Adrenergic regulation of the thyroid Axis: β- and α-Adrenergic signals as drivers of hormonal and metabolic homeostasis in lizard Podarcis siculus.

The adrenergic system is known for its ability to influence various physiological and metabolic processes; however, its involvement in thyroid function in non-mammalian vertebrates remains poorly explored. Adult male lizards were treated with β-adrenergic agonists (isoproterenol, terbutaline, and L-isopropylamino-3-(2-thiazoloxy)-2-propanol) and the α-adrenergic antagonist (phentolamine). The effects of β-adrenergic agonists and the α-adrenergic antagonist administered as single and repeated injections were evaluated on plasma TSH, thyroid hormones, hepatic 5'-T4 ORD (type II monodeiodinase) activity, hepatic thyroid hormone contents, blood glucose levels, and thyroid gland histology. β-adrenergic stimulation produced a dose-dependent decrease in plasma TSH, accompanied by significant increases in circulating T3 and T4 levels. These changes were associated with enhanced hepatic monodeiodinase activity, increased hepatic T3 content, reduced hepatic T4 levels, and histological features indicative of thyroid activation. Repeated administrations amplified these effects. In contrast, phentolamine treatment increased plasma TSH levels but reduced circulating and hepatic T3, increased hepatic T4, and tended to decrease deiodinase activity, indicating impaired peripheral thyroid hormone activation. Histological analysis showed reduced follicular synthetic activity, although colloid resorption vacuoles were still present. IPT treatment produced no significant effects. All treatments induced hyperglycaemia, with stronger responses after repeated administrations, suggesting coordinated regulation of thyroid function and energy metabolism by adrenergic pathways. The results demonstrate that β-adrenergic signaling promotes thyroid activation and peripheral hormone conversion, whereas α-adrenergic blockade disrupts thyroid hormone homeostasis. These findings highlight a key role of adrenergic mechanisms in the neuroendocrine and metabolic regulation of reptiles, supporting their importance in physiological adaptation to environmental variability.