Research published in Current Biology suggests that one reason we don't get hooked on on our endogenous "heroin" (endorphins and enkephalins) is that these natural ligands for the opioid receptor activate what can be seen as an "on/off" switch for the system, while morphine does not.
Consequently, the scientists engineered mice in which morphine does produce this effect-- and lo and behold, the mice were able to get pain relief from morphine, but developed less tolerance and fewer symptoms of withdrawal.
