I already tried this:
Changing trackpad scroll direction with AppleScript in Yosemite
But it didn't work.
What I need is when I use the internal macBook Pro trackpad, natural scroll direction is enabled, and when I use an external mouse, it is disabled.
If this is impossible, how about the following alternative: when I plug in a mouse switch to "non-natural" scroll direction, and when I disconnect the mouse switch back to natural.