UC San Diego

Thyrotropin-releasing hormone-degrading ectoenzyme (TRH-DE) specifically hydrolyzes released thyrotropin-releasing hormone (TRH), limiting TRH action. Past studies suggest released TRH exerts an anxiolytic- and antidepressant-like effect, Additionally, increased TRH promotes negative energy balance. TRH-DE has been shown to be highly expressed in brain regions that subserve emotional behavior. Furthermore, a past study in our lab found central trhde overexpression was associated with vulnerability to obesity. The following studies in this thesis tested the hypothesis that reducing TRH-DE function by targeted gene knockout in mice would show an antidepressant- or anxiolytic-like effect, and would be leaner and less fatty. Adult male and female mice of wild type (WT), heterozygous and trhde knock-out (KO) genotypes were analyzed for whole-body energy expenditure, food intake and body composition after chow or high fat-diet challenge. To assess anxiety-like and antidepressant-like phenotypes, adult male and female mice were tested in the elevated plus maze, light-dark box, and forced swim behavioral tests. Adult female and male mice of WT and KO genotypes were then compared for operant self-administration of food+water and a highly reinforcing sucrose(3%)-saccharin(0.125%) solution.

Energy expenditure (EE) analysis at 2 months old revealed KO mice have an altered circadian profile compared to WT, where KO have higher EE, with the greatest difference in the later half of the dark cycle. Body composition analysis after six weeks of high fat diet challenge revealed a significant Genotype (p<0.05) and Sex (p<0.05) effect where KO and HET gained less body fat than WT mice, with no difference in weight gained throughout the 6 weeks. Furthermore, the KO have a reduced high fat intake across all weeks compared to HET (p=0.038) and WT (p=0.006). Body composition analysis of adult mice weaned on chow showed that WT were heavier, fatter and had more lean mass than KO littermates (p<0.0001). This Genotype effect in body weight was more pronounced in older males than females.

The time spent in the open arm and light box for EPM and LD, respectively, did not differ by genotype, suggesting similar anxiety-like behavior. A main effect of Genotype was observed for forced swim immobility, however, whereby KO (p=0.01) and HET (p=0.02) mice were significantly less immobile than WT littermates. A main effect of Sex for climbing reflected that males climbed more than females (p<0.005).

With respect to operant self-administration behavior, Sex x Genotype interactions reflected that male, but not female, TRH-DE KO mice self-administered more food and water than WT controls following acquisition. Although there were no differences in later self-administration sessions with super-sac, males had exaggerated response after deprivation, but females had absent response. In conclusion, knocking out TRH-DE function had an anti-depressant-like effect on the FST, but did not show an anxiolytic-like effect, nor impair learning or memory in self-administration testing. Furthermore, even a partial trhde knockout resulted in less fat gain compared to WT on a high fat diet, even though there was no significant difference in amount of weight gained.