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|標題:||Hypothalamic circadian related gene expression pulsatility of meat- and egg-type hens in response to thyroid hormone and diurnal/nocturnal change|
meat- and egg-type hens
gonadotropin- releasing hormone
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|摘要:||現代雞隻演化可以追朔到8000年前，家雞起源於東南亞的紅色叢林雞，而在100多年前經由人為環境飼養與選拔育種下歧化出生長性強的肉雞(Broilers)與繁殖性強的蛋雞(Layers)。在20世紀初，為避免產肉及產蛋性狀間的內在衝突，藉由基因選拔方式分成商用肉雞與蛋雞。利用基因序列偵測出肉雞與蛋雞的差異點，包含生長、飲食、代謝等基因的調控，而促甲狀腺激素受體(thyroid stimulating hormone receptor, TSHR)影響途徑被認為是主要的關鍵之一。甲狀腺素除了主要調控能量代謝外，還能經由光照反應來調控換羽、鳥鳴、交配情形。本試驗主要目的在探討肉用與蛋用母雞其下視丘節律相關基因表現脈動對外源性甲狀腺素與晝夜改變之反應。試驗使用32週齡的紅羽土雞與海藍蛋雞，分成兩種處理，一開始在15:00給予任飼30分鐘，之後在17:00分別進行翼下靜脈注射生理食鹽水(1mL/kg)或三碘甲狀腺酪胺酸（3,5,3'-triiodothyronine, T3）(5μg/kg)做為控制組與處理組，犧牲採樣時間點為16:00、22:00、04:00、10:00、16:00。結果顯示，不論是蛋雞還是肉雞處理組血中TSH在早上5點時達到最高濃度，且其晝夜濃度波動均較控制組來的大（P < 0.05）。蛋雞與肉雞能量相關基因 (LKB1、UCP、PGC-1α) 表達與脈動模式直接受到甲狀腺處理影響，在22:00的表現量有顯著性提高。推測由白天進入晚上時施打甲狀腺素後，蛋雞與肉雞下視丘對體內能量變化的感測和調節上受到刺激，促使原本應在早上才表現的能量相關基因提早表現，以產生能量應付白天活躍狀態。處理組之肉雞下視丘GnRH與Dio2表現在22:00有顯著提升（P < 0.05），但是控制組GnRH表現在隔天16:00才有顯著增加（P < 0.05）。蛋雞處理組之GnRH與Dio2表現及波動則與控制組相似。蛋雞處理組晝夜節律基因Clock、Bmal1在白天時表現量有較顯著的脈動，但肉雞處理組則較平穩。肉雞處理組RORα基因在22:00達到波峰，但在蛋雞處理組RORα基因表現卻受到持續抑制。肉雞處理組Cry1及Cry2基因表現在4:00顯著提升，在16:00達到高峰（P < 0.05），但是蛋雞處理組Cry1及Cry2基因表現脈動則較平穩。蛋雞處理組TTF1基因表現在22:00達到波峰，而肉雞處理組TTF1基因表現在10:00受到抑制，但在隔天16:00顯著提升（P < 0.05）。歸納以上結果顯示: 造成肉雞處理組GnRH基因脈動較明顯是因為Cry1及TTF1晝夜脈動較大。甲狀腺素處理使得蛋雞血中TSH大量分泌，但對GnRH脈動則影響較小，因此推論蛋雞可以透過及時早期晝夜節律基因的表現，包括Clock及Bmal1來緩和甲狀腺素造成GnRH波動，而肉雞因甲狀腺素誘發晚期晝夜節律基因CRY1和TTF1表達延遲至白天，顯示容易受外源性因子影響而改變GnRH基因脈動。綜合以上; 肉雞的節律基因會因外在因子使得脈動更加明顯，且需一段時間回復，而蛋雞可以通過自我調控機制影響晝夜節律抵禦外部因子的刺激，說明了肉雞在維持晝夜節律基因表現較蛋雞不敏感。|
Evolution of modern chickens can be dated back to 8,000 years ago. House chicken was thought to be originated from Red Jungle fowls in Southeast Asia. The divergence of chicken breeds for egg laying and meat production was dated to about 100 years ago. During the 20th century, selection through population genetics produced the modern commercial strains of meat- and egg-type chickens. One of the most striking differences found between meat- and egg-type chickens was thyroid stimulating hormone receptor (TSHR) signaling, which has a pivotal role in metabolic regulation and photoperiod control of reproduction in birds. Thyroid hormone is the main hormone to regulate energy metabolism, molting, and mating in response to light cue. The aim of this study was to investigate the expression of circadian genes in the hypothalamus of meat- and egg-type hens in response to exogenous thyroid hormone. Hens at 32 weeks of age were injected with saline (1 mL/kg) or 3,5,3 '-triiodothyronine (T3, 5 μg/kg) through wing vein at 17:00 and sacrificed at 16:00, 22:00, 04:00, 10:00, and 16:00. Results showed that plasma TSH levels were higher in meat-type hens than that of egg-type hens before T3 treatment (P < 0.05). In contrast to meat-type hens, T3 treatment