Effects of water salinity on melatonin levels in plasma and peripheral tissues and on melatonin binding sites in European sea bass (Dicentrarchus labrax).

Sea bass is an euryhaline fish that lives in a wide range of salinities and migrates seasonally from lagoons to the open sea. However, to date, the influence of water salinity on sea bass melatonin levels has not been reported. Here, we evaluated the differences in plasma and tissue melatonin contents and melatonin binding sites in sea bass under four different salinity levels: seawater (36 per thousand), isotonic water (15 per thousand), brackish water (4 per thousand) and freshwater (0 per thousand). The melatonin content was evaluated in plasma, whole brain, gills, intestine and kidney, while melatonin binding sites were analyzed in different brain regions and in the neural retina. Plasma melatonin levels at mid-dark varied, the lowest value occurring in seawater (102 pg/mL), and the highest in freshwater (151 pg/mL). In gills and intestine, however, the highest melatonin values were found in the seawater group (209 and 627 pg/g tissue, respectively). Melatonin binding sites in the brain also varied with salinity, with the highest density observed at the lower salinities in the optic tectum, cerebellum and hypothalamus (30.3, 13.0, and 8.0 fmol/mg protein, respectively). Melatonin binding sites in the retina showed a similar pattern, with the highest values being observed in freshwater. Taken together, these results reveal that salinity influences melatonin production and modifies the density of binding sites, which suggests that this hormone could play a role in timing seasonal events in sea bass, including those linked to fish migration between waters of different salinities for reproduction and spawning.

Melatonin concentrations during larval and postlarval development of gilthead sea bream Sparus auratus: more than a time-keeping molecule?

In this study, melatonin (MEL) and thyroxine (T(4)) concentrations were measured during larval and postlarval development of gilthead sea bream Sparus auratus Hormones were measured in whole bodies of larvae or the head and trunk of postlarvae after 67 days of exposure to constant light, 24L:0D, constant darkness, 0L:24D or 12L:12D and in the plasma of 6 month juveniles kept under the 12L:12D, 0L:24D and 24L:0D regimes. High MEL concentrations in larvae suggested a distinct role of MEL in early organogenesis and development of S. auratus. In larvae, the gastro-intestinal tract seemed to be an important extrapineal and extraretinal source of MEL. No endogenous rhythm of MEL synthesis was demonstrated in 67 day larvae; however, in 6 month juveniles, it was evident. At early ontogenesis of S. auratus, the role of MEL is probably related mostly to the control of development and protection against free radicals, whereas its action as a time-keeping molecule develops later. The increase in T(4) concentration during the S. auratus larva-juvenile transition, i.e. between 50 and 70 days post-hatch, which was observed concurrently with the decrease of MEL concentration, may suggest an inverse relationship between T(4) and MEL.

Melatonin synthesis under calcium constraint in gilthead sea bream (Sparus auratus L.).

Brain or blood plasma melatonin was analysed as a measure for pineal melatonin production in sea bream. Access to calcium was limited by diluting the seawater to 2.5 per thousand and removing calcium from the diet or by prolonged feeding of vitamin D-deficient diet. Interactions/relations between melatonin and calcium balance and the hypercalcemic endocrines PTHrP and calcitriol were assessed. Restricting calcium availability in both water and diet had no effect on plasma melatonin, but when calcium was low in the water or absent from food, increased and decreased plasma melatonin was observed, respectively. Fish on a vitamin D-deficient diet (D- fish) showed decreased plasma calcitriol levels and remained normocalcemic. Decreased brain melatonin was found at all sampling times (10-22 weeks) in the D- fish compared to the controls. A positive correlation between plasma Ca2+ and plasma melatonin was found (R(2)=0.19; N=41; P <0.01) and brain melatonin was negatively correlated with plasma PTHrP (R(2)=0.78; N=4; P <0.05). The positive correlation between plasma levels of melatonin and Ca2+ provides evidence that melatonin synthesis is influenced by plasma Ca2+. The decreased melatonin production in the D- fish points to direct or indirect involvement of calcitriol in melatonin synthesis by the pineal organ in teleosts. The hypercalcemic factors PTHrP and calcitriol appeared to be negatively correlated with melatonin and this substantiates an involvement of melatonin in modulating the endocrine response to cope with hypocalcemia. It further points to the importance of Ca2+ in melatonin physiology.

Influence of opioids on LH secretion in gilts during the estrous cycle.

The effect of endogenous opioid peptides (EOP) on LH secretion is variable during different physiological states. A series of experiments concerning the role of EOP on LH secretion in cyclic gilts was performed. They were comprised of (1) an administration of an opioid antagonist or agonist in gilts during the estrous cycle and in ovariectomized (OVX) gilts in which the LH surge was induced with estradiol benzoate (EB) and (2) in vitro studies on GnRH release from the stalk median eminence (SME) of cyclic gilts and OVX estrogen and progesterone primed gilts in response to naloxone (NAL). Naloxone and met-enkephalin analogue (FK 33-824) administration as a single independent injections did not affect LH secretion during the early (Day 16) or late (Day 19 or 20) follicular phase. However, continuous infusion of FK 33-824 for 4 h decreased LH secretion during the infusion period on Day 19 of the estrous cycle. Morphine also exerts an inhibitory effect on the EB-induced LH surge during the positive feedback phase (60-64 h after EB administration) in OVX gilts. On the contrary, NAL infusion in OVX gilts during the negative feedback phase (30-34 h after EB administration) did not alter LH secretion. A single injection of FK 33-824 in luteal phase gilts decreased the number of LH pulses for a 3 h period. This allows to hypothesize that EOP participates in the regulation of pulsatile LH secretion in pigs during the luteal phase. In vitro studies indicate that influence of EOP on LH secretion also takes place at the SME level. GnRH efflux from the SME of gilts during the luteal and late follicular phases was augmented in the presence of NAL. Unexpectedly, the priming of OVX gilts with estrogens caused the highest increase in GnRH release from the SME in vitro in response to NAL. These results confirm the variety of functional links between the opioid system and LH secretion in gilts during different stages of the estrous cycle.