What i thought happened was that your brain alters the amount of opiate receptors available for the drug to bind to whilst addicted, and then after being clean for some time it goes back to normal (although not 100%). Then after that if you take one dosage of an opiate down the road, your brain picks up on it and reorganizes the receptor site back to how it was when you were addicted in full swing, so it primes you for that second dose which, if taken, leads to withdrawal effects. Is that not true?
It is not understood what exactly makes people get dependent/withdrawals far more easily when they have been dependent on opioids before. But it IS known that it happens. Once someone has been dependent on opioids their tolerance will go up
far more rapidly than tolerance rises in someone who has never been dependent on opioids. In a matter of days a substantial portion of their old tolerance will return. Withdrawal symptoms upon stopping will happen with much shorter length of use than with someone who has never been dependent on opioids (exact length of time is highly variable depending on the person, their history with opioids, what opioid it is and what dose, their environment, mindset, etc, but it's definitely not unheard of for very short periods of use to trigger withdrawal symptoms).
It is theorized to likely be a combination of things like conditioning - which plays a big role in withdrawals and is not something you can choose to control -, some kind of metabolic setpoint and some sort of "physical memory" which triggers receptor changes.
With withdrawal symptoms after just one single day of use I would hazard a
guess that conditioning could be the primary mechanism, and other mental stuff may be involved, like guilt, or a nocebo effect (attributing any unpleasant/sick feelings to the drug use, even if in reality they could be unrelated).
We are not rats and I don't think that's evidence to disprove that once you have been dependent on opioids you can get dependent again with much shorter periods of use. The only thing that research showed that I assume you are connecting with this thread, unless I missed something(?), was that rats who were made physically dependent on morphine for the first time (given morphine for just under 2 months) who had never quit before and were moved into the luxurious "rat park" upon quitting did not show signs of mental addiction and did show dependence and withdrawal symptoms but had milder withdrawal symptoms than "one hears about". It's simply showing the connection between environment and addiction. I would feel better than usual during withdrawal if I moved to a tropical island, that doesn't mean that physical dependence (or symptoms mimicking it) isn't more likely once you've been dependent before. Moving someone to an unfamiliar environment is also known to decrease their tolerance to opioids and is responsible for a fair number of overdoses. Opioid tolerance, dependence and withdrawal and complex and poorly understood. I don't see how Rat Park proves that receptor changes are not involved in the rapid tolerance and dependence people experience when they have been dependent on opioids in the past?