Swimming activity and behaviour of European Anguilla anguilla glass eels in response to photoperiod and flow reversal and the role of energy status.

To better understand migratory divergences among Anguilla anguilla glass eels, the behaviour of individuals caught at the time of their estuary entrance was studied through their response to a light:dark cycle and then to both water current reversal and light:dark cycle. In a first experiment, fish moving with the flow in response to dusk (M(+) fish) and fish that had not exhibited any movement (M(-) fish) were distinguished. Anguilla anguilla from these two groups were then individually marked and their response to water current reversal compared. M(+) individuals mainly exhibited negative rheotaxis with a tidal periodicity, whereas positive rheotaxis was mainly exhibited by M(-) individuals. Thus, M(+)A. anguilla glass eels showing negative rheotaxis appear to have the strongest propensity to migrate, the converse applies to M(-) ones showing positive rheotaxis. A small percentage of individuals (5%) were hyperactive, alternately swimming with and against the current with almost no resting phase. These fish lost c. 2 mg wet mass day(-1), whereas individuals which were almost inactive lost c. 1 mg day(-1). Wet and dry mass changes in relation to activity levels were compared with previous experiments and it was concluded that A. anguilla glass eel energy status might be involved in differences in migratory tendencies but other factors that might be important are discussed. It is proposed that any decrease in A. anguilla glass eel energy stores associated with global warming might lead to an increase in the proportion of sedentary individuals and thus be involved in the decrease in the recruitment to freshwater habitats.

Energy and migratory behavior in glass eels (Anguilla anguilla).

The influence of energy content of glass eels (Anguilla anguilla) on their migratory behavior was investigated in November 2004 and February 2005 under laboratory conditions. Fish were sorted according to their response to a decrease in light intensity: Glass eels becoming active at dusk and using water flow to move with the current were considered as "active", whereas those remaining in the substratum throughout the catching period (24 h) were categorized as "inactive". "Active" glass eels exhibited higher energy content than "inactive" ones, whatever the catching month, and were more pigmented. Differences in percentage dry weight between "active" and "inactive" fish were already significant in the first pigmentary stages, suggesting that energy content plays a causal role in the estuarine migration of glass eels. The possible impact of energy state on the development of different migratory tactics in glass eels is discussed.

Regulation of melatonin secretion by light in the isolated pineal organ of the white sucker (Catostomus commersoni).

The effects of different lighting conditions and physical parameters of light were investigated in the isolated pineal organ of the white sucker kept under static or superfusion culture. The secretion of pineal melatonin is directly controlled by the photoperiod and completely suppressed under constant illumination. When pineal organs are exposed to unexpected light at night, the secretion of melatonin is significantly reduced within 15 min and reaches basal value after 30-35 min of light treatment. The inhibition of melatonin secretion by unexpected light at night depends on the irradiance, duration, timing of the treatment and the lighting history of the pineal organ.

Demand-feeding rhythm in rainbow trout and European catfish. Synchronisation by photoperiod and food availability.

The effect of light-dark (LD) cycle and food availability was tested on the demand-feeding rhythm of single and groups of rainbow trout and European catfish. Under LD and free food access, most trout and catfish displayed, respectively, a diurnal and a nocturnal pattern of demand-feeding activity, whereas a few fish or groups of fish switched from diurnalism to nocturnalism or vice versa. In both species held under constant lighting conditions and a restricted feeding (RF) cycle (RF 20:4), the demand-feeding rhythm rapidly synchronised to food availability. The demand-feeding rhythm was under endogenous control and, in rainbow trout, periodogram analysis suggested the existence of two oscillators, one synchronised by photoperiod (LEO) and the other by food (FEO). When submitted to both LD and RF cycles, LD was, at least in the rainbow trout, the dominant zeitgeber synchronising the demand-feeding rhythm. In catfish, food availability rapidly synchronised demand-feeding rhythm. Finally, in both species, the synchronisation of single fish to LD or feed availability appeared slower than that of groups of fish, supporting the idea that social organisation affects the circadian activity in fish.

Toward a chronome of superfused pike pineals: about-weekly (circaseptan) modulation of circadian melatonin release.

