HYPOTHALAMIC SECRETION OF THYROTROPHIN RELEASING HORMONE IS DECREASED IN MALE JAPANESE QUAIL EXPOSED TO LONG DAILY PHOTOPERIODS

Abstract

Adult male Japanese quail held under short daily photoperiods (8 h light: 16 h darkness; 8L : 16D) had significantly higher plasma concentrations of thyroid-stimulating hormone (TSH), tri-iodothyronine (T3) and thyroxine (T4) than did those kept under long days (16L : 8D). When given a single s.c. injection of 50 μg thyrotrophin releasing hormone (TRH) the birds held under both the 8L : 16D and 16L : 8D photoperiods showed rapid increases in their blood concentrations of TSH, T4 and T3, the amplitude of the TSH response of the birds exposed to 16L : 8D being particularly marked. These results suggest that, in the male quail, long daily photoperiods produce a hypothyroid state as a result of diminished TRH secretion. The synthetic and secretory capacities of the thyroid gland and pituitary thyrotrophs are apparently unimpaired by long days.