A Closer Look at the Rat Park Experiment, Part 2
A Closer Look at the Rat Park Experiment, Part 2
By Kenneth Anderson, MA
Part 1
reviewed some of the historical background which led up to the rat park studies. Part 2 reviews the rat park studies themselves. Part 3 will take a look at where we have gone since.
Bruce K. Alexander’s first rat park study was published in 1978. The subjects were 32 albino Wistar rats (18 males and 14 females). After weaning, 10 of the rats (six males and four females) were placed in solitary confinement in standard laboratory cages. Twenty-two of the rats (12 males and 10 females) were placed in rat park. Rat park was an open-topped plywood box with 95 square feet of floor space covered in sawdust where rats could play, fight, and have sex with each other just like they did in their natural environment. Rat park contained several open topped cages which the rats could explore and a 2.3 foot high climbing pole. Each of the standard laboratory cages contained two drinking bottles: one containing tap water and the other containing a solution of 0.5 mg of morphine per milliliter of water. Rat park also contained two drinking dispensers: one containing tap water and the other containing a morphine solution. Rats in rat park each had an identifying mark and fluid consumption was tracked using a video camera.
Rat Park Experiment – The Initial Phases
The experiment began when the rats were a little over 100 days old. The experiment consisted of four phases. The first phase was called the limited access period and lasted for 22 days. Rats were given access to fluids seven hours each day. The first day, the rats had a choice between water or morphine solution. The next two days, the rats only had access to the morphine solution. This cycle was repeated for 22 days. The rats in rat park and the rats in the isolation cages reacted the same way on choice days: both groups stuck to water and drank very little of the morphine solution.
The second phase of the first rat park experiment was called the forced consumption period and lasted for 57 days. Rats were given a 24-hour choice between water and the morphine solution on days 16, 38, 42, and 43. On the other days they had access to the morphine solution all day long but no access to plain water. Two females in the social group and two females in the isolation group died during this period. On days 42 and 43, which were choice days, the rats in rat park consumed significantly less morphine than the rats in the isolation cages, basically just sticking to plain water. The rat park males consumed only 0.26 mg/kg (milligrams of morphine per kilogram of body weight) of morphine, whereas the isolated males consumed 4.09 mg/kg of morphine. Rat park females consumed 1.00 mg/kg of morphine, whereas isolated females consumed 6.96 mg/kg.
The third phase of the first rat park experiment was called the Nichols cycle period and lasted 28 days. The Nichols cycle was a technique for producing morphine addiction in pre-addicted rats; it had been published by John R. Nichols et al. in 1956. The Nichols cycle consists of multiple periods of three days each: on the first day of the cycle, the rats are given no fluids at all. On the second day, the rats are given only the morphine solution. On the third day, the rats are given only water. This teaches the rats that the bitter tasting morphine solution can relieve withdrawal symptoms. In the Nichols cycle period of the rat park experiment, the rats went through eight cycles of the Nichols cycle (24 days) plus four test days when the rats were given a choice between water and the morphine solution, making a total of 28 days. The test days took place after the second, fourth, sixth, and eighth cycles. On the four test days, when the rats could choose between morphine solution or water, the rat park males consumed an average of 0.8 mg/kg of morphine per day, whereas the isolated males consumed an average of 6.4 mg/kg. The rat park females consumed an average of 2.3 mg/kg of morphine per day, whereas the isolated females consumed an average of 19.7 mg/kg. These differences were statistically significant.
The fourth phase of the first rat park experiment was called the abstinence period; the morphine was removed, and the rats were given unlimited access to food and water. After two weeks of abstinence, there was a test day where the rats could choose between drinking water or drinking the morphine solution. There was another test day after five weeks of abstinence; one of the rat park females died prior to this test day. The rat park males consumed an average of 0.07 mg/kg of morphine per day on these two test days, whereas the isolated males consumed an average of 1.5 mg/kg of morphine. The rat park females consumed an average of 1.5 mg/kg of morphine per day on these two test days, whereas the isolated females consumed an average of 6.2 mg/kg of morphine. These differences were statistically significant.
