Abstract
Heat stress (HS) adversely impacts broiler health, productivity, and welfare, especially in market-age chickens. This study investigated the effects of early-life thermal conditioning (TC) on the expression of genes related to stress responses, thermoregulation, and metabolism in various tissues, including the hypothalamus, anterior pituitary gland, thyroid gland, adrenal gland, adipose tissue, liver, heart, breast, and thigh with and without HS in broilers. Male Ross 708 chicks were subjected to TC at 37.8°C or non-thermal conditioning (NTC) at 33°C for 24 h on d 3 after hatch. On d 31, birds from both groups were exposed to either HS (35°C) or non-heat stress (NHS; 22°C) conditions for 8 h, with collection of tissue samples for reverse transcription-quantitative PCR (RT-qPCR) analysis beginning after 4 h of HS. TC had no effect on production parameters, including BW, FI, BWG, and F/G. Key findings indicated that early-life TC significantly altered hypothalamic and pituitary expression of genes associated with stress adaptation under HS. Early-life TC increased hypothalamic neuropeptide Y expression under heat stress, which may contribute to improved stress adaptation. Additionally, TC resulted in higher heat shock protein 90 expression in thigh muscle and reduced heat shock protein 70 expression in breast muscle, highlighting tissue-specific adaptive responses. Interestingly, heart tissue from TC birds exhibited increased melanocortin 2 receptor mRNA levels under HS, suggesting improved cardiovascular resilience. Other tissues, including the adrenal gland, thyroid, liver, and adipose tissue, showed minimal gene expression changes in response to TC, implying selective tissue conditioning. These results suggest that early-life TC primes specific tissues to enhance HS resilience by modulating stress-response pathways, which may benefit poultry production in high-temperature environments. This research underscores the potential of TC as a management strategy for improving broiler response to HS challenges and circumventing the negative repercussions of HS on broiler health and performance.