The influence of dietary and genetic cues on the ability of rats to discriminate between the urinary odors of MHC-congenic mice.

This study directly compared the role of genetic and dietary factors in the production of individual urinary odors in mice by investigating the ability of male Long-Evans hooded rats to discriminate between urine samples from two strains of mice (C57BL/6-H-2Kb/J and C57BL/6-H-2Kbml/ByJ), which differ at only one locus of the major histocompatibility complex (MHC) and which were maintained on two different diets. Groups of rats were trained in an olfactometer on a go/no-go operant task with water as reinforcement to discriminate between one of the following four pairs of mouse urinary odors: individual mice differing at the MHC and maintained on the same diet (task 1), individual mice differing at the MHC and maintained on different diets (task 2), individual mice of the same MHC type maintained on the same diet (task 3), or individual mice of the same MHC type maintained on different diets (task 4). The urinary odors of mice on different diets (task 2 and 4) were more readily discriminable than those of mice maintained on the same diet (tasks 1 and 3), irrespective of genetic differences at the MHC. It was more difficult to discriminate between urinary odors of mice on the same diet whether they were genetically identical (task 4) or differed at the MHC (task 1). A second experiment revealed that it was no less difficult for rats to discriminate between the urinary odors of mice that differed at three MHC loci than it was for rats to discriminate between two mice that differed at one MHC locus. Thus, the results from Experiment 1 were not caused by the degree of genetic difference at the MHC. Overall, these results provide further evidence that dietary as well as genetic cues contribute to individual odors in rodents, and demonstrate the ability of rats to make very subtle discriminations between odors when trained in an olfactometer.