It depends on how you classify it.
You can say that the receptors are always as responsive. Just as you'll always respond to aspirin if you have a headache. But muscle gain is more than receptor acceptance. So what causes the big gains in the beginning -- fresh receptors, or the fact that you've never been enhanced before and the body reacts so well? Or...is the reason results slow down because of receptor acceptance or genetic limitation kicking it? It's kind of a moot point because the results are the same.
Having said that, I do believe there is a certain amount of tolerance developed to any drug and the the more you do, in both dosage and duration, the greater the tolerance. In that regard, juicing ought to feel like a big boost after a long layoff.