# Research Progress and Effects of Light on Poultry Circadian Rhythm Regulation Based on CiteSpace
## 4 Light Affects the Circadian Rhythm in Poultry by Regulating Melatonin Levels
### Regulation of Light on Circadian Rhythm in Poultry  
The retina and pineal gland are sensitive to light [75; 79]. The circadian pacemaker of poultry consists of the retina, pineal gland, and SCN, which cooperate with each other to form a complete input-pacemaker-output system [4; 80]. SCN acts as the central clock for daily rhythm that controls the circadian rhythm at the same step as the 24 h light-dark cycle. Exposure to constant light or even dim light at night leads to the desynchronization of SCN neurons, as well as the desynchronization of peripheral clocks in body tissues [81; 82; 83]. Light stimulates intrinsic photosensitive retinal ganglion cells (IpRGC) in the retina, and IpRGC releases melanin proteins, which depolarize glutamate, neuroactive peptide, and pituitary adenylate cyclase-activating peptide into retinal hypothalamic tract (RHT) neurons connected to the retinal sensing core of the SCN; then, RHT transmits signals to the SCN. SCN is hyperpolarized by K\({}^{+}\) at night and depolarized by Na\({}^{+}\) during the day, synchronized with day and night [4]. SCN induces circadian rhythm and sleep by driving the nocturnal synthesis and feedback of pineal melatonin [84]. During the day, SCN regulates sympathetic neural activity through the release of norepinephrine, thus inhibiting the production of melatonin in the pineal gland and retina [85]. Simultaneously, SCN neurons pass through a direct route (using the autonomic nervous system) and an indirect route (transmitting rhythmic information) to other brain regions and peripheral organs [86] to achieve the hormonal signals controlled by the SCN. Melatonin produced in the pineal gland can send signals back to SCN and other parts of the brain and peripheral tissues to help maintain the circadian rhythm [87]. The expression of the core clock genes has a circadian rhythm, and positive clock genes (cClock, cBmal1, and cBmal2) begin to increase during the day and decline after they peak in the morning or midway through the night [88]. By contrast, negative clock genes (cCry1, cCry2, cPer2, and cPer3) increase during the night and peak either early or midway through the day. Interestingly, melatonin levels are also low during the day, increase in the evening and peak midway through the night, and then decrease gradually [89]. Monochromatic light (Figure 6) also affects the core clock genes (cClock, cBmal1, cBmal2, cCry1, cCry2, cPer2, and cPer3) [90; 91] with increased expression of positive clock genes (cClock, cBmal1, cBmal2) and decreased negative clock genes (cCry1, cCry2, cPer2, and cPer3) under green light conditions [80]. Figure 6: Monochromatic light affects melatonin expression and negative feedback of transcription–translation. As reported, the hypothalamic clock gene Bmal1 expression for pigeons with monochromatic light exposure was involved in the birth rate [92]. The melatonin level of 10-week-old turkeys with a significant circadian rhythm was strongly affected by light [93]. Melatonin may be associated with an expression of the clock gene in the pancreas of chickens [94]. Light color affected the expression of cell clock regulators to regulate the expression of cAanat in the eyes [95]. As shown in Figure 6, at the beginning of the circadian rhythm, the positive regulatory factor outputs cycle protein CLOCK and BMAL1 to form a heterodimer, which binds to the E-box in the promoter region of negative feedback factor Per and Cry to promote their transcription [96]. The accumulation of Per and Cry can lead to the inactivation of the heterodimer to inhibit their transcriptional generation. The mRNA levels of Per and Cry decrease so that the circadian cycle begins again at dawn [97]. As the downstream gene of the Clock-Bmal1 heterodimer, when CRY is expressed in large quantities, their protein products will also form heterodimers and then re-enter the nucleus to inhibit the formation of the Clock-Bmal1 complex as an automatic regulation feedback loop [98].