Effects of γ-aminobutyric acid on the thymus tissue structure, antioxidant activity, cell apoptosis, and cytokine levels in chicks under heat stress

This study aims to investigate the effect of dietary γ-aminobutyric acid (GABA) on the development of thymus tissue structure and function in chicks under heat stress. One-day-old male Wenchang chicks were randomly divided into control group (CK), heat stress group (HS), and GABA+HS group. The chicks from GABA+HS group were administered 0.2 ml of GABA solution daily by oral gavage (50 mg/kg of body weight). Chicks from HS and GABA+HS groups were subjected to heat stress treatment at 40 ± 0.5°C for 2 h every day. Blood and thymus tissue were collected from the chicks at the end of weeks 1-6. Results showed that the thymus weight and index, thickness of cortex, cortex/medulla ratio, number of lymphocytes, activity of superoxide dismutase, total antioxidant capacity, and glutathione peroxidase, and plasma level of tumor necrosis factor-α in HS group were significantly lower than in CK group (P < 0.05). The Toll-like receptor 2 (TLR2) expression in the late stage of heat stress, malondialdehyde (MDA) content, thymocyte apoptosis rate, number of lymphocytes in the S and G2/M phases, and plasma levels of interleukin-4 and interferon-γ in HS group were significantly higher than in CK group (P < 0.05). In contrast, the integrity of thymus tissue structure of GABA+HS group was improved compared with HS group. The TLR2 expression in the early stage of heat stress and the activity of antioxidant enzymes in GABA+HS group were significantly higher than in HS group (P < 0.05), and the MDA content, thymocyte apoptosis rate, number of lymphocytes in the S and G2/M phases, and plasma level of IL-4 and IFN-γ in GABA+HS group were significantly lower than in HS group (P < 0.05). We concluded that heat stress caused structure damage to thymus tissue of chicks, changed the plasma levels of cytokines, reduced the antioxidant activity, and increased cell apoptosis in chick thymus. GABA alleviated the negative effects on the development of chick thymus, improved the immune function of thymus, and played a protective role by regulating the plasma levels of cytokines and antioxidant activity of thymus tissue.