Pentobarbital Sleep Time in Mouse Lines Selected for Resistance and Susceptibility to Fescue Toxicosis

In previous work with mouse lines selected for resistance (R) and susceptibility (S) to fescue toxicosis, R mice had higher activities of Phase II liver enzymes glutathione S-transferase and uridine diphosphate glucuronosyl-transferase than S mice. Objectives of this study were: 1. to determine whether selection for toxicosis response had also caused divergence between lines in hepatic Phase I enzyme activity (as assessed by sleep time following sodium pentobarbital anesthesia), 2. to determine whether sleep time differences between lines were modulated by fescue toxins or enzyme inducers in the diet, and 3. to determine whether sleep time differences among individual mice were correlated with the impact of a toxin-containing diet on their post-weaning growth.

In experiment 1, five dietary treatments were assigned to 24 male mice in each line: rodent food control, E+ (50% endophyte-infected fescue seed, 50% control), E+P (E+ with 1000 ppm phenobarbital), E- (50% endophyte-free fescue seed, 50% control), and E-P (E- with 1000 ppm phenobarbital). After four weeks on these diets, mice were challenged with a sleep time test. All mice were then switched to a pelleted rodent food diet. Each mouse then received a second sleep time test, a random 1/4 of the population after one, two, three, and four weeks on the standard diet. Results demonstrated that, regardless of dietary treatment, R mice had a shorter sleep time than S mice, suggesting higher activity of liver Phase I microsomal enzymes. Mice that were fed phenobarbital had significantly shorter sleep time than those whose diets did not include this microsomal enzyme inducer. Time interval between the first and second sleep time did not significantly impact the second sleep time, confirming line differences in the absence of toxins and inducers and with advancing age.

In experiment 2, male and female R and S mice were fed an E- diet for 2 weeks, then an E+ diet for 2 weeks, followed by a pelleted rodent food diet for 2 weeks. Mice were then administered a sleep time test. Their growth rate response to fescue toxicosis was quantified as the proportional reduction in gain during two weeks on the E+ diet, compared to gain on E- during the previous two weeks. Sleep time was significantly influenced by line but not by sex or the line x sex interaction. As in Experiment 1, S mice slept longer than their R counterparts. The residual correlation between reduction in gain associated with the E+ diet and sleep time was only 0.04. Thus, under these experimental conditions an individual animal's Phase 1 enzyme activity did not predict how severely its growth rate would be depressed by a toxin-containing diet.

Based upon these and previous studies, divergent selection for toxicosis response in mice was successful partially by causing divergence between lines both in Phase I and Phase II liver detoxification enzyme activities. If a heritable, practical, and economical criterion could be identified to quantify such differences in livestock species, then selection for toxicosis resistance might contribute to the solution of this important problem for American agriculture.