Ocular melatonin rhythms in teleost fish.

Melatonin (N-acetyl-5-methoxytryptamine) is synthesized in the pineal organ and the retina of vertebrates. In some teleost species, ocular melatonin levels can exhibit a circadian periodicity with elevated levels during the dark phase under light-dark (LD) cycles and this periodicity can persist even under constant dark (DD) cycles. However, reversed melatonin profiles and an absence of circadian ocular melatonin rhythms have also been reported.

Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou).

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8).

Melatonin binding sites in the brain of European sea bass (Dicentrarchus labrax).

Characteristics, day-night changes, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) modulation, and localization of melatonin binding sites in the brain of a marine teleost, European sea bass Dicentrarchus labrax, were studied by radioreceptor assay using 2-[(125)I]iodomelatonin as a radioligand. The specific binding to the sea bass brain membranes was rapid, stable, saturable and reversible. The radioligand binds to a single class of receptor site with the affinity (Kd) of 9.

Variations in plasma melatonin levels of the rainbow trout (Oncorhynchus mykiss) under various light and temperature conditions.

Daily variations in plasma melatonin levels in the rainbow trout Oncorhynchus mykiss were studied under various light and temperature conditions. Plasma melatonin levels were higher at mid-dark than those at mid-light under light-dark (LD) cycles. An acute exposure to darkness (2 hr) during the light phase significantly elevated the plasma melatonin to the level that is comparable with those at mid-dark, while an acute exposure to a light pulse (2 hr) during the dark phase significantly suppressed melatonin to the level that is comparable with those at mid-light.

In vitro photic entrainment of the circadian rhythm in melatonin release from the pineal organ of a teleost, ayu (Plecoglossus altivelis) in flow-through culture.

The effects of light on the circadian rhythm in melatonin release from the pineal organ of a teleost, ayu (Plecoglossus altivelis) were investigated in flow-through culture. Under the reversed light-dark (LD) cycle, the melatonin rhythm phase shifted as compared with those under the normal LD cycle. This phase shift persisted even under constant darkness (DD).

Circadian variations of melatonin binding sites in the goldfish brain.

Daily and circadian variations in melatonin binding sites in the brain of goldfish (Carassius auratus) were examined by radioreceptor assay. Under light-dark cycles of 12:12 h, the density (B(max)) of melatonin binding sites in the brain exhibited daily variations with a peak and a trough seen around the light offset and 2 h before light onset, respectively. The affinity (K(d)) exhibited no variation.

The effect of social interactions on circadian self-feeding rhythms in rainbow trout Oncorhynchus mykiss Walbaum.

Circadian feeding rhythms have been revealed in several fish species, but whether or not social interactions influence the expression of the rhythms remains largely unexplored. This paper reported such an exploration in rainbow trout. The experiment was conducted in two consecutive stages in two adjacent insulated rooms.