Melatonin Modulates Secretion of Growth Hormone and Prolactin by Trout Pituitary Glands and Cells in Culture

Abstract

In Teleost fish, development, growth, and reproduction are influenced by the daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated the pineal gland mediates the effects of these external factors, most probably through the rhythmic production of melatonin. The present investigation was aimed at determining whether melatonin acts directly on the pituitary to control GH and prolactin (PRL) secretion in rainbow trout. We show that 2-[125I]-iodomelatonin, a melatonin analog, binds selectively to membrane preparations and tissue sections from trout pituitaries. The affinity was within the range of that found for the binding to brain microsomal preparations, but the number of binding sites was 20-fold less than in the brain. In culture, melatonin inhibited pituitary cAMP accumulation induced by forskolin, the adenyl cyclase stimulator. Forskolin also induced an increase in GH release, which was reduced in the presence of picomolar concentrations of melatonin. At higher concentrations, the effects of melatonin became stimulatory. In the absence of forskolin, melatonin induced a dose-dependent increase in GH release, and a dose-dependent decrease in PRL release. Melatonin effects were abolished upon addition of luzindole, a melatonin antagonist. Our results provide the first evidence that melatonin modulates GH and PRL secretion in Teleost fish pituitary. Melatonin effects on GH have never been reported in any vertebrate before. The effects result from a direct action of melatonin on pituitary cells. The complexity of the observed responses suggests several types of melatonin receptors might be involved.

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

“…Clearly, our results indicate a positive correlation between a (presumedly) enhanced PRL activity in DSW and observed enhanced melatonin production. This in vivo result does not corroborate the observation by Falcon et al (2003) and suggests multivariable control; the increase in PRL in response to DSW exposure might over rule the inhibition of a PRL cell response to melatonin as observed in vitro.…”

“…An important factor in adaptation to hypo-osmo tic and hypocalcic conditions in euryhaline fishes is prolac tin (PRL; Flik et al, 1994), a hypercalcemic hormone in fish that is well-known for its key role in the control of low salinity adaptation. Falcon et al (2003) showed that melatonin reduced PRL secretion in cultured rainbow trout pituitary gland cells and provided the first evidence that melatonin modulates the secretion of PRL in teleosts. Clearly, our results indicate a positive correlation between a (presumedly) enhanced PRL activity in DSW and observed enhanced melatonin production.…”

Diurnal variation in phagocytic activity of splenic phagocytes in freshwater teleost Channa punctatus: melatonin and its signaling mechanism

“…Phagocytes incubated in medium alone for 90 min were considered as control. The concentration of melatonin for time-dependent experiment was determined based on literature available on in vitro effect of melatonin on fish leukocytes and pituitary cells (Falcón et al 2003, Cuesta et al 2007.…”

Circadian cycle of digestive enzyme production at fasting and feeding conditions in Nile tilapia, Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae)

“…The photoperiod is the most important factor that entrains animal rhythms, including enzymatic secretion cycles, tied to the daily melatonin rhythms (Bromage et al 2001). Fish exhibit photoperiod-dependent circadian cycles of several processes, including locomotion (zebrafi sh; Zhdanova et al 2001), development (zebrafi sh; Danilova et al 2004), growth (rainbow trout; Falcón et al 2003), reproduction (salmon; Amano et al 2000Amano et al , 2004Senegal sole;Vera et al 2007), control and regulation of reproductive hormones and melatonin (European seabass; Bayarri et al 2004), and digestive enzyme activity (Yúfera et al 2012). Also, they have oscillators and circadian photo transduction drag mechanisms, suggesting that circadian pacemaker functions can be spread throughout the animal (zebrafi sh; Cahill 2002).…”