Effect of Cyclic Heat Stress on Feeding-Related Hypothalamic Neuropeptides of Three Broiler Populations and Their Ancestor Jungle Fowl

Research Note: Carcass yield and meat quality in high- and low-water efficient broiler lines exposed to heat stress

Heat stress (HS) and water scarcity are significant challenges to sustainable poultry production worldwide. It is, therefore, critical to identify effective strategies to prevent, withstand, or adapt to these challenges. After four generations of divergent selection for water efficiency, the present study was undertaken to determine the effect of HS on meat quality and muscle myopathy incidences in high (HWE)- and low (LWE)-water efficient broilers. Day-old male chicks (240 chicks/line) were allotted randomly by line and body weight-matched groups to 12 controlled-environmental chambers (2 pens/chamber). At d29, birds were exposed to 2 environmental conditions (thermoneutral (TN), 25°C; or cyclic HS, 36°C, 9h/d) in a 2 × 2 factorial design. On d49, birds were processed, carcass parts were weighed, meat quality and muscle myopathy incidence were assessed. Processing data were analyzed by Two-way ANOVA and Tukey's HSD multiple comparison test, and frequency of muscle myopathy score between groups was determined using Chi-square and Fisher's exact test. Significance was set at P < 0.05. As no significant environment by line interaction was discerned, the 2 main factors were analyzed separately. High water efficient birds had significantly higher tender- and leg quarter (LQ)-weight as well as carcass without giblet (WOG), chilled carcass WOG (CWOG), wing, LQ, and rack yields compared to their LWE counterparts. Both abdominal fat content and yields were significantly greater in LWE than HWE chickens. Chronic HS exposure significantly decreased dock, WOG, fat, CWOG, breast, tender, wing, and LQ weights as well as breast yield. HWE chickens had a significantly lower b* value compared to the LWE birds and HS significantly reduced the drip loss and the b* value compared to TN condition. Compared to LWE, HWE birds had higher and lower incidence of severe woody breast (WB) and white striping (WS) under TN and HS, respectively. HS reduced the incidence of both myopathies in both lines. In conclusion, the genetic selection for water efficiency seems to improve carcass yield, reduce fat content, and decrease the breast b* value. HWE birds had higher incidences of WB and WS under TN, which is reversed under HS conditions.

Effect of heat stress on the hypothalamic expression profile of water homeostasis-associated genes in low- and high-water efficient chicken lines

With climate change, selection for water efficiency and heat resilience are vitally important. We undertook this study to determine the effect of chronic cyclic heat stress (HS) on the hypothalamic expression profile of water homeostasis-associated markers in high (HWE)- and low (LWE)-water efficient chicken lines. HS significantly elevated core body temperatures of both lines. However, the amplitude was higher by 0.5-1°C in HWE compared to their LWE counterparts. HWE line drank significantly less water than LWE during both thermoneutral (TN) and HS conditions, and HS increased water intake in both lines with pronounced magnitude in LWE birds. HWE had better feed conversion ratio (FCR), water conversion ratio (WCR), and water to feed intake ratio. At the molecular level, the overall hypothalamic expression of aquaporins (AQP8 and AQP12), arginine vasopressin (AVP) and its related receptor AVP2R, angiotensinogen (AGT), angiotensin II receptor type 1 (AT1), and calbindin 2 (CALB2) were significantly lower; however, CALB1 mRNA and AQP2 protein levels were higher in HWE compared to LWE line. Compared to TN conditions, HS exposure significantly increased mRNA abundances of AQPs (8, 12), AVPR1a, natriuretic peptide A (NPPA), angiotensin I-converting enzyme (ACE), CALB1 and 2, and transient receptor potential cation channel subfamily V member 1 and 4 (TRPV1 and TRPV4) as well as the protein levels of AQP2, however it decreased that of AQP4 gene expression. A significant line by environment interaction was observed in several hypothalamic genes. Heat stress significantly upregulated AQP2 and SCT at mRNA levels and AQP1 and AQP3 at both mRNA and protein levels, but it downregulated that of AQP4 protein only in LWE birds. In HWE broilers, however, HS upregulated the hypothalamic expression of renin (REN) and AVPR1b genes and AQP5 proteins, but it downregulated that of AQP3 protein. The hypothalamic expression of AQP (5, 7, 10, and 11) genes was increased by HS in both chicken lines. In summary, this is the first report showing improvement of growth performances in HWE birds. The hypothalamic expression of several genes was affected in a line- and/or environment-dependent manner, revealing potential molecular signatures for water efficiency and/or heat tolerance in chickens.

Response of growth performance and cecum microbial community to cyclic heat stress in broilers

Heat stress (HS) can affect growth performance through alterations in specific gut microbiota, which greatly threatens poultry production. How HS affects the mechanisms of microbial changes in the poultry cecum and the complex interactions between cecal microbial changes and growth performance have not yet been well evaluated. This study was conducted to examine the changes in growth performance and cecal microbiotal community in cyclic heat stress (CHS)-treated broilers. A total of 200 twenty-eight-day-old female Arbor Acres (AA) broilers were equally allotted into neutral ambient temperature group (TN group, 24 ± 1°C, 24 h/day) and CHS group (33 ± 1°C, 8 h/day) with five replicates of 10 broilers each, respectively. Growth performance, cecum microbial diversity, flora composition, and community structure were analyzed on days 35 and 42. The decreased average daily feed intake (ADFI), average daily gain (ADG), and the increased feed/gain ratio (F:G) were observed in heat-stressed broilers on days 35 and 42. The alpha and beta diversity index had no significant changes at the two experimental periods (P > 0.05). At the genus level, CHS significantly increased the relative abundance of Enterococcus at 42 days (P < 0.05). Based on the analysis of linear effect size feature selection, CHS made an enriched Reyranella and a reduced Romboutsia and Ruminiclostridium at 35 days of age (P < 0.05). CHS made an enriched Weissella and Enterococcus at 42 days of age (P < 0.05). The present study revealed that CHS reduces broiler growth performance and alters the microbial community of the cecum microbiota and the abundance of species. These findings are of critical importance to alleviate the negative effects of CHS on broiler chickens' growth performance by maintaining gut microbial balance.

