Secretion of the Methoxyindoles Melatonin, 5-Methoxytryptophol, 5-Methoxyindoleacetic Acid, and 5-Methoxytryptamine from Trout Pineal Organs in Superfusion Culture: Effects of Light Intensity

Abstract

The teleost pineal organ is a photoreceptive endocrine organ that synthesizes the hormone melatonin through specialized intrapineal photoreceptor cells in a light-dependent manner. The present study investigated whether the methoxyindoles 5-methoxytryptophol (5-MTOL), 5-methoxyindoleacetic acid (5-MIAA), and 5-methoxytryptamine (5-MT) correspond to melatonin secretion synthesized and released from explanted, superfused pineal organs in response to direct light stimulation and whether their release is correlated with the level of dark adaptation. Melatonin release in superfusion cultures using Hank's buffer was clearly dependent on irradiance of the incident light and increased with decreasing light intensity from an average release of 1 ng/pineal/hr in light-adapted pineal glands to about 9 ng/pineal/hr in dark-adapted pineal glands. The maximal release could be further enhanced if Medium 199 was used as the superfusion medium. Although the other methoxyindoles showed a less clear response to light and their production was clearly dependent on the external medium, their mean secretion rate in the light-adapted state was considerably higher than that of melatonin. In Hank's superfusion culture, 5-MTOL production decreased with decreasing light irradiances, but 5-MT and 5-MIAA concentrations remained almost constant. Using Medium 199 as the superfusion culture medium, concentrations of 5-MTOL and 5-MT showed no changes with light intensity, but 5-MIAA production clearly increased with decreasing irradiances. Pargyline, a monoamine oxidase inhibitor, significantly reduced the secretion of 5-MIAA and 5-MTOL, but caused a dramatic increase of 5-methoxytryptamine concentrations. The production of melatonin was not affected. A slight decrease of 5-MTOL secretion was also observed after the addition of 1 μMeserine, an inhibitor of the melatonin deacetylase, to the superfusion medium, whereas melatonin release remained constant. These results indicate that the pineal gland of the trout releases the methoxyindoles 5-MTOL, 5-MIAA, and 5-MT in addition to melatonin. The release appears to be only partly light dependent. Furthermore, the present results suggest that methoxyindoles released from the trout pineal gland are independent from melatonin deacetylation.