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 microgram 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.

Some Effects of Photoperiod on the Pituitary/Thyroid Axis of Castrated Cockerels

“…The TRH decrease in Coturnix with long photoperiod (Almeda, 1982) appears to contradict the finding of an increased thyroid activity, but it is in agreement with our findings of decreased pituitary activity in photostimulated cockerels. Further investigation of this problem should include measurement of TSH during long photoperiod.…”

“…Consequently, reduced LH secretion, depressed growth of the testes, and low plasma testosterone levels associated with nonstimulatory photoperiods (Wilson and Follett, 1974;Stokkan and Sharp, 1980;Gibson et al, 1975;Nicholls and Storey, 1976) may be due to the extreme sensitivity of the hypothalamus to androgen inhibition allowing the pituitary-gonadal axis to function at a low level. Reduction in TRH with long photoperiod in Japanese quail (Almeda, 1982) suggests that both gonadotrophs and thyrotrophs are inhibited by long photoperiod. The failure of 16-week-old photostimulated cockerels to maintain an elevated pituitary protein kinase activity ratio beyond 7 days indicates a weak suppressive effect of testosterone on hypothalamic-pituitary activity in the intact cockerel.…”