Selective pressure on a saccharin intake phenotype and its correlates: a replication study

The Occidental High- and Low-Saccharin rats (respectively, HiS and LoS lines) were selectively bred for decades to examine mechanisms and correlates of a saccharin intake phenotype. Observed line differences ranged from taste and eating to drug self-administration and defensive behavior, paralleling human research on relationships between gustation, personality, and psychopathology. The original lines were terminated in 2019, and replicate lines (HiS-R and LoS-R) were selectively bred for 5 generations to test for reproducible, rapid selection for the phenotype and its correlates. The line differences chosen for replication included intake of tastants (saccharin, sugars, quinine-adulterated sucrose, sodium chloride, and ethanol) and foods (cheese, peas, Spam, and chocolate) and several noningestive behaviors (deprivation-induced hyperactivity, acoustic startle, and open field behavior). The HiS-R and LoS-R lines diverged on intake of saccharin, disaccharides, quinine-adulterated sucrose, sodium chloride, and complex foods, and open field behavior. Differences from the original lines also were observed. Reasons for and implications of the pattern of replication and lack thereof in 5 generations are discussed.

Eating and wheel running across the estrous cycle in rat lines selectively bred on a taste phenotype

The Occidental Low- and High-Saccharin-Consuming rats (respectively, LoS and HiS) have been selectively bred for decades to study the relationship between taste and behaviors in and beyond the ingestive domain. Whether the saccharin phenotype is associated with behavioral periodicities tied to reproductive status is not known. Here we describe for the first time variation across the estrous cycle in chow intake and wheel running by LoS and HiS rats. This study also shed light on why rats, humans, and some other mammals eat less and become more active as fertility increases. Wheel running increases when eating is reduced through restricted chow access, more so in LoS rats than in HiS rats (Dess et al., 2000). If the decrease in food intake from diestrus through estrus causes the increase in running (Eat Less → Run More hypothesis, ELRM), then the running peak should follow the eating nadir and be greater in LoS rats. Bayesian cyclic regression showed that estrous cycles were shorter in LoS rats than in HiS rats; implications are discussed. Contrary to ELRM, the running peak did not follow the eating nadir, and cycle amplitude did not distinguish LoS rats from HiS rats. These results indirectly support the No Time To Eat hypothesis (Fessler, 2003), according to which the periovulatory eating nadir and running peak reflect fitness-enhancing consequences of shifts away from eating and toward mating as fertility increases.

"Us vs. Them" Pair Housing: Effects on Body Weight, Open Field Behavior, and Gut Microbiota in Rats Selectively Bred on a Taste Phenotype

Taste is increasingly recognized as being related to reward, risk, and social processes beyond the ingestive domain. Occidental High (HiS) and Low (LoS) Saccharin Consuming rats have been selectively bred for more than 25 years to study those relationships. The present study examined LoS and HiS rats' sensitivity to a social partner's lineage. The role of gut microbiome transfer between lines was also explored as a possible mediating mechanism. Rats were pair-housed with a rat from either their own line (same-line condition) or the other line (other-line condition); weight gain, saccharin intake, acoustic startle, and open field behavior were measured. Results show for the first time that the lines express different behavioral strategies in a novel open field. In addition, weight gain and open field measures indicate that other-line housing was stressful. Saccharin intake, however, was unaffected by housing condition. A previous finding that the lines possess different gut microbiota was replicated. Although microbial transfer occurred between social partners, no clear evidence was obtained that housing-condition effects on weight gain or behavior were mediated by microbial transfer. Overall, these findings add to the characterization of non-gustatory correlates of a taste phenotype and suggest that rats differing strikingly on the taste phenotype and/or its correlates may be socially incompatible.