Photoperiod-dependent negative feedback effects of thyroid hormones in Fundulus heteroclitus

In Fundulus heteroclitus, an annual cycle in the response of the thyroid to ovine thyroidstimulating hormone (oTSH) is characterized by maximal thyroxin (T 4) secretion in midwinter and minimal T 4 secretion in summer. Four daily injections of oTSH, given in winter caused serum T 4 to plateau at elevated levels for several days, while in summer fish similar treatment resulted in far more fluctuating titers of serum T 4; maximum levels were similar in both groups. The difference in sustenance rather than magnitude of Peak T 4 led to an examination of the negative feedback effects of thyroid hormones as they might relate to these seasonal changes. Radioiodine uptake by thyroid follicles served as a simple, but effective bioassay for endogenous TSH. Fish collected in summer were more sensitive to negative feedback of T 3 than those collected in winter; feedback effects of T 4 in the two groups were not significantly different. The effects of specific photoperiods on negative feedback sensitivity to T 3 and T 4 were also tested. Exposure of winter fish for one month to long days (LD 14:10) enhanced the degree of reduction of iodine uptake caused by T 4 in the aquarium water (10 μg/100 ml). Negative feedback in short-day (LD 8:16) winter fish was not demonstrated. It is concluded that long days increase and short days diminish the negative feedback sensitivity of the hypothalamus-pituitary axis to thyroid hormones in F. heteroclitus. Such photoperiodically induced changes may act to aid in the year-round maintenance of T 4 levels necessary for seasonal adaptation and survival.