Incorporation, distribution, and metabolism of polyunsaturated fatty acids in the pineal gland of rainbow trout (Oncorhynchus mykiss) in vitro

The in vitro incorporation of H-3-radio-labeled arachidonic (20:4n-6) and docosahexaenoic (22:6n-3) acids by the photosensitive trout pineal gland was visualized using photon and electron microscopy. After 6 hr of incubation, H-3-20:4n-6 appeared distributed in photoreceptors, as well as in glial cells, whereas H-3-22:6n-3 was preferentially taken up by photoreceptors, mainly in the apical part (including the photoreceptive outer segment). Radioactivity was mainly seen over membranes of glia when the incorporation of H-3-20:4n-6 was followed by 12-18 hr of chase. We also report differences in the incorporation of C-14-radio-labeled linoleic (18:2n-6), linolenic (18:3n-3), eicosapentenoic (20:5n-3), 20:4n-6, and 22:6n-3 acids into the lipids in glands cultured under different lighting regimes. Phosphatidylcholine and triacylglycerols contained most of the radio-labeled polyunsaturated fatty acids (PUFA) incorporated. The proportion of incorporated 20:4n-6 recovered in phosphatidylinositol was always significantly higher than that found with the other PUFA. At least 10% of radioactivity from each incorporated substrate, except 22:6n-3, was recovered in elongation products. It is concluded that the pineal gland of the trout can assimilate exogenous PUFA into cellular lipids in vitro, in a manner consistent with our previous in vivo findings. In terms of lipid composition, the trout pineal gland resembles more the vertebrate retina than the rat pineal gland. This might be related to the loss of direct photosensitivity of the mammalian pineal. Together, the differences reported herein between 22:6n-3 and 20:4n-6 suggest the former plays an important role in the phototransduction process, whereas the latter might be more specifically involved in the production of phosphoinositide-derived second messengers.