In continuous darkness for about 5.5 days in eleven replications, the pineal shows neither only a decreasing trend nor only a damped circadian rhythm; there is an infradian modulation: after an initial decline, melatonin production increases again, with a pattern compatible with the assumption, for at least one cycle, that a built-in pineal "week" has a larger extent of predictable change than that of a pineal "day". Melatonin release from the isolated pike pineal reveals a circadian rhythm which, on the average, is damped, but demonstrable in the second half of a time series covering 5.5 days (P < 0.05). At fixed environmental temperatures of 10, 15, 19 or 20 degrees C (the latter two pooled), single and population mean cosinor analyses reject the assumptions of zero circaseptan and circadian amplitudes in the chronome of the pineal from the pike (Esox lucius L.). The circaseptan pattern is confounded by possible trends. The time series analyzed are brief, yet validate, by a one-way analysis of variance in plexograms as well as by cosinor, an about-weekly pattern, if not (yet) rhythm, on the basis of 745 determinations. The circaseptan- and circasemiseptan-to-circadian amplitude ratios are numerically larger than unity, on the average (2.014; 95% confidence interval, CI: 1.120 to 3.641 and 1.107; CI: 0.724 to 1.693, respectively). A positive correlation of the urinary excretion rate of a major breakdown product of melatonin of a cancer patient with a circulating cancer marker serving as a gauge of disease progression lends an oncologic perspective to the amplitude ratios of melatonin release in vitro here reported.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical study of the development of serotoninergic neurons in the brain of the brook trout Salvelinus fontinalis.

Serotonin is believed to play an important role in the neuronal development of various invertebrates and mammals, but only one study has yet investigated the development of serotoninergic neurons in the brain of a teleost. In the present study, we investigate the development of serotoninergic neurons in the brain of the brook trout, Salvelinus fontinalis, by immunohistochemistry, from fertilisation to the resorption of the yolk sac and in juveniles for reference. Maintaining trout embryos and larvae in 9L:15D or in complete darkness made no difference in the appearance and distribution of serotonin immunoreactive (5-HTir) neurons. On day 56, the first 5-HTir perikarya, indicative of the primordia of the nuclei raphe medialis/dorsalis, have stained in the presumptive isthmus region. At hatching time (days 95-115), all the 5-HTir nuclei reported in the juvenile are present, except for the population observed in the lobus inferior hypothalami. We observed that the nucleus recessus lateralis stained on day 58, the raphe caudal nucleus on day 60, the nuclei raphe pallidus and obscurus on day 65, the area praetectalis on day 80, and the nucleus recessus posterioris and the anterior group of the nucleus recessus lateralis on day 92. At the complete resorption of the yolk sac (days 175-190), the nucleus reticularis paragigantocellularis, the nuclei raphe pallidus and obscurus and the caudal raphe of the brainstem are very weakly immunoreactive, and in juveniles, only one caudal immunoreactive nucleus is still present. A comparison of our result with those previously reported in the three-spined stickleback, Gasterosteus aculeatus, suggest a similar distribution of 5-HTir neurons in the brain of both species but also reveal species differences in the sequence of appearance of 5HT immunoreactivity in the different neuronal populations.

Melatonin secretion in vitro from the pineal complex of the lamprey Petromyzon marinus.

Melatonin secretion from cultured pineal complexes of the lamprey, Petromyzon marinus, was measured by radioimmunoassay (RIA) for 6 days under either a 12:12 light:dark cycle (L:D) or continuous darkness (D:D) at either 10 degrees or 20 degrees. Under the L:D cycle, melatonin release was completely suppressed during daytime and high during night-time, with larger amplitude at 20 degrees than at 10 degrees. Under D:D conditions, no significant circadian rhythmicity in secretion could be revealed by periodogram analysis at either 10 degrees or 20 degrees. This is the first direct evidence of melatonin secretion in a lamprey, suggesting that both light and temperature can affect in vitro melatonin secretion.

Partial characterization of serotonin N - acetyltransferases from northern pike (Esox lucius, L.) pineal organ and retina: effects of temperature.

