Roles of Direct Photoreception and the Internal Circadian Oscillator in the Regulation of Melatonin Secretion in the Pineal Organ of the Domestic Turkey: A Novel In Vitro Clock and Calendar Model

Abstract

The regulation of melatonin secretion in the avian pineal organ is highly complex and shows prominent interspecies differences. The aim of this study was to determine the roles of direct photoreception and the internal oscillator in the regulation of melatonin secretion in the pineal organ of the domestic turkey. The pineal organs were collected from 12-, 13- and 14-week-old female turkeys reared under a 12 L:12 D cycle with the photophase from 07.00 to 19.00, and were incubated in superfusion culture for 3–6 days. The cultures were subjected to different light conditions including 12 L:12 D cycles with photophases between 07.00 and 19.00, 13.00 and 01.00 or 01.00 and 13.00, a reversed cycle 12 D:12 L, cycles with long (16 L:8 D) and short (8 L:16 D) photophases, and continuous darkness or illumination. The pineal organs were also exposed to light pulses of variable duration during incubation in darkness or to periods of darkness during the photophase. The secretion of melatonin was determined by direct radioimmunoassay. The turkey pineal organs secreted melatonin in a well-entrained diurnal rhythm with a very high amplitude. Direct photoreception as an independently acting mechanism was able to ensure quick and precise adaptation of the melatonin secretion rhythm to changes in light-dark conditions. The pineal organs secreted melatonin in circadian rhythms during incubation in continuous darkness or illumination. The endogenous oscillator of turkey pinealocytes was able to acquire and store information about the light-dark cycle and then to generate the circadian rhythm of melatonin secretion in continuous darkness according to the stored data. The obtained data suggest that the turkey pineal gland is highly autonomous in the generation and regulation of the melatonin secretion rhythm. They also demonstrate that the turkey pineal organ in superfusion culture is a valuable model for chronobiological studies, providing a highly precise clock and calendar. This system has several features which make it an attractive alternative to other avian pineal glands for circadian studies.

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

“…The daily profile of plasma MLT concentration described in the present study was similar to the diurnal cycle of MLT secretion in previous in vitro studies that were performed on the pineal organs of turkeys at a similar age [32,33]. However, the ratio between the maximum and minimum concentrations of MLT in the plasma was significantly lower than the amplitude of the MLT rhythm reported in the superfusion culture (18-fold in vivo vs. 40-fold in vitro).…”

“…Similar data were obtained from younger turkeys, in which the circadian rhythm of MLT secretion persisted for 7 days under continuous dim red light and had an amplitude that was 50-80% lower than the rhythm in the 12L:12D cycle [27,41]. The circadian rhythm of MLT secretion with gradually decreasing amplitude was also observed in the superfusion culture of the turkey pineal organs for 5 days in continuous darkness [33]. However, the amplitude of this rhythm was very low starting from the third day of culture.…”

“…Two-hour exposure to light caused a decrease in the plasma MLT concentration close to the day-time level. The light-evoked decrease in vivo was significantly higher than that observed in similar in vitro experiments [32,33]. Results of the in vitro studies showed that even 3-h exposure to light did not decrease the MLT secretion to the level characteristic of the light phase of the cycle.…”

“…However, it decreased quickly in birds, in which sympathetic innervation of the pineal gland was interrupted [22,42]. In the superfusion culture, the circadian rhythm of MLT secretion persisted longer in the experiments with the turkey pineal organs than the chicken pineal organ [22,32,33,43]. In contrast, the circadian rhythm of MLT secretion from the pineal glands of Japanese quail cultured in constant darkness was very weak or completely abolished [44].…”

“…After 7 days of housing of 10-week-old turkeys under a reversed dark-light cycle (12D:12L), the diurnal rhythm of the plasma MLT concentration was adapted to the new lighting conditions. In an in vitro experiment, turkey pinealocytes entrained their secretory activity to such changes on the second day of culture [32,33]. The adaptation of the chicken pinealocytes to the 12D:12L cycle was observed on the third day of culture [43].…”