ABSTRACT: Embryonic heat conditioning (EHC; exposing the chick embryo to elevated temperatures transiently during incubation) has been associated with greater stress resiliency later in life, although mechanisms are unclear but likely involve the hypothalamus. The objective of this study was to determine the effects of an acute heat challenge at day 4 post-hatch on the transcriptome of several brain nuclei associated with thermal regulation, stress, and appetite, including the paraventricular nucleus of the hypothalamus (PVN), pre-optic anterior/hypothalamic area (POAH), and the nucleus of the hippocampal commissure (nCPa), in broilers that were subjected to control or EHC. The nuclei were collected by punch biopsy at 3 time points relative to the start of heat challenge (0, 2 and 12 hours), total RNA was isolated, and RNA-sequencing was performed. Transcript abundance was quantified and differentially expressed genes (DEG) identified, and gene ontology analyses were performed. In the nCPa, 469 DEGs were identified across the 3 timepoints. There were 0 DEGs at hour 0, 2 at hour 2, and 467 at hour 12. The gene ontology analysis on nCPa hour 12 samples revealed enrichment of 5 biological processes including mitochondrial electron transport, mitochondrial respiratory chain complex 1 assembly, synaptic vesicle lumen acidification, protein export from nucleus, and aerobic respiration. Most of these genes were down-regulated suggesting that these processes were less functional in EHC chicks. In the POAH, total 18 DEGs were identified, with 0 at hour 0, 18 at hour 2, and 0 at hour 12. Fewer differences were observed in the PVN, with only 4 DEGs identified. All 4 were up-regulated in the EHC group, with 2 being involved in hypothalamic thermal responses: vasoactive intestinal peptide transporter 1 (VIPR1) and caprin family member 2 (CAPRIN2). We also compared expression across time. There were no differences identified in the POAH or PVN. In the nCPA, no differences were detected for hour 2 vs. hour 0, however; the comparison between hour 12 and 2 yielded 9 DEGs. All except 1 were down-regulated at hour 12. The hour 12 vs. 0 comparison revealed 49 DEGs of which 24 were downregulated at hour 12. In conclusion, results revealed some pathways associated with energy metabolism that were altered in response to EHC, with most differences in the nCPa. Surprisingly, fewest differences were observed in the PVN. Findings provide some future target regions, such as the nCPa, and metabolic pathways to better understand how EHC affects stress responses and energy homeostasis later in life.
