Photoperiod and bromocriptine treatment effects on expression of prolactin receptor mRNA in bovine liver, mammary gland and peripheral blood lymphocytes

Abstract

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.

Prolactin Mediates Photoperiodic Immune Enhancement: Effects of Administration of Exogenous Prolactin on Circulating Concentrations, Receptor Expression, and Immune Function in Steers1

“…Real-time PCR was performed as described in Auchtung et al [3]. Briefly, RNA was extracted from lymphocytes using Trizol reagent (Gibco BRL, Grand Island, NY) and stored at Ϫ80ЊC until further processing.…”

“…Taqman (Applied Biosystems, Foster City, CA) probes and primers were designed using Primer Express software for both the long-and short-form of PRL-R, and 18S rRNA was amplified as the internal reference. Detection was performed using an ABI PRISM 7700 Sequence Detector (Applied Biosystems) and validation was conducted according to Auchtung et al [3]. Calculated input amounts were normalized to the 18S values.…”

“…In contrast, short-day photoperiod ex- posure (SDPP; 8L:16D) during late gestation, when lactation has ceased (i.e., the dry period), increases milk yields in the next lactation compared with cows exposed to LDPP [2]. Further, our laboratory has recently observed an enhancement of cellular immune function in dairy cattle [3,4] under SDPP similar to the positive effects of SDPP seen in rodents [5,6]. The endocrine mechanism that underlies these responses to photoperiod, however, is unclear.…”

“…Sensitivity to PRL is also a potential mechanism for altered immune function. Peripheral lymphocytes express PRL-R mRNA in many species, including the bovine [9], and receptor expression in lymphocytes is reduced by PRL administration in rodents [10] and increased by bromocriptine treatment [3,10]. We hypothesize that the sensitivity of an animal to shifts in PRL perceived by the differing expression of long and short forms of PRL-R with different photoperiod treatments is the mechanism by which the animal's immune system is influenced.…”

“…Changes in photoperiod result in robust alterations in circulating concentrations of prolactin (PRL) [7] and expression of its receptor (PRL-R) [3,4] in the bovine. Although postpartum exogenous PRL administration does not appear to alter lactational performance in dairy cows [8], PRL has been implicated in changes of mammary function in the peripartum period.…”