How can I adjust brightness when shooting high FPS video?
Two ways

1) Larger aperture (the lower the number the larger the aperture) letting in more light, but also narrowing your depth of field). 

2)
I would suggest a higher ISO, don't understand why that does not work for you. The current crop of camera's now are absolutely amazing at capturing low light environments that you cannot see with the naked eye.

One of the best cameras for this is the Sony a7s, just released (s for sensitivity).

It needs to be seen to be believed.

                
            

Sony a7s high ISO video link