This invention provides a method of treating pain in a mammal that includes administering to a mammal in need of such treatment a pain treating effective amount of a compound of the formula I: or a pharmaceutically acceptable salt thereof, wherein each of R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, n, and m is as defined and described herein. The present invention also provides pharmaceutical compositions for treating pain containing a pain treating effective amount of a compound of formula I.