Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Albert H. Meier


Circadian rhythms of hormone secretion and responsiveness to hormones were examined in the gulf killifish. The findings provide additional information to support the hypothesis that reproductive, metabolic and behavioral activities are regulated in part by a temporal interaction of circadian rhythms. These findings include: (1) Daily injections of drugs that influence serotonergic (5-hydroxytryptophan, 5-HTP) and dopaminergic (dihydroxyphenylalanine, DOPA) activities induce conditions appropriate to either summer or winter, and can determine the manner in which a fish reacts reproductively and metabolically to water temperature, the main environmental determinant. (2) Tryptophan-enriched diets stimulated increases in plasma triiodothyronine and somatomedin-C levels, dramatically stimulated growth and altered the circadian rhythms of plasma cortisol and thyroxine concentrations. Tryptophan probably exerts its major effects by acting on the neuroendocrine system. (3) Prolactin and growth hormone injections at several times of day induce variable somatomedin-C and thyroid hormone responses as a function of the time of day the injections were made, the time of blood sampling and the hormone assayed. The temporal interaction of circadian stimulus and response rhythms, especially prolactin, may well have major roles in supporting changing complexes of physiological and behavioral conditions associated with season. (4) There were marked daily variations in plasma thyroid and steroid hormone concentrations in male and female gulf killifish sampled at 3 separate days during the semilunar spawning cycle: at the GSI peak, at midcycle and at one day prior to the GSI peak. The phases of most of the hormone rhythms differed during the cycle indicating possible circadian neuroendocrine organization of the cycle.