I've searched the studies of endorphins and found one article from Lehigh University website, done by Sarah Willett. In article, the studies shown that how endorphins affect us relating to runner's high phenomenon and possibilities of connection between mood changes and endorphins. Endorphin is a morphine like chemical messengers(a natural hormone) in our brain which change the mood and feeling. It is interacted with opiate sensor neurons. Endorphins are released during long workout such as marathons, soccer, and other exercises. Our bodies feel tired when we started to run, but as time goes by running, body and mind are highly stimulated. Due to increase level of endorphins, we feel like we can do workout up to maximum potential. But we don't always feel good. Sometimes it changes to negative mood.
However, the association of endorphins, runner's high and our feeling is publicized only by the physical evidences and personal experiences. The feeling can vary depends on personal states and it leads to the question 'real existence of runner's high'. I love running and I think I've experienced this feeling. It was very interesting article which indicates a lot of 'good to know' information and concerns about runner's high.