Hypothalamic gonadotrophin-releasing hormone and pituitary and plasma FSH and prolactin during photostimulation and photorefractoriness in intact and thyroidectomized starlings (Sturnus vulgaris)

Abstract

Changes in concentrations of hypothalamic gonadotrophin-releasing hormone (GnRH) and pituitary and plasma FSH and prolactin were measured in intact and thyroidectomized female starlings (Sturnus vulgaris) after transfer from short to long photoperiods. In intact birds, hypothalamic GnRH did not increase significantly during the first 6 weeks of photo-stimulation, but by 12 weeks, as birds became photorefractory, it had decreased to levels significantly lower than those before photostimulation. In thyroidectomized birds, which did not become photorefractory, hypothalamic GnRH remained high after 12 weeks of photostimulation. Pituitary FSH increased in both intact and thyroidectomized birds; it then decreased to low levels in intact photorefractory birds, but remained high in thyroidectomized birds. Plasma FSH increased to a peak after 2 weeks, but by 6 weeks it had decreased to low levels in both groups. In intact birds there was a 70-fold increase in pituitary prolactin during the first 6 weeks, and levels were still high after 12 weeks of photostimulation. In thyroidectomized birds, pituitary prolactin remained low. The results suggest that while the initial effect of long daylengths is to cause gonadal maturation, the ultimate effect is to switch off the reproductive system.

Seasonal Changes in Brain GnRH Immunoreactivity and Song-Control Nuclei Volumes in an Opportunistically Breeding Songbird

“…In these species, refractoriness typically correlates with a dramatic decrease in the amount of GnRH in the hypothalamus, as compared to levels in photosensitive and/or photostimulated birds [reviewed in Ball and Hahn, 1997;Hahn et al, 1997]. This centrally-mediated down-regulation of reproductive physiology has been documented in European starlings [Sturnus vulgaris;Dawson et al, 1985;Foster et al, 1987], garden warblers [Sylvia borin; Bluhm et al, 1991], dark-eyed juncos [Junco hyemalis ;Saldanha et al, 1994;, house sparrows [Passer domesticus; Hahn and Ball, 1995], American tree sparrows [Spizella arborea; Reinert and Wilson, 1996;Wilson and Reinert, 1996], house finches [Carpodacus mexicanus; Cho et al, 1998] and Gambel's white-crowned sparrows [Zonotrichia leucophrys gambelii; Wingfield and Farner, 1993; but see Meddle et al, 1999]. On the whole, these data suggest that the hypothalamo-pituitary-gonad axis might be switched off at the hypothalamic level during refractoriness in many species, either precluding or reducing reproductive responses to environmental cues.…”

Photoperiodic induced changes in reproductive state of border canaries (Serinus canaria) are associated with marked variation in hypothalamic gonadotropin-releasing hormone immunoreactivity and the volume of song control regions

“…For example, a report on the American Singer canary breed (Bentley et al, 2003) suggested that when this strain is chronically photostimulated they do not exhibit absolute photorefractoriness --defined by a regression of the gonads after prolonged photostimulation that cannot be reversed even with 24 hours of light (Hamner, 1968). Additionally, GnRH content in the brain does not change in these birds as they transition from reproductive to non-reproductive conditions (Bentley et al, 2003), as it does in many absolutely refractory species both within and outside the cardueline subfamily (e.g., Dawson et al, 1985, Goldsmith et al, 1989, reviewed in Ball and Hahn, 1997. As discussed in Bentley et al (2003), it is possible that this breed has lost a wild-type pattern of photoperiodic responsiveness along with the associated neuroendocrine plasticity, but this cannot be asserted in the absence of knowledge about plasticity in GnRH in other canary populations.…”

Thyroidectomy results in termination of photorefractoriness in starlings (Sturnus vulgaris) kept in long daylengths

“…The onset of photorefractoriness in starlings is known to be dependent on the thyroid gland because thyroidectomized birds do not become photorefractory when transferred to long daylengths (Wieselthier & van Tienhoven, 1972;Goldsmith & Nicholls, 1984a; Dawson, Follett, Goldsmith & Nicholls, 1985) and because photorefractoriness can be induced by treatment with thyroxine (Goldsmith & Nicholls, 1984b;Nicholls, Goldsmith, Dawson, Chakraborty & Follett, 1984). The effect of thyroidectomy on birds that have already become photorefractory is unclear, although the results of Wieselthier & van Tienhoven ( 1972) suggested that it might result in gonadal growth.…”