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Photoperiod manipulations during the dry period significantly impact mammary circadian clock in goats
Exposing goats to short day photoperiod (SDPP; 8 h light:16 h dark) during the dry period increases milk production compared to long day photoperiod (LDPP; 16 h light:8 h dark) exposure, due in part to increased mammary cell proliferation rates. Photoperiod information is sent to the master clock in the suprachiasmatic nuclei (SCN) via the retinal nerve. In turn the SCN sends temporal information out to peripheral clocks located in every tissue of the body to synchronize physiological systems to time of day and season. Studies support mammary clock regulates cell proliferation, thus we hypothesized photoperiod effects on milk production are mediated in part by changes in the molecular clock located in mammary gland. The objective of this study was to determine the effect of photoperiod manipulation during the dry period in goats on core clock gene expression in mammary gland. Multiparous Israeli Saanen goats (n = 6) were blocked at dry off (∼45 d prepartum) into 2 treatments: LDPP (n=3) and SDPP (n=3) based on body weight and previous milk production. All goats were housed in metabolism chambers equipped with two separate but identical environmentally controlled rooms in which photoperiod was adjusted according to the treatment. Goats were fed a total mixed ration in two equal meals at 0800 and 1500 to meet nutritional demands. Serial mammary biopsy were taken over a 24 h period from each goat during three weeks prepartum at 4 h intervals (0900, 1300, 1700, 2100, 0100, 0500). Tissue was placed in Trizol and immersed in liquid nitrogen. Total RNA was isolated and q-PCR was used to measure expression of two reference genes (BACTIN and GAPDH) and the core clock genes CLOCK and ARNTL. Relative gene expression was calculated using delta-delta CT method with mean of SDPP treatment as normalizer. Exposure to SDPP significantly increased ARNTL (P <0.05) while significantly decreasing CLOCK gene expression. CLOCK and ARNTL heterodimerize to function as a transcription factor, thus changes in their abundance due to photoperiod manipulation will affect expression of target genes, including those that regulate cell proliferation.
Keywords: photoperiod, CLOCK, ARNTL