Thermal responses of achromatic ganglion cells in the photosensory pineal organ of rainbow trout oncorhynchus mykiss

Transient Receptor Potential-Vanilloid (TRPV1-TRPV4) Channels in the Atlantic Salmon, Salmo salar. A Focus on the Pineal Gland and Melatonin Production

Fish are ectotherm, which rely on the external temperature to regulate their internal body temperature, although some may perform partial endothermy. Together with photoperiod, temperature oscillations, contribute to synchronizing the daily and seasonal variations of fish metabolism, physiology and behavior. Recent studies are shedding light on the mechanisms of temperature sensing and behavioral thermoregulation in fish. In particular, the role of some members of the transient receptor potential channels (TRP) is being gradually unraveled. The present study in the migratory Atlantic salmon, Salmo salar, aims at identifying the tissue distribution and abundance in mRNA corresponding to the TRP of the vanilloid subfamilies, TRPV1 and TRPV4, and at characterizing their putative role in the control of the temperature-dependent modulation of melatonin production-the time-keeping hormone-by the pineal gland. In Salmo salar, TRPV1 and TRPV4 mRNA tissue distribution appeared ubiquitous; mRNA abundance varied as a function of the month investigated. In situ hybridization and immunohistochemistry indicated specific labeling located in the photoreceptor cells of the pineal gland and the retina. Additionally, TRPV analogs modulated the production of melatonin by isolated pineal glands in culture. The TRPV1 agonist induced an inhibitory response at high concentrations, while evoking a bell-shaped response (stimulatory at low, and inhibitory at high, concentrations) when added with an antagonist. The TRPV4 agonist was stimulatory at the highest concentration used. Altogether, the present results agree with the known widespread distribution and role of TRPV1 and TRPV4 channels, and with published data on trout (Oncorhynchus mykiss), leading to suggest these channels mediate the effects of temperature on S. salar pineal melatonin production. We discuss their involvement in controlling the timing of daily and seasonal events in this migratory species, in the context of an increasing warming of water temperatures.

Secretion of methoxyindoles from trout pineal organs in vitro: indication for a paracrine melatonin feedback

Synthesis and release of the pineal hormone melatonin is in all vertebrates primarily regulated by the light/dark cycle. In pineal organs of teleost fish, like in other non-mammalian vertebrates, melatonin formation is regulated by a direct photoreception of the pineal organ. We performed measurements in explanted, perifused pineal organs of the rainbow trout, Oncorhynchus mykiss, to examine whether melatonin can influence its own production. For this purpose we have continuously perifused isolated pineal organs under light- and dark-adapted conditions and measured the release of melatonin and other methoxy-indoles by HPLC with electrochemical detection. Addition of 2-iodomelatonin to the perifusate in a concentration of 2 ng/ml significantly inhibited melatonin release in light-, as well as in dark-adapted organs. The release of 5-methoxytryptamine and 5-methoxytryptophol was also significantly reduced in light-adapted organs. These results indicate that extracellular melatonin may act as a paracrine or autocrine feedback signal and may be important for the illumination-dependent melatonin production.

Benzodiazepines influence melatonin secretion of the pineal organ of the trout in vitro

The effect of benzodiazepines (BZP) on melatonin release was investigated in the pineal gland of the rainbow trout, Oncorhynchus mykiss, maintained under in vitro perifusion culture conditions. Melatonin and the methoxyindoles 5-methoxytryptophol (5-MTOL), 5-methoxyindoleacetic acid (5-MIAA), and 5-methoxytryptamine (5-MT) were determined directly in samples of the superfusion medium by HPLC with electrochemical detection. Melatonin release was significantly increased by addition of diazepam and clonazepam in a dose-related and reversible manner. The effects of benzodiazepines were more pronounced in light-adapted pineal organs, when melatonin secretion is low, than under scotopic conditions. When the perifusion medium was replaced by a medium containing low calcium, high magnesium concentrations, melatonin release was considerably decreased by 70% in light-adapted and 20% in dark-adapted pineal organs. Addition of diazepam to low Ca2+, high Mg(2+)-medium reversed the decrease of melatonin release and produced a clear rise in its secretion rate. Addition of the BZP antagonist flumazenil to the perifusion medium slightly decreased melatonin release in the light- and dark-adapted state, whereas the peripheral receptor antagonist PK 11195 did not alter melatonin release. The effect of diazepam is reduced by simultaneous addition of flumazenil to the superfusion medium, suggesting that the effects of diazepam are receptor-mediated. The methoxyindoles 5-MTOL, 5-MIAA, and 5-MT showed no significant changes of their release pattern after diazepam application in light- and dark-adapted pineal organs. These results suggest that BZP can influence melatonin production and release by an intrapineal action possibly on the melatonin synthesizing photoreceptor cell.