In vertebrates, the nocturnal rise in pineal organ and retinal melatonin synthesis results from the increase in the activity of the serotonin N-acetyltransferase (NAT), a cAMP-dependent enzyme. In the fish pineal organ in culture, light and temperature act in a similar manner on cAMP content and NAT activity. It is not known whether the effects of temperature are mediated through cAMP or through modifications of NAT kinetics. The present study was designed: (1) to find out whether NAT activity from pineal organ homogenates is similar to NAT activity from pineal organs in culture, with regard to variations in temperature, and (2) to compare NAT activity from the pineal organ and the retina. Pineal organ and retinal NAT activity increased linearly with protein concentrations. Higher activities were obtained with 0.2 mol/l of phosphate buffer, pH 6. Higher molarity or a higher pH induced a decrease in retinal and pineal organ NAT activity: retinal NAT was more sensitive than pineal organ NAT to changes in molarity, whereas the opposite held true as far as pH was concerned. Pineal organ and retinal NAT obeyed the Michaelis-Menten equation with respect to increasing concentrations of acetyl-coenzyme A. With increasing concentrations of tryptamine: (1) pineal organ NAT activity increased in a manner suggesting positive co-operativity, (2) retinal NAT displayed, after an initial increase, inhibition by substrate. The kinetics of the reactions were temperature dependent. Maximal activities were reached at 18/20 degrees C in the pineal organ and at 37 degrees C in the retina. The present study is the first to describe the optimum conditions for the assay of NAT activity in homogenates from the retina of fish and from the pineal organ of poikilotherms, and also the first to compare some characteristics of NAT activity from these two analogous organs. Our results suggest that the effects of temperature on melatonin production are mediated, at least in part, through modifications of NAT kinetics. Future studies will aim to clarify whether the activities measured in the pineal organ and retinal homogenates reflect the presence of one or of several enzymes.

Effects of photoperiod and temperature on rhythmic melatonin secretion from the pineal organ of the white sucker (Catostomus commersoni) in vitro.

The secretion rate of melatonin from cultured pineal organs of the white sucker was examined for several days under either a 12:12-hr light:dark (LD) cycle or continuous darkness (DD) at either 10 degrees or 20 degrees. The incubation medium was changed at 3-hr intervals and secreted melatonin was measured by RIA. Under a 12:12-hr LD cycle (0800 light on, 2000 light off) melatonin secretion was suppressed during the day and highly active at night, with larger amplitudes at 20 than at 10 degrees. In DD at 10 degrees no circadian rhythmicity in secretion was found in October or January, whereas at 20 degrees a circadian-like pattern was detected in pineals which were derived from animals reared at either 10 degrees or 20 degrees for 1 week prior to the experiment in October or January. The pineals in the DD experiment still responded to an additional 24-hr LD cycle at both temperatures even after 6 or more days. These results clearly reveal the influence of photoperiod and temperature on melatonin secretion of organ-cultured pineal glands. The existence of a circadian oscillator for melatonin secretion in the pineal gland of the white sucker is suggested.

Rhythmic melatonin secretion in different teleost species: an in vitro study.

The rhythmic production of melatonin is governed by intrapineal oscillators in all fish species so far investigated except the rainbow trout. To determine whether the latter represents an exception among fish, we measured in vitro melatonin secretion in pineal organs of nine wild freshwater and six marine teleost species cultured at constant temperature and under different photic conditions. The results demonstrate that pineal organs of all species maintain a rhythmic secretion of melatonin under light:dark cycles and complete darkness, and strongly suggest that most fish possess endogenous intrapineal oscillators driving the rhythm of melatonin production, with the exception of the rainbow trout.

Photoreceptor cells of the pike pineal organ as cellular circadian oscillators.

