A comparative ex vivo and in vivo study of day and night perception in teleosts species using the melatonin rhythm Introduction The pineal photoreceptor of fish is a photo-neuro-endocrine cell which releases at least two types of messages in response to photoperiodic information: a neurohormonal (melatoninergic) signal and a neural (excitory neurotrans- mitter) signal. Over the 500 million years of evolution of vertebrates only the melatoninergic information has been preserved [1]. In fish, melatonin rhythms, shown both in vivo [2–5] and ex vivo [6–8], are believed to entrain the temporal co-ordi- nation of many physiological processes, including smolti- fication and reproduction [9]. Indeed, exposing fish to artificial photoperiods has been used extensively to control the timing and completion of smoltification and gonadal development in salmonids [10–13] and sea bass [14, 15]. In all vertebrates, elevated melatonin production accurately reflects the length of the light:dark cycle [16], however, unlike mammals, the pineal in fish is directly photosensitive [2–5, 17]. Varied responses in growth and reproduction in Atlantic salmon to different intensities of additional night-time illumination has been reported [18, 19]. It was shown that plasma melatonin synthesis decreases with increasing light intensity and seems to be dependent on light irradiance [20]. It is thought that plasma melatonin may have threshold levels, which define the response of biological functions in fish to environmental influences [5, 19]. Although some electrophysiological studies have described the luminance and chromatic response of the pineal [9], better knowledge of such light requirements for melatonin regulation is needed especially regarding the quality of the light, as the aquatic environment acts as a potent filter, which modifies both the light spectrum and intensity. One of the major questions relating to light intensity in fish is what levels of illumination are perceived as day and what are seen as night? As plasma melatonin may have to be suppressed below a certain threshold before artificial photoperiods are capable of altering physiological functions, not only the photoperiod but also the power, colour and orientation of submerged lamps should be considered when designing an effective artificial lighting system for fish [21]. To fully understand and determine light intensity thresh- old levels, it is important to consider the light directly perceived by the pineal gland, i.e. that penetrates through the overlying tissue (pineal window) into the pineal fossa. The relative transmission of light at a variety of wave- lengths was quantified in rainbow trout [6]. It suggested that only 1% of the relative light entered the pineal fossa at 400 nm, rising to 2.5% at 450 nm, 7.5% at 550 nm, 12.5% at 650 nm and 16% at 700 nm. Transmission of the longer wavelengths (650–700 nm) through cranial bones also Abstract: The purpose of this study was to determine and compare the light sensitivity of two commercially important, phylogenetically different teleost species in terms of melatonin production. Three series of experiments were performed on both Atlantic salmon and European sea bass. First, a range of light intensities were tested ex vivo on pineal melatonin production in culture during the dark phase. Then, light transmission through the skull was investigated, and finally short-term in vivo light sensitivity trials were performed. Results showed that sea bass pineal gland ex vivo are at least 10 times more sensitive to light than that of the salmon. Light intensity threshold in sea bass appeared to be between 3.8 · 10 )5 and 3.8 · 10 )6 W/m 2 in contrast to 3.8 · 10 )4 and 3.8 · 10 )5 W/m 2 in salmon. These highlighted species-specific light sensitivities of pineal melatonin production that are likely to be the result of adaptation to particular photic niches. Light transmission results showed that a significantly higher percentage of light penetrates the sea bass pineal window relative to salmon, and confirmed that penetration is directly related to wavelength with higher penetration towards the red end of the visible spectrum. Although results obtained in vivo were comparable, large differences between ex vivo and in vivo were observed in both species. The pineal gland in isolation thus appeared to have different sensitivities as the whole animal, suggesting that retinal and/or deep brain photoreception may contribute, in vivo, to the control of melatonin production. H. Migaud 1 , J.F. Taylor 1 , G.L. Taranger 2 , A. Davie 1 , J.M. Cerda ´- Reverter 3 , M. Carrillo 3 , T. Hansen 2 and N.R. Bromage 1 1 Reproduction and Genetics group, Institute of Aquaculture, University of Stirling, Stirling, UK; 2 Institute of Marine Research, Bergen, Norway; 3 Institute of Aquaculture Torre de la Sal, Castello ´n, Spain Key words: light intensity, melatonin, pineal gland, salmon, sea bass, spectrum Address reprint requests to Herve Migaud, Institute of Aquaculture, University of Stirling, FK9 4LA, Stirling, UK. E-mail: herve.migaud@stir.ac.uk Received January 2, 2006; accepted March 6, 2006. 42 J. Pineal Res. 2006; 41:42–52 Doi:10.1111/j.1600-079X.2006.00330.x Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Munksgaard Journal of Pineal Research J. Pineal Res. 2006; 41:42–52 Doi:10.1111/j.1600-079X.2006.00330.x Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Munksgaard Journal of Pineal Research J. Pineal Res. 2006; 41:42–52 Doi:10.1111/j.1600-079X.2006.00330.x Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Munksgaard Journal of Pineal Research