Gut melatonin: A potent candidate in the diversified journey of melatonin research

After being discovered from the bovine pineal gland by Aaron Lerner and co-workers in the year 1958, various distinguished researchers have reported melatonin (5-methoxy-N-acetyl-tryptamine) from several extra-pineal sources, including the gastrointestinal tract (GIT). In the year 1974, Raikhlin and Kvetnoy first detected this molecule in the gastrointestinal tissue. Later, within the last 45 years, many renowned investigators found that the GIT is a rich source of melatonin, in addition to the pineal gland. In the carp gut, the estimation of Arylalkylamine-N-acetyltransferase (AANAT) mRNA/protein levels, which is the rate-determining enzyme for melatonin biosynthesis in the pineal gland, confirmed the endogenous synthesis of melatonin. The remarkable feature of the pineal gland melatonin is its rhythmic synthesis with a peak at dark-phase and lowest at light-phase in synchronization with seasonal environmental light-dark (LD) cycle. Recent studies on carp demonstrated that the melatonin concentrations and the AANAT protein intensities in different gut segments underwent significant daily fluctuations. However, compared to the melatonin rhythm in the pineal gland, the melatonin profiles in gut tissue displayed daily rhythm in parallel with the feeding cycle of the carp, irrespective of LD conditions of the environment. Notably, in carp, the temporal pattern of the gut melatoninergic system found to vary with the environmental non-photic signal(s), such as food entrainment factors (viz. availability of food, timing of food supply, number(s) of feed per day, quality of food) those act as the most dependable synchronizer(s) in daily rhythm characteristics of gut melatonin and AANAT. Thereby in this review, it appears meaningful to highlight the existing data on the mode of synthesis of melatonin in cells of the digestive tract, and most importantly, the regulation of its synthesis. Finally, in comparison with the dynamic actions of melatonin derived from the pineal gland, this review will lead to underline the role of gut-derived melatonin in a variety of physiological functions.

Development of sperm cryopreservation protocol of endangered spiny eel, Mastacembelus armatus (Lacepede 1800) for ex-situ conservation

The study was conducted to develop sperm cryopreservation protocol for the endangered spiny eel, Mastacembelus armatus as well as to use the cryopreserved sperm in breeding. Activation of sperm was evaluated by observing motility in different osmolalities of NaCl solution (48-319 mOsmol/kg) and found that sperm motility decreased with increasing osmolality of the NaCl solution and it was totally inhibited at 319 mOsmol/kg. The highest motility and swimming duration of sperm were observed as 14.5 ± 0.5 min and 4.75 ± 0.25 min at 128 mOsmol/kg and 67 mOsmol/kg, respectively. The concentration of sperm was found to be 1.75 × 10¹⁰ to 2.25 × 10¹⁰ cells/ml. Evaluation of toxicity of cryoprotectants of dimethyl sulfoxide (Me₂SO) and methanol to sperm was performed at 5, 10 and 15% concentrations during 5-30 min incubation period and found that cryoprotectants with 5 and 10% concentrations produced better motility up to 10 min incubation. In the selection of suitable diluent for cryopreservation of sperm, Alsever's solution with 10% Me₂SO performed best producing 91.67 ± 1.67% and 65 ± 2.89% equilibration and post-thaw motility, respectively. In breeding trials, sperm preserved with Alsever's solution plus Me₂SO produced more or less similar fertilization (82.25 ± 4.46%) and hatching (12 ± 0.65%) to those preserved with 0.9% NaCl solution plus Me₂SO as 79.88 ± 4.67% and 11 ± 0.46% fertilization and hatching respectively. Fresh sperm used as control produced 87.63 ± 3.6% and 14 ± 0.5% fertilization and hatching respectively. Both cryopreserved and fresh sperm-derived fry performed well but the control fry showed a significantly better (P < 0.05) growth. Fry derived from four diluents did not show any significant (P > 0.05) growth difference but Alsever's solution plus Me₂SO produced better results.

