Variation in gonadotrophin-releasing hormone-1 gene expression in the preoptic area predicts transitions in seasonal reproductive state.
ABSTRACT In many seasonally reproducing animals, the experience of prolonged exposure to constant photoperiods results in the induction of a state of photorefractoriness, which is defined as a lack of responsiveness to a previously stimulatory photoperiod. The physiological and genetic processes that control photorefractoriness are not well understood; however, the hallmark of photorefractoriness is an endogenous change in the physiological response to a constant photoperiod. It is already known that preoptic area (POA) gnrh1 gene expression declines during the development of refractoriness to long-day stimulation in European starlings. We employed in situ hybridisation histochemistry to characterise changes in POA gnrh1 mRNA expression during the reinstatement of photosensitivity in female starlings. Photorefractory starlings moved to short days (8L:16D) increased optical density of gnrh1 expressing cells within 10 days. Exposure to 30 short days resulted in greater visible gnrh1 cell numbers, with no detectable change in measures of ovarian follicular volume and oviduct mass. We subsequently examined the extent of gnrh1 expression in response to photostimulation after incremental periods on short day lengths. A significant long-day-induced increase in both gnrh1 expression and ovarian and oviduct mass occurred only after at least 30 short days. These findings demonstrate that the recovery of photorefractoriness involves an increase in gnrh1 mRNA expression and expands upon our previous knowledge that the development of photosensitivity is associated with an increase in both the precursor proGnRH1-GAP and GnRH1 peptides in the POA. Importantly, the change in the brain sensitivity occurs well before such changes can be detected via variation in ovarian activity.
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ABSTRACT: Cardueline finches (canaries, goldfinches, and rosefinches, etc.) vary widely in the degree to which their natural reproductive schedules track seasonal changes in photoperiod. In this study, we tested for photo-induction of reproductive development and photorefractoriness in males of three cardueline finch species: pine siskins (Carduelis pinus), common redpolls (Carduelis flammea), and white-winged crossbills (Loxia leucoptera). Exposure to long days (20L:4D) in winter induced gonadal growth and elevation of circulating luteinizing hormone (LH) in all three species. After 4.5 months on constant long days, gonadal regression was complete in redpolls and siskins, but only partial in crossbills. Feather molt was most advanced in redpolls, slightly less advanced in siskins, and least advanced in crossbills. These results indicate that the reproductive systems of all three species were stimulated by long days, but that the crossbills, which are temporal opportunists, either did not become absolutely photorefractory, or developed refractoriness more slowly than did the other two species. Reproductive development of controls held for 4.5 months on constant short days (5L:19D) was negligible in redpolls and crossbills, but substantial in siskins, suggesting that of the three species, pine siskins may be the least dependent on long days for reproductive development. Changes in fat deposition and body mass also differed among species. Short day redpolls tended to be fatter and heavier than long day redpolls, and long day crossbills tended to be fatter and heavier than short day crossbills. Body mass and fat depot of siskins remained high irrespective of photoperiod. These results illustrate substantial variation among these close relatives, and are consistent with the idea that differences in apparent reproductive flexibility among cardueline taxa relate to interspecific differences in responsiveness to environmental cues, not simply to differences in the environments experienced. This kind of information is critical to an understanding of the mechanistic bases of natural variation in reproductive schedules, and of how different species may be affected by modifications to the environment.General and Comparative Endocrinology 06/2004; 137(1):99-108. · 3.27 Impact Factor
Physiological Reviews 02/1988; 68(1):133-76. · 26.87 Impact Factor