Comment by __blockcipher__

The short answer is "yes". The long answer is "that's a philosophical question".

Downregulation is a natural homeostatic mechanism that happens with almost everything. Any time a certain receptor gets stimulated above baseline, over the long-term it's going to get downregulated. The actual neurobiology of how this works is enormously complicated and beyond my (and probably almost anyone's) understanding. I do recall that NMDA receptors have a critical role to play, given that NMDA receptor antagonists can attenuate some (presumably not all) of the down or up regulation