“Environment Is Not the Most Important Variable in Determining Oral Morphine Consumption in Wistar Rats”, 1996-04-01 (; backlinks; similar):
The role of differential housing on sucrose-morphine consumption in outbred Wistar rats was investigated in two studies.
The results of earlier research, indicating rats housed in a quasi-natural colony drank statistically-significantly less sucrose-morphine than rats isolated in standard laboratory cages, could not be replicated, as the consumption of sucrose-morphine by the isolated animals in the present two studies was reduced.
It is possible that during a colony conversion the supplier inadvertently introduced strain differences making the present rats more resistant to xenobiotic consumption.
Discussion documents the role of genetics in morphine consumption.