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Wall, Emma

Animal Science

2005

MS

Mammary gland development and function are critical for successful lactation in dairy cows. Exposure to short day photoperiod (SD; 8h light:16h dark) during the dry period increases milk yield of dairy cows in the subsequent lactation. During early lactation, increased milking frequency results in a persistent increase in milk yield. The mechanism(s) by which these factors affect milk yield are unknown. We hypothesized that sensitivity of the bovine mammary gland to prolactin (PRL) is increased by exposure to SD during the dry period and increased milking frequency during early lactation. Two animal trials were conducted in order to test our hypothesis. For the first trial, multiparous Holstein cows were dried off 60 days before expected parturition and assigned to long day (LD; 16h light:8h dark) or SD during the dry period. Mammary biopsies were obtained at -40, -20, -10 and + 10 days relative to expected calving. The second trial involved twenty multiparous Holstein cows that received 1 of 4 treatments from d 2 to 21 postpartum: twice daily milking with (2X+PRL) or without (2X) exogenous PRL, or four times daily milking with (4X +BROMO) or without (4X) bromocriptine treatment. Mammary biopsies were obtained at 7 DIM. For both trials, mammary tissue was incubated in media containing [³H]-thymidine to determine incorporation into DNA in vitro. Autoradiograms were prepared to identify proliferating cells, and TUNEL was performed to identify cells undergoing programmed cell death. Real time RT -PCR was used to quantify expression of PRL-regulated genes. Cell proliferation increased in SD cows from -40d to -20d and was higher at -20d than in LD cows. For both groups, cell proliferation decreased during lactation. The apoptotic index in mammary epithelial cells was lower overall in SD cows compared to LD cows. Expression of IGF-II mRNA increased through the dry period and into lactation and was higher in SD cows. Expression of both forms of PRL-R mRNA increased during the dry period and was higher in SD cows relative to LD cows. In both treatment groups, expression of suppressors of cytokine signaling. (SOCS) increased over time and expression of SOCS-3, SOCS-2 and CIS RNA was higher in LD cows relative to SD. In the second trial, cell proliferation was increased by 50% with PRL treatment, and analysis of autoradiograms revealed that stromal cell proliferation increased with increased milking frequency. Mammary expression of SOCS-3 mRNA decreased with increased milking frequency or PRL treatment, whereas SOCS-2 mRNA increased with 4X milking or PRL treatment. Abundance of PRL-R mRNA was higher in 2X than 2X+PRL cows and higher in 4X than 4X +BROMO cows. Expression of -lactalbumin mRNA was increased by PRL treatment. We conclude that exposure to SD during the dry period enhances mammary growth relative to LD. In addition, sensitivity of the mammary gland to PRL may be increased by exposure to SD as a result of decreased SOCS expression. Differences between treatments in the temporal pattern of proliferation suggest the existence of a critical period wherein photoperiod affects mammary gland development during the dry period. We also conclude that the bovine mammary gland is responsive to PRL during early lactation. Differences in expression of PRL-responsive genes suggest interactive effects of PRL and milk removal in the regulation of mammary response to increased milking frequency.