Abstract
Melatonin is a highly conserved hormone in evolutionary history. It occurs in numerous organisms and plays a role in the endocrine and immune systems. Locomotor behavior is a basic behavior in animals and is an important indicator of circadian rhythms, which are coordinated by the nervous and endocrine systems. To date, the effect of melatonin on locomotor behavior has been studied in vertebrates, including syrian hamsters, sparrows, rats, zebrafish, goldfish, and flatworms. However, there have been few studies of the effects of melatonin on locomotor behavior in marine invertebrates. The goals of present study were to show the existence of melatonin in the sea cucumber Apostichopus japonicus and to evaluate its effect on locomotor activity. In addition, muscle tissues from control and melatonin-treated sea cucumbers were tested using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to determine the changes of metabolic activity in muscle. Melatonin was present in the coelomic fluid of A. japonicus at a concentration of ∼135.0 ng/L. The total distance traveled and number steps taken over 9 h after melatonin administration decreased with increasing concentration of the melatonin dose. Mean and maximum velocity of movement and stride length and stride frequency also decreased, but their differences were not statistically significant. Overall, these results suggest that melatonin administration had a sedative effect on A. japonicus. The levels of 22 different metabolites were altered in the muscle tissues of melatonin-treated sea cucumbers. Serotonin, 9-cis retinoic acid, all-trans retinoic acid, flavin mononucleotide in muscles were downregulated after melatonin administration. Moreover, a high free fatty acid (FFA) concentration and a decrease in the adenosine 5′-triphosphate (ATP) concentration in the muscle tissues of the melatonin-treated group were detected as well. These results suggest that the sedative effect of melatonin involves some other metabolic pathways, and the reduced locomotor modulator—serotonin, inhibited fatty acid oxidation and disturbed oxidative phosphorylation are potential physiological mechanisms that result in the inhibitory effect of melatonin on locomotion in sea cucumbers.
Highlights
In 1958, melatonin was discovered from the pineal gland of cows (Lerner et al, 1958), and it subsequently was confirmed to be an indole neuroendocrine hormone (Reiter, 1995)
Melatonin was present in the coelomic fluid of normal A. japonicus at concentrations ranging from 120.01 to 165.38 ng/L (Table 1)
The total distance moved by sea cucumber during the 9 h after melatonin administration significantly decreased with increasing melatonin dose, ranging from 22.53 ± 4.35 m in the control group to 9.93 ± 2.57 m in the 1000 μM melatonin group (Figure 1A; F3,12 = 12.828, p < 0.01)
Summary
In 1958, melatonin was discovered from the pineal gland of cows (Lerner et al, 1958), and it subsequently was confirmed to be an indole neuroendocrine hormone (Reiter, 1995) This discovery triggered studies of the characteristics, physiology, medical applications, and behavioral functions of this special molecule. Melatonin can be produced in the gastrointestinal system (Bubenik, 2002) and even the retina (Do Carmo Buonfiglio et al, 2011), but its main source of production is the pineal gland In invertebrates, this hormone has been detected in numerous organs in insects (including the brain, palp, eyes, ovipositor, hind-leg, and ovary) and in the gonad of sea stars (Vivien-Roels and Pévet, 1993; Itoh et al, 1995; Peres et al, 2014). The expression of melatonin in most invertebrates does not show obvious rhythmicity (Tanaka et al, 2007)
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