Influences of thyroidectomy and thyroxine replacement on photoperiodically controlled reproduction in quail

Abstract

The breeding season in quail ends when they become relatively refractory to long photoperiods. The processes underlying the development of this refractoriness are dependent upon the thyroid gland. Thyroidectomized male Japanese quail transferred from short (8 h light: 16 h darkness; 8L: 16D) to long (20L:4D) daily photoperiods mature their gonads and develop the androgen-dependent cloacal gland at rates only marginally slower than controls. However, when the birds are retransferred to short days thyroidectomized individuals do not show the rapid testicular regression and moult which occurs in controls under such conditions. The testes remain large for a long period, eventually decreasing in size slowly and irregularly. Once such thyroidectomized birds are transferred back to short days the administration of thyroxine has little effect upon the slow rate of gonadal regression but if the hormone is administered during the preceding period of long days normal regression does occur under short days, even though treatment has ceased. This suggests that a process dependent upon thyroid hormones takes place under long day-lengths that ensures the termination of reproduction when they are exposed either to short daylengths or to decreasing daylengths after the summer solstice. This view was substantiated by further experiments in which thyroidectomy was performed after the birds had been exposed to long daylengths. When transferred to 8L:16D normal testicular regression and moult occurred rapidly. Presumably the thyroid-dependent process had proceeded to an adequate extent in these quail before the thyroid glands were destroyed. Further support came from treating intact quail chronically with thyroxine. Its administration to quail on 20L:4D had no observable effect; the testes grew rapidly to maximal size and remained large. However, if the birds were given thyroxine under 12L:12D the gonads first developed to maximal size and then, after about 12 weeks of treatment, regressed spontaneously whether or not thyroxine was still being administered. It seems that thyroxine had made the quail so relatively refractory that 12L:12D was no longer sufficiently stimulatory to overcome the developing inhibition on the photoperiodic system. The results in quail are very reminiscent of the finding that the absolute photorefractoriness and spontaneous testicular regression which develops in starlings under long daylengths is thyroid dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of Methimazole-induced Hypothyroidism on Adrenal and Gonadal Functions in Male Japanese Quail (<i>Coturnix japonica</i>)

“…In birds which are long day breeders, thyroidectomy prevents refractoriness developing, and this results in the birds remaining in a breeding condition indefinitely under long days [22][23][24]. In rats, previous studies have shown that hypothyroidism decreases the plasma concentrations of LH and FSH [25,26] and reduces the number and size of gonadotropes [27], suggesting that hypothyroidism is associated with gonadal dysfunctions.…”

“…Other studies in birds support the hypothesis that the thyroid axis plays a permissive role in the reproduction of seasonally breeding species [11][12][13]. Therefore, the aim of this study was to investigate the effects of hypothyroidism on the adrenal and gonadal functions in adult male Japanese quail.…”

Effect of Exogenous Thyroxine on Timing of Seasonal Reproductive Transitions in Ewes1

“…For example, thyroidecAccepted March 23, 1993. Received December 15, 1992 tomy before the onset of the breeding season blocks the transition into the nonbreeding season in certain birds [10][11][12], sheep [13][14], and deer [15]. After thyroidectomy, such animals are maintained in a state of continuous reproductive activity.…”

Molecular basis for regulating seasonal reproduction in vertebrates

“…Furthermore, removal of the thyroid gland prevents seasonal development/regression of gonads in several avian species and in sheep (Moenter et al 1991, Dawson 1993, 1998, Parkinson & Follett 1995. Administration of TH ameliorates the effect of thyroidectomy and mimics exposure to light conditions that induce gonadal development (Follett & Nicholls 1985, Goldsmith & Nicholls 1992, Wilson & Reinert 2000.…”