Testing the correlation between experimentally-induced hypothyroidism during pregnancy and autistic-like symptoms in the rat offspring
Thyroid hormones are important for the development of the central nervous system. Since the fetal thyroid gland is not functioning until mid-gestation, transport of maternal thyroid hormones across the placenta is essential during the early phases of gestation. Maternal thyroid deficiency has been associated with a higher incidence of neurodevelopmental disorders in the newborns. The relationship between maternal hypothyroidism and the onset of autism spectrum disorders (ASD) in the offspring, however, is still debated. To address this issue, we used a validated animal model of prenatal hypothyroidism based on the administration of the thyroid peroxidase inhibitor methimazole (MMI, 0.02 g/100 ml in tap water) to rat dams from gestational day 9 up to delivery. The offspring was tested in behavioral tasks during infancy (PNDs 5, 9, 13) and adolescence (PND 35-40) to capture some of the core and associated symptoms of ASD. MMI-exposed pups were able to vocalize as controls when separated from the nest, and showed intact social discrimination abilities in the homing behavior test. At adolescence, the offspring from both sexes did not show an anxious-phenotype in the elevated plus maze and showed intact object recognition. However, MMI-exposed male rats showed increased novelty-directed exploratory behaviors: they solicited their partner to play more and showed more interest for novel rather than familiar objects compared to control rats. Our results show that prenatal MMI-induced hypothyroidism does not cause in the rat offspring behaviors that resemble core and associated ASD symptoms, like deficits in communication and social interaction and anxiety.