Melatonin - Direct effects on gonadotrope cells in the fish pituitary?

Today, three major bottlenecks prevent efficient development and cultivation of marine species, namely improved quality of juvenile fish, improved disease control, as well as problems related to the control of sexual maturation. In this project I will investigate basic mechanisms controlling sexual maturation. Through puberty, increased activity in the brain-pituitary-gonadal axis govern the transition from juvenile to adult. From the brain, maturation is controlled through the signal molecule Gnrh, which in turn stimulates the production and release of the two peptide hormones Fsh and Lh. These two hormones are produced in the pituitary and responsible for controlling different stages of egg and sperm production. Nutritional status and external environmental factor such as temperature and photoperiod helps in the regulation and fine-tuning of maturation. In particular, at northern (and southern) latitudes where there is great variation in day and night cycles throughout the year, light plays an important role. Melatonin is the hormone that translates day and night cycle to internal physiological processes. During dark, the brain produces melatonin. However, when photosensitive cells in the eye, and also in fish pineal gland located close to the skull, are stimulated by light melatonin levels will again decrease. Melatonin may be either stimulatory or inhibitory, depending on which melatonin receptor a cell expresses. Today, artificial light is typically used to expose maturation of various fish species. Based on these observations, I want to examine how melatonin along with Gnrh affects production and release of Fsh and Lh from the pituitary gland. Common to cells which stores peptide hormones is that they use calcium to release the hormone into the bloodstream. How the cell regulate calcium signal plays a crucial role in hormone secretion, and thus the maturation process. I want to find out how melatonin may modulate calcium signal in Fsh-and Lh-producing cells. The cells can regulate their calcium level by regulating the electrical activity over the cell membrane. In my studies, I found that the electrical activity of these two cells that produce and release Fsh and Lh varies throughout the day. In particular, I have seen that Lh producing cells have the highest activity in the middle of the day, in time where Lh release is known to be at its maximum. In addition, I have found that the Lh producing cell responds differently to Gnrh throughout the day. By adding melatonin, I could reduce the electrical response to Gnrh and replicate the response observed at night and early morning. Further studies will be important to determine the causes of how melatonin reduces the activity of Lh producing cells

Improved control of sexual maturation is essential for further development of intensive fish farming, both in terms of precocious maturation in the production line, and timing of ovulation in the brood stock. This project aims to generate novel knowledge to face the future demands of efficient aquaculture by elucidating molecular mechanisms behind sexual maturation. In particular, the project will investigate how ion channels controlling excitability and hormone release in Fsh- and Lh- producing cells are regulated by melatonin. Melatonin serves as a mediator of environmental cues (mostly photoperiod and temperature) to endogenous signals that regulate the HPG-axis. In mammals there is a direct effect of melatonin on gonadotrope cells, with melatonin binding leading to inhibition of Gnrh-induced release of luteinizing hormone. Single-cell Ca2+ and electrophysiological recordings revealed a reduction in gonadotropin release due to melatonin induced inhibition of calcium channels and intracellular signaling. It is not clear if melatonin can act as a direct regulator on gonadotrope cells in fish, as it does in mammals. Moreover, the mechanism for melatonin effects on mammalian gonadotrope cells currently is not known. In this project, I will test the hypotheses that melatonin directly regulates gonadotrope cells, and whether this would be exerted through regulation of specific ion channels. If we are able to dissect the response of melatonin directly on gonadotropes and understand how this is modulated through development we hope that this can lead to target specific treatments to reduce the incidence of precocious puberty in farmed fish.