P. Gaildrat et al., DAILY AND CIRCADIAN VARIATIONS IN 2-[I-125]-IODOMELATONIN BINDING-SITES IN THE PIKE BRAIN (ESOX-LUCIUS), Journal of neuroendocrinology, 10(7), 1998, pp. 511-517
The fish pineal organ, through its 24 h rhythmic release of melatonin,
acts as a transducer of the photoperiod, influencing different physio
logical functions (e.g. reproduction, growth). We have investigated th
e binding of 2-[I-125]iodomelatonin to whole brain membrane preparatio
ns from pikes (Esox lucius L., teleost) maintained for 24-48 h under d
ifferent photoperiodic conditions. Specific binding was stable, revers
ible, saturable and sensitive to the presence of a GTP analogue. Scatc
hard analysis revealed one class of binding sites. Displacement experi
ments suggested the presence of two components with affinities in the
femtomolar and nanomolar range of concentrations, respectively. The B-
max exhibited monophasic nycthemeral variations, with higher values at
the light-to-dark transition (34.0 +/- 4.5 fmol/mg protein) and low v
alues during the second half of night (10.0 +/- 1.0 fmol/mg protein).
Under the same conditions, the K-D exhibited biphasic variations: valu
es were low during daytime and at the middle of the dark phase (approx
imately 100 pM); they were high at the beginning (approximately 225 pM
) and at the end (approximately 330 pM) of the night. These variations
were maintained under constant light (LL) and constant darkness (DD).
Thus, the variations in the number and affinity of the melatonin bind
ing sites were controlled by circadian oscillators, synchronized by th
e photoperiod, The nature of these oscillators is not known. Therefore
, in fish, we suggest that the photodependent effects of melatonin res
ult from the circadian variations of both its production by the pineal
and its binding sites in the brain.