Effects of short day photoperiod on prolactin signaling in dry cows: a common mechanism among tissues and environments?

Photoperiodic manipulation has dramatic physiological and production effects in dairy cows. During lactation, exposure to long day photoperiod (LDPP) increases milk yield and circulating IGF-I and prolactin (PRL) concentrations. Conversely, cows housed under a short day photoperiod (SDPP) during the dry period produce more milk in the subsequent lactation than cows exposed to LDPP or natural photoperiod. Exposure to SDPP depresses PRL secretion but increases PRL receptor mRNA levels in mammary, immune, and hepatic tissues. In dry cows under SDPP, PRL signaling is a potential mechanism to drive more extensive mammary cell differentiation and growth relative to LDPP. In mammary biopsies taken during the dry period and into lactation, the amount of IGF-II mRNA was greater in SDPP vs. LDPP cows during the dry period, whereas IGFBP-5 mRNA increased in both groups during lactation even though photoperiodic treatments ended at parturition and all cows were on an ambient lighting schedule when lactating. Levels of IGF-I mRNA did not differ over time or between treatments; however, during the dry period, lower IGFBP-5 and increased IGF-II expression in SDPP cows may enhance mammary cell growth and survival. Key among the potential modulators of PRL signaling is the suppressors of cytokine signaling (SOCS) family. Mammary transcription of mRNA for SOCS proteins was low during the dry period but increased in lactation. During the dry period, SOCS mRNA level in the mammary gland of cows on SDPP was reduced compared with cows on LDPP, which may enhance PRL-induced proliferation and subsequent milk production. However, improved mammary capacity and immune function alone are likely insufficient to support increased milk yield. Using improved milk yield as a functional indicator of greater animal well-being during the transition, it is clear that some metabolic accommodation is necessary for expression of that capacity. Emerging evidence supports a link between PRL signaling and hepatic lipid metabolism, with decreases in PRL being beneficial to lipid metabolism. Extending that concept to broad environmental responses, it can be speculated that altered PRL signaling impairs lipid metabolism, mammary growth, and immune function under conditions of stress (e.g., heat stress) also. Thus, shifts in gene expression related to PRL signaling may provide an environmentally mediated mechanism to alter production and health in cows as they transition into lactation.