Peripheral and Central Impact of Methionine Source and Level on Growth Performance, Circulating Methionine Levels and Metabolism in Broiler Chickens

The present study was designed to evaluate the effects of DL-methionine (DL-Met) 2-hydroxy-4-(methylthio) butanoic acid (HMTBa), or S-(5'-Adenosyl)-L-methionine chloride (SAM), using feeding trial and central administration, on live performance, plasma metabolites, and the expression of feeding-related hypothalamic neuropeptides in broilers raised to a market age (35 d). Final average body weight (BW) and feed conversion ratio (FCR) from the feeding trial exceeded the performance measurements published by the primary breeder. At d35, the MTBHa group had better BW and lower feed intake, which resulted in a better FCR than the DL-Met group at 87 TSAA to lysine. At the molecular levels, the expression of hypothalamic neuropeptide (NPY) and monocarboxylate transporter (MCT) 2 did not differ between all treated groups; however, the mRNA abundances of hypothalamic MCT1 and orexin (ORX) were significantly upregulated in DL-Met- treated groups compared to the control. The ICV administration of SAM significantly reduced feed intake at all tested periods (from 30 to 180 min post injection) compared to the aCSF-treated group (control). The central administration of HMTBa increased feed intake, which reached a significant level only 60 min post administration, compared to the control group. ICV administration of DL-Met slightly increased feed intake compared to the control group, but the difference was not statistically discernable. Quantitative real-time PCR analysis showed that the hypothalamic expression of NPY, cocaine- and amphetamine-regulated transcript, MCT1, and MCT2 was significantly upregulated in the ICV-HMTBa group compared to the aCSF birds. The hypothalamic expression of the mechanistic target of rapamycin (mTOR), AMP-activated protein kinase (AMPKα1), D-amino acid oxidase, and hydroxyacid oxidase was significantly upregulated in DL-Met compared to the control group. The mRNA abundances of ORX were significantly increased in the hypothalamus of both DL-Met and HMTBa groups compared to the aCSF birds; however, mTOR gene expression was significantly downregulated in the SAM compared to the control group. Taken together, these data show, for the first time, that DL-Met and HMTBa have a common downstream (ORX) pathway, but also a differential central pathway, typically NPY-MCT for HMTBa and mTOR-AMPK for methionine.

Effect of Cyclic Heat Stress on Hypothalamic Oxygen Homeostasis and Inflammatory State in the Jungle Fowl and Three Broiler-Based Research Lines

Heat stress (HS) is devastating to poultry production sustainability due its detrimental effects on performance, welfare, meat quality, and profitability. One of the most known negative effects of HS is feed intake depression, which is more pronounced in modern high-performing broilers compared to their ancestor unselected birds, yet the underlying molecular mechanisms are not fully defined. The present study aimed, therefore, to determine the hypothalamic expression of a newly involved pathway, hypoxia/oxygen homeostasis, in heat-stressed broiler-based research lines and jungle fowl. Three populations of broilers (slow growing ACRB developed in 1956, moderate growing 95RB from broilers available in 1995, and modern fast growing MRB from 2015) and unselected Jungle fowl birds were exposed to cyclic heat stress (36°C, 9 h/day for 4 weeks) in a 2 × 4 factorial experimental design. Total RNAs and proteins were extracted from the hypothalamic tissues and the expression of target genes and proteins was determined by real-time quantitative PCR and Western blot, respectively. It has been previously shown that HS increased core body temperature and decreased feed intake in 95RB and MRB, but not in ACRB or JF. HS exposure did not affect the hypothalamic expression of HIF complex, however there was a line effect for HIF-1α (P = 0.02) with higher expression in JF under heat stress. HS significantly up regulated the hypothalamic expression of hemoglobin subunits (HBA1, HBBR, HBE, HBZ), and HJV in ACRB, HBA1 and HJV in 95RB and MRB, and HJV in JF, but it down regulated FPN1 in JF. Additionally, HS altered the hypothalamic expression of oxygen homeostasis- up and down-stream signaling cascades. Phospho-AMPK^Thr172 was activated by HS in JF hypothalamus, but it decreased in that of the broiler-based research lines. Under thermoneutral conditions, p-AMPK^Thr172 was higher in broiler-based research lines compared to JF. Ribosomal protein S6K1, however, was significantly upregulated in 95RB and MRB under both environmental conditions. HS significantly upregulated the hypothalamic expression of NF-κB2 in MRB, RelB, and TNFα in ACRB, abut it down regulated RelA in 95RB. The regulation of HSPs by HS seems to be family- and line-dependent. HS upregulated the hypothalamic expression of HSP60 in ACRB and 95RB, down regulated HSP90 in JF only, and decreased HSP70 in all studied lines. Taken together, this is the first report showing that HS modulated the hypothalamic expression of hypoxia- and oxygen homeostasis-associated genes as well as their up- and down-stream mediators in chickens, and suggests that hypoxia, thermotolerance, and feed intake are interconnected, which merit further in-depth investigations.