the rats who were brought up in cages but moved to Rat Park before the experiment began. These animals rejected the morphine solution when it was stronger, but as it became sweeter and more dilute, they began to drink almost as much as the rats that had lived in cages throughout the experiment. They wanted the sweet water, he concluded, so long as it did not disrupt their normal social behavior. Even more significant, he writes, was that when he added naloxone, a drug which negates the effects of opioids, to the morphine-laced water, the Rat Park rats began to drink it. In another experiment, he forced rats in ordinary lab cages to consume the morphine-laced solution for 57 days without other liquid available to drink. When they moved into Rat Park, they were allowed to choose between the morphine solution and plain water. They drank the plain water. He writes that they did show some signs of dependence. There were "some minor withdrawal signs, twitching, what have you, but there were none of the mythic seizures and sweats you so often hear about ..." The authors concluded that isolated cages, as well as female sex, caused an increased consumption of morphine. The authors advised that it is important to consider the conditions of testing, as well as the sex of the animals, when exploring self-administration of morphine. Further experiments Studies that followed up on the contribution of environmental enrichment to addiction produced mixed results. A replication study found that both caged and "park" rats showed a decreased preference for morphine compared to Alexander's original study; the author suggested a genetic reason for the difference Alexander initially observed. Another study found that while social isolation can influence levels of heroin self-administration, isolation is not a necessary condition for heroin or cocaine injections to be reinforcing. Other studies have reinforced the effect of environmental enrichment on self-administration, such as one that showed it reduced re-instatement of cocaine seeking behavior in mice through cues (though not if that re-instatement was induced by cocaine itself) and another that showed it can eliminate previously established addiction-related behaviors. Furthermore, removing mice from enriched environments has been shown to increase vulnerability to cocaine addiction and exposure to complex environments during early stages of life produced dramatic changes in the reward system of the brain that resulted in reduced effects of cocaine. Broadly speaking, there is mounting