In the pike pineal, the rhythm of melatonin (MEL) secretion is driven by a population of cellular circadian oscillators, synchronized by the 24 h light/dark (LD) cycle. Because the pineal photoreceptor contains both the input and output pathways of the clock, this cell is likely to be a cellular circadian system by itself. To support this idea, we have dissociated and cultured pike pineal cells as well as purified photoreceptors. In culture, the pineal cells reassociated in follicles, surrounded by collagen fibres. At the electron microscopic level, they appeared well preserved. Total cells consisted mainly of photoreceptors and glia. Purified cells corresponded exclusively to photoreceptors. Under LD, MEL production was rhythmic. Under constant darkness (DD), the rhythm was well sustained for at least six 24 h cycles (tau = 24/27 h) with 1 x 10(6) total cells/well or below; with 2 x 10(6) total cells/well, a strong damping occurred towards high levels as soon as after the second cycle. At the density of 0.5 x 10(6) cells/well, purified photoreceptors produced less MEL than an equivalent amount of total cells. However, the pattern of the oscillations was similar to that observed with 2 x 10(6) total cells, i.e. a damping occurred rapidly. Decreasing the density to 0.125 x 10(6) photoreceptors/well resulted in a loss of homogeneity among replicates. The production of melatonin by single photoreceptors was monitored by means of the reverse haemolytic plaque assay. Both under LD and under DD, the number of photoreceptors releasing melatonin was higher during the (subjective) dark than during the (subjective) light. The results provide strong support to the idea that the pike pineal photoreceptor is a cellular circadian system. Expression of the oscillations seemed to depend on several factors, including cell to cell contacts between photoreceptors. There is indication that also MEL and glia might be involved.

Multiple circadian oscillators in the photosensitive pike pineal gland: a study using organ and cell culture.

The fish pineal organ contains typical and, in some species, modified photoreceptor cells involved in the photoperiodic control of melatonin production. In the majority of species studied, the rhythm in melatonin production is driven by an intra-pineal circadian oscillator synchronized by the light:dark cycle. In the present study, it is shown that the endogenous rhythm in melatonin release of superfused pike pineals maintained under constant darkness is expressed at temperatures of 19 degrees C, 20 degrees C, 25 degrees C, and 30 degrees C (period > 24 hr), but not at temperatures of 10 degrees C and 15 degrees C. Under constant darkness, pineal fractions containing either typical photoreceptors, modified photoreceptors, or both behaved like total organs. Dissociated pike pineal cells, cultured statically at 20 degrees C, expressed a high amplitude rhythm in melatonin secretion under a light:dark cycle. Under constant darkness, circadian oscillations, which appeared better sustained than in organ culture, were also observed. This study provides the first evidence that the rhythmic production of melatonin, by a fish pineal, is driven by a population of circadian oscillators or clocks. It is hypothesized that each typical and modified photoreceptor might be the locus of a circadian clock. Damping of the overall rhythm under constant darkness might reflect the desynchronization (uncoupling) between these clocks and/or damping of individual oscillators.

Rhythmic secretion of melatonin by the superfused pike pineal organ: thermo- and photoperiod interaction.

In the pineal organ of the pike (Esox lucius, teleost), the rhythmic production of melatonin by the photoreceptor cells is governed by a population of endogenous circadian oscillators, synchronized and entrained by the light/dark (L/D) cycle. Production of melatonin is inhibited by light and highly increased in the dark. In the present study, isolated and superfused pike pineals were exposed to a 24-hour temperature cycle of high (10 degrees C) or low (4 degrees C) amplitude, either under L/D, or under constant darkness. Under L/D, photoperiod is the dominant entraining stimulus for the melatonin secretion rhythm. It was high during the dark phase and low during the light phase, either under cold light/warm dark or under warm light/cold dark conditions. Under a warm light/cold dark cycle, the amplitude of the melatonin rhythm was reduced. In pineals cultured at 10 degrees C, a 20 degrees C temperature pulse potentiated or reduced the night-induced rise in melatonin production when applied, during the dark or during the preceding light phase, respectively. Under constant darkness, high- or low-amplitude temperature cycles could synchronize the rhythmic production of melatonin, which peaked with the high temperature. However, the shape of the oscillation could appear modified under warm subjective-L/cold subjective-D, depending on the experimental conditions. Finally, the rhythmic release of melatonin synchronized by a temperature cycle under constant darkness was no longer observed after removal of the external zeitgeber. The present study provides direct evidence that the pineal acts as a photothermotransducer.