Photoperiod as a neuroendocrine regulator: A review on pineal-melatonin signaling and physiological plasticity in poultry.

Photoperiod, the cyclical exposure to light and darkness, is a master regulator of neuroendocrine function in poultry, primarily via the pineal-melatonin axis. The pineal gland translates photic cues into rhythmic melatonin secretion, which synchronizes circadian and seasonal biological processes, linking environmental signals to systemic homeostasis. This review systematically explores pineal-melatonin signaling network, highlighting its pivotal role in regulating reproduction, growth, metabolic homeostasis, immune competence, and stress resilience. Mechanistically, melatonin modulates core clock genes (BMAL1, PER, CRY), interacts with hypothalamic-pituitary-gonadal and adrenal axes, and activates antioxidant, metabolic and epigenetic pathways, thereby integrating environmental cues with physiological adaptation. Photoperiodic manipulation leverages this axis to enhance energy allocation, feed efficiency, and adaptability to environmental fluctuations. Recent advances reveal that melatonin-mediated effects extend to nutrigenomic regulation and epigenetic reprogramming, offering precision strategies for optimizing poultry productivity, welfare, and health. Moreover, controlled light regimes enhance melatonin-dependent benefits, including improved egg production, enhanced meat quality, and overall welfare promotion. By integrating molecular, cellular, and applied perspectives, this review provides a comprehensive framework for understanding how the pineal-melatonin axis translates temporal environmental information into adaptive physiological outcomes, informing sustainable light management strategies in both commercial and experimental poultry systems.