Opioid mechanisms suppress the expression of milk proteins in pre-parturient mammary explant tissue

Abstract

Animal Production in Australia 1998 Vol. 22 OPIOID MECHANISMS SUPPRESS THE EXPRESSION OF MILK PROTEINS IN PREPARTURIENT MAMMARY EXPLANT TISSUE P.A. SHEEHYA, K.R. NICHOLASB, J.M. GOODENA and P.C.WYNN A B A Department of Animal Science, University of Sydney, Camden, NSW 2570 Victorian Institute of Animal Science, Attwood, Vic 3049 The process of mammogenesis involves the proliferation of mammary alveoli and their constituent epithelial cells which acquire the capacity for the synthesis of milk and its proteins. The lactational potential of a cow is not attained if these mechanisms are not fully functional at the time of peak lactation. We have identified a key period of 30 to 40 days pre-calving during which milk protein gene expression is low but inducible in mammary tissue explants in vitro with the appropriate lactogenic hormones, insulin (I), cortisol (F) and prolactin(P) (Sheehy et al. 1997). The identification of factors that limit gene induction during this period will be of commercial importance to the dairy industry. In this study we have investigated the impact of the stress-related hormone endorphin (Ep) on the ability of I, F and P to induce and casein gene expression in mammary explants from biopsies collected 30 days pre-calving. Mammary explants (each of 1mg; 20 per well) were floated in cell culture medium (M199) for 4 days in the presence of I (5�g/mL) and F (500ng/mL). P (500ng/mL) was then added together with increasing concentrations of Ep (50, 500 and 5000pg/mL) for a further 2 days. Total RNA was analysed for milk protein mRNA expression by incubating membranes from slot blots with specific cDNA probes and quantitating the signal by densitometry. The addition of P induced and casein gene expression. Co-incubation with Ep (50pg/mL) reduced this expression (Figure 1). To assess if Ep was operating through the � opioid receptor we substituted the specific � agonist D-Ala-Met-enkephalin-Gly-ol (DAMGO: 5, 50, 250, 2500 pg/mL) for Ep, which also suppressed this gene expression (Figure 2). To confirm the specificity of the response tissue maintained with I, F and P was incubated with DAMGO together with the specific � receptor antagonist D-Phe-Cys-Tyr-DTrp-Arg-Thr-Pen-Thr-NH2 (CTAP), which binds only to the � receptor. This antagonist prevented the DAMGO-induced suppression of gene expression. Figure 1. The influence of b endorphin on on k casein gene expression. Figure 2. The influence of DAMGO and CTAP o n k casein gene expression. The perception of stress resulting in endorphin release in cows 30 to 40 days pre-calving may suppress milk protein synthesis. We speculate that the timing of this stress may be important in determining its impact on milk protein synthesis for the entire lactation. SHEEHY, P.A., NICHOLAS, K.R., GOODEN, J.M. and WYNN, P.C. (1997) In Recent Advances in Animal Nutrition in Australia, pp. 176-184. (University of New England: Armidale NSW). 369