Photoperiod-induced phase-shifts of the endogenous clock controlling reproduction in the rainbow trout: a circannual phase-response curve.

Different groups of winter-spawning female rainbow trout that had been maintained under seasonally changing daylength and temperature were exposed to 2 months of continuous light at different times of the year. The same photoperiod produced advances in the time of spawning of up to 232 days and delays of up to 80 days, depending upon the timing of exposure in relation to the phase of the reproductive cycle. The proportion of fish spawning in each group varied from 18% to 100%, again dependent on the timing of exposure to continuous light. The photoperiod-induced changes in spawning time can be interpreted as phase-dependent phase-shifts of an endogenous circannual clock controlling maturation. It is proposed that long days, occurring earlier or later than they would under a natural photoperiod, were perceived as indications that the clock was running slow or fast, thus initiating corrective forward adjustments (advance phase-shifts) or backward adjustments (delay phase-shifts), respectively. Collectively, these responses can be described in the form of a circannual phase-response curve.

Melatonin rhythms in Atlantic salmon (Salmo salar) maintained under natural and out-of-phase photoperiods.

Diel changes in circulating melatonin were measured in juvenile Atlantic salmon, Salmo salar, maintained under natural and out-of-phase seasonal photocycles. Under natural daylengths of autumn, winter, spring, and summer circulating melatonin levels were inversely related to light intensity, with levels low during the day and high at night. The duration of the nocturnal increase in circulating melatonin was related to the duration of darkness, i.e., longer in winter than in summer. Under simulated seasonal photocycles circulating melatonin concentrations measured in August, October, and December were also elevated for the duration of darkness, irrespective of whether the photoperiods were synchronized or 6 months out-of-phase with the natural light and temperature cycles. Circulating melatonin also provided an accurate representation of the prevailing photoperiod in fish initially maintained on simulated natural photocycles, either synchronized or 6 months out-of-phase with the natural light cycle, and then held for 3 months on daylengths approximating the summer and winter solstices. Well-defined melatonin rhythms were always present, irrespective of time of year, photoperiod, and temperature. The amplitude of the nocturnal increase in circulating melatonin was similar in groups of fish maintained under simulated seasonal photoperiods 6 months out-of-phase with each other, but otherwise identical conditions, indicating that daylength per se did not influence the amplitude of the melatonin rhythm. The amplitude of the melatonin rhythm was slightly higher during the summer months, suggesting that temperature may modify circulating melatonin levels. These results demonstrate that circulating melatonin profiles always reflect the prevailing daylength and hence have the potential to provide the Atlantic salmon with accurate information on daily and calendar time, which could be utilised to time daily and seasonal events.

Central melatonin binding sites in rainbow trout (Onchorhynchus mykiss).

A combination of in vitro autoradiography and membrane homogenate receptor assays has been used to localize and characterized 2-[125I]iodomelatonin binding sites in the brain of the rainbow trout (Onchorhynchus mykiss). Specific 2-[125I]iodomelatonin binding, defined as that displaced by 1 microM melatonin, increased linearly with increasing protein concentration in membrane homogenates of whole trout brain. Specific binding was both time and temperature dependent and reversible in the presence of 1 microM melatonin. Binding was saturable at between 100-150 pM 2-[125I]iodomelatonin and Scatchard analysis of saturation isotherms revealed a dissociation constant (Kd) of 15.00 +/- 0.95 pM and a maximum receptor number (Bmax) of 42.35 +/- 2.70 fm/mg protein (n = 16). Addition of 10(-4) M GTP gamma S (an analogue of guanosine triphosphate) to saturation isotherms apparently reduced the Bmax by 75% on average with no apparent change in the affinity of the binding. Scatchard analysis of saturation isotherms generated from whole brain membrane homogenates of trout kept on long days (15 hr light:9 hr dark) and killed either during the midlight or middark phase showed no significant differences in either the Kd or the Bmax of 2-[125I]iodomelatonin binding, although a robust rhythm in melatonin concentration was confirmed in these fish. Displacement of 2-[125I]iodomelatonin binding with increasing concentrations of competing ligands gave an order of potency of 2-iodomelatonin > melatonin >> 5-HT. Localization of specific central 2-[125I]iodomelatonin binding in the rainbow trout showed high levels of binding associated with neuronal areas involved in the processing of visual signals, particularly the optic tectum and nucleus rotundus.(ABSTRACT TRUNCATED AT 250 WORDS)