Results of Rat Park Experiment, No. 2
The results of the second rat park experiment were published in 1979. The subjects were 36 albino Wistar rats (18 male and 18 female). After weaning, 18 rats (nine male and nine female) were placed in solitary confinement in individual cages, where they remained throughout the duration of the experiment. Eighteen rats (nine male and nine female) were placed in rat park, where they remained throughout the duration of the experiment. The experiment began when the rats were a little less than 90 days old and lasted for 38 days. The experiment was divided into eight phases; the first phase lasted three days and the subsequent seven phases lasted five days each. During each phase, the rats were given a choice between drinking plain water or a solution; the composition of the solution was different in each phase. Phase one was a pre-test phase and the solution consisted of a 5% solution of sugar water; there was no morphine in this solution. Phase two was 0.5 mg of morphine per milliliter of plain water. Phase three was 0.5 mg of morphine per milliliter of 1% sugar water. Phase four was 0.5 mg of morphine per milliliter of 5% sugar water. Phase five was 0.5 mg of morphine per milliliter of 10% sugar water. Phase six was 0.3 mg of morphine per milliliter of 10% sugar water. Phase seven was 0.15 mg of morphine per milliliter of 10% sugar water. Phase eight was a post-test phase using a 10% solution of sugar water with no morphine. The sugar was added to make the morphine solution more palatable to the rats. A statistically significant effect of housing (solitary confinement vs. rat park) was found at phase two, phase five, phase six, and phase seven. At phase seven, rat park males consumed an average of 1.46 mg/kg of morphine per day, whereas isolated males consumed an average of 24.09 mg/kg of morphine per day: a sixteen-fold difference. Rat park females consumed an average of 7.29 mg/kg of morphine per day, whereas isolated females consumed an average of 36.64 mg/kg of morphine per day: a five-fold difference. However, as we shall see below, the second rat park study was not replicable.
Results of Experiment, No. 3
The results of the third rat park study were published in 1981. This study attempted to identify whether the housing effect (rat park vs. solitary confinement) impacted adult rats or if it impacted rats during their formative years. This was based on the idea that adverse childhood events affect humans negatively when they are adults.
The subjects of rat park experiment three were 32 albino Wistar rats (16 male and 16 female). After weaning (about 21 days of age), 16 rats (eight male and eight female) were placed in solitary confinement in standard laboratory cages, and 16 (eight male and eight female) were placed in rat park. When the rats were 65 days old, half the rats from rat park were moved to individual cages and half the rats from the individual cages were moved into rat park. This gave us four different groups of eight rats each: those which had spent their entire lives in rat park (four male and four female), those which had spent their entire lives in solitary confinement in cages (four male and four female), those which had spent their youth in rat park but were put in solitary confinement in cages when they reached adulthood, (four male and four female), and those which had spent their youth in solitary confinement in cages but were put into rat park when they reached adulthood.
The experiment began when the rats were 80 days old and lasted for 31 days, divided into seven phases. As in the second rat park experiment, rats in the third rat park experiment were given a choice between drinking plain water or various mixtures of sugar water and morphine. Phase one was a pre-test phase, phases two through six were experimental phases, and phase seven was a post-test phase.
Rats living in rat park drank significantly less of the morphine-sugar solution during phases four, five, and six, regardless of whether their youth in rat park or in solitary confinement. The effect of early life experience only showed a significant effect in phase six. In phase six, rats which had grown up in solitary confinement but spent their adulthood in rat park consumed significantly more of the morphine solution than rats which had spent their entire lives in rat park. However, seven days’ worth of data were lost from this experiment due to malfunctioning equipment.
The rat park experiment was first presented to the general public in Stanton Peele’s 1985 book The Meaning of Addiction. It later reached even more of the general public via Gabor Mate’s 2008 book In the Realm of Hungry Ghosts and Johann Hari’s 2015 book Chasing the Scream.
NEXT: Part 3, a look at where we have gone since rat park.