T.L. Auchtung

Cows exposed to short day photoperiod during the dry period produce significantly more milk in their subsequent lactation than cows exposed to long days. The mechanism(s) underlying this effect are unknown. Because concentrations of prolactin (PRL) in circulation are consistently affected by changes in photoperiod, we hypothesized that alterations in the prolactin axis and sensitivity of the mammary gland to prolactin signaling may mediate photoperiodic effects in dry cows. The objective of this study was to determine the effects of exposure to different lengths of daylight during the dry period on circulating PRL and PRL receptor (PRL-R) mRNA expression in lymphocytes and mammary tissue during the transition to lactation. Multiparous Holstein cows were dried off 62 d before calving and assigned to long day (16 h light: 8 h dark) or short day photoperiod (8 h light: 16 h dark). During the dry period, PRL and PRL-R mRNA were analyzed biweekly in plasma and lymphocytes, respectively. Expression of PRL-R mRNA was assessed in mammary biopsies during the dry and periparturient periods. Dry matter intake (DMI) was recorded through 21 d of lactation, and milk yield was recorded until 120 d in milk. Short day photoperiod was associated with reduced PRL, whereas milk yield and expression of PRL-R mRNA in lymphocytes and mammary tissue were increased. Cows on short days had higher DMI during the dry period but did not differ in DMI after parturition. These data support the concept that greater responsiveness and sensitivity to PRL during transition to lactation may be associated with an increase in subsequent milk yield.

Exposure to short day photoperiod (SD; 8 h light:16 h dark) during the dry period increased milk yield of cows in the subsequent lactation. We hypothesized that this effect is due to increased growth of mammary cells in response to enhanced prolactin signaling to influence the insulin-like growth factor (IGF) axis. Multiparous Holstein cows were dried off 60 d before parturition and assigned to long day photoperiod (LD; 16 h light:8 h dark) or SD during the dry period. Mammary biopsies were obtained at approximately -40, -20, -10 and +10 d relative to expected calving. Expression of IGF-I, IGF-II, and IGF binding protein-5 mRNA was assessed by real time reverse transcription-polymerase chain reaction. In SD cows, incorporation of [3H]-thymidine in vitro increased from -40 d to -20 d and was greater at -20 d than in LD cows. A later increase in proliferation was observed at -10 d in LD cows. For both groups, cell proliferation decreased during lactation. Analysis by terminal deoxynucleotidyl transferase dUTP nick end labeling revealed that the apoptotic index in mammary epithelial cells was less in SD cows than in LD cows. Expression of IGF-II mRNA increased during the dry period and into lactation and was greater in SD cows. Expression of IGF binding protein-5 mRNA increased during lactation, but was unaffected by day length. Expression of IGF-I did not differ over time or between treatments. We concluded that exposure to SD during the dry period enhanced mammary growth relative to LD, and this may be related to increased expression of IGF-II. Treatment differences in the temporal pattern of proliferation indicated the existence of a critical period wherein photoperiod affects mammary gland development during the dry period.

Recent studies suggest that exposure of cattle to photoperiod can influence immune function. The objective of this study was to determine whether treatment of cows with short day photoperiod (SDPP; 8 h light: 16 h darkness) during the dry period alters immune function, relative to cows subjected to a long day photoperiod (LDPP; 16 h light: 8 h darkness). Multiparous Holstein cows (n = 39) were dried 62 d before calving and exposed to photoperiod treatment until parturition; thereafter, cows were exposed to natural photoperiod. General health was monitored weekly during the dry period and cellular immune function was examined monthly during the dry period and at calving. Concentrations of prolactin and cortisol were measured from 10 d before calving to 2 d after calving. The periparturient prolactin surge in plasma was greater in LDPP cows (54.6 ng/ mL) than SDPP (22.4 ng/mL). Relative to LDPP cows, neutrophil chemotaxis and lymphocyte proliferation were enhanced in SDPP cows during the dry period. Neutrophil chemotaxis averaged 142.5 and 178.8 cells/ well during the dry period for LDPP and SDPP, respectively. Lymphocyte proliferation during the dry period averaged 197.6 and 326.5% for LDPP and SDPP cows, respectively. Physiological characteristics of the cows were not affected by treatment during the dry period. However, differences between treatments were observed within 2 d of parturition. Potential implications of photoperiod management for cow health and well-being merit further investigation.

