Diurnal Rhythm of Plasma Melatonin Concentration in the Domestic Turkey and Its Regulation by Light and Endogenous Oscillators

Abstract

The aim of this study was to characterize the diurnal rhythm of plasma melatonin (MLT) concentration and its regulation by light and endogenous oscillators in 10-week-old domestic turkeys. Three experiments were performed to examine (i) the course of daily changes in plasma MLT concentration in turkeys kept under a 12 h light: 12 h dark (12L:12D) cycle; (ii) the influence of night-time light exposure lasting 0.5, 1, 2, or 3 h on the plasma MLT level; and (iii) the occurrence of circadian fluctuations in plasma MLT levels in birds kept under continuous dim red light and the ability of turkeys to adapt their pineal secretory activity to a reversed light-dark cycle (12D:12L). The plasma MLT concentration was measured with a direct radioimmunoassay. The plasma MLT concentration in turkeys kept under a 12L:12D cycle changed significantly in a daily rhythm. It was low during the photophase and increased stepwise after the onset of darkness to achieve the maximal level in the middle of the scotophase. Next, it decreased during the second half of the night. The difference between the lowest level of MLT and the highest level was approximately 18-fold. The exposure of turkeys to light during the scotophase caused a rapid, large decrease in plasma MLT concentration. The plasma MLT concentration decreased approximately 3- and 10-fold after 0.5 and 1 h of light exposure, respectively, and reached the day-time level after 2 h of exposure. In turkeys kept under continuous darkness, the plasma MLT level was approximately 2.5-fold higher at 02:00 h than at 14:00 h. In birds kept under 12D:12L, the plasma MLT level was significantly higher at 14:00 h than at 02:00 h. The results showed that plasma MLT concentrations in 10-week-old turkeys have a prominent diurnal rhythm, which is endogenously generated and strongly influenced by environmental light.

Metabolism of Melatonin Synthesis-Related Indoles in the Turkey Pineal Organ and Its Modification by Monochromatic Light

“…The increase in AA-NAT activity at night resulted also in intensive utilization of 5-HT, and significant decrease in the level of this amine and the levels of the indoles derived from 5-HT oxidative deamination (5-HIAA, 5-HTOL, 5-MIAA, and 5-MTOL). The high amplitude daily fluctuations in the activity of AA-NAT and in the contents of NAS and MLT found in this study concur with our previous data showing the prominent rhythms in the secretion of MLT from the turkey pineal organ in superfusion culture [28] and in the MLT concentration in blood plasma [29].…”

“…Previous studies demonstrated that the pineal organ of the domestic turkey is a very attractive model for chronobiological studies [ 28 , 29 ]. Although both turkeys and chickens belong to the Galliformes family, the histological organization and ultrastructure of the turkey pineal organ differ greatly from the chicken pineal organ [ 30 , 31 , 32 , 33 ].…”

Effects of Extreme Light Cycle and Density on Melatonin, Appetite, and Energy Metabolism of the Soft-Shelled Turtle (Pelodiscus sinensis)

“…Similar results are also revealed in domestic turkey ( Meleagris gallopavo gallopavo var. domesticus ) with higher melatonin concentrations during nighttime than during daytime after 7 days of continuous dim red light treatment [ 9 ], which was regarded as endogenously generated diurnal rhythm. Strangely, the circadian rhythm of the sleepy lizard ( Tiliqua rugosa ) was eliminated by 3 days of constant light [ 10 ], while higher plasma melatonin concentrations at nighttime remained after a month of constant light treatment in our present study.…”

“…All turtles were reared for 28 days under corresponding light and stocking density conditions, and the other conditions were kept the same as the period of acclimation. Removing uneaten food and feces and refreshing water under constant darkness were performed in dim red light with an intensity of 3 lux [ 9 ]. Food intake of the intermediate 3 days in each week was recorded for each replicate (one replicate inside one rearing tank).…”

“…Generally, melatonin secretion occurs during the dark phase and is suppressed by light, regardless of the habitat of animals [ 8 ]. Interestingly, published results differ in whether melatonin secretion is also regulated by endogenous factors [ 9 , 10 , 11 , 12 , 13 ]. In addition, numerous studies have demonstrated that melatonin exerts profound biological functions, such as food intake [ 7 , 14 ], energy balance regulation [ 15 , 16 ], sleeping regulation [ 17 ], antioxidant defense [ 18 , 19 ], immune system activity [ 20 ], and reproduction [ 21 ].…”