Research reportICV melatonin reduces acute stress responses in neonatal chicks
Introduction
Melatonin is synthesized from the amino acid precursor l-tryptophan primarily in the pineal gland and, to a lesser extent, in the retina of various vertebrate species including chickens. At least four enzymes are known to be involved in this process. Among them, serotonin N-acetyl-transferase, or arylalkylamine N-acetyltransferase (EC 2.3.1.87; AANAT), is considered the rate-limiting enzyme in the regulation of melatonin biosynthesis as changes in its activity and mRNA expression parallel alterations in melatonin concentrations [3], [34], [35]. Light is one of the key regulators of melatonin synthesis with peak concentrations occurring during the dark period and a nadir during the daytime [19], [34], [35]. This is believed to be the result of inhibition of melatonin synthesis during the daylight via degradation of AANAT [4], [12], [28], [34]. Exposure to light at night when AANAT activity levels are high results in a drastic reduction in both AANAT activity and melatonin concentrations in the pineal gland, retina and plasma [12], [36].
Melatonin is involved in many biological actions including food intake, sleep, and stress [5], [28]. In rats, intrahypothalamic microinjection of melatonin induced sleep in a dose-dependent manner [21]. Melatonin has similar actions in avian species. In pigeons receiving intravenous melatonin infusions during the daytime, Mintz et al. [22] demonstrated that melatonin induced sleep, increased electroencephalograph slow wave activity, and decreased body temperature and locomotor activity. Likewise, intraperitoneal administration of melatonin dose-dependently decreased food intake and induced sleep in layer-type chickens [2]. Melatonin has also been shown to act as an anti-stress agent. Melatonin reduced stress-induced increases in rat plasma corticosterone concentrations, and animals subjected to stress showed altered circadian patterns in plasma melatonin with elevated corticosterone concentrations [1], [15], [16], [23].
However, little is available as to whether or not melatonin is directly involved in stress in neonatal chickens. To determine a better model, the present study compared melatonin concentrations in brain tissues between broiler and layer chickens raised under either 12 h light:12 h dark cycle or continuous illumination. Although melatonin concentrations were much higher in broilers than layers at night under a 12 h light:12 h dark cycle, these differences disappeared under the 24 h illumination. We previously found that neonatal layer chicks were more susceptible to stress than broilers [30], [31]. We thus chose neonatal layers for a test system. We then tested if intracerebroventricular (ICV) injection of melatonin was able to modulate plasma corticosterone concentrations in layers under continuous illumination. Our findings provide a direct evidence that melatonin is able to reduce corticosterone concentrations associated with isolation-induced stress and corticotrophin-releasing factor (CRF).
Section snippets
Animals
Day-old male layer (Julia®, Murata Hatchery, Fukuoka, Japan) and broiler chicks (Cobb®, Mori Hatchery, Fukuoka, Japan) were housed in cages in a room with 30 ± 1 °C ambient temperature and either a 12 h light:12 h dark cycle (Experiment 1) or 24 h illumination (Experiments 2–4). They were given free access to a commercial starter diet (Toyohashi Feed and Mills Co. Ltd., Aichi, Japan) and water. This study was performed according to the guidance for Animal Experiments in the Faculty of Agriculture and
Experiment 1
In general, differences between both strains were more pronounced in the dark than light phase, which was consistently seen in all four-brain tissues tested (Fig. 1A–D). There were dramatic changes in melatonin concentrations in the pineal gland of both broiler and layer chicks, which were significantly affected by chicken type [F(1,16) = 11.1, P = 0.004] and lighting condition [F(1,16) = 25.0, P = 0.0001]. These changes were more pronounced in broilers than in layers, indicating a significant
Discussion
Our findings demonstrating that melatonin blocked the enhanced corticosterone concentrations observed after layers were either exposed to a novel, open field or injected ICV with CRF, providing direct evidence that melatonin plays a role in regulation of the stress response in neonatal chicks. This result confirms and extends the previous findings [24] demonstrating that intraperitoneal injection of melatonin reduced distress vocalization of isolated chicks. Furthermore, it was shown that daily
Acknowledgement
This work was supported by Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (No. 16380191 and 17208023).
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