in egg layers resulted in a more dramatic increase of plasma TSH level peaked at 05:00. Expression of energy-related genes, liver kinase B1 (LKB1), uncoupling protein (UCP), and peroxisome proliferator-activated receptor-γ coactivator (PGC-1α) in T3-treated hens exhibited a rhythmic pattern in both types of hens and reached the peak at 22:00 (P < 0.05), suggesting that T3 stimulation mimics the light cue, which in turn drives the hypothalamus to reply an early ignition of gene expressions. Expression of GnRH and Dio2 (deiodinase 2) in T3-treated meat-type hens exhibited an early peak at 22:00, but the control hens showed a dramatic increase of GnRH transcription at 16:00 in the next day (P < 0.05). In egg-type hens, GnRH and Dio2 (deiodinase 2) expression oscillated at a relatively stable pattern in response to T3 treatment. T3 treatment induced a more fluctuating pattern of CLOCK and Baml1 expression during the day in egg-type hens, whereas meat-type hens showed a relative stable pulsatility in response to T3 induction. In meat-type hens, T3 treatment induced a more rhythmic RORα expression to reach the peak at 22:00 (P < 0.05), whereas layers exhibited persistently suppressed RORα expression. Cry1 and Cry2 expression climbed up at 04:00 and peaked at 16:00 in T3-treated meat-type hens, but egg-type hens showed a stable pattern. Per2 pulsatility remained stable by T3 injection in both types of hens. T3 treatment induced a more rhythmic TTF1 (thyroid transcription factor 1) expression to the peak at 22:00 (P < 0.05) in the layers, whereas meat type hens displayed suppressed TTF1 expression down to the wave trough at 10:00 and up to the plateau at 16:00 (P < 0.05). In contrast to layers, therefore, a more dramatic oscillation of GnRH expression in response to T3 treatment in meat-type hens is attributed to the late diurnal fluctuation of Cry1 and TTF1. Based on the results, thyroid hormone regulation on TSH secretion was more dramatic in egg-type hens, but the following effect of GnRH oscillation was not observed, suggesting that egg-type hens may operate through early circadian gene rhythmicity including CLOCK and Baml1 at night to withstand the effect of variable factors on the oscillation of GnRH expression, while meat-type hens exhibited a late diurnal fluctuation of circadian Cry1 and TTF1 expression in response to T3 induction, and thereby indicating an more oscillating GnRH secretion in response to external cues. In conclusion, the circadian gene expression patterns of meat-type hens apparently are more fluctuating in response to stimuli, and resume to the basal pattern in a sluggish way. Thus, the egg-type hens can manage a homeostatic circadian rhythmicity through compensatory mechanisms to withstand the variable cues, while the meat-type hens are less sensitive in maintaining circadian rhythmicity.
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