Recent evidence suggests that photoperiod influences immune function. Interestingly, photoperiod has profound effects on concentrations of prolactin (PRL), a hormone also known to be involved in fluctuations of the immune system. However, the impact of photoperiod on PRL receptor (PRL-R) expression is poorly understood, particularly in tIssues of the immune system. Two experiments were performed to increase the general understanding of how photoperiod interacts with the immune system. Our first objective was to determine the effects of photoperiod on PRL-R mRNA expression and cellular immune function. Lymphocytes were isolated from blood collected from calves (n=10) and PRL-R mRNA expression of both long and short forms was quantified using real-time PCR. Lymphocytes expressed PRL-R mRNA, suggesting that PRL could act directly on these cells. To determine the relationship between photoperiod and PRL-R mRNA expression in other tIssues, hepatic and mammary biopsies were collected after calves were exposed to long days (LDPP; 16 h light:8 h darkness) or short days (SDPP; 8 h light:16 h darkness). Relative to LDPP, SDPP decreased circulating PRL, but increased expression of both forms of PRL-R mRNA in liver, mammary gland and lymphocytes. Short days also increased lymphocyte proliferation compared with long days. Reversal of photoperiodic treatments reversed the effects on circulating PRL, PRL-R mRNA expression and lymphocyte proliferation. Our second objective was to manipulate PRL concentration in photoperiod-treated animals, using bromocriptine. Concentrations of PRL in LDPP animals injected daily with bromocriptine for 1 week were decreased compared with LDPP controls, to a level similar to SDPP animals. Receptor expression was increased in LDPP+bromocriptine-treated animals relative to LDPP controls, as was lymphocyte proliferation. Overall, our results indicate that photoperiodic effects on PRL-R mRNA expression were inverse to those on circulating PRL, with short days stimulating expression of both forms of PRL-R mRNA. Expression of PRL-R mRNA changed in the same direction as lymphocyte proliferation with regard to photoperiod treatment, suggesting a link between photoperiodic effects on PRL sensitivity and immune function. Thus, PRL signaling may mediate photoperiodic effects on immune function.

Changes in photoperiod can significantly impact the physiology of many species. For example, we have observed an improvement in cellular immune function in cattle on short-day photoperiod (SDPP) relative to long-day photoperiod (LDPP). In addition, prolactin (PRL) and PRL receptor (PRL-R) are affected by photoperiod management. Our hypothesis is that the inverse relationship observed between PRL and PRL-R mRNA expression during photoperiod treatment alters the sensitivity of the animal to PRL, thereby affecting the changes in their cellular immune function. The objective of this study was to determine the effects of exogenous PRL on photoperiodic-mediated immune responses. Eight Holstein steers received each of four treatments: LDPP (16L:8D), SDPP (8L:D), SDom (SDPP plus PRL via osmotic minipump for 10 days), and SDinj (SDPP plus PRL via 3x daily injections for 10 days). Steers on SDPP had decreased PRL relative to the other treatments. Expression of PRL-R mRNA was increased in SDPP animals relative to LDPP, SDom, and SDinj. Prior to PRL treatment, SDPP animals had greater lymphocyte proliferation and neutrophil chemotaxis relative to LDPP animals. Following PRL treatment, cellular immune function of SDom and SDinj animals was reduced to the level of LDPP animals. Addition of PRL to the in vitro lymphocyte proliferation did not alter response of LDPP animals but increased proliferation of lymphocytes from SDPP animals. The results of these experiments suggest that an animal's responsiveness to PRL correlate to changes in cellular immune function that occur with photoperiod manipulation.