Reward System – just as the structures of the brain reward system encourage adaptive behaviors such as seeking food and sex, endogenous proteins called endorphins also motivate behaviour (1,3). The “runner’s high” is thought to be related to the production of such endogenous opiate compounds or endorphins.  In addition, place preference (animals prefer the environment where the drug is administered to other environments) is elicited when endorphins are applied to the NA (1,2).  Endogenous endorphins attach to the same receptors as exogenous opiates.  Through the same mechanism, they both increase dopamine in the brain reward pathway (1,2,3).

Reference: http://ibgwww.colorado.edu/cadd/a_drug/essays/essay4.htm