Differential mRNA Expression of Glucocorticoid Bioavailability Modulators and a Neurotrophic Factor in Three Organs Implicated in the Chicken Stress Response

The hypothalamus-pituitary-adrenal (HPA) axis remains active despite the elevated corticosterone (CORT) levels during immobilization stress (IMS). This indicates that the HPA axis activity is dependent not only on CORT concentrations but also on the availability of free active CORT that is unbound of corticosteroid-binding globulin (CBG) and is activated by the 11β-hydroxysteroid dehydrogenase (11β-HSD) enzyme. The study examined the CORT levels in the blood and the mRNA expression of both proopiomelanocortin gene in the anterior pituitary gland (APit) and brain-derived neurotrophic factor (BDNF) in the septohypothalamus during IMS. Additionally, the expression of glucocorticoid receptor (GR), 11β-HSD, and CBG were analyzed in the septohypothalamus, APit, and liver. The experiment included three male Cobb 500 chicken groups, a control group, and two treatment groups exposed to 60 or 120 minutes of IMS. Blood, brain, APit, and liver were collected at 35 days of age (N= 12 samples/group). CORT concentrations in blood were quantified using radioimmunoassay, while reverse transcription-quantitative PCR was used to measure mRNA levels of CBG, 11β-HSD1, 11β-HSD2, and BDNF in the septohypothalamus, APit, and liver. The findings suggested that the IMS activated the HPA axis, as demonstrated by increased CORT levels and changes in proopiomelanocortin expression within the APit of stressed chickens compared to unstressed ones. The septohypothalamus of stressed chickens showed an increase in the CBG, BDNF, and 11β-HSD1 mRNA levels, whereas 11β-HSD2 and GRs expression remained stable compared to the control group. Although CBG and BDNF expression decreased from peak levels, their mRNA remained significantly elevated in the 120-minute group. In liver tissue, the treatment groups showed higher levels of 11β-HSD1 and CBG expression, but 11β-HSD2 expression decreased. Overall, CORT levels and the expression of GR and CORT modulators seemed to have a significant influence on the stress response. Notably, increased mRNA levels of CBG and 11β-HSD1 could improve the availability of free active CORT. Furthermore, a positive correlation between CORT levels and BDNF expression was demonstrated, highlighting the role of BDNF in neuronal protection during IMS. Additionally, the liver may contribute to stress regulation through the functions of CBG and 11β-HSD, which are vital for CORT activation and transport.