Thyroid hormones are essential for normal development and their absence or deficiency during fetal and neonatal periods results in physical and mental retardation (Cherella and Wassner, 2020). Congenital hypothyroidism is a disorder of thyroid hormone deficiency that begins in the fetal period and is present at birth (Brady et al., 2021). Dietary iodine insufficiency and chronic autoimmune thyroiditis are the most common causes of maternal hypothyroidism in pregnant women (Ghanbari and Ghasemi, 2017). Treatment of hyperthyroid pregnant mothers with anti-thyroid drugs, such as propylthiouracil (PTU), is also associated with an increased prevalence of congenital hypothyroidism (Andersen et al., 2013). Thyroid hormones play an important role in regulating the expression of genes involved in the maturation of the developing brain areas (Morte et al., 2010). Prenatal thyroid hormone deficiency has detrimental effects on the development of the central nervous system (CNS), including organization of neural circuits, myelination, and alterations in the neurotransmitter systems (Sui et al., 2005). Also, delay in treatment and restoring thyroid hormone concentrations to normal could be associated with persistent neurological disorders (Brady et al., 2021; Morte et al., 2010).

Glutamate is the most abundant excitatory neurotransmitter in the CNS and plays a major role in a wide variety of normal brain functions. Once released into the synaptic cleft, glutamate is removed by five different types of excitatory amino acid transporters (EAAT1–5). Their rodent homologs are termed glutamate-aspartate transporter (GLAST), glutamate transporter-1 (GLT-1), excitatory amino acid transporter 1 (EAAC1), EAAT4, and EAAT5, respectively. Astrocytes uptake glutamate by EAAT1 and EAAT2, whereas glutamate is taken up by EAAT3–5 which are expressed on the nerve terminals. Astrocytes in the CNS protect neurons against excitotoxicity by removing glutamate and regulating its extracellular level. EAAT2 is responsible for the majority of glutamate clearance from the synaptic cleft (Bak et al., 2006; Danbolt, 2001; Zare et al., 2017).

Increased risk of cognitive and mood disorders had been reported in congenital hypothyroid infants; however, regarding the effects of developmental thyroid hormone deficiency on anxiety-related behaviors, contradictory results have been reported. While some studies have reported increases in anxiety levels (Shimokawa et al., 2014; Venero et al., 2005), others have documented either no change (Galton et al., 2007) or anxiolytic effects (Navarro et al., 2015; Sala-Roca et al., 2002; Stohn et al., 2016). Moreover, several animal and human studies have shown that normal concentrations of thyroid hormones are also necessary for the proper function of the adult brain and changes in their concentrations can lead to an increased risk of neuropsychiatric disorders (Bauer et al., 2008; Liu and Brent, 2021; Vasilopoulou et al., 2016). To determine the developmental effects of thyroid hormone deficiency on anxiety levels, the present study was conducted to evaluate anxiety-related behaviors in congenital hypothyroid male rat offspring using elevated plus maze (EPM) and light/dark box tests.

A balance between excitatory and inhibitory neurotransmission in the brain is critical in modulating anxiety-related behaviors (Bergink et al., 2004); thus, any intervention in this balance could be useful in the treatment of anxiety disorders (Wierońska and Pilc, 2013). Thyroid hormones contribute to the regulation of the brain's excitatory-inhibitory balance (Navarro et al., 2015). Hippocampus, a major limbic structure of the brain, has been indicated to be involved in a wide variety of brain processes, including cognitive and mood functions. In addition, hippocampus contains high levels of glutamate transporters and is known to be sensitive to thyroid hormone deficiency (Sui et al., 2005; Venero et al., 2005). Given the role of the brain glutamatergic system in modulating anxiety behaviors (Faria et al., 2016; Rivera et al., 2020), protein expression of neuronal (EAAC1) and both astrocytic (GLAST and GLT-1) glutamate transporters in the hippocampus were also investigated.

Physical activity, a non-pharmacological approach, has received a growing interest against numerous disorders over the past years. It has been reported that physical activity has anti-inflammatory, anti-oxidative, and anti-apoptotic effects in several experimental models (Lovatel et al., 2013; Lu et al., 2017). In addition, several studies support the idea that exercise improves mood disorders and enhances cognitive functions in both humans and animals (Malian et al., 2021; Tai et al., 2020). Given the importance of physical activity in the regulation and/or modulation of emotional behaviors, we aimed to determine whether treadmill exercise could alter anxiety-related behaviors and protein expression of hippocampal glutamate transporters in congenital hypothyroid rats.