Melatonin: a potent candidate in the regulation of fish oocyte growth and maturation

Recent studies on several fish species, especially carp, implicated pineal hormone melatonin (N-acetyl-5-methoxytryptamine) as a potent candidate in the regulatory mechanism of seasonal reproduction. Under natural conditions, the temporal pattern of serum melatonin varied with daily light-dark cycle and the reproductive status of the fish as well. Carefully controlled study revealed that exogenous administration of melatonin may result in stimulation or inhibition or no influences at all on the gonadal functions depending on the reproductive status of fish. Cross-talk between the melatonin and ovarian steroid has been evident from in vitro study, in which melatonin accelerated the action of 17α,20β-dihydroxy-4-pregnen-3-one or maturation inducing hormone (MIH) on meiotic cell cycle resumption in carp oocytes by formation of maturation promoting factor (MPF) - a complex of two proteins, cyclin B and cyclin dependant kinase Cdk1. While several lines of evidence suggest melatonin effects on hypothalamo-hypophyseal-gonadal axis, localization and dynamics of a 37-kDa melatonin receptor protein in carp oocytes argued in favor of extra-hypothalamic direct action of melatonin on fish reproduction. A recent study in carp indicated that influences of an identical regimen of photoperiods in different parts of annual cycle on ovarian functions vary in relation to the profiles of serum melatonin, but not to any rhythm parameters of MT1 or MT2 receptors on the gonad or brain. The purpose of this short review is to bring together the current knowledge on the biological effects of melatonin on fish reproduction mainly focusing the recent findings on carp.

Effect of photoperiod on growth and feed conversion of juvenile wild carp, Cyprinus carpio

Because of the effect of photoperiod on physiological and biochemical processes in fish, this study aimed to evaluate the effect of manipulated photoperiod on growth, feed conversion and survival of wild carp, Cyprinus carpio. Fish received six photoperiod regimes (light:dark cycle) including: natural photoperiod (control), 24L:0D, 16L:8D, 12L:12D, 8L:16D and 0L:24D by the three replications. Regulated photoperiods as a 16L:8D or 12L:12D light/dark cycle significantly improved growth rate and food conversion ratio of wild carp.

Changes in Mesopotamian spiny eel, Mastacembelus mastacembelus (Bank & Solender in Russell, 1794) (Mastacembelidae) milt quality during a spawning period

This study was carried out to determine the milt quality in male Mesopotamian spiny eel (Mastacembelus mastacembelus; Bank & Solender in Russell, 1794) during spawning season. Review was performed using 58 M. mastecembulus males captured from Atatürk Dam Lake in Turkey. Milt samples were collected and evaluated daily during sampling period. In collected milt, volume of milt, sperm motility percentage, the duration of forward motility, sperm concentration, milt production and milt pH were evaluated. On June 10, 2005, 72% of the males were spermiating, which increased to 100% in the middle of June and dropped to 72% by late June. Sperm motility percentage showed a significant improvement after beginning of spermiation (P<0.001). Sperm concentration changed significantly (P<0.001) in milt collected during sampling period. Duration of sperm motility increased significantly (P<0.001) from beginning (8.22 min) through end (37.33 min) of spermiation. Milt production was maximal on June 15th, decreasing from a mean value of 3.12-0.46 x 10(9)/ml. The high pH in milt was observed during the week of spermiation (P<0.001). In conclusion, the results show that season has a significant influence on semen quality in male M. mastecembulus corresponding to spawning season in hot arid. Milt of good quality was especially collected in the middle of June. Spawning season in this fish species was found to be short.

Implication of melatonin in oocyte maturation in Indian major carp Catla catla

Importance of melatonin (N-acetyl-5-methoxytryptamine) in the regulation of oocyte maturation has been studied in a carp Catla catla. Melatonin secretory cells were immunocytochemically localized only in the end vesicle. Diurnal and seasonal studies indicated that the serum levels of melatonin exhibit a minimum diurnal value in the mid-day of all seasons, but the peak value is attained either at mid-night or just before the onset of light. Moreover, highest seasonal value of melatonin was noted in the post-spawning phase. Administration of melatonin at different doses (25, 50, or 100 mug/100 g body weight) for 1, 15, or 30 days resulted in either pro- or anti-gonadal effects depending on the reproductive seasons. In vitro study revealed that incubation of oocytes with melatonin 4h prior to addition of MIH in the medium led to an accelerated rate of oocyte maturation through the formation of a complex of two proteins (MPF), cyclin B and cyclin dependant kinase, Cdc2. Moreover, melatonin pre-incubation considerably increased MIH stimulation of histone H1 phosphorylation as compared to MIH alone. Taken together, gathered information promotes the idea of a physiological role of melatonin in the maturation of oocytes in Catla catla.