In this study, we observed that constant light exposure increased ADFI and FCR, but did not affect body weight in 7 d and 21 d chickens. Similar results have been reported by Lewis, who found that compared with photoperiods in 12 h, photoperiods in 24h decreased feeding rate, increased food intake, but there was no significantly differences on body weight in 21 d males Cobb 500 and Ross 308 broiler [3]. However, recent study reported that compared with intermittent lighting, constant lighting in 30 lx decreased ADFI and did not affect FCR in Lingnan Yellow broiler chicks from 1-21 d [17]. The reason why these results were different from the present study may be due to the different broiler breeds and light intensity.
In the present study, behavior tests results found that constant light exposure in early life induced anxiety and depressive-like behavior in 7 d chickens. These results consistent with previous study, which found that constant light exposure increased anxiety and depressive-like behavior in mice [18]. Meanwhile, we found that constant light exposure induces higher plasma CORT levels both in 7 d and 21 d chickens. Higher plasma CORT levels have detrimental physiological and cognitive effects, which were linked with depression-like behavior in weanling mice [19] and adult rats [20] exposed to dim light at night. CORT levels are regarded as an important indicator of depression-like behavior in rodents [21]. Thus, in this study, we speculate that constant light increased anxiety and depressive-like behavior may be directly or indirectly induced by higher plasma CORT levels in chicken.
Melatonin plays a key role in controlling circadian behavioral responses [22]. It also acts as a potent antioxidant by scavenging reactive oxygen species [23]. Previous studies reported that constant light induced depressive-like behavior in rodents is usually associated with low melatonin and high oxidative stress levels [24, 25]. Indeed, we detected lower plasma melatonin and 5-HT levels, accompanied by lower hippocampal 5-HT receptor mRNA expression both in 7 d and 21 d of constant exposed chickens. Similar results have been reported by Lauber, who found that compared with photoperiods in 12 h, photoperiods in 23 h significantly reduced plasma melatonin in 7-week-old broiler chickens [26]. NF-E2-related factor 2 (Nrf2) and its endogenous inhibitor, Kelch-like ECH-associated protein 1 (Keap1) plays a critical role in counteract oxidative stress [27]. In this study, constant light exposure reduced Nrf2/Keap1 and superoxide dismutase (SOD) mRNA expression, as well as Nrf2 protein expression both in 7 d and 21 d chickens hippocampus. Agree with our present study, it was reported that constant light induced higher oxidative stress in rat hippocampus, cortex and cerebellum [28], and lower Nrf2 expression in rat thymus which could rescued by melatonin [29].
A number of studies indicate that constant light exposure influences the mRNA expression of clock-related genes, including Clock, Bmal1, Cry, and Per [30, 31], which is associated with impaired hippocampal neurogenesis and depressive-like behavior [32, 33] in mice. Similar with previous reported, we also found that constant light exposure significantly increased the mRNA expression of clock-related genes Cry1, Cry2 and Rev-erbα both in 7 d and 21 d chickens hippocampus. In mammals, Crys are important clock genes involved in the regulation of circadian rhythm [34]. Besides, Crys serve as photoactive pigments and important circadian photoreceptors for light entrainment of the circadian rhythm [35]. Mice lacking Cry1 and Cry2 genes lose completely the free-running circadian rhythmicity in wheel-running behavior [36] and the circadian oscillations in the electric activity of the suprachiasmatic nucleus under constant darkness [37]. Thus, we speculate that higher photoreceptor Cry expression may be involved in
BDNF/TrκB/ERK pathway is critical for the hippocampal neurogenesis [8], which plays an important role in regulated depressive-like behavior in animal [7]. In the present study, we found that constant light exposure significantly suppressed BDNF/TrκB/ERK pathway both in 7 d and 21 d chicken’s hippocampus. These results consistent with our previous study, which reported that constant light suppressed BDNF/TrκB/ERK pathway in mice hippocampus [11]. In addition, melatonin could rescue chronic restraint stress induced depressive-like behavior via activated the BDNF/TrκB signaling pathway [38]. And CORT exposure reduced hippocampal neurogenesis through suppressed BDNF/ERK pathway [39]. Thus, combined with the results in present study, we speculate that constant light exposure induces depressive-like behavior in chickens via modulation of hippocampal BDNF/TrκB/ERK pathway.