Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Victor De Vlaming
Robert H. Fitts
Daily oscillations in endocrine, metabolic and behavioral patterns occur in all vertebrate group9. The existence and significance of endocrine and physiological circadian rhythms has only recently begun to receive attention (see below). The significance of daily rhythms is to provide temporal physiological organization. Although timing mechanisms for diurnal rhythms reside in individual cells, the cellular rhythms must be synchronized to yield organismal rhythms. The neuroendocrine system plays an important role in synchronizing cellular oscillations into organismal rhythms which are appropriately integrated with environmental cycles. For example, circadian oscillations in the endocrine system are crucial in the control of vertebrate reproduction and metabolism. The time of day (with respect to the onset of the light phase) when a particular hormone titer peaks, and the phase relationships among hormone rhythms are of particular significance in the control of physiological functions. Centers which are responsible for the control of these endocrine circadian rhythms have not been clearly established. Biological rhythms are frequently entrained (or synchronized) by environmental oscillations (e.g., light-dark cycles). This entrainment allows the animals to organize and coordinate physiological functions with respect to time and seasonal environmental changes.
Many biological rhythms are synchronized by light-dark cycles. This implies that a photoreceptor system is involved in the circadian organization of an animal's physiology. Therefore, the eyes or an extra-optic photoreceptor or both must be involved in the entrainment of physiological rhythms to light-dark cycles. Extra-optic photoreception in nonmammalian vertebrates may involve the pineal organ or other photosensitive regions of the brain.
The pineal organ is an outgrowth of the roof of the diencephalon and is present in all vertebrate species. Considerable evidence indicates that the teleostean pineal is a photoreceptive organ and an endocrine gland. Therefore the teleostean pineal organ may function as a photoreceptor and/or a neuroendocrine transducer of photoperiod information. The cyclic nature of pineal chemistry implies that this organ is also important in the circadian organization of an animal. Some studies imply that the teleostean pineal participates in regulating reproductive function of fishes. Virtually nothing is known about the relationship of teleostean pineal to metabolism and its possible role in influencing biological rhythms. The proposed study is designed to elucidate the potential role of the pineal and retinal pathways in influencing metabolism (levels and daily variations of specific metabolites) and endocrine rhythms in fish. More specifically, the following questions will be addressed:
(1) Do specific parameters of carbohydrate and lipid metabolism in goldfish undergo diurnal variations? The specific parameters will include both tissue (liver glycogen , lipid) and plasma (glucose, total lipid , cholesterol , organophosphate, protein) metabolites. Experiments will be designed to test whether the levels and/or the daily variations of these metabolites are affected by photoperiod-temperature interactions. These studies are essential as photoperiod effects must be established before determining if the pineal organ or retinal pathways are involved in photoperiod mediation. The ability of feeding time and seasonal changes to alter the levels and rhythms of metabolites will also be determined. These studies should provide basic information on carbohydrate and lipid metabolism in teleosts. Little information is available regarding photoperiod, temperature and seasonal effects on metabolite levels in fish. If the levels of these metabolites vary throughout the day, anyone studying these parameters must be aware of this cycling or the data will be potentially biased.
(2) Does the pineal influence carbohydrate and lipid metabolism in teleosts? The role of the pineal in teleost metabolism will be assessed by studying the effects of pineal removal on levels and daily variations of liver and plasma metabolites (see above). Experiments will also be designed to determine if the pineal influences metabolism directly or by mediating photoperiod information. The ability of retinal pathways to mediate photoperiod information on liver and plasma metabolites will also be studied. Plasma corticoid titers will be measured in these experiments as corticoids have a prominent role in lipid and carbohydrate metabolism. Hopefully this will provide some insight into the mechanism by which light and the pineal influence metabolism.
(3) Is the pineal involved in the circadian organization of teleost metabolism? Experiments designed to study the effect of pineal removal on the daily variations of metabolites should determine whether the pineal is involved in the circadian organization of goldfish. The pineal may be involved in physiological timing even though it does not transduce photoperiod information. Results from these experiments should demonstrate whether or not the pineal is a "cog" in the timekeeping mechanism.