Setting mwheel down/up to new function

[burlo_stumproot]

I would like to change the behavior of mouse-wheel up and mouse-wheel down.
I though it would be nice to map them to gnus-article-goto-prev-page and
gnus-summary-next-page. I tried putting the following in my .gnus
but aparently it's completly ignored. 

(global-set-key [mouse-4] 'gnus-summary-next-page)
(global-set-key [mouse-5] 'gnus-article-goto-prev-page) 
 
Any hints ?