Content area
Full Text
Abstract
The aim of study was to determine the mechanisms regulating melatonin secretion in the pineal organs of 1-day-old and 9-month-old domestic ducks. The pineals were cultured in a superfusion system under different light conditions. Additionally, some explants were treated with norepinephrine.
The pineal glands of 1-day-old ducks released melatonin in a well-entrained, regular rhythm during incubation under a 12 hrs light : 12 hrs dark cycle and adjusted their secretory activity to a reversed 12 hrs dark : 12 hrs light cycle within 2 days. In contrast, the diurnal changes in melatonin secretion from the pineals of 9-month-old ducks were largely irregular and the adaptation to a reversed cycle lasted 3 days. The pineal organs of nestling and adult ducks incubated in a continuous light or darkness secreted melatonin in a circadian rhythm. The treatment with norepinephrine during photophases of a light-dark cycle resulted in: 1) a precise adjustment of melatonin secretion rhythm to the presence of this catecholamine in the culture medium, 2) a very high amplitude of the rhythm, 3) a rapid adaptation of the pineal secretory activity to a reversed light-dark cycle. The effects of norepinephrine were similar in the pineal organs of nestlings and adults.
In conclusion, melatonin secretion in the duck pineal organ is controlled by three main mechanisms: the direct photoreception, the endogenous generator and the noradrenergic transmission. The efficiency of intra-pineal, photosensitivity-based regulatory mechanism is markedly lower in adult than in nestling individuals.
Key words: pineal gland, avian pinealocytes, melatonin, diurnal rhythm, circadian rhythm, norepinephrine, duck
Introduction
The pineal organ via its hormone - melatonin (MLT) - is involved in the control of daily and seasonal rhythms of many physiological processes and behavioral activities, and therefore it participates in adaptation to changing environmental conditions. The morphological organization of the pineal organ differs extremely between vertebrate classes and even within the class, especially in reptiles and birds (Vollrath 1981, Przybylska-Gornowicz et al. 2005, 2012, Prusik and Lewczuk 2008a, Lewczuk and Przybylska-Gornowicz 2013). Likewise, the mechanisms regulating MLT biosynthesis and simultaneously the output of this lipophilic hormone from parenchymal cells vary significantly among species (Maronde and Stehle 2007, Prusik and Lewczuk 2008b, Lewczuk et al. 2014b, McStay et al. 2014).
The avian pineal organ shows enormous structural...