I don't know if men's nipples are anywhere on the same par of sensitivity as those of women's, but although there is obviously no way of reaching them with my tongue I occasionally press my fingertip gently against them, whilst moving around, repeatedly in small circles. By doing this, after a while the nipples tend to swell & harden slightly, in the same sort of way as I have seen happen with female partners I've been with when they're aroused, although not quite to the same level of extremes. Once in this aroused condition I find they can get extremely sensitive - tender, yet somewhat pleasureable at the same time - much like the glans of the penis.
