Abstract
Constitutive loss of the type 3 deiodinase (DIO3) causes abnormally increased levels of thyroid hormone action in the developing and adult brain, leading to an array of behavioral abnormalities. To determine to what extent those phenotypes derive from a lack of DIO3 in the adult brain, versus developmental consequences, we created a mouse model of conditional DIO3 inactivation. Mice carrying "floxed" Dio3 alleles and a tamoxifen-inducible cre transgene were injected with tamoxifen at two months of age. Compared to oil-injected controls, the brain tissue of these mice showed a 75-80% decrease in DIO3 activity and 85-95% Dio3 mRNA was expressed from recombinant alleles. Mice with adult DIO3 deficiency did not show significant differences in growth, serum thyroid hormone parameters or behaviors related to anxiety and depression. However, female mice exhibited elevated locomotor activity and increased marble-burying behavior. They also manifested relatively modest alterations in the expression of T3-dependent genes and genes related to hyperactivity in a sex- and region-specific manner. Upon thyroid hormone treatment, the expression response of T3-regulated genes was generally more pronounced in DIO3-deficient female mice than in female controls, while the opposite effect of altered genotype was noticed in males. The extent of the molecular and behavioral phenotypes of adult-onset DIO3 deficiency suggests that a substantial proportion of the neurological abnormalities caused by constitutive DIO3 deficiency has a developmental origin. However, our results show that DIO3 in the adult brain also influences behavior and sensitivity to thyroid hormone action in a sexually dimorphic fashion.