Are there some known biochemical mechanisms, that induces desire for/pleasure of physical exercise in mammalians and humans? I have heard, that horses can not live without physical exercise and they should regularly be allowed some runs, some rats have those fitness rounds and so on. Maybe there are some biochemical mechanisms that induces this desire and maybe there are some potential drugs that can induces those mechanisms?
I know the longing and pleasure for mental activity and something that is called "flow" of intellectual work. But I have hard time to warm up for my physical exercises and sometimes I have hard time to enjoy them although the mental activities (and a bit leisure) forms essentially largest part of my life and hence I should be longing for some exercises sometimes.
This my question is not about drugs and fitness, it is about biochemistry and the good answer would include pointers to some important proteins, pathways, molecular mechanisms, some modelling results and so on. I like to take things from the ground up and take control in my hands that is why I am starting from so deep levels of exploration.