i said ACTIVITY, not AFFINITY.
that means, assuming that morphine would activate the receptor to about 100% (meant as a reference value), then o-desmethyltramadol and buprenorphine would do the same in the range of 20%. these values mean maximum stimulation, that's also the reason why buprenorphine has a ceiling effect at higher dosages, when all receptors are saturated but all do only 20% of what they actually were capable of doing.
@jamshyd: well, your definition of withdrawal might be another one than mine. but i insist on the fact, that tramadol-withdrawal does not produce any real existing symptoms, just like caffeine-withdrawal doesn't.
of course, if you stop your daily tramadol (or coffee) consumption, you are going to be in a bad mood, unmotivated for most things, quickly upset and sometimes even aggressive. but that's not what i would call a "withdrawal" in its usually accepted meaning.
for further reading regarding the fact that even the withdrawals of harder opiates are in their most extent not much more than conditioning, i.e. imagination (or in a larger sense, maybe mass psychology), i suggest this:
amitai et al.
discrete cues paired with naloxone-precipitated withdrawal from acute morphine dependence elicit conditoned withdrawal responses.
behav. pharmacol., 2000, may p. 213-222
this article basically reports about inducing an opiate "withdrawal" out of pure imagination. maybe you find more such studies, i'd be glad if you'd post them
puuuuuh, you drove me to writing a whole novel with your critizism 8)