The melatoninergic system in the European sea bass: effects of pinealectomy

The time-keeping hormone melatonin represents a conserved feature in vertebrates and especially in fish it exerts a central role by entraining key physiological functions such as reproduction, feeding and rest with external cues, mainly photoperiod and then temperature. The aim of this study was to gain deeper insight into the melatoninergic system in a fish species of commercial interest, the European sea bass. To accomplish this target a number of surgical procedures (pinelectomy, opthalmectomy, pinealectomy + opthalmectomy, pinealectomy + opthalmectomy + melatonin implant, pinealectomy + melatonin implant) were performed, focusing on pineal removal, in order to analyse the response of several parameters, under natural photoperiod, including plasma melatonin levels, gene expression of melatonin enzymes and its main receptor MT1 in the diencephalon and locomotor activity. Our results support evidence that the pineal gland is directly photosensitive and further, suggested that it is able to re-grow, at least in part. Accordingly, neither opthalmectomized nor pinealectomized individuals exhibited diminished plasma melatonin levels with respect to control groups. Gene expression analyses did not detect any significant variation and basically removal of light sensitive organs did not affect the expression of melatonin enzymes in the diencephalon. However, in the pinealectomy + opthalmectomy + melatonin implant group higher expression at night was a common pattern, particularly pronounced for the melatonin receptor MT1, and we would suggest a time-dependent action of melatonin in this group. Finally, locomotor activity was not affected significantly by surgery and even in the absence of the pineal gland a day/night pattern was still present among groups, indicating that the pineal was not acting as a central pacemaker as the SCN in mammals.