Abstract
Pineal photoreceptor cells produce the neurohormone melatonin, a major "Zeitgeber" of the organism. This compound is involved in the control of development, growth, sexual maturation, and seasonal reproductive cycles. The present study reports that the photosensitive pineal organ and the retina of the trout express a 3.5-kb mRNA corresponding to the estradiol-17β receptor. The effects of estradiol-17β on melatonin production by cultured pineal photoreceptor cells were also studied. Under a light/dark (LD:12/12) cycle, these cells maintained a rhythmic secretion of melatonin, with higher amounts being released during the dark phase. The amplitude of the rhythm tended to increase with time in culture. Application of estradiol-17β during the dark phase of an LD cycle (i.e., for 12 hr) affected melatonin release in a dose-dependent manner; low concentrations (10 -10 to 10 -8 mol/liter) were inhibitory, whereas high concentrations (over 10 -7 mol/liter) were stimulatory (when compared to the values obtained at 10 -9 mol/liter). When estradiol-17β was given continuously for several 24-hr LD cycles, the inhibitory effect observed during the first dark phase disappeared in subsequent dark periods. In the presence of estradiol-17β, at concentrations ranging from 10 -10 to 10 -5 mol/liter, a high amplitude rhythm in melatonin secretion returned more rapidly than in controls. The present results suggest that estradiol-17β receptors are expressed in the fish pineal and retina and that estradiol-17β modulates melatonin secretion by cultured pineal photoreceptors. Sex hormones seem therefore to exert a control on the pineal. Pineal photoreceptors appear as multieffector cells which transduce information from both physical (photoperiod) and internal (chemical) factors.
Published Version
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